Review Article: Green Synthesis of Silver Nanoparticles and Their Application

Nanotechnology is important science field which deals with production, manipulation and use of material ranging in nanometers. ”Nano” is derived from the Greek word “nanos” meaning dwarf, tiny or very small (Rai et al., 2008). Nanotechnology has emerged as a dynamically developing area of scientific interest within the world. Nanoparticles are defined as a nanoscale particle of size within 1 to 100 nm. In nanotechnology, a particle is defined as is a small object that behaves as a whole unit respect to its transport and properties. Nanoparticles had a wide variety of application in the major fields of medicine, therapeutics, and diagnostic agents (Colvin et al., 1994, Wang and Herron, 1991, Schmid.G, 1992, Hoffman et al., 1992, Hamilton and Baetzold, 1979, Mansur et al., 1995, Senapati.S, 2005).

Different types of nanomaterials like copper, zinc, titanium, magnesium, gold, alginate & silver have come up but silver nanoparticles have proved to be most effective against bacteria, viruses & other eukaryotic microorganisms. Biologically synthesized silver nanoparticles (SNPs) are being widely used. Nanoparticles have been widely used for disinfection of water and to remove arsenic from water. Agnanomaterials also have many other applications in various fields, such as nanoscale detection and solar cells.

Generally there are two approaches which are involved in the syntheses of silver nanoparticles, either from „„top to bottom‟‟ approach or a „„bottom to up‟‟ approach as depicted in Fig.1 (Ahmed et al., 2010). In bottom to up approach, nanoparticles can be synthesized using chemical and biological methods by self-assemble of atoms to new nuclei which grow into a particle of nanoscale. In bottom to up approach, chemical reduction is the commonest scheme for synthesis of silver nanoparticles (Elghanian et al., 1997; Hurst et al., 2006).

Different organic and inorganic reducing agents, like sodium borohydride (NaBH4), sodium citrate, ascorbate, elemental hydrogen, Tollen‟sreagent and N, N-dimethyl formamide (DMF) are used for reduction of silver ions (Ag+) in aqueous or non-aqueous solutions (Tran et al., 2013; Iravani et al., 2014).In case of top to bottom approach; nanoparticles are generally synthesized by evaporation–condensation method. Also in top to bottom approach, suitable bulk material breakdown into fine particles by size reduction with various lithographic techniquese.g. grinding, milling, sputtering and thermal/laser ablation (Elghanian et al., 1997; Hurst et al., 2006).

The use of green materials like plant extracts, microbial extracts, algal extracts and fungal extracts are used for synthesis of Silver nanoparticles as described in Fig 2.Various reducing agents present in the green extracts are responsible for synthesis of Silver nanoparticles (Roy et al., 2019). FIGURE 1: Different approaches of synthesis of silver nanoparticles (Ahmed et al., 2010).

FIGURE 2: Schematic representation of the procedure for green synthesis of silver nanoparticles using various biological entities (Roy et al., 2019).

The biological method is also referred as green synthesis method. The biological method includes microorganisms like bacteria, fungi, algae, yeast and viruses and also plant extracts. Microorganisms and plants play role in absorption and accumulation of inorganic metallic ions from their surrounding (Shah et al., 2015). They secrete different enzymes in large quantities which have ability to hydrolyze metals and bring reduction of metals ions (Chokriwal et al., 2014). It is found that the reduction rate of metal ion using biological method is quicker compared to physical and chemical method. The synthesis or production of silver nanoparticles by using biological means is not much expensive as compared to physical and chemical method. It is necessary to develop an environmental and economically friendly method for production of nanoparticle without involving any toxic chemicals, (Iravani S, 2014).