Green-Synthesized Silver Nanoparticles Mediated by Aqueous Extracts of Kappaphycus alvarezii Biomass

The macroalga Kappaphycus alvarezii is a renewable source of bioactive compounds with potential in nanobiotechnology, particularly the green synthesis of metallic nanoparticles, where its extracts act as reducing and stabilizing agents. The aim of this study was to investigate the green synthesis of silver nanoparticles (AgNPs) using aqueous extracts of K. alvarezii, evaluating the influence of pH, silver nitrate (AgNO3) and extract concentration, temperature, and reaction time, through two 23 factorial designs with central points, as well as characterizing the obtained AgNPs by ultraviolet–visible spectroscopy (UV-Vis), dynamic light scattering (DLS), zeta potential, nanoparticle tracking analysis (NTA), and transmission electron microscopy (TEM). The selected synthesis conditions were identified as pH 13, 5.0 mg mL−1 extract, and 2.5 mM AgNO3, with two effective combinations of temperature and reaction time: 50 °C for 30 min or 60 °C for 15 min. The formation of AgNPs was confirmed by a surface plasmon resonance band at ~420 nm, showing spherical morphology, a negative zeta potential, and particle sizes ranging from 93–100 nm (NTA) to 174–192 nm (DLS), the latter reflecting the presence of aggregates in suspension. Colloidal behavior indicated partial stability, with a tendency toward aggregation during prolonged storage. Overall, these findings demonstrate the potential of K. alvarezii as a sustainable source of bioactive compounds for the synthesis of AgNPs, supporting its applicability as an antioxidant nanomaterial platform. Further biological validation and comprehensive sustainability assessments are required to evaluate its potential applications in sectors such as health, cosmetics, and agriculture. This approach reinforces the principles of Green Chemistry and aligns with several Sustainable Development Goals (SDGs), including SDG 3, 9, 12, and 14, particularly regarding the sustainable sourcing of the biomass used.