Metal nanoparticles fabricated by green chemistry using natural extracts: biosynthesis, mechanisms, and applications

Abstract

Nanoparticles (NPs) are new inspiring clinical targets that have emerged from persistent efforts with unique properties and diverse applications. However, the main methods currently utilized in their production are not environmentally friendly. With the aim of promoting a green approach for the synthesis of NPs, this review describes eco-friendly methods for the preparation of biogenic NPs and the known mechanisms for their biosynthesis. Natural plant extracts contain many different secondary metabolites and biomolecules, including flavonoids, alkaloids, terpenoids, phenolic compounds and enzymes. Secondary metabolites can enable the reduction of metal ions to NPs in eco-friendly one-step synthetic processes. Moreover, the green synthesis of NPs using plant extracts often obviates the need for stabilizing and capping agents and yields biologically active shape- and size-dependent products. Herein, we review the formation of metallic NPs induced by natural extracts and list the plant extracts used in the synthesis of NPs. In addition, the use of bacterial and fungal extracts in the synthesis of NPs is highlighted, and the parameters that influence the rate of particle production, size, and morphology are discussed. Finally, the importance and uniqueness of NP-based products are illustrated, and their commercial applications in various fields are briefly featured.

Fig. 1. Suggested general mechanisms for the synthesis of NPs.

Fig. 2. Structures of NADH and NAD⁺, and the mechanism of Ag⁺ reduction by NADPH-dependent reductases to form AgNPs.

Fig. 3. Hypothetical mechanism for the extracellular synthesis of NPs.

Fig. 4. Schematic illustration of the proposed mechanism for the intracellular synthesis of AuNPs. Adapted with permission from ref. 98; Copyright 2012, American Chemical Society.

Fig. 5. Keto–enol tautomerism and its effects on the synthesis of Ag NPs. Adapted with permission from ref. 35; Copyright 2010, Elsevier.

Fig. 6. Proposed mechanism for the formation of AgNPs via the reduction of Ag⁺ ions by the reducing steroidal saponin diosgenin. The OH groups are deprotonated to yield reducing anions, which convert Ag⁺ into Ag⁰. The neutral Ag⁰ then reacts with an Ag⁺ ion to form the comparatively stable Ag²⁺ cation. Ag²⁺ ions then dimerize to yield Ag₄²⁺. Adapted with permission from ref. 145; Copyright 2014, Elsevier.

Fig. 7. Suggested mechanism explaining the involvement of two proteins (32 and 35 kDa) in the synthesis of AgNPs. Adapted with permission from ref. 33; Copyright 2011, Royal Society of Chemistry.

Fig. 8. Synthesis of biofunctionalized AuNPs using a designed peptide. Adapted with permission from ref. 153; Copyright 2015, American Chemical Society.

Fig. 9. Silver nanocrystal formation mediated by silver-binding peptides. Adapted with permission from ref. 150; Copyright 2002, Nature Publishing Group.

Fig. 10. Structure of tripeptide-1 and the self-assembly of AuNPs.

Fig. 11. Results obtained in the attempted synthesis of AuNPs using some glucose derivatives.

Fig. 12. (a), (b) and (c) Secondary metabolites used in the synthesis of metallic NPs.

Fig. 13. Factors affecting the biosynthesis of NPs.

Fig. 14. Photo-induced production of AgNPs with various morphologies. Adapted with permission from ref. 175; copyright 2015, Springer.

Fig. 15. (a) Tyrosine-catalyzed formation of AgNPs at high pH and (b) formation of Au-core–Ag-shell NPs under basic conditions. Adapted with permission from ref. 183. Copyright 2004, American Chemical Society.

Fig. 16. Effects of varying the concentration of C. albicans cytosolic extract on the morphology of the AuNPs.

Fig. 17. Applications of metallic NPs.