Green Synthesis of Gold and Silver Nanoparticles Using Plant Extracts, Fungal Filtrates, and Bacterial Cultures

Green synthesis of metal nanoparticles (NPs) using biological systems provides a sustainable alternative to conventional chemical fabrication methods. This protocol presents reproducible experimental and analytical procedures for the synthesis of gold (Au) and silver (Ag) NPs using bacterial cultures, fungal filtrates, and plant leaf extracts as reducing and stabilizing agents. AgNPs were synthesized using extracts from Psidium guayaquilensis, Acanthophora spicifera, and Earliella sp., exhibiting characteristic ultraviolet-visible (UV-Vis) absorption bands between 405 and 425 nm that confirmed NP formation. A design-of-experiments approach was implemented to evaluate the influence of environmental factors, including oxygen conditions, pH, Au concentration, cell concentration, electron donor type, and temperature, on the synthesis of AuNPs by Shewanella oneidensis and Cupriavidus metallidurans. Under ideal conditions (0.2 mM Au and pH 5), S. oneidensis produced predominantly spherical AuNPs with an average size of 43.6 ± 11.0 nm and a characteristic absorption peak at 520 nm. NP formation and morphology were confirmed using UV-Vis spectroscopy and transmission electron microscopy. This workflow provides a reproducible platform for biogenic NP synthesis and supports applications in biosensing, bioremediation, and antimicrobial technologies.