Synthesis and Characterization of Silica Sol-Gel Material

Synthesis and Characterization of Silica Sol-Gel Material Hypotheses are the scaffolds which are erected in front of a building and removed when the building is completed. They are indispensable to the worker; but the worker must not mistake the scaffolding for the building. Johann Wolfgang von Goethe

4.1. Introduction Sol-gel method has been used to formulate materials which can be used as new laser materials, nanocomposites, biomimetic systems, and so on. Since, potential in the creation of a wide variety of new materials, understanding of the sol-gel method has become the center of interdisciplinary research comprising physics, chemistry, biotechnology, biochemistry, electronics, and related engineering branches. Sol-gel materials have a wide range of applications, from environmental monitoring to biosensors for health care for sensing clinically-important analytes. Sol-gel derived silica thin films have gained prominence interest because of the mild processing conditions and widespread understanding of the sol-gel chemistry. The sol-gel method is receiving the worldwide attention of scientists in the field of material science as in arrears to its versatility in synthesizing inorganic nanomaterials. High purity, controlled porosity, homogeneity, stable temperature, physical rigidity, nanoscale structuring, high photochemical and thermal stability and excellent optical transparency are the most remarkable features offered by this method for generating highly sensitive and selective matrices to incorporate in several biomedical applications [1]. The inorganic nanomaterials produced by the sol-gel method are particularly remarkable for fabrication of biosensors since they possess a wide range of physical attributes such as ph, temperature, water, viscosity and solvent content. These experimental conditions are used to tailor different property sol-gel materials for biosensor application. In the synthesis of silica sol-gel materials, control over porosity is important for maximum loading of enzyme and less likely to leach the encapsulated enzymes. The porosity of sol-gel thin films allows small reactant molecules to diffuse into the matrix while the large enzyme macromolecules get entrapped in the pores. The reaction in the pores is studied by using the Center for Interdisciplinary Research, DYPU Kolhapur 93

transparency of thin films which can be used for optical biosensor [2]. Thus, the study of the synthesis conditions and the related resulting properties of the xerogel are very important to understand the hybrid sol-gel systems for application in biosensor. 4.2. Mechanism of Sol-Gel Reaction Sol-gel reaction refers to the transition of definite compositions of inorganic alkoxides precursors from liquid sol phase to solid gel phase. The sol-gel term was known for more than 150 years, first coined in the late 1800s. It generally mentions to a low-temperature method using inorganic precursors that can produce ceramics and glasses with better purity and homogeneity. Great concerted efforts of researchers from multidisciplinary fields have been taken to transform sol-gel science to technology. Several products are already commercially available for applications in optical coatings, biomedical, and health care. Sol-gel method involves the transformation of a sol from a liquid sol into gel phase and understanding of this summarized in various books and reviews [1, 3]. The main reactions of sol-gel process are the hydrolysis and polycondensation of metal alkoxides that result in a cross linked silica network. A sol is first formed by mixing an alkoxide precursor like TEOS with water as a co-solvent and an acid or base as catalyst at room temperature. The sol-gel reaction starts when water is mixed with alkoxide precursor. Hydrolysis occurs by nucleophilic attack of the oxygen atom of water molecules on a silicon atom of alkoxide. Subsequently, condensation reaction takes place producing siloxane bonds and polymerization of monomers to polymer concurred. Following three reactions describes a sol-gel mechanism, Center for Interdisciplinary Research, DYPU Kolhapur 94

