Sustainable green synthesis of metallic nanoparticle using plants and microorganisms: A review of biosynthesis methods, mechanisms, toxicity, and applications

Green synthesis provides a sustainable approach to producing metallic nanoparticles (MNPs) using biological entities such as plants, algae, bacteria, yeast, and fungi. While extensive research has explored these biosynthetic processes, an integrated review is needed to systematically consolidate knowledge on biosynthesis mechanisms, key synthesis parameters, and the comparative advantages and limitations of green versus chemical synthesis methods. This review addresses these gaps by examining the roles of biological entities and their metabolites in reducing and stabilizing MNPs. Plants use polyphenols and sugars to reduce metal ions, while algae utilize compounds such as chlorophylls and carotenoids. Bacteria produce enzymes like nitrate reductase to reduce metal ions inside and outside the cell. Yeast, for instance, employs nitrate reductase for extracellular synthesis and metallothioneins for intracellular synthesis while fungi use enzymes like laccase and reductase to reduce metal ions and stabilize MNPs. It also examines how reaction factors—such as solvent type, pH, precursor concentration, and temperature—affect size, shape, and stability. The comparative analysis highlights the structural, functional, and environmental differences between green and chemical synthesis, emphasizing that green-synthesized MNPs exhibit improved biocompatibility and biological activity. While green synthesis avoids toxic chemicals and harsh conditions, reducing environmental impact, it may result in broader size distributions and less precise shape control compared to chemical methods. This review also addresses current limitations, including batch variability, differences in biological extracts, and challenges in maintaining consistent MNP properties. It emphasizes the need for advanced characterization techniques for reproducibility and quality control, proposing solutions such as bioprocess engineering, real-time monitoring, and lifecycle assessments to improve industrial scalability. In summary, this review provides a comprehensive resource for researchers and industries seeking to use green synthesis for sustainable, large-scale applications in medical, environmental, and biotechnological fields, supporting global sustainability goals and green chemistry principles. • The biosynthesis mechanisms used by plants, algae, bacteria, yeast, and fungi to produce MNPs are explained • The influence of physicochemical parameters on the biosynthesis and stability of MNPs is analyzed • Comparative studies between green vs chemically synthesized MNPs are presented • The biocompatibility, toxicity, and biodistribution of green MNPs, in vitro and in vivo, are examined • Challenges and limitations of green synthesis are discussed, and new strategies are proposed.