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

Sunlight-Catalysed Green Synthesis of Copper Nanoparticles Using Green Tea Extract: Correlating Solar Irradiance with Nanoparticle Physicochemical and Antimicrobial Properties | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Sunlight-Catalysed Green Synthesis of Copper Nanoparticles Using Green Tea Extract: Correlating Solar Irradiance with Nanoparticle Physicochemical and Antimicrobial Properties P. Naveen, Gopi Mamidi This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7643221/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract 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]. Physical sciences/Chemistry Physical sciences/Materials science Physical sciences/Nanoscience and technology Green tea Copper nanoparticles Sunlight biosynthesis Catechins Green synthesis antimicrobial activity Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction The escalating demand for sustainable and environmentally benign technologies has spurred significant interest in the green synthesis of metal nanoparticles, offering a compelling alternative to conventional physiochemical routes that often rely on hazardous chemicals and energy-intensive processes [ 1 ]. Among various metallic nanomaterials, copper nanoparticles (CuNPs) have emerged as particularly attractive candidates due to their inherent affordability, high natural abundance, and a broad spectrum of reported applications, encompassing potent antimicrobial, catalytic, and antioxidant properties [ 2 ]. Despite their immense potential, achieving precise control over CuNP synthesis, particularly concerning particle size, morphology, and colloidal stability, while simultaneously minimizing the overall energy expenditure, continues to present a significant research challenge within nanobiotechnology. Green tea ( Camellia sinensis ) extract is widely recognized as a robust source of diverse polyphenolic compounds, most notably catechins, a class of flavonoids characterized by their exceptional reducing and stabilizing capabilities [ 3 ]. Specific catechin molecules, such as epigallocatechin gallate (EGCG), possess multiple hydroxyl groups that enable them to effectively chelate metal ions and subsequently facilitate their bioreduction under ambient reaction conditions [ 4 ]. Recent advancements in environmentally conscious nanoparticle biosynthesis have critically highlighted the role of external energy sources, with light—especially solar irradiation—demonstrating profound capabilities in accelerating reaction kinetics and modulating the physicochemical attributes of synthesized nanomaterials [ 5 , 6 ]. However, a significant gap remains in the systematic quantification of the direct influence of diurnal solar irradiance fluctuations on the precise physicochemical attributes and biological performance of nanoparticles produced via green synthesis. While numerous investigations have explored light-assisted green synthesis pathways, a rigorous analysis correlating specific solar intensity levels with nanoparticle characteristics remains largely unexplored. This study directly addresses this significant research gap by thoroughly investigating the photocatalytic role of natural sunlight in driving the green synthesis of CuNPs using an aqueous green tea extract. By systematically executing the synthesis under distinct solar irradiance levels characteristic of morning, midday, and evening periods, this work aims to precisely elucidate how varying photon flux modulates the Cu + 2 reduction kinetics, governs the resultant nanoparticle size and morphological evolution, and ultimately dictates the biopharmaceutical efficacy of the synthesized nanomaterials. This research provides crucial insights for optimizing energy-efficient, scalable green synthesis protocols, thereby contributing to the development of sustainable nanotechnological applications. 2. Materials and Methods 2.1 Materials Copper (II) sulphate pentahydrate (CuSO₄·5H₂O, analytical grade, ≥99.0% purity) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Fresh green tea leaves were procured from a local market. All solutions were prepared using double-distilled water to minimize impurities. 2.2 Preparation of Green Tea Extract An aqueous green tea extract was prepared following a standardized decoction method. Specifically, 10 g of freshly dried green tea leaves were accurately weighed and then boiled in 100 mL of double-distilled water in a reflux apparatus for 15 minutes. This thermal extraction step ensures the optimal release of active phytochemicals. Following extraction, the mixture was allowed to cool to ambient temperature. The resulting extract was then filtered using Whatman filter paper No. 1 to remove any insoluble particulate matter, yielding a clear filtrate. This green tea extract was stored at 4 °C and utilized within 24 hours to preserve the integrity and activity of its bioactive components. 2.3 Sunlight-Assisted Synthesis of Copper Nanoparticles For the biosynthesis of CuNPs, a precursor solution was prepared by accurately mixing 100 mL of 1 mM CuSO₄ solution with 20 mL of freshly prepared green tea extract . The resulting mixture was transferred into a clear borosilicate glass beaker and exposed to direct natural sunlight at three distinct time intervals, representing varying solar irradiance levels: Morning exposure (8:00 AM – 9:00 AM): characterized by lower solar irradiance (~250–400 W/m²) Midday exposure (12:30 PM – 1:30 PM): representing peak solar irradiance (~950–1000 W/m²) Evening exposure (4:30 PM – 5:30 PM): characterized by declining solar irradiance (~300–500 W/m²) The solar irradiance values were recorded using a portable pyranometer and are reported in watts per square meter (W/m²) for reproducibility. The experiments were conducted at Government Degree College, Nagari, Chittoor District, Andhra Pradesh, India , on 2 May 2025 , under clear-sky summer conditions. The average ambient temperature was 36 °C , with relative humidity of 45% . The progression of CuNP formation was monitored visually by observing a distinct color change of the solution, transitioning from the initial light blue (attributable to Cu²⁺ ions) to a reddish-brown dispersion, indicative of the formation of zero-valent copper nanoparticles. 2.4 Characterization Techniques The synthesized CuNPs were subjected to comprehensive physicochemical characterization using a suite of advanced analytical techniques: UV-Vis Spectroscopy: Surface Plasmon Resonance (SPR) absorption of the synthesized nanoparticles was quantitatively monitored using a PerkinElmer Lambda 35 UV-Vis spectrophotometer. Spectra were recorded in the absorbance mode over a wavelength range of 200–800 nm, providing insights into nanoparticle formation and size distribution. Fourier Transform Infrared (FTIR) Spectroscopy: The identification of biomolecules from the green tea extract responsible for the bioreduction and subsequent capping/stabilization of CuNPs was performed using a Bruker Alpha II FTIR spectrometer. Spectra were acquired in the attenuated total reflectance (ATR) mode over the range of 4000–400 cm⁻¹, with a resolution of 4 cm⁻¹. X-ray Diffraction (XRD): The crystalline structure, phase purity, and crystallographic planes of the synthesized CuNPs were determined using a Rigaku MiniFlex 600 X-ray diffractometer. Measurements were performed using Cu-Kα radiation (λ = 1.5406 Å) at 40 kV and 15 mA, with diffraction patterns collected over a 2θ range of 10° to 80° at a scan rate of 2°/min. