Exploring the green synthesis of silver nanoparticles using natural extracts and their potential for cancer treatment

Abstract

Silver nanoparticles (AgNPs) have attracted increasing attention in nanomedicine, with versatile applications in drug delivery, antimicrobial treatments, and cancer therapies. While chemical synthesis remains a common approach for AgNP production, ensuring environmental sustainability requires a shift toward eco-friendly, “green” synthesis techniques. This article underscores the promising role of plant extracts in the green synthesis of AgNPs, highlighting the importance of their natural sources and diverse bioactive compounds. Various characterization methods for these nanomaterials are also reviewed. Furthermore, the anticancer potential of green AgNPs (Gr-AgNPs) is examined, focusing on their mechanisms of action and the challenges to their clinical implementation. Finally, future directions in the field are discussed.

Introduction

Over the past decade, considerable efforts have focused on using nanosized materials in cancer treatment. As a result, various nanomaterial-based cancer therapies have been successfully tested in clinical settings (Heydari and Rashidipour 2015; Vanlalveni et al. 2021). Silver nanoparticles (AgNPs) have attracted considerable attention for their potential as drug delivery systems and as therapeutic agents with antimicrobial, anticancer, and wound-healing properties. AgNPs can be synthesized through chemical, physical, or biological methods. Chemically synthesized AgNPs are typically produced by reducing Ag⁺ ions using a range of organic and inorganic reducing agents, such as sodium citrate, sodium borohydride, sodium ascorbate, N,N-dimethylformamide, elemental hydrogen, and polymers. This process reduces Ag⁺ ions to Ag⁰. In addition, surfactants or polymers are required to stabilize AgNPs and prevent their aggregation (Abdellatif et al. 2021a, b; Abdellatif et al. 2016). Moreover, physical synthesis methods include evaporation–condensation and laser ablation of Ag⁺ in solution. However, both physical and chemical approaches commonly involve the use of hazardous chemical reagents. Consequently, there is a pressing need to develop safer, more environmentally friendly alternatives that enhance efficiency while minimizing reliance on toxic and time-consuming chemicals.

Green synthesis involves the design and production of eco-friendly chemical compounds or products that aim to reduce or eliminate the use of toxic or hazardous substances, thereby promoting both human health and environmental well-being. By leveraging natural resources such as plant extracts, green synthesis offers an environmentally sustainable alternative to conventional nanoparticle (NP) synthesis, considerably reducing dependence on harmful chemicals and decreasing the overall environmental footprint. However, despite its contributions to sustainability, green synthesis still faces several challenges that need to be addressed. Key challenges include inconsistencies in the quality and reproducibility of natural extracts, difficulties in scaling up production, and the need for stringent standardization in industrial applications. Addressing these hurdles is essential for the broader adoption and long-term success of green synthesis in sustainable nanotechnology (Heydari and Rashidipour 2015). This approach can also be applied to the synthesis of AgNPs using natural resources, such as plant extracts and microorganisms, to induce the bioreduction of silver ions into metallic nanosilver (Iravani et al. 2014; Abdellatif et al. 2022a, b, c, d, e). As a result, the concept of “green silver nanoparticles” (Gr-AgNPs) has attracted considerable attention.

Many researchers have used plant extracts to identify reducing agents for synthesizing Gr-AgNPs (Bagherzade et al. 2017; Raja et al. 2017; Patil et al. 2018). Gr-AgNPs have shown considerable potential as anticancer therapeutics against a wide range of cancer-cell types (Abdellatif et al. 2021a, b; Abdellatif et al. 2022a, b, c, d, e). Despite its promise, the clinical translation of this research is still obstructed by several challenges, including difficulties in synthesis, scalability, biosafety concerns, and regulatory approval hurdles (Younis et al. 2022; Abdellatif et al. 2022a, b, c, d, e).

In this article, we emphasize the role of plant extracts in the synthesis of Gr-AgNPs, reviewing various examples from the literature and focusing on key subclasses of natural products, such as polyphenols, proteins, and alkaloids, which act as bioreducing and stabilizing agents in Gr-AgNP synthesis. In addition, we examine the different characterization tools used for Gr-AgNPs, discussing their limitations. The anticancer potential of Gr-AgNPs is also thoroughly explored, with particular attention to their mechanisms of action and the challenges impeding clinical translation. Finally, we propose future directions for advancement in this promising field.

Green synthesis of AgNPs using plant extracts

The green synthesis of AgNPs using plant extracts takes advantage of the rich phytochemical content of plants, including alkaloids, phenolic compounds, flavonoids, terpenoids, catechins, saponins, tannins, enzymes, proteins, and polysaccharides. These phytochemicals serve a dual function: they act as reducing agents to convert Ag⁺ into metallic Ag⁰, forming AgNPs, and as stabilizing agents for the newly formed NPs. AgNPs typically form through redox reactions, where electrons combine with Ag⁺ to generate nanoseeds. These nanoseeds then aggregate to create AgNPs, which are subsequently hydrated, stabilized, and coated with plant-derived compounds, enhancing the stability of the nanosilver (Afshinnia and Baalousha 2017), as depicted in Fig. 1.

Fig. 1.

An illustration showing the principle of green synthesis of AgNPs using plant extracts. The figure was created with BioRender.com, with a publication license

This section summarizes the role of some subclasses of plant extracts in the green synthesis of AgNPs.

Polyphenols

Polyphenols, known for their powerful antioxidant properties, have been successfully used in NP synthesis. Their ability to donate electrons makes them excellent reducing agents, enabling the conversion of metal ions into metallic NPs. A prime example is the use of tannic acid, a polyphenol, in synthesizing AgNPs. The hydroxyl groups in polyphenols promote the reduction of Ag⁺ ions, while the polyphenolic structure helps stabilize the NPs, preventing both oxidation and aggregation (Sivalingam et al. 2023). Swilam and Nematallah (2020) identified 23 polyphenolic compounds in the extract of pomegranate (Punica granatum) leaves, with ellagic acid being the most abundant. Using this extract for AgNP synthesis produced polygonal NPs with an average size of 26.22 nm, demonstrating versatile applicability. In another study, Alowaiesh et al. (2023) used olive leaf waste extract to synthesize AgNPs that exhibited anticancer, antibacterial, and antifungal properties. High-performance liquid chromatography analysis of this extract revealed a wide range of polyphenolic compounds, including gallic acid, chlorogenic acid, rutin, naringenin, catechin, and propyl gallate.

Proteins

Proteins, characterized by their complex structures and functional diversity, are highly effective in NP synthesis. They serve dual roles as both reducing agents and stabilizers, and their biocompatibility makes them particularly well-suited for biomedical applications of NPs (Alabdallah and Hasan 2021). Amino acids have been widely studied for their ability to function as both reducing and stabilizing agents in the green synthesis of NPs. The unique functional groups present in amino acids, such as amine and carboxyl groups, facilitate the reduction of metal ions and help stabilize the resulting NPs. For example, research has shown that L-cysteine and L-glutamine can be used to synthesize AgNPs, where the thiol and amino groups are crucial for reducing Ag⁺ and capping the NPs, effectively preventing aggregation (Shankar and Rhim 2015). In a model study, Li et al. (2007) used Capsicum annuum L. extract for the green synthesis of AgNPs, with the amino groups in the proteins of the extract acting as both reducing and stabilizing agents. This dual functionality was validated through various electromagnetic and electrochemical measurements, as well as observations of changes in the secondary structures of the proteins in the extract. Furthermore, plant enzymes represent another class of plant proteins that can facilitate the green synthesis of AgNPs. For instance, Das et al. (2023) extracted the enzyme α-amylase from germinated green gram (Vigna radiata) and used it as a reducing agent to convert AgNO₃ into AgNPs. A considerable advantage of this method was that the entire reaction took place at room temperature, conserving energy and enabling the use of thermolabile components that may be negatively impacted by the conventional heating conditions typically used in AgNP synthesis. In addition, the synthesized AgNPs exhibited stability, with an average particle diameter of 36 nm, and displayed various versatile biological effects.

