Green synthesis of CuNPs using Camellia sinensis and Ocimum sanctum under UV light

Abstract

The development of environmentally benign routes for the synthesis of metallic nanoparticles is an important objective in contemporary nanomaterials research, particularly to minimize reliance on toxic chemical reductants and stabilizers. In the present study, copper nanoparticles (CuNPs) were synthesized using aqueous extracts of Camellia sinensis (green tea) and Ocimum sanctum (tulsi), which functioned simultaneously as reducing and capping agents. Two synthesis routes were systematically investigated: a conventional phytogenic reduction process and an ultraviolet (UV-A, 365 nm) assisted photoreduction approach. In the conventional method, gradual reduction of Cu²⁺ ions over 3 h yielded predominantly spherical CuNPs with particle sizes in the range of 20–50 nm and a zeta potential of −29.5 mV, indicating moderate colloidal stability. In contrast, UV-assisted synthesis significantly accelerated nanoparticle formation, completing the reaction within 15–20 min and producing smaller particles (8–25 nm) with improved stability (zeta potential between −30 and −38 mV). UV–visible spectroscopy revealed distinct surface plasmon resonance bands for both samples, with the UV-assisted CuNPs exhibiting a sharper and slightly blue-shifted peak, consistent with reduced particle size and narrower size distribution. Fourier transform infrared spectroscopy confirmed the involvement of polyphenolic and phenolic constituents, including catechin- and eugenol-type moieties, in the reduction and stabilization of CuNPs. X-ray diffraction analysis verified the formation of crystalline face-centered cubic copper with minor contributions from Cu₂O, while transmission electron microscopy corroborated the enhanced uniformity of UV-assisted nanoparticles. The antibacterial activity of the synthesized CuNPs was evaluated against Escherichia coli and Staphylococcus aureus, revealing effective growth inhibition for both bacterial strains. UV-assisted CuNPs exhibited marginally higher antibacterial efficacy, which is attributed to their smaller size and improved surface stabilization. Overall, this study demonstrates that UV-assisted phytogenic synthesis offers a rapid and reproducible route to stable copper nanoparticles, while maintaining the advantages of green chemistry. The findings highlight the potential of combining plant-derived reductants with photochemical activation to achieve controlled nanoparticle synthesis for antimicrobial and related applications.