From Past to Present: Gold Nanoparticles (AuNPs) in Daily LifeSynthesis Mechanisms, Influencing Factors, Characterization, Toxicity, and Emerging Applications in Biomedicine, Nanoelectronics, and Materials Science

Abstract

Gold nanoparticles (AuNPs) are renowned for their unique optical, electronic, and biocompatible properties, making them ideal for applications in drug delivery, diagnostics, imaging, and materials science. With green chemistry-based biological synthesis methods gaining popularity, eco-friendly alternatives to traditional chemical and physical methods techniques have emerged. Plant extract-derived AuNPs stand out for their remarkable medicinal properties, stability, and low reactivity, enhancing their biological applicability. This review explores the different synthesis methodschemical, physical, and biologicalhighlighting their advantages and challenges. We summarize the latest research (Updated until March, 2025), focusing on the most recent developments in the synthesis and multifield application of AuNPs, offering a comprehensive perspective on their potential.

1. Introduction

The current era is observing a widespread use of nanotechnology and nanomaterials. The ever-increasing demand has motivated researchers to explore and develop novel nanomaterials with enhanced properties for diverse applications. , Nanotechnology is highly versatile, with applications spanning multiple fields such as electronics, sensors, optics, mechanics, catalysis, chemistry, cosmetics, pharmaceuticals, medicine, biomedical sciences, food technology, and environmental science. − Metal nanoparticles are extensively utilized across various industries due to their exceptional physicochemical properties, , including a high surface-area-to-volume ratio, enhanced catalytic activity, and tunable optical and electronic characteristics. − Gold nanoparticle chemistry and physics have developed into a significant subdiscipline within the field of nanotechnology and metal nanoparticles. The unique set of optical properties of small gold particles, the size-dependent electrochemical behavior, and their remarkable chemical stability have established them as the preferred model system for investigating a variety of phenomena. − Gold nanoparticles (AuNPs) are tiny gold particles, typically ranging from 1 to 100 nm synthesized through different methods such as physical, chemical, or green synthesis. They have the unique property of plasmon resonance, which enables applications in sensing, catalysis, imaging, and energy conversion. These properties are influenced by quantum effects and advanced physical processes. At the macroscopic level, gold is recognized for its distinct yellow color, chemical stability, and high redox potential. At the nanoscale, the properties of AuNPs (Figure ) arise from a blend of its electronic structure and additional effects that are associated with its extremely small size. These include a high ratio of surface atoms to bulk atoms, electromagnetic confinement due to localized plasmon resonance when interacting with optical waves, and quantum effects, which, for example, account for the transition from metallic to semiconducting behavior. , Gold nanoparticles (AuNPs) have gained significant attention in theranostic applications for cancer therapy. In the biomedical field, they are used for highly sensitive biomolecular screening, selective destruction of cancer cells via photothermal therapy, targeted protein labeling, and specific cellular drug delivery.

1.

Various properties of gold nanoparticles (AuNPs).

2. Synthesis of AuNPs

2.1. Physical and Chemical Methods of Synthesis

Gold nanoparticles can be synthesized through a variety of methods (Figure ). Physical methods such as micropatterning, sputtering, , pyrolysis, , laser ablation, − and ball milling, , use mechanical forces or energy-based processes to generate nanoparticles (Table ). Due to the absence of any chemical moiety or process involved, the nanoparticles are high in purity along with a minimum amount of contamination. However, the imperfect surface structure of the nanoparticles and the high cost of the process, which requires a massive amount of energy to maintain high-pressure and high-temperature conditions, have particular demerits. For instance, in a study by Sylvestre et al., gold nanoparticles were synthesized using femtosecond laser ablation in aqueous solutions, hence leading to partial oxidation and a negatively charged surface. This charge, enhanced by Cl^–, OH^–, or N-propylamine, prevented the coalescence and controlled the nanoparticle size. Oxidation increased the reactivity, aiding functionalization through covalent and electrostatic interactions allowing for precise control over nanoparticle growth, producing stable, uniformly sized particles (5–8 nm) suitable for various applications in nanotechnology and materials science. In another study by Mafuné et al. AuNPs were synthesized via laser ablation of a gold plate in sodium dodecyl sulfate solution. Their absorption matched chemically prepared nanoparticles. Increased surfactant concentration reduced particle size, stabilizing them above 10^–5 M. Larger nanoparticles (>5 nm) were fragmented into 1–5 nm sizes using a 532 nm laser. Similarly, Resta et al. synthesized AuNPs via pulsed laser deposition (PLD) on amorphous carbon/glass and single-crystalline MgO substrates. Despite similar nucleation rates, nanoparticle shape varied: quasi-spherical on amorphous substrates and faceted on MgO. High-energy ions (≥200 eV) enhanced nucleation, while epitaxial growth on MgO enabled shape control, following the Wulff-Kaichew theorem. Similarly, Donnelly et al. used nanosecond pulsed laser deposition (PLD) on Si and sapphire substrates in a vacuum for the synthesis of AuNPs. Atomic force microscopy confirmed nanoparticle formation below 5 nm thickness, with increasing deposition leading to coalescence. Optical absorption showed surface plasmon resonance, shifting to longer wavelengths as film thickness increased. Jankowsk et al. utilized high-voltage AC arc discharge in distilled water to synthesize AuNPs, with characterization through X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), ultraviolet–visible (UV–vis), and inductively coupled plasma-mass spectrometry (ICP-MS) confirming nanoparticle formation, crystallinity, and stability. The synthesis process showed that increasing the discharge time resulted in larger nanoparticles, with AuNPs-2 having a crystallite size of 8.4 ± 3.2 nm and AuNPs-5 exhibiting a size of 11.6 ± 2.8 nm. This cost-effective and environmentally friendly method provides controlled nanoparticle synthesis without the need for stabilizers, making it a viable alternative to DC methods. Endla and Radhika utilized high-energy ball milling to convert microsized Au powder into nanoparticles over a 20-h process. Particle size decreases from 115 to 22 nm, with increased strain and B values. The energy of vacancy formation and Debye temperature were evaluated, showing significant effects on particle size and strain. Hatakeyama et al. synthesized AuNPs using sputter deposition in the ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate. The size of AuNPs was influenced by target temperature, applied voltage, and capture medium temperature, while sputtering time, working distance, and discharge current, had minimal effects. Lower temperatures and higher voltages produced smaller NPs. In a study by Gromov et al., AuNPs and Au–Cu nanoalloys were synthesized using thermal evaporation, condensation, and heating in a vacuum. Due to size effects, their melting point decreased, influencing phase formation and structural deviations from bulk phase diagrams. TEM analysis revealed nanoparticle migration and fusion, affecting their final composition and stability. On the other hand, chemical-based methods (Table ) utilize the unique properties of different chemicals to mediate the synthesis of AuNPs. They are synthesized chemically by reducing gold salts like HAuCl₄ using agents such as sodium citrate or NaBH₄. The reduction leads to nucleation, followed by controlled growth. Stabilizing agents prevent aggregation, thus ensuring desired size and shape formation through surface interactions. Some techniques such as microemulsion, − electrochemical methods, radiation-induced synthesis, , Turkevich synthesis, Brust–Schiffrin synthesis, seeding-growth technique, ascorbic acid mediated synthesis, , and synthesis with NaBH₄, , have been widely used to synthesize AuNPs.

2.

Synthesis method for AuNPs: top-down and bottom-up approach.

1. Physical Methods for the Synthesis of Gold Nanoparticles.

Sr. No.	method	principle methodology	parameters and conditions	size & shape	properties	applications	references
1	Pulsed Laser Ablation in Liquid (PLAL)	High-energy laser pulses ablate gold in water to form NPs via plasma and condensation.	1064 nm Nd:YAG; 15 and 30 min; well-dispersed; eco-friendly	5.20 and 7.46 nm; spherical	Low MIC (1.719 μg/mL S. aureus); biocompatible	Antibacterial dental implants
2	Laser Ablation in Liquid (LAL)	Ablation in MEM & DW using 532/1064 nm lasers to form colloids.	8.84 nm (MEM); 16.3 nm (DW); 600 μg/mL for photothermal	Spherical	Cytotoxic to MDA-MB-231 cells; ∼43–58% ablation	Photothermal cancer therapy
3	Pulsed Laser Ablation in Liquid (PLAL) (Core–Shell Au@Fe3O4)	Gold and iron oxide ablated together in liquid to form core–shell particles.	1200 mJ laser; porous silicon substrate; XRD, PL, TEM used	Core–shell; 25 nm	Bandgap 2.38–2.53 eV; high photodetector responsivity (0.189 A/W)	Photodetectors, biosensors
4	Pulsed Laser Ablation in Liquid (PLAL)	Laser ablation of gold foil in DI water; analyzed for biological response.	1064 nm, 250 mJ, 6 Hz, 300 pulses; drop-cast; FE-SEM, UV–vis, XRD	Spherical; Avg. 29 nm (crystalline: 24.6 nm); SPR@550 nm	Stable; Creatine Kinase levels increase with dose in rats	Enzyme modulation therapy; possible drug dosage control
5	PLAL-assisted Doping	Au target irradiated (500–2500 pulses) in CdS sol–gel to dope thin films.	1064 nm Nd:YAG, 700 mJ, 9 Hz; 500–2500 pulses; spin-coated films	CdS: 6.6–9.3 nm (doped); grain size down to 25.8 nm	Decreased bandgap (2.39 → 2.33 eV); nonlinear refraction and absorption	Optical modulators, photonic devices, nonlinear optics
6	PLAL + Photoelectrochemical (PEC) Etching	Au-NPs deposited on nanostructured porous Si made via photoelectrochemical etching.	400–1000 mJ; bandgap: 1.82–1.77 eV; max responsivity: 0.086 A/W at 1000 mJ	Spherical; SPR at 500–580 nm	Visible-NIR detection; high surface-to-volume photoresponse	Quantum photodetectors; UV–vis-NIR sensors
7	PLAL (Au@CuO Core–Shell)	Gold core ablated, CuO forms nanoshell in liquid; deposited on porous silicon.	1200 mJ Nd:YAG; cubic Au, monoclinic CuO; bandgap: 2.53–2.82 eV; PL + UV–vis	Core–shell, spherical; Avg. 41 nm	High photodetector responsivity (0.194A/W@548 nm); broad-spectrum absorption	Wearable optoelectronics, photodetectors, plasmonic devices
8	PLAL (AuNP–doped CNF)	Au ablated into CNF dissolved in DMF by 500–1500 pulses to tune doping.	1064 nm, 500–1500 pulses, 500–1500 mJ; CNF roughness ↓ from 158 to 106 nm; SPR @530 nm	AuNPs ∼ 50.5 nm; spherical; embedded in fibrous CNF matrix	Sensitivity ↑ (7.5–19.2%), recovery and response time ↑; antibacterial zone 27.10 mm (S. aureus)	Gas sensors (200 °C); antibacterial coatings; flexible electronics
9	PLAL (Au@Al2O3 Core–Shell)	Au plate ablated in DI water; Al placed in same jar under same laser to form core–shell NPs.	532 nm Nd:YAG, 250 pulses, 1 Hz, 9 ns; PECE PS etched with 20% HF, 20 mA/cm2, 15 min	Core–shell; spherical; size increases with laser energy	Bandgap 3.93–4.42 eV; max responsivity 0.084 A/W at 1000 mJ; UV–IR detection	Optoelectronics, photodetectors, UV–Vis–IR sensing
10	Synthesis of Si@Au Core–Satellite Nanostructures	Synthesis of Si@Au Core–Satellite Nanostructures	Synthesis of Si@Au Core–Satellite Nanostructures	Synthesis of Si@Au Core–Satellite Nanostructures	Synthesis of Si@Au Core–Satellite Nanostructures	Synthesis of Si@Au Core–Satellite Nanostructures
11	Coprecipitation + Ultrasonic Spray Pyrolysis (USP)	Coprecipitation forms Fe3O4 NPs via base-induced precipitation of iron salts; USP thermally decomposes gold precursors into AuNPs that nucleate and coat the magnetic core	Fe2+/Fe3+ with NH4OH yields Fe3O4; AuNPs formed on Fe3O4 via USP; separate AuNPs also observed; crystalline structure; particle size 10–200 nm; poor zeta stability (ζ ≈ 0 mV)	Crystalline, mostly spherical; 10–200 nm	Superparamagnetic core; reduced saturation magnetization (0.1 emu/g); poor colloidal stability; visible aggregation; AuNPs exhibit plasmonic response	Biomedical imaging, drug delivery, plasmon-enhanced magnetic diagnostics
12	Coprecipitation + Ultrasonic Spray Pyrolysis (USP) for LFIA	USP thermally decomposes HAuCl4 to form AuNPs in PVP-stabilized DI water; conjugated with SARS-CoV-2 S1 antigen and Rabbit IgG for diagnostic use.	AuNPs (40–60 nm); 204 ppm in PBS; conjugated with 100 μg S1 antigen; borate buffer pH 8.5; incubated 30 min at 22  °C; OD change 0.91 → 0.33.	Crystalline; spherical; Avg. 60  ±  20 nm	Monodispersed; stable postlyophilization; effective protein binding; UV–vis confirmed conjugation; OD ↓ indicates binding strength	LFIA rapid diagnostics for SARS-CoV-2; virus antigen detection; pandemic monitoring
13	Physical Sputtering + Annealing	Au film sputtered onto substrates is thermally agglomerated into nanostructures via annealing, forming SERS-active surfaces with tunable morphology and enhancement properties.	10–20 mA current, 20–40 s sputtering; annealed at 300–500 °C for 1 h; substrates: silicon/quartz; cooling in air; Raman enhancement: 100–200×.	Irregular Au; size and spacing increase with film thickness; roughness varies	Morphology-dependent enhancement; tunable by current, time, and annealing; simple and scalable fabrication; 100–200× Raman signal boost	Low-cost, scalable SERS substrates for biomedical, chemical and industrial trace analysis
14	Combinatorial Magnetron Sputtering + Annealing	Au layers of graded thickness sputtered on fused silica, then annealed to form nanoparticles with spatially varying optical and sensing characteristics.	Deposited Au thickness: 1.6–7 nm; annealed postdeposition; gradient profile; optical scanning and FEM modeling used to determine optimal sensing zones for Raman, gas, and molecular detection.	Size varies with thickness; denser packing at 2–4 nm; optimal plasmon peak at 7 nm	Tunable SERS and gas sensitivity; near-field enhancement optimized at specific thicknesses; supported by FEM modeling; high-throughput mapping	Optimized SERS and molecular/gas sensing, database generation, and machine learning input for sensing material design
15	Magnetron Sputtering + Thermal Dewetting	Ultrathin Au films (9–18 s sputter) deposited on glass, annealed at 500 °C for 2 h, forming tunable Au NPs with LSPR properties dependent on deposition time.	Sputtering: 9–18 s; annealing: 500 °C, 2 h; RI sensitivity: 36.118–92.165 nm/RIU; optimized LOD: 1:5120 dilution for ASFV antibodies; stable and regenerable for 1 month	Tunable 527–563 nm LSPR peak; NP diameter, density, spacing dependent on Au thickness	High sensitivity (LOD: 5.92 ng/mL), regenerability (93% over 5 cycles), reproducibility, and long-term stability	ASFV antibody biosensing, veterinary diagnostics, POF devices, medical diagnostics, food safety, environmental monitoring
16	Femtosecond Laser Ablation + White-Light Growth	Two-step laser process: seed formation via femtosecond ablation in biopolymer solution, followed by white-light-induced size-controlled growth without surfactants.	Laser: 800 nm, 110 fs, 1 kHz; spot: 180 μm; polymer: chitosan, PEG, etc.; NP size 2–80 nm controlled by polymer/gold ratio; dispersion ∼20%	Spherical; tunable 2–80 nm; low polydispersity (∼20%)	Surfactant-free, size-controlled, biopolymer-functionalized, nontoxic; stable; suitable for aqueous biosystems	Biosensing via gold-dextran–concanavalin A aggregation model; biomedical and diagnostic applications
17	Laser-Induced Fragmentation (with/without Graphene Oxide)	Picosecond laser-induced phase explosion fragments AuNPs; graphene oxide sheets electrostatically confine fragments, influencing diffusion, growth, and final cluster size.	1064 nm, 10 ps, 9.6 mJ, 100 kHz; pH 9, 0.3 mM NaCl; graphene oxide: 0.3 g/L; sizes: 5–20 nm (no GO) vs 40–70 nm (with GO)	Spherical; with graphene oxide: larger ∼ 60 nm; without: smaller 5–20 nm	Surfactant-free; graphene oxide enhances size by hindering diffusion; phase-explosion governed; size control via additives	Nanocatalysis, electrochemical devices, graphene-based hybrids; platform for fragmentation studies in colloids
18	Microchip Laser Pulsed Laser Ablation in Liquid (MCL-PLAL)	Laser ablation generates nanoparticles in liquid using focused high-energy pulses	Gold target ablated in toluene with 10–400 mM polystyrene; 1064 nm laser, 104–550 mW, 900 ps, 100 Hz; 60 min irradiation at room temperature with stirring at 200 rpm	1.7–2.8 nm, spherical, carbon-encapsulated	Size and carbon thickness tunable via laser power and polymer; strong photoluminescence with excitation-dependence	Optoelectronics, photonic materials, sensing, bioimaging
19	Pulsed Laser Ablation in Liquid (PLAL) + Pulsed Laser Deposition (PLD)	Laser ablation generates AuNPs in liquid; PLD forms ZrO2 films on porous Si	AuNPs synthesized via PLAL; drop-cast on ZrO2 films prepared by PLD (600–1000 mJ); applied on porous Si; 3 V bias; tested for UV responsivity (350 nm)	10–15 nm, spherical	Crystalline FCC Au; monoclinic ZrO2; uniform dispersion; high responsivity (0.17 A/W); QE up to 60.7%	UV photodetectors, optoelectronics, high-sensitivity sensors
20	Pulsed Laser Ablation in Liquid (PLAL)	Sequential laser ablation enables core–shell nanostructure formation in liquid medium	Au@WO3 synthesized using Nd:YAG laser (1064 and 532 nm), 19.10 J/cm2, 1200 pulses; particle size 35–97 nm; size/shell thickness depend on laser wavelength	35–97 nm, spherical, core–shell (Au@WO3)	Tunable bandgap (3.17–3.0 eV), SPR-enhanced absorption (250–550 nm), monoclinic WO3 phase, strong PL at 371–372 nm	UV–Vis photodetectors, plasmonics, photocatalysis

2. Chemical Methods for the Synthesis of Gold Nanoparticles.

Sr. No.	synthesis method	parameters and conditions	size and shape	properties	application suggested	references
1	Turkevich method	HAuCl4 (5 mM), 3 mL added to 250 mL flask with 54 mL water; Boil while stirring (400 rpm), 3 mL α-amino acid salt (20 mM) added	Quasi-spherical AuNPs (several to tens of nm), irregular AuNPs (20–60 nm, for l-tyrosine, l-lysine), nanoclusters (25–95 nm, for l-(4)-hydroxyproline)	Good stability, variable optical properties	Therapeutics, diagnostics
2	Turkevich method with citrate	Use of trisodium citrate as reducing agent, forming complex with Au3+ to Au+ species	Highly stabilized AuNPs, 5–150 nm in size	Stable, reliable synthesis, size variability	Common in nanoparticle synthesis, diagnostics, and sensors
3	Seed-mediated growth (Turkevich method)	Use preprepared citrate-stabilized Au NP seeds, controlled addition of HAuCl4, reflux at 125 °C with 500 mM sodium citrate (2.5 mM final concentration), stirring at 480 rpm	Spherical, monodisperse AuNPs, size range 21–53 nm	Water-dispersible, stable, uniform growth	Biosensing, drug delivery, biomedical applications
4	Seed-growth method (Turkevich)	2.2 mM trisodium citrate heated to boiling, add 8.4 mM chloroauric acid, cycles of addition and heating	Gold NPs of 63.9 nm for conjugation with IgY anti-N-SARS-CoV-2	Economical, reproducible, antibody conjugation	SARS-CoV-2 detection, biosensors, diagnostic tools
5	Brust-Schiffrin method	HAuCl4 in aqueous phase, phase-transfer agent (TOAB), reducing agent (NaBH4), alkanethiols as stabilizers	Monodisperse AuNPs, size tunable between 1–5 nm	Chemical stability, monodispersity, surface functionalization, stable in organic solvents	Catalysis, sensing, biomedical use, nanoelectronics
6	Aqueous synthesis of tiopronin-capped gold nanoclusters	Gold precursor (HAuCl4) dissolved in water, tiopronin and sodium hydroxide added for deprotonation, followed by reduction with NaBH4	Tiopronin-capped gold nanoclusters/nanoparticles, size 2.1–2.5 nm	Dialysis-free synthesis, stable nucleation, photoluminescence, hydrodynamic size control	Drug delivery, biomedical applications, biomolecule detection
7	Gold Nanostars (AuNSt) using PVP in DMF	PVP synthesized with H2O2 or AIBN, pH 8–9, 70 °C, bases NH4OH or Na2CO3. Seed prep with citrate-capped AuNP.	AuNSt (sharp tips) or spherical NPs	Shape influenced by PVP impurities; H2O2 leads to sharp-tipped AuNSt, AIBN gives spherical NPs.	Nanotechnology, biosensing, drug delivery.
8	Photochemical reduction of gold ions using CW & femtosecond lasers	445 nm CW laser for AuNW in HAuCl4; 800 nm femtosecond laser for AuNR, Au nanoplates	Gold NPs on AuNW, Au nanoplates	Hot electron-induced reduction; femtosecond laser enables precise shape control	Nanophotonics, laser-assisted nanofabrication
9	Paper-based UV dosimeter via solid-state photochemical reduction of Au-complexes	UV light exposure (254–365 nm), AuCl4–CTAB complex, phosphate buffer (pH 8), hydration step	Gold NPs on paper	UV-induced gold reduction, hydrochromic response upon hydration, color change on demand	UV exposure monitoring, sterilization, UV phototherapy
10	Seed-mediated colloidal synthesis using chiral amino acids and peptides	Octahedral seeds and chiral agents like l-GSH or d-GSH used. Synthesis done at room temperature in 8 h with optimized seed and peptide concentration.	432-symmetric helicoid III nanoparticles with well-defined chiral morphology.	High g-factor (∼0.2), tunable CD peak, and consistent reproducibility. Chiroptical properties depend on seeds and synthesis optimization.	Chiral sensing, polarization control, and biomedical applications.
11	Chemical synthesis using shape-directing agents	Gold nanospheres, rods, stars, and prisms synthesized by adjusting reducing agents, surfactants, and reactants. Conditions tailored to control morphology and optical response.	Nanospheres, nanorods, nanostars, and nanoprisms with controlled size and shape.	Nanoprisms showed highest RIS (691.5 nm/RIU) and FoM (3.4). Sensitivity increases with particle size and is strongly shape-dependent.	LSPR-based and SERS-based biosensing using shape-optimized gold nanostructures.
12	Surfactant-free colloidal synthesis at room temperature	AuNPs synthesized using 0.5 mM HAuCl4, 2 mM NaOH in ethanol–water. Size controlled by shaking, stirring, or sonication. Simple, green, and reproducible approach.	Quasi-spherical gold nanoparticles ranging from 5–22 nm.	Smaller particles (5 nm) had highest electrochemical activity. Method enables clear observation of size effects in catalysis.	Alcohol electro-oxidation, green synthesis for catalysis, size-effect studies.
13	Turkevich method using sodium citrate	Reduction of NaAuCl4 with sodium citrate at ∼90 °C. Simple, rapid, and didactic synthesis with visual process steps and detailed mechanism explanation. Characterized using SEM, AFM, XRD, UV–vis, and DLS.	Spherical gold nanoparticles, average size 12–25 nm.	Stable AuNPs with SPR peak at 521 nm, FCC structure, good dispersity, and citrate capping. Ideal for modifying electrochemical sensors due to biocompatibility and size.	Electrochemical sensors, teaching demonstrations, biomedical and plasmonic applications.
14	Seed-mediated growth using Turkevich-prepared Au seeds	Presynthesized citrate-stabilized Au seeds grown by adding HAuCl4 at boiling temperature. Size controlled by adjusting precursor, seed, temperature, and reducing agent. One-step synthesis enables simplicity.	Monodisperse spherical AuNPs ranging from 21–53 nm.	Citrate-stabilized, water-dispersible AuNPs. Size uniformity improves with boiling. Particles suitable for functionalization with biomolecules for advanced biological applications.	Drug delivery, biosensing, and bioconjugation in medicine and biology.
15	Turkevich (labile) and Ranelate-based (inert) AuNPs	Turkevich: 1 mL 40 mM sodium citrate into 10 mL 1 mM HAuCl4 at boiling. Ranelate: 0.1 mL 20 mM lithium ranelate in 9.9 mL 0.202 mM HAuCl4.	Both produce quasi-spherical nanoparticles; diameter not explicitly stated but suitable for SERS studies.	AuNP@cit (labile) allows strong SERS via chemical/electromagnetic mechanisms; AuNP@ran (inert) suppresses SERS due to ligand rigidity, blocking electronic contact with probe molecules.	SERS-based detection, understanding plasmonic interactions, chemical and electromagnetic enhancement studies.
16	Turkevich + DNA Conjugation	AuNPs synthesized via Turkevich method at 92 °C with trisodium citrate, followed by DNA initiator functionalization and TdT enzyme-based enzymatic DNA elongation on surface.	∼58.3 nm, spherical, PDI < 0.2	High monodispersity, stable DNA conjugation, suitable for enzymatic synthesis platforms	Enzymatic DNA synthesis, DNA data storage, DNA origami, nanorobotics, biosensing, drug delivery
17	Turkevich method (with and without PVA/PEG stabilizers)	Gold nanoparticles synthesized using boiling HAuCl4 with trisodium citrate; PVA or PEG (0.03%) added to enhance stability; 20 min reflux; stored at 4 °C	Bimodal size distribution; hydrodynamic diameter ∼32 nm; z-average ∼ 20 nm; PDI 0.55	PVA and PEG coatings enhanced colloidal stability and shelf life but reduced oligonucleotide sensing ability	Colorimetric detection of Hepatitis C virus (HCV); visual and spectroscopic distinction of samples
18	Modified Brust–Schiffrin method using amine-functionalized liquid crystals	Chloroauric acid transferred to organic phase via TOAB; reduced with LC ligand; excess LC removed via ethanol washing and centrifugation; DCM used as solvent	Not explicitly stated	Enhanced mesophase thermal range, Fano-like resonance in UV–vis due to 1D superstructure; improved electro-optic properties	Electro-optic materials, photonic devices, plasmonic systems
19	Modified Brust–Schiffrin with ligand exchange	AuNPs synthesized using TOAB as phase transfer agent; DDT as capping ligand; NaBH4 as reducing agent; purified with methanol at –20 °C; 3-azopyridine ligand exchanged postsynthesis.	∼2 nm, spherical	Hydrophobic; excellent dispersibility in organic solvents; ligand photoswitching (E/Z isomerism) unaffected by surface conjugation.	Photoresponsive materials; reversible nematic-to-isotropic transitions in liquid crystalline hosts under UV/green light irradiation.
20	Citrate reduction + ligand exchange (TDG); MPA-based conjugation (EDCl/NHS, DCC/DMAP)	AuNPs synthesized using 300 mg HAuCl4·3H2O, 157 μL MPA in 150 mL methanol; NaBH4 (6–12 mmol, 30 mL) added dropwise. TDG: 0.2% w/v in DMSO; conjugation via esterification with 100 mg DG, 85 mg DCC, 20 mg DMAP. Purified by centrifugation at 13,000–14,500 rpm.	MPAm1:27.8 nm (spherical), MPAm2:58.8 nm (polycrystalline); MPAm1-DG: ∼ 53 nm, ζ-potential –30 mV	SP-ICP-MS confirmed narrow size distribution; 27.6% loading; DG conjugates showed improved cancer cell selectivity and reduced toxicity vs TDG; >90% synthetic yield	Targeted prostate cancer therapy; safer diosgenin delivery; advanced nanomedicine with selective cytotoxicity to DU-145 cells

2.2. Green Synthesis of AuNPs

Apart from the traditional physical and chemical methods of synthesis, green synthesis (Table ) has gained a lot of attention primarily due to its essence of sustainability and low toxicity. There are several eco-friendly and biocompatible techniques to synthesize AuNPs that utilize plant extracts, bacteria, and microbes. The chemical composition and concentration of reducing agents in organic extracts are significant and can vary, which in turn influences the characteristics of the final product. These variations lead to differences in size and shape, which in turn affect the function and application of the material. Many biomolecules including, phenols, flavonoids, amino acids, proteins, enzymes, amines, aldehydes, ketones, carboxylic acids, and alkaloids act as electron donors, which enable the reduction of cationic gold to form AuNPs. The properties of the resulting nanoparticles are determined by factors such as the plant extract concentration, the type of metal salt used, the pH of the reaction mixture, and the reaction temperature. ,