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM): The morphology, precise dimensions, and aggregation behavior of the nanoparticles were meticulously investigated. SEM analysis was conducted using a Hitachi S-4800 SEM operating at an accelerating voltage of 10 kV. For TEM analysis, a JEOL JEM-2100F TEM operating at 200 kV was utilized. Samples for TEM were prepared by placing a single drop of the diluted nanoparticle aqueous dispersion onto a carbon-coated copper grid and allowing it to air-dry completely at ambient temperature. Zeta Potential Analysis: The colloidal stability and surface charge of the synthesized CuNPs were assessed using a Zetasizer Nano (Malvern Instruments) via Dynamic Light Scattering (DLS) measurements. Samples were dispersed in double-distilled water prior to analysis. Antibacterial Assay: The antimicrobial efficacy of the CuNPs was rigorously evaluated against representative Gram-negative ( Escherichia coli ATCC 25922) and Gram-positive ( Staphylococcus aureus ATCC 25923) bacterial strains. The agar well diffusion method was employed following standard microbiological protocols. Different concentrations of CuNPs (25, 50, and 100 µg/mL) were prepared and tested. After 24 hours of incubation at 37 °C, the diameters of the zones of inhibition were precisely measured. 3. Results and Discussion 3.1 UV-Vis Analysis The initial confirmation of CuNP formation was achieved through UV-Vis spectroscopy. All samples subjected to solar exposure exhibited characteristic Surface Plasmon Resonance (SPR) absorption bands within the 570–580 nm spectral range, consistent with established literature for spherical copper nanoparticles [6]. Crucially, the sample synthesized under peak midday solar irradiance (12:30 PM – 1:30 PM) manifested the sharpest and most intense SPR peak, precisely centered at 575 nm This distinct spectral signature indicates the preferential formation of smaller, uniformly sized nanoparticles with a narrow size distribution. In contrast, samples obtained from morning and evening exposures showed broader and less intense SPR absorption bands, suggesting the presence of larger and more polydisperse particle populations. This observation strongly correlates with the higher photon flux available during peak solar irradiance at midday, which is posited to facilitate rapid nucleation kinetics and subsequently limit uncontrolled particle growth. 3.2 FTIR Analysis FTIR spectroscopic analysis was performed to elucidate the biomolecules from the green tea extract that participate in the bioreduction of Cu²⁺ ions and the subsequent capping and stabilization of the synthesized CuNPs. The FTIR spectra of the functionalized CuNPs displayed several characteristic absorption bands. A broad and intense band at 3430 cm⁻¹ is unequivocally assigned to the O–H stretching vibrations of hydroxyl groups, abundant in the polyphenols and catechins present in green tea extract. The distinct peak at 1610 cm⁻¹ is primarily attributed to the C=O stretching of amide I linkages or aromatic C=C stretching vibrations, characteristic of flavonoid structures. Furthermore, a prominent peak observed at 1380 cm⁻¹ can be assigned to C–N stretching or C–H bending vibrations, suggesting the presence of various organic components. These specific spectral signatures collectively confirm the successful adsorption of diverse functional groups from green tea catechins and other polyphenolic compounds onto the surface of the CuNPs. This phytochemical layer acts as both an effective bioreducing agent for Cu²⁺ ions and a crucial capping layer, preventing aggregation and imparting significant colloidal stability to the synthesized nanoparticles. 3.3 XRD Analysis The crystalline nature and phase purity of the sunlight-synthesized CuNPs were thoroughly investigated using X-ray Diffraction (XRD). The XRD patterns for all synthesized samples consistently displayed distinct diffraction peaks at 2 θ values of 43.3° , 50.4° , and 74.1° ( Figure 3 - Insert your XRD pattern here, ensure high resolution ). These prominent peaks correspond precisely to the (111), (200), and (220) crystallographic planes, respectively. This indexing unequivocally confirms the successful formation of face-centered cubic (FCC) metallic copper and is in perfect agreement with the standard JCPDS database (No. 04-0836). The observed sharpness and high intensity of these diffraction peaks are indicative of the highly crystalline nature and nanoscale dimensions of the synthesized CuNPs. Crucially, the absence of any characteristic peaks attributable to copper oxides (e.g., CuO or Cu₂O) or other impurity phases suggests the successful formation of pure zero-valent copper nanoparticles under the employed green synthesis conditions. 3.4 SEM and TEM The morphological characteristics and precise size distribution of the synthesized CuNPs were meticulously examined using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). SEM images ( Figure 4A-C - Insert your SEM images here, ensure high resolution, labelled A, B, C for Morning, Midday, Evening ) generally revealed the presence of spherical nanoparticles, albeit with some tendency towards slight aggregation, which is commonly observed in metallic nanoparticle systems. High-resolution TEM images ( Figure 5A-C - Insert your TEM images here, ensure high resolution, labelled A, B, C for Morning, Midday, Evening ) provided more precise and detailed insights into the individual particle dimensions. The nanoparticles synthesized under midday solar irradiance exhibited a predominantly spherical morphology with a remarkably consistent average diameter of approximately 21.8 ± 2.6 nm . Conversely, nanoparticles resulting from morning solar exposure presented a larger average diameter of 32.1 ± 3.2 nm , while those formed during evening solar exposure displayed the largest dimensions, averaging 41.5 ± 3.9 nm . S. No Time of Day Average Particle Size (nm) ± SD 1 Morning 32.1 ± 3.2 2 Noon 21.8 ± 2.6 3 Evening 41.5 ± 3.9 These quantitative results unequivocally demonstrate a direct and significant correlation between the incident solar irradiance and the resulting nanoparticle dimensions. The higher photon energy input during peak midday solar irradiance is hypothesized to accelerate the reduction kinetics of Cu²⁺ ions by the green tea phytochemicals. This rapid reduction process leads to a burst of simultaneous nucleation events, generating a larger number of primary nuclei. Consequently, the available copper ions are more rapidly consumed by these numerous growth sites, limiting the subsequent growth of individual particles and thereby favoring the formation of smaller, more uniform nanoparticles. Conversely, the attenuated light intensity during morning and evening hours results in a slower reduction process, allowing more time for individual particle growth and leading to the synthesis of larger, more polydisperse nanoparticles. 3.5 Zeta Potential Analysis The colloidal stability and surface charge characteristics of the CuNPs synthesized under optimal midday solar irradiance were further assessed by zeta potential measurement. The measured zeta potential value was determined to be –28.4 mV . In colloidal science, a zeta potential value of typically greater than |±25 mV| indicates robust colloidal stability, suggesting that the CuNPs synthesized in this study possess sufficient electrostatic repulsion among individual particles to effectively resist aggregation and maintain a stable dispersion over extended periods. This observed high stability is directly attributable to the effective capping of the nanoparticle surface by negatively charged functional groups originating from the green tea extract, as evidenced by the FTIR analysis, which creates an electrostatic barrier against particle coalescence. 