Eclipta prostrata leaf extract derived carbon dots integrated TiO₂ nanocomposites using for enhanced 4-nitrophenol reduction, antioxidant and antibacterial activity

Abstract

Water contamination by organic pollutants and microbes causes serious environmental problem and risks the life of living things. 4-Nitrophenol is a major constituent of several industrial effluents can damage the health of human. This study presents the development of a titanium dioxide (TiO₂) nanoparticles integrated with carbon dots (CDs) of Eclipta prostrata leaf extract designed for the catalytic reduction of 4-nitrophenol in water and antibacterial applications. CDs/TiO₂ nanocomposites were synthesized via a green, hydrothermal method. The structural and morphological analysis were performed by X-ray diffraction and microscopic analysis. The optical properties and stability of CDs/TiO₂ nanocomposites was studied using UV–visible and zeta potential measurements. The CDs/TiO₂ nanocomposite exhibited concentration-dependent antioxidant activity, results 90–95% radical scavenging efficiency, compared with the performance of ascorbic acid at 125 µg/mL in both 2,2-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing antioxidant power assays. Antibacterial studies showed strong activity against Staphylococcus aureus and Klebsiella pneumoniae, with zone of inhibition of 18.4 mm and 19.3 mm, respectively, at 50 µg/mL. The catalytic activity of CDs/TiO₂ nanoparticles were tested through the reduction of 4-nitrophenol (4-NP) into 4-aminophenol through hydrogenation. CDs/TiO₂ nanocomposites possess excellent catalytic activity in the reduction of nitrophenol, achieving near-complete conversion within 20 min with a rate of 0.168 min⁻¹ follows pseudo first order kinetic reaction. These findings suggest the enhanced catalytic and antibacterial property of CDs/TiO₂ nanocomposites can be potentially employed for the remediation of 4-NP and microbial contaminated water.

Supplementary Information

The online version contains supplementary material available at 10.1186/s11671-026-04703-6.

Introduction

Nanotechnology has emerged as a transformative field with profound implications for environmental remediation, biomedical and industrial applications [1–5]. The ability to synthesize and tailor nanoparticles structures with at least one dimension in the 1–100 nm range enables the exploitation of unique physicochemical properties in comparison with bulk materials. Among the various nanomaterials, titanium dioxide (TiO₂) nanoparticles have garnered significant attention due to their favourable photocatalytic activity, chemical stability, biocompatibility, and cost-effectiveness. TiO₂ nanoparticles capacity to harness light energy for the generation of electron–hole pairs,as a key candidate for applications ranging from pollutant degradation to antibacterial coatings [6–8].

The photocatalytic behaviour of TiO₂ is primarily attributed to its wide bandgap and high surface area, which facilitate the formation of reactive oxygen species (ROS) upon illumination with UV or visible light. These ROS, including hydroxyl radicals and superoxide anions, play a crucial role in breaking down organic pollutants and inactivating microorganisms [9–11]. Incorporating carbon dots (CDs) derived from biologically available source into TiO₂ nanocomposites enhances catalytic activity through extended visible-light absorption and up-conversion by CDs, which sensitize TiO₂ and inject photoexcited electrons into its conduction band, while the CD/TiO₂ interface promotes charge separation to suppress electron–hole recombination, prolonging carrier lifetimes for faster electron transfer [12–14]. For antibacterial activity, the nanocomposites generates higher reactive oxygen species (ROS) like –OH and O₂⁻ under visible light due to improved photoexcitation and charge transfer, damaging bacterial membranes, proteins, and DNA more effectively against both Gram-positive and Gram-negative strains. Enhanced dispersibility, tuned surface charge and close nano-bio contact from CDs facilitate uniform ROS delivery and membrane disruption, while bioactive phenols and flavonoids on CD surfaces improves synergy for broad-spectrum bactericidal effects superior to unmodified TiO₂ [15–18].

