Optimized green synthesis of silver nanoparticles from guarana seed skin extract with antibacterial potential

Abstract This study optimizes the green synthesis of silver nanoparticles (AgNPs) using seed skin extract of guarana ( Paullinia cupana ) as a natural stabilizing and reducing agent. Employing a Taguchi design, nine experiments were conducted across three levels for four key factors: reaction temperature, pH, silver nitrate concentration, and guarana seed skin extract concentration to minimize AgNP size. Optimal conditions – pH 11, 5 mM silver nitrate, 50°C reaction temperature, and 1% (m/v) guarana extract – produced quasi-spherical AgNPs with an average size of ∼26 nm. Chemical analysis revealed caffeine as the main organic compound and potassium oxide as the primary inorganic component. UV-vis spectra showed an absorption peak at 438 nm, and X-ray diffraction confirmed typical AgNP peaks. Further analysis identified polyphenols, alkaloids, and flavonoids as reducing and stabilizing agents. A high AgNP concentration (180.0 ± 0.2 ppm) was confirmed through ICP-OES, and AgNPs demonstrated a significant antibacterial activity against Escherichia coli and Staphylococcus aureus , with a minimum inhibitory concentration of 50 μg·mL −1 . This study underscores the sustainability of green synthesis as a promising alternative to traditional nanoparticle production methods.