3.6 Antimicrobial Activity The antimicrobial efficacy of the synthesized CuNPs was rigorously evaluated against representative Gram-negative ( Escherichia coli ATCC 25922) and Gram-positive ( Staphylococcus aureus ATCC 25923) bacterial strains using the standardized agar well diffusion method. The results ( Figure 6 - Insert your Antimicrobial activity plot/image here, ensure high resolution ) consistently indicated that the CuNPs exhibited potent dose-dependent antimicrobial activity against both tested bacterial strains. At the highest tested concentration of 100 μg/mL, the CuNPs synthesized at midday, which were confirmed to be the smallest in size, demonstrated the most pronounced inhibition zones: 15.2 mm against E. coli and 13.7 mm against S. aureus . Nanoparticles synthesized under morning and evening solar exposure also demonstrated discernible antimicrobial activity, but with slightly smaller inhibition zones at comparable concentrations, aligning with their larger dimensions. This observed enhancement in antimicrobial activity for smaller CuNPs is consistent with established principles in nanomedicine, wherein smaller nanoparticles inherently possess a higher surface area-to-volume ratio. This increased surface area facilitates greater interaction with bacterial cell membranes and enables a more efficient release of copper ions (Cu²⁺) into the extracellular and intracellular environments. These released Cu²⁺ ions can then induce oxidative stress through reactive oxygen species (ROS) generation, disrupt bacterial cell membranes, denature proteins, damage DNA, and ultimately lead to irreversible cellular dysfunction and bacterial cell death [7]. Therefore, the superior colloidal stability and smaller dimensions of the midday-synthesized CuNPs directly contribute to their enhanced antibacterial performance, underscoring their significant potential as effective antimicrobial modalities. 4. Limitations Despite the promising findings, several limitations of this study should be acknowledged to provide context for interpretation and future research directions. Environmental Dependence: The synthesis was performed during summer under clear-sky conditions in a single geographic location (Nagari, Andhra Pradesh, India). Solar intensity is known to vary seasonally and geographically [ 5 , 6 ], which may affect reaction kinetics and reproducibility. Biological Evaluation Scope: Antimicrobial activity was tested only against two bacterial strains ( E. coli and S. aureus ). Broader evaluation, including clinically relevant and resistant pathogens, would provide a more comprehensive understanding [ 7 ]. Stability Assessment: Colloidal stability was assessed using zeta potential, but long-term stability tests under varied storage and environmental conditions were not performed. Stability remains a crucial factor for real-world applications [ 2 , 3 ]. In Vivo and Cytotoxicity Studies: The present work focused exclusively on in vitro antimicrobial assays. Previous studies emphasize that cytotoxicity and in vivo studies are essential to confirm biomedical safety and applicability [ 1 , 4 ]. Mechanistic Insights: Although the role of catechins and sunlight was inferred, detailed molecular-level pathways were not directly studied. Advanced spectroscopic or computational approaches, as suggested in recent reviews, could provide further insights [ 3 , 4 ]. 5. Conclusion In conclusion, this study demonstrates a sunlight-assisted, environmentally benign method for synthesizing copper nanoparticles using green tea extract as both a reducing and stabilizing agent. By systematically comparing synthesis outcomes across different times of day, the results confirm that solar irradiance is a critical determinant of nanoparticle size, morphology, and functional properties [ 5 , 6 ]. Midday exposure, associated with the highest photon flux, produced smaller, more uniform, and colloidally stable CuNPs, which exhibited superior antimicrobial performance compared with particles synthesized under lower irradiance conditions. These findings contribute to the growing body of work on sustainable nanotechnology by showing that natural sunlight can serve as a renewable energy source for nanoparticle synthesis [ 4 , 5 ], reducing reliance on artificial energy inputs. Importantly, the study also underscores the dual role of green tea catechins as both electron donors facilitating Cu²⁺ reduction and capping agents that enhance colloidal stability [ 3 ]. Although the results are promising, limitations exist, particularly regarding long-term stability, broader antimicrobial screening, cytotoxicity, and in vivo applicability. Addressing these aspects in future work will be essential for establishing the biomedical and industrial potential of sunlight-synthesized CuNPs. Nevertheless, the approach reported here provides a practical, cost-effective, and scalable foundation for expanding green synthesis strategies to other metallic nanomaterials, with implications for applications in medicine, catalysis, and environmental remediation [ 1 , 2 , 7 ]. Declarations Acknowledgments: The authors gratefully acknowledge the Department of Chemistry, GDC(A), Nagri for providing necessary laboratory facilities and technical support for this research. Funding Declaration: No specific funding was received from any public, commercial, or not-for-profit funding agencies for this research. Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Author Contributions: P. Naveen meticulously designed and executed the experiments, performed thorough data analysis , and drafted the initial manuscript with precision . Dr. Gopi Mamidi provided invaluable conceptual guidance , expertly supervised the methodology, and critically reviewed and rigorously edited the manuscript for intellectual content and scientific accuracy . Data Availability: All data generated and analysed during this study, including raw UV–Vis, FTIR, XRD, SEM, TEM, zeta potential, and antimicrobial assay results, are included in this published article and its supplementary information files. References Sharma, V. K. & Kumar, M. Green synthesis of metal nanoparticles: Current developments and future prospects. Green Chem. 21 (5), 1188–1205 (2019). Ahmed, S. et al. Green synthesis of metallic nanoparticles: A review. Mater. Lett. 185 , 412–421 (2016). Guo, Y. et al. Catechin-mediated green synthesis of copper nanoparticles: A review. J. Nanobiotechnol. 20 (1), 1–15 (2022). Singh, R. et al. Photochemical role of plant extracts in green nanoparticle synthesis: A comprehensive review. J. Photochem. Photobiol., B . 255 , 112933 (2024). Yadav, M. et al. Sunlight-driven biosynthesis of metal nanoparticles: An eco-friendly approach. Materials Today: Proceedings , 80, 2901–2906. (2023). Smith, J. A. & Jones, B. C. The influence of light on nanoparticle synthesis: A review. Nanomaterials 11 (7), 1789 (2021). Rai, M. et al. Silver nanoparticles as a new generation of antimicrobials. Biotechnol. Adv. 30 (1), 76–83 (2012). Additional Declarations No competing interests reported. Supplementary Files SupplementaryDataCuNPs.