Environmental contamination seriously harms both the environment and human health [19, 20]. 4-Nitrophenol (4-NP) has been regarded as the most prevalent aromatic pollutant because of its high toxicity and good chemical and biological stability in an aqueous solution. It is widely used as a raw material or intermediate in a variety of industrial applications [21, 22]. In order to purify water and address environmental contamination, it is critical to remove 4-NP from wastewater. Because of the easy handling, high catalytic efficiency, and affordable cost, the catalytic reduction of 4-NP to 4-aminophenol (4-AP) using metal catalysts has been regarded as the most widely used technique to remove 4-NP in wastewater [23, 24]. CDs incorporation into TiO₂ nanoparticles framework boost substrate adsorption via π–π stacking with the nitrophenyl ring and hydrogen bonding from surface –OH, –COOH groups, concentrating 4-NP and BH₄⁻ near active sites to accelerate the pseudo-first-order reduction kinetics and increase turnover frequency compared to bare TiO₂ [14, 25]. Gregory et al. reported the Au/TiO₂ nanoparticles for the reduction of 4-NP with a rate of 1.3 × 10^–2 min⁻¹ under green light [26]. Mergenbayeva et al. reported the reduction of 4-nitrophenol by Ag-doped TiO₂ nanoparticles as a photocatalyst with 98% degradation in the presence of NaBH₄ and their antibacterial activity also examined [27]. Behra et al. reported the CuO/TiO₂/ZnO NPs anchored on exfoliated graphene for the reduction of 4-nitrophenol results in 98.4% of degradation at pH 7.1 [28].

The present study demonstrates a novel green synthesis approach for the fabrication of CD/TiO₂ nanocomposites using E. prostrata leaf extract, highlighting an eco-friendly and sustainable route compared to conventional chemical methods. The novelty of this work lies in the integration of bio-derived carbon dots with TiO₂ to develop a multifunctional nanocomposite exhibiting enhanced antibacterial, antioxidant, and catalytic activity toward 4-nitrophenol reduction. The synthesized CDs@TiO₂ NPs shows wider band gap was characterized by UV–visible spectroscopy studies. The morphology and structural analysis were analysed through XRD, FE-SEM and DLS measurements. The primary objective is to synthesize a cost-effective nanomaterial and systematically evaluate its physicochemical properties and functional performance, while also understanding the synergistic role of carbon dots in improving charge separation and surface reactivity of TiO₂. The advantages of this system include its green synthesis, low toxicity, improved electron transfer, enhanced reactive oxygen species generation, and broad applicability in environmental remediation and biomedical fields. However, certain limitations are also acknowledged, including the limited mechanistic validation using advanced spectroscopic techniques and lack of large-scale or real-time application studies. Overall, this work provides a promising platform for the development of sustainable nanomaterials with multifunctional performance.

Experimental methodology

Materials

Titanium isopropoxide (purity 97%), 4-nitrophenol (purity ≥ 99%) and sodium hydroxide (purity ≥ 99%) were purchased from Sigma-Aldrich, USA. 2,2-diphenyl-1-picrylhydrazyl (DPPH), ferric chloride (purity ≥ 99.99%), 2,4,6-tripyridyl-s-triazine (purity ≥ 98%), Acetate buffer were purchased from Sigma-Aldrich, USA. Distilled water was used throughout the experiment. All the chemicals were used as such received without further purification.

Preparation of TiO₂ nanoparticles

Approximately, 1.0 mmol of Titanium isopropoxide was dissolved in 100 mL of double distilled water. The whole mixture was kept under stirring at 800 rpm at room temperature. To that, dropwise addition of 100 mL of 2M NaOH was added and stirred for 8 h. After completion of reaction, the mixture was centrifuged and the white precipitate was repeatedly washed with ethanol and water. The precipitated sample was dried at 50 °C for overnight. The dried sample was annealed for 2 h at 500 °C at muffle furnace. The annealed sample was used for further studies.

Preparation of CDs/TiO₂ nanocomposites

The leaves of Eclipta prostrata (L.) L. was collected from Taif Region, (18°16′02″N 42°22′05″E) Saudi Arabia. The collection of E. prostrata plant materials were performed according to institutional, national, and international guidelines. Our collection methodology follows best practices for sustainable harvesting to ensure no long- term impact on the local ecosystem. Moreover, the collected plant are not rare or endangered plants, rather they are common plants available in Taif region, Saudi Arabia. The plant was authenticated by Botanist at University of Tabuk, Saudi Arabia. The voucher specimen no (EP/Taif/2015) was deposited in the herbarium of the Entomology and Toxicology Laboratory, University of Tabuk, Saudi Arabia. The leaf extract of E. prostrata was prepared by boiling 1 g of dried leaves in a 10 mL of double distilled water. From that, 2 mL of extract was mixed with the 1g of TiO₂ nanoparticles and annealed again at 150 °C for 1 h. The resultant CDs integrated TiO₂ nanocomposites was utilized for further characterizations and studies.

