Researchers in Andhra Pradesh, India, have shown that the intensity of natural sunlight can measurably shape the size and antibacterial performance of copper nanoparticles made with green tea extract. Working under clear summer skies on a single day in May 2025, the team synthesized copper nanoparticles from copper sulphate and an aqueous extract of Camellia sinensis at three times of day, recording solar irradiance with a calibrated pyranometer. Their statistical analysis of more than 200 particles per sample found a strong negative correlation between irradiance and particle size, with midday sunlight producing the smallest particles at 18.6 ± 3.1 nanometers.
The study, published as open access research in the journal Bioscience Nanotechnology on 15 June 2026, addresses a gap the authors identify in earlier green synthesis work, where sunlight exposure is typically described only qualitatively. Instead of simply noting that a reaction took place in natural light, the team measured irradiance directly: roughly 420 watts per square meter in the morning, 925 watts per square meter at midday, and 480 watts per square meter in the evening. They then linked these measurements to reaction kinetics, particle dimensions, colloidal stability, and biological activity.
The synthesis itself was deliberately simple. Ten grams of dried green tea leaves were boiled in double-distilled water for fifteen minutes, filtered, and mixed with a dilute copper sulphate solution at a final pH of about 5.3. The mixture, held in a clear borosilicate beaker, was placed in direct sunlight. Within minutes the solution shifted from pale green to reddish brown, a visual sign that copper ions had been reduced to zero-valent metallic copper nanoparticles. Ultraviolet-visible spectroscopy confirmed the transformation, showing a surface plasmon resonance band between 565 and 580 nanometers, characteristic of metallic copper nanoparticles.
Reaction speed tracked the sun closely. Under midday light the spectra stabilized within roughly twenty minutes, while morning and evening reactions took nearly an hour. Statistical analysis of irradiance against the observed reaction rate constant revealed a strong positive correlation, with a Pearson coefficient of 0.92 and a p-value below 0.01. Crucially, control experiments ruled out simple heating as the driver: mixtures kept in the dark, and in temperature-matched environments, developed no plasmon band even after extended times. Solutions of copper sulphate alone in sunlight, or tea extract alone in sunlight, likewise produced no nanoparticles, indicating that copper ions, tea phytochemicals, and photons all had to act together.
Transmission electron microscopy revealed predominantly spherical particles with mean diameters of 29.4 ± 5.2 nanometers for the morning batch, 18.6 ± 3.1 nanometers at midday, and 25.8 ± 4.7 nanometers in the evening. One-way analysis of variance confirmed significant differences among the groups, and post hoc comparisons singled out the midday sample as distinct from both lower-irradiance batches. The correlation between irradiance and mean particle size was strongly negative, at r = −0.97, p < 0.01. The authors interpret this as evidence that higher photon flux accelerates nucleation, creating many growing nuclei at once and leaving less material for each particle to accumulate, which restricts final growth.
X-ray diffraction showed peaks at 2θ values of 43.3, 50.4, and 74.1 degrees, matching the (111), (200), and (220) planes of face-centered cubic metallic copper, with no detectable copper oxide phases. Peak broadening suggested crystallite sizes of roughly 17 to 25 nanometers, consistent with the electron microscopy results. The authors caution, however, that XRD probes bulk structure, and a thin surface oxide layer below the detection limit cannot be excluded; they did not use surface-sensitive methods such as X-ray photoelectron spectroscopy, which they list as a limitation and a priority for future work. Infrared spectroscopy detected hydroxyl, carbonyl, and carbon-nitrogen bands attributable to tea polyphenols capping the particle surfaces.
High-performance liquid chromatography identified epigallocatechin gallate (EGCG), epigallocatechin, and epicatechin gallate as the extract’s major catechins, with EGCG at approximately 164 micrograms per milliliter and EGC at about 97. EGCG, with its multiple hydroxyl groups, is proposed as the principal electron donor reducing Cu2+ to Cu0 while simultaneously capping the growing particles. The authors stress that this photochemical mechanism is inferred from phytochemical identification and prior literature rather than directly demonstrated; techniques such as X-ray photoelectron spectroscopy, electron paramagnetic resonance, or electrochemical analysis would be needed to verify oxidation-state transitions and transient radical species.
The particles also proved colloidally robust. Zeta potential measurements gave −28.4 millivolts for the midday sample and −25.6 millivolts for the morning and evening batches, indicating strong electrostatic repulsion. After thirty days of storage, ultraviolet-visible spectra showed negligible redshift, less than 3 nanometers, and hydrodynamic diameter increased by under 5 percent, suggesting good resistance to aggregation and oxidation under laboratory conditions. The authors attribute this stability to catechin-mediated surface capping combined with the negative surface charge, while noting that longer-term behavior under varying light, temperature, and ionic strength was not tested.
Antibacterial testing against Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923 showed clear, concentration-dependent inhibition. Disc diffusion inhibition zones grew from 9 to 18 millimeters for E. coli and from 8 to 16 millimeters for S. aureus as nanoparticle concentrations rose from 25 to 100 micrograms per milliliter. Minimum inhibitory concentrations were 62.5 micrograms per milliliter for E. coli and 78.1 for S. aureus, with minimum bactericidal concentrations of 125 and 156 micrograms per milliliter respectively. Ciprofloxacin served as a positive control, and both sterile solvent and tea extract without nanoparticles acted as negative controls, confirming that the inhibition arose from the particles themselves. Midday-synthesized particles performed comparatively best, which the authors link to their smaller size and higher surface reactivity.
Mechanistic assays pointed to reactive oxygen species and membrane damage. A DCFH-DA fluorescence assay showed a 2.4-fold increase in signal relative to untreated cells, indicating oxidative stress, while scanning electron micrographs of treated bacteria revealed cell wall deformation, pore formation, and cytoplasmic leakage. The authors propose that particles first attach to bacterial membranes electrostatically, then surface redox reactions and gradual release of copper ions generate superoxide and hydroxyl radicals that damage lipids, proteins, and nucleic acids. They emphasize that specific radical species were not quantitatively identified, and that these are in vitro results against only two model strains, not evidence of clinical efficacy; cytotoxicity, hemocompatibility, and testing against resistant clinical strains would be needed before any biomedical application. The synthesis itself was demonstrated only at laboratory scale on a single clear-sky day, so seasonal and geographic variability remain untested, and pilot-scale solar reactor studies and techno-economic assessment are flagged as necessary next steps before the approach could move beyond the bench.
Subject of Research: Sunlight-driven green synthesis of copper nanoparticles using green tea extract and their antibacterial activity
Article Title: Sunlight-driven green synthesis of copper nanoparticles using green tea extract and their antibacterial activity
Article References: Pathivada, N., Mamidi, G., Amancharla, I. P., & Swathi, G. (2026). Sunlight-driven green synthesis of copper nanoparticles using green tea extract and their antibacterial activity. Bioscience Nanotechnology, 2(1), Article 15. https://doi.org/10.1186/s44331-026-00019-5
Image Credits: AI Generated
DOI: 10.1186/s44331-026-00019-5
Keywords: copper nanoparticles, green synthesis, green tea extract, solar irradiance, EGCG, antibacterial activity, reactive oxygen species, photoreduction, colloidal stability, Escherichia coli, Staphylococcus aureus, sustainable nanotechnology
