ReviewPreparation of ormosil and its applications in the immobilizing biomolecules
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.
Section snippets
Chemistry of sol–gel process
The sol–gel chemistry paves a versatile path for the low temperature synthesis of silica matrices. In typical sol–gel process alkoxide monomers (TMOS or TEOS) undergo hydrolysis to form silanols, silanols then link together to form siloxanes, finally through condensation silanols react with siloxanes to form porous sol–gel matrices after aging and drying processes under ambient atmospheres [28]. The chemistry of such a process can be expressed in Fig. 1. Sol is the dispersion of colloidal
Characterization of ormosil films
Although many reports indicated silica net works are able to retain the structure and activity of a wide variety of enzymes [49], [50], some proteins may completely unfold upon encapsulation as reported for apomyoglobin by Eggers and Valentine [51]. Biomolecules are highly sensitive and fragile in nature; hence their vicinity should be mild and closer to the native environment (inside the cell) after immobilization. Commonly the factors such as polarity, local microviscosity and interactions
Advances
Inorganic sol–gel matrices are not highly biocompatible and bristle in nature. Organic modification in sol–gel precursor may provide better way of controlling nanoporous geometry of ormosil suitable for sensor design [81], [82]. The addition of synthetic or natural polymers such as poly(ethylene glycol) (PEG) to TEOS and organosilane-derived sol–gel material offers enhanced material properties such as optical clarity and dehydration/rehydration stability, results in significant improvement in
Ormosil based biosensors
The use of sol–gel glass for the development of electrochemical biosensors has received great attention because of its sturdiness and possible commercial applications. A number of publications are available on the applications of sol–gel glass for the development of electrochemical sensors [90]. Four approaches dependent on the convenience, stability and response can be used for immobilizing the enzyme on electrode surface: entrapment of enzyme in ormosil film, attachment of enzyme on the
Other applications
Ormosils have also been used in other different fields such as manufacture of new contact lenses and fresnel lenses, preparation of laser components for opticsm, second-order non-linear optically active nanocomposites and bone repairing materials, synthesis of photochromic coatings and porous solvent absorbers, and so on [29], [124], [125], [126], [127], [128], [129]. The biomolecules or dyes entrapped in ormosil nanoparticles can be efficiently employed in drug delivery and other
Conclusions
This review article discussed the ormosil basic chemistry, characterization, advances and biosensor/biological applications. Ormosil matrices offer tailorable hydrophilic, hydrophobic, ionic, and H-bonding capacities as well as electrochemical activities and controllable porosity and are highly stable, inert and non-biodegradable. By doping conductive materials and natural polymers into ormosil matrices their conductivity and biocompatibility can be further improved. By creating more reactive
Acknowledgements
We gratefully acknowledge the financial support of the Distinguished Young Scholar Fund to H.X. Ju (20325518), the National Natural Science Foundation of China (20275017). V.S. Tripathi and V.B. Kandimalla are highly thankful to Nanjing University for providing Post-doctoral fellowships.
Vijay Shyam Tripathi received his MSc degree in chemistry and PhD degree in analytical chemistry from Banaras Hindu University, Varanasi, India in 1997 and 2003, respectively. He is currently working as a Post-doctoral fellow in the Department of Chemistry, Nanjing University, China. His research interest includes electroanalytical chemistry, sol–gel chemistry, biosensor and bioelectronics.
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2017, Advances in Colloid and Interface ScienceCitation Excerpt :These features facilitate the loading of various drug molecules and are useful for studies of drug release kinetics [36]. The organic/inorganic hybrid material ORMOSIL is an organically modified sol–gel silica synthesized by the incorporation of organic functional groups in inorganic alkoxides [37]. There are several attractive features of such materials compared to the features of inorganic sol-gels.
Separation of oligopeptides, nucleobases, nucleosides and nucleotides using capillary electrophoresis/electrochromatography with sol–gel modified inner capillary wall
2017, Journal of Chromatography ACitation Excerpt :The advantages of the sol–gel method in preparing of stationary phases for CEC are summarized in Ref. [2], and the focusing on OT-CEC technique is very broadly described in Ref. [1]. The entrapment of biomolecules in sol–gel matrices and their applications as biosensors and other wide applications was also published and is far beyond the scope of this paper [3,4]. Sol–gel glass offers an easy way to immobilize biomolecules within its porous optically transparent matrix and demonstrate functional activity of encapsulated biomolecules [5,6].
Vijay Shyam Tripathi received his MSc degree in chemistry and PhD degree in analytical chemistry from Banaras Hindu University, Varanasi, India in 1997 and 2003, respectively. He is currently working as a Post-doctoral fellow in the Department of Chemistry, Nanjing University, China. His research interest includes electroanalytical chemistry, sol–gel chemistry, biosensor and bioelectronics.
Vivek Babu Kandimalla received his BSc degree (1993) in biology and MSc degree (1996) in biotechnology from Nagarjuna University, Nagarjuna Nagar, India and PhD in biotechnology in 2002 form Andhra University, Visakhapatnam, India. He is a Post-doctoral fellow in the Department of Chemistry, Nanjing University, Nanjing China. His research interests include the development of electrochemical biosensors, antibodies production, quantum dots (QDs) conjugation with antibodies and aptamers, cellular imaging and in situ imaging tools design.
Huangxian Ju received his BSc (1986), MSc (1989) and PhD (1992) degrees in chemistry from Nanjing University, Nanjing, China. He was appointed as a Research Scientist at this University in 1992 and became a Full Professor in 1999. During January 1996 to July 1997 he was a Post-doctoral fellow in the Department of Chemistry, Montreal University, Canada. His research interests include analytical biochemistry, biosensors, electroanalysis and clinical chemistry.