Antioxidant activity of CDs/TiO₂ nanocomposites

The method of DPPH was carried out as per Blois (1958) with slight modifications [29]. A freshly prepared 0.1 mM DPPH solution in methanol was used. Solutions of different concentrations (25, 50, 75, 100 and 125 µg/mL) of the CDs/TiO₂ nanocomposites were procured. To each test tube was added 1 mL of DPPH solution and 1 mL of NPs solution. The mixture was briefly vortexed and kept at room temperature in the dark for 30 min. Blank samples were prepared using methanol, and the positive control was ascorbic acid. The absorbance was read at 517 nm after incubation on a UV–Vis spectrophotometer. Percent of inhibition was calculated by the Eq. (1):

1

All experiments were conducted in triplicate. The results are expressed as mean ± standard deviation (SD). Statistical analysis was performed using one-way ANOVA followed by Tukey’s post-hoc test. A p value < 0.05 was considered statistically significant.

Antibacterial test of CDs/TiO₂ nanocomposites

The antibacterial property of CDs/TiO₂ nanocomposites was evaluated by the agar well diffusion method against gram-negative Klebsiella pneumoniae and gram-positive Staphylococcus aureus. A 100 µL of microbial cultures standardized at approximately 10⁶ CFU/mL were spotted on freshly prepared nutrient agar plates. Sterile nutrient agar plates were prepared and uniformly inoculated by spreading 100 µL of the standardized bacterial culture using a sterile cotton swab to ensure even distribution. The nanoparticles were diluted in sterile distilled water, and the dilutions were tested at 25 and 50 µg/mL. A standard antibiotic (streptomycin) was used as a positive control, while sterile distilled water served as a negative control. After the wells were punched on the agar, each NPs concentration was loaded into a well of 6 mm diameter. The plates were kept for 24 h at 37 °C. The antibacterial activity of each well was determined by the diameter sizes of zones of inhibition around its post-incubation. NPs antimicrobial activity was confirmed by containing control wells with distilled water only. All experiments were performed in triplicate to ensure the accuracy and reproducibility of the data.

The minimum inhibitory concentration (MIC) of the CDs/TiO₂ nanocomposites was determined using the standard broth microdilution method. Briefly, bacterial strains S. aureus and K. pneumoniae were cultured in nutrient broth. Serial twofold dilutions of CDs/TiO₂ nanocomposites were prepared in sterile nutrient broth to obtain concentrations ranging from 0.39 to 200 µg/mL. Each well of a sterile 96-well microtiter plate was filled with 100 µL of nanoparticle suspension and 100 µL of bacterial inoculum. Positive control wells contained bacteria with standard antibiotic (streptomycin), while negative control wells contained broth without bacteria. The plates were incubated at 37 °C for 24 h, and bacterial growth was assessed visually and by measuring absorbance at 600 nm using a microplate reader. For determination of the minimum bactericidal concentration (MBC), aliquots (10 µL) from wells showing no visible growth were plated onto fresh nutrient agar plates and incubated at 37 °C for 24 h.

Degradation of 4-nitrophenol by using CDs/TiO₂ nanocomposites as a catalyst

Photocatalytic degradation of 4-nitrophenol (4-NP) using CDs/TiO₂ nanocomposites was carried out by dispersing a 0.5 mg of the catalyst in a 50 mL of aqueous 4-NP solution of predetermined concentration. Initially, the suspension was magnetically stirred in the dark for 30–60 min to establish adsorption–desorption equilibrium between 4-NP molecules and the catalyst surface in the presence of excess of NaBH₄(0.01M) solution. Subsequently, the reaction mixture was exposed to visible light under continuous stirring to initiate catalysis. At regular time intervals, aliquots were withdrawn and centrifuged to remove the catalyst, and the residual concentration of 4-NP was determined using UV–visible spectrophotometry by monitoring the characteristic absorption peak of 4-NP. The decrease in absorbance with irradiation time indicated the progressive degradation of 4-NP.

