Green Nanotechnology and Phytosynthesis of Metallic Nanoparticles: The Green Approach, Mechanism, Biomedical Applications and Challenges

Abstract

The synthesis of nanoparticles is generally divided into bottom-up and top-down approaches which involve physical, chemical, and biological methods. The physical and chemical methods are associated with issues such as high cost, non-ambient reaction conditions, and toxicity. Biological methods or green-methods, using plants, bacteria, algae, and fungi have been developed in recent years to overcome the issues associated with conventional methods. The synthesis of nanoparticles using plants and plant-products as chemical reducing and stabilizing agents from metal precursors is termed phytosynthesis. Plants are available in plenty, safe to handle and contain a wide variety of water-soluble metabolites such as anthocyanins, flavonoids, polyphenols, alkaloids, and terpenoids, that act as excellent chemical reducing and stabilizing agents. These metabolites reduce the metal precursors to metal nanoparticles in a much shorter time as compared to bacteria and fungi. Furthermore, both bacteria and fungi require much longer incubation time for the chemical reduction process. A wide variety of plants have been used to synthesize nanoparticles, oxides, and alloys of gold, silver, titanium, platinum, palladium, copper, cobalt, selenium, zinc, titanium, and iron for various biomedical applications. The types of plants and parts used such as stem, leaf, flower, fruit, pods, and peel have significant effects on the size and shape of the synthesized nanoparticle. Although the phytosynthesis method is advantageous in many aspects of synthesis, there are challenges associated with scale-up process for larger scale production which could be overcome in the future. This review summarizes the phytosynthesis process, green nanotechnology, characterization methods, mechanisms, various biomedical applications, and challenges.