Eco-friendly synthesis of nanosized Ag and Cu for the catalytic disintegration of 4-nitrophenol

An eco-friendly strategy was developed for the preparation of silver and copper nanoparticles using Tecoma stans (TS) flower extract. The synthesized nanostructures exhibited predominantly spherical morphologies, with particle sizes of 9.8 nm and 3.6 nm for silver and copper, respectively. The X-ray diffraction (XRD) analysis confirmed the face-centered cubic (fcc) of the nanoparticles. The catalytic activity of prepared nanoparticles was assessed toward 4-nitrophenol (4-NP) reduction under the presence of NaBH₄. Compared with sivler nanoparticles, the copper nano catalysts demonstrated excellent catalytic activity achieving rapid conversion of 4-NP to 4-aminophenol (4-AP) with 93.5% catalytic efficiency. Furthermore, the catalysts demonstrated good reusability, and retaining the conversion efficiency of 95% even after five reaction cycles. The environmentally friendly synthesis, high catalytic performance, and recyclability highlight the ability of Ag and Cu nanoparticles for diverse catalytic applications.

Introduction

4-Nitrophenol (4-NP) is an aromatic phenolic substance widely utilized in the manufacturing of pharmaceuticals, pesticides, analgesics, dyes, pigments, fungicides, explosives, and leather processing (Cardoso Juarez et al., 2024). However, its release into industrial wastewater poses a serious environmental threat. Due to its high solubility, chemical stability, and poor biodegradability, 4-NP persists in aquatic ecosystems, where it exhibits bioaccumulative and toxic properties (Farré et al., 2008). Prolonged exposure can severely affect multiple organs, such as lungs, kidneys, brain, liver, and eyes, and may lead to health complications such as anemia, cyanosis, hepatotoxicity, mutagenesis, and carcinogenesis (Dinari et al., 2024; Tazi et al., 2024). Consequently, the US-EPA has classified 4-nitrophenol as a hazardous organic pollutant, limiting its permissible intensity in water to 10 ppb (Yahya et al., 2021). Effective elimination of 4-nitrophenol from wastewater is, therefore, an urgent environmental priority.

Several treatment methods, including adsorption (Yao et al., 2014), electrocoagulation (Zhang et al., 2023), photocatalytic degradation (Kadam et al., 2021), biodegradation (Sarkar and Dey, 2020), and advanced oxidation processes (Nie et al., 2016), have been investigated for 4-NP removal. However, these approaches often suffer from constraints such as tedious reaction times, high energy consumption, high cost instrumentation, and the exploitation of harmful reagents (Tchieno and Tonle, 2018; Farajollahi and Poursattar Marjani, 2024). This creates an attention on the development of simple, cost-profitable, and ecologically benign techniques for efficient degradation of 4-NP. Nanomaterials have arose as favorable alternatives in catalysis and environmental remediation, owing to their high surface area, flexible physicochemical behaviours, and enhanced catalytic activity (Baig et al., 2021). Specifically, noble and transition metal nanoparticles including Au (Mikami et al., 2013), Ag (Bindhu and Umadevi, 2015), Cu (Gawande et al., 2016), Pt (Januszewska et al., 2013), Ru have shown excellent catalytic activity (Asadzadeh and Poursattar Marjani, 2025). Among them, silver and copper nanoparticles are attractive due to their optical and electronic properties, thermodynamic stability, low toxicity, biocompatibility, and natural abundance (Maliki et al., 2022; Shahzadi et al., 2025). Although silver nanoparticles have been extensively studied as catalysts for nitrophenol reduction, conventional synthesis methods often involve toxic reagents, high-temperature conditions, or complex procedures, limiting their sustainability (Payamifar et al., 2025). In this context, plant-mediated synthesis, or “green synthesis,” has gained significant attention as an eco-friendly approach, (Shafey, 2020; Singh et al., 2023). In this study, a simple and environmentally benign strategy for synthesizing Ag and Cu nanoparticles using Tecoma stans flower extract has reported. The biosynthesized nanoparticles were systematically characterized and their catalytic activity for the removal of 4-nitrophenol to 4-aminophenol was investigated. This work not only demonstrates the effectiveness of TS flower as a bio-reductant but also establishes the potential of such nanomaterials in sustainable wastewater treatment.