The catalytic efficiency was quantitatively evaluated using the following relation (Eq. 2).

2

where C₀ and C_t represent the initial and time-dependent concentrations of 4-NP respectively, which are proportional to their corresponding absorbance values.

The reaction kinetics were analysed by fitting in a pseudo-first-order model (Eq. 3),

3

where k is the apparent rate constant (min⁻¹) and t is the reaction time. A linear plot of ln(C₀/C_t) vs time was used to determine the rate constant from the slope.

Results and discussion

The UV–visible absorption spectrum of E. prostrata leaf extract, TiO₂ NPs and CDs/TiO₂ nanoparticles was show in Fig. 1. The E. prostrata leaf extract exhibits strong absorption in the UV region with a shoulder around 322.1 nm, which can be attributed to π → π* transitions of aromatic compounds and n → π* transitions of functional groups present in phytochemicals [30]. For TiO₂ nanoparticles, a broad absorption can be seen in UV region confirms their characteristic absorption. CDs/TiO₂ nanoparticles exhibits the pronounced absorption in the UV region at 272.6 nm, confirming the incorporation of carbon dots with TiO₂ nanoparticles. The intense absorption below 300 nm arises mainly from the intrinsic band-to-band transition of TiO₂, while the distinct peak at 272.6 nm can be attributed to the n–π* or π–π* transitions associated with surface functional groups and conjugated domains in the CDs coupled to the TiO₂ matrix. The broad absorption seen in visible region might be arise from the defects state induced by CDs on incorporation with TiO₂ nanoparticles. The optical band gap calculated from the Tauc plot is 3.47 eV, which is slightly red-shifted compared with typical TiO₂ (3.12 eV) as shown in Fig. 1b, implying a modest narrowing of the band gap due to the interaction between CDs and TiO₂ [31]. This band-gap modification can be ascribed to charge transfer between CDs and TiO₂ and to the formation of composite interface states, which facilitate photon absorption at lower energies and can improve charge separation efficiency under UV or near-visible irradiation.

Fig. 1.

a UV–visible absorption spectrum of leaf extract, TiO₂ and CDs/TiO₂ NPs and b Tauc plot represents the bandgap energies of TiO₂ and CDs/TiO₂ NPs

The XRD pattern shown in Fig. 2 confirms that the CDs/TiO₂ nanocomposites crystallize in the rutile phase of TiO₂, as all diffraction peaks match well with the standard JCPDS card no. 21–1276 for rutile TiO₂, and the intensive peaks located at 2θ values indexed to the (110), (101), (111), (211), and (301) planes are characteristic of tetragonal rutile TiO₂ and agree with literature data [32]. The relatively sharp peaks suggest good crystallinity of the TiO₂ lattice, while the broad background hump at low angles is attributed to the amorphous nature of the carbon dots, and the absence of a clear graphitic carbon (002) peak near 24–27° along with the dominance of TiO₂ reflections indicates that the CDs are highly dispersed and confined on the TiO₂ surface, existing mainly in a semi-crystalline state. Any slight change in peak intensity compared to pure TiO₂ can be associated with surface modification by CDs, which may introduce lattice defects or microstrain without producing a new bulk crystalline phase. The retention of the pure rutile TiO₂ phase after CDs loading implies that the preservation of crystalline framework which is significant for maintaining photocatalytic stability.

Fig. 2.

