GREEN SYNTHESIS OF SILVER NANOPARTICLES MEDIATED BY AQUEOUS EXTRACT OF Pyropia yezoensis ALGAE: OPTIMIZATION OF SYNTHESIS CONDITIONS AND EVALUATION OF ANTIOXIDANT ACTIVITY

Marine algae are promising sources of bioactive compounds presenting potential applications in biotechnology, pharmacology, and food industry, and for the development of nanomaterials. In this context, Pyropia yezoensis (Nori) stands out as a biomass of interest for obtaining compounds capable of promoting the green synthesis of metallic nanoparticles. This study aimed to investigate the use of aqueous extract of P. yezoensis for the synthesis of silver nanoparticles (AgNPs), to evaluate the influence of experimental conditions on the formation of these nanomaterials, and to determine the antioxidant activity of both extract and AgNPs through DPPH free radical scavenging assay. To prepare aqueous extract, the biomass was dried, grounded, and diluted in water at a 1:30 (w/v) ratio. The mixture was sonicated at 50 Hz for 5 minutes, heated at 60°C for 20 minutes, and then vacuum filtered. For the green synthesis process, various parameters were evaluated including extract concentration, silver nitrate (AgNO₃) concentration, pHs, temperature and reaction time. The AgNPs formation was monitored through visual color changes and ultraviolet-visible (UV-Vis) spectrophotometry. The results indicated that a combination of 25% extract, 2.5 mM AgNO₃, and pH 12 promoted AgNPs formation. A characteristic absorption band was observed in the 400-500 nm region, with a peak at approximately 430 nm at room temperature, and at 420 nm when the colloidal solution was heated at 50°C for 30 minutes. Antioxidant evaluation was performed via UV-Vis spectrophotometry at 517 nm, following a 30-minute incubation of samples with DPPH in dark. The aqueous extract exhibited antioxidant activity of 17.83±2.68%, 11.97±2.23%, 8.88±0.82%, and 5.10±0.37% at concentrations of 100%, 50%, 25%, and 15%, respectively. AgNPs synthesized at room temperature and at 50°C, exhibited respectively 25.45±1.75% and 55.32±5.39% antioxidant activity. These results indicate higher antioxidant activity for the AgNPs compared to extract used for the synthesis. It is possible to note that AgNPs sample obtained under heating showed an greater antioxidant activity. These findings suggest that green synthesis using aqueous extract of P. yezoensis is a promising strategy for obtaining nanomaterials with antioxidant activity, highlighting the importance of optimizing synthesis conditions. Therefore, further studies are required to characterize the physicochemical properties of the AgNPs, investigate the mechanisms involved in the activity exhibited in the present study, and evaluate their safety and potential biotechnological applications.

Acknowledgments: FAPESP Process number: 2024/10127-2 and 2024/ 21456-7.