Sunlight-Catalysed Green Synthesis of Copper Nanoparticles Using Green Tea Extract: Correlating Solar Irradiance with Nanoparticle Physicochemical and Antimicrobial Properties

This study presents a green, sunlight-assisted route for the synthesis of copper nanoparticles (CuNPs) using aqueous green tea (Camellia sinensis) extract as a natural reducing and stabilizing agent. Particular emphasis was placed on examining the influence of diurnal variations in solar irradiance on nanoparticle formation, morphology, and antimicrobial performance. The synthesis was carried out under three distinct sunlight exposures—morning, midday, and evening—to capture natural differences in photon flux. UV–Vis spectroscopy confirmed the formation of CuNPs, with surface plasmon resonance bands appearing between 570–580 nm, consistent with earlier reports on copper nanostructures [6]. Particles synthesized at midday, under the highest irradiance, displayed sharper absorption peaks and more uniform size distribution compared with morning and evening syntheses. TEM analysis revealed that midday-synthesized CuNPs had the smallest average size (21.8 ± 2.6 nm), whereas morning and evening syntheses produced larger particles (32.1 ± 3.2 nm and 41.5 ± 3.9 nm, respectively). FTIR spectra confirmed the presence of functional groups from green tea catechins on the nanoparticle surface, indicating their role in reduction and stabilization [3]. XRD analysis demonstrated the crystalline FCC structure of metallic copper [5], and zeta potential measurements (–28.4 mV) suggested good colloidal stability. Antimicrobial assays against Escherichia coli and Staphylococcus aureus revealed dose-dependent inhibition, with the smallest, midday-synthesized CuNPs exhibiting the strongest activity. Collectively, these results indicate that solar irradiance is a critical factor influencing nanoparticle synthesis and functional performance, and suggest that sunlight-assisted green synthesis offers a sustainable, cost-effective pathway for producing CuNPs with potential biomedical applications [2,4,7].