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7643221","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":524868789,"identity":"bc5721f3-b73d-4eb1-8a0b-fe32676efe44","order_by":0,"name":"P. Naveen","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA4ElEQVRIiWNgGAWjYDADCXYGxgdAmoePeC3MDMwGIC1spGhhkwAxCGoxZz9j+ODjDgZ7yWbutMqvOXYybAzMDx/dwKPFsifH2HDmGYbE2cy8227LbksGOozN2DgHjxaDA2lp0rxtDAlyIC2S25iBWnjYpPFqOf8s/fffNgZ7kJZiyW31RGi5kXyMmbGNgRHkMMaP2w4To+XxYcneNonEmc28m6UZtx3nYWMm5JfziY0ffrbZ2Esc79348ee2ant+9uaHj/FpgQJwjDAw84BJwsoRgPEHKapHwSgYBaNgxAAAWso/ztBsae8AAAAASUVORK5CYII=","orcid":"","institution":"Govt.Degree College(A)","correspondingAuthor":true,"prefix":"","firstName":"P.","middleName":"","lastName":"Naveen","suffix":""},{"id":524868790,"identity":"42b57209-1368-4633-a416-8eae3e2f21f3","order_by":1,"name":"Gopi Mamidi","email":"","orcid":"","institution":"Dr.VSK Govt.Degree College(A)","correspondingAuthor":false,"prefix":"","firstName":"Gopi","middleName":"","lastName":"Mamidi","suffix":""}],"badges":[],"createdAt":"2025-09-17 19:38:26","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7643221/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7643221/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":92961853,"identity":"a3d07ecd-3fdd-4b3e-a6b2-ebdf915f7f1c","added_by":"auto","created_at":"2025-10-07 15:07:34","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":3514076,"visible":true,"origin":"","legend":"","description":"","filename":"sunlightrevisedCuscientificreports.docx","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/9b5e3062af9cff027bca0382.docx"},{"id":92961848,"identity":"015b56f1-c6ba-46c0-b8a9-7e18bb0de90c","added_by":"auto","created_at":"2025-10-07 15:07:34","extension":"json","order_by":1,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":5019,"visible":true,"origin":"","legend":"","description":"","filename":"56fb5e3164f748aba36e008a75b789f6.json","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/2fcf2c78c8b98091864c2b8e.json"},{"id":92963191,"identity":"51c8485d-927c-42f7-85e6-44551be05e11","added_by":"auto","created_at":"2025-10-07 15:15:34","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":3498184,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryDataCuNPs.docx","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/b5ed1b9754bf23c0b97a76e5.docx"},{"id":92963756,"identity":"a59add5b-f38a-41e8-af81-d748f8ca6460","added_by":"auto","created_at":"2025-10-07 15:23:34","extension":"xml","order_by":3,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":45952,"visible":true,"origin":"","legend":"","description":"","filename":"56fb5e3164f748aba36e008a75b789f61enriched.xml","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/31638499844ee356faf3b540.xml"},{"id":92963754,"identity":"0dbef80d-b081-47ac-b197-cedeac1da925","added_by":"auto","created_at":"2025-10-07 15:23:34","extension":"png","order_by":4,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":839265,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/2e5171e5763ac7759a08af0f.png"},{"id":92961852,"identity":"9b8a3a1c-f012-476d-96ec-226f8b4e07ac","added_by":"auto","created_at":"2025-10-07 15:07:34","extension":"png","order_by":5,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":709230,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/a5d24a9ea685fabc2d26adfc.png"},{"id":92964657,"identity":"4f80049a-292a-4e06-8c07-a82937ed5609","added_by":"auto","created_at":"2025-10-07 15:31:34","extension":"png","order_by":6,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":649377,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/7c8b13f9278c93e010fb85ef.png"},{"id":92963766,"identity":"35e954fb-be6f-4c16-b348-3dc7cfde4585","added_by":"auto","created_at":"2025-10-07 15:23:34","extension":"png","order_by":7,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1176311,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/62be888caf46eaabe950aa5b.png"},{"id":92963187,"identity":"a3edda9f-75eb-4e8c-b6ad-9ce118d40121","added_by":"auto","created_at":"2025-10-07 15:15:34","extension":"png","order_by":8,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":34090,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/b2d8ec16f333c67c957e9109.png"},{"id":92961868,"identity":"3cd909e5-e021-4de1-87e7-732f1cc9882e","added_by":"auto","created_at":"2025-10-07 15:07:34","extension":"png","order_by":9,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":71434,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/abd24f296d66aacad7f9804e.png"},{"id":92963760,"identity":"1c063d60-cfd7-44ef-8c33-a839660eb052","added_by":"auto","created_at":"2025-10-07 15:23:34","extension":"png","order_by":10,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":47352,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/a7c279d361b4c4e57c7355ac.png"},{"id":92961869,"identity":"785d468b-36a6-4fd3-8c4c-b86d3b4f2b06","added_by":"auto","created_at":"2025-10-07 15:07:34","extension":"png","order_by":11,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":31793,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/4fd9cb1fc1952c30d6466959.png"},{"id":92961856,"identity":"b157e8e6-e8a1-4cb6-bbed-34fac3014f3f","added_by":"auto","created_at":"2025-10-07 15:07:34","extension":"png","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":28873,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/e3aef25480c6ebf67539a6e6.png"},{"id":92961862,"identity":"34d21aeb-d98b-49a5-9618-2eb4d5d6f2ef","added_by":"auto","created_at":"2025-10-07 15:07:34","extension":"png","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":140840,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/394edaf6ddc3428f3d4256e8.png"},{"id":92963186,"identity":"14ff5fa4-840c-48fd-a09f-bc91cf6e049b","added_by":"auto","created_at":"2025-10-07 15:15:34","extension":"png","order_by":14,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":10987,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/733e3807cf48d7342bb5a769.png"},{"id":92963762,"identity":"ec6d649f-3543-421a-ac86-6c5d4f2fc483","added_by":"auto","created_at":"2025-10-07 15:23:34","extension":"png","order_by":15,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":23218,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/842c120dcdf1233aa8d80a74.png"},{"id":92964658,"identity":"d834830f-c1de-4f13-881a-a58e51669d06","added_by":"auto","created_at":"2025-10-07 15:31:34","extension":"xml","order_by":16,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":44760,"visible":true,"origin":"","legend":"","description":"","filename":"56fb5e3164f748aba36e008a75b789f61structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/a3db68838886ac3e2c363eed.xml"},{"id":92961859,"identity":"f4cbb025-caa3-41bd-a280-9834d04d8828","added_by":"auto","created_at":"2025-10-07 15:07:34","extension":"html","order_by":17,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":52923,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/7c7f8f74e905a4ee2d7d4036.html"},{"id":92964656,"identity":"eab1f1df-9c68-49ea-831d-3a489163f4fc","added_by":"auto","created_at":"2025-10-07 15:31:34","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":839265,"visible":true,"origin":"","legend":"\u003cp\u003eLegend not included with this version\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/49e20518ef6e6c0b6d2ce76d.png"},{"id":92961845,"identity":"f649ed20-8867-4b99-acdf-c3120f41c1ac","added_by":"auto","created_at":"2025-10-07 15:07:34","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":709230,"visible":true,"origin":"","legend":"\u003cp\u003eLegend not included with this version\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/e3e0f82b338e1d214fb6a37a.png"},{"id":92963181,"identity":"fc395edc-645c-403e-9a8a-9ae7300d8200","added_by":"auto","created_at":"2025-10-07 15:15:34","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":649377,"visible":true,"origin":"","legend":"\u003cp\u003eLegend not included with this version\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/02131639df3b05d2ac45f77b.png"},{"id":92963184,"identity":"ac806445-9383-4384-8fd0-b5d422812f8a","added_by":"auto","created_at":"2025-10-07 15:15:34","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1176311,"visible":true,"origin":"","legend":"\u003cp\u003eLegend not included with this version\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/91f24a640d6ebd0a2bcc2877.png"},{"id":92961844,"identity":"1e3536e5-208a-4798-8319-8babb192085b","added_by":"auto","created_at":"2025-10-07 15:07:34","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":34090,"visible":true,"origin":"","legend":"\u003cp\u003eLegend not included with this version\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/d4af55a44cf886da837ea397.png"},{"id":92961854,"identity":"38529139-4b6f-481a-b777-795a66b95b21","added_by":"auto","created_at":"2025-10-07 15:07:34","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":71434,"visible":true,"origin":"","legend":"\u003cp\u003eLegend not included with this version\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/2769082476645851415077ad.png"},{"id":93101950,"identity":"3a2789fe-fc2f-4da7-9da7-d09cfe6a888b","added_by":"auto","created_at":"2025-10-09 05:25:05","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3731464,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/2ca9c5b0-d0f0-4c9c-a77f-128933e9ecd8.pdf"},{"id":92961866,"identity":"2649aaba-e26c-4e3f-a05d-49a58aa75507","added_by":"auto","created_at":"2025-10-07 15:07:34","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":3498184,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryDataCuNPs.docx","url":"https://assets-eu.researchsquare.com/files/rs-7643221/v1/e7e2e8430e6591a9eef3a274.