3. Green Synthesis of AuNPs: Plant type, Biomolecules, Reaction Conditions, Shape and Size, and Applications.

Sr. No.	plant/source used	plant part used	biomolecules responsible	gold salt used	reaction conditions	size & shape of AuNPs	method	characterization techniques	applications	references
1	Ziziphus spina-christi	Leaves	Alkaloids, phenols, flavonoids, proteins, vitamins	HAuCl4·3H2O (Gold Chloride)	70 °C, 60 min, aqueous extract	31.26–58.06 nm, spherical	Green synthesis using aqueous plant extract under controlled temperature and time, ensuring nanoparticle formation with stability and bioactivity for therapeutic applications.	TEM, FE-SEM, AFM, XRD	Effective in photothermal therapy for breast cancer treatment, leveraging near-infrared radiation to induce localized hyperthermia and apoptosis in cancer cells.
2	Cordia myxa L.	Leaves	Flavonoid glycosides, phenolic derivatives	HAuCl4·3H2O (Gold Chloride)	80 °C, 90 min, aqueous extract	56.49–89.38 nm, spherical	Green synthesis involving biomolecule-assisted reduction of gold salts, yielding nanoparticles with controlled morphology and enhanced biocompatibility for medical applications.	TEM, FE-SEM, AFM, XRD	Utilized in targeted cancer therapy by combining gold nanoparticles with near-infrared irradiation to enhance cytotoxicity and induce apoptosis in breast cancer cells.
3	Capsicum annum	Fruit (Dried)	Phenols, flavonoids, capsaicin, secondary metabolites	HAuCl4 (Gold Chloride)	90 °C, rapid mixing, aqueous extract	20–30 nm, spherical	Green synthesis via aqueous extract of dried fruit powder, acting as both reducing and stabilizing agent to form highly stable gold nanoparticles with biomedical relevance.	TEM, FT-IR, EDAX, XRD, ζ-potential	Demonstrates strong antioxidant, anti-inflammatory, and antiangiogenic properties, making it useful in cancer treatment by inhibiting blood vessel formation and oxidative damage.
4	Rutin extract	Flavonoid (Rutin)	Rutin (Flavonoid)	HAuCl4 (Gold Chloride)	532 nm laser, 150 J/cm2, 300 s exposure	40 nm, spherical	Green synthesis using Rutin as a stabilizing and reducing agent, forming plasmonic nanoparticles suitable for photothermal applications.	TEM, UV–vis, MTT assay	Effective in plasmonic photothermal therapy (PPTT) for breast cancer, with high selectivity for MCF-7 cancer cells and minimal cytotoxicity to normal cells. Potential for antibody-functionalized targeted therapy.
5	Mentha spicata L.	Essential Oil	Monoterpenes (Menthol, Carvone, Limonene)	HAuCl4 (Gold Chloride)	Room temperature, 24 h reaction time	19.61 nm, spherical	Green synthesis using essential oil as a reducing and stabilizing agent at room temperature, ensuring eco-friendly nanoparticle formation with enhanced bioactivity.	TEM, DLS, FT-IR, XRD, UV–vis, MTT assay	Exhibits antibacterial, antioxidant, and cytotoxic properties, with potent free radical scavenging ability and selective cytotoxicity against HEPG-2 liver cancer cells. Suitable for biomedical and pharmaceutical applications.
6	Chitosan & Reduced Graphene Oxide (rGO)	Chitosan (Different MW)	Chitosan (Polysaccharide)	HAuCl4 (Gold Chloride)	Varying MW of chitosan, room temperature	9.29–13.03 nm, spherical, immobilized on rGO	Green synthesis using chitosan as a reducing and stabilizing agent, preventing AuNP aggregation and enhancing catalytic efficiency.	FT-IR, XRD, XPS, SEM, FESEM, EDS, TEM, HRTEM, TGA	Highly efficient and stable catalyst for the reduction of 4-nitrophenol (4-NP) in the presence of NaBH4. Potential application in catalysis for reducing aromatic nitro compounds.
7	Halymenia pseudofloresii (Red Algae)	Whole Algae (Dried)	Phenolic compounds, carboxyl groups	HAuCl4 (Gold Chloride)	60 °C, 20 min, aqueous extract	27 nm, cubic and rectangular	Green synthesis using seaweed extract as a reducing and capping agent, ensuring bioactive-stabilized nanoparticle formation.	UV–vis, XRD, FTIR, SEM	Exhibits strong antioxidant, antibacterial, and anticancer activities, effectively inhibiting Staphylococcus aureus, Lactobacillus, and Pseudomonas aeruginosa. Shows potent cytotoxicity against A549 lung cancer cells (IC50 = 19.02 μg/mL) and LN-18 glioblastoma cells (IC50 = 32.46 μg/mL). Induces apoptosis via ROS generation, making it a potential anticancer agent.
8	Ziziphus zizyphus	Leaves	Phenolic compounds, flavonoids, alkaloids	HAuCl4·3H2O (Gold Chloride)	Boiling extraction, 5 min	50 nm, spherical	Green synthesis of gold nanoparticles using Ziziphus zizyphus leaf extract, followed by purification via dialysis and column separation.	SEM, TEM, DLS, ζ-potential, ICP-MS, Histopathology	Evaluated for acute and chronic toxicity in albino mice. Gold accumulation was highest in the spleen, liver, and kidneys. Mild to moderate histopathological changes were observed in kidney tissue, with minimal effects on heart and blood glucose levels. Suggests need for further toxicity assessments for biomedical applications.
9	White Pitaya (Hylocereus undatus)	Extract (Pulp)	Phenolic compounds, polysaccharides	HAuCl4	Magnetic stirring (30 min) + heating (65 °C, 2 min)	18.0 ± 0.7 nm, spherical	Green synthesis via the reduction of gold ions using bioactive compounds in H. undatus extract under controlled thermal and magnetic stirring conditions, leading to the formation of stable AuNPs.	UV–vis Spectroscopy, TEM	Electrochemical sensor for hydroxychloroquine detection, pharmaceutical analysis, clinical diagnostics, biosensing, environmental monitoring, and biomedical applications
10	Andrographis paniculata	Leaves	Polyphenols, flavonoids, triterpenoids	HAuCl4	1.5 mL extract, 0.5 mM HAuCl4, 60 °C, 30 min, pH 6	10–15 nm, spherical	Green synthesis through the bioreduction of Au3+ to Au0 by polyphenolic and flavonoid compounds in A. paniculata extract under optimized reaction temperature and pH, ensuring uniform nanoparticle dispersion.	UV–vis Spectroscopy, TEM, FTIR, XRD, EDX, DLS, ζ-potential, SEM	Lead ion detection, dye degradation (methylene blue, methyl orange, crystal violet), antibacterial (B. subtilis, Salmonella enterica, S. aureus, Escherichia coli), antioxidant activity, biosensing, wastewater treatment
11	Glaucium flavum	Leaves	Alkaloids (glaucine, aporphine, protoberberine, protopine), polyphenols	HAuCl4	1 mL extract +1 mL 10 mM HAuCl4, stirred for 1 h	32 nm, spherical	Green synthesis utilizing alkaloid-rich G. flavum extract as a reducing and stabilizing agent, forming monodisperse AuNPs through a controlled nucleation and growth mechanism.	UV–vis Spectroscopy, TEM, DLS, SEM, XRD, FTIR, ζ-potential	Antiviral (H1N1 virus), antioxidant, biosensing, drug delivery, biocompatibility studies, biomedical applications
12	Ephedra	Whole plant extract	Alkaloids (ephedrine, pseudoephedrine), polyphenols, tannins, flavonoids, glycosides	HAuCl4·3H2O	4 mL extract +6 mL 10–3 M HAuCl4, stirred at 25 °C for 3 h, pH 4	1.3–15.6 nm, spherical and triangular	Green synthesis through the reduction of gold salt by alkaloids and polyphenols in Ephedra extract at ambient temperature, promoting nanoparticle stabilization via capping biomolecules.	UV–vis Spectroscopy, FTIR, TEM, HRTEM, XRD, EDX, XPS, ζ-potential	Antioxidant, antipyretic (reduced fever by 83.3%), antiasthmatic, antibacterial (S. aureus, B. subtilis, L. monocytogenes, E. coli , P. aeruginosa, S. typhimurium), antifungal (Candida albicans, A. nigra, A. flavus), biosensing, drug delivery
13	Oroxylum indicum	Plant extract	Flavonoids	HAuCl4	Microwave-assisted synthesis, 3 min at 800 W	5.25 ± 1.00 nm, spherical	Microwave-assisted green synthesis utilizing O. indicum extract, where flavonoids facilitate the rapid reduction of gold ions under electromagnetic radiation, enhancing nanoparticle formation kinetics.	UV–vis Spectroscopy, FTIR, XRD, HRTEM, EDX, XPS, ζ-potential	Antibacterial (S. aureus, E. coli), catalytic reduction of 4-nitrophenol, potential biomedical applications
14	Satureja khuzestanica	Aerial parts	Phenolic compounds, flavonoids, terpenoids	HAuCl4	Extract and gold salt mixed, ultrasonic treatment (40 min, 60 °C), followed by rotary evaporation	20–30 nm, cubic	Green synthesis employing ultrasonic-assisted reduction, where bioactive phytochemicals in S. khuzestanica extract act as reducing and capping agents, ensuring controlled morphology and stability of AuNPs.	UV–vis Spectroscopy, SEM, XRD, FTIR	Antiparasitic (hydatid cyst protoscoleces), apoptosis induction via caspase-3 activation, ultrastructural cell damage, low cytotoxicity toward normal human cells, potential biomedical and antiparasitic applications
15	Prunus persica (Peach)	Pulp extract	Phenolic compounds (chlorogenic acid, catechins, epicatechins, anthocyanins), polyphenol oxidase enzyme	HAuCl4	1 mL peach extract +0.25 mL 6.0 mM HAuCl4, stirred at 500 rpm for 3 min at 25 °C	11.5 ± 1.5 nm, spherical	Green synthesis via the bioreduction of Au3+ using phenolic compounds and polyphenol oxidase enzyme in P. persica extract. The synthesized AuNPs were further incorporated into graphene to enhance sensor conductivity.	UV–vis Spectroscopy, ATR-FTIR, TEM, SEM, EIS, Cyclic Voltammetry	Electrochemical sensor for butylated hydroxyanisole (BHA) detection in food matrices, biosensing, environmental analysis, and antioxidant screening
16	Moringa oleifera + Titanium dioxide (TiO2)	Leaf extract	Flavonoids, polyphenols, alkaloids	HAuCl4·3H2O	Extract heated (100 °C, 1 h), cooled, filtered; Gold salt added to TiO2 (1:1:1), stirred (60 °C, 1 h), then calcined (200 °C, 2 h)	10 ± 5 nm, uniformly dispersed	Green synthesis of TiO2@Au nanocomposite using M. oleifera extract as a reducing and stabilizing agent. The synthesized AuNPs were uniformly deposited onto TiO2 for enhanced photocatalytic activity.	UV–vis Spectroscopy, FTIR, Raman, XRD, XPS, TEM	Photocatalytic degradation of ciprofloxacin under visible light, environmental remediation, wastewater treatment, phytotoxicity studies with Pistia stratiotes
17	Thyme (Thymus kotschyanus)	Extract	Polyphenolic compounds	HAuCl4	Green synthesis using plant extract, room temperature	20–25 nm, spherical	Green synthesis using Thymus kotschyanus extract as a natural reducing and stabilizing agent, ensuring controlled nucleation, preventing aggregation, and forming monodisperse AuNPs without harsh chemicals under mild reaction conditions.	UV–Vis, FT-IR, XRD, TEM	Used as catalysts in the reduction of 4-nitrophenol to 4-aminophenol, a crucial step in wastewater treatment. Catalysts were recyclable for six cycles with over 90% efficiency.
18	Green tea (Camellia sinensis)	Leaves	Polyphenols, flavonoids	HAuCl4	Green synthesis using aqueous extract, room temperature	Spherical, stable	Green synthesis using Camellia sinensis extract as a biocompatible reducing agent, avoiding toxic precursors, ensuring high stability, and enabling large-scale production under mild conditions.	UV–Vis, SEM, Optical fiber analysis	Functionalized for biosensing of CD44 cancer biomarker, significantly enhancing sensitivity in optical fiber biosensors, achieving ultralow detection limits (0.111 pM), and enabling real-time cancer biomarker detection.
19	Chitosan	Biopolymer	Chitosan (reducing and capping agent)	HAuCl4	Heating for 15 min in 1% acetic acid	18 ± 4 nm (AuNPs), 25 ± 5 nm (6MP-AuNPs), spherical	Green synthesis using chitosan as a reducing and stabilizing agent, ensuring high stability, biocompatibility, and eco-friendly nanoparticle production. 6MP drug was loaded via sonication.	UV–Vis, ζ-potential, SEM	Developed as a hybrid nanocomposite (6MP-AuNPs) for combinatorial chemo-photothermal therapy. Demonstrated enhanced anticancer efficacy with laser irradiation, reducing cell viability by 63% at 1.25 μM.
20	Garlic (Allium sativum)	Cloves	Sulfur compounds, organosulfur compounds	HAuCl4	Boiling in water, reaction at 95 °C, color change to wine red within 15 min	7–21 nm, spherical	Green synthesis using Allium sativum extract as a reducing agent, enabling biocompatible, eco-friendly synthesis with high yield and stability.	UV–Vis, HRTEM, FESEM, EDX	Exhibited strong antifungal activity against Candida species (C. albicans, C. krusei, C. tropicalis, C. guilliermondii). MIC80 values ranged from 40 to 0.31 μg/mL, with antifungal mechanisms involving ROS generation, membrane disruption, and metabolic inhibition. Showed low toxicity on human cells, highlighting potential for biomedical applications.
21	Marine soft coral (Sarcophyton crassocaule)	Whole coral extract	Organic compounds (polysaccharides, proteins, polyphenols)	HAuCl4·3H2O	Reaction at 40–45 °C, color change to ruby red at 540 nm, completed in 30 min	5–50 nm, spherical and oval	First-time green synthesis using Sarcophyton crassocaule extract, ensuring biofabrication under mild reaction conditions with high stability.	UV–vis, TEM, SEM, FT-IR, ζ-potential, TGA	Exhibited strong antibacterial (V. cholerae, S. aureus), antioxidant (DPPH: 85%, RP: 82%), and antidiabetic (α-amylase inhibition: 68%, α-glucosidase inhibition: 79%) properties. Also functioned as a catalytic agent for the degradation of 4-NP, MR, and BPB, showing 91% catalytic efficiency. Potential for therapeutic applications against type II diabetes and as an eco-friendly material for pollutant degradation.
22	Marine algae (Solieria tenuis)	Whole algae extract	Phycoerythrin (PE)	HAuCl4·3H2O	Incubation at 4 °C for 48 h, controlled PE-to-gold ratios	10–25 nm, avg. Sixteen nm, spherical	Green synthesis using Solieria tenuis extract to mediate the formation of Au@PE NPs, leveraging PE as both reducing and capping agents to enhance solubility and catalytic activity.	UV–vis, Fluorescence spectroscopy, FT-IR, TEM, XPS, XRD	Efficient catalyst for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) and degradation of cationic dyes (MB, Safranine O) in wastewater. Demonstrated high catalytic efficiency (k298K = 0.36× 10 2 s –1 ), offering an eco-friendly solution for environmental remediation.
23	Amphipterygium adstringens extract	Bark	Polyphenols, flavonoids, tannins	HAuCl4	Shock wave-assisted and ultrasound-assisted extraction using aqueous, ethanolic, and methanolic extracts	10 nm (maceration), 50–100 nm (shock wave, quasi-spherical), 100–150 nm (ultrasound, irregular/triangular)	Green synthesis using A. adstringens extracts obtained through shock wave and ultrasound-assisted extraction	UV–vis, XRD, FTIR, TEM, ζ-potential	Cytotoxic activity against leukemia cells (Jurkat), IC50: 87 μM (ultrasound-assisted AuNPs), 94.7 μM (shock wave-assisted AuNPs); potential as drug nanocarriers for cancer therapy
24	Azadirachta indica extract	Leaf	Polyphenols, flavonoids	HAuCl4	Reduction using aqueous leaf extract	73.74–171.45 nm, crystallite size 5.68–13.25 nm, alloy and core–shell structures	Green synthesis of Au-based bimetallic nanoparticles (BNPs)	UV–vis (SPR: 423–557 nm), XRD, FTIR, ζ-potential (−17.96 to −33.11 mV)	Nanocatalysts for textile dye degradation (Rh–B and MO), recyclable and reusable
25	Elaeis guineensisJacq. (Oil Palm)	Leaf Extract	Flavonoids, Phenols	HAuCl4	0.69 mM HAuCl4, Room Temp	10–30 nm, Spherical	Green synthesis using E. guineensis extract as a reducing agent	UV–Vis, FT-IR, TEM, XRD, ζ-potential, Raman Spectroscopy	Used in SERS for enhanced Raman intensity and sensitive molecular detection.
26	Coleus scutellarioides (L.) Benth	Leaf Extract	Phenolic Compounds	HAuCl4	1 mM HAuCl4, 25 °C, Stirring 3 min	40.10 nm, Spherical	Green synthesis using C. scutellarioides extract as a bioreductant and stabilizing agent	UV–Vis, XRD, TEM, DLS, FT-IR, FESEM, EDX	Exhibits significant anticancer activity against MDA-MB-231 cells and antioxidant properties.
27	Allium cepa (Red Onion Peel)	Peel Extract	Flavonoids, Polyphenols	HAuCl4·3H2O	75 mM HAuCl4, Room Temp	6.08–54.20 nm, Spherical	Green synthesis using A. cepa peel extract as a bioreductant	UV–vis, XRD, FT-IR, TEM, EDX, TGA-DTA, AFM	Displays antibacterial, antifungal, antioxidant, and anticholinesterase activities for Alzheimer’s therapy.
28	Pseudomonas aeruginosa	Bacterial Culture	Proteins, Enzymes	HAuCl4	pH 9.0, 24 h at 58 °C	Small, Spherical, Less Aggregated	Microbial green synthesis using P. aeruginosa for optimized redox potential	ATR-FT-IR, TEM, UV–vis	Shows high antitumor efficacy against PC3 prostate cancer cells, with reduced IL-6 levels.
29	Artemisia annua	Hairy Root Extract	Hydroxybenzoic Acids, Flavonoids, Caffeic Acid	HAuCl4	Ethanolic (70%) extract, controlled conditions	15–40 nm, Triangular and Spherical	Green synthesis using A. annua extract as a reducing agent	HPLC, MALDI-MS, TEM, UV–vis	Exhibits strong photocatalytic activity for Methylene Blue degradation and potential biomedical applications.
30	Halimeda macroloba (Green Macroalgae)	Whole Extract	Bioactive Compounds	HAuCl4	Sunlight exposure, natural bioreduction	18.72 nm, Spherical	Green synthesis using H. macroloba extract for efficient nanoparticle production	UV–vis, FT-IR, TEM, XRD, SEM-EDX, XPS, PSD	High photocatalytic degradation (97.23% MB, 89.91% MO) for industrial dye removal under sunlight.
31	Citrus sinensis (Orange Peel)	Peel Extract	Polyphenols, Flavonoids	HAuCl4	1 mM HAuCl4, 100 °C, 30 min	37 nm, Spherical	Green synthesis using C. sinensis peel extract as a reducing agent	UV–vis, XRD, SEM, FT-IR, TEM	Development of a highly sensitive, noninvasive electrochemical glucose sensor for diabetes monitoring in saliva.
32	Peganum harmala	Leaf Extract	Hydroxyl, Carboxyl, Polyphenolic Groups	HAuCl4	1 mM HAuCl4, optimal ratio 4:1, pH 14, 12 h stirring	12.6–35.7 nm, Oval and Spherical	Green synthesis using P. harmala leaf extract as a reducing and stabilizing agent	UV–vis, FT-IR, SEM, EDX	Significant antibacterial activity against E. coli (25 mm) and antifungal efficacy against Candida albicans (31 mm).
33	Passiflora ligularis (Sweet Granadilla)	Pulp Extract	Polyphenols, Antioxidants	HAuCl4·3H2O	1 × 10–3 M HAuCl4, pH 6, 25 °C, 6 h	8.43–13 nm, Spherical and Triangular	Green synthesis using P. ligularis extract for controlled nanoparticle formation	UV–vis, XRD, HRTEM, EDX, ζ-potential	Strong neuroprotective effects against autism, reducing neuroinflammation, oxidative stress, and apoptosis in brain tissue.
34	Citrus sinensis (Orange Peel)	Peel Extract	Polyphenols, Hydroxyl Groups	HAuCl4	0.165 mM HAuCl4, 100 °C, 1 h	Spherical, Stable in Water	Green synthesis using C. sinensis extract as a reducing and capping agent	UV–vis, FT-IR, DLS, SEM	Selective colorimetric detection of Pb2+ ions in water with LOD of 0.05 μM, applicable for real water samples.
35	Equisetum diffusum (Horsetail)	Leaf Extract	Polar Bioactive Compounds	HAuCl4·3H2O	1.26 mM HAuCl4, Sunlight exposure (4–5 min), 30 °C	Cubic, Well-Dispersed	Green synthesis using E. diffusum extract under sunlight irradiation for nanoparticle formation	UV–vis, FT-IR, SEM-EDX, PXRD, DLS, ζ-potential	Highly selective Pb2+ colorimetric sensor (LOD 4.4 μM), antimicrobial activity, and 96.5% photocatalytic degradation of methylene blue.
36	Curcumin and Doxorubicin-Loaded AuNPs	-	Trisodium Citrate, Curcumin, Doxorubicin	HAuCl4	2.0 mM HAuCl4, 19.4 mM trisodium citrate, 0.2 mM drug	Spherical, Stable	Green synthesis using anticancer drug-loaded gold nanoparticles	UV–vis, DLS, TEM, Hyperspectral Microscopy, MTT Assay	Ultrasound-triggered drug release for targeted cancer therapy, effective intracellular uptake, and enhanced cytotoxicity.
37	Curcuma longa (Turmeric Root)	Root Extract	Curcuminoids, Polyphenols	HAuCl4	1% turmeric extract, 1–5 mM HAuCl4, 24 h in dark	8.5 nm, Spherical	Green synthesis using C. longa extract for controlled nanoparticle formation	UV–vis, FT-IR, TEM, XRD	Sensing of Al3+ and Cr3+ ions (LOD: 0.83 and 1.19 nM), catalytic reduction of 4-NP, degradation of MB and MO dyes in wastewater.
38	Lilium longiflorum (Easter Lily)	Stem Extract	Organic Compounds	HAuCl4·3H2O	1 mM HAuCl4, 25 °C, 6 mL extract	4.97 nm, Spherical	Green synthesis using L. longiflorum extract as reducing and capping agent	UV–vis, FT-IR, TEM, SEM, EDX, HRTEM	High catalytic efficiency in 4-NP and MB reduction (rate constants 1.50 min–1 and 1.29 min–1), promising for environmental remediation.
39	Aconitum violaceum Jacquem. ex Stapf	Crude Extract	Alkaloids, Flavonoids	HAuCl4	Stirring for 50 min, pH 9, Centrifugation (6000 rpm for 15 min)	Spherical, Triangular, <100 nm	Green synthesis using A. violaceum extract as a reducing and capping agent for stable AuNPs formation under controlled pH conditions.	SEM, TEM, EDX, XRD, UV–vis, FTIR, XPS, ζ-potential	Effective antibacterial agent against L. acidophilus and E. coli, strong antioxidant, and efficient photocatalyst for methylene blue degradation under UV light.
40	Annona muricata	Leaf Extract	Phytoconstituents	HAuCl4	Agitation at room temperature until deep purple color appears, incubation for 24 h	Not specified	Green synthesis using A. muricata leaf extract as a reducing and capping agent for stable AuNP formation under ambient conditions.	UV–vis, TEM, SEM, XRD	Exhibits significant anticancer activity against SCC-15 cells by inducing apoptosis via upregulation of Bax and p53 genes and downregulation of Bcl-2.
41	Acorus calamus	Leaf Extract	Bioactive Compounds	HAuCl4·3H2O	5 mL extract mixed with 95 mL of 1 mM gold salt, stored at room temperature for 30 min until ruby red color appears	Hexagonal, Face-Centered Cubic, 30–50 nm	Green synthesis using A. calamus leaf extract as a reducing and stabilizing agent for controlled AuNP fabrication with optimized reaction parameters.	XRD, UV–vis, FTIR, TEM	Demonstrates neuroprotective potential by inhibiting cholinesterase activity, crossing the blood-brain barrier, and reducing oxidative stress for potential Alzheimer’s treatment.
42	Catha edulis (Khat)	Leaf Extract	Phytochemicals	HAuCl4	0.75 mL extract added to 30 mL of 1 mM boiling HAuCl4, stirred for 45 min, color change to wine red, centrifuged after 24 h	Spherical, 17.3 ± 3.7 nm	Green synthesis using Khat leaf extract as a reducing and capping agent for AuNP formation under controlled conditions.	UV–vis, TEM	Developed a colorimetric nanosensor for BPA detection in plastic bottled water with high sensitivity, low detection limit (0.09 ng/mL), and recovery of 86.7–98.0%.
43	Quercetin (Flavonoid)	-	Quercetin	HAuCl4	Quercetin solution (0.05%) mixed with water, gold salt added, stirred at RT until color change from yellow to red/purple	Spherical, ∼10 nm	Green synthesis using quercetin as a reducing agent to form Au/TiO2 hybrid nanocomposites with controlled gold loading	UV–vis, TEM, ζ-potential, Band Gap Energy Measurement	Enhanced photocatalytic degradation of ciprofloxacin antibiotic under visible and UV light, reducing TiO2 band gap from 3.05 to 2.85 eV, achieving up to 95% degradation efficiency.
44	Diospyros kaki L. (Persimmon)	Leaf	Phytochemicals from leaf extract	HAuCl4·3H2O	0.1 and 0.5 mL extract added to 0.5 mM HAuCl4·3H2O at 25 °C, continuous stirring, microwave extraction	25.99 nm (XRD), 45.44 ± 1.095 nm (Zetasizer), spherical/elliptical	Green synthesis using aqueous leaf extract	UV–vis, XRD, TEM, FTIR, Zetasizer	Biomedical, catalysis, and environmental applications
45	Ginkgo biloba	Leaf	Flavonoids, terpenoids, polyphenols	HAuCl4	1:1 ratio with 1 mM HAuCl4, pH 5, 50 °C, magnetic stirring (500 rpm)	18 ± 4 nm, monodispersed, crystalline	Green synthesis using aqueous leaf extract	FTIR, UV–vis, XRD, TEM, ζ-potential	Anticancer therapy via apoptosis induction in nasopharyngeal carcinoma cells
46	Synechococcus moorigangae strain InaCC M208 (Marine Microalga)	Whole cell (Ethanol extract)	Phytoconstituents (Flavonoids, proteins, polysaccharides)	HAuCl4	1:1 ratio with 1 mM HAuCl4, 37 °C, 22 h incubation	8.2 nm, spherical, SPR at 552 nm, ζ-potential – 21.5 mV	Green synthesis using ethanol extract	UV–vis, TEM, XRD, FTIR, PSA, EDX	Antibacterial and antioxidant properties for nanomedicine applications
47	Spirulina platensis (Cyanobacterium)	Whole cell (Aqueous extract)	OH-, carbonyl, amino acids	AgNO3	1 mL extract mixed with 5 mL of 1 mM AgNO3, exposed to sunlight for 10 min, incubated in dark at 30 °C for 24 h	50–70 nm, spherical	Green synthesis using S. platensis extract	UV–vis, SEM, FTIR	Antibacterial, antioxidant, and biocompatibility for biomedical applications
48	Scutellaria barbata D. Don (Scu)	Whole plant (Aqueous extract)	Reducing sugars, flavonoids, polyphenols	HAuCl4	12 mL of 2 mM HAuCl4 mixed with 2 mL of extract, reaction at 60 °C for 30 min	50 nm, spherical	Green synthesis using Scu extract as reducing and stabilizing agent	TEM, DLS, XRD, XPS, UV–vis	Photothermal and chemo-photothermal therapy for cancer treatment, enhancing apoptosis and necrosis in tumor cells
49	Tangerine (Citrus reticulata) Peel Extract (TPE)	Peel	Phytochemicals (Polyphenols, Flavonoids, Terpenoids)	HAuCl4	10 mM HAuCl4 in 10 mL water, mixed with 10 mL TPE extract, reaction at 25 °C for 60 min	26 ± 5 nm, spherical	Green synthesis using TPE as reducing and capping agent	UV–vis, XRD, FTIR, EDX, FESEM, TEM	Antibacterial, antifungal, antioxidant (92.7% DPPH scavenging), photocatalytic degradation of dyes (Methyl orange & Rhodamine B)
50	Plum (Prunus domestica) Peel Extract (PPE)	Peel	Phenolic compounds (Hydroxyl, Carboxyl, Polyphenols)	HAuCl4·3H2O	5 mL of 1 mM HAuCl4 mixed with 30 mL of DES (Betaine-Urea & Choline chloride-Urea), heated at 65 °C for 45 min	5–12 nm, spherical	Green synthesis using DES-based PPE as reducing and stabilizing agent	UV–vis, FTIR, SEM, XRD, EDX, ζ-potential	Antioxidant (ABTS IC50 = 110 mg/mL), skincare (moisturizing cream with 0.5–2% AuNPs), antibacterial (E. coli, B. subtilis)
51	Cucurbita moschata (Pumpkin) Fruit Peel Extract (CM)	Peel	Phytochemicals (Polyphenols, Flavonoids, Carboxyl & Hydroxyl groups)	HAuCl4·3H2O	1:3 ratio of 250 mL of 10 mM HAuCl4 solution and 750 mL plant extract, stirred at 50 °C for 15 min at 750 rpm	18.10 nm, spherical, cubic crystal structure	Green synthesis using CM peel extract as reducing and stabilizing agent	UV–vis, FTIR, TEM, AFM, SEM, EDX, TGA-DTA, XRD, ζ-potential	Antimicrobial (S. aureus, E. coli, C. albicans at 0.004–0.64 μg/mL), Anticancer (IC50 at 50 μg/mL for cancer cell lines Sk-Ov-3, CaCo2, A549)
52	Strobilanthes kunthiana	Leaf extract	Polyphenols, flavonoids, alkaloids	HAuCl4	pH 7, 70 °C, 1 h reaction time	31 nm, spherical	Green synthesis using Strobilanthes kunthiana leaf extract as a bioreducing and stabilizing agent for eco-friendly gold nanoparticle production.	UV–vis, FTIR, XRD, FE-SEM, TEM, DLS, EDX	Used in dopamine sensing, electrochemical biosensors, neurochemical detection, and medical diagnostics for precise and selective neurotransmitter quantification.
53	Halodule uninervis (HUE)	Ethanolic extract	Phytochemicals (hydroxyl, carbonyl groups)	HAuCl4·3H2O	70–80 °C, 30 min sonication, color change to dark purple, centrifugation, lyophilization	Small, spherical, <50 nm, ZP	Green synthesis using H. uninervis extract as a reducing and stabilizing agent for eco-friendly and cost-effective gold nanoparticle production.	UV–vis, SEM, DLS, XRD, TGA, FTIR	Biogenic AuNPs exhibit anticancer properties via apoptosis induction, making them potential candidates for targeted cancer therapy and drug delivery.
54	Punica granatum (Pomegranate)	Juice	Polyphenols (punicalagin, ellagic acid, flavonoids), carbohydrates	HAuCl4	Room temperature, 4 h stirring (120 rpm), centrifugation, solvent washing	Nanostructured AuNPs, 20–50 nm, tunable characteristics, ZP	Green synthesis using pomegranate juice as an effective reducing and stabilizing agent, minimizing environmental impact and utilizing agri-food waste.	UV–vis, MALDI FT-ICR MS, LC-ESI-MS/MS	Biogenic AuNPs show potential in food packaging and dermatology, with antimicrobial and antioxidant properties aiding in shelf life extension and skincare formulations.
55	Anemopsis californica	Isopropanoic extract	Bioactive phytochemicals aiding in nanoparticle stabilization	HAuCl4	24 h agitation with graphene-based material, maintained as colloidal solution or vacuum filtered	Homogeneous distribution of AuNPs on graphene-based material, ∼10–40 nm, ZP	Green synthesis using A. californica extract for in situ nanoparticle formation and integration with graphene for enhanced nanocomposite properties.	UV–vis, TEM, SEM-EDS, XPS, Raman	Graphene-based AuNPs nanocomposites enhance SERS for chemical sensing, biomedical diagnostics, and environmental monitoring, offering improved sensitivity and detection limits.
56	Punica granatum (Pomegranate)	Juice	Carbon dots (CDs) from polyphenols and sugars	HAuCl4	180 °C, 6 h hydrothermal treatment, followed by gold salt reduction using NaBH4	AuNPs-modified carbon dots (CDs), 2–10 nm, ZP	Green synthesis of CDs from pomegranate juice followed by AuNPs decoration to create optical neurotransmitter nanosensors for dopamine detection.	UV–vis, SEM, TEM, FL	AuNPs-CDs sensor efficiently detects dopamine in biological samples with high sensitivity and selectivity, enabling advanced diagnostics for neurological disorders.
57	Madhuca indica	Flower extract	Phytochemicals acting as reducing and stabilizing agents	HAuCl4	Rapid green synthesis, UV absorbance at 550 nm	Spherical, 20.34 ± 4.36 nm, ZP	Green synthesis of AuNPs using Madhuca indica flower extract, providing a cost-effective and eco-friendly approach for nanomaterial synthesis.	UV–vis, XRD, FTIR, TEM, FE-SEM, DLS, EDX	MI-AuNPs exhibit strong anticancer effects by targeting cancer stemness and EMT in HNSCC, demonstrating therapeutic potential with minimal toxicity.
58	Arctium lappa	Flowers	Quercetin (Flavonoid)	None (Laser Ablation)	Soxhlet extraction, hydrolysis, crystallization, laser ablation	Spherical AuNPs 80–160 nm, AgNPs 20–40 nm, ZP	Green synthesis of AuNPs and AgNPs using quercetin extracted from A. lappa, ensuring eco-friendly nanoparticle production.	UV–vis, HPLC, FTIR, NMR, SEM, TEM, XRD	AuNPs and AgNPs exhibit high antioxidant activity, with AgNPs showing superior free radical scavenging, highlighting their potential for antioxidant therapy and biomedical applications.
59	Psidium araca (Sour Guava)	Peel extract	Polyphenols (Phenolic acids, flavonoids)	HAuCl4·3H2O	50 °C, 1 h stirring, response surface optimization, NaBH4 addition	Gold nanorods (AuNRs), ∼30 nm, LSPR 700–800 nm	Green synthesis using Psidium araca peel extract as a weak reducing agent for gold nanorods biosynthesis, replacing ascorbic acid.	UV–vis, TEM, LSPR	Optimized synthesis of anisotropic AuNRs for photothermal therapy, targeted drug delivery, and diagnostic imaging in biomedical applications
60	Sphaeranthus amaranthoides	Leaf extract	Phenolic compounds	Chloroauric acid (HAuCl4)	Sunlight exposure for 2 h, PVA stabilization	Anisotropic nanoparticles	Green synthesis using Sphaeranthus amaranthoides leaf extract as a natural reducing agent under solar irradiation	SPR, HRTEM, ζ-potential analysis	Biocompatible gold nanoparticles for targeted drug delivery, exhibiting cytotoxicity against THP-1 Acute Monocytic Leukemia cells
61	Sodium lignosulfonate	Biopolymer	Lignosulfonate	Chloroauric acid (HAuCl4)	Ultrasonic irradiation at 60 °C for 30 min	20–30 nm, spherical	Green synthesis using sodium lignosulfonate under ultrasonic conditions for gold nanoparticle stabilization and reduction	UV–vis, FT-IR, FE-SEM, TEM, EDX, ICP-AES	Catalysis in Suzuki–Miyaura coupling, antigastric cancer agent via PI3K-Akt-mTOR modulation, high biocompatibility for biomedical applications
62	Croton draco	Branch extract	Polyphenols, flavonoids	Chloroauric acid (HAuCl4)	Different extraction methods, 25 ± 2 °C, UAE method preferred	10–50 nm, spherical	Green synthesis using Croton draco aqueous extracts with high antioxidant capacity for reducing and stabilizing AuNPs	UV–vis, TGA, DLS, SEM, XRD, FT-IR	Excellent antibacterial (99% Gram-positive, 87% Gram-negative) and anti-inflammatory (97%) properties, potential biomedical applications
63	Stachytarpheta jamaicensis	Whole plant extract	Polyphenols, flavonoids	Chloroauric acid (HAuCl4)	Room temperature, plant extract-mediated reduction and stabilization	60.3 nm, spherical	Green synthesis using Stachytarpheta jamaicensis extract for eco-friendly nanoparticle formation with therapeutic potential	UV–vis, FT-IR, SEM, PSA	Strong anticancer activity in MCF7 breast cancer cells (IC50 = 19.53 μg/mL), downregulation of MYC and CCND1 oncogenes, potential in nanomedicine
64	Scrophularia striata Boiss	Flower and stem extract	Polyphenols, flavonoids	Silver nitrate (AgNO3)	Ultrasonication for 15 min at room temperature, optimized plant extract stored at 4 °C	56.1 nm (stem), 62.5 nm (flower), spherical	In-situ green synthesis of Ag NPs on polyamide (PA) fabric using plant extract as a reducing and stabilizing agent	UV–vis, DLS, SEM, XRD	Enhanced dyeing performance (K/S increase of 64–100%), excellent antibacterial activity (100% inhibition against E. coli and S. aureus), retained 88–79% activity after 10 washes, potential for medical and hygienic textile applications
65	Tangerine peel extract	Peel extract	Flavonoids, polyphenols	Chloroauric acid (HAuCl4)	40 °C, 15 mM gold concentration, 60 min	26 ± 5 nm, spherical	Green synthesis using Tangerine peel extract (TPE) as a reducing and capping agent	UV–vis, XRD, FT-IR, EDAX, FE-SEM, TEM	Strong antibacterial (MIC: 31.25–62.5 μg/mL), antifungal (Candida albicans), 92.7% antioxidant (DPPH assay),6effective photocatalytic degradation (88.6–93.2% UV, 67.3–74.1% sunlight)
66	Croton Caudatus Geisel	Leaf extract	Polyphenols, antioxidants	Chloroauric acid (HAuCl4), Hexachloroplatinic acid (H2PtCl6·6H2O)	60 °C, reaction time 30 min	12–33 nm, rectangular	Single-step green synthesis of Au–Pt bimetallic nanoparticles without external capping or stabilizing agents	UV–vis, FT-IR, PXRD, TEM, EDAX, SAED	Strong antibacterial, antifungal, and anticancer effects (IC50 = 37.17 μg/mL in HeLa cells), potential for bimetallic nanoparticle drug development
67	Euphorbia acaulis	Leaf extract	Polyphenols, flavonoids	Silver nitrate (AgNO3), Chloroauric acid (HAuCl4)	Room temperature, optimized leaf extract-mediated reduction	1–100 nm, spherical	Green synthesis of silver (SNPs) and gold nanoparticles (GNPs) using E. acaulis as a natural reducing and capping agent	UV–vis, HR-SEM, EDX, TEM, AFM, XRD, FTIR	Anticancer (MCF-7, SNP IC50: 59.87 μg/mL, GNP IC50: 91.074 μg/mL), apoptosis and DNA damage induction, superior antibacterial (E. coli 17 mm, S. aureus 10.77 mm), larvicidal (Aedes aegypti, SNP LC50: 20.81 mg/L, GNP LC50: 51.10 mg/L), photocatalytic methylene blue degradation

For instance, Srinath et al. reported the biosynthesis of biocompatible gold nanoparticles (AuNPs) using Brevibacillus formosus isolated from the Hutti gold mine, India. The bacterial metabolites acted as reducing and stabilizing agents. Synthesized AuNPs (5–12 nm, spherical) were characterized using UV–vis, Fourier transform infrared (FTIR), dynamic light scattering (DLS), and TEM and also exhibited strong antibacterial activity against S. aureus along with biocompatibility with chicken RBCs, highlighting their potential for biomedical applications. In another study by Singh and Kundu, biosynthesis of gold nanoparticles (AuNPs) using Pseudomonas aeruginosa and Rhodopseudomonas capsulata was carried out. The pH significantly influenced size and shape, with spherical 10–20 nm AuNPs at pH 7 and nanoplates at pH 4. Altaf et al. reported the green synthesis of gold nanoparticles (AuNPs) using Iris kashmiriana rhizome extract with spherical AuNPs (∼80 nm). These nanoparticles exhibited strong antibacterial, antibiofilm, and antiadherence properties, particularly against Streptococcus mutans, making them effective for coating orthodontic appliances. Daramola et al. evaluated the bacteria-mediated synthesis of gold nanoparticles (AuNPs) using Bacillus subtilis and P. aeruginosa. The process involved the microbial reduction of gold salts, producing stable, biocompatible nanoparticles of size 118 nm. Abu-Elghait et al. developed optimal conditions for the biosynthesis of gold (AuNPs) using Trichoderma saturnisporum. The ideal conditions for AuNPs were pH 6.94, 33.2 °C, and 1.21 mmol, offering a more efficient alternative to traditional synthesis methods. Another study, Omole et al. explored the microbial synthesis of gold nanoparticles (AuNPs) using Lysinibacillus fusiformis and HAuCl4 solution. The synthesized AuNPs were characterized by UV–vis, SEM, EDX, DLS, TEM, XRD, and FT-IR analyses, revealing spherical particles with a mean size of 121.2 nm. These AuNPs demonstrated significant antibacterial activity against multidrug-resistant bacteria from chronic wounds, indicating their potential for pharmaceutical applications in treating infections caused by Multi-Drug Resistant (MDR) bacteria. Table effectively summarizes all advantages and disadvantages of physical, chemical and various green synthesis methods in synthesis of AuNPs.