Considering that the water is a byproduct of the polycondensation reaction, the H 2 O/alkoxide molar ratio (n=2) is theoretically sufficient to promote a complete hydrolysis and condensation to yield anhydrous silica as illustrated by equation [1]. where R is stands for methyl or ethyl group. An increase in H 2 O/alkoxide molar ratio (n) promotes hydrolysis reaction. Under stoichiometric addition of water, when the alcohol producing condensation reaction is predominant; while at water forming condensation reaction is favored. Higher H 2 O/alkoxide molar ratio leads to more complete hydrolysis of monomer before sufficient condensation occurs. However, it was reported that variations in H 2 O/alkoxide molar ratios during the synthesis of pure inorganic silica, results in change in the porosity and pore size distribution of the obtained xerogel [4, 5]. The intrinsic characteristics of silica sol-gel matrix such as porosity, polarity, surface area and rigidity mainly dependent on the progress of hydrolysis and condensation reaction as illustrated in Equations 4.1-4.3. Moreover, properties of silica sol-gel material influenced by choice of precursor, the molar ratio of H 2 O/alkoxide (n), type of solvent and co-solvent, pressure, temperature, aging, drying and curing conditions. Physical properties and internal microenvironment of sol-gel material is mainly dependent on aging i.e. storage condition. As crosslinking of network increases with expelling of internal solvent from pores of matrix results in change in internal Center for Interdisciplinary Research, DYPU Kolhapur 95

polarity and viscosity. Furthermore, average pore size decreases that leads to entrapped species inaccessible to the external analyte. This is the most important concerns of silica sol matrices for biosensor application. Reactions in acidic environments, the oxygen atom in Si-OH or Si-OR is protonated and H-OH or H-OR are good leaving groups as demonstrated in Figure 4.1. The electron density is shifted from the Si atom, making it more accessible for reaction with water (hydrolysis)) or silanol (condensation). Reactions in basic environments are based on nucleophilic attack by OH- or Si- O- on the central Si atom. These species are formed by dissociation of water or Si-OH. The reactions are of SN 2 type where OH- replaces OR- (hydrolysis) or silanolate replaces OH- or OR- (condensation). Acid catalyzed sol-gel exhibit irregular chains due to the electrophilic reaction of H +. Whereas F - and OH - anions directly attack Si(OR) 4 groups by nucleophilic substitution which involves the temporary expansion of the coordination number of silicone from four to five or six which causes rapid hydrolysis and condensation reaction thus affect the pore size, shape and distribution. Figure 4.1: Acid and Base catalyzed reaction mechanism of sol-gel method Center for Interdisciplinary Research, DYPU Kolhapur 96

Sol-gel method is a versatile process for material synthesis, which exploits mild processing conditions such as ambient temperature and pressure creating amorphous materials with high homogeneity and purity. Sols are colloidal suspensions of solid particles in a liquid; prepared from metal alkoxide or inorganic metal salts, water, ethanol and a catalyst. If the sol is allowed to settle in a container, form a gel which is a porous three dimensional network spans the entire volume. These gels are biphasic (both liquid and solid phases) and can either collapse or maintain its structure depending on the evaporation conditions. When the liquid is removed xerogels or aerogels are produced under natural evaporation or supercritical conditions, respectively. The physical properties of these materials can be tailored by manipulating process conditions such as concentration and types of precursors used. The aqueous sol-gel process is defined as the conversion of a precursor solution into an inorganic solid through inorganic polymerization reactions induced by water. In general, the precursor or starting compound is either an inorganic metal salt (chloride, nitrate, sulfate, etc.) or a metal organic compound such as an alkoxide. Metal alkoxides are the most widely used precursors, because they react readily with water and are known for many metals [6]. Center for Interdisciplinary Research, DYPU Kolhapur 97

Fig. 4.2: Schematic representation of sol-gel method 4.2): In general, the sol-gel process consists of the following steps (Figure 1. Preparation of a homogeneous solution either by dissolution of precursors in an organic solvent or by dissolution of inorganic salts in water; 2. Conversion of the homogeneous solution into a sol by treatment with a suitable reagent (generally water and with or without any acid/base catalyst) 3. Aging; 4. Shaping; and 5. Thermal treatment/drying. The first step in a sol-gel reaction is the formation of an inorganic polymer by hydrolysis and condensation reactions, i.e., the transformation of the molecular precursor into a highly cross-linked solid. Hydrolysis leads to a sol, a dispersion of colloidal particles in a liquid, and further condensation results in a gel, an interconnected, rigid and porous inorganic network enclosing a continuous liquid phase. This transformation is called as the sol-gel transition. There are two possibilities to dry the gels. Upon removal of the pore Center for Interdisciplinary Research, DYPU Kolhapur 98