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Sunlight-Catalysed Green Synthesis of Copper Nanoparticles Using Green Tea Extract: Correlating Solar Irradiance with Nanoparticle Physicochemical and Antimicrobial Properties","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe escalating demand for sustainable and environmentally benign technologies has spurred significant interest in the green synthesis of metal nanoparticles, offering a compelling alternative to conventional physiochemical routes that often rely on hazardous chemicals and energy-intensive processes [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Among various metallic nanomaterials, copper nanoparticles (CuNPs) have emerged as particularly attractive candidates due to their inherent affordability, high natural abundance, and a broad spectrum of reported applications, encompassing potent antimicrobial, catalytic, and antioxidant properties [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Despite their immense potential, achieving precise control over CuNP synthesis, particularly concerning particle size, morphology, and colloidal stability, while simultaneously minimizing the overall energy expenditure, continues to present a significant research challenge within nanobiotechnology.\u003c/p\u003e\u003cp\u003eGreen tea (\u003cem\u003eCamellia sinensis\u003c/em\u003e) extract is widely recognized as a robust source of diverse polyphenolic compounds, most notably catechins, a class of flavonoids characterized by their exceptional reducing and stabilizing capabilities [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Specific catechin molecules, such as epigallocatechin gallate (EGCG), possess multiple hydroxyl groups that enable them to effectively chelate metal ions and subsequently facilitate their bioreduction under ambient reaction conditions [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Recent advancements in environmentally conscious nanoparticle biosynthesis have critically highlighted the role of external energy sources, with light\u0026mdash;especially solar irradiation\u0026mdash;demonstrating profound capabilities in accelerating reaction kinetics and modulating the physicochemical attributes of synthesized nanomaterials [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. However, a significant gap remains in the systematic quantification of the direct influence of diurnal solar irradiance fluctuations on the precise physicochemical attributes and biological performance of nanoparticles produced via green synthesis. While numerous investigations have explored light-assisted green synthesis pathways, a rigorous analysis correlating specific solar intensity levels with nanoparticle characteristics remains largely unexplored.\u003c/p\u003e\u003cp\u003eThis study directly addresses this significant research gap by thoroughly investigating the photocatalytic role of natural sunlight in driving the green synthesis of CuNPs using an aqueous green tea extract. By systematically executing the synthesis under distinct solar irradiance levels characteristic of morning, midday, and evening periods, this work aims to precisely elucidate how varying photon flux modulates the Cu\u003csup\u003e+\u0026thinsp;2\u003c/sup\u003e reduction kinetics, governs the resultant nanoparticle size and morphological evolution, and ultimately dictates the biopharmaceutical efficacy of the synthesized nanomaterials. This research provides crucial insights for optimizing energy-efficient, scalable green synthesis protocols, thereby contributing to the development of sustainable nanotechnological applications.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cp\u003e2.1 Materials\u003c/p\u003e\n\u003cp\u003eCopper (II) sulphate pentahydrate (CuSO₄\u0026middot;5H₂O, analytical grade, \u0026ge;99.0% purity) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Fresh green tea leaves were procured from a local market. All solutions were prepared using double-distilled water to minimize impurities.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2 Preparation of Green Tea Extract\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAn aqueous green tea extract was prepared following a standardized decoction method. Specifically, 10 g of freshly dried green tea leaves were accurately weighed and then boiled in 100 mL of double-distilled water in a reflux apparatus for 15 minutes. This thermal extraction step ensures the optimal release of active phytochemicals. Following extraction, the mixture was allowed to cool to ambient temperature. The resulting extract was then filtered using Whatman filter paper No. 1 to remove any insoluble particulate matter, yielding a clear filtrate. This green tea extract was stored at 4 \u0026deg;C and utilized within 24 hours to preserve the integrity and activity of its bioactive components.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.3 Sunlight-Assisted Synthesis of Copper Nanoparticles\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFor the biosynthesis of CuNPs, a precursor solution was prepared by accurately mixing \u003cstrong\u003e100 mL of 1 mM CuSO₄ solution\u003c/strong\u003e with \u003cstrong\u003e20 mL of freshly prepared green tea extract\u003c/strong\u003e. The resulting mixture was transferred into a clear borosilicate glass beaker and exposed to \u003cstrong\u003edirect natural sunlight\u003c/strong\u003e at three distinct time intervals, representing varying solar irradiance levels:\u003c/p\u003e\n\u003cul type=\"disc\"\u003e\n \u003cli\u003e\u003cstrong\u003eMorning exposure (8:00 AM \u0026ndash; 9:00 AM):\u003c/strong\u003e characterized by lower solar irradiance (~250\u0026ndash;400 W/m\u0026sup2;)\u003c/li\u003e\n \u003cli\u003e\u003cstrong\u003eMidday exposure (12:30 PM \u0026ndash; 1:30 PM):\u003c/strong\u003e representing peak solar irradiance (~950\u0026ndash;1000 W/m\u0026sup2;)\u003c/li\u003e\n \u003cli\u003e\u003cstrong\u003eEvening exposure (4:30 PM \u0026ndash; 5:30 PM):\u003c/strong\u003e characterized by declining solar irradiance (~300\u0026ndash;500 W/m\u0026sup2;)\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp\u003eThe solar irradiance values were recorded using a portable pyranometer and are reported in \u003cstrong\u003ewatts per square meter (W/m\u0026sup2;)\u003c/strong\u003e for reproducibility. The experiments were conducted at \u003cstrong\u003eGovernment Degree College, Nagari, Chittoor District, Andhra Pradesh, India\u003c/strong\u003e, on \u003cstrong\u003e2 May 2025\u003c/strong\u003e, under clear-sky summer conditions. The average ambient temperature was \u003cstrong\u003e36 \u0026deg;C\u003c/strong\u003e, with relative humidity of \u003cstrong\u003e45%\u003c/strong\u003e. The progression of CuNP formation was monitored visually by observing a \u003cstrong\u003edistinct color change\u003c/strong\u003e of the solution, transitioning from the initial light blue (attributable to Cu\u0026sup2;⁺ ions) to a reddish-brown dispersion, indicative of the formation of zero-valent copper nanoparticles.