4. Comparative Overview of Physical, Chemical, and Green Synthesis Methods for Gold Nanoparticles (AuNPs) .

method type	synthesis method	principle	advantages	disadvantages	cost	scalability	purity issues	biocompatibility	limitations	references
Physical	Laser Ablation	A pulsed laser is focused on a bulk gold target submerged in liquid, causing vaporization and nanoparticle formation via plasma-induced cavitation, without chemical reagents.	High purity, no chemicals, clean process	Expensive, low yield, not scalable	High	Low	Very low	Very High	Poor scalability, costly equipment, difficult size control	−
	Ball Milling	Mechanical grinding of bulk gold using high-energy balls in a rotating chamber produces nanoparticles by collision and shear forces, typically in inert conditions.	Simple, scalable, solvent-free	Contamination from media, broad size distribution	Low	High	Moderate	Moderate	Particle aggregation, contamination from grinding media
	Evaporation–Condensation	Gold is evaporated in a high-temperature zone and condensed into nanoparticles in a colder inert atmosphere, avoiding chemical reagents or solvents.	High purity, solvent-free	Needs vacuum, poor control	High	Low	Low	High	Needs inert gas, limited shape control
	Ultrasonic Spray Pyrolysis	Ultrasonic waves generate aerosol droplets from gold precursor solutions; these undergo pyrolysis in a high-temperature furnace to yield gold nanoparticles.	Continuous, scalable, uniform synthesis	Requires high T, complex setup	Moderate	High	Moderate	Moderate–High	Agglomeration risk, byproduct formation
	Sputter Deposition	Gold atoms are ejected from a gold target in vacuum using energetic ions and deposited onto a substrate or dispersed in a liquid medium.	High purity, precise control	Slow process, expensive setup	High	Low	Very low	High	Limited for large-scale production
	Microwave Irradiation	Microwave energy rapidly heats gold precursor solutions via dielectric heating, inducing nucleation and growth of gold nanoparticles without conventional thermal conduction.	Fast, uniform heating, simple	Size variability, depends on solvent	Low–Moderate	Moderate	Moderate	High (in aqueous setup)	Uneven nucleation, less suitable for scale-up
	Arc Discharge	High-voltage electric arc vaporizes gold electrodes in a gas environment, with subsequent cooling and condensation forming gold nanoparticles.	High temperature, no reagents	Broad size, low yield, safety issues	Moderate	Low	Low–Moderate	High	Risk of spark ignition, broad distribution
	Thermal Decomposition (Physical)	Solid gold compounds are thermally decomposed at elevated temperatures to form gold nanoparticles without solvents, often under inert or reducing atmospheres.	Simple, solvent-free	Needs high T, low shape control	Moderate	Moderate	Low	High	Not suitable for heat-sensitive applications
Chemical	Turkevich Method (Citrate)	Gold salt (e.g., HAuCl4) is reduced in aqueous medium using trisodium citrate, which also stabilizes the formed nanoparticles via surface adsorption.	Easy, aqueous, good control	Surface-bound citrate may interfere	Low	High	Moderate	High (after washing)	Citrate residues may hinder functionalization	−
	Brust-Schiffrin Method	A two-phase system uses NaBH4 to reduce Au3+ in the presence of alkanethiols, forming stable organic-phase monolayer-protected nanoparticles.	Stable, tunable size	Toxic organics and thiols	Moderate	Moderate	Moderate–High	Moderate	Limited biocompatibility without purification
	Seed-Mediated Growth	Small gold seeds are used to control the anisotropic or isotropic growth of larger nanoparticles using additional precursors and surfactants.	Excellent size/shape control	Multistep, impurity-sensitive	Moderate	High	Moderate	High	Requires precise conditions and surfactant use
	Microemulsion	Nanoscale water droplets within surfactant-stabilized micelles act as confined reactors for the reduction of gold salts to form nanoparticles.	Narrow size distribution	Surfactant removal is difficult	Moderate–High	Low–Moderate	Moderate	Moderate–High	Thermodynamically unstable systems
	Sonochemical (Chemical)	Acoustic cavitation from ultrasonic waves generates radicals and extreme conditions that drive the reduction of gold ions into nanoparticles.	Fast, fine dispersion	Requires optimization, broad distribution	Low–Moderate	Moderate	Moderate	Moderate–High	Cavitation effects difficult to control
	Electrochemical Reduction	Gold ions in solution are reduced to metallic nanoparticles at the cathode under applied potential in an electrochemical cell.	High purity, no chemicals	Electrode degradation, low throughput	Low–Moderate	Low–Moderate	Very low	High	Limited industrial viability
	Photochemical Reduction	Light-sensitive compounds generate reducing agents upon UV or visible irradiation, which then reduce Au3+ to Au0 nanoparticles in solution.	Mild, tunable via light	Needs photoactive reagents	Moderate	Low–Moderate	Moderate	High	Requires UV/visible light control
	Radiolytic Reduction	High-energy radiation (e.g., γ-rays) induces solvent radiolysis, generating radicals that reduce gold ions to form ultrapure nanoparticles.	High purity, no reducing agents	Requires radiation source, expensive	High	Low	Very low	High	Inaccessible in many laboratories, safety concerns
	Polyol Method	Gold salt is reduced by polyols (e.g., ethylene glycol) at high temperature, which also act as stabilizers to control growth.	Good size control, no extra reductants	Organic solvent removal required	Low–Moderate	Moderate	Moderate	Moderate	Viscous solvent complicates purification
	Alcohol Reduction	Alcohols like ethanol or methanol reduce gold salts upon heating, with temperature and alcohol concentration influencing nucleation and growth.	Simple, low cost	Needs heating, larger size distribution	Low	Moderate	Low–Moderate	High	Temperature-sensitive reaction
	Chemical Vapor Deposition	Gold precursor vapor is decomposed or reacts on a heated substrate to form nanoparticles or thin films under controlled atmosphere.	Pure, controllable	High vacuum, substrate limited	High	Low	Very low	High	Industrial-scale usage limited to thin films
	Sol–Gel Method	Gold precursors are hydrolyzed and condensed to form gels, which on drying and calcination yield gold nanoparticles embedded in matrix.	Morphology control, embedding capability	Long processing time	Moderate	Moderate	Moderate	Moderate–High	Solvent residues, drying/cracking during gelation
Green	Plant Extract-Mediated Synthesis	Phytochemicals like flavonoids and polyphenols in plant extracts reduce gold ions and stabilize nanoparticles without toxic chemicals.	Eco-friendly, abundant raw materials, high biocompatibility	Batch-to-batch variability, poor size/shape control	Very Low	Moderate	Low–Moderate	Very High	Reproducibility issues, extract complexity	−
	Microbial Synthesis (Bacteria)	Bacterial enzymes or metabolites mediate reduction of Au3+ ions to Au0, with intracellular or extracellular nanoparticle formation.	Sustainable, extracellular synthesis possible, biosafe	Requires sterile growth, slow process, difficult purification	Low	Low–Moderate	Moderate	High	Contamination risk, low yields
	Fungal-Mediated Synthesis	Fungal species secrete redox-active biomolecules that facilitate gold ion reduction and nanoparticle stabilization, often extracellularly.	Higher yield than bacteria, extracellular synthesis	Long incubation, complex downstream processing	Low	Low–Moderate	Moderate	High	Optimization of culture conditions needed
	Algal-Mediated Synthesis	Algae contain reducing metabolites (e.g., polysaccharides, proteins) capable of converting gold ions into nanoparticles under mild conditions.	Eco-friendly, renewable, low cost	Less explored, slow kinetics	Low	Low	Low–Moderate	High	Lower efficiency, extraction of active compounds needed
	Enzyme-Mediated Synthesis	Purified enzymes catalyze the bioreduction of Au3+ to Au0 under aqueous and controlled conditions, yielding pure and uniform nanoparticles.	High selectivity, clean mechanism	Enzyme purification, high cost	Moderate	Low	Very low	Very High	Enzyme stability and reusability
	Polysaccharide-Assisted Synthesis	Natural polysaccharides (e.g., starch, cellulose) reduce and stabilize AuNPs via hydroxyl or aldehyde functional groups in water.	Biodegradable, safe, stable dispersion	Some polysaccharides may remain bound	Low	Moderate	Low–Moderate	Very High	Surface passivation may hinder activity
	Amino Acid/Protein-Based Synthesis	Amino acids or proteins like BSA act as reducing and stabilizing agents under mild conditions to form biocompatible nanoparticles.	Mild conditions, excellent biocompatibility	Size distribution varies, protein denaturation risk	Low	Moderate	Moderate	Very High	Functional group interference, protein cost
	Honey-Mediated Synthesis	Natural sugars and enzymes in honey serve as reducing agents for gold ions, forming capped nanoparticles with inherent antimicrobial properties.	Natural, accessible, inherently antimicrobial	Viscosity interferes with uniform synthesis	Low	Moderate	Moderate	High	Consistency issues due to honey composition variability
	Natural Gum/Resin-Assisted Synthesis	Natural gums or resins contain polysaccharides and polyphenols that facilitate reduction and stabilize AuNPs in aqueous systems.	Eco-safe, good stabilizing properties	May alter NP surface chemistry	Low	Moderate	Low–Moderate	Very High	Surface capping may affect functionalization
	Biopolymer-Assisted Synthesis	Biopolymers like chitosan or gelatin reduce and stabilize gold nanoparticles through functional group interactions under aqueous or mild conditions.	Biodegradable, low toxicity, stable nanoparticles	May interfere with surface functionality	Low	Moderate	Low–Moderate	Very High	Batch inconsistency, polymer–metal interactions complex

^aComments based on available literature.

2.3. Biosynthetic Mechanism of AuNPs

The biosynthesis of gold nanoparticles (AuNPs) typically follows a straightforward two-step process that does not require extreme temperature or pressure conditions. − Initially, a biological extractsuch as one derived from plants, bacteria, or fungiis combined with a solution of chloroauric acid (HAuCl₄). This interaction leads to the reduction of gold ions (Au³⁺) to elemental gold atoms (Au⁰). In the second phase, the nucleated gold atoms undergo growth and stabilization, resulting in the formation of AuNPs, as illustrated in Figure .

3.

Schematic representation of AuNPs biosynthesis mechanism and mechanism of the formation and stabilization of gold nanoparticles by polyphenolic compounds. Adapted with permission from ref . Copyright 2022, MDPI, Basel, Switzerland.

The synthesis is often visually confirmed by a distinct color change in the solution, indicating nanoparticle formation. The chemical reduction of Au³⁺ to Au⁰ in the presence of water can be represented by the reactions depicted in above Figure .

2.3.1. Reducing Potential of Plant Phytochemicals

Phytochemicals present in plant extractssuch as amino acids, proteins, carbohydrates, phenolic acids, flavonoids, and terpenoidsplay crucial roles in the green synthesis of metal and metal oxide nanoparticles. These bioactive compounds act as natural reducing and stabilizing agents, enabling nanoparticle formation under mild, eco-friendly conditions without the need for toxic chemicals or harsh physical processes. The literature suggests that the extent of synthesis of AuNPs is directly linked with reduction potential of plant extract. Nevertheless, these plant extracts have varieties of phytomolecules of different classes such as polyphenols, terpenes, etc. It has been seen that there are 3 key factors influencing the efficiency of this biosynthesis process: (1) the degree to which metal ions are reduced by compounds in the extract, (2) the concentration of reducing agents, and (3) the composition of bioactive compounds that stabilize the resulting AuNPs. It is very likely that higher the amount of content of reducing substances, higher would be rate of formation of AuNPs and promotes the creation of smaller nanoparticles, and enhances their stability. − , Amino acids and proteins reduce metal ions and stabilize nanoparticles through electron donation and capping, while carbohydrates (e.g., polysaccharides) contribute to particle size and morphology control due to their hydrophilic and catalytic properties. Phenolic acids and flavonoids facilitate metal ion reduction via hydrogen or electron transfer, often through redox or keto–enol transitions. Huang et al. elaborated in details mechanisms of phenolic acids in biosynthesis of AuNPs (Scheme ). One of such representative mechanism is illustrated below, which was proposed by Manyuan and Danwanichakul. In concern work, they illustrated synthesis of AuNPs with the help of spent coffee ground extract.

1. Proposed Reactions Involved in the Formation of AuNPs from HAuCl₄ via Phenolic Acids.

Terpenoids, commonly found in essential oils, also assist in nanoparticle synthesis, particularly silver nanoparticles apart from AuNPs, due to their strong reducing capacity. Hossanisaadi et al. reviewed studies assessing the ability of plant extracts to reduce gold ions. The review examined extracts from 27 plant speciesincluding Rosa damascena, Juglans regia, Caccinia macranthera, etc.many of which are traditionally used in Middle Eastern medicine. Extracts were prepared from different plant parts, and the study identified 28 additional plant species with effective gold ion-reducing capabilities. These extracts successfully facilitated gold nanoparticle synthesis. , Additionally, the success of metal nanoparticle (MNP) biosynthesis depends on the electrochemical potential of the specific metal ion involved. Noble metal salts have reduction potentials ranging from 0.35 to 1.0 V, and metal ions can be reduced to nanoparticles if the extract’s reduction potential exceeds +0.16 V. , Table gives an idea about the reducing potentials of various phytochemicals reported for the synthesis of AuNPs.

5. Reducing Potentials of Plant Phytochemicals Reported in the Literature .

plants	compounds/phytochemicals	particle size (nm)	shape	references
Parkia roxburghii	Proteins	5–25	Spherical
Murraya koenigii	Polyphenols, flavonoids, alkaloids	20	Spherical, triangle
Mentha longifolia	Phenolic flavonoids, tannins, etc.	10.23 ± 2	Oval
Anacardium occidentale	Proteins, polyols, gallic acid, tannins	17	Spherical
Prunus serrulata	Phenolic compounds, proteins	20–50	Hexagonal
Zingiber officinale	Ascorbic acid, oxalic acid	5–20	Spherical, irregular (hexagonal, triangular)
Indigofera tinctoria	Polyphenols, flavonoids, etc.	6–29	Spherical, hexagonal, triangular
Bauhinia purpurea	Polyphenols	20–100	Triangular, hexagonal, nanorods
Cibotium barometz	Flavonoids, phenolic acids, fatty acids	5–20	Spherical
Jasminum sambac	Polyphenols, flavonoids, terpenoids	20	Spherical
Coleus forskohlii (1)	Phenolic compounds	10–30	Spherical
Coleus forskohlii (2)	Forskolin, proteins	15–35	Hexagonal
Memecylon edule	Saponin	10–45	Triangular, circular, hexagonal
Mussaenda glabrata	Alkaloids, tannins, flavonoids, steroids	10.59	Spherical
Stereospermum chelonoides	Polyphenolic compound (lignans)	27.19 ± 5.96	Spherical
Rivea hypocrateriformis	Polyphenols	20–30	Spherical
Gloriosa superba	Glycosides, tannins	20–50	Triangular, spherical
Glycyrrhiza uralensis	Flavonoids, polyphenols, glycyrrhizin	10–15	Spherical
Aerva lanata	Polyphenols, flavonoids, alkaloids, etc.	10–30	Spherical, hexagonal, triangular plate
Dendropanax morbifera	Polysaccharides	10–20	Polygonal, hexagonal
Angelica pubescens Maxim	Flavonoids, phenols, sesquiterpenes	10–30	Spherical, icosahedral
Amorphophallus paeoniifolius	Phenolic compounds (quercetin)	13.3	Spherical, polygonal
Asparagus racemosus	Phenolics, flavonoids, etc.	10–50	Spherical
Chrysopogon zizanioides	Alkaloids, phytosterols	123–138	Cubic
Memecylon umbellatum	Saponins, phenolics, proteins, quinones	15–25	Spherical, triangular, hexagonal
Platycodon grandiflorum	Triterpenoidal platycodon saponin	14–15	Spherical (major), triangular (minor)
Pulicaria undulata	Phenolic compounds (quercetin, kaempferol, etc.)	5–12	Regular (spherical), irregular (triangular, hexagonal, plates)
Rhodiola rosea	Flavonoids, polyphenols, terpenoids, etc.	12–18	Irregular shape
Garcinia mangostana	Flavonoids, phenolics, carbohydrates, glycosides	15.37–44.20	Spherical
Stemona tuberosa Lour	Not specified	20–30	Irregular shape
Actinidia deliciosa	Proteins	7–20	Spherical
Rosa canina L.	Phenolic compounds	26	Quasi-spherical
Aspalathus linearis	Polyphenols, aspalathin	7.5 ± 0.34	Hydra-like
Ficus retusa	Phenolic compounds	10–25	Spherical
Mukia maderaspatana	Flavonols (quercetin, phloroglucinol)	20–50	Spherical, triangular, circular
Clerodendrum inerme	Phenolics, flavonoids, etc.	5.82	Spherical
Trapa natans var. bispinosa Roxb.	Phenolic compounds (gallic acid, quinones)	25 ± 2	Spherical
Panax ginseng Meyer	Ginsenosides, polyphenols, reducing sugars, acidic polysaccharides	5–10	Spherical
Eleutherococcus senticosus	Phenolic compounds, reducing sugars, proteins	189	Face-centered cubical
Cornus mas	Polyphenolic compounds	5–30	Pseudospherical

^aAdapted and modified with permission from ref . Copyright 2023, MDPI.

2.3.2. Reducing Potential of Microbial Enzymes

The biosynthesis by these microorganisms can occur via two distinct pathways: extracellular and intracellular, depending on where the nanoparticle formation takes place (Figure ). In extracellular biosynthesis, metal ions are reduced outside the bacterial cell through enzymatic activity, leading to nanoparticle formation in the surrounding medium. In contrast, intracellular biosynthesis involves the uptake of metal ions into the cell, where enzymatic processes reduce them internally to form nanoparticles ,

4.

Schematic representation of AuNPs biosynthesis mechanism through microorganisms. Adapted from ref . Copyright 2022, MDPI, Basel, Switzerland.

Gholami-Shabani and colleagues developed a cell-free method for synthesis of AuNPs using α-NADPH-dependent sulfite reductase purified from E. coli. An average sized AuNPs of 10 nm, were confirmed from this novel route. In another review, Shedbalkar et al. covered detailed aspects of mechanisms of microbial AuNPs synthesis. Physicochemical parameters like temperature, pH, and substrate concentration influence intracellular AuNP synthesis and morphology, with monodispersity achievable through their optimizationthough comprehensive studies are lacking. Proteins and amino acids such as cysteine, tyrosine, and tryptophan play key roles in AuNP biosynthesis and stabilization. Free amino or cysteine groups bind AuNPs for stabilization, while tyrosine and tryptophan contribute to NP formation and reduction at high pH through specific binding and redox activity. Protein type also affects AuNP capping and stability. Several other review articles have also been published detailing recent advances in microbe assisted AuNP synthesis. , Chaurasia and colleagues beautifully summarized list of various microbial sources known for producing extracellular and intracellular enzymes involved in AuNPs synthesis along with their influence of particle sizes and shapes. (Table )

6. Overview of Key Microbial Sources Known for Producing Extracellular and Intracellular Enzymes Involved in Gold Nanoparticle Synthesis .

Sr. No.	microbial source	shape	size (nm)
1	Coprinus comatus		<100
2	Cladosporium sp.	Spherical	5–10
3	Bacillus subtilis		20–25
4	Pleurotus ostreatus	Uneven spherical	10–30
5	Trichoderma harzianum	Spherical	32–44
6	Rhizopus oryzae	Flower-like	43 ± 19
7	Cladosporium oxysporum AJP03	Quasi-spherical	72.32 ± 21.80
8	Magnusiomyces ingens LH-F1	Spherical, plates (triangle, hexagon, etc.), irregular	80.1 ± 9.8
9	Acinetobacter	Spherical, triangular, polyhedral	19
10	Klebsiella pneumonia	Spherical	10–15
11	Ureibacillus thermosphaericus	–	50–70
12	Shewanella oneidensis	Round	12
13	Neurospora crassa	Round	20–50
14	Brevibacterium casei	–	10–50
15	Yarrowia lipolytica	Triangles	15
16	E. coli	Hexagonal, triangular	20–30
17	Rhodopseudomonas capsulata	Round	10–20
18	Pseudomonas aeruginosa	–	15–30
19	Sargassum wightii	Planar	8–12
20	Plectonema boryanum	Cubic	<10–25
21	Fusarium oxysporum (Au–Ag alloy)	Round	8–14
22	Rhodococcus sp.	Round	8–12

^aAdapted and reproduced with permission from ref Copyright 2022, Elsevier.

Table explains various factors affecting synthesis of AuNPs.

7. Factors Affecting the Synthesis of AuNPs .

factor	details
pH	Affects NP shape, size, and biomolecule interaction. Acidic conditions (pH 2.5–5) promote faster aggregation and growth of medium to large NPs; basic pH (7–9) slows growth.
Temperature	Influences nucleation rate, particle growth, and monodispersity. Higher temperatures yield smaller, more uniform NPs. Green synthesis methods operate below 100 °C; physical methods may exceed 350 °C. Below 50 °C can cause polydispersity and slow reactions.
Pressure	Affects NP morphology and growth rate. Ambient pressure settings favor biological reduction processes and lead to more efficient NP synthesis.
Time	Reaction duration influences NP size, aggregation, and completion of gold ion reduction. Example: Full reduction achieved after 60 min in palm leaf-based synthesis, beyond which wavelength stabilizes. Prolonged time may cause aggregation or degrade stability.
Concentration of Plant Extract/Biomass	Higher concentrations typically lead to enhanced NP formation and morphological variations. Optimal extract levels must be established for consistent size and shape.
Salt Concentration	Regulates NP yield and characteristics. Increased salt (e.g., 0.7 mM) enhances synthesis efficiency, but excessive salt may destabilize particles.
Reducing and Stabilizing Agent Concentration	Agents like trisodium citrate influence reduction rate and NP stability. Higher citrate levels lead to smaller, more uniform NPs due to better control over nucleation and stabilization. The adsorption rate of the stabilizer is key in determining NP size and preventing aggregation.
Synthesis Method	Chemical (e.g., citrate reduction), physical, or green synthesis methods each have distinct requirements for temperature, pH, and reactants. Method selection impacts reproducibility, particle size control, and environmental impact.

^aWell explained in the ref .

2.4. Effect of Different Parameters on the Appearance and Functionality of Gold Nanoparticles

A wide number of factors such as size, shape, and type of environment are responsible for the interaction of the AuNPs with light. A coordinated oscillation of electron charge, which is in resonance with the frequency of visible light, is generated by the interaction of free electrons with the oscillating electric fields of a light ray traveling near a colloidal nanoparticle. The size or form of the nanoparticles can be changed to modify the surface plasmon resonance, creating particles with customized optical properties for various purposes. The unique interaction with light, surface plasmon resonance, changes depending on particle sizes. The smaller ones appear red, while larger ones reflect bluish or purplish hues. Shape plays a key role in how cells absorb these particles, with triangular and rod-shaped nanoparticles being taken up more efficiently than star-shaped ones. While larger particles tend to circulate longer and serve as better delivery agents, smaller ones can be more toxic. Yue et al. showed that gold nanoparticle size and shape affect siRNA delivery. Larger particles (50 nm spheres, 40 nm stars) had higher cellular uptake and escaped endosomes, unlike smaller 13 nm spheres. This highlights the importance of nanoparticle design in enhancing functionality for effective gene delivery applications. Xie et al. evaluated how shape influences gold nanoparticle uptake in RAW264.7 cells. Star, rod, and triangle-shaped nanoparticles showed varying internalization, with triangles achieving the highest uptake. Different shapes engaged distinct endocytosis pathways, demonstrating that nanoparticle geometry significantly affects cellular uptake and can guide effective drug delivery system design. These characteristics make gold nanoparticles highly adaptable for medical applications such as imaging, therapy, and drug delivery. Figure gives an idea about various kinds of Au nanostructures.

5.

Transmission electron microscope (TEM) images for various kinds of Au nanostructures (a) nanospheres, (b) nanodisks, (c) nanorods, and (d) cubic nanocages. Reproduced from ref . Copyright 2021, with permission of Elsevier.

2.5. Characterization of AuNPs

The characterization of AuNPs is usually done via modern analytical techniques mentioned in Figure .

6.

Various Characterization techniques used for AuNPs.

2.4.1. UV–Visible Absorption Spectroscopy

UV visible absorption spectroscopy is usually performed to study the optical properties and band gap of nanoparticles. When a PerkinElmer 2 spectrometer was used for characterization, it showed a peak in range of 300–900 nm via capturing surface plasmon resonance (SPR) feature for gold nanoparticles. It has been observed that AuNPs shows a strong absorption peak around 520 nm due to SPR and it remains the same irrespective of change in particle shape or the surrounding material. Similarly, on application of gold nanoparticle on APS (aminopropylsilane) -treated glass, we can see the SPR peak shifting to a higher wavelength (red shift) followed by the band broadening due to the electromagnetic interactions between nearby particles. , This is one of the most helpful techniques to find the size, concentration, and aggregation level of gold NPs. While operating, spectrum registration may take some time and the extinct spectra of AuNPs can be derived via Mie theory. Furthermore, in one case, an Electrochemiluminescence biosensor based on gold nanoparticles was synthesized and on characterization, it was observed that a 5 nm gold nanoparticle showed a peak at 515 and 20 nm at 535 nm (Figure A,B). This demonstrated the size dependent optical property because of the localized SPR (LSPR). The 20 nm one was more effective in increasing the electrochemiluminescence signals leading to a better electron transfer. Iqbal et.al, synthesized sodium alginate (SA) coated AuNps for delivering the natural anticancer compound T-res and on characterization, a peak at 526.4 nm indicated the presence of bare gold nanoparticles, whereas SA coated nanoparticles showed a blue shift that is decrease in peak. This was because the SA improved the nanoparticles dispersion and stability and reduced the interparticle interactions or aggregation. Absorption intensity increases with heat due to improved SPR While analyzing AuNPs, a blue shift was observed for a smaller particle and greater pH resulting in increase in the peak, i.e., from 530 to 640 nm. Overall, UV–vis is an efficient method as it does not require any sample preparation and gold nanoparticles can be analyzed straight way after synthesizing.

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UV–vis absorption spectra of Au NPs (5 nm) (A), Au NPs (20 nm) (B), and sodium tetrachloroaurate and gold nanoparticles (C). Panel (A) and (B) Reproduced from ref . Copyright 2025, with permission of Elsevier; Panel (C) Reproduced from ref . Copyright 2023, with permission of MDPI.

2.4.2. Fourier Transform Infrared Spectroscopy (FTIR)

This characterization technique identifies the presence of functional group. In the case of gold nanoparticles synthesized from plant extracts, this method showed the functional groups involved in reduction, capping and stabilization of gold nanoparticles. It also confirmed the presence of phytochemicals by showing a common peak at 3389 cm^–1 indicating O–H stretch, alcohol, 2919–2844 cm^–1 C–H stretch, 1458 cm^–1 (N–H bending), 1700 cm^–1 (CC stretch). This is a highly sensitive method for nanoparticles. Nowadays FTIR also shows the interaction of gold nanoparticles with the reactive agents in few seconds. Similarly, advanced version is called attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), which is extremely useful for nanoparticles as it helps in understanding the composition and behavior of these materials. Key absorption bands include: 3286 cm^–1, attributed to −OH stretching vibrations of phenolic compounds and other phytochemicals; 2928 and 2875 cm^–1, corresponding to −CH stretching of alkanes; 1636 cm^–1, associated with −NH bending and −CO stretching; 1512 cm^–1, related to −CH vibrations of alkanes and −NO stretching of nitro compounds; 1454 cm^–1, indicative of −OH bending and – CO stretching in phenolics and similar compounds; and 1387 cm^–1, assigned to −CN stretching of aromatic amine groups (Figure A).

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(A) ATR-FTIR spectrum and (B) XRD spectrum of the synthesized Au NPs. Reproduced from ref . Copyright 2022, with permission of MDPI.

2.4.3. Atomic Force Microscopy

Commonly known as AFM, is a technique used to determine the structural properties of gold nanoparticles via three-dimensional (3D) image showcasing particle’s size, shape and surface feature. This technique is vital for their use in different fields. When this technique was performed on untreated, gold-coated silanized glass plates with scans at 2.0 Hz with 256 pixels per line. AuNPs appeared roughly spherical with an initial average diameter of 100 nm with a height profile approximately 409 nm after extended soaking. Surface roughness for silanized glass without gold was observed to be 0.755 nm which increases with gold deposition showcasing the nanoparticle attachment. Maximum roughness is observed at 2 h. AFM can be performed in various environments including ambient, gas, liquid and requires minimum surface preparation. Its only disadvantage is its limited scanning size. However, it also provides insights about the ligand binding at nanoscale making it one of the most vital methods. Figure revealing individual spherical gold nanoparticles (AuNPs) and their aggregates. The measured average diameter of the AuNPs ranged from approximately 11 to 19 nm.

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AFM phase images of AuNPs on a bare graphite substrate. The circled regions exhibited gold nanoparticles and their diameter (particles around 11–19 nm). Reproduced from ref . Copyright 2023, with permission of MDPI.

2.4.4. X-ray Diffraction (XRD)

This technique is performed to identify the crystal structure, phase and spatial arrangement of an atom. The X-ray is irradiated on the gold nanoparticle, and the intensity of scattering angle of X-ray from gold nanoparticle is measured. For gold nanoparticles it shows a distinct peak which matches the cubic structure of gold (Figure B). If a broad peak appears then it indicates the smaller particle size because of the quantum confinement. It is an important characterization technique as it helps to find out the nanoparticle size and structural changes via laser energy. XRD for gold nanoparticle entrenched in a silica xerogel matrix showcased the crystalline nature of the gold nanoparticles, displaying peaks at 2θ = 38.21, 44.45, 65.04, and 77.65° which is corresponding to the (111), (200), (220), and (311) planes of face-centered cubic (FCC) gold. These peaks confirmed the formation of metallic and crystalline AuNps. But on XRD of AuNPs coated with SA, we could observe no significant change in the crystal structure. Further to estimate the particle size, Debye–Scherrer equation should be used. Danchova et.al, study showcased the XRD of AuNPs in a nanocomposite of silica, proving crystalline AuNPs at 111 reflections. It also showed weak peaks of Au-thiolate complexes and at high temperature low cristobalite phase was formed. Similar results were obtained when C. limon juice and E. prostrata leaves were used as reducing agents along gold nanoparticles and a constant, strong peak around 534 nm was observed in all such cases. Overall it is a very versatile technique and as per the XRD card (JCPDS no. 65–2870), the prominent peak at 20 = 38.31, 44.47, 64.58 and 77.43°, indexed to the planes of (111), (200), (220), and (311) is for gold nanoparticles. Only disadvantage of this technique is the XRD peaks are too broad for particles with size less than 3 nm and it cannot detect the amorphous materials.

2.4.5. Scanning Electron Microscopy (SEM)

Commonly called SEM is a technique used to identify the morphology and size of particles. It generates three types of images - external X-ray maps, backscattered electron images, and secondary electron images.

It needs simple sample preparation by putting a drop of solution on silica plane other conductive substrates and then removing the excess solution. In case of gold nanoparticles to confirm its presence, we can observe spherical particles and enlarged size in SEM images due to the laser energy. It can show variety of geometry like square, rectangle, cubic, triangular all nearly at 60 nm diameter.These nanoparticles can be easily analyzed as SEM does not need any intense sample preparation. It also shows the surface texture, structure and composition. When gold nanoparticles were synthesized using knotweed extract in Pulit et.al, study, we could clearly see spherical rod like structures ranging from 20–200 nm in SEM image indicating the presence of AuNPs. Similarly, gold nanoparticles synthesized using Nepenthes khasiana leaf extract, showed cluster with particle sizes between 50–80 nm. Overall, green synthesized gold nanoparticles tend to form spherical shapes, irrespective of any conditions. Only limitation is that it does not give any details on internal structure and is a very labor intensive and expensive technique. Figure displays the obtained images for tetrachloroaurate­(III) dihydrate (NaAuCl₄) appears as cubic crystals. At a magnification of 30,000×, two crystals measuring approximately 0.38 and 0.67 μm were observed.

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SEM images at 2000× and 30,000× magnification of NaAuCl₄ and AuNP samples. Reproduced from ref . Copyright 2023, with permission of MDPI.

2.4.6. X-ray Photoelectron Spectroscopy (XPS)

This spectroscopy is usually used to analyze the surface composition and chemical state of gold nanoparticles (AuNPs) immobilized on silanized glass substrates. Sharp peaks for Au 4f, Au 4d, and Au 4p and Au 4f_7/2 at 84 eV, confirms the presence of gold. It has a superior capacity to ascertain chemical states, extensive applicability, and nondestructive characteristics. XPS applications encompasses the assessment of oxidation states in nanoscale particles. It also offers insights on nanoparticle surfaces and chemistry, majorly in comparison to uncoordinated ligands. In Seit’s study, there was a significant increase in oxygen-containing carbon species on gold-coated samples, which proves the chemical modification due to the presence and attachment of AuNP. In Tendo et.al, study XPS confirmed the chemical state and single layer orientation of flat gold films and AuNPs. It also helps to identify the effective molecular layer thickness by comparing C 1s and Au 4f peak intensities and gives 10 Å (MP) and 19 Å (MBP) confirming the AuNP films. Similarly on narrow scans for AuNPs synthesized via bacteria, peaks were visible with binding energy between 84 to 88 eV corresponding to aryl shell features. Furthermore, for AuNPs made from Ulva linza, Au was clearly observed due to the presence Au 4f_7/2 and Au 4f_5/2 peaks. Carbon, Nitrogen, Sulfur was also detected, which came from the Ulva linza and helped in reduction of gold ions to nanoparticles to prevent surface clumping and interaction. In some studies, where gold nanoparticle was capped with cysteine or glutathione (that is thiol containing compounds) XPS showed S 2p binding energies (161–164 eV), confirming strong Au–S bonding, essential for nanoparticle stabilization. It also detects the surface contamination by detecting elements like Cl, Na, or C. Furthermore, in bimetallic nanoparticles like Au–Ag or Au–Pd, XPS differentiates interaction and elemental distribution by showcasing shell composition or surface enrichment of one metal over the other. Various studies indicate that, AuNPs synthesized in a green manner showcases peaks with higher binding energy due to surface oxidation and interaction. The prominent shift in peaks denotes the partial oxidation or rigid, stubborn, stable capping interaction.

