Sol-Gel Method

In general, CVD is the primary method for synthesis of silica nanoparticles (Silva 2004). However, there is a limitation of CVD to control the particle size: its morphology and phase composition (Kempster 1992). As an alternative to CVD, the sol-gel method is applied for the production of silica-based materials. This process can form pure and homogenous products at mild reaction conditions. In this process, hydrolysis and the condensation of tetraethylorthosilicate (TEOS) or sodium silicate salt has been obtained with a mild acidic or basic condition. In the basic condition, ammonia (NH3) is widely used as a catalyst (Hench and West 1990: Stober et al. 1968).

In the formation of silica structure, siloxane bridges are formed due to the hydrolysis of TEOS and condensation of silanol groups. Further, the formation of silica nanoparticles comprises two steps: nucleation and growth. The two models are proposed to understand the growth pattern of silica nanoparticles:

  • (1) monomer addition (Matsoukas and Gulari 1988)
  • (2) controlled aggregation (Bogush et al. 1988; Bogush and Zukoski 1991).

In monomer addition, hydrolyzed monomers are added to the reaction after completion of the nucleation process. Meanwhile, in controlled aggregation, the resulting nuclei are aggregated to form nanoparticles. Both models are applicable for the formation of spherical or gel structure. Researchers have worked widely to understand the size of nuclei and primary particle (Bailey and Mecartney 1992; Green et al. 2003). Rahman et al. (2007) synthesized 7.1 ± 1.9 nm of nearly monodispersed and stable silica nanoparticles. Many synthesis processes of nanosilica have evolved by following the Stober method. An advantage of following the Stober method is the ability to form homogeneous and spherical silica nanoparticles in comparison to the acid-catalyzed method that mostly produces a gel structure. In this method, fabrication or functionalization of silica nanoparticles has been performed by chemical modification of the silica surface. Surface modifications of silica nanoparticles have been demonstrated to improve the affinity between inorganic and organic phases and also enhance the dispersion ability of silica nanoparticles (Kickelbick 2003; Wei et al. 2011; Shu et al. 2008; Bailly et al. 2010). Generally, silane coupling agents are preferred to modify the silica surface. Silane coupling agents (Si(OR),R) facilitate bond formation between inorganic silica and other organic material like resins. In this reaction, Si(OR), reacts with inorganic material and the R group reacts with the organic phase. This reaction generally conducted in aqueous or nonaqueous solvent systems that are known as post-modification. Nonaqueous systems are mostly used to functionalize APTS molecules onto the silica surface. The major advantage of a nonaqueous system is that it prevents hydrolysis. Silanes like APTS have the property to hydrolyze in an uncontrolled manner that leads to polycondensation in an aqueous system. Therefore, use of organic solvent facilitates a good control of reactions and it is preferred to utilize hydrolysis-prone silane coupling agents. The silane molecules are functionalized to silica through a direct condensation reaction under the nonaqueous system (Vansant 1995). In contrast, the aqueous system is preferred for large-scale production. In the aqueous system, silanes first hydrolyze and then condense before coating on the silica surface. The alkoxy silanes are hydrolyzed and self-condensed to form siloxane bonds with silanol groups of silica particles. Aminopropylmethydiethoxy silane (APMDS) and methacryloxypropyltriethoxysilane (MPTS) are also used as silane molecules to modify the surface of silica nanoparticles (Kang et al. 2001; Yu et al. 2003). The use of these silane molecules resulted inan increased size of particles. Besides that, amino groups carrying aminoethylaminopropyltrimethoxy-silane (AEAPTS) and 3-glycidyloxypropyltriethoxysilane (GPTS) with epoxy groups are also utilized to functionalize silica nanoparticles. Pre-treatment of nanosilica with lower silane molecules via sonication and posttreatment with epoxy silane for a longer time period resulted in monodispersed silica nanoparticles. In this reaction, both treatmentswere performed in an aqueous system. In addition, the advantage of epoxy silane is described as being that its enhanced dispersion of nanosilica in comparisonto amino silanes is due to the absence of H-bonding between silica nanoparticles (Sun et al. 2005). Pharm et al. also modified nanosilica of 30 nm size by using 3-aminopropyltrimethoxysi- lane (APTS) and aminopropyldimethylmethoxysilane (APMS) molecules via aqueous route (Pham et al. 2007). Trimethoxy silane or monomethoxy silane molecules were utilized in less concentration in respect to silica molecules, to avoid irreversible aggregation of nanosilica. Therefore, these studies suggest the use of a low concentration of silane agents with a longer reaction period for the synthesis of monodispersed nanosilica. Vejaykumaran et al. also fabricated 7 nm-sized silica particles with APTS (Vejayakumaran et al. 2008). They applied a one- pot synthesisapproach that is an alternative method to reduce time, energy, and overcome the disadvantages of the post-modification method. In this approach, co-condensation is applied to modify the silica nanoparticles. However, co-condensation methods were mainly used to synthesis porous silica nanoparticles and very few reports are present with the modification of nanosilica (Kobler and Bein 2008; Suzuki et al. 2008). Kobler and Bein demonstrated a co-condensation reaction to synthesize ultrasmall nanosilica by using triethanolamine as a catalyst and phenylethoxysilane as a coupling agent (Kobler and Bein 2008). A one pot sol-gel method was also reported to prepare amino- functionalized nanosilica by using tetraethoxysilane and amino- propyltriethoxysilane (Chen et al. 2008). They prepared mixtures of both precursors in ethanol/water solutions and obtained a 200 nm size of silica nanoparticles that depends on mixing the ratio of precursors. A one-pot microemulsion method was also reported to functionalize silica nanoparticles by utilizing mixtures of TEOS and various organosilanes(Naka et al. 2010). The one-pot synthesis method and post-modificationapproach are both useful methods to functionalize silica nanoparticles. The one-pot methodhasan advantage over the post-modificationapproach in the synthesis of monodispersed and low aggregated particles. On the other hand, post-modification methods have a better ability to functionalize small-size particles with less effect on its size. In the one-pot method, the presence of NH2 group enhances hydrolysis rate that leads to an increase in particle size. Therefore, a low concentration of silane molecules is advantageousfor control of the size of particles. Amino-functionalized nanosilica have potential bio-medical applications to be utilized as nanocarriers for drugs, DNA, and enzymes.

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