Alkaloids

Alkaloids, a diverse group of naturally occurring compounds, have also been used in the green synthesis of NPs. Their nitrogen-containing heterocyclic structures allow them to function as both reducing and capping agents (Zuhrotun et al. 2023). Almadiy and Nenaah (2018) synthesized green AgNPs using two steroidal alkaloids, α-chaconine and α-solanine, extracted from potatoes (Solanum tuberosum L.). The resulting AgNPs had a particle-size range of 40–80 nm and demonstrated high biosafety. In a more recent study (Gumy et al. 2023), Gumy and coworkers discovered that alkaloids extracted from Catharanthus roseus could be used to synthesize AgNPs. In this process, the alkaloids not only reduced silver ions to form AgNPs but also provided stability through capping, effectively preventing NP aggregation. In addition to the abovementioned examples, other studies have highlighted the use of various plant constituents, including flavonoids, as well as the synergistic application of multiple phytochemicals, in stabilizing and shaping AgNPs (Mirgorod and Borodina 2013). The formation of AgNPs consists of three main steps: nucleation, crystal growth, and particle stabilization. Nucleation is the initial phase of NP formation, during which small clusters of atoms or molecules (nuclei) aggregate to form a stable seed that serves as the basis for further growth. This process begins when silver ions in the solution are reduced to metallic nanosilver, facilitated by the applied plant extract. Nucleation is crucial for determining the size and distribution of the resulting NPs (LaMer and Dinegar 1950; Thanh et al. 2014). Subsequently, the nuclei, often referred to in the literature as “nanoseeds,” start to grow and form oligomeric clusters—small assemblies of molecules or ions typically composed of a few units—that form during the early stages of NP synthesis. These clusters serve as intermediates before the final formation of NPs, and their stability and behavior greatly influence the growth and eventual morphology of the NPs (Henglein 1993; Watzky et al. 2008). Ultimately, the formed AgNPs are stabilized through a capping process in which components from plant extracts coat the NPs, preventing excessive aggregation or re-oxidation back into Ag⁺. Several technical parameters influence the formation of AgNPs during these processes, including temperature, pH, the phytochemical composition of the plant used, stirring speed, the type of plant, extraction method, microwave assistance, Ag⁺ concentration, and synthesis duration (Ivanova et al. 2019). These factors affect the nucleation of Ag⁺ ion reduction and the subsequent agglomeration into oligomeric clusters, collectively influencing the colloidal stability and physicochemical properties of the resulting AgNPs. Research has shown that reaction time, temperature, and extraction methods of specific plants, such as Solanum xanthocarpum fruit, can enhance and accelerate the reduction of Ag⁺ ions, thereby impacting the size and shape of the produced AgNPs (Amin et al. 2012). The synthesized AgNPs measured approximately 10 nm in size and displayed uniformity, homogeneity, monodispersity, and a spherical shape. The surface properties of AgNPs are essential for their stability, compatibility, application, and safety. The characteristics of the nanosurface considerably influence how it interacts with ligands, proteins, and the surrounding environment (Kim et al. 2013).

Characterization of Gr-AgNPs

Gr-AgNPs can be characterized through a diverse range of methods and parameters, including size, surface area, shape, coating, and charge. Techniques such as zeta potential measurements, color change observations, redox processes, conductivity assessments, surface plasmon resonance (SPR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), inductively coupled plasma-mass spectrometry (ICP-MS), and inductively coupled plasma–optical emission spectroscopy (ICP–OES) are commonly used for this characterization (Abdellatif et al. 2016; Younis et al. 2022; Abdellatif et al. 2020a, b, 2022a, b, c, d, e; Abdellatif and Tawfeek 2018; Islam et al. 2019; Li et al. 2017). Table 1 summarizes the most commonly used techniques for the characterization of AgNPs.

Table 1.

The most commonly-used techniques for the characterization of AgNPs

Technique	Applications	References
TEM	Particle size, morphology, and sample purity	(Kovacs et al. 2022)
SEM	Surface properties, particle shape, and sample purity	(Kovacs et al. 2022)
UV‒VIS	The maximum absorbance wavelength, particle shape, drug content, stability, surface properties, and purity	(Aljohani et al. 2022)
XRD	Particle morphology, size distributions, crystal structure, phase identification, crystallinity, and purity of the sample	(Holder and Schaak 2019)
FT-IR	Identification of surface residues, functional groups, and compatibilities	(Titus, James Jebaseelan Samuel, and Roopan 2019)
EDS	Determination of the elemental compositions	(Iqbal et al. 2019)
DLS	Particle size, particle-size distribution (diameter), zeta potential, and stability determination	(Carvalho et al. 2018b)
AFM	Size distributions, morphology, surface properties, stability, and purity of the sample	(Klapetek et al. 2011)
ICP-MS ICP‒OES	Surface structure and chemical composition, quantification of biomolecules and proteins	(Yang et al. 2012)
TGA	Chemical composition, the amount of coating layer on the surface, and the thermal stability	(Loganathan et al. 2017)
Raman spectroscopy	The particle surface residues and identification of functional groups	(Gouadec and Colomban 2007)
XPS	Chemical composition and shift, surface sensitivity, and determination of chemical bonds	(Korin et al. 2017)

TEM transmission electron microscopy, SEM scanning electron microscopy, UV–VIS ultraviolet–visible spectroscopy, XRD X-ray diffractometry, FT-IR fourier-transform infrared spectroscopy, EDS energy dispersive spectroscopy, DLS dynamic light scattering, AFM atomic force microscopy, ICP-MS inductively coupled plasma-mass spectrometry, ICP-OES inductively coupled plasma atomic emission spectroscopy, TGA Thermogravimetric analysis, XPS X-ray photoelectron spectroscopy

The surface coating of AgNPs can be influenced by the synthesis process (such as one-step crosslinking) or through additional treatments after separation (multistep capping process) (Ivanova et al. 2019). The size, shape (morphology), and charge of AgNPs vary with the composition of nanopreparative colloids, reflecting changes in these parameters (Abdellatif et al. 2020a, b). Steinigeweg et al. (2012) noted that the reaction involving AgNPs was indicated by a rapid change in color in the reaction medium, attributed to variations in the absorption wavelengths of the AgNPs. In addition, as the diameter of the AgNPs increases, the maximum wavelength of the absorption band shifts. Ranoszek-Soliwoda et al. (2017) observed that the maximum absorption band (λmax) at 432 nm at 100 °C influenced the synthesis of AgNPs, confirming the role of temperature in the synthesis and its dependence on AgNP size. In addition, the size of AgNPs is related to the concentration of the raw materials and the Ag⁺ source (e.g., AgNO₃). (Fuku et al. 2013) reported that the relationship between the size of AgNPs and their color is attributed to changes in the SPR-absorption wavelengths. The blueshift and redshift of AgNP absorption are determined by the redox reactions and increases in AgNP size (Fuku et al. 2013; Shirahata et al. 2013). Scattering methods are widely used to investigate the structure and properties of materials by observing the deflection of incident particles (such as light, electrons, or neutrons) from their original trajectory. It is essential to differentiate among the various types of scattering, as summarized below (Anzini et al. 2022).

• Light scattering: This involves the deflection of light waves by particles in a sample and is commonly used to study the size distribution and aggregation state of NPs, as seen in dynamic light scattering (DLS).

• Electron scattering: This involves the interaction of a beam of electrons with a material, yielding detailed information about the material’s atomic structure. Techniques such as TEM and SEM rely on electron scattering.

• Neutron scattering: This technique uses neutrons to investigate the atomic and magnetic structure of materials, although it is less frequently used in NP research compared to light and electron scattering.

