Nanotechnology: Basic approaches and common methods used to synthesize nanoparticles

1. Introduction:

Nowadays, nanotechnology is the most emerging area of research in nearly all the disciplines of science in synthetic and biological applications, ranging from nano medicines to nano fertilizers, smart devices and the list can go on, due to huge area of applications. Nano materials replace their bulk counterparts in almost all products because nano-particles conspicuously possess incredible and extraordinary properties and abilities as compared to bulk material of same substance. Specific surface are of nano ranged particles is high, which contributes to the increase in the surface area of solid particles per unit mass. Owing to large surface area, less quantity of nano material is used which make the final product smart in manufacturing and handling, as well as cost-effective and affordable, as compared to conventional stuff. Applications of nanotechnology have been spread in each and every category of research and industry, for instance, information technology, environmental remediation, biotechnology pharmaceutical and drug industry, textile industry, nano batteries, metallurgy, smartphones, electrical devices, smart phones and so on.

2. Approaches for the fabrication of nanoparticles:

There are two basic approaches used to fabricate nano products, which are discussed here. All the synthetic methods of nano materials comes under these two approaches

1.     Top-down approach

2.     Bottom-up approach

2.1. Top-down approach:

This approach works on the principle of exfoliation of massive or bulk compounds to nano sized particles using various synthesis routes. Different techniques classified under this approach are mechanical, chemical and electrical exfoliation of bulk samples to nano products. The basic mechanism involved in top-down approach is to convert the bulk material having low surface area to nano sized material possessing large surface area by exfoliating the raw material used.

2.2 Bottom-up approach:

Paradoxically to aforementioned top-down approach, it works on the amalgamation of small entities (below nano range) to acquire the requisite product. The basic mechanism of bottom-up approach starts when atoms stacked together, resulting in crystal planes that combine further to give rise particles of nano range. To illustrate it further in simple words, in this approach small units combine to form building blocks which combine to fabricate the final product. Techniques discussed under bottom-up approach are categorized under two main types which are gas phase and wet chemical synthesis methods for example hydrothermal or solvothermal method, sol gel method, vapor deposition etc. The most promising edge of bottom-up approach over top-down one is that, it offers synthesis of homogenized as well as orderly arranged particles with fewer chances of defects and byproducts. Figure shows a schematic diagram of classification of synthetic approaches for nanoparticles. Figure 1 shows the schematic diagram of classification of synthetic approaches of nano particles.

Fig: 1. Approaches to synthesize nanoparticles

3. Methods used in the synthesis of nanoparticles

Different methods for the synthesis of nanoparticles are used according to need of the product and to get the required properties and dimensions of final product like size, surface to volume ratio, morphology, porosity, adsorption and catalytic properties etc. These methods are classified under three main types according to mode of action and mechanism and given as follows

1.   Physical methods

2.  Chemical methods

3.  Biological methods

3.1 Chemical methods:

This method deals with the interaction of various reactants under specified conditions of temperature or pressure. Solvent act as media in these reactions, and stirring is used to mix the reactants to get a homogenized product. This technique considerably relies on the use of chemicals, nevertheless, has become the most commonly used method in the fabrication of nano materials. The most contributing factor in increased use of these methods is that it provides fine product with controlled morphologies, size, dimensions and properties, in a cost-effective way.
One of the commonly used chemical methods is solvothermal method in which specific solvent is used as media and the reaction mixture is treated under high pressure in autoclave at specified temperature, usually above the boiling point of solvent used as media. In this way uniform conditions of temperature and pressure are provided to all the reactants, thus, the resulted product is homogenized. This method is usually called hydrothermal method when water is used as media. Sol gel is another chemical method used for the synthesis of nano products which involves agitation of the reaction mixture at specified temperature to form a gel and then subjected to centrifugation for further treatment. Ultrasonic and microwave methods involve the usage of sonochemical and microwave assisted methodologies to synthesize the required product in nano range. Figure 2 illustrates different chemical methods used to synthesize nanoparticles.

Fig: 2. Chemical methods to synthesize nanoparticles

3.2. Physical methods:

Mechanical and electrical methodologies are subjected to synthesize nano ranged products. The most highlighted benefit to use physical method is that it provides chemical free and environmentally safe route of synthesis, but at the same time this method does not guarantee the controlled synthesis of nanoparticles with requisite and prescribe morphology and properties, that is the main drawback of this technique.

In contrast to chemical methods, that only involve bottom-up approach, physical methods obey both bottom-up and top-down approaches. The much frequently used physical technique is chemical/physical vapor deposition. The other commonly used physical method is ion beam method that involves synthesis of nanoparticles via ion beam deposition, implantation and sputtering. Electric arc method is also one of the mostly used physical methods. Figure 3 shows schematic diagram of various physical methods used for the synthesis of nanoparticles.

Fig: 3. Physical methods for the synthesis of nanoparticles

3.3. Biological methods:

Biological techniques are used in the green synthesis of nanoparticles that involve environment friendly, safe and cheap synthesis with only minute use of hazardous chemicals. The use of natural resources as raw material makes this technique more affordable, safe and easy to use and handle. In this method, not only different plant extracts as well as agricultural waste are used as raw material, which otherwise pollute the environment by dumping in soil or water. The use of enzymes for the synthesis of nanoparticles is another emerging field of research which combines biotechnology to nanotechnology. Figure 4 shows schematic diagram of some biological methods employed in the synthesis of nanoparticles.

Fig: 4. Biological methods for the synthesis of nanoparticles