**3. Green synthesis of nanoparticles**

Green synthesis of nanoparticles refers to the synthesis of nanoparticles through biological routes such as those with the help of microorganisms, enzymes, fungus plants or using various plant products [16, 17] Conventional physical or chemical methods of nanoparticle synthesis often produce byproducts that are hazardous to the environment which is one of the key reasons to opt for a more suitable alternative, that is, the green synthesis or green technology [16]. Other aspects by which green synthesis is more superior than the physical and chemical methods are that they are cost efficient and consume less energy [16].

Bottom- up approach is employed in biological- based synthesis of nanoparticles that requires the use of stabilizing and reducing agents [16]. The process of biologically synthesizing nanoparticles is basically divided into three steps: (i) the choice of a suitable solvent medium used, (ii) the choice of a suitable reducing agent that is eco- friendly and environmentally benign, and (iii) the choice of a non- toxic capping agent that can stabilize the synthesized nanoparticles [16].

Prokaryotes as well as eukaryotes are used in the green synthesis of metallic nanoparticles such as silver, gold, platinum, iron, and metal oxides such as zinc oxide and titanium oxide [17].

### **3.1 Biological components for green synthesis**

*Bacteria:* Prokarytic bacteria and actinomycetes are widely used in the synthesis of metal and metal oxide nanoparticles as they have the potential to reduce metal ions and therefore, are suitable candidates for the preparation of nanoparticles [18] . The fact that it is relatively easier to manipulate bacteria is a key point in employing them in nanoparticle synthesis [18].

*Fungi:* Another popular choice for the biological synthesis of metal and metal oxide nanoparticles is fungi as they behave as better biological agents because they have diverse intracellular enzymes [18]. It is also reported that fungi can comparatively synthesize more amounts of nanoparticles than bacteria which could also be because of the fact that fungi have various enzymes/proteins/reducing components on the surface of their cells [18].

*Yeast: Saccharomyces cerevisiae* has found to be quite effectively employed in the synthesis of silver and gold nanoparticles as reported in numerous studies [18].

*Plants:* The most simple, efficient, cost effective and feasible method of biosynthesis of metal and metal oxide nanoparticles is using plants and plant extracts as biological agents. Biomolecules such as carbohydrates, proteins and coenzymes extracted from plants are employed to reduce metal salt into nanoparticles [18].