DLS, also known as quasi-light scattering, is a technique used to measure hydrodynamic particle size, particle-size distribution (or polydispersity), and zeta potential by analyzing the deflection of a light beam after its interaction with the particles (Carpenter 1977; Bhattacharjee 2016). In additional, DLS can directly measure the particle size before and after coating the AgNPs, enabling the indirect evaluation of shell thickness Titus et al. 2019. However, DLS does not provide information about the homogeneity of the shell. Furthermore, DLS assumes a spherical morphology for the NPs, which may not accurately represent the actual morphology of the AgNPs in question (Dhas et al. 2018). SEM and TEM are used to examine the morphology of AgNPs through their interaction with an electron beam. While they provide detailed morphologic data, these techniques are labor-intensive, require extensive sample preparation, and can analyze only a limited number of particles compared to DLS. In addition, the resulting micrographs depict only the metallic core of the AgNPs. It is important to note that electron microscopy operates under dry conditions. Consequently, the coating layer may collapse and be removed during sample preparation. Furthermore, the instrument operates as a high-vacuum chamber, which can deplete the coating layer during electron imaging. This leads to discrepancies in size when comparing data obtained from TEM and DLS, with particle sizes measured by TEM often appearing considerably smaller than those measured by DLS (Fissan et al. 2014; Tawfeek et al. 2023). While DLS and TEM are often used together in laboratory research, correlating their outputs can be challenging owing to the discrepancies in particle-size data mentioned earlier and differences in their operational environments. Nevertheless, these two techniques can be effectively integrated. For instance, DLS measurements can be considerably influenced by particle agglomeration, which increases the apparent hydrodynamic radius, while TEM is capable of resolving individual particles (Kirby 2010). In addition, DLS offers an ensemble average of particle sizes, which may overlook smaller particle populations or subtle size variations. In contrast, TEM, with its high resolution, can effectively resolve these differences but may not represent the overall size distribution as accurately as DLS (Carvalho et al. 2018a). Therefore, leveraging the distinct advantages of each tool is the most effective approach for a comprehensive characterization of AgNPs.

Cancer-targeted Gr-AgNPs

Anticancer potential of Gr-AgNPs

Numerous studies have demonstrated that Gr-AgNPs enter cells via endocytosis, becoming localized in the perinuclear area of the cytoplasm and in the endolysosomal compartment (Asharani et al. 2009). Gr-AgNPs generate reactive oxygen species that impact cellular respiration. They can inflict damage on cancer cells, leading to oxidative stress, DNA damage, apoptosis, and mitochondrial impairment (Sakthi Devi et al. 2022). In addition, research has indicated that the vascular endothelial growth factor (VEGF) activity, which is crucial for angiogenesis, may be influenced by Gr-AgNPs (Kalishwaralal et al. 2009). Owing to their anticancer properties, ability to facilitate targeted drug delivery, and function as bioimaging carriers, green-synthesized AgNPs possess considerable theranostic applications. Gr-AgNPs offer a cost-effective, secure, simple, and environmentally friendly approach to anticancer therapy (Kalishwaralal et al. 2009). Figure 2 summarizes some of the applications of Gr-AgNPs in anticancer treatment.

Fig. 2.

Some applications of green AgNPs as anticancer therapeutic modality. The figure was created with BioRender.com, with a publication license

Gr-AgNPs have been reported as effective treatments for colon and other gastrointestinal cancers. Kuppusamy et al. (2016) reported that Commelina nudiflora was used to reduce AgNO₃ into potent anticancer AgNPs, which exhibited lower toxicity compared to gold nanoparticles (AuNPs). The resulting Gr-AgNPs showed an inhibitory effect with an IC₅₀ of 100 µg/mL. Prabhu et al. (2013) reported on the green synthesis of Gr-AgNPs using Vitex negundo, which inhibited cell proliferation with an IC₅₀ of 20 µg/mL, categorizing it as an antiproliferative agent. Artemisia turcomanica leaf extract was also used to reduce AgNO₃ to AgNPs, resulting in compounds with an IC₅₀ value of 4.88 μg/mL. This study demonstrated that even the lowest concentration of Gr-AgNPs was effective in inhibiting cell growth, unlike commercial AgNPs (Mousavi et al. 2018). In addition, the extract from Teucrium polium produced AgNPs with a higher potency, exhibiting an IC₅₀ value of 68.2 μg/mL after 48 h of cell exposure (Hashemi et al. 2020). Additional examples of the use of Gr-AgNPs in the treatment of GIT cancers are listed in Table 2.

Table 2.

Examples of studies recruiting green AgNPs in the treatment of GIT cancers

Plant	Extraction method	Part of plant used	Physical conditions of synthesis	Cell-line model	Anticancer activities	References
Commelina nudiflora	Methanolic extract	Whole plant	80 °C, pH 8, 24-h incubation	HCT-116	Antiproliferative, apoptosis induction	(Kuppusamy et al. 2016)
Vitex negundo	Aqueous extract	Leaves	Room temperature, 12–h incubation	HCT15	Cytotoxicity, ROS generation	(Prabhu et al. 2013)
Artemisia turcomanica	Aqueous extract	Leaves	90 °C, pH 7, 48-h incubation	AGS	Cytotoxicity, cell-cycle arrest	(Mousavi et al. 2018)
Teucrium polium	Ethanolic extract	Leaves	70 °C, pH 9, 36-h incubation	MNK45	Antiproliferative, apoptosis induction, mitochondrial dysfunction	(Hashemi, Tasharrofi, and Saber 2020)
Camellia sinensis	80% methanol	Leaves	80 °C, pH 6, 4 h under reflux	Caco-2	Inhibited cell proliferation	(Mueed et al. 2023)
Curcuma longa	Hot Aqueous extraction	Rhizomes	25 °C, pH 7.4, 24 h	HT-29	Induced oxidative stress	(Barbinta-Patrascu et al. 2021)
Moringa oleifera	Aqueous extraction	Leaves	Room temperature, pH 7, 6-h stirring	HT-29	Induced cell-cycle arrest	(Susanto et al. 2024)

Gr-AgNPs have also demonstrated anticancer effects against various lung cell lines. For instance, Dimocarpus longan Lour reduced AgNO₃ to Gr-AgNPs, achieving a potent IC₅₀ of 5.33 ± 0.37 μg/mL. These NPs effectively suppressed tumor growth in H1299 cell models in mice (He et al. 2016). Similarly, Gr-AgNPs synthesized using Erythrina indica exhibited considerable anticancer activity, reducing cell viability to 13.86 ± 0.95% at a concentration of 25 µg/mL. This root-mediated synthesis is pivotal in developing cancer-chemotherapy agents (Rathi Sre et al. 2015). Most studies on Gr-AgNPs have focused on lung cells, particularly the A549 cell line (adenocarcinoma human alveolar basal epithelial cells). A549 cells have previously been used to demonstrate the effects of various drugs, including paclitaxel, docetaxel, and bevacizumab, both in vitro and in vivo. Gr-AgNPs can also be used to construct pedigree trees and uncover behavioral correlations between sister cells through single-cell monitoring of A549 cells (Korsnes and Korsnes 2018). Vijayan Joseph et al. (2018) found that Gr-AgNPs reduced the anticancer efficacy of Indigofera tinctoria leaf extract, with an IC₅₀ value of 71.92 ± 0.76 μg/mL, attributed to ROS generated by AgNPs. In addition, Kanipandian et al. (2014) synthesized Gr-AgNPs using Cleistanthus collinus extract, indicating that this toxic plant could serve as an anticancer agent at low doses; specifically, a concentration of 30 µg/mL was associated with an acceptable inhibitory concentration (IC₅₀). Moreover, in vivo histopathological analyses of mice treated with Gr-AgNPs showed no signs of organ edema or inflammation, indicating their potential for both diagnostic and therapeutic applications. However, it is essential to lower the concentration of the cytotoxic extract to ensure safety for normal cells. Gengan et al. (2013) used Albizia adianthifolia leaf extract to produce AgNPs coated with this extract and noted a considerable cytotoxic effect, achieving 79% inhibition at a concentration of 10 μg/mL. Similar results were observed by Annu et al. (2018), who used P. granatum to synthesize Gr-AgNPs. The resulting Gr-AgNPs exhibited a considerable cytotoxic effect on lung cancer cells (A549) while sparing normal cells. Notably, a 50% growth inhibition was observed after 48 h of incubation at a low concentration of 5 μg/mL. Furthermore, Gr-AgNPs combined with sucrose demonstrated considerable antitumor activity against vincristine and methotrexate in squamous cell lung carcinoma (H157 cell line) (Nazir et al. 2011), suggesting that green AgNPs synthesized from plant extracts show superior anticancer activity compared to chemical drugs. Additional examples of the application of Gr-AgNPs in lung-cancer treatment can be found in Table 3.

