Biohybrids for Environmental Remediation and Biosensing 267

their applicability for various practical applications. Availability of silanol groups on silica’s

surface helps in attachment of biomolecules to develop biohybrids and it could be also utilized for

functionalization using chemical agents (Zucca and Sanjust 2014).

15.4.2.2 Silica-based Supports

Silicates are the most common component on the surface of Earth. Silica is one form of the silicate

(general chemical formula SiO2 or SiO2.xH2O) which exists in two main forms crystalline and

amorphous (Zucca and Sanjust 2014). Silica surfaces appear from the dehydration of hydrated silica

preparations (“silicic acids”) or after grinding bulky silica. Usually, two main functional groups

silanols (Si–OH) and siloxanes (Si–O–Si) are available on the surface of silica. Silanols (Si–OH)

are acidic and impart a negative charge to the silica surface in a wide range of pH (Davydov 2000).

Due to the presence of siloxane motifs, some silica show a hydrophobic character (Rimola et al.

2013). As a result of availability of both hydrophilic and hydrophobic sites on surface and the

characteristics such as weakly acidic and high tendency to take part in hydrogen bonding, silica

shows high adsorption character. The property of adsorption plays a very important role as support.

15.4.2.3 Silica Nanoparticles

Silica has been extensively used throughout history because of its accessibility and ease of recovery.

In recent years, colloidal silica has come up as an ideal support in the field of material science as

nano- and microparticle (Bergna and Roberts 2006). The reason behind wide applicability of colloidal

silica in research is the unique properties which they possess like high colloidal stability, optical

transparency, chemical and thermal stability low toxicity, high surface area, biocompatibility and

well-known surface chemistry (Hyde et al. 2016). Colloidal silica has a wide range of applications

like catalytic supports (Chen et al. 2011, Xie et al. 2015), biosensor supports (Zhao et al. 2013, Wang

et al. 2014), drug carriers (Tang et al. 2012, Rajanna et al. 2015), antifouling coatings (Zhu et al.

2014) and additives to paints/lacquers/coatings (Puig et al. 2014). As a result of many applications,

colloidal silica production has become a large industry. Commercially available colloidal silica is

preferred over naturally available mineral silica. As, natural silica is contaminated with various

metal ions, it offers less surface area and is available in a crystalline form which is not suitable

for health applications (Rahman and Padavettan 2012). Therefore chemically synthesized colloidal

silica is a preferred choice for various applications.

Silica nanoparticles could be of following different types:

• Fumed (pyrogenic) silica

• Precipitated silica

• Silica gel

• Mesoporous silica nanoparticles

• Biogenic silica

Fumed silica has uses as an additive (Boldridge 2010) and used for the production of silicone

rubber for medical applications (Taikum et al. 2010). However, due to its high cost and a smaller

number of silanol groups, application of fumed silica is limited in research and development

(Prasertsri and Rattanasom 2012). In comparison to fumed silica, precipitated silica is widely used

for various applications (Kim et al. 2004, Sun et al. 2013, Pattanawanidchai et al. 2014). Silica gel

offers high specific surface area, used as a desiccant (drying agent), food additive (approved by the

FDA) and as a humidity indicator. Mesoporous silica is another form of silica

which has gained lot

of interest in nanotechnology

. A wide range of mesoporous silica is available while MCM-41 and

SBA-15 are the most used mesoporous silica (Katiyar et al. 2006). These have applications in the

field of medicine, biosensor, imaging, etc. (Valenti et al. 2016). Biogenic silica is synthesized through

the process of biosilication, wherein biomolecules are involved in the synthesis of silica. Various

specialized proteins (silaffins, silacidins and cinguliums) are also known to be responsible for the