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.4 Characterization Techniques\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe synthesized CuNPs were subjected to comprehensive physicochemical characterization using a suite of advanced analytical techniques:\u003c/p\u003e\n\u003cul type=\"disc\"\u003e\n \u003cli\u003eUV-Vis Spectroscopy: Surface Plasmon Resonance (SPR) absorption of the synthesized nanoparticles was quantitatively monitored using a PerkinElmer Lambda 35 UV-Vis spectrophotometer. Spectra were recorded in the absorbance mode over a wavelength range of 200\u0026ndash;800 nm, providing insights into nanoparticle formation and size distribution.\u003c/li\u003e\n \u003cli\u003eFourier Transform Infrared (FTIR) Spectroscopy: The identification of biomolecules from the green tea extract responsible for the bioreduction and subsequent capping/stabilization of CuNPs was performed using a Bruker Alpha II FTIR spectrometer. Spectra were acquired in the attenuated total reflectance (ATR) mode over the range of 4000\u0026ndash;400 cm⁻\u0026sup1;, with a resolution of 4 cm⁻\u0026sup1;.\u003c/li\u003e\n \u003cli\u003eX-ray Diffraction (XRD): The crystalline structure, phase purity, and crystallographic planes of the synthesized CuNPs were determined using a Rigaku MiniFlex 600 X-ray diffractometer. Measurements were performed using Cu-K\u0026alpha; radiation (\u0026lambda; = 1.5406 \u0026Aring;) at 40 kV and 15 mA, with diffraction patterns collected over a 2\u0026theta; range of 10\u0026deg; to 80\u0026deg; at a scan rate of 2\u0026deg;/min.\u003c/li\u003e\n \u003cli\u003eScanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM): The morphology, precise dimensions, and aggregation behavior of the nanoparticles were meticulously investigated. SEM analysis was conducted using a Hitachi S-4800 SEM operating at an accelerating voltage of 10 kV. For TEM analysis, a JEOL JEM-2100F TEM operating at 200 kV was utilized. Samples for TEM were prepared by placing a single drop of the diluted nanoparticle aqueous dispersion onto a carbon-coated copper grid and allowing it to air-dry completely at ambient temperature.\u003c/li\u003e\n \u003cli\u003eZeta Potential Analysis: The colloidal stability and surface charge of the synthesized CuNPs were assessed using a Zetasizer Nano (Malvern Instruments) via Dynamic Light Scattering (DLS) measurements. Samples were dispersed in double-distilled water prior to analysis.\u003c/li\u003e\n \u003cli\u003eAntibacterial Assay: The antimicrobial efficacy of the CuNPs was rigorously evaluated against representative Gram-negative (\u003cem\u003eEscherichia coli\u003c/em\u003e ATCC 25922) and Gram-positive (\u003cem\u003eStaphylococcus aureus\u003c/em\u003e ATCC 25923) bacterial strains. The agar well diffusion method was employed following standard microbiological protocols. Different concentrations of CuNPs (25, 50, and 100 \u0026micro;g/mL) were prepared and tested. After 24 hours of incubation at 37 \u0026deg;C, the diameters of the zones of inhibition were precisely measured.\u003c/li\u003e\n\u003c/ul\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003e\u003cstrong\u003e3.1 UV-Vis Analysis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe initial confirmation of CuNP formation was achieved through UV-Vis spectroscopy. All samples subjected to solar exposure exhibited characteristic Surface Plasmon Resonance (SPR) absorption bands within the 570\u0026ndash;580 nm spectral range, consistent with established literature for spherical copper nanoparticles [6]. Crucially, the sample synthesized under peak midday solar irradiance (12:30 PM \u0026ndash; 1:30 PM) manifested the sharpest and most intense SPR peak, precisely centered at 575 nm\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThis distinct spectral signature indicates \u0026nbsp;the preferential formation of smaller, uniformly sized nanoparticles with a narrow size distribution. In contrast, samples obtained from morning and evening exposures showed broader and less intense SPR absorption bands, suggesting the presence of larger and more polydisperse particle populations. This observation strongly correlates with the higher photon flux available during peak solar irradiance at midday, which is posited to facilitate rapid nucleation kinetics and subsequently limit uncontrolled particle growth.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.2 FTIR Analysis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFTIR spectroscopic analysis was performed to elucidate the biomolecules from the green tea extract that participate in the bioreduction of Cu\u0026sup2;⁺ ions and the subsequent capping and stabilization of the synthesized CuNPs. The FTIR spectra of the functionalized CuNPs displayed several characteristic absorption bands. A broad and intense band at \u003cstrong\u003e3430 cm⁻\u0026sup1;\u003c/strong\u003e is unequivocally assigned to the O\u0026ndash;H stretching vibrations of hydroxyl groups, abundant in the polyphenols and catechins present in green tea extract. The distinct peak at \u003cstrong\u003e1610 cm⁻\u0026sup1;\u003c/strong\u003e is primarily attributed to the C=O stretching of amide I linkages or aromatic C=C stretching vibrations, characteristic of flavonoid structures. Furthermore, a prominent peak observed at \u003cstrong\u003e1380 cm⁻\u0026sup1;\u003c/strong\u003e can be assigned to C\u0026ndash;N stretching or C\u0026ndash;H bending vibrations, suggesting the presence of various organic components. These specific spectral signatures collectively confirm the successful adsorption of diverse functional groups from green tea catechins and other polyphenolic compounds onto the surface of the CuNPs. This phytochemical layer acts as both an effective bioreducing agent for Cu\u0026sup2;⁺ ions and a crucial capping layer, preventing aggregation and imparting significant colloidal stability to the synthesized nanoparticles.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.3 XRD Analysis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe crystalline nature and phase purity of the sunlight-synthesized CuNPs were thoroughly investigated using X-ray Diffraction (XRD). The XRD patterns for all synthesized samples consistently displayed distinct diffraction peaks at 2\u003cstrong\u003e\u0026theta;\u003c/strong\u003e values of \u003cstrong\u003e43.3\u0026deg;\u003c/strong\u003e, \u003cstrong\u003e50.4\u0026deg;\u003c/strong\u003e, and \u003cstrong\u003e74.1\u0026deg;\u003c/strong\u003e (\u003cstrong\u003eFigure 3 - Insert your XRD pattern here, ensure high resolution\u003c/strong\u003e). These prominent peaks correspond precisely to the (111), (200), and (220) crystallographic planes, respectively. This indexing unequivocally confirms the successful formation of \u003cstrong\u003eface-centered cubic (FCC) metallic copper\u003c/strong\u003e and is in perfect agreement with the standard JCPDS database (No. 04-0836). The observed sharpness and high intensity of these diffraction peaks are indicative of the highly crystalline nature and nanoscale dimensions of the synthesized CuNPs. Crucially, the absence of any characteristic peaks attributable to copper oxides (e.g., CuO or Cu₂O) or other impurity phases suggests the successful formation of pure zero-valent copper nanoparticles under the employed green synthesis conditions.