Table 3.

Examples of previous studies reporting on the applicability of Gr-AgNPs against lung cancer

Natural extract used for the preparation of AgNPs	Cell-line model	References
Dimocarpus longan Lour fruit	H1229	(He et al. 2016)
Erythrina indica root	HepG2	(Rathi Sre et al. 2015)
Indigofera tinctoria leaves	A549	(Vijayan et al. 2018)
Cleistanthus collinus leaves	A549	(Kanipandian et al. 2014)
Albizia adianthifolia leaves	A549	(Gengan et al. 2013)
Punica granatum fruit peel	A549	(Annu et al. 2018)
Inonotus obliquus mushroom	A549	(Nagajyothi et al. 2014)
Sucrose	H157	(Nazir et al. 2011)

Gr-AgNPs were also tested on human breast cancer to assess their anticancer effects. The MCF-7 cell line was chosen as the primary model for breast cancer research owing to its widespread use and effectiveness in studies related to human breast cancer. These cells are estrogen receptor alpha positive (ER-α⁺) and are classified as noninvasive (Sundararajan et al. 2014). Plant extracts from Erythrina indica, Dendrophthoe falcata (L.f.), and Datura inoxia serve as exemplary models for the green synthesis of AgNPs with anti-breast cancer properties. The synthesized Gr-AgNPs effectively inhibited cancer-cell lines by inducing apoptosis mediated by caspase-3 (Gajendran et al. 2014). Zhang et al. (2016a, 2016b) reported that the cytotoxic effects of Gr-AgNPs were due to the inhibition of LDH, which increases ROS production, resulting in apoptosis. Abdellatif et al. (2022a, b, c, d, e) demonstrated the considerable inhibition of TNF-α by AgNPs. Gurunathan et al. (2013) reported that Gr-AgNPs enhanced the production of hydroxyl radicals and O₂, thereby reducing cell viability. They trigger apoptosis by activating caspase-9 and inducing DNA fragmentation. The resulting Gr-AgNPs exhibit a considerable and potent effect on breast cancer-celllines, making them promising candidates for future formulations in the treatment of breast cancer and other malignancies. Additional examples of Gr-AgNPs in breast cancer research are summarized in Table 4.

Table 4.

Examples of Green synthesized AgNPs prepared using different natural product extracts with a potential against the human breast cancer

Natural resource	Cell-line model	Therapeutic outcomes	References
Erythrina indica root	MCF-7	Reduced cell viability to 23.89 ± 0.39% at 25 μg/mL	(Rathi Sre et al. 2015)
Dendrophthoe falcata leaves	MCF-7	Enhanced cytotoxicity at 5 μg/mL	(Sathishkumar et al. 2014)
Datura inoxia leaves	MCF-7	Cell-cycle arrest, inhibited growth, apoptosis, IC50 = 20 μg/mL	(Gajendran et al. 2014)
Padina tetrastromatica seaweed	MCF-7	Concentration-dependent cytotoxicity, Caspase-3-mediated apoptosis	(Gajendran et al. 2014)
Juglans regia (walnut fruits)	MCF-7	Significant cytotoxicity at 60 μg/mL	(Khorrami et al. 2018)
Trigonella foenum-graecum (seeds)	MCF-7	Significant inhibition of TNF-α	(Abdellatif et al. 2022a, e, b, c, d)
Ethyl cellulose	MCF-7	Significant inhibition of TNF-α	(Abdellatif et al. 2021a, b)
Inonotus obliquus mushroom	MCF-7	Significant cytotoxicity	(Nagajyothi et al. 2014)
Ganoderma neo-japonicum Imazeki mushroom	MB-231	Increased ROS production, Caspase-9 activation, and DNA fragmentation	(Gurunathan et al. 2013)
Acalypha indica leaves	MB-231	Cytotoxicity at 100 µg/mL	(Krishnaraj et al. 2014)
Melia dubia leaves	KB	IC50 = 31.2 μg/mL	(Patil et al. 2018)

In addition to the abovementioned examples, several studies have reported the efficacy of Gr-AgNPs against a broader range of cancer-celllines. For instance, Clinacanthus nutans extract was reacted with Ag⁺ to produce Gr-AgNPs, which exhibited a considerable cytotoxic effect on HSC-4 cells (an oral squamous cell carcinoma line) at a concentration of 1.61 ± 0.14 μg/mL by inhibiting the release of Bcl-2 protein (Yakop et al. 2018). Gr-AgNPs prepared using Pimpinella anisum showed a lower cytotoxic effect on human neonatal skin stromal cells (hSSCs) (Alsalhi et al. 2016). In addition, Phyllanthus emblica-reduced AgNPs showed a potent cytotoxic effect with an IC₅₀ of approximately 20 µg/mL (Rosarin et al. 2013). However, the plant extract exhibited no noticeable cytotoxic effects on C₂C₁₂ cells. In addition, when applied in vivo to zebrafish embryos, Gr-AgNPs demonstrated greater cytotoxicity, achieving 100% mortality at just 3 μg/mL, whereas AuNPs only reached 100% mortality at considerably higher concentrations (300 mg/mL) (Ramachandran et al. 2017). Additional data on various cancer types from different organs, including laryngeal carcinoma cells (HepG2 cell line), cervical cancer (HeLa cell line), Ehrlich ascites carcinoma tumor-bearing mice, myoblast cancer (mouse C₂C₁₂ cell and zebrafish), cervical cancer (SiHa human cervical cancer-cell line), human hepatocellular carcinoma (HepG2 cell line), malignant skin melanoma (HT144 cell line), and human myeloblastic leukemia (HL60 and HeLa cells), can be found in Table 5.

Table 5.

Additional examples for the applicability of Gr-AgNPs against a wide diversity of cancerous cell lines

Natural resource	Cell-line model	Therapeutic outcomes	References
Clinacanthus Nutans leaves	HSC-4	Cytotoxicity at 1.61 ± 0.14 μg/mL by suppressing Bcl-2 protein release	(Yakop et al. 2018)
Phyllanthus emblica (PE) fruit	HepG2	IC50 = 20 μg/mL	(Rosarin et al. 2013)
Curcumin derivative (ST06)	HeLa	IC50 = 1 μg/mL	(Sakthi Devi et al. 2022)
Spinacia oleracea leaves	C2C12 cells, zebrafish	100% growth inhibition at 20 μg/mL	(Ramachandran et al. 2017)
Argemone Mexicana leaves	SiHa	Reduced cell viability by 70–80% at 100 µg/mL	(Gengan et al. 2013)
Pterocladiella capillacea red seaweed	HepG2	IC50 = 3.7 μg/mL	(Yang et al. 2020)
Sucrose	HT144	Strong anticancer activity compared to methotrexate and vincristine	(Nazir et al. 2011)
Taraxacum officinale leaves	HepG2	Significant cytotoxicity	(Saratale et al. 2018)
Sargassum vulgare seaweed	HL60, HeLa	Apoptosis induction	(Govindaraju et al. 2015)
Piper longum leaves	HepG2	Cytotoxicity of 94% at 0.5 mg/mL due to ROS generation	(Jacob et al. 2012)

There is a need to advance to the next step by testing the obtained Gr-AgNPs in clinical trials. While there are various AgNP products available in the pharmaceutical market or currently undergoing clinical trials for antimicrobial and aseptic applications, further investigation is warranted (Younis 2024). The clinical translation of Gr-AgNPs remains sluggish. A promising example is Nowarta 110, the world’s first topical botanical silver medication designed to treat human papillomavirus-related plantar wartscompleted phase I/II randomized controlled double-blind study (NCT02338336) and demonstrated a lesion clearance rate of 64.3% without any adverse events. According to the investigators (Yousefpour et al. 2023), a phase III clinical study was scheduled for completion in 2023; however, as of August 2024, the results have not yet been released.