2.4.7. Transmission Electron Microscopy (TEM)

TEM is widely used to study the internal structure, surface morphology, shape, particle size distribution and dispersion of gold nanoparticles. Since electrons have much shorter wavelengths than light, TEM is ideal as it gives better resolution for nanoparticles with nano sizes (Figure ). Its advanced version is the High-resolution TEM (HRTEM) which shows clear images of the gold core and helps to identify whether particles are crystalline or amorphous. To operate, this method requires a sample preparation, which may lead to gold NP aggregation. Surface alterations are undetectable by normal or ordinary TEM techniques and to overcome this issue glycerol spraying/low-angle rotating metal shadowing TEM and cryo-TEM, is necessary. For AuNPs, a 5–100 nm, spherical shapes are observed on TEM images. Usually polydispersity is confirmed at 30–100 nm range and on heating, gold nanoparticles agglomerate making it easy to identify on TEM images. Similarly, TEM analysis of AuNPs synthesized via bacteria, showed spherical and triangular sites and these nanoparticles synthesized at different conditions had different sizes. For example, AuNPs synthesized at 25 °C had a size of approximately 39.0 ± 9.1 nm and for 42 °C, 36.7 ± 7.7 nm. It was noticed that at pH 12.7, the size was decreased. Recently, AuNPs were synthesized from T. farfara flower buds and was charactersied using TEM which revealed particle size 15–20 nm, indicating efficient size control via plant extract. Overall, TEM images prove that the green-synthesized GNPs are usually spherical, but also show hexagonal, pentagonal, and triangular shapes.

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TEM images of Au NPs (5 nm) (A, B) and Au NPs (20 nm) (C, D). Reproduced from ref . Copyright 2025, with permission of Elsevier.

2.4.8. Dynamic Light Scattering (DLS)

Dynamic Light Scattering is one of the common and widely used technique to identify the size distribution, average particle size and polydispersity index (PDI) of gold nanoparticles dispersed in a colloidal medium. Unlike electron microscopy techniques that provide direct imaging, DLS offers hydrodynamic diameter measurements that include the particle core, surface coating like capping agents and solvent layer. This makes it valuable for tracking size changes during synthesis, bioconjugation, or storage. This method is based on simple principle which analyses the intensity of light scattering when nanoparticles undergo Brownian motion in suspension. Another study reported synthesis of AuNPs using Evolvulus alsinoides extract and on characterization, the DLS revealed sizes ranging from 50–100 nm with a dominant size near 80.29 nm. Similarly, Mapala et al. found that Mimosa pudica flower extract-refereed AuNPs showed an average size of 24 nm with a narrow range (15–62 nm), which was later confirmed by TEM analysis. DLS also monitors the gold nanoparticles stability by measuring the ζ-potential, which reflects surface charge. High absolute ζ-potential values (typically more than or equal to ± 30 mV) indicate strong interparticle repulsion and colloidal stability, while low values suggest potential aggregation making it advantageous. DLS analysis of AuNPs reported by Oliveira et al. is depicted in Figure . But it may overestimate size in polydisperse or aggregated samples and assume spherical shape during analysis, making other techniques like TEM or SEM essential for accurate morphological validation.

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(a) DLS analysis of particle size distribution and ζ-potential and (b) schematic representation of the double layer that surrounds the nanoparticle in aqueous medium. Reproduced from ref . Copyright 2023, with permission of MDPI.

2.4.9. Energy-Dispersive X-ray Spectroscopy (EDX)

This technique is widely used to detect the elemental composition of gold nanoparticles (AuNPs). It provides qualitative, semiquantitative, and quantitative data by detecting X-rays emitted from the sample during electron beam interaction and is often performed with electron microscopy like SEM or TEM. EDX also helps in plotting the spatial distribution of other elements present, making it easy to identify the presence of gold. It is major disadvantage is precision of chemical analysis and quantification which is often used in conjunction with X-ray diffraction (XRD) for a more complete understanding of nanoparticle composition and crystallinity. Hence, a combination of XRD and EDS is the best for characterization of gold nanoparticle. For gold nanoparticles and ions, EDX shows a signature peak near 2 to 3 keV. Some reports also show an additional peak near 9 keV, depending on the instrumentation and settings. For AuNPs mediated using T. argentiea flower extract, the EDX depicted a strong gold peak at 3 keV, along with minor signals from carbon, oxygen, and the gold content was estimated to be around 52.27%. ,,

3. Applications of AuNPs

3.1. Antimicrobial Activity of AuNPs

A number of applications of AuNPs have been shown in Figure . The growing resistance of pathogenic microorganisms to a wide range of antibiotics, including the latest ones, presents a major challenge in clinical medicine. One potential solution lies in the use of metal nanoparticles, particularly gold nanoparticles (AuNPs). Gold Nanoparticles are widely used for antimicrobial applications (Table ) due to their small sizes, high surface area, and unique ability to interact with microbial cells. The antimicrobial effectiveness (Figure ) of AuNPs is influenced by factors such as the synthesis method, particle size, shape, and the concentration of biologically synthesized nanoparticles. One of the crucial and unique properties of AuNPs is its positively charged nature, which enables them to interact with Gram-positive and Gram-negative bacteria. However, due to their thinner cell walls, Gram-negative bacteria are more susceptible to AuNP penetration. In contrast, Gram-positive bacteria possess a thick peptidoglycan layer that acts as a barrier, limiting nanoparticle entry. , Gold nanoparticles supported on clinoptilolite, mordenite, and faujasite zeolites effectively eliminated E. coli and Salmonella typhi. Faujasite-supported AuNPs (5 nm) showed the highest dispersion and efficiency, reducing bacterial colonies by 90–95%. The zeolite support significantly influenced nanoparticle size, roughness, and biocidal activity. Functionalized gold nanoparticles (AuNPs) effectively combat multidrug-resistant (MDR) bacteria, targeting both Gram-negative and Gram-positive uropathogens. Cationic and hydrophobic AuNPs suppressed 11 MDR clinical isolates with minimal toxicity to mammalian cells. Their surface chemistry plays a crucial role in antimicrobial activity, offering a promising long-term strategy against bacterial resistance.

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Multifield applications of AuNPs.

8. Antimicrobial Activity of AuNPs.

Sr. No.	gold nanoparticle type	microbe species	efficacy/activity	application	references
1	Gold Nanostars (AuNSTs)	E. coli, S. aureus	ZOI: 20.0 ± 0.54 mm (E. coli), 23.0 ± 0.35 mm (S. aureus); MIC: 1.25 μg/mL (E. coli), 0.625 μg/mL (S. aureus)	Antibacterial treatment
2	Actinomycetes-based Gold Nanoparticles (Ac-AuNPs)	Streptomyces sp. ASM19	IC50: 3.77 mg/mL (SCC9), 1.56 mg/mL (SCC25); Induced apoptosis (26.37% SCC9, 32.08% SCC25); G2/M cell cycle arrest	Biomedical applications (nano drug delivery, anticancer therapy)
3	Cassia sericea-derived AuNPs	S. aureus, Pseudomonas aeruginosa, E. coli	82.45% wound closure (antibacterial and wound healing)	Antimicrobial & wound healing agent
4	AuNRs@PDA@AgNPs nanocomposites	E. coli, S. aureus	MIC: 16 μg/mL; 5.3 and 2.0% bacterial viability for E. coli & S. aureus	Synergistic photothermal and chemical antimicrobial agent
5	AMP-AuNCs (Antimicrobial Peptide-functionalized Gold Nanoclusters)	E. coli, six common bacterial species	100% accuracy in bacterial ID, 86.7% accuracy for clinical E. coli isolates	Rapid bacterial diagnostics for clinical applications
6	AuNP/WO3 NPL/r-GO ternary heterostructure	Carbapenem-resistant E. coli (CRE E. coli), Methicillin-resistant S. aureus (MRSA)	100% inactivation of CRE E. coli and MRSA superbugs after 60 min of sunlight exposure	Photothermal-photocatalytic antimicrobial and antibiotic degradation agent
7	Polyphenol Nanoparticles (PNPs)	Micrococcus luteus, E. coli	Growth inhibition: 50.7% (M. luteus), 12.1% (E. coli)	Antimicrobial coatings to prevent biofilm formation
8	HBPAA-capped Ag@AuNPs-coated cotton fabric	E. coli, S. aureus	MIC: 5 mg/L; 100% bactericidal efficacy even after 50 washes with 900 mg/kg Ag@Au content	Washable antimicrobial textiles with organic–inorganic coupling antibacterial properties
9	Cyphostemma adenocaule (CA)-AgNPs	S. aureus, E. coli	88% S. aureus biofilm inhibition (125 μg/mL), broad-spectrum antibacterial activity	Antibacterial, antibiofilm, antioxidant agent for food, cosmetics, and medicine
10	Iris kashmiriana-derived AuNPs	E. coli, S. aureus, Pseudomonas aeruginosa, Streptococcus mutans	MIC-based antibacterial activity, ROS-mediated mechanism, antibiofilm and antiadherence effects	Antibacterial coatings for orthodontic appliances (wires and brackets)
11	Eisenia bicyclis-gold nanoparticles (EB-AuNPs)	Listeria monocytogenes, S. aureus, Pseudomonas aeruginosa, Klebsiella pneumoniae, Candida albicans	MIC: 512–2048 μg/mL; Biofilm inhibition: 91.13% (S. aureus), 58.60% (K. pneumoniae), 57.22% (P. aeruginosa), 33.80% (L. monocytogenes)	Antibacterial, antibiofilm, antivirulence agent against bacterial and fungal pathogens

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Mechanism of antibacterial action of AuNPs and unreduced gold ions. Reprinted with permission from ref . Copyright 2022 MDPI.

3.2. Antioxidant Activity of Gold Nanoparticles

A study by Katas et al. showed powerful antioxidant effects toward AuNPs, helping to protect cells from harmful free radicals. This ability can reduce the oxidative stress-related damage. It contributes to aging and diseases like cardiovascular and neurodegenerative treatments. Gold nanoparticles (AuNPs) stabilized with carboxymethylated frankincense resin (CMFR) showed antioxidant activity by neutralizing DPPH free radicals. AuNPs synthesized from CMFR-AuNPs (7–10 nm) displayed dose-dependent antioxidant properties. Due to the large surface area and strong stability free radical scavenging. CMFR-AuNPs showed potential therapeutic advancements. For instance, a study by Faraday et al. examined that gold nanoparticles affect light transmission and color based on their size and dispersion. The study also examined the gold’s stability and reactivity in different conditions, which shows antioxidant behavior. Gold nanoparticles (AuNPs) produced through the electrodeposition were evaluated using the DPPH assay. Suliasih et al. found that smaller AuNPs with greater particle density exhibited higher antioxidant activity. AuNPs synthesized at −1 V depicted the highest inhibition (36.80%) of free radicals. In another study, Ipek et al. synthesized Gold nanoparticles (AuNPs) using Allium cepa L. peel extract. The analysis identified their spherical shape, ranging from 6.08 to 54.20 nm. Antioxidant tests (DPPH, ABTS, and CUPRAC) showed moderate free radical scavenging ability, less effective than BHA and α-tocopherol. AuNPS synthesized from Spirulina platensis depicted strong antioxidant activity. The DPPH assay showed that at 300 μg/mL, achieved 70% inhibition, effectively neutralizing free radicals. The results showed Spirulina-based AuNps as a natural antioxidant with potential benefits in biomedical and pharmaceutical fields. Another study compared the antioxidant properties of chitosan and green-synthesized gold nanoparticles (AuNPs). Chitosan acted as both a reducing and stabilizing agent for AuNPs and was measured using DPPH, ABTS, hydroxyl radical scavenging, and FRAP assays. Low molecular chitosan (47.8 kDa) showed the highest antioxidant effect. Another study by Radadi et al. showed AuNPs synthesized from Annona muricata leaf extract through ABTS assay, with a maximum of 96.4 μg/mL. Characterization of nanoparticles using UV–vis, FTIR, TEM, and XRD exhibited spherical shape and crystalline structure. Their antioxidant properties are linked to polyphenols and flavonoids in the extract, which enhance the free radical scavenging. Gold nanoparticles (AuNPs) synthesized using Vibrio alginolyticus were tested using DPPH and metal chelating assays. Characterization depicted irregular shapes, ranging from 100 to 150 nm. The nanoparticles showed greater antioxidant effectiveness at lower concentrations. It emphasizes the eco-friendly synthesis of AuNPs. For instance, another study of AuNPs synthesized using Alternaria chlamydospore was tested using the DPPH assay, achieving 71.2% inhibition at 500 μL. Nanoparticles confirmed their spherical shape ranging from 12–15 nm with a high surface area-to-volume ratio, enhancing free radical neutralization. The IC₅₀ value was 224.97 μg/mL, demonstrating strong antioxidant activity. Gold nanoparticles (AuNPs) results suggest that they could be valuable for biomedical applications.

3.3. Anti-Inflammatory Activity

Gold NPs have been extensively used for their anti-inflammatory benefits. Synthesis is done by using various methods such as green synthesis with plant extracts, microbial sources, and chemical modifications. For instance, Au-NPs synthesized using Saussurea costus extract exhibited strong anti-inflammatory effects due to the presence of bioactive polyphenols that stabilize and enhance their biological activity. Similarly, Capsicum annum fruit extract-mediated Au-NPs showed notable anti-inflammatory and antiangiogenic properties. Au- NPs derived from Chaetomium globosum extract demonstrated potent inhibition of inflammatory mediators, making them a viable option for anti-inflammatory treatment. Sivakavinesan et al. synthesized Au-NPs via Citrus sinensis fruit peel extract, which exhibits strong antioxidant and anti-inflammatory activities Similar study was conducted by Gao et al. in which gold nanoparticles were synthesized using citrus peel extract with the ultrasound-assisted method and showed enhanced anti-inflammatory activity. Additionally, AuNPs synthesized from papaya peel extract demonstrated antimicrobial, antioxidant, and anti-inflammatory effects, reinforcing their relevance in pharmaceutical and biomedical fields In a study by Elizalde-Mata et al. AuNPs derived from Croton draco extract exhibited strong antioxidant and anti-inflammatory properties, making them promising for therapeutic applications. Similarly, Eltahir et al. reported that phenolic compounds from Glycyrrhiza glabra showed notable anti-inflammatory effects, suggesting their potential in treating inflammation-related disorders. In another study conducted by Nasab et al. conjugated Au-NPs with cortistatin peptides were successfully employed for targeted drug delivery to asthmatic lung tissues, offering a novel approach for respiratory inflammatory diseases. The Saussurea Costus gold nanoextract demonstrated significant antioxidant, antidiabetic, anti-Alzheimer, and anti-inflammatory activities, making it a potential therapeutic agent for managing oxidative stress related diseases. Prasad et al. synthesized gold nanoparticles using an aqueous extract of Commiphora wightii which were potential anti-inflammatory agents. Hongsa et al. developed chitosan-collagen coated gold nanoparticles for targeted drug delivery of 5-fluorouracil (5-FU), a chemotherapy drug that exhibited controlled and sustained drug release, anti-inflammatory activity, and cytotoxicity studies confirmed their anticancer potential

3.4. For Skin Disease Treatment

Gold nanoparticles (AuNPs) have emerged as a promising tool in skin disease treatment due to their biocompatibility, enhanced skin penetration, and controlled drug release properties. They offer therapeutic benefits for conditions like psoriasis, atopic dermatitis, skin cancer, and microbial infections by reducing inflammation, stabilizing the skin barrier, and enhancing drug delivery. Qui et al. successfully synthesized antibacterial photodynamic gold nanoparticles (AP-AuNPs) for treating skin infections. These nanoparticles, combining antibacterial peptides and photodynamic effects, showed significant antibacterial activity against S. aureus and E. coli, while also promoting wound healing in infected skin tissues. Cuyler et al. explored the potential of gold nanoparticles synthesized using Bulbine Frutescens to treat Eczema. The nanoparticles demonstrated significant wound closure and inhibited histamine production, suggesting their potential to alleviate skin damage associated with atopic dermatitis Gold nanostructures can also be implemented in melanoma therapy. Leu et al. studied the effect of AuNPs combined with antioxidant epigallocatechin gallate and α-lipoic acid on wound healing in mice, demonstrating anti-inflammatory and antioxidative effects Another study conducted by Oliveira et al. utilized AuNPs in combination with photobiomodulation (low-power laser therapy) to treat dermonecrotic lesions, specifically those caused by the venom of the spider Loxosceles simillis. It effectively reduced necrotic tissue and erythema in animal models Poomrattanangoon and Pissuwan evaluated the wound healing ability of gold nanoparticles coated with collagen-I. Collagen-I@AuNPs reduced the levels of inflammatory cytokines and induced the growth factors involved in wound healing. Dong et al. developed mussel-inspired electroactive, antibacterial, and antioxidative composite membranes with gold nanoparticles and antibacterial peptides for enhancing skin wound healing. The composite membranes promoted cell proliferation and migration and exhibited enhanced antibacterial and antioxidant effects Wu et al. developed an injectable, antibacterial hydrogel based on gold nanorods and an N-calamine polymer for bacteria-infected skin wound healing. The hydrogel demonstrated excellent in vitro antibacterial capacity and promoted wound healing Abbas et al. investigated the efficacy of alginate-coated gold nanoparticles against antibiotic-resistant Staphylococcus and Streptococcus strains that cause acne. The results concluded the potential of Gold NPs as a potential antimicrobial agent to combat antibiotic resistance in acne treatment

3.5. Ocular Disease Treatment

Gold has long symbolized nobility and attracted attention due to its shiny appearance, it has excellent ductility, biocompatibility, and molecular recognition, and gold is widely used in various fields. Gold nanoparticles (AuNPs) exhibit unique physical and chemical properties, enabling biomedical applications. AuNPs easily bind to proteins and antibodies, aiding in disease diagnosis, gene detection, and cancer treatment. They also show cytotoxic effects on cancer cells and possess antioxidant and anti-inflammatory properties. Research highlights their therapeutic potential in treating retinopathy, neurological diseases, cancers, cardiovascular diseases, infections, and metabolic disorders. Delivering drugs through the ocular route holds great potential for treating various eye diseases, including diabetic retinopathy. This condition is primarily driven by inflammation and elevated vascular endothelial growth factor (VEGF) levels, leading to abnormal blood vessel growth (neovascularization). Ocular drug delivery systems aim to effectively target these factors, improving treatment outcomes for such vision-threatening disorders. Ocular drug delivery is an effective method for treating eye diseases like ocular neovascularization in diabetic retinopathy, which is caused by inflammation, retinal ischemia, and the accumulation of advanced glycation end-products. Elevated vascular endothelial growth factor (VEGF), interleukins, and reactive oxygen species (ROS) also contribute to disease progression. Gold nanoparticles (GNPs) possess antioxidant and antiangiogenic properties, making them ideal for ocular drug delivery. They are biocompatible, easy to synthesize, and can be functionalized to improve movement across ocular barriers. Apaolaza et al. conducted a study where low molecular weight hyaluronan (HA) was used to enhance nanoparticle stability, mobility, and targeting via CD44 receptor interaction. HA-GNPs effectively reached deeper retinal layers, inhibiting neovascularization and protecting retinal pigment epithelial cells. Despite a slight reduction in antioxidant activity, longer studies are needed to assess their full potential in delivering antiangiogenic treatments for intraocular vascular diseases. HA-AuNPs showed excellent colloidal stability, with the HA coating preventing aggregation in the trabecular meshwork of ex vivo porcine eyes. Nanoparticles (120 nm) accumulated the most in this region. Advancements in nanotechnology have improved nanoparticle stability, while coatings like poly­(ethylene glycol) (PEG) enhance biocompatibility, reducing toxicity and increasing circulation time. Although AuNPs show significant potential in biomedical applications, further safety studies are required before clinical implementation. Further research is needed to clarify ocular distribution and potential toxicity, as current biocompatibility data remain conflicting.

3.6. For Diabetes Treatment

Diabetes mellitus, commonly known as diabetes, occurs due to reduced insulin secretion by the pancreatic islet cells, leading to elevated blood glucose levels (hyperglycemia). It is characterized by symptoms such as unexplained weight loss, excessive urination (polyuria), increased thirst (polydipsia), and an excessive. Diabetes mellitus is classified into Type 1, Type 2, and Gestational diabetes. Type 1 diabetes results from an autoimmune attack on pancreatic β cells, leading to insulin deficiency. For children, the term juvenile diabetes is used. Type 2 diabetes is caused by insulin resistance and reduced insulin secretion due to defective insulin receptor response. Gestational diabetes occurs during pregnancy when hormonal changes reduce insulin sensitivity and increase blood sugar levels. Management includes dietary modifications, exercise, insulin therapy, and oral medications. Recent nanomedicine research focuses on using nanoparticles for effective insulin delivery in Type 1 diabetes treatment. Nanoparticles offer promising advancements in controlled insulin release, improved bioavailability, and fewer side effects, providing a potential breakthrough in diabetes management and treatment strategies for better patient outcomes. Gold nanoparticles (AuNPs) have shown potential as therapeutic agents for diabetes treatment. As they exhibited anti-inflammatory, antioxidant, and antihyperglycemic effects, disrupting key disease pathways. However, their safety, optimal size, and dosage require further detailed study. Proper characterization is essential to ensure safe applications for treating diabetes and related microvascular complications. Daisy et al. prepared an aqueous extract using Cassia fistula stem bark and synthesized gold nanoparticles (AuNPs) to assess their hypoglycaemic effects. Synthesized AuNPs were characterized using spectroscopy and electron microscopy. In streptozotocin-induced diabetic rats, AuNPs significantly lowered blood glucose, improved liver and kidney function, enhanced lipid profiles, and increased body weight more effectively than the aqueous extract. Phytochemically synthesized AuNPs demonstrated excellent antidiabetic properties by stabilizing serum biochemistry and reversing renal dysfunction. This study confirms that Cassia fistula-derived AuNPs are promising therapeutic agents for diabetes mellitus treatment, outperforming conventional plant extracts in improving metabolic health. AuNPs also reduces hyperglycaemia, oxidative stress, inflammation, and the proteolytic pathway. Gold nanoparticles (AuNPs) in conjugation with natural products have demonstrated promising antidiabetic properties, such as reduction in glycated hemoglobin levels and anti-inflammatory effects. Opris et al. conducted a study aimed to evaluate the therapeutic potential of AuNPs functionalized with Sambucus nigra L. (SN) extract in an experimental rat model of diabetes. Diabetes was induced in 18 male Wistar rats using a single streptozotocin injection. The diabetic rats were then treated with SN extract, gold nanoparticles (AuNPs), or saline for 2 weeks via oral gavage. Another 18 nondiabetic rats received the same treatments. After treatment, blood, liver, and muscle samples were analyzed for oxidative stress markers, liver MMP-2/-9 activity, COX-2 and NFKB expression, and histopathological changes. Serum glucose, cholesterol, ALAT, and ASAT levels were also measured to assess the effects of AuNPs and Sambucus nigra L. extract on diabetes-related metabolic and inflammatory changes. The administration of AuNP-SN extract significantly increased the muscle and systemic GSH/GSSG ratio in diabetic rats compared to untreated diabetic (p < 0.03) and nondiabetic vehicle-treated groups (p < 0.05). Malondialdehyde (MDA) levels were reduced in the AuNP-treated diabetic group compared to nondiabetic controls (p < 0.05). Additionally, COX-2 expression (p < 0.0001) and proMMP-2 activity (p < 0.05) were decreased, along with a significant reduction in Kupffer cell percentage (<0.001). Histopathological analysis revealed no structural abnormalities in liver tissue. These findings suggest that AuNPs functionalized with Sambucus nigra L. extract possess strong potential as adjuvants in diabetes therapy by enhancing antioxidant defenses, reducing MMP activity, and mitigating inflammation in liver tissue. Further research is warranted to explore their clinical applications in diabetes management.

3.7. Cardiovascular Disease Treatment

Gold nanoparticles (AuNPs) are now being widely used for treating cardiovascular diseases (CVD) due to their ability to increase the drug delivery to a targeted site, reducing side effects, and maximizing bioavailability (Table ). AuNPs which are in functionalized form are capable of delivering drug molecules to targeted tissues with enhanced specificity, offering controlled release as well as longer action. These nanoparticles are ideal for controlling atherosclerosis, thrombosis, and myocardial infarction and provide a probable pathway for prospective cardiovascular therapies due to their anticoagulant, antioxidant, and anti-inflammatory properties. Second, due to their unique optical, electronic, and biological properties, they also show potential in recovering cardiovascular disease. They can target selectively atherosclerotic plaques, provide controlled drug release, and function as imaging agents for early diagnosis. They can also decrease oxidative stress, regulate inflammatory reactions, and enhance endothelial function, thereby providing cardio protection. Among the diverse theragnostic nanomaterials, gold nanoparticles (AuNPs) are remarkable because of their special optical and physicochemical properties. AuNPs can facilitate the diagnosis of CVDs by computed tomography (CT) and can carry out photothermal therapy to induce plaque ablation. Their therapeutic efficacy with a reduced side effect is increased due to the ability to deliver drugs directly to the infected effects. Nanotechnology here plays a critical role in the improvement of CVD management as these particles increase therapeutic efficiency, minimize side effects, and allow for controlled drug release. On using branched polyethylenimine-coated gold nanoparticles (bPEI-AuNPs) in collagen hydrogels, it greatly improves the drug delivery, mechanical properties, and conductivity. These hydrogels also improve cardiomyocyte function, inducing rhythmic and synchronized beating. In fact, on incorporating gold nanoparticles (AuNPs) into the biosensor architecture, sensitivity and specificity increase due to their conductivity, extensive surface area, and biocompatibility. The performance of the biosensor, such as its low detection limit (LDL) and high specificity, shows its promise for clinical use, offering a reliable means for evaluating cardiovascular risk and individualized treatment plans. The measurements indicated remarkable blood velocity, pressure, and temperature variations, and additionally, the in vivo biodistribution of AuNP-miR-67, administered subcutaneously, indicated the particles were being cleared mainly through the liver and kidneys within 11 days. Ven et. al points to the possibility of the PNP-AuNP-miRNA hydrogel platform in controlled, sustained miRNA delivery with the additional advantage of being minimally invasive. Future advancements can include the inclusion of targeting ligands for cell-specific delivery, and increasing therapeutic efficiency for targeted treatment of cardiovascular disease and other conditions.

9. Role of AuNPs in Cardiovascular Disease Treatment.

cardiovascular disease	role of AuNPs	mechanism of action	references
Atherosclerosis	Targeted Drug Delivery, Plaque Reduction, Anti-inflammatory Effects	Functionalized with anti-inflammatory agents and statins to reduce cholesterol, inhibit foam cell formation, and stabilize plaques.
Myocardial Infarction (MI)	Cardiac Tissue Regeneration, Drug Delivery, Theranostics	Loaded with growth factors, cytokines, and drugs to enhance tissue repair, reduce apoptosis, and promote angiogenesis.
Hypertension	Antihypertensive Drug Delivery, Vasodilation	Functionalized with NO donors for sustained vasodilation and endothelial function improvement.
Thrombosis	Thrombolytic Therapy, Clot Dissolution	Functionalized with thrombolytic agents for targeted clot lysis and prevention of rethrombosis.
Ischemia or Reperfusion Injury	Reduction of Oxidative Stress	Conjugated with antioxidants and antiapoptotic agents to scavenge ROS, reduce inflammation, and prevent cell death.
Coronary Artery Disease	Diagnostic Imaging, Plaque Detection, and Treatment	AuNPs enhance imaging via CT, MRI, and photoacoustic techniques for precise localization and treatment.
Arrhythmias	Electrophysiological Regulation, Drug Delivery	Conjugated with antiarrhythmic drugs to regulate cardiac rhythm and prevent arrhythmias.
Stroke	Thrombolytic Therapy, Neuroprotection	AuNPs conjugated with neuroprotective agents to reduce brain damage and restore blood flow.
Venous Thromboembolism	Clot Targeting, Thrombolytic Therapy	AuNPs functionalized with anticoagulants to prevent clot formation and enhance clot dissolution.
Cardiovascular Imaging	Enhanced CT and MRI Imaging, Real-time Monitoring	AuNPs provide high-contrast imaging for cardiovascular diagnosis, identifying plaques and occlusions.
Pulmonary Hypertension	Antihypertensive Drug Delivery, Vasodilation	Functionalized with vasodilators and NO donors for targeted therapy.

3.8. Anticancer Applications

Gold nanoparticles (AuNPs) have become attractive nanoplatforms for cancer diagnosis and therapy because of their size, shape, and optical tunability (Table ). Their use in photothermal therapy, drug delivery, and imaging has exhibited increased tumor targeting (Figure ) with low systemic toxicity. Yet, issues like biodistribution, cytotoxicity, and immunogenicity must be explored before they can be translated to the clinic. Optimizing AuNP formulations to enhance efficacy and safety in cancer treatment will be a task for future studies. These nanoparticles play a vital role as an anticancer due to their size, shape and physiochemical properties they inhibit. Properties like biocompatibility, high surface area to volume ratio and ease on functionalization makes them suitable for delivery of chemotherapeutic agents on tumor sites. They also help in minimizing the damage to healthy cells and reducing systemic toxicity. By offering controlled drug release, targeted delivery, and reduced side effects, gold nanoparticles represent a multifaceted tool in advancing more effective and personalized cancer treatments. Gold nanoparticles are ideal carriers for targeted drug delivery as they passively accumulate in tumor tissues due to leaky vasculature and poor lymphatic drainage. This is due to an effect known as ‘Enhanced Permeability and Retention Effect’. Second, due to their strong optical properties, they are widely used in CT, MRI, and photoacoustic imaging which are all real-time imaging techniques. Some of its applications in different types of cancer are given in the table below.

