Preparation of ormosil and its applications in the immobilizing biomolecules

Abstract

Biomolecules such as enzymes, antibodies, etc., are highly sensitive and specific in catalysis and recognition. These characteristics make them as potential recognition and catalytic agents in different fields. Attempts have been made to utilize their harness by immobilizing them in suitable matrices/supports. In recent years sol–gel technology has appeared as a greatly promising tool in entrapment of active biomolecules. The introduction of various organic functional groups, such as amino, glycidoxy, epoxy, hydroxyl, etc., into alkoxide monomers leads to organically modified sol–gel glasses (ormosil). The preparation of such organic/inorganic composites provides a means to produce silicate materials with continuously tunable chemical and physical properties by simply changing the precursors employed, their molar ratio, or both. Recently ormosils have been employed in multifarious applications in industrial and medical fields and show promising results in preserving native activity of biomolecules. This review article discusses about the basic chemistry, characterization, advances and biosensor applications of ormosil. The attractive features of ferrocene linked/entrapped ormosil are also incorporated.

Introduction

The developments in sol–gel technology have permitted the formation of ceramic materials in desired shapes at low temperature and triggered several domestic and technological applications [1], [2]. The preparation of silica gel via hydrolysis of tetraethyl orthosilicate, [Si(OC₂H₅)₄] in acidic medium resulted in the productions of glass-like materials in various forms like fibers, monolithic optical lenses and composite glass [3]. Major limitation in preparation of conventional ceramic materials is the need of high temperature during processing and the difficulty in forming desired complex geometrical configurations of the materials. During 1970s Roy and his co-workers [4] recognized the potential of sol–gel process for achieving very high level of chemical homogeneity. They synthesized a large number of ceramic oxide compositions involving Al, Si, Ti, Zn, etc., which could not be produced by traditional ceramic technology. The pioneering work of Iler's group [5] in silica chemistry led to commercial development of colloidal silica powders [6]. This concept has led to the production of a wide variety of composites with controlled morphologies and particle size [6], [7].

The most widely used starting precursors for the fabrication of silica-based materials (sol–gels) are tetramethoxysilane (TMOS) and tetraethoxysilnae (TEOS). The introduction of various organic functional groups into inorganic alkoxide has led to organically modified sol–gel glasses, known as ormosils. Ormosils have several attractive features compared to inorganic sol–gels. Firstly, they allow specific binding of an enzyme to the silica network, for example on silica grafted with aminosilane by Michael coupling through glutaraldehyde to an amine bearing enzyme. Secondly, they allow the encapsulation of catalysts with effective retention properties in case of strong interaction with the organic branch, or better, the covalent bonding of a charge transfer cofactor to the composite material via a chemical reaction with the previously grafted groups. And they make it possible to tune the wettability of composite material by a judicious choice of the ratio of hydrophilic to hydrophobic monomers [8], [9]. With respect to analytical applications, ormosil derived materials can be designed to a controlled active thickness of the sensing device and controlled porosity and provide a versatile way to prepare modified electrodes [10], [11], [12]. The major advantages of the ormosil technology are that the materials can be prepared at a relatively low temperature and their compositions can be easily changed according to the applications. For example, it is possible to prepare the electrodes using different conducting species, organosilanes or polymer additives, redox mediators and several enzymes, each of which can be used to fine tune the properties of the electrode. The conducting species contain graphite and palladium and the incorporation of gold colloids that was reported in recent work [13]. Ormosils and polymers are good to modulate enzyme activity. For example, some of the researchers demonstrated that the use of polycationic polymers into ormosil materials could improve the performance of flavoproteins [14], [15], while some studies have demonstrated that the incorporation of copolymers into silica-based glasses can improve the activity of entrapped glucose oxidase for amperometric detection of glucose [16]. The biomolecules such as atrazine chlorohydrolase [17], lipase [18], lipase and human serum albumin [19] entrapped in ormosils show improved performances including storage stability, excellent activity retention, etc. By taking these advantages and utilizing the advances in ormosil technology in last two decades several enzymes have been successfully encapsulated into ormosil and employed in design of biosensors [9]. In recent years several reviews were also appeared on sol–gel encapsulated biomolecules [20], [21], [22], [23], [24], [25], [26], [27], and few on ormosil [9], [28]. This review tries to describe all important aspects and reports of the ormosil with emphasis on active biomolecules.