The cellular uptake process: how do Gr-AgNPs fit in?

A comprehensive understanding of the biological interactions between tissue cells and their subsequent internalization is crucial for minimizing potential adverse effects on human health and enhancing the therapeutic efficacy of Gr-AgNPs (Shang et al. 2014). Small molecules, including oxygen, carbon dioxide, water, and small nonpolar molecules or NPs ranging from 8 to 30 nm in size, can easily traverse cell membranes. There are multiple pathways for the uptake of particulate matter, such as lipids, proteins, glucose, and other extracellular substances, which are essential for enhancing cellular quality. Endocytosis refers to the process through which materials are internalized via the plasma membrane-derived endoplasmic reticulum (ER) (Conner and Schmid 2003). Endocytosis encompasses several methods, including phagocytosis, which involves the ingestion of particles by specialized cell types such as monocytes, macrophages, and neutrophils. The second method, pinocytosis, pertains to the uptake of extracellular material and soluble substances. Pinocytosis can be further categorized into four pathways based on vesicle size and the proteins involved in vesicle formation. The first pathway, macropinocytosis, is an actin-dependent mechanism characterized by the extension of the plasma membrane, resulting in the formation of large vesicles known as macropinosomes. The second pathway is clathrin-mediated endocytosis, also known as receptor-mediated endocytosis, which involves the internalization of clathrin-coated vesicles containing biomolecules via specific membrane receptors. Once internalized, the vesicle forms an endosome that may lead to lysosomal degradation of the encapsulated particle unless it can rupture the endosomal membrane and access the cytosol, a process known as endosomal escape. This mechanism is the primary method for the internalization of functionalized nanomedicines, including Gr-AgNPs (Abdellatif et al. 2022a, b, c, d, e). The third pathway is caveolae-mediated endocytosis, in which caveolae—cholesterol-rich, flask-shaped membrane invaginations—encapsulate the NP and facilitate its internalization. This process is similar to that of endosomes, but the resulting vesicles are referred to as caveosomes. Caveosomes are believed to have a less destructive fate compared to endosomes (Zhao et al. 2011). Meanwhile, the fourth pathway is clathrin–caveolin-independent endocytosis, which is less prevalent than the other mechanisms. In addition to the internal endocytic delivery process and the transcellular delivery pathway, vesicles can also be transported to the opposite end of the cellular membrane and released into the extracellular environment. A passive transport mechanism for molecular distribution known as the paracellular delivery channel has also been identified. In this pathway, particles pass through tight junctions that have openings with a maximum width of 15 Å and negative charges, which regulate the transport of particles between neighboring cells (Hayashi et al. 1997). Each type of NP exhibits a preferred mechanism for cellular uptake. Various studies have demonstrated that factors such as size, surface characteristics, morphology, surface modifications, and the protein corona can influence the entry and eventual cytosolic access of AgNPs. Figure 3 shows the various internalization mechanisms associated with AgNPs and how their physicochemical properties impact these processes.

Fig. 3.

The most reported mechanisms for the cellular internalization of Gr-AgNPs. Phagocytosis is commonly associated with phagocytic cellular populations such as macrophages, monocytes, and antigen-presenting cells of the immune system (e.g., dendritic cells). Meanwhile, pinocytosis mediates the uptake of nanoparticles in most cellular populations and can be subclassified into macropinocytosis, clathrin-mediated endocytosis, caveolin-mediated endocytosis, and clathrin/caveolin-independent endocytosis. Most functionalized Gr-AgNPs are recognized by specific cell-membrane receptors and internalized through the clathrin-mediated pathway. In addition, ultra-small Gr-AgNPs (4–10 nm) can be internalized through passive diffusion. The mechanism of cellular transport is largely affected by the physico-chemical properties of Gr-AgNPs and their surface characteristics. Endogenous proteins have also a pivotal role in determining the fate of Gr-AgNPs through the formation of protein corona that can either facilitate the targeting of Gr-AgNPs through exploiting the endogenous machinery, or on the other hand, can negatively impact their stability by inducing agglomeration

Mechanistic aspects of the Gr-AgNPs for cancer treatment

Peer-reviewed studies suggest multiple pathways through which AgNPs may exert anticancer effects. Gr-AgNPs interact efficiently with cancer cells owing to their highly porous structure and enhanced permeability and retention (EPR) effect. This leads to a greater accumulation of AgNPs in the body, either destroying cancer cells or inhibiting their uncontrolled proliferation (Sundararajan et al. 2014). AgNPs modulate physiologic-signaling pathways by both upregulating and downregulating them, leading to early apoptosis or slowing the proliferation of rapidly dividing tumor cells. In addition, research indicates that AgNPs activate p53, caspase-3, and p-ErK1/2, ultimately inducing apoptosis and regulating cell division through various cellular mechanisms (Sundararajan et al. 2014; Gurunathan et al. 2015). In vitro cytotoxicity tests using AgNP–poly(vinyl pyrrolidone) (AgNP–PVP) demonstrated a considerable reduction in TNF-α levels and a considerable decrease in IL-6 production. AgNP–PVP shows potential as a treatment for human prostate cancer and as an anti-IL-6 agent in both malignant and noncancerous cells (Abdellatif et al. 2022a, b, c, d, e). Another anticancer mechanism of AgNPs involves their impact on nuclear factor kappa-B (NF–kB), a key-transcription factor responsible for the production of various inflammatory mediators, including tumor necrosis factor (TNF-α). AgNPs were shown to inhibit PMA-induced NF-kappaBp65 activation, reducing both TNF-α mRNA and protein expression (Abdellatif et al. 2020a, b). The anticancer mechanism of AgNPs on human colorectal cancer-cell lines (HT-29 and SW620) was investigated. AgNP–CEPA inhibited cell proliferation and modulated the expression of Bcl2-family genes. Moreover, AgNP–CEPA considerably enhanced apoptosis induction. These findings suggest that AgNP–CEPA holds potential as a promising anticancer agent for colorectal cancer treatment by promoting apoptosis through the downregulation of Bcl2-family gene expression (Abdellatif et al. 2022a, b, c, d, e). The mechanism of action of AgNPs in breast cancer (SKBR3) cell lines was explored through complex and challenging methods. A considerable reduction in cell motility and inhibition of metalloproteinases (MMPs) were observed. Treatment with AgNPs also induced notable morphologic changes in cancer cells, such as cell shrinkage, irregular shape, cytoplasmic blebbing, altered intracellular vacuole structures, and chromatin condensation. These effects, along with oxidative stress and cell death, were linked to the generation of ROS. In addition, Gr-AgNPs decreased the expression of autophagic markers HSP90, AKT, P62, and p-AKT while upregulating markers such as LC3-II, ATG7, Beclin-1, and ATG5 (Buttacavoli et al. 2018). It has been reported that cancer cells exhibit an EPR effect, leading to increased production of cytotoxic Ag⁺ and greater uptake of AgNPs (Torchilin 2011). This increased absorption induces oxidative stress in tumor cells, damaging vital biomolecules, disrupting various physiologic processes and molecular pathways, and destroying critical cellular organelles and biomembranes, ultimately resulting in tumor-cell death (Sundararajan et al. 2014; Al-Sheddi et al. 2018).

