Silica Nanoparticles: Methods of Fabrication and Multidisciplinary Applications

Atul Dev, Mohammed Nadim Sardoiwala, and Surajit Karmakar

Introduction

The field of nanotechnology is multidisciplinary, which provides the force of development and innovation. Silica, one of the most abundant materials on earth with very little application in its natural form, converted to different nanoscale structures using nanotechnology approaches.Variation in its nanoscale size and physical state resulted in new unique properties, which were not present in its natural state. These newly added properties have revolutionized its application window. Silica nanoparticles are prepared in solid to mesoporous form depending upon the applicability. This chapter focuses on the various fabrication and functionalization strategies of silica nanoparticles, which resulted in widespread applications of the material.

Strategy for Functionalization/Fabrication of Silica Nanoparticles

Functionalization is useful in the advancement of the utility of silica nanoparticles. Functionalized nanoparticles are applicable in the food, agriculture, environment, and bio-medical sectors with the purpose of imaging, delivery, sensing, diagnosis, and treatment. There are various methods for the synthesis of silica nanoparticles, some of the most common methods are explained in detail.

Stöber Method

The Stober method was first introduced in 1968 (Stober et al. 1968). This procedure is used for the synthesis of solid colloidal silica particles with a large size distribution: a size range from

Stober method for silica nanoparticle synthesis

FIGURE 11.1 Stober method for silica nanoparticle synthesis.

nanoscale to micron. In this method, a silica alkoxide precursor such as TEOS (tetraethoxysilane) is used which results in the formation of monodisperse silica particles upon hydrolysis and condensation in a mixture of ethanol and ammonium hydroxide (Figure 11.1).

Chemical Vapor Deposition (CVD)

CVD is a route to deposit a solid material onto a substrate via surface reaction in the gas phase. Various forms of CVD have been utilized, butmost commonly, firstly precursors are vaporized. Sonication, thermal heating, and pressure reduction are common methods to be used for vaporization (Licausi et al. 2011). In the CVD process, after vaporization, reactants are activated by using heating (Shi et al. 2011), electromagnetic radiation (Santucci et al. 2010), and plasma activation (Wang et al. 201 lb). Hydrophobic material deposition with CVD is a challenging process. In general, the outcome of CVD provides a flat and chemically homogeneous deposition. So, surface roughness has been achieved by the further introduction of nanoparticles (Wang et al. 2011a). In the case of silica coating, surface chemical composition has been modified with post-treatment. Silica surface is generally hydrophilic due to the presence of hydroxyl groups on their surface that favors reaction with water molecules (Laskowski and Kitchener 1969). Lower surface energy and reduced interaction with water help to achieve hydrophobic silica. Commonly, FAS molecules are applied to obtain a superhydrophobic surface of silica (Xu et al. 2009), which is costly. Hence, new and cost-effective routes are required to deposit superhydrophobic material by using CVD. Generally, surface coating of SiO, is performed via CVD using SiCl4. The reaction is bifurcated into two reactions as follows (Klaus et al. 1997):

The requirement of a high temperature around 600-800 К and HC1 production during reaction limits the use of this reaction for coating of soft materials or biomaterials. However, the use of TEOS has overcome one of the limitations of this reaction by preventing the production of HC1. The reaction is as follows:

The use of the amine catalysts has been proven to reduce the higher temperature requirement, and also a modification of silica surface with vinyl alkoxysilanes helps in obtaining reaction at the optimum temperature (Effati and Pourabbas 2012). Further, advancesin the CVD process for the coating of complex geometries and substrates for the modification of nanoparticles has been studied recently. In the study, a new one-step CVD modification method for silica nanoparticles synthesis is demonstrated. In this method, deposition and modification of a silica coating was performed with an all-gas phase to overcome the limitations of CVD. Post-treatment of silica nanoparticles, using ammonia and the advancesin the CVD process facilitated the utilization of temperature-sensitive substrates or biomaterials.

 
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