XRD pattern of CDs/TiO₂ nanoparticles

The FTIR spectrum of plant extract and CDs/TiO₂ nanocomposite were shown in Fig. S1 & 3). Figure 3 exhibits a broad band at 3574.7 cm⁻¹ assigned to O–H stretching vibrations of surface hydroxyl groups on TiO₂. The band at 3029.4 cm⁻¹ attributed to aromatic or unsaturated C–H stretching that evidences organic moieties from the plant extract and the carbon framework anchored on TiO₂. The peak at 1520.2 cm⁻¹ corresponds to C = C/C=O and N–H bending vibrations of aromatic, amide, or conjugated carbonyl groups, confirming successful incorporation of CDs into the hybrid material, while the band at 1008.2 cm⁻¹ arises from C–O/C–O–C stretching of alcohols, ethers, or glycosidic linkages, further supporting the presence of oxygenated organic groups originating from both the leaf extract and CDs on the TiO₂ surface. The intense low-wavenumber band at 618.8 cm⁻¹ is characteristic of Ti–O/Ti–O–Ti lattice vibrations, with possible contributions from Ti–O–C bonds, demonstrating that the TiO₂ crystal framework is retained. Overall, the coexistence of Ti–O lattice vibrations with pronounced CDs bands indicates that TiO₂ nanoparticles are effectively decorated by hydrophilic, π-conjugated CDs and phytochemical residues from the leaf extract, providing abundant surface functional groups, enhance interfacial charge transfer, improve aqueous dispersibility, and furnish active adsorption sites that are advantageous for the photocatalytic and antimicrobial performance of the CDs/TiO₂ nanocomposite.

Fig. 3.

FTIR spectrum of CDs/TiO₂ nanoparticles

Figure 4 & S2 represents the FE-SEM and EDX image of the CDs/TiO₂ nanocomposites reveals closely packed, quasi-spherical nanoparticles with a relatively uniform morphology, indicating that the applied synthesis conditions facilitate homogeneous nucleation and growth of TiO₂ and the presence of carbon dots. The annealing temperature regulates the structure and pore size of TiO₂ NPs. The particle size distribution in the inset shows that most particles fall within a narrow nanoscale range of 100–200 nm with an average size of 146 nm, which is beneficial for providing a high specific surface area and abundant active sites for photocatalytic reactions. The EDX (Fig. S2(c)) and elemental mapping further confirms that Ti, O, N, and S are uniformly distributed over the entire scanned region, demonstrating successful incorporation of heteroatom-doped CDs onto the TiO₂ surface without detectable phase segregation. The homogeneous presence of N and S suggests that these dopants are associated with the carbon dots anchored on TiO₂, which can introduce mid energy states, enhance visible-light absorption, and facilitate charge separation by acting as electron donors/acceptors [33, 34]. The combined FE-SEM morphology and elemental mapping results therefore support the formation of a well-dispersed CDs/TiO₂ nanocomposite.

Fig. 4.

a FE-SEM image of CDs/TiO₂ nanoparticles, inset image shows the particle size distribution image and b Elemental mapping analysis of Ti, O, N and S present in CDs/TiO₂ nanoparticles

The HR-TEM image of CDs/TiO₂ nanoparticles was shown in Fig. 5, reveals the morphological and structural features of the nanocomposites. The micrograph shows aggregated, quasi-spherical nanoparticles with a relatively uniform distribution, forming clustered assemblies. The individual particles are observed to be in the nanoscale range, which is significantly smaller than the agglomerated sizes observed in FE-SEM and DLS analyses, indicating that the larger sizes obtained earlier correspond to particle aggregation in the aqueous medium.

Fig. 5.

HR-TEM image of CDs/TiO₂ nanoparticles

Dynamic light scattering analysis represented in Fig. 6a revealed a narrow monomodal size distribution with an average hydrodynamic diameter of about 157.5 nm for the CDs/TiO₂ nanocomposite, indicating the formation of colloidally stable nanoaggregates in the aqueous medium and suggesting that the CDs efficiently decorate the TiO₂ surface without causing extensive agglomeration. The relatively small and uniform particle size is consistent with the FE-SEM results. Zeta potential measurements (shown in Fig. 6b) exhibits a negative surface charge of − 24.7 mV, reflecting deprotonated hydroxyl, carboxyl, and other oxygenated groups originating from the leaf-extract-derived CDs and TiO₂ surface, and the potential value indicates good electrostatic stabilization against aggregation in suspension. The negatively charged surface can further promote dispersion in water, facilitate electrostatic attraction toward positively charged pollutants or microbial cell components, and thus is expected to contribute significantly to the improved functional performance of the CDs/TiO₂ nanocomposite.

