Microbial Synthesis of Precious Metal Nanoparticles and Their Applications: A Review.

Precious metal nanoparticles (PMNPs), particularly silver, gold, palladium, and platinum nanoparticles, have attracted considerable attention owing to their unique physicochemical properties and broad applications in catalysis, environmental remediation, and biomedicine. Conventional physical and chemical synthesis methods often require substantial energy input, harsh reaction conditions, and generate large volumes of metal-containing wastewater, raising concerns regarding sustainability and environmental impact. Microbial synthesis provides a sustainable alternative by using microorganisms as natural biofactories to convert toxic precious metal ions into valuable nanoparticles under mild conditions. This review summarizes recent advances in the microbial synthesis of PMNPs (Bio-PMNPs), focusing on biosynthetic mechanisms in bacteria, algae, and fungi. Bio-PMNPs formation involves both extracellular and intracellular reduction processes, coupled with electron transfer mediated by reductases and other redox-active biomolecules. Functional groups present on microbial cell walls, as well as proteins, polysaccharides, enzymes, and other metabolites, play important roles in the adsorption, reduction, stabilization, and growth of nanoparticles. We further highlight the applications of Bio-PMNPs in antimicrobial activity, cancer therapy, pollutant degradation, heavy-metal removal, and catalytic enhancement of organic synthesis. Despite substantial progress, challenges remain in controlling nanoparticle size and morphology, elucidating biosynthetic mechanisms, and achieving large-scale production. Future integration of synthetic biology, metabolic engineering, and process optimization is expected to improve the controllability, stability, scalability, and biosafety of Bio-PMNPs production.