liquid under hypercritical conditions, the network does not collapse and aerogels are produced. When the gel is dried under ambient conditions, shrinkage of the pores occurs, yielding a xerogel. One of the highly attractive features of the sol-gel process is the possibility to shape the material into any desired form such as monoliths, films, fibers, and mono sized powders, and subsequently to convert it into a ceramic material by heat treatment [3]. Some important terms in sol-gel chemistry are described in following sections: Aging When a gel is preserved its pore liquid, its structure and properties continue to change long after the gel point which is called aging. Four processes occur, separately or simultaneously, during aging, comprising polycondensation, synerisis, coarsening, and phase transformation. Polycondensation reactions continue to occur within the sol-gel network as long as neighboring silanols are close enough to react. This increases the connectivity of the network and its fractal dimension. Syneresis It is the spontaneous shrinkage of the gel and resulting expulsion of liquid from the pores. Coarsening is the irreversible decrease in surface area through dissolution and precipitation processes. Syneresis in alcoholic gel systems is generally attributed to formation of new bonds through condensation reactions, which increases the bridging bonds and causes contraction of the gel network. In aqueous gel systems, or colloidal gels, the structure is controlled by the balance between electrostatic repulsion and attractive van der Waals forces. Therefore, the extent of shrinkage is controlled by additions of electrolyte. The rate of contraction of silica gel during syneresis has a minimum at the isoelectric point (IEP). For silica this point is at a ph of 2, at which the silicate Center for Interdisciplinary Research, DYPU Kolhapur 99

species are uncharged. Since the condensation is the slowest at that point, this suggests that the shrinkage is driven by the condensation reaction. Drying In drying, several things may happen when the liquid is removed from the gel. When the liquid in the gel is replaced by air, the structure is maintained and aerogel is formed. Xerogel is formed if the structure collapses. Normal drying of the gel results in structural collapse due capillary forces drawing the walls of the pores together and reducing the pore size. OH groups on opposite sides may react and form new bonds by condensation. If the tension in the gel is large that at which it cannot shrink any more cracking is occur. The reaction kinetics of two similar tetraalkoxy silanes: TMOS and TEOS can be distinctly different under identical reaction conditions. Under acid catalyzed reaction conditions, a TMOS sol-gel undertakes both water and alcohol producing condensation reactions whereas a TEOS sol-gel undergoes only water producing condensation reaction. The early time hydrolysis and condensation reactions of a TMOS sol-gel are statistical in nature and can be quantitatively described by a few simple reaction rate constants while the reaction behavior of a TEOS sol-gel is markedly no statistical. TEOS is an effective precursor for synthesis of Silica sol-gel material; it has some advantages such as convenience in controlling size distribution and pretty compatibility with other organic additives. Also, it has more reactivity towards condensation reaction and high affinity towards enzymes as compared to other precursor. TEOS derived thin films are optically transparent and porous hence suitable for the immobilization of enzymes. Sol-gel method has several flexibilities and unique properties that are of noteworthy in material science. The use of solutions allows the ready construction of thin films and fibers with the high purity. It is used for creating Center for Interdisciplinary Research, DYPU Kolhapur 100