Fig. 6.

a Size distribution (DLS) image and b zeta potential image of CDs/TiO₂ nanoparticles

The antioxidant activity of the leaf-extract-derived CDs/TiO₂ nanocomposite increased in a concentration-dependent manner in both DPPH and ferric reducing antioxidant power (FRAP) assays, demonstrating efficient free-radical scavenging and electron-donating ability comparable to the standard ascorbic acid at higher concentrations (Fig. 7a, b). The enhanced activity can be attributed to the presence of abundant surface functional groups such as –OH, –COOH, and conjugated π-domains originating from the carbon dots and phytochemicals of E. prostrata. These functional groups facilitate hydrogen atom transfer and single electron transfer mechanisms, which are responsible for neutralizing free radicals. In the DPPH assay, the percentage inhibition rose progressively from the 25 µg/mL to 125 µg/mL, where the nanocomposite approached the scavenging efficiency of ascorbic acid, indicating that surface-bound phenolic, carboxyl, and conjugated groups from the CDs and phytochemicals can effectively quench DPPH radicals by hydrogen atom or electron transfer. A similar dose-dependent trend was observed in the FRAP assay, in which the CDs/TiO₂ nanoparticles exhibited steadily increasing ferric-to-ferrous ion reducing power with concentration and showed only a modest difference from ascorbic acid at the highest dose, suggesting that the redox-active functional groups on the CDs/TiO₂ surface act as efficient electron donors and contribute to strong overall reducing capacity. These findings imply that decoration of TiO₂ with CDs enhances radical scavenging and reducing properties, which are beneficial for prospective biomedical applications where oxidative stress mitigation is required [35].

Fig. 7.

a DPPH and b ferric reducing Antioxidant power assay of CDs/TiO₂ nanoparticles. Values are represented mean ± SD

The CDs/TiO₂ nanocomposite exhibited pronounced antibacterial activity against both the gram-positive S. aureus and the gram-negative K. pneumoniae in the agar well diffusion assay, with a clear concentration-dependent increase in the zone of inhibition that was comparable to the positive control at the higher dose (shown in Fig. 8). For S. aureus, the nanocomposite produced zone of inhibition of 9.9 mm and 18.4 mm at 25 and 50 µg/mL, respectively, approaching the 19.3 mm zone obtained for the standard antibiotic streptomycin, indicating that CDs/TiO₂ is highly effective against gram-positive bacteria. In the case of K. pneumoniae, zones of 14.6 mm and 19.3 mm were obtained at 25 and 50 µg/mL, respectively, which are close to the 20.2 mm inhibition shown by the positive control, demonstrating strong activity also toward gram-negative bacteria despite their more complex outer membrane barrier. The absence of any inhibition in the negative control confirms that the observed bactericidal effect arises solely from the CDs/TiO₂ nanocomposite, and the slightly larger zones against K. pneumoniae suggest that the nanocomposite can effectively interact with and disrupt both cell wall types, likely via a combination of reactive oxygen species generation, membrane damage, and interaction of negatively charged, functionalized CD surfaces with bacterial cell components [36, 37]. The antibacterial activity of TiO₂ and CDs/TiO₂ nanocomposites was assessed against bacterial strains were shown in (Fig. S3). The results clearly indicate that the CDs/TiO₂ nanocomposite exhibits significantly improved antibacterial performance compared to bare TiO₂ nanoparticles.

Fig. 8.

Antibacterial activity of CDs/TiO₂ nanoparticles against a Gram positive Staphylococcus aureus and b Gram-negative Klebsiella pneumoniae and c shows the corresponding zone of inhibition values

The antibacterial efficacy of CD/TiO₂ nanoparticles was evaluated through MIC and MBC determination against Klebsiella pneumoniae and Staphylococcus aureus. The results (Table S1) clearly demonstrate concentration-dependent antibacterial activity of the nanocomposite. For K. pneumoniae, the MIC was observed at 6.25 µg/mL, indicating that bacterial growth was effectively inhibited at this low concentration. The MBC was determined at 12.5 µg/mL, confirming complete bactericidal activity at slightly higher concentration. Similarly, for S. aureus, the MIC and MBC values were found at 12.5 µg/mL and 25 µg/mL, respectively. These results suggest that K. pneumoniae is slightly more susceptible to CD/TiO₂ nanoparticles compared to S. aureus. The lower MIC values indicate strong antibacterial potency, while the relatively close MIC and MBC values suggest that the nanocomposite exhibits bactericidal rather than merely bacteriostatic behaviour.

