Nanoparticle synthesis approaches at a glance

a Department of Biotechnology, Quaid-i-Azam University 45320, Islamabad, Pakistan. b College of Pharmacy, Chung-Ang University, Seoul 06974, Republic of Korea. c Department of Chemistry, Quaid-i-Azam University 45320, Islamabad, Pakistan d Department of Pharmacy, Quaid-i-Azam University 45320, Islamabad, Pakistan e Post Graduate and Research Department of Physics, Kongunadu Arts and Science College, Coimbatore, Tamil Nadu 641029, India *Corresponding Author wali.biotech5511@gmail.com (Wali Muhammad) Tel# +92-345-5224994


Nanoparticles
The last few years have captivated a number of researchers from a variety of fields to synthesize NPs especially that of zero-valent metals owing to their unique properties exhibited in both biological and physical systems. The term NP is used to describe particles that vary in size from 1-100 nm [7]. They are considered as the spearheads of the exponentially growing nanotechnology discipline [8]. Due to its smaller size and large surface area, characterization and formulation of nano-metallic particles has become centre of contemplation as compared to their bulk counterparts as it offers improved applications and enhanced properties. Several nanometallic particles have been formulated so far out of which, NPs like gold, platinum, silver and copper have been abundantly reported [9]. There are four main types NPs includes silver and gold etc. [10]. The most studied type of NPs among these are the metallic NPs [11], the studies on which can be dated back to 19th century when Michael Faraday synthesized gold colloid solution in 1850s [12]. Interaction of metallic NPs with light was demonstrated by Mie [13]. Literature also shows that gold NPs have been used by ancient romans for preparation of stained glass [14]. There are two approaches currently under the use for NP formulation and fabrication namely top down and bottom up approaches [15], that have been described briefly in Figure 1. Top-down approach, also known as microfabrication method [8] utilizes physical approaches like cutting and milling in order to shape the materials into desired conformation and order. It usually involves techniques like lithography, sputtering, etching, and ball mill assisted grinding etc. [16]. Bottom up approach for preparation of NPs is the most widely used method that involves self-assembling of tiny molecules or atoms to form larger subunits. Such procedures help in effectively controlling the shape and size of desired materials in accordance with desired application by varying the reaction conditions like pH, temperature etc.
or the concentration of precursor material. Products with high precision accuracy can be formulated with such approaches [8].

Physical Approaches
Various physical approaches including laser ablation and evaporation-condensation are used for NP synthesis resulting in production of variety of nanometallic particles such as gold, silver, fullerene, lead sulfide and cadmium sulfide have been synthesized through physical processes. Physical methods poses several advantages over chemical counterparts like solvent contamination nonappearance in thin film preparations and uniform distribution of nano-metallic particles [17].
Silver NPs synthesis have been achieved using small ceramic heaters having a local heating originator.
Vapors that evaporate cools down rapidly since the temperature gradient of the heater surface within the vicinity is steep when compared to a tube furnace. This results in production of large concentrations of min sized NPs. This type of physical method is useful for generation of NP in long-term experiments [18]. Laser ablation technique can also be utilized for synthesizing silver NPs from metallic bulk materials [19].

Chemical Approaches
Chemical approaches for NP synthesis focuses on While using chemical approaches for NP formulation, protective agents must be used to improve stability of NPs.
These stabilizing agents prevent NPs from binding to the surrounding NPs and helps in preventing NPs agglomeration [23].

Biological Methods
Biological approaches involves the uses of microorganisms like fungi, bacteria and plants for the synthesis of NPs. It also involves biomolecules, protein and enzymes. Biologist mainly performed this method and considered it as an ecofriendly method. In biological methodology, mostly plants are using nowadays due to easily availability and also rich source of metabolites. This simple method is known as "green synthesis".

Green synthesis
The remarkable potential of Nanotechnology, since its dawn, has exhibited its concrete worth in numerous unlikely areas as NPs. Catauro et al. (2004) stated that the physical or chemical methods comprising of toxic chemicals, which pave a way as precursors to convert approaches [29]. These approaches along with the possible applications of their resulting product have been briefly described in Figure. 2 [46]. Figure 3 shows some of the major NPs synthesized with the help of plant material while Figure 4 shows the generalized process of NP production and its potential applications.   reported that methanolic extract of Eucalyptus hybrid has been exploited for silver NPs green synthesis [48]. A list of plant mediated synthesis of NPs is given in Table 2. Punica granatum (peel extract) Cu [55] Physalis alkekengi (plant Zn [56] extract)

Conclusion
Nanotechnology is buzzword in scientific area nowadays with its multiflorous applications. Nanoparticles have greater surface area and significant outcomes as Various research presented potential applications of green synthesized nanoparticles. More studies are recommended to minimize the toxicity of these NPs and further advancement in this field will revolutionize the human civilization from every aspect.