10. Anticancer Potential of AuNPs.

type of AuNPs	targeted cancer type	observed effects	references
Gold Nanorods modified with Silica	Hepatocellular Carcinoma	Induced localized hyperthermia leading to necrotic tumor cell death while sparing healthy tissue.	,
PEGylated AuNPs Loaded with miRNA	Breast Cancer	Restored tumor-suppressive miRNA expression, leading to reduced proliferation
Hollow Gold Nanospheres	Brain Tumors	Facilitated blood-brain barrier penetration and enhanced accumulation at tumor sites.
Resveratrol-loaded gold nanoparticles	Pancreatic Cancer	Induced apoptosis via mitochondrial pathway and reduced tumor volume in vivo.
AuNPs Functionalized with Anti-EGFR Antibodies	Non-Small Cell Lung Cancer	Enabled precision delivery and EGFR signaling inhibition, resulting in tumor regression.
Gold nanostructures	Breast Cancer	Synergistic photothermal and chemotherapeutic effects led to enhanced cell death.
Gold Nanoparticles coated on magnetic Fe3O4 nanoparticles	Leukemia	ultrasensitive and selective electrochemical detection of leukemia cells
Albumin conjugated Gold Nanoparticles	Prostate Cancer	moderately reduced adhesion in prostate cell lines suggesting potential to combat cancer.
Gold Nanoparticles Embedded in Liposomal Formulations	Brain cancer	enhanced brain delivery of docetaxel, achieving up to 4.08-fold higher accumulation than the marketed formulation Docel.
ScFv-Decorated Bimodal Nanoprobes from Au	Breast Cancer	Specific recognition and internalization in HER2-overexpressing cells; inhibited proliferation.	,
AuNPs (2–6 nm, microwave-synthesized)	Breast (MCF-7), Colon (HCT-116)	ROS-mediated cytotoxicity, impaired cell migration, upregulation of apoptotic genes, DNA damage
AuNPs synthesized using Antigonon leptopus extract	Breast adenocarcinoma (MCF-7)	GI50 = 257.8 μg/mL; inhibited cell growth; higher free radical scavenging activity than extract alone
Biogenic AuNPs synthesized using Brazilian Red Propolis (BRP)	Bladder (T24), Prostate (PC-3)	Dose-dependent cytotoxicity; highest effect from AuNP dichloromethane and AuNP extract; apoptosis-related cell death
Solanum xanthocarpum (Sx)-derived AuNPs	Nasopharyngeal cancer	Induced apoptosis via autophagy and mitochondrial pathway; reduced viability and colony formation
Shape-dependent AuNPs (rods, stars, spheres)	Osteosarcoma; Pancreatic	AuNPs stars showed highest cytotoxicity and anticancer potential; upregulated Bax, downregulated Bcl-2
Biosynthesized AuNPs (Curcumin, Turmeric, Quercetin, Paclitaxel conjugated)	Breast cancer	Synergistic inhibition of growth and angiogenesis
Mango seed–derived AuNPs	General (in vitro)	Moderate cytotoxicity and antioxidant activity
Sargassum glaucescens–stabilized AuNPs	Cervical, Liver, Breast, Leukemia	Induced intrinsic apoptosis (via caspase-3/9), no toxicity to normal cells
Mushroom-extract biosynthesized AuNPs	Breast cancer	Hexagonal AuNPs (∼13 nm) showed significant cytotoxicity
Combretum glutinosum based AuNPs	Lung cancer	Multishaped AuNPs showed cytotoxicity; also, part of a bi- and trimetallic system with catalytic and antibacterial properties
Nothapodytes fetida leaf extract AuNPs	Osteosarcoma, Lung Cancer	Spherical AuNPs showed significant anticancer and wound-healing properties
Curcumin-INH functionalized AuNPs (AuNPsCur@INH)	Lung squamous carcinoma	Showed selective cytotoxicity via ROS-mediated apoptosis
Panax notoginseng-mediated AuNPs	Pancreatic cancer	AuNPs induced ROS generation and intrinsic apoptosis
Pomegranate seed oil (PSO)-capped AuNPs	Lung and colon cancer	AuNPs (70 nm, +34 mV) in functional yoghurt reduced cell viability to ∼25–28%
Satureja rechingeri-mediated AuNPs	Cisplatin-resistant ovarian cancer	Spherical AuNPs (∼15.1 nm) induced apoptosis, cell cycle arrest (G0/G1), and gene regulation; effective against drug-resistant cells
Moringa oleifera leaf-extract AuNPs	Breast cancer	Exhibited anticancer activity with IC50 = 67.92 μg/mL
Ziziphus nummularia leaf-extract AuNPs	Cervical and breast cancer	Spherical AuNPs showed dose-dependent cytotoxicity to cancer cells, nontoxic to normal fibroblasts
Peptide-functionalized AuNPs (TAP@AuNPs)	Breast cancer	Targeted nucleolin receptors; induced apoptosis via cytochrome c and caspase-3/7 activation
Chemicaly synthesized AuNPs	Bladder cancer	upregulated Bax, downregulated Bcl-2 & VEGFA, reduced migration
Alternanthera sessilis-extract AuNPs	Cervical cancer	Induced cytotoxicity and apoptosis via intrinsic apoptotic pathway
Coleus scutellarioides leaf-extract AuNPs	Breast cancer	nontoxic to normal Hs-27 cells; also exhibited antioxidant activity
AuNPs loaded with copper(I) complexes	General anticancer	AuNPs improved bioavailability and controlled release of Cu(I)-based drugs; AuNPs–A system showed 90% drug loading and slow release over 4 days
Theaflavin-conjugated AuNPs	Ovarian cancer	Enhanced apoptosis via ROS generation and mitochondrial depolarization
Pm-AuNPs (from Pterocarpus marsupium bark)	Oral squamous cell carcinoma	Showed cytotoxicity in OSCC cell lines (IC50: 25–75 μg/mL); induced ROS generation, mitochondrial dysfunction
Dracocephalum kotschyi leaf AuNPs (d-GNPs)	Leukemia (K562), Cervical (HeLa)	Spherical AuNPs (∼11 nm) showed dose-dependent cytotoxicity (IC50: 196.32 and 152.16 μg/mL)
Trachyspermum ammi seed-extract AuNPs	Liver cancer	Spherical AuNPs induced ROS-mediated apoptosis
Magnolia officinalis-derived AuNPs	Lung cancer	Induced cytotoxicity and apoptosis by modulating apoptotic gene expression
Amygdalus communis (AC) leaf-extract AuNPs	Colorectal (CaCo-2), glioma (U118), ovarian	inhibited CaCo-2 proliferation
Crassocephalum rubens (AECR)-intermediated AuNPs	Breast cancer; Colon Cancer	showed significant dose- and time-dependent cytotoxicity
Carboxymethyl cellulose-AuNPs (CMC-AuNPs)	Breast cancer	induced apoptosis via caspase-8/9 activation and VEGFR-2 downregulation
Dunaliella salina-extract AuNPs	Breast cancer	Photoinduced AuNPs selectively killed cancer cells
Clerodendrum trichotomum (CTT)-AuNPs	Breast cancer	Reduced cell proliferation to 32.67% at
Marine algae-extract biosynthesized AuNPs	Breast cancer	showed strong anticancer activity (up to 92.13%) at 188 μg/mL
Equisetum diffusum-extract AuNPs	Liver cancer	Dose-dependent cytotoxicity; max 47.62% inhibition at 200 μg/mL; also shows antioxidant, antibacterial, and antidiabetic activity
AuNPs from Mimusops elengi in PVA/PCL nanofibers	Skin cancer	less toxic to normal fibroblasts (3T3); effective drug delivery system
M. elangi leaf-extract AuNPs in PVA/PCL nanofibers with curcumin	Skin cancer	Showed selective cytotoxicity via apoptosis; lower toxicity to normal cells; sustained drug release
Muntingia calabura (Mc)-AuNPs	Laryngeal carcinoma	AuNPs induced apoptosis, disrupted membrane, altered nuclear morphology, and caused G2 phase arrest
Cyclodextrin-covered AuNP	Breast (MCF-7), Lung (A549) cancer	Encapsulated b-lapachone; targeted delivery via EGFR; enhanced apoptosis
AuNPs with c-MWCNTs on GCE	Cervical cancer	Enabled sensitive detection of HeLa cells; used to evaluate anticancer activity of pinoresinol
Polygala elongata leaf-extract AuNPs	Lung cancer	Spherical AuNPs (10–20 nm) showed dose-dependent cytotoxicity with IC50
Acanthophora spicifera-mediated AuNP	Colon cancer	Spherical AuNPs (<20 nm) induced strong cytotoxicity (IC50 = 21.86 μg/mL) against HT-29 colon cancer cells, demonstrated antioxidant and antibacterial properties too
Ganoderma lucidum AuNPs–Doxorubicin conjugate	Drug-resistant breast cancer	enhanced drug delivery and gene (ABCB1) regulation
AuNPs synthesized from Exiguobacterium aestuarii	Breast cancer	Spherical AuNPs showed strong cytotoxic and apoptotic effects against cancer cells
Rauwolfia serpentina-mediated AuNPs	Cervical cancer	potential for novel targeted therapies
Euphorbia antiquorum-extract AuNPs	Breast cancer	Showed anticancer potential; active compounds (euphol, euphorbol) may inhibit VEGFR-2, ERK-2.
Mn(I)-carbonyl functionalized AuNPs	Lung cancer	Dual red light photoactivation induced CO release and singlet oxygen generation
T. capensis-extract AuNPs	Breast cancer	preventing the development and proliferation of human breast cancer cells
Kaempferol glycoside-mediated AuNPs	Breast cancer	showed mild to low cytotoxicity; strong antioxidant and catalytic activity
Sodium borohydride-synthesized AuNPs	Lung cancer	AuNP induced apoptosis and low toxicity to normal cells
AuNPs@ZnO (gold–zinc oxide hybrid)	Breast, Liver	showed anticancer potential but higher safety than ZnO alone
P. oleracea-extract AuNPs	Breast, liver, cervical, colon	showed stronger anticancer activity than crude extract via ROS-induced apoptosis
Bombax ceiba leaf-extract AuNPs	Ovarian cancer	Anisotropic AuNPs showed anticancer activity
Starch-reduced bimetallic AuNPs	Melanoma	9.7 nm AuNPs showed dose-dependent cytotoxicity toward melanoma cells, while being cytocompatibility with normal human dermal fibroblasts (HDF)
Physalis minima-extract capped AuNPs	Breast cancer	Spherical AuNPs (22–32 nm) showed anticancer activity at 50 μg/mL; also exhibited strong antimicrobial and antioxidant effects
Levan-capped AuNPs loaded with DOX	Breast cancer	High drug encapsulation
HACD-AuNPs (β-CD–adamantane based supramolecular conjugates)	General (e.g., DOX-tested tumor cells)	Enabled targeted, pH-responsive delivery of multiple anticancer drugs with reduced toxicity and effective tumor inhibition
Achillea biebersteinii flower-extract AuNPs	Testicular embryonic carcinoma	apoptosis confirmed via gene expression analysis
Garcinia mangostana-derived Au–Ag core–shell NPs loaded with protocatechuic acid	Colorectal cancer	>80% inhibition at 15.63 μg/mL; higher selectivity over normal cells
β-lactoglobulin (BLG)-conjugated AuNPs	Breast cancer	Enhanced drug binding/stability with curcumin and gemcitabine
Cyanthillium cinereum leaf-extract AuNPs	Lung cancer	enhanced apoptosis compared to pure plant extract
Citrus macroptera fruit juice-derived AuNPs	Liver, lung, breast cancer	acted as antibiofilm agent
Pholiota adiposa polysaccharide (PAP-1a)–mediated AuNPs	Hepatic carcinoma	Enhanced immune regulation and antitumor effect
Enterococcus-mediated AuNPs	Colorectal cancer	Induced ROS, caspase-3 expression, mitochondrial dysfunction
M. tenacissima-extract AuNPs	Lung cancer	50 nm AuNPs showed dose-dependent cytotoxicity, activated caspase and downregulated antiapoptotic proteins
Catharanthus roseus (CR)-AuNPs	Cervical cancer	Induced mitochondrial-mediated apoptosis via ROS
Curto-Cumin AuNP (CC-AuNP) biosynthesized using Nigella sativa and Curtobacterium proimmune	Gastric adenocarcinoma	Selectively cytotoxic to AGS cells; induced apoptosis via p53/Bax
Sasa borealis leaf-extract AuNPs	Gastric cancer	Oval/spherical AuNPs (10–30 nm) induced apoptosis in AGS cells
Plant-extract AuNPs (from Abutilon, Origanum, Euphorbia, Senna)	Breast cancer	Senna-based AuNPs most selective and potent
GMS-AuNPs and GMS-AuNPs@CS (Thyme-synthesized)	Breast cancer	Showed dose and time-dependent cytotoxicity; no toxicity to normal breast cells
Streptomyces flavolimosus-mediated AuNPs	Breast (MCF-7), Cervical (HeLa), EAC (in vivo)	reduced tumor growth in mice
Turmeric rhizome extract-based AuNPs	Lung (A549) and Prostate (PC3) cancer	phytochemical content influenced by altitude
A. muricata leaf-extract synthesized AuNPs	Tongue squamous cell carcinoma (SCC-15)	Induced apoptosis via upregulation of p53, Bax and downregulation of Bcl-2
Sonneratia alba fruit-extract AuNPs (SF-AuNPs)	Lung cancer	∼30 nm AuNPs showed dose-dependent inhibition (up to 59.38% at 400 nM); exhibited antioxidant, anti-inflammatory, and anticancer properties
Camptothecin-loaded AuNPs	Breast cancer	apoptosis induction, antioxidant enzyme modulation, and PARP/Bax gene expression changes
Abies spectabilis-extract AuNPs	Bladder cancer	induced apoptosis via upregulation of Beclin-1
AuNPs@saponins niosomes (Sapindus mukorossi)	Breast cancer	Demonstrated anti-inflammatory potential with an IC50 of 30.08 μg/mL and showed anticancer activity in a dose-dependent manner.
AuNPs–PVP–Withaferin A–FA nanoconjugate	Breast cancer	pH-responsive and shows targeted delivery and anticancer efficacy
Marine bacteria (Vibrio alginolyticus)-synthesized AuNPs	Colon carcinoma	Induced apoptosis via nuclear condensation
Elephantopus scaber-extract AuNPs	Breast (MCF-7), Lung (A-549), Oral (SCC-40), Colon (COLO-205)	synergistic effect with Adriamycin enhanced anticancer activity across all tested lines
PSP-AuNPs (from Polygonatum polysaccharide)	Hepatic carcinoma	Showed strong anticancer effects in vivo with immune regulation

15.

Schematic mechanism of anticancer activity for AuNPs. Reprinted with permission from ref . Copyright 2021 Elsevier.

2.8.1. Enhanced Permeability and Retention Effect (EPR) and Tumor Targeting

Gold nanoparticles (AuNPs) show excellent stability, biocompatibility, and interesting optical properties, making them suitable as candidates for drug delivery to tumors. Their nanometer dimensions enable preferential tumor site accumulation through the EPR effect, wherein the leaky tumor vasculature traps nanoparticles while minimizing systemic clearance. Targeting ligand conjugation, e.g., antibodies, peptides, or folic acid, increases specificity and cancer cell uptake, enhancing therapeutic efficacy. Additionally, AuNPs act as photothermal agents, enabling targeted tumor ablation using plasmonic heating upon NIR illumination. These characteristics make AuNPs valuable nanomedicine tools for imaging and targeted therapy.

3.8.2. Imaging Application of AuNPs in Cancer

Gold nanoparticles possess unique optical properties, particularly surface plasmon resonance (SPR), which make them suitable for imaging applications in biomedical research. Their tenable size, shape, and surface chemistry allows for efficient functionalization with imaging ligands, enabling their use as contrast agents in molecular imaging and nuclear medicine (Table ). They can easily modify their surface with polymers, drugs, antibodies, and proteins, and they have become widely used as delivery vehicles. Their prolonged circulation time, enabling extended imaging and enhanced targeting efficiency make them suitable for CT imaging. AuNPs offer approximately 2.7 times greater imaging contrast than iodinated agents. Some traditional iodinated agents like iopamidol and iodixanol often cause allergic reactions and are unsuitable for patients with renal issues due to rapid clearance.

11. Summary of Imaging Applications of AuNPs Reported in the Literature.

imaging category	AuNP formation method	targeted area	impact	quantitative data	limitation	references
compound Tomography (CT)	Citrate reduction followed by conjugation with 2-deoxy-d-glucose using mercaptosuccinic acid	Human Lung A549 cells	Enhanced tumor contrast on CT scans due to increased uptake of glucose- conjugated AuNPs	CT intensity ∼900 HU with AuNP-DG (vs <300 HU with regular AuNPs)	Uptake pathway not clearly understood; no in vivo testing yet
Photoacoustic Imaging (PAI) and OCT	Femosecond laser ablation Chain like AuNP cluster assembled using CALNN + cysteamine, conjugated with PEG & RGD peptide	Eye neovascularization in rabbit model (CNV)	Strong dual imaging signal; excellent biodistribution and target specificity	176% increase in OCT signal	Slightly complex synthesis Human translation still pending
MRI	24 nm sized AuNP were made via citrate reduction method	Breast cancer cells	Improved targeting and MRI contrast with low toxicity; promising for early tumor detection		Signal intensity lower than commercial MRI agents; purification step required
Photoacoustic Imaging & Photothermal Therapy (NIR-I and NIR-II)	Seed mediated synthesis of nanorods and dumbbell-shaped, AuNPs, PEGylated for biocompatibility	4T1 Breast tumor in mice	NIR-II window particles showed deeper tissue imaging and stronger heat generation for therapy	AuNR1P, AuND1P, AuNR2P, and AuND2P exhibited high photothermal conversion efficiency values of 18.9%, 17.9%, 23.9%, and 25.3%, respectively.	NIR-II AuNPs had slightly lower heating in tumors than NIR-I under same laser settings
Theranostics/Glioma Imaging & Therapy	Various synthesis (e.g., Turkevich, Frens methods); functionalized with antibodies/peptides for brain targeting	Glioblastoma (GBM) and CNS tumor	AuNPs enable targeted therapy across blood-brain barrier, with potential for image-guided delivery	No exact numeric data; qualitative confirmation of GNPs aiding brain-targeted delivery	Difficulty in penetrating BBB; lack of standardized clinical formulations
Multimodal imaging (PAI,CT,MRI,PDT)	AuNPs(spherical or rod shaped) engineered with antibodies/liposomes for selective targeting	General cancer imaging (e.g., breast, glioma, colorectal)	Demonstrated high specificity, stable imaging signals, and enhanced therapeutic payload delivery	PAI, CT, MRI confirmed effective in vitro/in vivo models (no consolidated data set)	Toxicity risks, particle aggregation, and immune clearance limit some applications
Fluorescence	Hybrid QD@MSN-EPI-AuNPs, PEGylated with EpCAM aptamer	Colorectal cancer (HT- 29 CELLS in mice)	Targeted multimodal imaging and drug delivery	Cumulative drug release (ph- 5.4): 76.33% in 6 days	-
				Emission peak shift from 610 to 630 nm
Photoacoustic Imaging	Seedless synthesis of Au Nanostars with MOPS Buffer	Deep tissue imaging	High contrast signal 25 times better than spherical AuNPs	AuNPs-MOPS produced 9.0 ± 0.0 PA units vs 4.7 ± 0.1 (EPPS), 4.0 ± 0.1 (HEPES), and ∼0.4 (spherical AuNPs); melanin coating enhanced signal 4.5 times	Morphological heterogeneity may reduce reproducibility
PAI Guided therapy	Janus chitosan-Au nanorods via counterion complexation + PEG	Breast Cancer	Synergistic gene/photothermal therapy; imaging + treatment	Temperature elevation after 10 min NIR irradiation (808 nm): J-Au-CS = 30.4 °C vs Au NRs = 25.2 °C in cell	Structural complexity may limit scalability
PAI	Gold nanospheres coated with polydopamine-HA modifies	Endometriosis lesions	Targeted imaging of deep-seated lesions, enabled lesion volume tracking	After irradiating with 808 nm laser for 10 min, temperature of 200 μg mL–1 PDA@HA nanoparticles increased by ≈22 °C, PA signal peaked at 8 h postinjection; lesion volume reduced by 66.93% (NAC group)	Limited imaging depth due to NIR I absorption

3.8.3. Gold Nanoparticles in Drug Delivery

Gold NPs have gained significant attention for drug delivery due to their unique structural and functional properties. They have been explored fir drug delivery, particularly in cancer treatments and studies are being conducted on structure-efficacy relationships to optimize their design and application. Additionally, hybrid systems combining AuNPs with other therapeutic agents are being developed. A study conducted by Zazo et al. developed a gold nanoparticle-based system to improve the delivery of stavudine, an antiretroviral drug. The AuNPs used were citrate stabilized and 40 nm in size and prepared by incubating in stavudine solution for 24 h at room temperature. The formulation was tested for drug release at different pH levels and cellular uptake in human macrophages. In vivo studies in wistar rats showed that AuNPs increased stavudine accumulation in HIV (Human Immunodeficiency Virus) reservoirs like the liver, spleen, and macrophages. The liver showed a notable increase in MTR (mean residence time) from 1.28 to 5.67 h and partition coefficient was changed from 0.27 to 0.55. This system had enhanced biodistribution, prolonged drug residence and better targeting of latent HIV sites. No major limitations were directly reported, but the study emphasizes the need for further validation and clinical development. Sun et al. developed a thermosensitive nanoplatform combining AuNPs, hexanoyl glycol chitosan (HGC), a chemotherapeutic agent doxorubicin (DOX) and a photothermal; dye indocyanine green (ICG). AuNPs were synthesized via a citrate reduction method and them coated with HGC through electrostatic interaction, encapsulating DOX and ICG within the HGC shell. This assembly allowed drug release to be triggered by heat generated under 808 nm laser irradiation. It effectively targeted tumors and showed strong cytotoxicity in HeLa and SiHa cells and tumor-bearing mice. Advantages included enhanced therapeutic efficacy, targeted release, and minimal side effects. No major limitations were reported. Similarly, in the research conducted by Devi et al. AuNPs were synthesized using citrate and DOXmediated reduction, then functionalized with BP100 which is a cell penetrating peptide or RGD (targeting peptide). BP100@AuNPs-DOX showed high DOX loading (19.2 μM) with 96% encapsulation efficiency and up to 85% drug release at pH 4.5, better than RGD@AuNPs (70% release). BP100 systems showed superior cellular uptake and anticancer efficacy in HeLa cells. In HeLa cell assays, free DOX showed 32.1% killing, while DOX@AuNPs alone caused 27.2% death. Peptide-functionalized versions significantly improved efficacy-BP100@AuNPs-DOX and RGD@AuNPs-DOX killed 51.4 and 37.3% of cells, respectively. Overall, BP100@AuNPs-DOX exhibited the best anticancer activity, though high DOX concentrations reduced nanoparticle stability. In an experiment carried out by Zheng et al., 214 nm alginate-cysteine nanogels were embedded with 20 nm AuNPs. These DOX@ACA nanogels had 89.6% encapsulation efficiency and 3.7% drug loading. DOX release reached from 42.4 to 67.2% under 532 nm laser irradiation. DOX-AuNPs were highly responsive and effective but synthesis complexity and charge repulsion were the only noted limitations.

2.8.4. Limitations of AuNPs in Drug Delivery Applications

Although gold nanoparticles offer various benefits as drug delivery vehicles, their broader clinical application is primarily limited by concerns over safety. Factors including particle size, morphology, surface ligands, nuclei acid conjugates, dosage levels, and degradability influence their toxicity. To ensure reproducibility and consistent performance of gold nanoparticles (AuNPs) in drug delivery, maintaining uniformity in their size and shape is essential. Additionally, strategies for encapsulation, surface functionalization, and drug release must be specifically designed to match the physicochemical characteristics of the therapeutic agents being delivered. Release mechanism should be precisely controlled. A further challenge is ensuring the biological stability of AuNPs within physiological environments. Variations in temperature, pH, and the presence of ions or other biomolecules should not compromise the nanoparticle’s functionality or cause premature release or degradation of the loaded drug. Although these challenges demand rigorous design and testing, the unique properties of AuNPs continue to present significant opportunities in advancing drug delivery systems. While numerous studies have shown that AuNPs are generally safe, some have reported toxic effects. For example, a study conducted by Feng et al., involving bacteria such as Shewanella oneidensis (Gram-negative) and Bacillus subtilis (Gram-positive), gold nanoparticles coated with cationic or polyelectrolyte substances were found to exhibit higher toxicity compared to those functionalized with anionic ligands like 3-mercaptopropionic acid or the cationic ligand 3-mercaptopropylamine. Another study explored the use of gold nanoparticles embedded in or added to the soaking solution of contact lenses to enhance sustained release of Bimatoprost for glaucoma treatment. The researchers evaluated drug uptake, release duration, lens transparency, oxygen permeability, and protein adherence. However, certain limitations were identified. GNP- Laden contact lenses caused a high initial burst release of bimatoprost which may lead to side effects like eye redness. While drug release lasted up to 72 h, it dropped after 48 h, potentially below therapeutic levels. There is also concern about nanoparticle leakage, and long-term safety studies are still needed. The EPR effect was a pivotal discovery in nano-oncology, enabling passive targeting of nanoparticles to tumors. However, its clinical applicability has fallen short of expectations, as high accumulation of nanoconjugates in tumors via EPR alone is often inconsistent. , Consequently, the need for active targeting strategies remains critical to improve specificity and therapeutic efficacy in cancer drug delivery. ,

3.9. As Potent Quorum Quenchers

Gold nanoparticles (AuNPs) exhibit promising quorum quenching capabilities by disrupting bacterial quorum sensing (QS) pathways, which control virulence and biofilm formation (Table ). AuNPs inhibit QS by downregulating genes linked to bacterial communication, thereby reducing pathogenicity. These nanoparticles offer a potential alternative to antibiotics, especially against multidrug-resistant bacteria. Biosynthesized AuNPs, due to their eco-friendly nature and cost-effectiveness, are emerging as effective antimicrobial agents in combating bacterial infections and managing biofilms. These have shown promising potential as quorum quenchers, effectively inhibiting quorum sensing (QS) and virulence factors in Pseudomonas aeruginosa. Biosynthesized using Streptomyces isolate S91, these monodispersed AuNPs disrupted QS-related traits like pyocyanin, protease, and elastase production. The inhibitory effects were confirmed using RT-PCR, highlighting the potential of AuNPs as novel anti-QS agents for managing microbial resistance and chronic infections. Qais et al., showcased gold nanoparticles synthesized using Capsicum annuum extract demonstrated potent quorum-quenching capabilities against Pseudomonas aeruginosa PAO1 and Serratia marcescens MTCC 97. The AuNPs-CA effectively inhibited QS-regulated virulence factors, including pyocyanin, pyoverdin, elastase, and rhamnolipid production, along with biofilm formation. , Khosravi et al., study highlights the potential of green-synthesized AuNPs as biofilm, paving the way for new strategies to combat multidrug-resistant bacterial infections.

12. AuNPs as Potent Quorum Quenchers, Mechanisms, and Target Bacteria.

type of AuNPs/AuNCs	functionalization/conjugation	target bacteria	mechanism/outcome	references
Ultrasmall AuNCs (<2 nm)	Acyl Homoserine Lactones (C-6, C-8, C-12)	E. coli, C. sakazakii, P. aeruginosa	Selective inhibition; C-8 AHL specific to C. sakazakii.
Chitosan AuNPs	Chitosan, Antibiotics	E. coli, S. aureus, P. aeruginosa	pH-responsive Cur release; electrostatic bacterial targeting; ROS & heat generation kill
AuNPs (10–30 nm)	N-acyl homoserine lactonase (AiiA)	Proteus spp. (MDR)	Inhibited QS signals, reduced biofilm formation, metabolic activity
Spherical AuNPs (168  ±  52 nm)	Tetramethylpyrazine	P. aeruginosa, S. aureus, S. mutans, K. pneumoniae, L. monocytogenes, E. coli	Biofilm and virulence gene suppression; inhibits early/mature biofilms; reduces motility and toxins
SB-AuNPs (∼31.6 nm, aggregated spherical)	Synthesized using quorum-quenching Salmonella bongori	Pseudomonas fluorescens, Serratia marcescens	Significant antibacterial, antibiofilm, and antiproliferative activity; AHL degradation
Biogenic AuNPs	Ethnobotanical plant extracts	Aeromonas hydrophila	Biofilm and quorum sensing inhibition; enhanced antivirulence activity without resistance
AuNCs in Cs-GA copolymer composite	Chitosan–Gum Acacia polymer matrix	Pseudomonas aeruginosa	Inhibits quorum sensing via LasR suppression; reduces biofilm, motility, protease; biocompatible

3.10. For Cosmeceutical Applications

Lately, Gold nanoparticles (AuNPs) are increasingly being incorporated in cosmeceuticals due to their antioxidant, anti-inflammatory, and antiaging properties. (Table and Figure ) outlines the various applications of AuNPs in cosmetics. AuNPs enhance skin permeability, deliver active ingredients, and offer resistance to environmental insult. AuNPs also promote collagen synthesis which reduces wrinkles and improves skin elasticity. However, further research is needed to know long-term safety and efficacy in cosmetic formulations. In cosmeceutical (that is cosmetic + pharmaceutical) applications, gold nanoparticles (AuNPs) offer promising potential for skin delivery systems. Curcumin which is a bioactive component of turmeric inhibits antioxidant and anti-inflammatory properties, with low bioavailability and instability. On encapsulating curcumin in chitosan-gold nanoparticles, the stability, absorption, and therapeutic activity are enhanced, and efficiency increases. Using chitosan derived from Oryctes rhinoceros beetle offers a green approach that converts pests into useful resources, gives remarkable stability, mild toxicity, and uniform nanosized particles, which are ideal for future cosmeceutical products. Nowadays, plant extract-mediated green synthesis of gold nanoparticles (AuNPs) is becoming increasingly popular because it is eco-friendly and does not utilize toxic chemicals. Despite limited studies, these are beneficial in antiaging, rejuvenation, and acne treatment creams and products. Due to their size and big surface-to-volume ratio, AuNPs penetrate skin layers with ease, facilitating the delivery of active components more efficient. Nevertheless, incorporating AuNPs in cosmeceuticals necessitates sensitive care of their environmental footprint, with a focus on green synthesis methods and lifecycle studies This approach ensures that the cosmetic industry can harness the unique benefits of AuNPs while prioritizing sustainability and minimizing ecological impact. Recent advancements include stimuli-responsive AuNP-collagen hydrogel nanoparticles (Au-CHPs), which enable controlled delivery of therapeutic proteins like fibroblast growth factor, superoxide dismutase, and epidermal growth factor. These Au-CHPs enhance wound healing, reduce oxidative stress, and promote skin regeneration, highlighting their potential for targeted, on-demand treatments in skincare. Singh et.al, in his research formulated a cosmeceutical peel-off mask by copolymerizing poly­(vinyl alcohol) (PVA) with sodium alginate, hydroxypropyl methylcellulose, or hydroxyethyl cellulose and incorporating silver nanoparticles (AgNPs). This approach was seen to enhance the antibacterial properties of the mask while maintaining its stability and biocompatibility. Similarly, gold nanoparticles (AuNPs) hold potential in cosmeceuticals due to their antioxidant, antiaging, and anti-inflammatory properties, enabling effective skin rejuvenation, wrinkle reduction, and enhanced product stability.

13. AuNPs in Cosmeceutical Applications.

methodology	application	key findings	references
AuNPs conjugated with hyaluronic acid for topical application	Targeted, pH-sensitive drug delivery system for cosmeceuticals and oral formulations.	High encapsulation (94%) and sustained drug release; minimal release in acidic (gastric) and enhanced in basic (intestinal) pH.
AuNPs incorporated into botanical extracts	antiaging skincare formulations to enhance cellular penetration and efficacy.	improve antiaging activity of natural extracts, showing lower IC50 and higher inhibition than extracts alone.
AuNPs synthesized using marine algae and formulated into lotions	As an antioxidant, and antibacterial cosmeceutical agent.	Exhibited strong α-amylase & α-glucosidase inhibition, high antioxidant activity (DPPH & FRAP)
Green synthesis of AuNPs using extract of Hubertia ambavilla.	Antiaging and dermoprotective cosmetic formulations.	Nontoxic to human dermal fibroblasts; strong antioxidant and UVA-protective properties;
Green synthesis of AuNPs using Punica granatum juice (PGJ)	Used as a natural antioxidant and SPF booster in sunscreen formulations.	showed strong antioxidant activity, Improved SPF (3–18) depending on AuNP concentration.
Curcumin-loaded chitosan–AuNPs (CCG-NPs) synthesized using chitosan from Oryctes rhinoceros	Stabilization and delivery of curcumin for cosmetic formulations using insect-derived chitosan.	Spherical nanoparticles (∼128 nm), stable, mildly toxic (IC50 ∼ 58%), enhanced curcumin bioavailability and stability.
Green synthesis using Panax ginseng leaf extract.	Antioxidant, moisturizing, and skin-whitening agent in cosmetics.	Pg AuNPs showed dose-dependent antioxidant activity, moisture retention
Green synthesis of AuNPs using Helichrysum odoratissimum extract with/without gum arabic as stabilizer	Antiacne cosmeceutical targeting Cutibacterium acnes adhesion	HO-AuNPs significantly inhibited C. acnes adhesion, suggesting selective antiadhesion

16.

Cosmeceutical applications of AuNPs.

3.11. Gold Nanoparticle Application in the Treatment of Brain Dysfunction

Gold nanoparticles (AuNPs) are proven to be beneficial in treating brain dysfunctions due to their unique structure and properties, such as the ability to cross the blood-brain barrier (BBB) and deliver therapeutic agents directly to the brain. Some research and studies have shown that AuNPs are responsible for enhanced drug efficacy, reducing oxidative stress, and modulating neuroinflammation, offering neuroprotection in conditions like Alzheimer’s and Parkinson’s disease. However, further research is required to understand their long-term safety, biodistribution, and precise therapeutic mechanisms. Gold nanoparticles (AuNPs) also offer promising potential for treating neurological disorders due to their unique physicochemical properties. They can easily penetrate across the blood-brain barrier (BBB), facilitating drugs’ targeted delivery and minimizing off-target effects. In Alzheimer’s disease AuNPs inhibit amyloid-β aggregation, remove oxidative stress, and regulate neuroinflammation. In Parkinson’s disease (PD), they are neuroprotective through the removal of reactive oxygen species and inhibition of dopaminergic neuron loss. Nevertheless, challenges in toxicity, stability, and controlled release must be addressed for clinical use. Further research is necessary to optimize AuNPs for Safe and effective neurological interventions. In a recent study, AuNPs synthesized via microwave radiation and stabilized with dextrin were administered to diabetic rats. Diabetes can be mitigated by this treatment as it reduces oxidative stress, inflammation, and neurotransmitter imbalances. The higher dose (that is 2 mg/kg) was more effective than the lower dose (1 mg/kg), highlighting AuNPs’ potential in neuroprotection and managing diabetic brain dysfunctions. Gold nanoparticles (GNPs) linked with sodium diclofenac and/or soy lecithin were evaluated for safety and therapeutic potential in treating obesity-related inflammation. The 18 nm GNPs, administered intraperitoneally for 14 days, accumulated significantly in tissues without causing hepatic or renal toxicity. In obese mice, GNPs reduced food intake, and alleviated inflammation and oxidative stress, but did not reverse mitochondrial dysfunction. These findings suggest GNPs could be promising therapeutic agents, pending further safety assessments.

3.12. Photothermal Therapy

Gold nanoparticles inserted into gelatin hydrogels demonstrate strong photothermal properties under near-infrared laser light. These hydrogels enhance MC3T3-E1 preosteoblast, supporting bone regeneration. It produces heat in the localized area promoting cell growth and making this system promising in bone tissue engineering. They provide a minimally invasive and efficient way to improve bone healing through controlled thermal stimulation. AuNPs due to their adjustable optical properties and biocompatibility, are effectively used in plasmonic therapy. The radiation emitted from the laser converts light into localized heat, enabling the targeted destruction of cancer cells. Their surface can be modified for targeting tumors and reducing the damage to healthy tissues. Another study by Faid et al. presents hybrid chitosan-coated AuNPs as effective agents in photothermal therapy. They exhibit biocompatibility and stability, efficiently converting near-infrared light into heat to destroy cancer cells. The chitosan shells support in cellular uptake and prolongs circulation time. It shows significant potential in minimally invasive cancer treatment. A study by Frantellizzi et al. presented 99 mTc-labeled keratin-coated gold nanoparticles (Ker-AuNPs) designed for cancer photothermal therapy and imaging. They show biocompatibility and renal-clearance properties, modeled through a nephron-like system, ensuring minimal toxicity. This dual-function nano platform depicts integrated diagnosis and photothermal treatment in oncological applications. Similar study by Darvish et al. highlighted the use of keratin- coated AuNPs for photothermal therapy. The functionalized AuNPs serve as promising agents for cancer therapy and therapeutic effects. The study investigates anisotropic gold nanoparticles stabilized with choline carboxylic acid ionic liquids for photothermal therapy. They show excellent thermal conversion under infrared light, enabling targeted cancer cell ablation. Their unique structure enhances the stability, biocompatibility, and efficiency. They are effective agents in noninvasive cancer treatments through localized heat generation. Green synthesis of gold nanoparticles for photothermal therapy in combination with chemotherapy. They generate heat to kill cancer cells, enhance drug uptake and therapeutic efficiency. The biocompatible approach reduces side effects and improves the cancer treatment. Another study discussed the nanocellulose-based codelivery system that enhances photothermal therapy. Gold nanoparticles (AuNPs) combined with curcumin are integrated with carboxymethylated cellulose nanofibrils, enhancing the stability, solubility, and release rate. The system boosts photothermal effects under near-infrared irradiation, leading to efficient cancer cell destruction. This method offers a biocompatible and synergistic treatment combining thermal and chemotherapeutic effects with minimal side effects.

3.13. Antibacterial Properties

4-mercaptobenzoic acid (MBA)-functionalized gold nanoparticles (AuNPs) have demonstrated potent antibacterial activity (Table ), particularly against multidrug-resistant (MDR) S. aureus and S. epidermidis. Similarly, 4,6-diamino-2-pyrimidine-thiol (DAPT) functionalized gold nanoparticles embedded in a silk fibroin (SF) membrane showed good antibacterial activity against drug-resistant and drug-sensitive strains of E. coli. Controlled release of the DAPT-AuNPs was provided by the hydrophilic nature of the SF membrane. In vivo experiments with SD rats revealed quick healing of the wound in 3 μg/cm²-treated groups compared with control groups, or groups treated with plain SF membranes or gauze. Besides surface changes, particle shape is also an important determinant of the antibacterial activity of AuNPs. Irregular AuNPsrod, star, peanut, and porous sphereswere found to cause membrane integrity loss of E. coli and P. aeruginosa. Irregular shapes facilitate interaction with bacterial surfaces, enhance the generation of reactive oxygen species (ROS), and enhance antibiofilm efficacy. All these mechanisms contribute to efficient inhibition of bacteria and biofilm formation inhibition.