Experimental section

Preparation of plant extract

Fresh Tecoma stans flowers were harvested from St. Xavier’s College, Palayamkottai, Tamil Nadu, India. The flowers were thoroughly cleaned with D.I H₂O to remove dust and impurities. Approximately 5 g of finely chopped fresh flowers were immersed in 50 mL of D.I H₂O and refluxed for 30 min at 80 °C. Then the clean extract was obtained by filtrate the above extract using Whatman filter paper and centrifugation. Finally, the resultant extract was preserved at 5 °C for future use.

Synthesis of silver nanoparticles (AgNPs)

For AgNP synthesis, 5 mL of 0.01 M AgNO₃ solution was added in 3 mL of T. stans flower extract. The T. stans flower extract contains flavonoids and polyphenols, which serve as natural capping and reducing agents. The pH of the reaction mixture was adjusted to 8 by the dropwise addition of 2 M NaOH under magnetic stirring for 10 min. This pH increases the ability to reduce bioextract by enhancing deprotonation of carboxyl and hydroxyl groups, thereby increasing the metal-ligand interactions, accelerating the Ag + reduction and stabilizing the nanoparticles formation. Then the mixture was subjected to microwave irradiation (1000 W) for 45 s at a temperature of 75 °C. Finally, the product cools naturally to room temperature centrifuged, washed repeatedly with D.I H₂O and dried at 60 °C. The maximum yield of 15 mg was obtained by one experiment. The utilization of microwave irradiation offers uniform and rapid volumetric heating to the reaction solution, assisting to minimize the overall reaction time, increases the nucleation rate, and narrow particle size distribution. Because of these tremendous advantages, microwave irradiation has been considered as a key step contributor in the preparation.

Synthesis of copper nanoparticles (CuNPs)

For CuNP synthesis, 3 mL of flower extract was added in 10 mL of 1 mM CuSO₄·5H₂O. The pH of the reaction mixture was altered to 11 using 2 M sodium hydroxide, followed by microwave irradiation for 2 min (1000 W) at a temperature of 75 °C. Final product was allowed to cool room temperature and collected, centrifuged, washed several times with DI water, and dried at 60 °C. The yield of 24 mg was obtained in this process. Both Ag and Cu nanoparticles were redispersed in D.I H₂O and preserved at 5 °C for characterization and further applications.

Characterizations

The Ag and Cu nanoparticles formation was primarily identified using UV–visible spectrophotometry (Shimadzu UV-1700) at the wavelength range of 200–800 nm. The functional groups were labeled using FT-IR spectroscopy (Shimadzu IR-100) in the range of 4000–400 cm^-1 by making KBr pellets in the presence of prepared nanoparticles. The crystalline character was determined using XRD, Bruker Eco D8, diffractometer (CuKα radiation (λ = 1.54178 Å). Morphological features were studied using SEM, JSM-5600LV, JEOL Ltd., Japan and TEM, JEOL 211, operating at 90 kV.

Evaluation of catalytic activity

The Ag and Cu nanoparticles catalytic activity was evaluated via the reduction of 4-NP with the addition of NaBH₄. An 1 mM aqueous solution of 4-NP was added in 0.1 M NaBH₄ solution and aqueous nanoparticle catalyst (1 mL, prepared by dispersing 0.5 mg nanoparticles in DI water) was introduced into the above mixture by the dropwise addition. The reduction progress was monitored by recording UV–Vis spectra at regular intervals. For reusability studies, the catalysts were collected after the reaction by centrifugation, and thoroughly washed with D.I H₂O. The dried catalysts were reused for subsequent catalytic cycles under identical conditions. The catalytic efficiencies of respective catalysts were identified by the following equation.