Uptake modality and cellular effects of Gr-AgNPs

Gr-AgNPs can rapidly traverse physiologic barriers, reaching key organs through the exposure pathway. Their interactions with cells, uptake mechanisms, biocompatibility, solubility, concentration, and surface functionalization are influenced by various NP-related factors, including size, morphology, zeta potential, surface coating, lipophilicity, concentration, and surface modification. In addition, experimental conditions or cell types—such as particle distribution, entry mode, mechanism of action, growth media, exposure duration, and cell type—also play an important role. Human lung fibroblasts (IMR-90) and glioblastoma cells (U251) internalize AgNPs in the brain via lysosomal or endosomal endocytosis. In addition, glioma cells, fibroblasts, and macrophages continue to absorb AgNPs through clathrin-mediated pathways, scavenger receptors, and macropinocytosis (Panzarini et al. 2018; Asharani et al. 2009). Hsiao recently investigated the absorption and toxicity of 10 nm AgNPs in different types of brain cells (ALT, BV-2, and N2a cells). Cell viability was linked to uptake levels, with uptake patterns being both dose- and cell-dependent. ALT cells absorbed the highest amount of AgNPs, followed by BV-2 and N2a cells. Furthermore, BV-2 cells activated by lipopolysaccharide (LPS) exhibited greater AgNP uptake compared to their inactivated counterparts. However, no considerable difference in NP absorption was observed between normal and LPS-activated ALT and N2a cells. The selected internalization mechanisms for ALT cells include clathrin- and caveolae-independent endocytosis and phagocytosis, whereas BV-2 cell uptake involves macropinocytosis and clathrin-dependent endocytosis. AgNP uptake is dose-dependent. ALT took up the greatest quantity of AgNPs (Hsiao et al. 2017; Panzarini et al. 2018). After absorption, AgNPs migrate to the mitochondria and nucleus, depending on their type and surface properties. This leads to changes in cell morphology, oxidative stress, DNA damage, inflammation, genotoxicity, and mitochondrial dysfunction, ultimately triggering apoptosis or necrosis (Zhang et al. 2014). In spermatogonial stem cells, smaller AgNPs demonstrate a higher ability to penetrate the plasma membrane and localize in the cell, resulting in greater toxicity. Their larger surface area enhances the release, absorption, and cytotoxicity of Ag⁺ ions in the cell culture medium (Braydich-Stolle et al. 2010; Gliga et al. 2014). In contrast, Chen et al. (2015) used 15, 50, and 100-nm AgNPs and demonstrated that the smallest 15 nm AgNPs were the most lethal, while the larger 50-nm AgNPs exhibited the highest adsorption and passive absorption in red blood cells. In addition, 100-nm AgNPs were found to have limited ability to cross the plasma membrane. Smaller AgNPs displayed high transit rates across biological barriers, resulting in NP accumulation that contributed to cytotoxicity in lung, stomach, breast, and endothelial cells (Chen et al. 2016; Stoehr et al. 2011). Moreover, AgNPs exhibit considerably higher cytotoxicity compared to Ag nanowires, which have relatively few Ag wires (length: 1.5–25-μm; diameter: 100–160 nm). A549 cells were successfully endocytosed with spherical AgNPs (30 nm); however, only a small fraction was detected inside the cells. This limited internalization may be attributed to contact between the silver nanowires and the plasma membrane rather than the endocytosis mechanism (Stoehr et al. 2011). AgNPs can be coated with various compounds to improve their stability, dispersibility, and agglomeration, thereby imparting new functionalities. This modification can also influence cellular absorption. For example, albumin and other human serum proteins inhibit the uptake of citrate/PVP-coated AgNPs, while these NPs are more readily absorbed by human lung cells. Conversely, serum proteins enhance the internalization of silica-coated NPs. This discrepancy may be attributed to the increased affinity of negatively charged cell membranes for positively charged AgNPs, which facilitates their internalization and intracellular bioavailability (Cho et al. 2009; Suresh et al. 2012). According to previous studies, PEG binding to NP surfaces inhibits nonspecific internalization by cells (Owens and Peppas 2006; Poulose et al. 2012; Kim et al. 2010; Gopee et al. 2009). The uptake of AgNPs through surface receptors can be enhanced by direct interactions between coated particles and receptors facilitated by ligands attached to NPs. The PEG layer also provides steric hindrance during interactions with surfaces such as the cell membrane and protein adsorption (Kelf et al. 2010; Osaki et al. 2004; Hild et al. 2008). In the lung, inflammatory sites, liver, spleen, bone marrow, and malignant regions, NPs tend to target only a limited number of organs. Once injected into the bloodstream, polymeric NPs show potential for site-specific cell delivery (Fan et al. 2012; Yoon et al. 2012; Xu et al. 2012). Polymeric NPs can target various ocular sites, including the cornea, retina, and choroid, via topical applications and intravitreal injections. Nanosuspensions formulated with MBA demonstrated faster therapeutic effects and prolonged anti-inflammatory activity compared to aqueous suspensions of the same drug (Oh and Park 2014). The cytotoxicity of AgNPs is linked to oxidative stress and the release of Ag⁺ ions resulting from the dissolution of AgNPs. These Ag⁺ ions can interact with the thiol groups of various antioxidant molecules, including superoxide dismutase (SOD), thioredoxin, and glutathione (GSH). Such interactions may increase oxidative stress and cause DNA damage, ultimately triggering apoptosis (Arora et al. 2008). Research shows a relationship between cell ROS release and mitochondrial dysfunction (Chairuangkitti et al. 2013). AgNPs considerably affect mitochondria, leading to the disruption of the respiratory chain, the induction of oxidative stress, and the inhibition of ATP production, which activates the apoptotic pathway (Foldbjerg et al. 2012). Compounds smaller than 10 nm can penetrate nuclear pores, enter the nucleus, cause DNA damage, and lead to cell-cycle arrest (Asharani et al. 2009). AgNPs have been shown to induce genotoxic effects in HepG2 cells, hMSCs, and PBMCs (Kawata et al. 2009). In addition, adenosine nucleophosphates can cause DNA damage in human cells by upregulating DNA repair genes and increasing micronuclei formation in cells treated with AgNPs. AgNPs impact cell structure, disrupt cytoskeletal components, and displace pre- and postsynaptic proteins, leading to mitochondrial dysfunction in neurons, all of which are detrimental (Xu et al. 2013). AgNPs may also reduce cell viability by causing apoptosis through the mitochondrial pathway (Panzarini et al. 2018). They also trigger an inflammatory response, enhance the secretion of proinflammatory cytokines, increase blood–brain barrier permeability, and increase immunotoxicity in a size-dependent manner in rat brain microvessel endothelial cells (Panzarini et al. 2018; Luo et al. 2015; Trickler et al. 2010). In addition, AgNPs are important for angiogenesis. Gurunathan et al. (2013) demonstrated that they acted as antiangiogenic factors in cells by activating the phosphatidylinositol-3-kinase/protein kinase B (PI3K/Akt) pathway, which reduces cell proliferation and migration induced by VEGF. In another study, Sriram et al. (2010) demonstrated that AgNPs inhibited the proliferation of cancer cells (Dalton’s lymphoma ascites) in a dose-dependent manner by activating caspase-3 and fragmenting DNA. In addition, AgNPs suppressed the expression of HIF-1 and its downstream targets, further supporting their role in cytotoxicity and angiogenesis. Several studies have shown that the morphology and viability of Gr-AgNPs do not considerably impact stem cells. To mitigate ion release, AgNPs can be stabilized with specific surface capping materials or through functionalization (Panzarini et al. 2018). AgNPs specifically target lysosomes and endosomes. They can induce oxidative damage to proteins and other organelles, including mitochondria, by interacting with the acidic lysosomal environment. The release of ROS from AgNPs and Ag⁺ into the cytoplasm disrupts calcium homeostasis. ROS can also lead to the release of Ca²⁺ from the ER, resulting in a Ca²⁺ imbalance.