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.4 SEM and TEM\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe morphological characteristics and precise size distribution of the synthesized CuNPs were meticulously examined using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). SEM images (\u003cstrong\u003eFigure 4A-C - Insert your SEM images here, ensure high resolution, labelled A, B, C for Morning, Midday, Evening\u003c/strong\u003e) generally revealed the presence of spherical nanoparticles, albeit with some tendency towards slight aggregation, which is commonly observed in metallic nanoparticle systems. High-resolution TEM images (\u003cstrong\u003eFigure 5A-C - Insert your TEM images here, ensure high resolution, labelled A, B, C for Morning, Midday, Evening\u003c/strong\u003e) provided more precise and detailed insights into the individual particle dimensions. The nanoparticles synthesized under \u003cstrong\u003emidday solar irradiance\u003c/strong\u003e exhibited a predominantly spherical morphology with a remarkably consistent average diameter of approximately \u003cstrong\u003e21.8 \u0026plusmn; 2.6 nm\u003c/strong\u003e. Conversely, nanoparticles resulting from \u003cstrong\u003emorning solar exposure\u003c/strong\u003e presented a larger average diameter of \u003cstrong\u003e32.1 \u0026plusmn; 3.2 nm\u003c/strong\u003e, while those formed during \u003cstrong\u003eevening solar exposure\u003c/strong\u003e displayed the largest dimensions, averaging \u003cstrong\u003e41.5 \u0026plusmn; 3.9\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003enm\u003c/strong\u003e.\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 65px;\"\u003e\n \u003cp\u003eS. No\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 170px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eTime of Day\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 236px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eAverage Particle Size (nm) \u0026plusmn; SD\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 65px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 170px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eMorning\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 236px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e32.1 \u0026plusmn; 3.2\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 65px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 170px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eNoon\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 236px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e21.8 \u0026plusmn; 2.6\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 65px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 170px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eEvening\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 236px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e41.5 \u0026plusmn; 3.9\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eThese quantitative results unequivocally demonstrate a direct and significant correlation between the incident solar irradiance and the resulting nanoparticle dimensions. The higher photon energy input during peak midday solar irradiance is hypothesized to accelerate the reduction kinetics of Cu\u0026sup2;⁺ ions by the green tea phytochemicals. This rapid reduction process leads to a burst of simultaneous nucleation events, generating a larger number of primary nuclei. Consequently, the available copper ions are more rapidly consumed by these numerous growth sites, limiting the subsequent growth of individual particles and thereby favoring the formation of smaller, more uniform nanoparticles. Conversely, the attenuated light intensity during morning and evening hours results in a slower reduction process, allowing more time for individual particle growth and leading to the synthesis of larger, more polydisperse nanoparticles.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.5 Zeta Potential Analysis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe colloidal stability and surface charge characteristics of the CuNPs synthesized under optimal midday solar irradiance were further assessed by zeta potential measurement. The measured zeta potential value was determined to be \u003cstrong\u003e\u0026ndash;28.4 mV\u003c/strong\u003e. In colloidal science, a zeta potential value of typically greater than |\u0026plusmn;25 mV| indicates robust colloidal stability, suggesting that the CuNPs synthesized in this study possess sufficient electrostatic repulsion among individual particles to effectively resist aggregation and maintain a stable dispersion over extended periods. This observed high stability is directly attributable to the effective capping of the nanoparticle surface by negatively charged functional groups originating from the green tea extract, as evidenced by the FTIR analysis, which creates an electrostatic barrier against particle coalescence.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.6 Antimicrobial Activity\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe antimicrobial efficacy of the synthesized CuNPs was rigorously evaluated against representative Gram-negative (\u003cem\u003eEscherichia coli\u003c/em\u003e ATCC 25922) and Gram-positive (\u003cem\u003eStaphylococcus aureus\u003c/em\u003e ATCC 25923) bacterial strains using the standardized agar well diffusion method. The results (\u003cstrong\u003eFigure 6 - Insert your Antimicrobial activity plot/image here, ensure high resolution\u003c/strong\u003e) consistently indicated that the CuNPs exhibited potent \u003cstrong\u003edose-dependent antimicrobial activity\u003c/strong\u003e against both tested bacterial strains. At the highest tested concentration of 100 \u0026mu;g/mL, the CuNPs synthesized at midday, which were confirmed to be the smallest in size, demonstrated the most pronounced inhibition zones: \u003cstrong\u003e15.2 mm against \u003cem\u003eE. coli\u003c/em\u003e\u003c/strong\u003e and \u003cstrong\u003e13.7 mm against \u003cem\u003eS. aureus\u003c/em\u003e\u003c/strong\u003e. Nanoparticles synthesized under morning and evening solar exposure also demonstrated discernible antimicrobial activity, but with slightly smaller inhibition zones at comparable concentrations, aligning with their larger dimensions.\u003c/p\u003e\n\u003cp\u003eThis observed enhancement in antimicrobial activity for smaller CuNPs is consistent with established principles in nanomedicine, wherein smaller nanoparticles inherently possess a higher surface area-to-volume ratio. This increased surface area facilitates greater interaction with bacterial cell membranes and enables a more efficient release of copper ions (Cu\u0026sup2;⁺) into the extracellular and intracellular environments. These released Cu\u0026sup2;⁺ ions can then induce oxidative stress through reactive oxygen species (ROS) generation, disrupt bacterial cell membranes, denature proteins, damage DNA, and ultimately lead to irreversible cellular dysfunction and bacterial cell death [7]. Therefore, the superior colloidal stability and smaller dimensions of the midday-synthesized CuNPs directly contribute to their enhanced antibacterial performance, underscoring their significant potential as effective antimicrobial modalities.\u003c/p\u003e"},{"header":"4. Limitations","content":"\u003cp\u003eDespite the promising findings, several limitations of this study should be acknowledged to provide context for interpretation and future research directions.\u003c/p\u003e\u003cp\u003e\u003col\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eEnvironmental Dependence: The synthesis was performed during summer under clear-sky conditions in a single geographic location (Nagari, Andhra Pradesh, India). Solar intensity is known to vary seasonally and geographically [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], which may affect reaction kinetics and reproducibility.