$\text{Catalytic} \text{efficiency\hspace{0.17em}}\left(\%\right)=\left(\frac{A_0-A_t}{A_0}\right)\times 100$

Result and discussion

Biogenic nanoparticles were successfully synthesized using the Tecoma stans flower extract under microwave irradiation. As drawn in Figure 1, the flower extract exhibited a defined absorption peak at 326 nm, attributed to π–π* transitions of phenolic groups in flavonoids and polyphenolic compounds (Gendo et al., 2024). The HPLC was used to identify the major phytoconstituents in the extract (Supplementary Figure S1). The chromatograms exposed the existence of gallic acid and quercetin, with retention factor (Rf) values of 0.47 and 0.61, respectively (Mehesare et al., 2022). These results are consistent with previous reports and reveal the occurrence of flavonoids, terpenoids, phenolic acids and alkaloids (Senapati et al., 2013). Such phytochemicals are known to play key roles in reducing and stabilizing the nanoparticles. The silver and copper nanoparticles formation was determined by distinct absorption peaks at 422 nm (Das et al., 2010) and 545 nm (Mandke and Pathan, 2012), respectively (Figure 1), corresponding to their SPR bands.

Figure 1

Figure 1. UV-vis spectrum of Tecoma stans flower extract, Ag and Cu nanoparticles.

The formation of Ag nanoparticles proceeds via an electron-donation mechanism, wherein the hydroxyl groups (–OH) of phenolic acids undergo oxidation to quinones, thereby releasing electrons that reduce Ag⁺ ions to Ag⁰ (Figure 2a). Phenolic acids first form Ag⁺–polyphenol complexes, which then undergo electron transfer, producing Ag⁰ nuclei (Ozdemir-Sanci et al., 2025). As nucleation proceeds, phytochemicals cap the nanoparticle surfaces through hydrogen bonding and π–π interactions, preventing aggregation and controlling particle size (Hosseinzadeh et al., 2023). Likewise, the formation of Cu nanoparticles occurs via reduction by flavonoids (Figure 2b). Flavonoids such as quercetin undergo tautomeric transformation from the enol to keto form, releasing reactive hydrogen atoms and electrons that reduce Cu²⁺ to Cu⁰ (Khan et al., 2018). In addition, the aromatic ring of quercetin donates π-electrons to the partially filled d-orbitals of copper, further facilitating reduction. The carbonyl and hydroxyl groups of flavonoids then coordinate with Cu⁰ atoms, acting as capping agents that stabilize the nanoparticles and protect them from oxidation (Veisi et al., 2015).

Figure 2

Figure 2. Illustration of plausible (a) Ag and (b) Cu nanoparticles formation mechanism.

The SEM was employed to study the morphology and particle size of the synthesized silver (AgNPs) and copper nanoparticles (CuNPs). The SEM micrographs revealed that the biosynthesized AgNPs were predominantly spherical in shape (Figure 3). Energy-Dispersive X-ray Analysis (EDAX) further confirmed the elemental arrangement, validating the existence of silver in the nanoparticles (Supplementary Figure S2). Similarly, the morphology of the CuNPs demonstrated that the particles were spherical and exhibited a tendency to form agglomerates. EDAX analysis confirmed the presence of copper, along with signals for carbon (C) and oxygen (O). The detection of these additional elements suggests that bio-organic compounds resulting from the Tecoma stans flower extract were adsorbed on the surface of copper nanoparticles, likely contributing to their stabilization. The particle sizes of the Ag and CuNPs were assessed to be about 62 and 174 nm. The ICP-OES analysis confirmed that the AgNPs contained 95 wt% Ag and 91 wt% Cu indicating high-purity metallic silver and copper nanoparticles with only trace contributions from biomolecular capping agents.

Figure 3

Figure 3. SEM morphological results of (a,b) Ag and (c,d) Cu nanoparticles.

In XRD analysis, for AgNPs, four prominent peaks were observed at 38.09°, 44.12°, 64.52°, and 77.35°, corresponding to the reflection planes of (111), (200), (220), and (311), respectively (Figure 4). These peaks match the characteristic Bragg reflections of the face-centered cubic (fcc) structure of silver (Thirumagal and Jeyakumari, 2020). The sharpness of the peaks opens up a high degree of nanoparticles crystallinity, while slight broadening reflects the influence of biocompounds on the surfaces of Ag NPs (Wei et al., 2012). For CuNPs, XRD peaks were obtained at 2θ values of 35.49°, 36.27°, 42.05°, and 52.49°, corresponding to the planes (111), (200), (220), and (110). These values align with the JCPDS card No. 03-5645, corroborating the fcc structure of copper (Dong et al., 2018).