Challenges hampering the use of Gr-AgNPs in cancer therapy

Despite their promising potential, several challenges must be addressed to fully harness the capabilities of green-synthesized AgNPs in cancer therapy. One of the primary challenges is ensuring the safety of Gr-AgNPs for human use. While Gr-AgNPs have demonstrated anticancer properties, they can also be toxic to healthy cells if not synthesized correctly. Therefore, researchers must conduct thorough investigations into the effects of green-synthesized AgNPs on both cancerous and healthy cells to assess their safety and efficacy (Kovacs et al. 2022; Ratan et al. 2020). Furthermore, considering that Gr-AgNPs are primarily synthesized using plant extracts, their potential immunogenicity is a considerable concern. For example, the green synthesis of AgNPs using plant extracts such as Aloe vera, Azadirachta indica, and Camellia sinensis has attracted considerable interest owing to their eco-friendly properties and potential biomedical applications. However, the immunogenic responses triggered by these plant extracts pose a considerable challenge to their clinical translation (Simon et al. 2022). Aloe vera is well-known for its medicinal properties, including its anti-inflammatory and wound-healing effects. However, it has also been associated with contact dermatitis in sensitive individuals owing to the presence of anthraquinones, which may trigger allergic reactions when Aloe vera-extracted compounds are used in NP synthesis (Ferreira et al. 2007). Similarly, Azadirachta indica (Neem) offers therapeutic benefits but has been linked to allergic reactions such as skin irritation and hypersensitivity. These reactions may restrict the use of Neem-based Gr-AgNPs, particularly in individuals who are predisposed to allergies or have compromised immune systems (Ambooken et al. 2017). Camellia sinensis (green tea) is another widely used extract in the green synthesis of NPs. While generally considered safe, there have been instances of allergic reactions, particularly in individuals sensitive to tea components or related plants. These immunogenic concerns may impact the safety profile of NPs synthesized using Camellia sinensis (Kajita et al. 2021). Given these potential immunogenic responses, it is essential to evaluate the safety and biocompatibility of green-synthesized AgNPs in preclinical models before proceeding to clinical trials. Comprehensive studies should assess the risk of immune responses upon introduction of these NPs into the human body. In addition, strategies to mitigate these responses, such as surface modification of NPs to reduce immunogenicity, should be explored to enhance their translational potential (Younis 2024). Another challenge is establishing a reliable and cost-effective method for synthesizing Gr-AgNPs. Although natural extracts are effective in synthesizing NPs, this process can often be time-consuming and costly. Therefore, researchers need to develop more efficient methods for synthesizing Gr-AgNPs to enhance their viability as options for cancer therapy (Ratan et al. 2020, Vishwanath and Negi 2021). The green synthesis of AgNPs largely depends on the phytochemicals found in plant extracts, which serve as reducing and stabilizing agents. However, the chemical composition of these extracts can vary considerably owing to factors such as geographic source, environmental conditions, and extraction methods. This variability presents challenges for the reproducibility and scalability of NP synthesis, both of which are essential for clinical applications. The phytochemical composition of plant extracts can be affected by various factors, including the region of cultivation, soil composition, climate, and altitude. For example, Aloe vera cultivated in arid regions may exhibit different concentrations of bioactive compounds compared to those grown in temperate climates, resulting in variations in the size, shape, and stability of the synthesized Gr-AgNPs (Kumar et al. 2017). In addition, cultivation conditions, including the use of fertilizers, irrigation practices, and harvesting times, can also influence the concentration of active compounds in plant extracts. For instance, Azadirachta indica extracts may vary in azadirachtin levels, an important bioactive compound, depending on the agricultural practices used (Gahukar 2014). Such variations can lead to inconsistencies in the synthesis process, resulting in AgNPs with different physicochemical properties that may affect their therapeutic efficacy and safety. Furthermore, the extraction method—whether aqueous, alcoholic, or using other solvents—can significantly affect the yield and composition of phytochemicals in the extract. For example, Camellia sinensis extracts obtained using different solvents can exhibit varying antioxidant capacities and polyphenol content, both of which are essential for the reduction and stabilization of Gr-AgNPs (Chen et al. 2022). As a result, inconsistent extraction methods can introduce variability in nanoparticle synthesis, complicating efforts to standardize production processes for clinical applications. Overall, the variability in natural extracts used for green synthesis can impede the reproducibility of NP production, which is crucial for the regulatory approval of nanomedicines. To address this challenge, standardized protocols that consider the variability in plant sources and extraction methods are essential. Furthermore, comprehensive characterization of the phytochemical content in extracts should be performed before their use in NP synthesis to ensure consistency and reliability, thereby facilitating the transition from laboratory to clinical settings (Singh et al. 2023). Finally, delivering NPs to cancer cells presents a considerable challenge. Although AgNPs are effective at killing cancer cells, they must specifically target these cells to prevent damage to healthy tissues (Kovacs et al. 2022; Ratan et al. 2020). Researchers are investigating various delivery methods, including the attachment of NPs to antibodies that specifically target cancer cells, to ensure that the NPs are delivered directly to malignant cells (Yao et al. 2020). Despite these challenges, the potential benefits of Gr-AgNPs in cancer therapy are substantial. These NPs not only effectively kill cancer cells but also exhibit the potential for reduced toxicity compared to traditional chemotherapy treatments. Furthermore, the incorporation of natural extracts in the synthesis process renders Gr-AgNPs more environmentally friendly than conventional chemical treatments (Gomes et al. 2021). The development of Gr-AgNPs using natural extracts is a promising direction in cancer therapy research. Although several challenges must be addressed to fully harness their potential, the advantages of using these NPs for cancer treatment are considerable. With ongoing research and development, Gr-AgNPs may become an important tool in the fight against cancer.

Discussion

Cancer remains one of the leading causes of death globally, prompting researchers to continuously seek new methods to enhance cancer therapy. One promising approach involves the synthesis of AgNPs using natural extracts, which have demonstrated potential in cancer treatment owing to their unique properties. While AgNPs can be synthesized through various methods, including chemical and physical techniques, these often use toxic chemicals and can be costly. In contrast, the green synthesis of AgNPs using natural extracts offers an eco-friendly, cost-effective, and safer alternative (Jabeen et al. 2021). Plants possess various chemicals that stabilize Ag⁺, which are then reduced to create AgNPs. For instance, Capsicum annuum L. is a plant known to produce AgNPs. This plant contains compounds such as polyphenols, flavonoids, and alkaloids that serve as reducing agents and stabilize AgNPs (Iravani et al. 2014). In addition, proteins play a crucial role in the green synthesis of AgNPs. The amine groups in protein function as reducing agents during the synthesis process, resulting in structural changes. This can lead to the formation of protein-coated AgNPs, enhancing their stability and biocompatibility (Zhang et al. 2016a, b).

One of the key advantages of Gr-AgNPs is their low toxicity. Toxicity remains a considerable concern when using NPs in cancer therapy because they can harm healthy cells. However, Gr-AgNPs synthesized from natural extracts are biocompatible and exhibit low toxicity, positioning them as safer alternatives (Ullah et al. 2020). Beyond their reduced toxicity, Gr-AgNPs possess unique characteristics that enhance their suitability for cancer therapy. AgNPs have a high surface area-to-volume ratio, facilitating improved interactions with cancer cells. In addition, they can penetrate cells and tissues more readily than larger particles, which promotes more effective drug delivery to cancerous cells (Abdellatif et al. 2021a, b, 2022a, e, b, c, d; 2023). AgNPs can also induce apoptosis, or programmed cell death, in cancer cells. This capability is crucial in cancer therapy because many cancer cells often display defects in their apoptotic pathways. By activating these pathways, AgNPs can trigger apoptosis in cancer cells, ultimately resulting in their death (Abdellatif et al. 2022a, e, b, c, d; Ullah et al. 2020). Another promising application of green-synthesized AgNPs is their use in combination with other cancer therapies. AgNPs can enhance the effectiveness of treatments such as chemotherapy and radiation therapy by improving drug delivery and promoting apoptosis in cancer cells (Tinajero-Diaz et al. 2021). Nevertheless, the application of natural extracts in Gr-AgNPs demonstrates considerable potential in cancer therapy owing to their low toxicity, unique properties, and capability to induce apoptosis in cancer cells. Further research is essential to fully understand their mechanisms of action and optimize their use in cancer treatment. However, these NPs present a promising alternative to conventional cancer therapies and may enhance patient outcomes.

