**2.1. General overview**

The sol-gel route [41, 42] involves the mixing of metal precursor into either water or organic solvent followed by formation of 3-dimensional network resulting in viscous gel, which in general results in amorphous powder after drying process. The process of mixing is known as hydrolysis, while formation of 3-dimensional network is called as condensation. These two processes are further controlled by many parameters including nature of metal precursor, ratio of precursor to solvent, nature of solvent, capping agents (surfactants), pH and temperature. **Figure 1** summarizes the various types of sol-gel routes.

Aqueous sol-gel route [41], as its name suggests, uses water as solvent to dissolve metal precursor and to complete hydrolysis process. The hydrolysis process is extremely fast due to high reactivity of water with precursors and therefore, generally there is little control over morphology and reproducibility. Nonaqueous sol-gel routes offer a good alternative to get rid of these difficulties [43, 44]. An organic solvent (alcohols, ketones, aldehydes or ethers) is used to complete the hydrolysis process instead of water. Besides, the oxygen required for metal-oxide formation is supplied by organic solvent in nonaqueous sol-gel route, whereas water plays the role of oxygen donor in aqueous sol-gel synthesis. However, inclusion or exclusion of some surfactant (consisting of hydrophilic and hydrophobic groups) in reaction solution further classifies the nonaqueous sol-gel route into surfactant assisted and solvent controlled (surfactant free) routes respectively. The main advantage of surfactant assisted nonaqueous sol-gel route is that the surfactant acts as capping agent and results in highly mono-dispersed nanoparticles. In addition a good control over particle size, morphology with outstanding reproducibility is direct consequence of surfactant-assisted nonaqueous sol-gel route. Moreover, the surface properties of nanoparticles can be easily tailored by exchanging surfactants with other functional groups. However this method is also prone to certain limitations like impurities in nanoparticles and toxic effects due to surfactants. These limitations impose restrictions on the surface sensitive applications (photocatalytic, biomedical and sensing) of nanoparticles.