a broad array of materials, especially oxides, in various forms including fibers, composites and monoliths, thin films and coatings, porous membranes, and powders. The formation of a gel requires the synthesis and the aggregation of nano particles in the range 1-10 nm. The aggregation mechanism leads to the synthesis of porous materials with small pore size, which is a nano-porous material. Sol-gel products find application in numerous sectors including abrasives, aerospace, agriculture, analytical chemistry, architecture, automotive, biomedical, chemical, construction, cosmetics, defense, food, optics, dentistry, electronics, electro-optics, environmental, beverages, refrigeration and textiles [7]. The major problems of sol-gel methods are based on the hydrolysis and condensation of molecular precursors is the control over the reaction rates. For most transition metal oxide precursors, these reactions are too fast, resulting in loss of morphological and also structural control over the final oxide material. Furthermore, the different reactivity of metal alkoxides makes it difficult to control the composition and the homogeneity of complex multi-metal oxides by the sol-gel process. One possibility to decrease and to adjust the reactivity of the precursors is the use of organic additives like carboxylic acids, functional alcohols, which act as chelating ligands and modify the reactivity of the precursors. An alternative strategy involves the slow release of water by chemical or physical processes, allowing control over the local water concentration and thus, over the hydrolysis of the metal oxide precursors. In spite of all these efforts, the strong sensitivity of aqueous sol-gel processes towards any slight changes in the synthesis conditions and the simultaneous occurrence of hydrolysis and condensation reactions make it still impossible to fully control the sol-gel processing of metal oxides in aqueous medium. Center for Interdisciplinary Research, DYPU Kolhapur 101

Silica sol-gel materials have many advantages and some disadvantages as displayed in Table 4.1. Table 4.1: Advantages and Disadvantages of Sol-gel material There are two classes of hybrid silica sol-gel materials. Class (I) materials are prepared by adding the dopant (biomolecules, dyes, etc.) material to the starting sol whereas class (II) materials are prepared by co-condensation of silicon alkoxides [Si(OR) 4 ] one or more organosilanes [RʹSi(OR) 3 (where Rʹ could be an alkyl group identical to R, a different alkyl group or a completely different functionality)]. In class (I) hybrid silica-based material the organic and inorganic precursors are linked via weak interactions. The class (II) hybrid silica-based material is composed of the organic and inorganic components are linked via strong covalent bonds. These hybrid materials are attractive because they Center for Interdisciplinary Research, DYPU Kolhapur 102

combine in a single solid both the properties of a rigid three-dimensional network with the specific functionality of the organic component. According to the IUPAC definition, the porous materials are classified into three classes: microporous (< 2 nm), mesoporous (2-50 nm) and macroporous (>50 nm). If the pore dimension is in the nanoscale range, such materials denoted as nanoporous materials [8]. The nanoporous materials have found growing interest in many applications, due to their large surface area, tailored pore size distribution, manageable pore structure and versatile composition. Exclusively for biosensors, the use of nanoporous materials can significantly improve the analytical performance of the biosensors since their large surface area and versatile porous structure are beneficial for the loading of large amounts of active catalysts and they have a fast diffusion rate [9]. The porosity, pore size and its distribution can be controlled by catalyst and experimental condition. The porous material has many advantages as an immobilization matrix owing to its high chemical and physical properties, very high chemical inertness and ageing behavior. A porous structure of materials is beneficial in the development of biosensor as it can enhance interfacial reaction and as a consequence improve sensitivity. As well, tailored permeability for substrate enhances selectivity of biosensor environment. The tailored porosity allows the small analyte molecule to diffuse into the matrix while large protein macromolecule remains physically entrapped in the pores. The transparent material is useful to characterize the reaction that occurs in the pores of matrices by optical spectroscopy. The porous Silica sol-gel matrices derived by HCl, HF, NaF, HNO 3 and NH 4 F catalysis is usually mesoporous with pore diameter in the range of 5-100 Center for Interdisciplinary Research, DYPU Kolhapur 103