AgNPs can be synthesized through a reduction‒oxidation reaction involving Ag⁺ and flavonoids. This reaction can be quite complex and involves interactions with both Ag⁺ and NPs (Mirgorod and Borodina 2013). AgNPs are formed from nanoseeds created by the interaction of electrons with Ag⁺. These NPs are then hydrated, aggregated, and stabilized using plant constituents (Afshinnia and Baalousha 2017). The use of plants for the green synthesis of AgNPs is becoming increasingly popular owing to their eco-friendliness, accessibility, cost-effectiveness, and suitability for large-scale production. Numerous researchers have used plant extracts, such as Crocus sativus L., Calliandra haematocephala, and Madhuca longifolia, as reducing agents in this process (Bagherzade et al. 2017; Raja et al. 2017; Patil et al. 2018). The rich abundance of phytochemicals is a key factor that makes the composition of these extracts suitable, indicating complex activities such as reduction, stabilization, and the formation of capping materials around Ag⁰. Consequently, AgNPs have the potential to be used in drug delivery for many applied nanomaterials (Vanlalveni et al. 2021; Abdellatif et al. 2022a, b, c, d, e; Ghani et al. 2022; Gur 2022). AgNPs are known for their antimicrobial properties and are widely used in various therapeutic applications. Their antimicrobial and cytotoxic effects position them as a promising option for cancer treatment (Abdellatif et al. 2021a, b). Gr-AgNPs are considered “nano” and contain natural reducing materials. It can be applied to deliver cytotoxic medication to cancer and other targeted cells (Abdellatif et al. 2022a, b, c, d, e). One of the most effective anticancer agents is the curcumin derivative ST06, which, when combined with a reduced amount of Gr-AgNPs (ST06-AgNPs), demonstrated considerable anticancer activity, inducing cell death in 50% of HeLa cells at a concentration of 1 μg/mL. In addition, intraperitoneal injection of 5 μg/mL ST06-AgNP effectively inhibited tumor growth in tumor-bearing mice without impacting their body weight (Sakthi Devi et al. 2022). In addition, the reduction of Gr-AgNPs in Spinacia oleracea leaves led to 100% growth inhibition at a concentration of 20 μg/mL, demonstrating strong anticancer activity. In comparison, a concentration of 100 μg/mL resulted in 20% viability (Ramachandran et al. 2017). The Gr-AgNPs have demonstrated potential as anticancer agents, exhibiting toxic effects comparable to traditional chemotherapy medications. As “nanologics,” AgNPs help mitigate the severity of undesirable side effects. Owing to their nanostructure, AgNPs can be targeted passively or actively to tumor cells, allowing them to accumulate in high concentrations at the site of the tumor (Kovacs et al. 2022). The passive accumulation of AgNPs is influenced by the morphology of tumor tissue, resulting in an atypical endothelial layer and fenestrated vasculature owing to neoangiogenesis and compromised lymphatic drainage. This process facilitates the penetration and accumulation of nanosized materials in cancerous tissues (Maeda et al. 2013) (EPR effect) (Kalyane et al. 2019). It has also been suggested that the shape, size distribution, and coating materials adhered to the surface of AgNPs can optimize their passive distribution (Kovacs et al. 2022). However, evidence suggests that the EPR effect enhances drug accumulation in tumor cells, although its effectiveness is considerably influenced by the specific anatomic characteristics of the tumor (Harrington et al. 2001). Although the EPR effect may be a key idea in AgNP-tumor targeting, it should be viewed cautiously because it has recently been scrutinized. AgNPs can specifically concentrate in malignant tissues. Techniques for passive and active targeting have been developed for Ag-based nanosystems. The interactions between noncancerous and cancerous cells in neoplastic tissues differ, influencing disease progression and medical outcomes (Younis et al. 2022; Abdellatif et al. 2022a, b, c, d, e). The size of AgNPs is the main factor affecting their uptake. Both the pathway used and the quantity of AgNPs depend on their size, tension, and the spreading of the internalized cells. Neutral and charged AgNPs typically use the paracellular route for transport, while cationic AgNPs commonly use the clathrin-mediated pathway (Harush-Frenkel et al. 2008; Zhang and Monteiro-Riviere 2009). Irregularly shaped AgNPs have a lower likelihood of internalization compared to spherical AgNPs. In addition, the accumulation of AgNPs in specific organs or tissues is influenced by their shape. For example, discoidal shaped AgNPs are more readily internalized in the lungs than spherical or cylindrical forms. Furthermore, fibrosarcoma and breast cancer cells tend to ingest nanoworms more than round AgNPs. Various properties of AgNPs, such as opsonization, surface conductivity, and permeability, can be modulated to enhance or hinder uptake. Incorporating molecules such as polyethylene glycol (PEG) or zwitterionic agents can help address this issue (Panzarini et al. 2018). There is a strong correlation between the uptake of metal NPs and cellular responses. The synthesized Gr-AgNPs can traverse the extracellular space at neutral pH and effectively pass through endosomes at pH 6 and acidic pH (4.5). In addition, the concentration of AgNPs is a critical factor influencing both the uptake and toxicity of Gr-AgNPs (Sabella et al. 2014). Consequently, cellular toxicity can be mitigated by incubating with specific ion chelators, which do not impact uptake efficiency or lead to an increase in the release of toxic ions (Flora and Pachauri 2010). Alternatively, lysosomotropic drugs can be used to address these conditions by neutralizing lysosomal acidity, thereby reducing the release of toxic ions (Panzarini et al. 2018). According to previous studies, VEGF induces angiogenesis—a critical process in the development and spread of malignancies—by promoting the formation of new blood vessels (Carmeliet 2005). Furthermore, AgNPs interfere with VEGF functions, acting as antiangiogenic agents (Kalishwaralal et al. 2009). In addition to enhancing the effectiveness and potency of anticancer drugs and preventing drug resistance in cancer cells, AgNPs facilitate the targeted delivery of these medications, thereby minimizing the adverse effects of chemotherapy on healthy tissues (Sundararajan et al. 2014; Patra et al. 2015). AgNPs act as anticancer agents by activating the ptdlns3K pathway in cancer cells, promoting autophagy. Furthermore, studies using a mouse melanoma cell model demonstrated enhanced efficacy in killing cancer cells when autophagy was inhibited by autophagic inhibitors such as wortmannin (B16 cell lines) (Sundararajan et al. 2014; Lin et al. 2015). One of the most effective plants for the green synthesis of AgNPs is Moringa oleifera. Native to the Indian subcontinent, this tree’s leaves, fruits, and seeds have been used for centuries owing to their medicinal properties (Mohammed and Hawar 2022). The leaves of Moringa oleifera are particularly rich in antioxidants, vitamins, and minerals, making them highly suitable for the green synthesis of AgNPs. This eco-friendly process uses Moringa oleifera leaf extracts for AgNP synthesis, offering a simple, cost-effective alternative that avoids harmful chemicals. Alongside Moringa oleifera, other plants recognized for their effectiveness in the green synthesis of AgNPs include Aloe vera, Azadirachta indica (neem), and Ocimum sanctum (holy basil) (Tailor et al. 2020;

Concluding remarks

Green-synthesized AgNPs derived from natural extracts exhibit a wide range of applications in cancer therapy. These NPs effectively inhibit cancer-cell growth, induce apoptosis, and minimize the side effects associated with chemotherapy. Although the use of Gr-AgNPs in cancer treatment is still in its early stages, initial results are promising. Gr-AgNPs have shown efficacy in treating various cancers, including breast, lung, colon, and liver cancers. A considerable advantage of using Gr-AgNPs in cancer therapy is their safety for human use. Unlike conventional chemotherapies, which often lead to considerable side effects, Gr-AgNPs do not harm healthy cells. The green synthesis of AgNPs using natural extracts represents a promising advancement in cancer therapy. This approach leverages plants in a straightforward, cost-effective, and environmentally friendly process that has the potential to transform cancer treatment. Although further research is required to fully explore the capabilities of Gr-AgNPs in cancer therapy, the preliminary results have been highly encouraging.

Funding

The authors declare that no funding was obtained for this study.

Data availability

Not applicable.

Declarations

Conflict of interest

The authors report no conflict of interests associated with this work.