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Bioremediation for Sustainable Environmental Cleanup

formation of biogenic silica. Silaffins are polypeptides rich in lysine and serine residues with a high

degree of post-translation modification (Kroger 2007). Sponges are the another major biosilica­

producing organism and silicatein isoforms have been identified in siliceous sponge species which

play role in synthesis of biosilica (Mishra 2019). Colloidal silica nanoparticles are commercially

available and are often used for research and development purposes. One of the widely used

commercially available colloidal silica is LUDOX^® colloidal silica which is aqueous dispersions of

silica nanoparticles in the nanometre size range. In India, various grades of silica nanoparticle in the

nm size range are commercially supplied by Visa chemical industries.

1.5.5 Ways/Methods to Synthesize Biohybrids

15.5.1 Synthesis of Hybrids/Bio-hybrids using Sol-gel Method

There are other methods also available for synthesis of hybrids/bio-hybrids however, sol-gel method

has been widely used (Mishra 2019, Tian et al. 1997). Due to its mild reaction conditions and

compatibility with a wide variety of solvents, it could efficiently combine silica (inorganic oxide)

with an organic phase/bio-component. The development of several novel polymeric-silica hybrid

materials with a good performance has been reported using sol-gel process (Yang et al. 2004).

There are also many reports wherein biological components, i.e., enzymes and microbes have

been associated with silica through sol-gel process (Avnir et al. 1994, Kim et al. 2000, Premkumar

et al. 2001).

Sol-gel method: The sol-gel process is widely used to synthesize silica, glass and ceramic materials

due to its ability to form pure and homogenous products. The role of the sol-gel method has been

studied since the mid 1800’s. The sol-gel technique is the method in which liquid (colloidal) sol

phase is transformed into a solid gel phase. This involves hydrolysis and condensation of metal

alkoxides (Si (OR)4) like tetraethylorthosilicate (TEOS, Si (OC2H5)4) or inorganic salts like sodium

silicate (Na2SiO3) in presence of acid (HCl) or base (NH3) which act as catalyst (Nandiyanto et al.

2009). This step causes the formation of a colloidal phase which is comprised of relatively higher

molecular weight intermediates and it is known as the sol phase. Intermediates formed during the

reaction undergo polycondensation reactions and escort to the formation of three-dimensional (3-D)

gel. Figure 15.3 shows the flow chart for the sol-gel technique.

Figure 15.3. Flow chart of sol-gel process.

The following processes are involved during the sol-gel process:

i. Hydrolysis of alkoxisilane to silanol

ii. Alcohol condensation: reaction between silanol and alkoxisilane

iii. Water condensation: reaction between silanols