Green synthesis of metal and metal oxide nanoparticles via plant extracts: an overview

Recently, a biological approach to synthesizing materials via environmentally friendly green chemistry-based techniques involving natural materials such as plants, bacteria, fungi, seaweed, polysaccharides, biodegradable polymers, plant-derived materials and algae has been employed as an alternative method for the synthesis of metal and metal oxide nanoparticles. With increasing enthusiasm for efficient green chemistry, biosynthetic routes for fabricating nanoparticles have aroused much interest because they are environmentally benign, simple, economic, and clean technology; they do not involve hazardous chemicals, and they have zero contaminants and by-products. Of these bio-entities, plant extracts have received great attention due to their ability to reduce and stabilize metal nanoparticles in a single-step synthesis using their distinct natural traits. Due to their diverse and complex compositions, natural organic phytoconstituent biomolecules existing in plant extracts such as alkaloids, flavonoids, saponins, steroids, terpenoids and tannins act as reducing and stabilizing agents. This paper provides an updated review of recent literature on metal and metal oxide nanoparticles, such as those containing silver, gold, palladium, platinum, zinc oxide, iron, titanium, ceria and magnetite, and the transformations, directions and current uses of green synthesis methods using plant extracts. The challenges, limiting factors and future direction of the plant-based synthesis of metal nanoparticles are also highlighted in this review.