Figure 4

Figure 4. XRD spectra of biosynthesized (a) Ag and (b) Cu nanoparticles.

TEM images of AgNPs revealed nearly spherical particles with slight distortion (Figure 5). The particles were well dispersed, with no significant aggregation. The average particle size was calculated to be 9.8 nm, indicating a moderately broad size distribution. The crystalline behaviour of the AgNPs was established by the Selected Area Electron Diffraction (SAED) pattern, which exhibited distinct diffraction rings. TEM analysis of CuNPs demonstrated spherical, uniformly dispersed nanoparticles with a size of about 3.6 nm. The SAED patterns further confirmed their crystalline nature, showing multiple diffraction rings corresponding to the crystalline planes of copper.

Figure 5

Figure 5. HRTEM image of as synthesized (a–c) Ag and (d–f) Cu nanoparticles.

The FT-IR spectrum of AgNPs exhibited a broad band at 3285 cm^-1, corresponding to O–H stretching vibrations, demonstrating the existence of phenolic and alcoholic compounds (Figure 6) (Pasieczna-Patkowska et al., 2025). A band at 2299 cm^-1 was represented to C–H stretching of methylene or aliphatic groups. A shift in the absorption band from 1516 cm^-1 to 1613 cm^-1 was observed, corresponding to aromatic C=C stretching vibrations (Vijayaraj et al., 2018). Additionally, a weak band at 1380 cm^-1 indicated C–N stretching vibrations of amine groups, while a band at 1045 cm^-1 was attributed to C–O stretching vibrations of phenolic compounds (Rezazadeh et al., 2020). These functional groups, originating from phytochemicals in the flower extract, are likely responsible for both reducing Ag⁺ ions to Ag⁰ nanoparticles and stabilizing the resulting nanostructures. A comparative analysis with the FT-IR spectrum of the pure extract confirmed these interactions, with only minor shifts in band positions. The FT-IR bands shift in the prepared materials was ascribed to the phytochemical functional group coordination with the Ag and Cu nanoparticles surface. During the Ag and Cu nanoparticles formation, the aromatic, hydroxyl, and amine groups react with metal atoms via metal-ligand bonding, reducing the bond force constants of those functional groups. This phenomenon involves in the corresponding bonding modes shift to the lower wavenumbers, revealing the participation of those functional groups in the Cu and Ag nanoparticles stabilization and reduction processes.

Figure 6

Figure 6. FTIR results of biosynthesized (a) Ag and (b) Cu nanoparticles.