14. Antibacterial Potential of AuNPs.

type of AuNPs	targeted bacteria	observed effects	references
Ellagic Acid-Modified AuNPs (EA-AuNPs)	Multidrug-resistant ESKAPE pathogens	High bactericidal efficacy; mitigated inflammatory responses
Aminophenol-Modified AuNPs (AP_Au NPs)	Multidrug-resistant bacteria	Damaged bacterial cell walls; bound to 16S rRNA, blocking protein synthesis; effective in vivo against abdominal infections.
Thioproline-Modified AuNPs	E. coli	Potent antibacterial activity; balance between monodispersity and aggregation crucial.
D-Maltose-Capped Au Nanoclusters with Thiourea (AuNC-Mal/TU)	Multidrug-resistant Pseudomonas aeruginosa	Effective at low MIC; inhibited thioredoxin reductase; interfered with Cu+ regulation; depleted ATP.
Graphene Oxide–Gold Nanoparticles (GO–Au NPs)	S. aureus, E. coli	Significant antibacterial activity; effective against both Gram-positive and Gram-negative bacteria.
Vitex negundo Leaf Extract Synthesized AuNPs	Salmonella typhi, Micrococcus luteus	Exhibited antibacterial activity; zones of inhibition observed; potential for biomedical applications.
Virola oleifera-Synthesized AuNPs	S. aureus, Pseudomonas aeruginosa	Significant antibacterial activity against both strains;
Phospholipid-Stabilized Gold Nanorods	E. coli, S. aureus	Effective photothermal antibacterial activity; stability and low toxicity
Codariocalyx motorius-Synthesized AuNPs	Various bacterial and fungal pathogens	Exhibited antibacterial and antifungal activities
Metallic-Core AuNPs	S. aureus, Klebsiella pneumoniae, Streptococcus pneumoniae, E. coli, Brucella abortus, Mycobacterium bovis	Displayed good antibacterial activity against all tested pathogens; potential alternative to traditional antibiotics.

3.14. As Antiviral Agents

Antiviral drugs and agents are in great demand throughout the world. Each year millions of infections occur due to the attack of these viruses thus causing infections. Metal nanoparticles have the intrinsic property of antiviral activity which provides a potential solution to mitigate these infections. AuNPs can attach to viral particles, thereby preventing their interaction with cellular or viral receptors and inhibiting the initiation of the viral replication cycle. AuNPs adhere to the cell surface and alter the membrane potential, thereby blocking the viruses from entering the cell. This antiviral action of AuNPs (Table ) has been attributed to several mechanisms, including the prevention of virus attachment and entry into host cells, interference with plasma membrane binding, inactivation of viral particles prior to entry, and interaction with double-stranded DNA. Gold nanoparticles inhibit a broad spectrum of viruses, including HIV, influenza, and herpes simplex virus due to their antiviral properties. In fact, some research showcased the antiviral potential of gold nanoparticles (AuNPs) against SARS-CoV-2, focusing on their ability to disrupt viral envelopes and prevent viral entry. Ayurvedic metal nanoparticles, such as Swarna Bhasma have unique properties like anti-inflammatory, immunomodulatory, and antiviral effects, making them promising agents for COVID-19 treatment. Furthermore, study is required to authenticate their therapeutic potential against SARS-CoV-2 and other viral infections leading to diseases. Glaucium flavum leaf extract was known to produce stable, spherical AuNPs with a mean size of 32 nm and low aggregation. It was synthesized in a green way and characterization techniques like FTIR, and GC-MS confirmed their stability and bioactivity. The potential of functionalized gold nanoparticles (AuNPs) as an effective antiviral agent against herpes simplex virus (HSV) by inhibiting viral entry and replication was observed by Ayipo et al., in his work. These nanoparticles undergo surface modification of AuNPs to enhance their binding affinity to viral proteins, allowing targeted. These findings highlight the potential of AuNPs in developing advanced antiviral therapies. Similarly, sulfonic group-modified gold nanoparticles (MDS_AuNPs) possess broad-spectrum via antiviral properties but lose efficacy in high-protein environments. To address this, sulfonic mixed-charge modified gold nanoparticles (MC_AuNPs) were developed by introducing positively charged ligands. MC_AuNPs retained antiviral activity even in a 10 mg/mL protein solution, unlike MDS_AuNPs, which failed in 1 mg/mL solutions. Chaika et al., in their work showcased that these gold nanoparticles (AuNPs) exhibit potent antiviral activity against adenovirus and H1N1 influenza virus, with 5 nm size indicating stronger virucidal effects compared to 20 nm AuNPs. The smaller nanoparticles were observed to disrupt viral structures within 2 h, while larger ones made shape changes. AuNPs are universal virucidal agents with low cytotoxicity and minimal reactive oxygen species (ROS) generation due to this physical adsorption mechanism.

15. AuNPs as Antiviral Agents, Target Virus, Cells, and Formulations.

AuNP formulation	target virus	synthesis method	size of AuNP (nm)	targeted cells/organisms	references
AuNPs functionalized with peptide, Mes, Mus, Ot	SARS-CoV-2	Molecular dynamics	2	Spike protein RBD; blocks ACE2 binding
PEG-AuNPs	PEG-AuNPs	Turkevich chemical reduction	2–10	HIV particles, HIV-infected CD4+ T cells
	HIV
AuNPs with sulfonated glucose/lactose ligands	DENV-2	Ligand coating + docking study	4–5	Hepatocytes; DENV-2 envelope protein
DNA-conjugated AuNP network	Human RSV	Self-assembly on cell membrane	13	Inhibits viral attachment, entry, budding, and spread
AuNPs using garlic extract (AuNPs-As)	Measles virus (MeV)	Green synthesis with Allium sativa	6	Vero cells; blocks viral particles (virucidal)
AuNPs–PNA conjugate	BVDV (HCV model)	PNA loading on AuNPs	13 ± 1 nm	Infected cells; targets viral RNA (5′-UTR)
Nonfunctionalized AuNPs	HSV-1	Turkevich method	10 or 16 Smaller AuNPs more effective	Vero cells; HSV-1 envelope
Citrated and PEG-dithiol functionalized AuNPs	BVDV	Citrate reduction; PEGylation	12 nm (citrated), 15 nm (PEGylated)	MDBK cells; inhibits viral attachment
HA–AuNP/IFNα complex	HCV	Chemical binding (thiolated HA) + physical loading of IFNα	22.16	Liver tissue
AuNPs/LDHs (e.g., MgFeLDH)	Hepatitis B Virus (HBV)	Self-assembly using LDH	AuNPs ∼ 3.5 nm on LDH ∼ 150 nm	HepG2.2.215 cells
Lactoferrin-modified AuNPs	HSV-2	Chemical + Lactoferrin coating	varied	Human keratinocytes
AuNPs coated with poly(styrenesulfonate) (PSS)	Broad-spectrum viruses	RAFT polymerization + gold salt reduction	3.1	Viral particles

3.15. Gold Nanoparticles in Chemical and Biological Sensing

Gold nanoparticles (AuNPs) are now being widely used in biological and chemical sensing due to their unmatched optical and electronic properties, which enable easy detection of analytes (Table ) Figure . AuNP-based sensors control the electron transport mechanisms and surface plasmon for electrical and optical readouts. Environmental monitoring, medical diagnostic and real-time detection in complex matrices due to its rapid response times, selectivity, and potential for miniaturization. Gold nanoparticles play a crucial role in biosensors due to their unique properties. Surface Plasmon Resonance (SPR) enables them to exhibit collective electron oscillations when exposed to specific wavelengths of light, allowing for the detection of subtle refractive index changes in the surrounding medium. This characteristic is essential for high-sensitivity biosensing. Additionally, their high surface area to volume ratio facilitates the attachment of numerous biomolecules, improving sensor performance. Fungal-mediated synthesis techniques offer precise control over nanoparticle size and shape, further enhancing surface area and making them highly effective for sensing applications. Moreover, Au NPs are biocompatible and nontoxic allowing functionalization with biomolecules like antibodies, DNA, and peptides without affecting their biological compatibility and making them ideal material for medical and environmental sensing.

16. Chemical and Biological Sensing Applications of AuNPs.

Type of AuNPs (with reason)	functionalization	target analyte	sensing mechanism	references
Spherical AuNPs (citrate-capped)Uniform size distribution and surface plasmon resonance (SPR) optimized for visible light detection.	Unmodified	Water Pollutants (Hg2+, Pb2+)	Colorimetric detection via SPR shift; aggregation causes color change
AuNPs conjugated with anti-SARS-CoV-2 spike antibodies via PEG linker (Ab-PEG-AuNPs)for specific viral protein recognition and enhanced signal transduction	Antibody (antispike) functionalized using EDC/NHS chemistry and PEG linker	SARS-CoV-2 virus (Spike protein)	Microwave sensing enhancement through specific antigen–antibody interaction; label-free detection
Gold Nanoclusters (2 nm, Ligand-Protected) - Atomically precise, fluorescence in NIR region.	Ionic Liquid Functionalization	Ascorbic Acid	Aggregation-induced quenching for fluorescence detection
Bimetallic gold nanostars coated with silver (BGNS-Ag)sharp spikes and Ag shell offer high SERS enhancement and reproducibility	Surfactant-free (unmodified)	Methylene blue, Thiram (pesticide)	SERS-based detection with ultrahigh sensitivity and minimal sample prep
AuNPs functionalized with mixed PEG-thiol ligandsstudy highlights limitations in uniform grafting of mixed layers.	Mixed thiol-PEG ligands (methoxy, carboxylate, alkyne ends)	Ligand grafting efficiency	Quantitative 1H NMR analysis to assess ligand density on AuNP surface
Colloidal Gold NanoparticlesEnhanced stability in physiological conditions.	Bioactive agents	Biomarkers	Electrochemical detection via enhanced electrocatalytic activity
Plasmonic AuNPs	Unmodified	Energetic charge carriers	Time-resolved emission upconversion microscopy via plasmon decay and carrier cooling

17.

Summary of different types of gold nanoparticle biosensors. Reproduced from ref . Copyright 2018, with permission of Elsevier.

3.16. Gold Nanoparticle-Based Plasmonic Organic Solar Cells

Plasmonic solar cells are an advanced type of photovoltaic technology that utilize the unique optical properties of metal nanoparticles (usually gold, silver, or aluminum) to enhance light absorption and increase solar cell efficiency (Table ). The key concept behind plasmonic solar cells is the Surface plasmon resonance (SPR), which refers to the collective oscillation of conduction electrons at the surface of metal nanoparticles when excited by incident light. Studies have shown that the incorporation of plasmonic nanostructures in solar cells significantly enhances their power conversion efficiency (PCE). The improvement in PCE largely depends on the type of solar cell, the material used, the shape and size of plasmonic nanoparticles, and their position within the device structure. Gold nanoparticles, despite having lower heat-generation capability than silver or copper, are preferred due to their chemical stability and resistance to oxidation. Heat generated through plasmonic effects is transferred from the nanoparticle to the surrounding medium, affecting the local temperature and thus enabling various applications such as photothermal therapy, catalysis, and sensing.

17. AuNPs Applications in Solar Cells and Their Impact on Performance.

Sr. No.	size	shape	solar cell composition	impact on performance	references
1	41 nm	Spherical	Organic (PBDTTT-CF:PC70BM)	Increased efficiency from 6.67 to 7.86%
2		Spherical	Perovskite (CdS-grated)	Improved Jsc, PCE, VOC value and FF
3	70 nm	Spherical	Schottky (silicon based)	47% increase in J sc with significant heat generation
4	Core–Shell (Au@TiO2, Au@SiO2)	Spherical	perovskite	Increased optical absorption, improved charge separation, and enhanced stability.
5	10–150 nm	Spherical	Perovskite-Organic Tandem	Mitigates optical losses, improves electron–hole recombination, and enhances current gain by >1.5 mA/cm2, achieving 25.34% efficiency.
6	Fe3O4@Au Core–Shell		Hybrid organic (magnetoplasmonic)	8.3% increase in PCE
7	16 nm	Spherical	Polymer (P3HT:PCBM)	16% PCE Improvement via plasmonic enhancement of light absorption
8	51 nm (AuMPA) and 12 nm (for AuOA,)	Spherical	Hybrid bulk heterojunction	Improved J sc, PCE of the device
9	87	spherical	Polymer bulk heterojunction	87 NM SIZED NPS due to the balanced contribution of near- and far-field plasmonic effects, have improved vertical coverage and better interfacial properties. Thus, improve efficiency
10		Truncated polyhedral	Organic (P3HT:PC71BM)	Higher PCE as compared to nanocubes due to better plasmonic coupling
11	Au and Ag NPs (1 wt%)	spherical	Organic(P3HT:PCBM)	PCE increased was observed from 2.11% to 2.55% (Au) and 2.23% (Ag) due to enhanced light absorption.
12		Decahedral	Polymer (P3HT:PC60BM, PBDT-TS1:PC70BM)	Improved light response across broadband wavelengths, improving PCE to 10.29%
13	10–20 nm	spherical	inverted bulk heterojunction organic solar cell (PTB7-Th:PC71BM)	24% efficiency improvement, PCE reaching to 11.8%, due to plasmonic enhancement in ZnO electron transport layer.

3.17. In Veterinary Medicine

Gold nanoparticles have gained attention in recent years for their multifunctional role in nanomedicine, particularly in drug delivery and theranostics. Their unique ability to be tailored in different size and shapes influences their efficiency in imaging, drug encapsulation, and targeted therapeutic applications. One of the notable uses of AuNPs in veterinary medicine is in rapid disease detection. Moongkardi et al. developed an immunochromatographic assay for detecting bacterial infections such as Salmonella enterica serovars Typhimurium and Enteritidis, in poultry. These assays provide a quick and reliable means of identifying pathogens and can help to improve food safety and animal health. Similarly, AuNPs-biosensors have been employed for the detection of haptoglobin in mastitic milk, thus helping in early detection of subclinical mastitis in dairy cows. AuNPs also play a crucial role in antimicrobial applications. Kumar et al. developed gold nanoparticle-based immunogens for the detection of colistin, a critical antibiotic in veterinary medicine in chicken liver samples. Additionally, AuNPs have been investigated for extending the shelf life of chilled minced meat by inhibiting bacterial growth, particularly against E. coli and Salmonella. In therapeutics, AuNPs have been studied for targeted drug delivery and immunomodulation. Loghmani et al. developed betaine-conjugated AuNPs in a murine model of heatstroke, where they significantly reduced inflammation and oxidative stress by modulating cytokine levels and enhancing immune function Moreover, they are also integrated into regenerative medicine strategies, such as stem cell therapy tracking and tissue repair, demonstrating their potential for veterinary application. The role of AuNPs in role regeneration and dental applications has also been explored, with studies investigating their ability to enhance osseointegration of dental implants in rabbit models. Furthermore, AuNP-based hydrogels have been developed for use in regenerative medicine, providing a biocompatible scaffold for tissue engineering applications.

3.18. AuNPs in Dental Biomaterials

Modern healthcare prioritizes increasing patient survival rates and improving quality of life, with a key focus on developing implantable materials that can mimic natural biological functions. Currently a range of materials such as metals, ceramics, carbon nanostructures, and polymers are widely used in orthopedic and oncological surgeries. However, these materials face challenges such as limited strength, poor biointegration, metal ion diffusion, and toxicity from degradation byproducts. To mitigate these complications, researchers are developing bioactive surface treatments that provide antimicrobial protection Several studies of use for titanium and titanium-based alloys are being reported. Noble metals like platinum, palladium, silver, and gold have gained attention due to their exceptional biocompatibility, resistance to corrosion, and antibacterial properties. Coating implant surfaces with ultrathin layers or nanoparticles of these metals has demonstrated benefits in promoting bone integration and reducing inflammation Due to its biocompatibility, and chemical and corrosion resistance in biological environments, metallic gold and its alloys are being used for implants and surgeries. There are two primary forms of gold known for their antibacterial properties. The first is nanoporous gold (NPG), which features a highly porous structure at the nanometre scale. This unique morphology enhances its surface area, enabling effective interaction with bacterial cells. Studies have demonstrated that gold nanoparticles exhibit antimicrobial effects against E. coli and Staphylococcus epidermidis, making it a promising material for medical and biomedical applications. Solanki et al. synthesized AuNPs using Triphala extract which were found to be biocompatible and possess strong antimicrobial properties, making them a potential alternative for oral care products. Researchers evaluated the cytotoxicity of these nanoparticles and an AuNP-based mouthwash using the Artemia salina (brine shrimp) assay. The mouthwash exhibited mild toxicity at higher concentrations (40–50 μL), with a mortality rate of 46.6%. At lower concentrations (20–30 μL), the mouthwash was found to be safe and effective, suggesting its suitability for use in orthodontic patients. Another study suggested that gold nanoparticles (AuNPs) enhance the biocompatibility, proliferation, and antioxidant properties of dental biomaterials, making them suitable for tissue regeneration and restorative applications Biz et al. combined gold nanoparticles with poly­(l-lysine) (AuNP-PLL) and introduced into dental pulp stem cells (DPSC) which resulted in high cellular uptake without compromising cell viability or inducing apoptosis. This highlights AuNPs’ potential for imaging and monitoring stem cells in regenerative endodontics Dharman et al., synthesized gold nanoparticles using curcumin which demonstrated excellent antimicrobial and anti-inflammatory properties and can be used for treating oral infections and mucosal lesion Dalavi et al., synthesized spherical AuNPs (70 nm) using a microwave irradiation method which exhibited strong antioxidant activity and cytocompatibility with IMR-32 cells and thus can be used for cosmeceuticals and pharmaceutical application. Dentures, which serve as artificial, nonshedding surfaces in the oral cavity, are primarily composed of poly­(methyl methacrylate) (PMMA). AuNPs were synthesized via ultrasonic spray pyrolysis and incorporated into PMMA poly­(methyl methacrylate) to improve its physical and mechanical properties, additionally enhancing its antimicrobial properties. The modified material exhibited similar density and microhardness to conventional PMMA but demonstrated a significant reduction in monomicrobial biofilms of Candida albicans, Streptococcus mitis, S. aureus, and E. coli Another study incorporating gold nps with PMMA resulted in enhanced antifungal activity and improve oral hygiene for denture wearers. These nanocomposites demonstrated significant antifungal activity against Candida albicans at concentrations above 2.0%, with minimal ion release.

3.19. In Active Food Packaging

Shelf life of food is one of the major global concerns for food loss which increases the importance of advanced packaging solutions. Nanotechnology, particularly metal-based nanoparticles, offers a promising approach to extending shelf life while addressing environmental issues. Nanotechnology in the food industry is predominantly used for processing, packaging, and detecting contaminants like toxins, microbes, pesticides, and food adulteration. Nanoparticles aid in enhancing the taste, smell, texture, look, and shelf life of food products, offering improved quality and safety throughout the supply chain. Green synthesis of gold nanoparticles (AuNPs) is gaining attention for its efficiency, reduced biohazards, and enhanced antimicrobial and antioxidant properties which will be beneficial for food packaging (Table ). These have remarkable antioxidant, antibacterial, antifungal, anticancer, and barrier properties along with being inert, nontoxic, hypoallergenic nature and biocompatible to humans. AuNPs can be incorporated into smart packaging systems to improve food safety, quality monitoring, and shelf life. However, factors such as nanoparticle concentration, food type, polymer composition, and storage conditions influence their effectiveness and migration into food, necessitating careful evaluation for safe application in packaging. One of the unique properties of AuNPs particularly are surface plasmon resonance (SPR), peroxidase-like activity and their influence on sensing performance. SPR-based sensors include aggregation, antiaggregation, etching, and growth-based methods, each with varying sensitivity and selectivity. Peroxidase-based (nanozyme) sensors offer a higher sensitivity but involve complex protocols. Smaller AuNPs (<10 nm) increase the catalytic activity, while larger ones (>10 nm) improve SPR-based detection. Despite strong potential, colorimetric sensors are less explored than electrochemical ones for inorganic ion detection. AuNP-based colorimetric sensors offer a simple, fast, and cost-effective method for food safety monitoring without any need for complex equipment. However, challenges remain, including interference from food matrices and the need for specific ligand modifications to enhance selectivity and sensitivity. Future advancements should focus on improving analyte extraction, developing highly specific aptamers, and incorporating signal amplification strategies to enhance detection in real-world food safety applications. Biogenic NPs improves the food shelf life and can serve as biosensors for real-time quality assessment. While promising, challenges remain regarding nanoparticle migration into food and potential toxicity, requiring further research to ensure safety in food-related applications. Sreelakshmi et al. explored the use of chitosan, derived from shrimp waste, as a reducing and capping agent in synthesizing gold nanoparticles (AuNPs) for smart packaging applications in the food and pharmaceutical industries. The reduction time for gold atoms varies depending on the chitosan type, ranging from 6 to 15 min. Higher concentrations of chitosan give rise to smaller, more uniform AuNPs. When exposed to freezing conditions (−18 °C ± 1 °C), the ruby red color of AuNPs shifted to bluish or colorless, which indicates the temperature fluctuation. This visible change confirms the potential of chitosan-based AuNPs as indicators for distinguishing between fresh and frozen products. Alghamdi et al. had also done a study which focuses on developing bionanocomposite films by incorporating gold nanoparticles (AuNPs) into a chitosan (CS) and polyacrylamide (PAM) polymer blend by application of solution casting method. AuNPs were synthesized using Chenopodium murale leaf extract. XRD analysis revealed a crystallinity reduction and an increase in amorphousness by addition of AuNPs. TEM images showed nearly spherical AuNPs, while FTIR spectra confirmed strong interactions between the nanoparticles and the polymer matrix, which was indicated by shifts and intensity changes in functional group bands. UV–vis analysis showed a decrease in both direct and indirect bandgap energies, along with an increase in Urbach energy, reflecting enhanced optical properties. TGA results showed improved thermal stability in Au-CS/PAM composites in comparison to the pure blend. Mechanical testing showed enhanced Young’s modulus, tensile strength, and elongation at break. Additionally, Au-CS/PAM nanocomposites exhibited impressive antimicrobial and antioxidant activity, pointing to their usefulness as active food packaging materials and in optoelectronic applications. Choudhary et al. conducted a study, where poly­(vinyl alcohol) (PVA) composite films incorporating gold nanoparticles (AuNPs) and graphene oxide (GO) was developed and cross-linked using glyoxal or glutaraldehyde (GA). FTIR analysis confirmed effective cross-linking through reduced hydroxyl group transmittance. The addition of AuNPs and GO enhanced mechanical and physical attributes such as tensile strength, Young’s modulus, water vapor transmission rate (WVTR), and water solubility. WVTR tests revealed that nanofillers contributed to reduced permeability by forming a complex, tortuous structure. SEM images showed compact pore morphology in cross-linked films in comparison to pure PVA. Antibacterial activity was observed in both AuNPs- and GO-based composites against E. coli, with PVA-glyoxal-AuNPs films showing a major inhibition zone, indicating superior antimicrobial performance. Besides that, this film extended the shelf life of bananas more effectively than others, confirming its potential for food packaging. In conclusion, the PVA-glyoxal-AuNPs composite exhibited promising structural, antimicrobial, and preservative properties for advanced food packaging applications. Mehmood et al. highlighted the potential of gold nanoparticles (AuNPs) conjugated with gallic acid (GA) as an effective strategy to combat reactive oxygen species (ROS), a major factor in packaged food spoilage. Amine-stabilized AuNPs were synthesized and functionalized with GA from Caesalpinia pulcherrima extract, reducing toxicity and improving antioxidant efficiency. GA-AuNPs exhibited a strong free radical scavenging ability, shown by cyclic voltammetry, with a low IC₅₀ (4 × 10^–9 g/mL) and high antioxidant coefficient. Even though antimicrobial activity was minimal, GA-AuNPs achieved 94.1% DPPH scavenging, outperforming GA and its derivatives. The study confirms GA-AuNPs to be a safe, multifunctional material for extending food shelf life and serving as a nontoxic antioxidant carrier in food packaging applications. Yang et al. developed a novel ratiometric electrochemical sensor for the simultaneous detection of two endocrine-disrupting compounds (EDCs) as 17β-estradiol (E2) and bisphenol S (BPS), using a composite of gold nanoparticles (AuNPs) and MIL-101­(Fe). Traditional electrochemical sensors have been commonly facing challenges in detecting EDCs in food due to poor reproducibility. To tackle this problem, the sensor utilized the Fe signal inherent to MIL-101­(Fe) as a built-in internal reference, replacing the need for external electroactive tags which are commonly used in ratiometric sensing. AuNPs were electrodeposited onto MIL-101­(Fe), which resulted in a composite material with enhanced electrochemical properties. The stability of the Fe reference signal and the oxidation responses of E2 and BPS has remarkably boosted due to synergistic effect between AuNPs and MIL-101­(Fe).The developed sensor has a wide linear detection range from 0.039 to 7.80 μM and achieved low detection limits of 9.8 nM for E2 and 11.2 nM for BPS. Application in milk sample analysis confirmed the sensor’s excellent reproducibility, sensitivity, stability, and practical performance. This work provides a promising direction for constructing ratiometric electrochemical sensors with the use of electroactive MOFs for rapid and accurate EDC detection in food safety monitoring.

18. Key Findings of AuNPs in Active Food Packaging.

Sr. No.	application area	key findings	references
1	Antibacterial Activities	AuNPs exhibit significant antibacterial, antifungal, and antibiofilm properties.	−
		Synergistic effects with chitosan, APTMS, and other compounds enhance antimicrobial activity.
		Effective against Gram-negative and Gram-positive bacteria.
		Shape and concentration-dependent antimicrobial properties.
2	Barrier Properties	AuNPs improve water vapor, gas (CO2, O2), and light barrier properties.	,,
		Enhanced molecular structure tortuosity reduces permeability.
		Optimal concentration is crucial to avoid agglomeration and maintain barrier efficiency.
3	Antioxidant Properties	AuNPs interact with free radicals, showing dose-dependent antioxidant potential.	−
		Green-synthesized AuNPs (e.g., using onion peel extract) exhibit notable radical scavenging capacity.
		High surface-to-volume ratio enhances antioxidant activity.
4	Biosensing	AuNPs are used in colorimetric sensors for detecting food spoilage and contamination.	−
		High sensitivity and rapid response due to large surface-to-volume ratio.
		Effective in detecting volatile biogenic markers and heavy metal ions.
5	General Benefits	AuNPs improve mechanical, thermal, and surface properties of packaging materials.	,
		Enhance shelf life and food safety.
		Migration behavior of AuNPs into food requires careful regulation due to potential toxicity concerns.

3.20. Agricultural Applications

The synthesis of nanoparticles (NPs) has garnered significant attention due to their distinctive properties, which make them valuable for a wide range of applications, including composite fibers, biosensors, cryogenic superconducting materials, cosmetics, and electronic components. However, in light of climate change and the depletion of natural resources, there is a growing emphasis on sustainable methods for producing gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs). One promising approach involves using plant extracts, particularly agricultural waste, as a green and eco-friendly alternative. This method aligns with sustainable development goals in agro-industrial practices. Given that plants serve as the foundation for this green synthesis, the resulting NPs are not only environmentally friendly but also exhibit low toxicity, making them suitable for various agricultural applicationsfrom soil treatment to food chain integration. , In June 2009, the Food and Agricultural Organization (FAO) and the World Health Organization (WHO) highlighted the potential of nanotechnology in food and agriculture. Their joint initiative identified several key areas for innovation, including nanostructured ingredients, nanosized biofortification, food packaging, nanocoating, and nanofiltration. NPs can boost productivity of specific plant tissues or structures by beneficial genes, delivering nutrients, or organic compounds. This capability positions NPs as advanced nanodelivery systems, specifically for enhancing crop nutrition and agricultural efficiency. In agriculture, AuNPs have been extensively studied for their direct applications, which include improving seed germination, promoting root growth, and understanding plant responses to metal NPs, also cellular oxidative stress and cytotoxicity. Nanofertilizers and nanopesticides are also being developed by direct use of metal Nanoparticles. Indirect applications of nanoparticles (NPs), using their antimicrobial properties, are primarily centered on advancements in food packaging. These advancements have been extensively imparted in the agricultural industry, with AuNP based products typically incorporating particles ranging from 100 to 250 nm in size, this size range enhances their water solubility and overall effectiveness. In short, the embedding of NPs into agriculture and food systems represents a novel and transformative approach, offering solutions that are both innovative and aligned with sustainability principles.

3.20.1. In Water Treatment

Water is a vital part of the ecosystem and its treatment highlights the need. Several techniques have been developed for this purpose. Agriculture accounts for almost 70% of the world’s renewable water resources which makes it important to consider for wastewater treatment of agricultural water. In order to enhance sustainable agricultural practices, it is necessary to assess the impact of agro nanobiotechnology on water conservation and its quality. To enhance sustainability and efficiency Nanotechnology is gaining attention in agriculture. A notable approach is the development of gold nanoparticles through environmentally friendly biogenic synthesis using natural sources like plants, fungi, and bacteria. These nanoparticles have strong antibacterial and antifungal properties, making them useful in safeguarding crops from harmful pathogens. Moreover, they can aid in pesticide detection and water purification, promoting safer and more effective farming methods. By incorporating nanotechnology into agricultural practices, researchers strive to improve crop protection and optimize resource use while maintaining environmental responsibility. Water pollution caused by nitrophenol, a nitrogen-containing pollutant, creates significant environmental concerns. A novel approach using thiourea-treated gold nanoparticles (AuNPs) to create nanoporous films via filtration was explored for efficient catalytic degradation of nitrophenol into the less toxic aminophenol. The study found that AuNP films treated with 20 μg/mL thiourea and 1000 mM NaBH₄ facilitated rapid conversion within 150 s. These films demonstrated remarkable structural stability and retained 90% catalytic efficiency after seven cycles. The stable mesoporous AuNPs film results in a cost-effective and sustainable solution for industrial wastewater treatment, making it a promising strategy for environmental remediation. A study by Francis et al. utilized a rapid microwave-assisted method to synthesize gold and silver nanoparticles using Mussaenda glabrata leaf extract as a reducing and stabilizing agent. Characterization techniques, including UV–vis, FT-IR, XRD, TEM, and AFM, confirmed the nanoparticles’ FCC crystal structure. Both nanoparticles exhibited strong antioxidant activity and antimicrobial properties against pathogens like Pseudomonas aeruginosa and E. coli. They showed effective degradation of pollutants such as rhodamine B, methyl orange, and 4-nitrophenol, making them promising catalysts for wastewater purification.

3.20.2. Nanofertilizers, Nanopesticides, and Nanoherbicides

Agriculture plays a vital role worldwide by ensuring food security and economic stability. Traditional ways for boosting crops include the usage of chemical fertilizers, insecticides, and herbicides. However, excessive use of these agrochemicals has led to soil degradation, loss of biodiversity, and environmental concerns. Nanotechnology approaches are used to enhance crop production, improve food security, and develop pest- and drought-resistant crops. Nanoengineered materials improve nutrient absorption, enable rapid disease detection, and function as nanofertilizers and nanopesticides, boosting productivity while minimizing soil and water contamination. These materials also protect against microbial diseases and pests which reduces chemical use and nutrient loss. Additionally, nanotechnology aids in soil quality monitoring, ensuring optimal crop yields. Ongoing research in agricultural nanotechnology is being used to enhance food quality, safety, and efficiency, supporting sustainable agricultural practices for a growing global population. Its applications have been used across multiple areas, such as precision farming, food preservation, and plant protection, due to the distinctive properties of nanomaterials. These materials are known for their controlled-release capabilities, targeted action on specific sites, and large surface area, making them highly efficient in agricultural use. Nanofertilizers, nano herbicides, and nano pesticides enhance plant growth, optimizing nutrient absorption, and provide effective pest and weed control, ultimately leading to higher yields with very less environmental impact. Over the past decade, nanotechnology in agriculture, specifically nanofertilizers and nanopesticides, has gained attention for its potential to transform farming. This analysis has reviewed scientific, regulatory, and commercial progress, highlighting emerging products, differing sector perceptions, and the challenge of risk-benefit assessment. It focuses on the need for improved formulations, clearer definitions, and smarter, sustainable agrochemical development.

3.21. Textiles Industry Applications

Nanotechnology has a vital role in expanding textile applications across various fields, such as protection, fashion, sports, healthcare, the military. By modifying textiles at the nanoscale with functional nanomaterials, it is possible to introduce new features while maintaining their comfort and usability. The textile industry has greatly advanced with the introduction of new applications, especially when combined with nanomaterials. Different types of textilessuch as woven, knitted, and nonwoven fabrics, as well as fibers, yarns, threads, nanofibers, scaffolds, and membranesare now used in high-tech and smart applications. Adding gold nanoparticles (AuNPs) to textiles brings exciting new features, including unique colors, improved filtration, antimicrobial properties, conductivity, UV protection, sensory functions, and catalytic abilities, as explored in this section.

3.21.1. Chemical Reduction Method without Pretreatments on Fabrics

Chemical reduction is one of the most used methods for production of textile-fictionalized gold nanoparticles (AuNPs), due to its effectiveness and simplicity. However, there is a growing shift toward greener and more cost-effective alternatives. Traditionally, researchers follow a two-step functionalization approach, where AuNPs are first synthesized and then applied to fabrics. Alternatively, some opt for an in situ approach, where synthesis and deposition occur in a single step. In the two-step method, sodium borohydride (NaBH₄) and sodium citrate used as common reducing agents. Various deposition techniques have been explored, such as dropwise deposition, exhaustion, padding, impregnation, and printing with AuNP dispersions. For example, Chan et al. (2016) synthesized AuNPs using chloroauric acid (HAuCl₄) as the precursor, NaBH₄ as the reducing agent, and sodium citrate as a capping agent. They applied the nanoparticles to cotton, silk, and wool fabrics using the dropwise deposition method. Zheng et al. synthesized Gold nanoparticles (AuNPs) via citrate reduction, were immobilized onto chitosan-treated soybean knitted fabric by use of the exhaustion method. Moderately polydisperse AuNPs had an average size of ∼35 nm. Successful immobilization was confirmed through spectrophotometric reflectance, X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). XPS analysis showed strong AuNP-chitosan binding. The coated fabrics exhibited enhanced thermal stability, ultraviolet protection (UPF 50+), and antimicrobial properties, effectively reducing S. aureus (99.94%) and E. coli (96.26%) adhesion. The coating also displayed durability, withstanding five washing cycles with minimal AuNP loss with additional benefit of coloration. XPS analysis suggested AuNPs bound to chitosan in a pure metallic state, though potential antimicrobial mechanisms involving oxidized Au species and reactive oxygen species require further study. These multifunctional fabrics show promise for biomedical applications due to their UV shielding, optical properties, and antimicrobial effectiveness. Shanmugasundaram and Ramkumar has utilized keratin with silver and gold nanoparticles to develop antibacterial wound-healing materials. Human hair, a major waste from barbershops, contains keratin (a biocompatible protein) with wound-healing and antibacterial properties. The nanoparticles, synthesized by chemical reduction, were characterized using UV–visible spectroscopy, particle size, and ζ-potential analysis. Silver and gold nanoparticles formed at 420 and 479 nm with average sizes of 71.8 and 14.59 nm, respectively, and negative ζ-potential values of −18.9 and −3.2 mV. FTIR confirmed the presence of keratin and nanoparticles in coated cotton fabrics. SEM images showed uniform, high-density coatings, while EDX confirmed high oxygen and carbon content. The coated fabrics exhibited excellent physical properties, such as air permeability, moisture content, and water absorbency. Additionally, they demonstrated superior antibacterial activity against burn wound bacteria, making them promising for biomedical wound-healing applications. Lin et al. synthesized Nylon fabrics through heat treatment with citrate assistance by in situ technique. The synthesized AuNPs impart bright colors to the fabrics due to their localized surface plasmon resonance (LSPR) properties. The optical characteristics were analyzed using color strength (K/S) curves, while SEM was used to observe surface conditions. The synthesis process was influenced by pH, with acidic conditions favoring AuNP formation. The treated fabrics exhibited excellent color fastness to washing and rubbing. Additionally, the coloration process significantly enhanced the UV-blocking properties of Nylon fabrics. This study provides both aesthetic and functional benefits, offering a promising approach for textile coloration and UV protection at the same time.