nm. HCl catalyzed material has less pore size, on the other hand ammonia catalysis tends to enlarge the pore size. Pore shapes are mainly controlled by the type of catalyst. The pores of narrow necked and large abdomen shapes with the free network structure of linear chains can be obtained by HCl catalysis, while fine cylindrical pores can be achieved by fluorine anion (HF, NaF and NH 4 F) and ammonia catalysis. Generally, Silica particles are positively charged at low ph and negatively charged at high ph. Basic to moderate acidic sols have a significant amount of deprotonated silanol groups (SiO - ) which increase the condensation rate causing formation of highly branched silica species. Gelation of these branched species results in the formation of a mesoporous region with pore size 2-50 nm. As the ph is lowered to the isoelectric point, the gelation time increased and this leads to linear or randomly branched Silica gel having a highly microporous structure with pore diameter 2 nm. At very high acid concentrations (ph 1), dried xerogel become more mesoporous. This is due to the protonation of silanols group to produce (SiOH 2+ ) groups which are acting to increase the rate of condensation. 4.3. Experimental 4.3.1. Materials Tetraethyl orthosilicate (TEOS) was purchased from Sigma Aldrich and was used without further purification. Methanol, iso-propanol, hydrochloric acid and nitric acid were of analytical grade, procured from SD Fine chemicals India. Microscopic slides (scientific plaza) were used as glass substrate. 4.3.2. Characterization of Silica sol-gel material X-ray diffraction measurement was carried out with INXITU x-ray diffractometer equipped with a crystal monochromator employing Cu-K α Center for Interdisciplinary Research, DYPU Kolhapur 104

radiation of wavelength 1.5406 Å in the 2θ range from 10 to 55º. The optical activity of TEOS sol-gel thin films were monitored by using a UV-visible spectrophotometer (Shimazdu model 1800). Fourier-transform infrared spectra of TEOS sol-gel thin films were recorded using a Nicolet FTIR spectrometer (Model 510 P). 4.3.3. Synthesis of Silica sol-gel material A. Preparation of silica stock solution The preparation of silica samples was carried out by the previously reported method [10] as illustrated in Figure 4.3. The sols were prepared by hydrolysis and condensation of TEOS in the presence of water and hydrochloric acid (0.1 M) with different TEOS concentrations. The samples were prepared for the various concentrations and thin films were prepared. Out of which, thin films prepared from samples A 1 and A 2 are crack free and uniform in nature. Hence, these are chosen for further study and explained in detail. The sample A 1 was prepared by mixing 4.5 ml of TEOS and 0.25 ml of 0.1 M HCL in 1.4 ml of double distilled water. Sample A 2 was prepared by adding 2 ml of TEOS and 0.25 ml of 0.1 M HCL in 1.5 ml of double distilled water. These reaction mixtures were placed in 25 ml stopper glass container and stirred for 5 h at 300 rpm until visible homogeneity was obtained. The samples A 1 and A 2 were further kept in a polystyrene container for aging for about 24 h and used as stock solutions for preparation of thin films. Center for Interdisciplinary Research, DYPU Kolhapur 105

Figure 4.3: Schematic illustration for synthesis of silica sol-gel material It was investigated that as the TEOS concentration decreases, the gelation times become larger as shown in Table 4.2. Table 4.2: Effect of silane content on viscosity and gelation time Silica Samples TEOS:H 2 O:HCl Molar ratio Stirring (h) Ageing (h) Viscosity (Pa s) Gelation Time t gel (day) A 1 18:5.6:1 5 24 14 5 A 2 8:5.6:1 5 24 16 3 B. Preparation of thin films I) Pre-treatment for glass plates Prior to casting, glass plates were sonicated in distilled water for 30 minutes to remove impurities. These glass plates were etched by treating with concentrated HNO 3 for 2 h. Further, the plates were washed many times with double distilled water. These pre-treated glass plates wetted with isopropanol for even spreading of stock solution. II) Deposition Method Various physical and chemical deposition methods have been reported to prepare thin films, including chemical bath deposition, spray pyrolysis method, dip coating and spin coating. However, spin coating method is widely Center for Interdisciplinary Research, DYPU Kolhapur 106