The CDs/TiO₂ nanocomposite exhibited potential antibacterial activity against both Gram-positive and Gram-negative bacteria, confirming its broad-spectrum efficacy. Primarily, the nanocomposite generates ROS, such as hydroxyl radicals (·OH) and superoxide anions (O₂⁻·), which induce oxidative stress in bacterial cells. The generated ROS attack cellular components including lipids, proteins, and DNA, leading to membrane disruption and eventual cell death. The incorporation of carbon dots enhances visible-light absorption and promotes efficient electron–hole separation in TiO₂, thereby increasing ROS generation. In addition to that, the nanoscale size and high surface area of the CDs/TiO₂ facilitate close interaction with bacterial cell membranes. The negatively charged surface of nanoparticles, as confirmed by zeta potential analysis, promotes electrostatic interactions with positively charged sites on bacterial membranes, leading to structural damage. The slightly higher activity observed against K. pneumoniae suggests that the nanocomposite effectively penetrates even the complex outer membrane of Gram-negative bacteria.

The catalytic reduction of 4-nitrophenol (4-NP) by NaBH₄ in the presence of CDs@TiO₂ nanoparticles was monitored by UV–Vis spectroscopy, and the results clearly demonstrate the high catalytic efficiency of the nanocomposite (represented in Fig. 9). Initially, a strong absorption band centered at about 400 nm corresponding to the nitrophenolate ion was observed, which gradually decreased in intensity with reaction time and almost disappeared within 12 min, indicating the rapid conversion of 4-NP to the corresponding 4-aminophenol (4-AP) (shown in Fig. 9a). Concurrently, the reaction mixture changed from intense yellow to colourless, further confirming the successful reduction of 4-NP to 4-AP. The C_t/C₀ versus time plot showed a continuous decline in the concentration of 4-NP with increasing reaction time, reflecting a smooth and uninterrupted reduction process governed by surface catalysis on CDs@TiO₂ (Fig. 9b). TiO₂ nanoparticles show a comparatively slower reduction, reaching only ~ 45% degradation within the same time frame. This clearly demonstrates the superior catalytic efficiency of the CDs/TiO₂ system. The linear relationship obtained in the plot of ln(C_t/C₀) against time indicates that the reduction of 4-NP follows a pseudo-first-order kinetic model with respect to 4-NP in the presence of excess NaBH₄ (Fig. 9c). The apparent rate constant (K_app) was calculated to be 0.168 min⁻¹ with an excellent correlation coefficient (R² = 0.988), suggesting that CDs@TiO₂ provides abundant active sites and efficient electron transfer pathways for the reduction reaction. Based on the UV–visible absorbance data (Fig. 8b), the CDs@TiO₂ nanocomposite exhibited 89.2% degradation of 4-NP within 12 min, confirming its excellent catalytic efficiency (shown in Fig. S4). The complete degradation of 4-NP was achieved at about 20 min. The rapid decrease in the characteristic absorption peak at ~ 400 nm indicates almost complete conversion of 4-NP into 4-aminophenol. Pitcheri et al. reported the green synthesized TiO₂ NPs for the degradation of 4-NP with the degradation efficiency of 98% in alkaline medium showed decreasing rate of 0.011 min⁻¹ [38]. Migdadi et al. reported the degradation of 4-NP with 94% reduction by TiO₂/rGO nanocomposites [39].

Fig. 9.