3.21.2. Chemical Reduction Method with Pretreatments on Fabrics

Radić et al. conducted study that evaluated two ambient air plasma treatments including volume dielectric barrier discharge (DBD) and diffuse coplanar surface barrier discharge (DCSBD) to enhance gold nanoparticle (AuNPs) deposition on polypropylene (PP) nonwovens. Plasma treatments used to improve surface wettability and sorption, increasing AuNP loading from 17 mg/kg (untreated) to up to 62 mg/kg (DBD-treated). DBD enhances the surface roughness, while DCSBD induces notable chemical changes. Antibacterial tests showed effective activity against S. aureus and E. coli, with higher sensitivity observed in S. aureus. Surprisingly, rinsing enhances the antibacterial properties without reducing AuNP content, due to altered nanoparticle clustering. These findings suggest that AuNPs-loaded plasma-treated PP nonwovens are promising for reusable antibacterial materials, with the choice of plasma method influencing surface and functional properties. Ikegami et al. developed a novel filter-type Au/ZrO₂ catalyst using PET nonwoven fabric as a lightweight, flexible support, offering advantages over conventional catalyst forms. ZrO₂ particles were deposited with a silane agent to create a thin, fish-scale-like layer, followed by gold nanoparticle deposition. This catalyst effectively removed 1000 ppm of CO (83% conversion in 20 min) and 140 ppm formaldehyde (90% removal, 68% oxidized to CO₂ in 90 min) at room temperature. Nearly 100% removal of 0.5 ppm formaldehyde was found in tests simulating indoor conditions for up to 136 h, demonstrating strong performance and long-term effectiveness for air purification applications. A flexible and sensitive surface-enhanced Raman spectroscopy (SERS) substrate was developed by depositing uniform Au nanoparticles onto polydopamine-coated cotton fabrics using Ag nanoparticles as catalytic hotspots. The in situ reduction process produced a dense, even layer of AuNPs which was confirmed by SEM, XRD, and XPS analyses. The substrate, CF/Ag/PDA/Au, showed strong and reproducible SERS signals using 4-MBA as a probe, detecting concentrations as low as 10^–9 M. It also successfully detected carbaryl pesticide residues on cucumbers reduced to 10^–6 M, below regulatory limits. This work highlights the potential of CF/Ag/PDA/Au as a flexible, durable, and effective SERS platform for real-world food safety monitoring. Hence Gold Nanoparticles can be seen as a greener and sustainable approach in the textile industry.

3.21.3. Electrochemical Synthesis Method for Textile Applications

In this there is integration of gold nanoparticles (AuNPs) into textiles to create antimicrobial fabrics, emphasizing eco-friendly and effective synthesis and deposition methods. Two main electrochemical approaches have been identified: A two-step process involving separate AuNP synthesis and deposition, and a more efficient one-step in situ synthesis directly on textiles. While the in situ method saves time but still relies on chemical agents for gold reduction. Innovative techniques like plasma treatment and thermal activation using silk fibers have shown promise in enhancing nanoparticle adhesion and reducing chemical usage. These electrochemical strategies have the potential for scalable, low-toxicity fabrication of antimicrobial textiles, especially for biomedical and hygiene applications. However, more research is needed to optimize these methods and evaluate long-term safety and effectiveness. Over the past two decades, gold nanomaterials (AuNMs) have gained remarkable attention due to their unique catalytic properties at the nanoscale. Electrodeposition is a highly controllable synthesis method, commonly used to create gold nanoparticles, nanoclusters, and nanowires. Enhancing both activity and stability by designing monodisperse nanoclusters, multimetallic nanoparticles, and tailoring surface-support interactions is an important area in this. AuNMs have illustrated versatility in chemical, photochemical, and electrochemical catalysis. Their biocompatibility also finds usage in biomedical, such as drug delivery and photodynamic cancer therapy. These advancements position AuNMs as highly promising materials for diverse applications across scientific field.

3.22. AuNPs as Wearable Sensor

A Study by Zhang et al. showed the development of self-healing and conductive elastomer for wearable sensors. The material consists of poly­(dimethylsiloxane) (PDMS) and gold nanoparticles (AuNPs) by utilizing sulfur–gold (S–Au) interactions. When AuNPs are exposed to near-infrared (NIR) light, the material repairs itself by achieving healing efficiency of 92%. The elastomers exhibit electrical conductivity and strain sensitivity, making it suitable for applications such as monitoring human joint movement and muscle activity in health monitoring and soft robotics. Another study depicted a wearable strain sensor based on the ligand-exchanged AuNPs for detecting human motion. It consists of thin layer of 9 nm gold nanoparticles (AuNPs) deposited on a poly­(dimethylsiloxane) (PDMS) substrate, offering high flexibility and sensitivity capturing small movements of fingers and wrist. With the help of the ligand exchange the conductivity is enhanced, making it well suited for biomedical applications. Another study by Khorablou et al. depicts highly sensitive and flexible sensor which detects methadone. It is built using gold nanoparticles (AuNPs) and polythiophene on a carbon cloth platform, improving sensitivity and enhancing electron transfer. It showed successful application in human blood and urine samples which helps to demonstrate real-time drug detection. For instance, another study discusses about the advancement of wearable sensor for detecting cardiovascular disease (CVD). AuNPs enhance the sensors’ conductivity, sensitivity, and biocompatibility allowing to monitor vital signs like pulse waves, heart sounds, and electrocardiogram (ECG) signals. Their integration into flexible, lightweight material supports real-time health tracking and early cardiovascular disease detection. They offer potential applications in continuous, noninvasive diagnostics and personalized healthcare. A study by Chen et al. demonstrates a wearable electrochemical biosensor utilizing gold nanoparticles (AuNPs) for in situ pesticide detection on crops. It is built on a flexible fiber membrane, a three-electrode system modified with acetylcholinesterase (AchE) and reduced graphene oxide (rGO) to enhance performance. It detects methyl parathion with a low detection limit of 0.48 ppb, providing a rapid and nondestructive method for agricultural monitoring. The development of wearable strain sensors using cross-linked gold nanoparticles (AuNPs) via contact printing method. These sensors are integrated into flexible polyimide (PI) and poly­(dimethylsiloxane) (PDMS) substrates. They demonstrate high strain sensitivity, durability over 10,000 usage cycles, and a rapid response making them well-suitable for healthcare applications. Another study depicted implantable sensors using AuNPs present hydrogel-embedded sensors for continuous biomarker monitoring. They utilize plasmon resonance shifts to detect analyte concentrations through the skin. The tests on anesthetized rats successfully detected kanamycin levels, depicting long-term stability and integration into tissue. A study by Chen et al. demonstrated a wearable glucose sensor designed for continuous monitoring through human sweat, incorporating gold nanoparticles (AuNPs) with aminated multiwalled carbon nanotubes (AMWCNTs) and cross-linked with XSBR and PEDOT: PSS. Integrated onto screen-printed electrodes, it shows high sensitivity, flexibility, and stability making it a potential tool for health tracking and diabetes management. The wearable capacitive sensor designed to monitor leaf moisture was created by depositing gold nanoparticles (AuNPs) onto a poly­(ethylene terephthalate) (PET) membrane using magnetron sputtering, ensuring stable conductivity and adaptability to plant surfaces. The change in capacitance provides a noninvasive method for assessing plant hydration. It has promising applications in agriculture and environmental monitoring. For instance, a study by Wang et al. introduced a sweatband sensor capable of real-time sodium ion, incorporating an all-solid-state ion-selective electrode (ISE) and a reference electrode (RE) by ensuring high sensitivity and stability. It is fabricated via electrodeposition on a flexible substrate, allowing continuous monitoring of hydration levels and electrolyte balance, showing potential applications in personalized healthcare, sports science, and medical diagnostics. A similar study by Dau et al. developed a wearable colorimetric sensor for glucose detection in sweat, integrated with an automated microfluidic chip. It features a glass fiber-based electrode enhanced with gold nanoparticles (AuNPs), improving color stability and sensitivity. This device is designed for continuous health track monitoring, diabetes management, and personalized healthcare applications.

3.23. Gold Nanoparticles in Nanoelectronics

Gold nanoparticles (AuNPs) are used in biomedicine due to their high stability, biocompatibility, and flexible properties (Table ). They play an essential role in drug delivery, biosensing, imaging, and photothermal therapy. Their surface modification enables targeted therapeutic applications by reducing its side effects. Advancements in nanotechnology are enhancing their efficiency for clinical use, medicine, and disease treatment. The plasmonic gold nanoparticles (AuNps) focus on the optical properties, method of synthesis and various biomedical uses. Their plasmonic properties enhance diagnostic techniques such as surface-enhanced Raman spectroscopy (SERS). A study by Oladipo et al. discusses about the biosynthesis of gold nanoparticles (AuNPs) using Datura stramonium seed extract. It is characterized via UV–vis, FTIR, SEM, and EDX, and depicted antifungal, antioxidant, anticoagulant, and thrombolytic activities. They are effective in inhibiting fungal growth, neutralizing free radicals, prevented blood coagulation and clot dissolution. It shows potential applications in biomedicine, particularly for antimicrobial and cardiovascular treatments. For instance, study by Clarance et al. depicted the synthesis of gold nanoparticles (AuNPs) using endophytic fungus Fusarium solani. AuNPs were characterized via various techniques and potent anticancer properties against breast cancer (MCF-7) and cervical (HeLa) cells. The biosynthesized AuNPs were found to trigger apoptosis, block cell proliferation, and induce cell cycle arrest. It shows potential chemotherapeutic agents. Another study by Das et al. explores the synthesis of gold nanoparticles (AuNPs) using Amaranthus plant extract. They exhibit various biomedical applications such as drug delivery, bioimaging, and antimicrobial activity. It shows antioxidant and anticancer properties making is well suited for therapeutic use. The research shows potential applications in nanomedicine, enhancing biocompatible nanomaterials for advanced medical treatments. Gold nanoparticles (AuNPs) have significant applications in nanoelectronics due to their excellent conductivity, stability, and customizable optical properties. They are used in sensors, transistors, and memory devices, allowing miniaturization and enhanced performance of electronic components. Functionalized nanoparticles with organic molecules further improve the efficiency of device and their self-assembly properties, making them ideal for nanocircuit fabrication. GNPs contribute to flexible electronics and molecular-scale computing, forwarding next-generation nanoelectronics technologies. A study by Babajani et al. focuses on the controlled stabilization in heterometallic nanogaps. They use various fabrication techniques, surface modifications, and their influence on electrical characteristics. It shows advanced nanoscale electronics, enabling advancements in molecular circuits, sensors, and quantum devices by distinctive optical and electronic characteristics. Another study investigates the electroless deposition (ELD) of silver thin films on SiO₂/Si surfaces activated by AuNPs. It depicted successful Ag film growth through distinct stages by improving their electrical and optical properties. AuNPs provide a cost-effective, result in rougher films as compared to traditional palladium activation. It improves film quality by using smaller AuNPs to refine surface morphology and conductivity. A similar study by Ruiz et al. demonstrates the use of DNA origami as a template, leveraging the self-assembling properties of DNA developing nanoscale and circular metallic structures. They enable the precise placement of gold nanoparticles, crucial for miniaturized electronic devices. The fabrication process using atomic force microscopy and gel electrophoresis, depicts the potential of DNA origami for future nanoelectronics application.

19. Gold Nanoparticles in Biomedical Applications.

Sr. No.	cell type	shape	size	synthesis	applications	references
1	HeLa and MCF-7 cells	Needle and Flower like	Size-40–45 nm	Green synthesis using Fusarium solani fungus	Shows anticancer by inducing apoptysis and cell cycle arrest, potential chemotherapeutic effect
2	Fibroblast (3T3 cells)	Spherical	Size varies- (AuNps-38 nm and keratin albumin-591 nm)	AuNps synthesized using Keratin and albumin	Used in drug delivery, Biocompatibility and potential therapeutic applications
3	Gloeocapsa sp. and Anabaena cylindrica	Spherical, cubical, and triangular	Size ranging from 2–100 nm	Synthesized via Extracellular or Intracellular using enzymes and proteins	Includes drug delivery, biosensing and anticancer treatment
4	HepG2 and A549 cells	Spherical	Size range between 2–10 nm	Synthesis using Asprgillus niger culture supernatant, reducing chloroauric acid and silver nitrate at pH 8, 100 °C	Highlights the potential in thrombosis treatment and cancer therapy
5	A549 and HepG2 cells	Cubic	16.51 nm	Green synthesis using Morchella esculenta hot water extract at room temperature	Biocompatible agents for therapeutic applications, including microbial inhibition and cancer treatment
6	Hela and A549 cells	quasi-spherical	49.72 ± 1.2 nm	Green synthesis using Euphrasia officianalis leaf extract and cholorauric acid reducion	Showed significant anticancer activity against HeLa cells (cervical cancer cells) but no cytotoxicity effect toward A549 (lung cancer cells)
7	Not specified	Cubic	range between 20–50 nm	Green bioreduction using Chrysopogon zizanioides aqueous extract and chloroauric acid	Shows significant antibacterial, antioxidant and cytotoxic activities
8	HT-29 cells	Spherical to oval	Gold nanoparticles size between (7.2–46.9 nm)	Synthesized using Caulerpa racemosa extract through a bioreduction process	Shows anticancer activity against HT-29 cells and antibacterial effect against A. veronii and S. agalactiae
9	Not specified(In vivo study in rats)	Spherical	Gold nanoparticles(12 nm)	Synthesized using the citrate reduction method and conjugated with peptides, labeled with [18F]-fluorobenzoate	Potential for imaging (PET), drug delivery and Alzheimer’s disease treatment
10	HepG2 cells	Spherical, Triangular, and hexaganol	size range ∼31 ± 1.6 nm	Synthesized using Eclipta Prostrata leaf extract and HAucl4 reduction	Potential use in cancer therapy, drug delivery and antimicrobial treatments
11	Not specified	Anisotropic	ranges between 10–18 nm	Rapid green synthesis using Adiantum philippense L. frond extract, reduction of tetrachloroauric acid at pH11	Potential for cancer therapy and antimicrobial treatments.
12	MCF-7 and NIH3T3 cells	Spherical	35 ± 16 nm	Synthesized using the Citrate reduction and precipitation method	Potential applications in drug delivery, cancer therapy, and biomedical imaging
13	Not specified	Spherical	Not specified	Synthesized using Colloidal methods for stable and functionalized structures	applications in medical diagnostics and imaging, MRI contrast enhancement
14	Not explicit but potentially in cancer cells and inflammatory cells	Spherical	20–30 nm	Synthesized using Capsicum annum fruit extract	potential use in cancer treatment, oxidative stress management, and inflammation reduction,
15	Various human and bacterial cells	Shapes include spherical, rod-shaped, and star-shaped nanoparticles	Sizes typically range from 1–100 nm	Green synthesis using plant extracts, microbial synthesis, and chemical reduction methods	Used in drug delivery, cancer therapy, bioimaging, biosensors, wound healing, and antimicrobial treatments.

4. Toxicity of AuNPs

Gold nanoparticles showed potential use in biomedical, but their toxicity can vary depending on the size, shape, surface charge, and functionalization (Table and Figure ). Smaller particles penetrate easily leading to oxidative stress, inflammation, or genetic damage. Surface modification improves the toxicity of AuNPs. Gold nanoparticles synthesized using melatonin reduced the toxicity and enhanced biocompatibility. Melatonin served as both a reducing agent and stabilizing agent, producing uniform nanoparticles with antioxidant properties. They show minimal cytotoxicity in cell viability assays by interacting safely with biological environments. This approach can reduce the negative impacts of AuNPs, depicting safer use in medical applications like drug delivery and imaging. Another study examines the size and surface chemistry of gold nanoparticles influencing their impact on neurons. The particles also build up in cell structures, disrupting their function. They highlight the importance of nanoparticles for safe use in treatments involving the nervous system. Biosynthesized gold nanoparticles using plant extracts exhibit reduced cytotoxicity due to their natural capping agents. Biocompatibility and eco-friendly synthesis make them suitable for biomedical applications. Tables , , and depicts toxicity of AuNPs tested on various mice models, Zebrafish models and on various cell line.

20. Factors Influencing the Toxicity of Gold Nanoparticles (AuNPs) as Described in the Literature.

factor	description & influence on toxicity
1. Assay Interference	In vitro assays (MTT, LDH, CFE, etc.) may give inconsistent results due to differences in AuNP size, shape, dose, surface chemistry, and cell types, limiting reliable toxicity evaluation.
2. Size	Smaller AuNPs (<5 nm) often show higher toxicity − due to larger surface area-to-volume ratio, increasing interaction with biomolecules; however, effects may vary between in vitro and in vivo models.
3. Shape	Spherical AuNPs tend to be more toxic than nanorods, often due to residual synthesis agents like CTAB.
	Spherical AuNPs are more commonly used due to ease of synthesis and lower toxicity.
4. Surface Chemistry	Functionalization (e.g., PEG, peptides, , nucleic acids) affects stability, targeting, and uptake.
	Surface coatings reduce aggregation and can minimize toxicity or alter cellular responses and biodistribution.
5. Surface Charge	Positively charged AuNPs show higher cellular uptake and toxicity due to interactions with negatively charged membranes,
	Neutral or negatively charged particles are generally less toxic.
6. Aggregation State	Aggregation increases toxicity; influenced by surface charge, size, and coating agents.
	Preventing aggregation (e.g., via PEG, BSA) reduces cytotoxicity. ,
7. Interactions with Media/Fluids	Proteins and salts in biological fluids form a protein corona, altering AuNPs’ charge, size, and behavior. ,
	This may reduce targeting ability, trigger immune responses, or influence biodistribution. ,
8. Sex Differences	In vivo toxicity may differ between sexes due to hormonal, anatomical, and metabolic differences.
	Some studies reported higher liver toxicity in male mice.
9. Dose	Toxicity is dose-dependent; higher doses may not reflect clinical relevance. Low-dose models offer better human relevance, and effects vary with route and exposure duration.
10. Route of Administration	IV injection generally causes lower toxicity; oral and intraperitoneal routes may cause more damage due to mucosal absorption and systemic exposure.
11. Uptake & Biodistribution	Uptake depends on particle size, shape, charge, and coating. ,
	Smaller particles show wider tissue distribution; liver and spleen show highest accumulation, which may or may not correlate with toxicity.
12. Clearance & Excretion	Particles <6 nm are cleared renally; larger particles are cleared hepatobiliary or retained in RES (e.g., liver, spleen).
	Surface chemistry (e.g., PEGylation) can improve clearance and reduce long-term toxicity.

18.

Toxicity mechanism of gold nanoparticle. Reprinted with permission from ref . Copyright Elsevier B.V.

21. Toxicity of AuNPs in Mice.

Sr. No.	organism	method of synthesis	particle size & shape	dosage & route	exposure duration	capping/reducing agent	ζ-potential	observed toxic effect	outcome	references
1	Male albino rats	Citrate reduction	Gold nanoparticles (AuNPs): 13 ± 4 nm, spherical	570 μg/kg/day, intraperitoneal	28 days	Citrate (reducing + capping agent)		Decreased body and testicular weight, reduced testosterone, sperm count/motility, abnormal sperm morphology, histopathological changes (seminiferous tubule disruption, Sertoli and Leydig cell degeneration, mitochondrial damage)	Reversible reproductive toxicity (partial recovery at 30 days, near-complete at 60 days)
2	Mice	Citrate & NaBH4 reduction	Gold nanoparticles (AuNPs): 3–100 nm, spherical	8 mg/kg/week, intraperitoneal	21 days	Sodium citrate		Sizes 8–37 nm caused systemic toxicity (fatigue, weight loss, fur changes, spinal deformities, high mortality); histopathology revealed liver Kupffer cell proliferation, lung damage, splenic disorganization	Toxicity was size-dependent; particles <8 or >37 nm showed no adverse effects. Immunogenic peptide coating reduced toxicity
3	Mice	Citrate reduction	Gold nanoparticles (AuNPs): 2, 40, 100 nm, spherical	5 doses, intratracheal	3 weeks	Citrate	Negative	No significant toxicity; 2 nm particles showed minor liver accumulation; larger particles remained lung-localized; macrophage uptake was prominent	Translocation was size-dependent; nanoparticles were largely retained in lung macrophages with negligible systemic toxicity
4	BALB/c mice, F344 rats	Commercial	Gold nanoparticles (AuNPs): 15 nm, spherical	1000 mg/kg, intravenous, single dose	Acute	Citrate & PEG	Negative	Mice: liver granulomas, IL-18 elevation, no lethality; Rats: spleen accumulation, fecal excretion, partial mortality	Demonstrated species-specific biodistribution and immunogenicity; inflammatory but nonlethal response in mice, higher toxicity in rats
5	Wistar rats	Citrate reduction	Gold nanoparticles (AuNPs): 10, 30, 60 nm, spherical	Intravenous (dose not specified)	-	Citrate		Smaller AuNPs (10 nm) caused higher DNA damage, oxidative stress, nuclear localization; accumulation noted in liver, spleen, kidney, and intestines; inflammation and tissue injury increased with smaller size	Demonstrated size-dependent toxicity; 10 nm particles induced stronger genotoxic and inflammatory effects compared to larger ones
6	Rats	Citrate reduction	Gold nanoparticles (AuNPs): 20 nm, spherical	0.01 mg/kg, intravenous	2 months	Trisodium citrate		No overt toxicity; however, liver and spleen showed gene expression changes related to metabolism, detoxification, and immune function; minimal testis accumulation; no brain distribution	Demonstrates long-term biodistribution with subcellular, organ-specific effects without clinical toxicity
7	C57BL6/J mice	Biosynthesis (P. pterocarpum leaf extract)	Gold (b-Au-PP): 54.2 nm, spherical (hydrodynamic)	Daily, intraperitoneal (7 days)	7 days	P. pterocarpum (biocapping/reducing)	–21.1 mV	No significant hematological, biochemical, or histological toxicity; nanoparticles were stable and well-tolerated	Demonstrated excellent short-term biocompatibility; biosynthesized AuNPs show potential for biomedical applications
8	C57/Bl6 mice	Antibody conjugation	Gold (AuNPs-Cetuximab) Core: 4–5 nm; Total: ∼26 nm, spherical	90 μg Au/mouse, intravenous, single dose	Up to 6 months	Poly allylamine, Cetuximab	–7.04 mV	No acute toxicity in major organs up to 4 weeks; kidney casts and splenic apoptosis observed at 6 months	Good short-term safety profile; potential long-term organ-specific effects warrant further evaluation
9	Wistar rats	Citrate reduction	Gold (AuNPs) 10, 50, 100, 250 nm, spherical	1 mL/rat, intravenous, single dose	24 h	Citrate		No overt systemic toxicity; 10 nm particles distributed to multiple organs including brain; larger particles mostly confined to liver, spleen, and blood	Biodistribution was size-dependent; smaller particles exhibited broader organ penetration without acute toxicity
10	Mice, fibroblast cells	Chemical or green synthesis	Gold nanoparticles (AuNPs): 25–30 nm, spherical	Systemic or dermal		Phytochemicals (capping/reducing agents)		ROS production, inflammation, apoptosis, DNA damage, epigenetic alterations, cellular dysfunction	Small size, crystalline structure, and surface charge influenced toxicity; surface modification recommended to reduce toxicity
11	Male Wistar rats	Not specified; characterized by TEM and UV–vis	Gold nanoparticles (AuNPs) 5–50 nm, spherical	25–250 mg/kg, intramuscular or intravenous				Reduced testosterone, altered liver enzymes (ALT, AST, ALP), altered kidney markers (urea, creatinine), histological changes in testes, oxidative stress, hormonal disruption, cellular damage	Tissue accumulation and hormonal imbalance highlight reproductive and hepatic toxicity concerns; size and surface chemistry influenced outcomes
12	Mice	Turkevich method	BSA-coated AuNPs ∼ 20 nm, spherical	1 mg/kg, intravenous (IV)	Up to 120 days	Bovine Serum Albumin (BSA)		Accumulation in liver, spleen, kidneys; kidney inflammation; liver/spleen fibrosis; fibronectin expression; inflammatory gene upregulation	Long-term retention led to subchronic toxicity; fibrotic changes in organs noted.
13	Pc-Au NCs Study (Rats)	Green synthesis (potato extract)	Gold nanoparticles: Spherical, 12 nm (AuNPs), 20 nm (Pc-Au NCs)	10 μg/kg/day, IP	3 weeks	Phytochemicals (potato extract), Phthalocyanine	–22.7 mV (AuNPs), –19 mV (Pc-Au NCs)	No toxicity; anti-inflammatory and protective effects	Safe profile at low dose
14	Male BALB/c mice	Citrate reduction	Gold Nanoparticles: Spherical; 10 nm (GnP10), 50 nm (GnP50), 100 nm (GnP100)	4 mg/kg IV (tail vein), alone and with drugs (cisplatin, paraquat, 5-ASA)	24 h	Citric acid	Negative (due to citrate coating)	GnP10: Strong nephrotoxicity when coadministered with drugs (↑IL-6, BUN, Cr); GnP50: mild effects with 5-ASA; GnP100: no toxicity, even with drugs	Toxicity was size-dependent: smaller particles (GnP10) showed strong interaction and toxicity; larger ones (GnP100) were biocompatible even with toxic drugs
15	Male ICR mice	Turkevich method	Gold nanoparticles (AuNPs): 13.5 nm, spherical	137.5–2200 μg/kg; oral, intraperitoneal, and intravenous (tail vein) routes	14–28 days	Citrate	Not numerically stated; negative due to citrate	Low doses: no toxicity; High doses: reduced body weight, RBC count, spleen index (especially oral/IP). IV had least toxicity	Toxicity was dose- and route-dependent; tail vein was safest, supporting its use for biomedical applications
16	Male albino rats	Commercial (citrate-stabilized)	Gold nanoparticles (GNPs): 10 nm, spherical	20 μg/kg, intraperitoneal injection	7 days	Citrate	Negative (citrate buffer)	Oxidative stress, lowered antioxidants (SOD, GST), decreased neurotransmitters, brain inflammation and damage	Sulforaphane reversed toxic effects by boosting Nrf2 activity, antioxidants, and neural function
17	Swiss albino mice	Sodium borohydride reduction	Thiol-PEG capped gold nanoparticles ∼4.5 nm, spherical	IV (dose not specified), single/acute exposure	Not specified (short-term)	Thiol-functionalized triethylene glycol	Not specified; improved stability	No cytotoxicity (in vitro), no histological or biochemical toxicity; distributed in liver, kidney, tumors	High safety; favorable biodistribution and nontoxic even in tumor-bearing mice at high concentrations
18	Male C57/BL6 mice	Citrate reduction (HAuCl4)	Gold nanoparticles (GNPs) ∼12.5  ±  1.7 nm, spherical	40, 200, 400 μg/kg/day, intraperitoneal injection	8 days	Sodium citrate	–53 mV	No toxicity; normal behavior, weight, serum biochemistry, hematology, histology, despite organ accumulation.	Dose-dependent bioaccumulation occurred, but no adverse effects; supports therapeutic applications.

22. Toxicity in Zebrafish.

Sr. No.	organism	particle size & shape	method of synthesis	dosage & route	exposure duration	capping/reducing agent	ζ-potential	observed toxic effect	outcome	references
1	Zebrafish embryos	Citrate/TPPMS AuNPs; 3–100 nm; spherical	Citrate reduction, tannic acid	0.25–250 μM; waterborne	4–120 hpf	Uncapped/TPPMS		<3% mortality, no morphological effects	Inert; low toxicity
2	Adult zebrafish	Biogenic AuNPs (Acalypha indica); <30 nm; spherical	Leaf extract (green synthesis)	9.7–58.2 mg/L; waterborne	96 h LC50: 41 mg/L; 14 days	Plant phytochemicals		Minimal gill/liver changes; no ROS or genotoxicity	High biocompatibility
3	Zebrafish embryos	TMAT-AuNPs; 1.3 nm; spherical	Ligand exchange	0.08–50 mg/L; injection		TMAT	Positive	Eye/behavioral defects, apoptosis, axonal inhibition, gene suppression	Surface charge key to toxicity
4	Zebrafish embryos	BRP-AuNPs; 6.34 ± 2.01 nm; spherical	Green synthesis with Brazilian red propolis	Dose-dependent; waterborne		Natural BRP ligands	–17.2 mV	Oxidative stress, developmental delays, morphology anomalies	Green synthesis reduces but not eliminate toxicity
5	Zebrafish embryos, ZF4	GO–Au nanohybrids; AuNPs embedded in GO	AuNPs into graphene oxide	≤150 μg/mL; cell/embryo exposure		GO–Au hybrid		Lower ROS/apoptosis than GO alone; improved dispersion	Safer nanohybrid alternative
6	Zebrafish embryos	Au@CT; 7.6 ± 2.2 nm; spherical	Marine algae extract (green synthesis)	1.25–2.5 μM; waterborne		Plant-based	–24.6 mV	Low embryotoxicity; minor heart rate/epiboly effects	Safer vs raw extract/Au@CB
7	Zebrafish embryos	Spinacia oleracea AuNPs; 20–30 nm; spherical	Leaf extract; HR-TEM, UV–Vis	Up to 300 μg/mL; waterborne		Plant phytochemicals	Likely reduced	100% mortality at high dose; moderate anomalies; better than AgNPs	Moderate toxicity; better than silver
8	Transgenic zebrafish	Functionalized AuNPs; 10–100 nm; spherical	PEG, TNFa, NHS/PAMAM	IV injection		PEG, TNFa, PAMAM	Variable	Kidney/gut uptake; no oxidative or renal toxicity	High uptake; low acute toxicity
9	Zebrafish embryos	Citrate AuNPs; 11.6 nm; spherical	Citrate reduction	Up to 1.2 nM; passive diffusion via chorion	120 hpf	None reported		24% mortality; 2% deformities; 74% normal	Random accumulation affects outcomes
10	Zebrafish (larvae/adults)	Casein-coated AuNPs; ∼5 nm; spherical	NaBH4 reduction with casein	Intracardial injection		Casein protein	Neutralized	Crossed BBB; no toxicity; reversed Aβ-induced damage	Potential for neurotherapy
11	Zebrafish embryos	Large AuNPs; 86.2 ± 10.8 nm; spherical	Citrate reduction	0–20 pM; aquatic incubation		Not reported	Implied stable	>96% developed normally; minimal deformation	Large particles may be more biocompatible
12	Zebrafish embryos	Ammannia baccifera AuNPs; size not reported; spherical	Green synthesis	Dose-dependent; waterborne		Plant phytochemicals	–25.801 mV	Accumulation; immune gene activation; moderate mortality	Dose threshold important
13	Zebrafish embryos	Drug-loaded AuNPs; 20–30 nm; spherical	Sunlight-mediated with 6-MP, folic acid	Immersion		Nutrient agar + drugs	Likely reduced	Minimal toxicity; low stress from fluorescence and heartbeat	Surface function lowers embryonic stress
14	Zebrafish embryos	Pectin-mediated AuNPs; 4–18 nm; polymorphic/polydisperse	Green synthesis using pectin	Dose-dependent; waterborne		Pectin	–33.4 mV	Higher doses increased mortality; minimal genotoxicity at low doses	Important to control concentration