The catalytic efficiency of the synthesized AgNPs was evaluated through the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) using sodium borohydride (NaBH₄) as the reducing agent (Figure 7a). Owing to the presence of an electron-withdrawing nitro group, 4-NP is generally resistant to biological oxidation, chemical degradation, and hydrolysis, making it an ideal probe molecule for assessing catalytic activity. The 4-NP (300 μL, diluted in 2 mL of water) displayed a characteristic absorption band at 319 nm, agreeing the n–π** electronic transition, with a faint yellow coloration (Liu et al., 2017). Upon the addition of NaBH₄ (100 µL), the solution turned dark yellow, owing to the deprotonation of 4-nitrophenol to the 4-nitrophenolate ion under strong alkaline condition, which has been confirmed from the observed bathochromic shift from 319 nm to 400 nm (Pradhan et al., 2002). When AgNPs were instilled into the reaction solution, a gradual reduction in the absorption intensity at 400 nm was detected, alongside a visible fading of the yellow color. Eventually, the reaction mixture became colorless, signifying the successful reduction of 4-NP to 4-AP (Zhou et al., 2020). A new band appeared at 316 nm, conforming the formation of 4-aminophenol. This process highlights the electron transference from BH₄⁻ ions (donor) to 4-nitrophenol (acceptor), facilitated by the surface of the AgNPs acting as an efficient electron relay. The temporal evolution of the UV–visible absorption spectra clearly demonstrate the catalytic transformation of 4-nitropheno to 4-aminophenol. The catalytic performance of biosynthesized CuNPs was also assessed for the 4-nitrophenol reduction in NaBH₄ (Figure 7b). The aqueous 4-nitrophenol solution displayed a clear absorption peak at 317 nm (Liu et al., 2017). Upon addition of NaBH₄, this peak red-shifted to 414 nm may be due to the following reasons: (i) creation of alkaline conditions by the excessive addition of NaBH₄, (ii) nanoparticles interaction with 4-nitrophenol, (iii) changes in the ionic and solvation strength, which collectively modifying the electronic structure of the nitrophenolate ion and generating the bathochromic displacement of the absorption band. In the absence of a catalyst, no noticeable reduction was observed, confirming the requirement of a catalytic surface for electron transportation. However, upon introducing CuNPs into the system, the intensity of the absorption band at 414 nm progressively diminished. Concurrently, a new band emerged at ∼300 nm, characteristic of 4-AP, confirming the catalytic conversion of 4-NP (Zhou et al., 2020). The complete reduction reaction was achieved within 50 min, validating the catalytic efficacy of the CuNPs. The overall 4-NP reduction efficiencies of 90% and 93.5% were achieved for Ag and Cu nanoparticles, respectively (Supplementary Figure S3). Compared with Ag and recently reported results, the synthesised Cu nanoparticles exhibit excellent catalytic efficiency as evidenced from comparison table presented in Supplementary Table S1. To find the kinetics of the reaction, the change in the uv absorbance with increasing time has been identified and the fitted to the following equation:

$\ln \left(\frac{A_t}{A_0}\right)=-k_{\text{app}}t$

Figure 7

Figure 7. (a) UV-Vis spectra results observed in the presence of (a) Ag and (b) Cu nanoparticles. (c) Proposed mechanism for the conversion of -nitrophenol to 4-aminophenol.

A linear plot of $\ln \left(A_t/A_0\right)$ versus time confirms the pseudo–first-order nature of the reaction. The proposed reaction mechanism is summarized, and a schematic representation of the product transformation is presented in Figure 7c.

Conclusion

This study demonstrates a cost efficient, simple, and environmentally benign approach for the ecosynthesis of nanosized silver and copper using the Tecoma stans flower aqueous extract, which served as a reducing, capping, and stabilizing agent. Structural characterization confirmed the nanomaterials through UV–visible spectroscopy, FT-IR analysis, SEM/TEM imaging, and XRD studies, revealing spherical nanostructures. The catalytic activity of the nanoparticles was assessed via the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH₄. Both AgNPs and CuNPs exhibited excellent catalytic activity, with AgNPs showing faster reduction kinetics compared to CuNPs. The catalytic mechanism is attributed to the surface-mediated electron transmission from BH₄⁻ ions to 4-nitrophenol, resulting in the formation of 4-AP. Overall, green-synthesized AgNPs and CuNPs not only offer promising catalytic potential but also highlight the role of bioactive phytochemicals in producing stable, functional nanomaterials through sustainable methodologies.

Data availability statement

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/Supplementary Material.

Author contributions

JI: Conceptualization, Writing – original draft, Writing – review and editing. SSi: Investigation, Resources, Writing – review and editing. SG: Formal Analysis, Methodology, Writing – review and editing. AP: Methodology, Validation, Writing – review and editing. SSe: Data curation, Writing – review and editing. GR: Formal Analysis, Writing – review and editing. SN: Conceptualization, Data curation, Formal Analysis, Resources, Software, Writing – review and editing. JV: Conceptualization, Data curation, Methodology, Writing – review and editing. DV: Formal Analysis, Writing – review and editing. MO: Formal Analysis, Writing – review and editing.

Funding

The author(s) declared that financial support was not received for this work and/or its publication.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2026.1741476/full#supplementary-material

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