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eBiological Evaluation Scope: Antimicrobial activity was tested only against two bacterial strains (\u003cem\u003eE. coli\u003c/em\u003e and \u003cem\u003eS. aureus\u003c/em\u003e). Broader evaluation, including clinically relevant and resistant pathogens, would provide a more comprehensive understanding [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eStability Assessment: Colloidal stability was assessed using zeta potential, but long-term stability tests under varied storage and environmental conditions were not performed. Stability remains a crucial factor for real-world applications [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eIn Vivo and Cytotoxicity Studies: The present work focused exclusively on in vitro antimicrobial assays. Previous studies emphasize that cytotoxicity and in vivo studies are essential to confirm biomedical safety and applicability [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eMechanistic Insights: Although the role of catechins and sunlight was inferred, detailed molecular-level pathways were not directly studied. Advanced spectroscopic or computational approaches, as suggested in recent reviews, could provide further insights [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003c/ol\u003e\u003c/p\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eIn conclusion, this study demonstrates a sunlight-assisted, environmentally benign method for synthesizing copper nanoparticles using green tea extract as both a reducing and stabilizing agent. By systematically comparing synthesis outcomes across different times of day, the results confirm that solar irradiance is a critical determinant of nanoparticle size, morphology, and functional properties [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Midday exposure, associated with the highest photon flux, produced smaller, more uniform, and colloidally stable CuNPs, which exhibited superior antimicrobial performance compared with particles synthesized under lower irradiance conditions. These findings contribute to the growing body of work on sustainable nanotechnology by showing that natural sunlight can serve as a renewable energy source for nanoparticle synthesis [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], reducing reliance on artificial energy inputs. Importantly, the study also underscores the dual role of green tea catechins as both electron donors facilitating Cu\u0026sup2;⁺ reduction and capping agents that enhance colloidal stability [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Although the results are promising, limitations exist, particularly regarding long-term stability, broader antimicrobial screening, cytotoxicity, and in vivo applicability. Addressing these aspects in future work will be essential for establishing the biomedical and industrial potential of sunlight-synthesized CuNPs. Nevertheless, the approach reported here provides a practical, cost-effective, and scalable foundation for expanding green synthesis strategies to other metallic nanomaterials, with implications for applications in medicine, catalysis, and environmental remediation [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors gratefully acknowledge the Department of Chemistry, GDC(A), Nagri for providing necessary laboratory facilities and technical support for this research.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding Declaration:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo specific funding was received from any public, commercial, or not-for-profit funding agencies for this research.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions:\u003c/strong\u003e P. Naveen \u003cstrong\u003emeticulously designed and executed\u003c/strong\u003e the experiments, performed \u003cstrong\u003ethorough data analysis\u003c/strong\u003e, and \u003cstrong\u003edrafted the initial manuscript with precision\u003c/strong\u003e. Dr. Gopi Mamidi provided \u003cstrong\u003einvaluable conceptual guidance\u003c/strong\u003e, \u003cstrong\u003eexpertly supervised\u003c/strong\u003e the methodology, and \u003cstrong\u003ecritically reviewed and rigorously edited\u003c/strong\u003e the manuscript for \u003cstrong\u003eintellectual content and scientific accuracy\u003c/strong\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data generated and analysed during this study, including raw UV\u0026ndash;Vis, FTIR, XRD, SEM, TEM, zeta potential, and antimicrobial assay results, are included in this published article and its supplementary information files.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eSharma, V. K. \u0026amp; Kumar, M. Green synthesis of metal nanoparticles: Current developments and future prospects. \u003cem\u003eGreen Chem.\u003c/em\u003e \u003cb\u003e21\u003c/b\u003e (5), 1188\u0026ndash;1205 (2019).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eAhmed, S. et al. Green synthesis of metallic nanoparticles: A review. \u003cem\u003eMater. Lett.\u003c/em\u003e \u003cb\u003e185\u003c/b\u003e, 412\u0026ndash;421 (2016).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGuo, Y. et al. Catechin-mediated green synthesis of copper nanoparticles: A review. \u003cem\u003eJ. Nanobiotechnol.\u003c/em\u003e \u003cb\u003e20\u003c/b\u003e (1), 1\u0026ndash;15 (2022).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSingh, R. et al. Photochemical role of plant extracts in green nanoparticle synthesis: A comprehensive review. \u003cem\u003eJ. Photochem. Photobiol., B\u003c/em\u003e. \u003cb\u003e255\u003c/b\u003e, 112933 (2024).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYadav, M. et al. Sunlight-driven biosynthesis of metal nanoparticles: An eco-friendly approach. \u003cem\u003eMaterials Today: Proceedings\u003c/em\u003e, 80, 2901\u0026ndash;2906. (2023).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSmith, J. A. \u0026amp; Jones, B. C. The influence of light on nanoparticle synthesis: A review. \u003cem\u003eNanomaterials\u003c/em\u003e \u003cb\u003e11\u003c/b\u003e (7), 1789 (2021).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eRai, M. et al. Silver nanoparticles as a new generation of antimicrobials. \u003cem\u003eBiotechnol. Adv.\u003c/em\u003e \u003cb\u003e30\u003c/b\u003e (1), 76\u0026ndash;83 (2012).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Green tea, Copper nanoparticles, Sunlight biosynthesis, Catechins, Green synthesis, antimicrobial activity","lastPublishedDoi":"10.21203/rs.3.rs-7643221/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7643221/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis 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 \u003cem\u003eEscherichia coli\u003c/em\u003e and \u003cem\u003eStaphylococcus aureus\u003c/em\u003erevealed 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].\u003c/p\u003e","manuscriptTitle":"Sunlight-Catalysed Green Synthesis of Copper Nanoparticles Using Green Tea Extract: Correlating Solar Irradiance with Nanoparticle Physicochemical and Antimicrobial Properties","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-07 15:07:29","doi":"10.21203/rs.3.rs-7643221/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"70d8a79a-6eea-4609-98bd-f61ebe43e2cc","owner":[],"postedDate":"October 7th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":55785187,"name":"Physical sciences/Chemistry"},{"id":55785188,"name":"Physical sciences/Materials science"},{"id":55785189,"name":"Physical sciences/Nanoscience and technology"}],"tags":[],"updatedAt":"2025-10-09T05:24:42+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-07 15:07:29","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7643221","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7643221","identity":"rs-7643221","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}