23. In Vitro Toxicity of AuNPs Tested on Various Cell Lines.

Sr. No.	organism	particle size & shape	method of synthesis	dosage & route	exposure duration	capping/reducing agent	ζ-potential	observed toxic effect	outcome	references
1.	BEAS-2B, HEK 293, CHO	AuNPs (14 and 20 nm, spherical)	Citrate reduction	In vitro, various doses	24 h	Citrate	–30 mV (typical)	Mild cytotoxicity in BEAS-2B/HEK 293, higher in CHO	Uptake rapid; size/toxicity correlation weak
2.	A549, NCIH441	AuNPs (27–28 nm, spherical)	Tetrachloroauric acid + sodium citrate	0.7 mM, in vitro	Up to 72 h	Sodium citrate	–13 mV (before) to –12 mV (after)	Viability >60%; A549 more sensitive	Surface chemistry affects toxicity
3	DU145, MDA-MB-231, L132	Aurovist (1.9 nm, spherical)	Commercial	Up to 13,300 μg/mL	24 h	Not specified	Highly negative (typical)	Tumor-specific apoptosis, ROS generation	High selectivity, dose- and cell-specific toxicity
4	Vero, NIH3T3, PK-15, MRC-5	Commercial AuNPs (∼10–40 nm, nanorod)	Commercial					Apoptosis/autophagy/cell cycle arrest	Cell-line specific stress responses.
5	HT29	AuNPs (<100 nm, spherical)	Not specified	2–10 μg/mL			Measured, not stated	Moderate cytotoxicity, no DNA damage	Oral route implications suggested
6	MCF-7, HepG2, HCT-116	AuNPs (3.4–6.9 nm, spherical/triangular)	Green synthesis using flaxseed extract	In vitro, various doses		Lignans (flaxseed)	Implied stable	Dose-dependent tumor cytotoxicity	Effective anticancer potential
7	MRC-5, Mice	AuNPs (15–20 nm, spherical)	Chemical synthesis	In vitro + oral (mice)		Aspartic acid, citrate, BSA	Varied; but not given in numeric	GNPA/GNPC mild liver/kidney toxicity; GNPB nontoxic	Capping agent crucial for stability
8	DU-145	AuNPs (20 and 200 nm, spherical)	Commercial	In vitro, high doses	48–72 h	Mannose-functionalized and uncoated		20 nm more toxic; enhanced uptake	Size & surface mod. affect diagnostics
9	MCF-7	AuNPs (∼20 nm, spherical)	Citrate reduction + alginate coating	12 nM, in vitro		Sodium alginate	Stable postcoating	Viability ∼29%; oxidative stress	pH-responsive release mechanism
10	Hepatocellular carcinoma (C3A)	AuNPs (40 and 80 nm, spherical)	Chemical	Not specified		BPEI, LA, PEG	Charge dependent	PEG-AuNPs biocompatible; BPEI toxic	Corona formation modulates toxicity
11	MCF-7, MDA-MB-231	AuNPs (∼narrow, spherical)	Green synthesis via *Curcuma caesia*	In vitro, low μg/mL	Not specified	Phytochemicals	Implied stable	Selective toxicity to cancer cells	Apoptosis-mediated effect
12	3T3 Fibroblasts	AuNPs (∼9 nm, spherical)	Chemical	In vitro		Glutathione	Implied stable	Minimal cytotoxicity; low ROS	Dual antioxidant and nanocarrier role
13	HepG2	AuNPs (118 ± 72 nm, irregular)	Green synthesis (*Verbascum speciosum*)	In vitro, various doses	Time/dose dependent	Phytochemicals	Not mentioned	Antiproliferative effects	Likely apoptosis/ROS-based
14	Not specific	AuNPs (<100 nm, various)	General/Review	Environmental	Chronic/unknown	Various	Unstable small ones risky	Oxidative stress, inflammation	Smaller poorly capped particles most toxic
15	HepG2, MCF7, HeLa, Vero	AuNPs (25–35 nm, spherical)	Green synthesis using *Cassia roxburghii*	Up to 75 μg/mL	In vitro	Phytochemicals	Not disclosed	Selective toxicity (IC50 30–50 μg/mL)	DNA fragmentation; apoptotic mechanism
16	THP-1	AuNPs (<50 nm, spherical)	Green synthesis using *Centaurea behen*	In vitro		Phytochemicals		IC50 ∼ 25 μg/mL	Mild antioxidant, strong anticancer
17	DU-145, PC-3, MDA-MB-231	GNR & GNS (shape-specific)	Chemical (CTAB & citrate)	0.55–34.5 μg/mL	In vitro	BSA	Surface coating dependent	GNR > GNS cytotoxicity; shape-specific	GNR reduced migration; therapeutic potential
18	8 Mammalian Cell Lines	Au-PEI (∼10–20 nm, cationic) & Au-PEG-COOH (anionic)	Chemical	Various doses	In vitro	PEI, PEG	Positive (PEI), Negative (PEG)	PEI cytotoxic; PEG biocompatible	Model systems for uptake/toxicity
18	H69	AuNPs (<40 nm, spherical)	Green (*Coffea arabica*), drug-loaded	In vitro, Dox-conjugated		Phytochemicals + MPSNa/MUA	Stable	Enhanced cytotoxicity vs free Dox	pH-dependent release; lower off-target toxicity
19	HepG2	AuNPs (10–50 nm, polydispersed)	Biosynthesis via *Enterococcus sp.*	In vitro, 5–15 μg/mL		Bacterial enzymes	-	Mitochondrial apoptosis; ROS, cytochrome c	Effective green therapy
20	HepaRG, Primary Rat Hepatocytes	AuNPs; 15 and 60 nm spheres/stars	Turkevich & Bastus	In vitro with/without FBS		Citrate, MUA	Inferred via stability	Smaller, citrate-capped AuNPs more toxic; MUA-coated safer	Serum impacts toxicity and uptake

24. Toxicity of AuNPs with Organism, Cell Type, Toxicity Effects, and Significance.

Sr. No.	organism	cell type	particle shape	toxicity and effects	particle size of particular type	significance	references
1	Zebrafish embryos	Embryonic cells	Spherical	Dose-dependent toxicity with significant effects above 40 mg/L; toxicity varies with coating agents and concentrations.	Biogenic gold nanoparticles (BRP-GNPs): Size 6.34 ± 2.01 nm (TEM), Hydrodynamic diameter 58.79 ± 1.69 nm	The need for concentration and coating agent considerations in assessing nanoparticle toxicity, especially for early stage developmental organisms.
2	Mice	Phagocytic cells (liver and spleen), Renal cells	Spherical	Accumulation in liver, spleen, and kidneys due to slow clearance, leading to mild structural changes and increased inflammatory markers.	BSA-coated gold nanoparticles (BSA-AuNPs): Gold core size 18.3 ± 1.5 nm; Hydrodynamic diameter: 38 ± 5 nm in water, 48 ± 23 nm in 5% glucose solution	Emphasizes the impact of slow nanoparticle clearance and accumulation in organs, raising concerns for long-term exposure and inflammatory responses.
3	Drosophila melanogaster	Hemocytes	Spherical	Citrate-capped AuNPs (15 nm) cause genotoxicity, leading to DNA damage and hereditary mutations.	Citrate-capped gold nanoparticles (Size: 15 nm)	It shows the potential genotoxic effects of nanoparticles, underlining the need for careful evaluation of their environmental and biological impact.
4	Wistar rats	Hepatocytes, Renal cells, Reproductive cells	Spherical	Accumulate in liver, kidneys, and reproductive organs, causing oxidative stress, inflammation, and organ damage.	Gold nanoparticles with a size of 5–50 nm	The toxic potential of nanoparticle accumulation in organs, with implications for long-term exposure risks in mammals.
5	DU-145, PC-3 (Prostate), MDA-MB-231 (Breast)	Cancer cells (Prostate, Breast)	Spherical	GNR-BSA had higher uptake and inhibited migration; GNS-BSA had minimal impact.	GNR-BSA (127.95 nm), GNS-BSA (35 nm)	The toxic potential of nanoparticle accumulation in organs, with implications for long-term exposure risks in mammals
6	Vero cells (African green monkey kidney cells)	kidney cells	Spherical	AuNPs antibacterial activity against drug-resistant bacteria.	Biosynthesized gold nanoparticles (Size: 7.1–40.0 nm, average 23.2 ± 10.7 nm)	The potential of AuNPs in combating bacterial resistance, with varying effects based on particle size.
7	RAW 264.7, HaCaT, NHDF, Mice	Macrophages, Keratinocytes, Fibroblasts	Spherical	Bifi-CKAuNPs showed no significant cytotoxicity, inhibited NF-κB/MAPK signaling, and reduced inflammation in mice lung, liver, and kidneys.	Bifi-CKAuNPs size is 10–25 nm	Anti-inflammatory applications with minimal cytotoxicity, highlighting the role of nanoparticles in modulating immune signaling pathways.
8	Male albino rats	Hepatocytes, Renal cells, Glial cells	Spherical	Accumulated in liver, kidney, and brain, with higher toxicity at 100 mg/kg, causing hepatic and renal changes. 10 mg/kg showed minimal toxicity.	Gold nanoparticles (Size: 7.8 nm)	Highlights the toxicity of high nanoparticle concentrations, stressing the need for dose optimization in therapeutic applications.
9	BEAS-2B, CHO, HEK 293	Human embryonic kidney cells	Spherical	Showed minimal toxicity, with CHO cells most sensitive to 20 nm particles. Uptake varied by cell type and time.	Citrate-stabilized gold nanoparticles (Size: 14 nm, 20 nm)	Emphasizes the effect of particle size and cell type on nanoparticle toxicity, suggesting the importance of size and cellular context in biomedical applications.
10	Rat ovarian granulosa cells	Ovarian granulosa cells	Spherical	Accumulated in mitochondria, lipid droplets, and vacuoles, causing structural changes. Short-term exposure increased estrogen secretion, but levels dropped after 24 h due to mitochondrial damage.	10 nm AuNPs in concentration 2.85 × 10 10/mL	Shows endocrine disruption from nanoparticles, especially in reproductive health, emphasizing the need for safety evaluations in reproductive biology.
11	DLA, EAC, Swiss albino mice	Normal peritoneal cells	Spherical	AuNPs showed no significant toxicity at 100 mM, with no metabolic changes, organ damage, or tissue abnormalities in mice.	Gold nanoparticles (Size: 4.5 nm)	Highlights the relatively low toxicity of small AuNPs at specific concentrations, suggesting potential biocompatibility for therapeutic uses.
12	Human (Homo sapiens)	HEK 293 cells	Spherical	Phosphine-stabilized AuNPs blocked hERG channels, indicating cardiotoxicity; Thiol-stabilized AuNPs had no effect.	Gold nanoparticles (1.4 nm), phosphine-stabilized and thiol-stabilized	Highlights the significance of surface stabilization in nanoparticle toxicity, particularly in relation to cardiovascular health.
13	Human (Homo sapiens) and Mice	MRC-5 human fibroblast cells	Spherical	No significant cytotoxicity in MRC-5 cells; GNPC and GNPA induced mild hepatotoxicity and nephrotoxicity in mice; GNPB was the most biocompatible; Stability varied with capping agents, influencing biomedical potential	Gold nanoparticles (15–20 nm), aspartic acid-capped (GNPA), trisodium citrate-capped (GNPC), and bovine serum albumin-capped (GNPB)	Emphasizes the importance of nanoparticle surface chemistry in biocompatibility and potential biomedical applications, such as drug delivery and therapy.
14	Human (Homo sapiens)	HT29 human colorectal adenocarcinoma cells	Spherical	Intracellular accumulation but did not induce significant DNA damage or apoptosis; silver and metal oxide nanoparticles caused higher cytotoxicity	Gold nanoparticles (32 nm)	Suggests that AuNPs may have lower toxicity Compared to other metal nanoparticles, emphasizing the importance of particle size in toxicity assessments.
15	Mouse	Balb/3T3 fibroblast cells	Spherical	AuNPs showed cytotoxicity at ≥50 μM after 72 h, disrupted actin cytoskeleton, caused cell contraction and morphology changes; 15 nm AuNPs were nontoxic; Both sizes were internalized via endocytosis, accumulating in endosomal compartments, with clathrin-mediated uptake and clathrin degradation observed	Citrate-stabilized AuNPs of 5 and 15 nm diameter in concentrations of 2, 10, 20, 39.2, 58.8 g/mL	Highlights size-dependent toxicity and cellular uptake mechanisms, stressing the need for further research on nanoparticle-cell interactions
16	Human (Homo sapiens)	HepG2 and PBMCs cells	Spherical	PAMAM dendrimers induced ROS production, DNA damage, and reduced cell viability; HepG2 cells were more sensitive than PBMCs, suggesting potential for cancer therapy with minimal effects on healthy cells	Gold nanoparticles (7–20 nm), citrate-coated and PAMAM dendrimer-coated	Demonstrates the role of surface chemistry in nanoparticle toxicity, suggesting cancer therapy applications with minimal toxicity to healthy cells.
17	Mammalian (CHO K1, A549, HeLa, NIH 3T3, HaCaT, SK-MEL-28, BEAS-2B, HepG2)	Various cell lines	Spherical	low toxicity up to 50 μg/mL; Toxicity was dose-dependent and linked to surface charge, with positively charged Au-PEI nanoparticles exhibiting higher cytotoxicity	Gold nanoparticles (11.5–14.3 nm), PEI- and PEG-COOH-functionalized	Emphasizes the role of surface charge and functionalization in modulating nanoparticle toxicity and their potential in targeted therapies.
18	Human (Homo sapiens)	Hela cells	Spherical	No significant cytotoxicity at high concentrations; glucose-functionalized nanoparticles showed higher uptake than PEG-coated ones	Gold-coated iron oxide nanoparticles (4.6–5.4 nm), PEG- and glucose-functionalized	Highlights the importance of surface functionalization for improving nanoparticle uptake and biocompatibility in biomedical applications
19	Human cells	LNCaP prostate cancer and HFF healthy fibroblast cells	Spherical	Concentration-dependent cytotoxicity; reduced cancer cell viability, but also affected healthy cells at higher doses; Protein corona reduced cytotoxicity on cancer cells	Gold nanoparticles (60–250 nm)	Highlights the importance of nanoparticle size, surface Interactions, and biological environment in toxicity, suggesting a need for tailored nanomedicine approaches in cancer therapy
20	Human cells	HaCaT human keratinocyte cells	Spherical	Positively and negatively charged AuNPs caused mitochondrial stress, apoptosis, and DNA damage; neutral AuNPs induced necrosis; Cytotoxicity was dose-dependent	The average particle sizes are reported as follows: 1.8 ± 0.7 nm for neutral particles (MEEE), 1.6 ± 0.8 nm for positive particles (TMAT), and 1.8 ± 0.7 nm for negative particles (MES).	Highlights the role of surface charge in nanoparticle-induced cytotoxicity, emphasizing the need for safety evaluations in biomedical applications.
21	Mytilus edulis (Blue Mussels)	Digestive gland cells	Spherical	It caused lipid peroxidation, decreased thiol-containing proteins, and reduced lysosomal membrane stability, indicating oxidative stress.	Gold nanoparticles (5.3 ± 1 nm)	Highlights the stronger oxidative stress induced by smaller AuNPs, emphasizing the need for further research on nanoparticle toxicity in marine organisms.
22	Rats	Liver and spleen cells	Spherical	Rapidly cleared from bloodstream, accumulated mainly in liver and spleen. After 28 days, spleen atrophy and mild anemia observed, especially with CALNN-coated AuNPs.	Gold nanoparticles (∼20 nm), Citrate- and pentapeptide CALNN-coated	Surface chemistry influences toxicity more than biodistribution, emphasizing the need for improved nanoparticle coatings for biomedical applications.
23	Nicotiana xanthi (Tobacco)	Seedling cells	Spherical	Smaller AuNPs (3.5 nm) entered roots and translocated to leaves, causing necrosis. Larger AuNPs (18 nm) remained on root surfaces, indicating size-dependent uptake and toxicity.	Gold nanoparticles (3.5 and 18 nm)	Highlights the importance of nanoparticle size in plant uptake and toxicity, emphasizing the need for further research on AuNP interactions with plants and their environmental impact.
24	Daphnia magna (Water flea)	Aquatic organism	Spherical	Toxicity varied with surface charge; positively charged PAH–AuNPs caused the highest mortality, while Cit–AuNPs and MPA–AuNPs were less toxic. Chronic exposure affected reproduction, indicating long-term impacts.	Gold nanoparticles (4–5 nm), PAH-, Cit-, and MPA-coated	Highlights the importance of surface chemistry and charge in nanoparticle toxicity, emphasizing the need for environmental risk assessment in aquatic ecosystems.
25	BALB/c mice	Various organs (Lungs, Kidneys, Liver, Spleen)	Spherical	No significant toxicity, immune response, or renal stress even at 60 μM concentrations. Rapid excretion through kidneys; initial accumulation in lungs and kidneys, later shifting to liver and spleen.	Glutathione-coated gold nanoparticles (1.2 ± 0.9 nm)	supports the use of glutathione as a biocompatible alternative to PEG for nanomedicine and targeted drug delivery.
26	Mice	Macrophages	Core–shell	Rapidly cleared by macrophages; mild inflammation and oxidative stress, resolved in 2 weeks. No major toxicity in biochemical or histological analyses. Rectal administration showed no systemic absorption or toxicity.	Raman-active silica-gold nanoparticles (60 nm gold core, 30 nm silica shell, total ∼120 nm)	Suggests that while intravenous exposure may cause temporary immune activation, localized administration could be safer for biomedical applications.
27	Human Cells	Lymphocytes, Fibroblasts	Spherical	Higher micronuclei formation in lymphocytes, slight DNA damage in fibroblasts; size- and concentration-dependent toxicity	Citrate-capped gold nanoparticles (AuNPs) with sizes of 15 and 47 nm	Greater genotoxicity in smaller 15 nm particles; critical for assessing biomedical applications of gold nanoparticles
28	3T3 Cells	Fibroblast Cells	Spherical	GSH-AuNPs significantly reduced oxidative stress compared to glutathione alone and nonstabilized AuNPs	Glutathione-stabilized gold nanoparticles (GSH-AuNPs) with an average size of 9 nm	Potential for reducing oxidative stress in ROS-related conditions, promising for biomedical applications

Many studies have been carried out to replace these nanoparticles with biobased materials to address toxicity-related issues in various applications.

4.1. Toxicity of AuNPs on Human Health

Gold nanoparticles (AuNPs) exhibit unique biomedical potential, yet their toxicity remains a concern, as size, shape, surface chemistry, and concentration significantly influence cellular uptake, oxidative stress, and cytotoxic responses. Table depicts various factors affecting toxicity of AuNPs. Gold nanoparticles (30, 50, 90 nm) exhibited dose- and time-dependent cytotoxicity in HL-60 and HepG2 cells. Cytotoxicity correlated with oxidative stress markers, including increased ROS, GSH depletion, and altered SOD (Superoxide Dismutase) activity. NAC conferred partial protection, particularly with 30 nm particles. Effects varied slightly with nanoparticle size and cell type. Gold nanoparticles (∼20 nm) with citrate or 11-MUA coatings were tested on HepG2 cells. Both showed no cytotoxicity, but citrate-AuNPs induced DNA damage at low concentrations. Internalization was similar for both, highlighting surface coating’s role in AuNPs’ biocompatibility and genotoxic potential despite negligible cytotoxic effects. Surface charge significantly influenced the cytotoxicity of 1.5 nm gold nanoparticles in human keratinocyte (HaCaT) cells. Charged AuNPs induced dose-dependent toxicity, mitochondrial dysfunction, altered gene expression, and apoptosis, while neutral AuNPs caused necrosis, highlighting surface charge as a key factor in AuNP–cell interactions. In HL7702 human liver cells, 8 nm gold nanoparticles (AuNPs) triggered early cytosolic glutathione (GSH) depletion, leading to mitochondrial depolarization and apoptosis. Strong Au–S interactions initiated Bax translocation, H₂O₂ buildup, and caspase-3 activation, confirming GSH loss as a key apoptotic trigger via the mitochondrial pathway. Gold nanoparticles (AuNPs) exhibited dose-dependent cytotoxic effects on human spermatozoa, significantly reducing motility and viability at higher concentrations. AuNPs were internalized by sperm cells, indicating potential reproductive toxicity and the need for further investigation into their genotoxic effects on germ cells. AuNPs induced oxidative stress and autophagy in MRC-5 human lung fibroblasts. Increased lipid peroxidation, autophagosome formation, and upregulation of MAP-LC3, ATG7, and stress response proteins suggest AuNPs trigger oxidative damage, with autophagy acting as a protective cellular response mechanism.

4.2. Toxicity of AuNPs on Environment

Gold nanoparticles (AuNPs) are often considered less toxic than silver counterparts. Their toxicity is primarily linked to oxidative stress, disruption of cell membranes, and DNA damage. They tend to accumulate in digestive tissues, causing lipid peroxidation and disrupting in antioxidant enzyme functions. At moderate concentrations AuNP depicts low cytotoxicity, increased reactive oxygen species shows long-term threats to the environment. They depict significant environmental toxicity due to their high mobility, reactivity, and accumulation of living organisms. In aquatic systems, they are absorbed by plankton and fish, disrupting vital functions and moving through the food web. In soil, AuNPs impact plant growth and disrupts the microbial activities, altering nutrient availability. Their persistence and capacity to penetrate biological barriers that may lead to oxidative stress and DNA damage in unintended organisms. Proper regulation and eco-safe design are essential to reduce the environmental risks. Gold nanoparticles (AuNPs) are used in various industries and can enter the environment via waste streams or incorrect disposal methods. Due to their small size, they can be easily absorbed by organisms, potentially disrupting biological activities. In aquatic environments, they threaten fish and microbes by causing oxidative stress and bioaccumulation. The lack of comprehensive data on their long-term effects raises concerns, stressing the need for monitoring, product labeling, and implementing proper nanowaste management regulations to limit the environmental damage. Another study shows variable toxicity in aquatic environments, based on factors like size, shape, and surface coating. Functionalized AuNPs developed for medical applications may pose ecological hazards. In studies with microalgae and Daphnia magna, AuNPs induced oxidative stress and DNA damage. Aggregation in water does not necessarily reduce toxicity, they can move through food webs. Gold nanoparticles synthesized using Carica papaya leaf extract demonstrated an eco-friendly and effective approach for photocatalytic degradation of methylene blue, achieving 95% reduction in 90 min and 99% removal of total organic carbon. Characterization confirmed their crystalline structure, morphological, and surface characteristics.

5. Regulatory Frameworks Governing Gold Nanomaterials

Global regulations for nanomaterials differ significantly across regions. Kus-Liśkiewicz et al., in their comprehensive review, have effectively detailed the global regulatory landscape surrounding nanomaterials (Table ). They highlight the contrasting approaches across regions, noting how stringent guidelines by agencies like the FDA and EMA differ from the more lenient or absent regulations in parts of Asia and South America. The U.S. FDA and Europe’s EMA mandate stringent nanosafety evaluations for nanomaterials, particularly in food and pharmaceuticals, requiring comprehensive physicochemical, toxicological, and ecotoxicological data. In contrast, countries like China, Japan, South Korea, and Brazil often lack specific regulatory frameworks for nanomaterials in cosmetics and food. The U.S. launched the National Nanotechnology Initiative (NNI) in the year 2000 to promote nanoscience research and define “nanopharmaceuticals”. Despite ongoing advancements, major challenges remain in the classification of novel nanomaterials and the development of standardized testing methods essential for ensuring their safe commercial and clinical application. , Regulations for gold nanoparticles are gradually evolving worldwide, aiming to ensure their safe use in medical, food, and consumer products. Standardized protocols and international harmonization are crucial for effective risk assessment and commercialization.

25. Regulating Authorities That Have Defined Specific Requirements for Various Groups of Nanoproducts .

Authority Name	regulations, procedures, standardisation, and references
Joint Research Centre (JRC)	Provides evidence-based scientific support in EU policymaking process.
European Chemicals Agency (ECHA)	Implements REACH: Registration, Evaluation, Authorisation, and Restriction of Chemicals.
Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR)	Gives opinions on emerging or complex health/environmental risks needing multidisciplinary risk assessment.
Scientific Committee on Consumer Safety (SCCS)	Advises on risks of nonfood consumer products like cosmetics, toys, textiles, etc.
European Food Safety Authority (EFSA)	Publishes nanosafety guidelines for food products, recommends analytical technologies.
European Medicines Agency (EMA)	Ensures scientific evaluation and supervision of medicines for human and animal health in the EU.
U.S. Food and Drug Administration (FDA)	Protects public health through safety and efficacy of drugs, food, medical devices, and cosmetics.
Quality Supervision, Inspection, and Quarantine (AQSIQ)	Oversees commodity inspection, food safety, certification, accreditation, and standardization in China Available online: https://www.aqsiq.net
Chinese National Nanotechnology Standardization Technical Committee (NSTC)	Reviews nanomaterial standards. Available online: http://english.nanoctr.cas.cn/
Standardization Administration of China (SAC)	Establishes standards for nanomaterials and their characterization. https://www.sac.gov.cn/
Brazilian Health Surveillance Agency (ANVISA)	Regulates research, production, disposal, and use of nanotechnologies. https://www.gov.br/anvisa/pt-br

^aAdapted and modified from ref . Copyright 2021, with permission of MDPI.

6. Limitations and Future Prospects

Even though AuNPs are considered generally safe and biocompatible and also act as promising agents in biomedical fields due to their unique physicochemical properties, factors like shape, size, drug dosage, electrical charge, and purity of the gold formation can play a role in determining the cytotoxicity. The toxicity can depend on the surface chemistry of the nanoparticles, including the type and concentration of capping ligands, which can also influence their interactions with biological systems. The nanoparticles can be penetrative in nature resulting in increased threat to humans, flora and fauna, and the environment. The synthesis methods should incorporate biobased mechanisms using biobased extracts, hence minimizing the toxic effects. Also, surface modifications to the AuNPs and aggregations significantly affect their functionality and reduce their effectiveness. New researchers should aim to find advanced and green methods of synthesis that aim to improve the health and drug delivery effectiveness, improve sensing applications, incorporate models for safe environmentally friendly catalytic applications, and reduce toxicity. , AuNPs have a high potential application in next-generation electrical sensors, biomedical devices, solar energy harvesting, precision gene therapy delivery methods, advanced anticancer diagnostics, and health and sanitation applications. Future research should focus on optimizing the synthesis strategies of gold nanoparticles (AuNPs) to enhance their stability, biocompatibility, and functionality for diverse biomedical applications. − Comparative studies, such as those evaluating sonochemical − and chemical reduction methods, have demonstrated that synthesis techniques directly influence the physicochemical characteristics and antibacterial activity of AuNPs. Building on this, integrating multifunctional nanostructures such as Fe₃O₄@Au hybrids holds promise for advanced applications in drug delivery, catalysis, hyperthermia, and imaging. , Recent advancements in gold nanoparticle (AuNP) research highlight green mycosynthesis using mushrooms like Agaricus bisporus and rapid sonochemical techniques as promising routes for eco-friendly and scalable production. These methods yield stable, crystalline AuNPs with diverse morphologies and excellent catalytic and biomedical properties, including CT contrast enhancement and methylene blue degradation. Core@shell Fe₃O₄@Au structures synthesized via optimized sonochemical methods show dual-mode MRI/CT imaging potential. Sonochemistry also outperforms laser ablation in purity and colloidal stability. Furthermore, the development of eco-friendly, green synthesis methods using biological extractssuch as plant or mushroom derivativesnot only reduces environmental toxicity but also improves the therapeutic potential of AuNPs in anticancer treatments. Gold nanoparticles (AuNPs) synthesized using fresh fruiting bodies of Enoki and Shiitake mushrooms as green reducing agents exhibited diverse shapes, good stability, and crystallinity, with sizes around 72–74 nm. These mycosynthesized AuNPs showed effective catalytic degradation of Methylene Blue and potential for biomedical applications. Sonochemical methods further enhanced nanoparticle dispersion and enabled Au coating on Fe₃O₄ via acoustic cavitation. − Ultrasmall AuNPs derived from fruit peels, for example, demonstrate strong photothermal effects and enhanced biocompatibility, offering a sustainable pathway for developing next-generation nanomedicine.

6.1. Emerging Trends

Nowadays, Two-dimensional (2D) gold (Au) nanocrystals have been an area of interest for researchers. Gold nanoparticles are also used in antiaging creams or masks having particle size 5–400 nm, which enhances the collagen formation by 20–200 times. Cosmetic companies such as L’Oreal and L’Core Paris have infused gold nanoparticles in their formulations making them more effective. Recent advancements in biosensor and bioelectronics technology have opened new possibilities in biomedical diagnostics, particularly for early cancer detection. Biosensors are being used for detection of cancer with the use of advance techniques. Fox et al. has done a detailed investigation into the optimized synthesis and mechanism of formation for ultrathin, free-standing two-dimensional gold nanosheets (AuNS), which are approximately 0.47 nm thick and shows aspiring results for applications in computing, biosensing, catalysis, and healthcare. The AuNS were synthesized by one-pot, seedless reaction using chloroauric acid (HAuCl₄) reduced by sodium citrate in the presence of methyl orange (MO). Spectrophotometric and TEM analyses confirmed that MO is rapidly oxidized to form 4-diazobenzenesulfonic acid (4-DBSA), and this oxidation is crucial to directing two-dimensional growth. However, by using 4-DBSA alone can yield low-quality AuNS, showing that MO’s self-assembly and in situ oxidation synergistically assists nanosheet formation. UV–vis and other kinetic studies showed the MO degradation during the process follows pseudo-first-order kinetics and occurs within 30 s. Optimization such as delaying the addition of reagents by 30 s and slow mixing were observed for higher yields. Also, by lowering the reaction temperature from 20 °C to 4 °C can enhance AuNS yield by 16 times. The synthesis was complete within 8 h, with nanosheets observable after just 1 h. Mild centrifugation (∼1000 g) was essential to isolate the AuNS over 3D structures. The resulting AuNS showed enhanced catalytic activity compared to spherical gold nanoparticles,which makes them highly attractive for use in catalytic, enzymatic, and diagnostic applications due to their large surface area-to-volume ratio. A study conducted by Huang et al. presents a novel yet cost-effective way to develop an electrochemical biosensor for the ultrasensitive detection of hydrogen peroxide (H₂O₂), a cancer biomarker, especially used in identifying colon cancer cells. The sensor utilizes a dual-nanozyme system by combination of ultrathin 2D conductive Cu-HHTP nanosheets with densely deposited ultrafine gold nanoparticles of size 3 nm. The Cu-HHTP nanosheets provide a large surface area and active Cu–O₄ metal sites which enhances catalytic activity, while the embedded AuNPs prevent from aggregation and further amplifying the response. This synergistic nanohybrid structure offers exceptional electrocatalytic efficiency, achieving an exceptionally low detection limit of 5.6 nM and a high sensitivity of 188.1 μA cm^–2 mM^–1. This biosensor successfully tracks real-time H₂O₂ release in human colon cells, by distinguishing cancerous cells from normal epithelial cells. It exhibits strong selectivity, reproducibility, and stability. This is a promising approach for advancing early cancer diagnostics and could inspire further development of nanozyme-based biosensors in biomedical and clinical applications. Another approach is made by Rauf et al. to develop an electrochemical biosensor using laser-scribed graphene modified with gold nanostructures (LSG-AuNS) to address the limitations of bare LSG electrodes in sensitivity and biomolecule immobilization. Gold nanostructures were electrodeposited onto LSG, which enhances the sensitivity and catalytic activity almost by 2-fold compared to unmodified LSG and commercial screen-printed gold electrodes. This 3D-porous LSG-AuNS platform was utilized to fabricate an aptamer-based sensor for detection of the breast cancer biomarker HER2, achieving a detection limit of 0.008 ng/mL and a broad linear detection range of 0.1–200 ng/mL. This sensor effectively detected HER2 in undiluted human serum with high accuracy. Furthermore, the system was integrated into a hand-held electrochemical point-of-care (POC) device, controlled via a custom mobile application. Although minor nonspecific protein adsorption was noted, the platform demonstrated high potential toward biosensing applications. A highly sensitive and multiplex electrochemical biosensor was developed for detecting multiple cancer-related microRNAs (miRNAs) in plasma by utilizing a dual amplification strategy. This approach by Mohammadniaei et al. combines MXene-Ti₃C₂T _x nanosheets decorated with 5 nm gold nanoparticles (AuNPs) for enhanced signal transduction and duplex-specific nuclease (DSN)-based target recycling for signal amplification. Colorimetric biosensors rely on the visible color change of gold nanoparticles (AuNPs) in response to the presence or absence of a target analyte. This method offers a simple but yet fast approach for cancer detection. AuNPs are widely used in these sensors due to their unique optical properties, allowing for rapid and easily detectable color shifts that provide a quick visual indication of the analyte. A novel and highly sensitive colorimetric biosensor was developed by Xiao et al. using gold nanoparticle-decorated Bi₂Se₃ (Au/Bi₂Se₃) nanosheets, synthesized through simple sonication of a gold precursor with Bi₂Se₃ in water. Bi₂Se₃ nanosheets exhibit excellent electron-donating properties also it synergizes with AuNPs to enhance catalytic performance, particularly in the 4-nitrophenol (4-NP) reduction by NaBH₄. The sensor works through a smart “on–off” mechanism: its catalytic activity is briefly turned off when it comes into contact with certain cancer-related antibodies, such as anti-CEA. However, this activity switches back on when the matching antigen like CEA is present. This clever response allows the system to precisely detect cancer biomarkers. Impressively, it can identify carcinoembryonic antigen (CEA) at concentrations as low as 160 picograms per milliliter, demonstrating strong potential for use in real-world clinical diagnostics. The biosensor is versatile for detection of other cancer markers like AFP and PSA. The work introduces a promising platform for biosensing, with future potential in diagnostics, biocatalysis, and smart biomaterial systems, Currently, breast cancer detection methods like mammography, MRI, and biopsy are expensive, invasive, and time-consuming, often yielding inconclusive results. To address these limitations, Selwyna et al. developed a sensitive, label-free electrochemical immunosensor that detects breast cancer by targeting the CA 15–3 biomarker in serum. The sensor uses gold nanoparticles coated with CA 15–3 antibodies, which enhance conductivity and provide a larger surface area for biomolecular interaction. In this setup, nanoparticles were anchored onto a screen-printed carbon electrode (SPCE), and the detection of antigen–antibody interactions is carried out with electrochemical impedance spectroscopy (EIS) and a potentiostat. The immunosensor effectively identifies CA 15–3 levels within a concentration range of 5 to 75 U/mL. Ranjan et al. developed a highly sensitive electrochemical immunosensor for detection of the CD44 antigen, a key breast cancer biomarker. The sensor was based on a hybrid nanocomposite comprising graphene oxide (GO), an ionic liquid (BMIM.BF₄), and gold nanoparticles (AuNPs), all immobilized on a glassy carbon electrode (GCE). Graphene oxide (GO) provides numerous oxygen-rich functional groups that facilitate effective antibody attachment. At the same time, the ionic liquid and gold nanoparticles (AuNPs) work together to enhance electron conductivity and expand the electrode’s surface area. These properties combination creates a synergistic effect that greatly improves the overall sensitivity and efficiency of the sensor. This sensor showed the detection range of 5.0 fg/mL to 50.0 μg/mL in both phosphate-buffered saline and human serum.

7. Conclusions

In conclusion, gold nanoparticles (AuNPs) have emerged as a highly versatile and valuable nanomaterial due to their unique physicochemical properties, such as high surface area, tunable size, and excellent biocompatibility. As a multidisciplinary subdomain, nanobiotechnology epitomizes the amalgamation of sciences and technologies to produce nano-objects with profound applications. Among nanomaterials, nanoparticles particularly gold nanoparticles (AuNPs) have taken center stage due to their remarkable properties and versatility. The fascination with AuNPs stems from their unique characteristics, which span a wide range of shapes and sizes, enabling diverse and impactful applications. The type of synthesis and reaction conditions mediate how the properties of the AuNPs formed vary. The applications of AuNPs span various fields, including healthcare, drug delivery, catalysis, textiles, agriculture, sensing, and electronics, which makes them pivotal in advancing nanotechnology. The ease of functionalization allows for targeted interactions in biological systems, while their optical properties enable sensitive detection methods. In this review, we have delved deep into the role of AuNPs in nanotechnology, from different types of synthesis routes and properties to the factors affecting them, and their versatile range of applications and toxicity. Overall, nanotechnology, particularly gold nanoparticles, continues to drive scientific research and technological innovation, paving the way for transformative applications across various disciplines. Continued research is essential to optimize their synthesis, minimize toxicity, and unlock their full potential across industries.

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A.S., S.M., and N.S. contributed equally to this work. Conceptualization, S.M., A.S., T.R., N.S., N.S. R.D., and S.A.; writingOriginal draft preparation, S.A. and A.P.; writingreview and editing, T.R. and A.S.; and visualization, A.P. and S.M. All authors have read and agreed to the submitted version of the manuscript.

Large Research Project under grant number RGP2/509/46.

The authors declare no competing financial interest.