Time-dependent UV–Vis spectral changes for the catalytic reduction of 4-nitrophenol using CDs/TiO₂ nanoparticles in the presence of NaBH₄ b corresponding C_t/C₀ decay profile of TiO₂ and CDs/TiO₂ nanoparticles, c pseudo-first-order kinetic plot with apparent rate constant, and d schematic illustration of the reaction pathway from 4-nitrophenol to 4-aminophenol via nitrophenolate intermediate

The schematic illustration shown in Fig. 9d further explains the plausible mechanism for the catalytic reduction of 4-NP by CDs@TiO₂ in the presence of NaBH₄. In alkaline medium, 4-NP is first converted to the nitrophenolate ion, which strongly adsorbs on the surface of CDs/TiO₂. The catalyst facilitates the transfer of electrons from the BH₄⁻ ions to the adsorbed nitrophenolate, leading to the formation of 4-AP and regeneration of the catalyst surface. The CDs/TiO₂ nanocomposite acts as an electron mediator, in the presence of NaBH₄, facilitating the electron transfer from BH₄⁻ ions to 4-NP molecules adsorbed on the catalyst surface. The CDs play a crucial role by enhancing electron mobility and reducing recombination losses, thereby accelerating the reduction process. In addition to that, the π–π interactions between the aromatic ring of 4-NP and the conjugated structure of carbon dots enhance adsorption of the pollutant onto the catalyst surface. The presence of surface functional groups on nanoparticles also promotes hydrogen bonding, thereby increasing the concentration of reactants near active sites facilitates the feasible reaction. During this process, ROS enhances the reaction kinetics by improving adsorption and transformation efficiency. The synergistic effect between carbon dots and TiO₂, including enhanced electron mobility and increased surface area, accounts for the high reduction rate and efficient recyclability of the catalyst in nitrophenol degradation applications.

Conclusion

In this study, E. prostrata leaf extract–derived carbon dots functionalized TiO₂ nanoparticles (CDs/TiO₂) were successfully synthesized via a green hydrothermal approach and systematically evaluated for their physicochemical, antioxidant, antibacterial and catalytic performances. UV–visible spectroscopy revealed a strong absorption at 272.6 nm with a calculated optical band gap of 3.47 eV, indicating effective interaction between CDs and TiO₂ and modest band gap modification. XRD analysis confirmed the retention of the rutile TiO₂ phase with good crystallinity, while the absence of distinct carbon peaks suggested uniform surface decoration of amorphous CDs. FTIR spectra evidenced abundant surface functional groups (–OH, C=O, C–O), supporting strong interfacial bonding and enhanced surface reactivity. FESEM and DLS analyses showed quasi-spherical nanoparticles with an average hydrodynamic diameter of ~ 157.5 nm, and a negative zeta potential of − 24.7 mV confirmed good colloidal stability. The CDs/TiO₂ nanocomposite exhibited concentration-dependent antioxidant activity, approaching the performance of ascorbic acid at 125 µg/mL in both DPPH and FRAP assays, highlighting efficient free-radical scavenging and reducing power imparted by phytochemical residues and carbonaceous domains. Antibacterial studies demonstrated strong activity against both S. aureus and K. pneumoniae, with zone of inhibition reaching 18.4 mm and 19.3 mm, respectively, at 50 µg/mL, comparable to standard antibiotics. Furthermore, the nanocomposite exhibited excellent catalytic efficiency for the reduction of 4-nitrophenol, achieving near-complete conversion to 4-aminophenol within 20 min and following pseudo-first-order kinetics with an apparent rate constant of 0.168 min⁻¹ (R² = 0.988). Overall, the synergistic integration of bio-derived carbon dots with TiO₂ significantly enhanced charge transfer, surface functionality, and catalytic as well as biological activity. The enhanced degradation performance indicates improved catalytic activity, which can be attributed to increased surface reactivity, effective charge separation, and higher generation of reactive oxygen species. These results demonstrate that CDs/TiO₂ nanoparticles are promising multifunctional materials for environmental remediation and antimicrobial applications, particularly in the treatment of organic pollutant and microbe contaminated water.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Conceptualization, Supervision, Methodology, Validation and Writing, review & Editing—RFA.

Funding

This article is derived from a research grant funded by the Research, Development, and Innovation Authority (RDIA)—Kingdom of Saudi Arabia, with grant number (13445-Tabuk-2023-UT-R-3-1-SE).

Data availability

The data used to support the findings of this study are included within the article. Further information can be made available upon request from corresponding authors.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent to publish

All authors read and approved the final manuscript for publication.

Competing interests

The authors declare no conflicts of interest.