employed for the highly reproducible fabrication of thin film over large areas with high structural uniformity and homogeneity at ambient temperature. Spin coating Preparation of silica sol-gel thin films by spin coating method is represented in Figure 4.4. The clear stock solutions of samples A 1 and A 2 further diluted in methanol (1:3). Methanol was used to decrease viscosity to enhance spreading of silica sol on the substrate. About 100 µl of the diluted stock solution was placed on glass plate (area about 4 cm 2 ) and spun at 2800 rpm for 15 min by using spin coater. These films were then dried at room temperature. Figure 4.4: Preparation of silica thin films by spin coating method 4.4. Results and Discussion 4.4.1. X - Ray Diffraction method The phase formation process in silica matrix triggered by sol-gel method was monitored using an x-ray powder diffraction method as shown in Figure 4.5. For the as-dried gel, a much broadened peak at the 2θ of 23 is observed, Center for Interdisciplinary Research, DYPU Kolhapur 107

corresponding to the amorphous silica matrix conforming to JCPDS file (79-1711) [11]. The figure shows that the silica film is in crystalline form. This indicates that no crystalline phase was formed during the initial drying of silica sol-gel material prepared with TEOS at room temperature. Therefore, FTIR spectroscopy was used in order to reconnaissance the thin film material. Figure 4.5: The X-ray diffraction pattern of silica sol-gel material 4.4.2. Fourier - transform infrared spectroscopy FTIR study is carried out to confirm the presence of the silica network in samples synthesized of two different concentrations of TEOS (Sample A 1 and A 2 ) as represented in Figure 4.6. The silica gel spectra illustrate several frequency regions as given below: A] 4000 cm -1-3000 cm -1 : In this spectral range, the bands are mainly due to overtones and combination of vibration of Si-OH or H 2 O. The band at 3430-3445 cm -1 corresponds to molecular water hydrogen bonded to Si-OH group [12]. Center for Interdisciplinary Research, DYPU Kolhapur 108

B] 3000 cm -1-1350 cm -1 : In this spectral range, the bands are due to the overtones and combination of vibration of organic residue, molecular water and SiO 2 network. The band at 1638cm -1 corresponds to vibration of molecular water. In the spectra, water bands observed at around 1640 cm 1 corresponding to bending vibrations indicate hygroscopic character of the powdered samples [13]. C] 1300 cm -1-400 cm -1 : This spectral region is associated with combinations of vibration of silica network. The band at 1082 cm -1 assign to asymmetric stretching vibrations of Si-O-Si bridging sequences. The band at 937-944 cm -1 attributed to stretching vibrations of the free silanol group on the surface of silica network. The band at 791-795 cm -1 corresponds to bending vibration of C-H of CH 3 -Si group. The band at 470-474 cm -1 associated with out of plane deformation of Si-O bonds. It is clear that the effect of water content in sol is pronounced for gel. The samples A 1 and A 2 show slight differences in intensity of peaks. The broad peak in 1400-1600 regions confirm the presence of SiO 4. The intensity of absorption decreases on the low frequency side of 1082-1032 cm -1 and 944-937 cm -1 bands and increases at high frequency peak 3430-3445 cm -1, 1635-1638 cm -1, 791-795 cm -1, 470-474 cm -1. Center for Interdisciplinary Research, DYPU Kolhapur 109

Figure 4.6: FTIR spectra of silica sol-gel thin films for a) sample A 1 and b) A 2 As shown in Table 4.3, the FTIR spectra show characteristic vibrational bands at 1082, 799 and 470 cm -1 ; corresponding to the stretching, bending and out of plane deformation of Si-O bonds, respectively. The position and shape of the main Si-O-Si vibrational band at 1082 cm -1, and the well pronounced shoulder suggests a stoichiometric silicon dioxide structure. Table 4.3: Assignments of band in the FTIR spectra in silica thin films Wavenumber Mode Comment (cm 1 ) 3440 ν Si-OH or OH stretching H 2 O 1082 ν a Si-O-Si Network Si-O-Si stretching 1035 ν a Si-O-Si Silicon sub-oxide, Si-O-Si angle <144 o 944 ν s Si-O-R R: H;C 2 H 5 799 ν Si-C, ρ a CH 3 Si-O bending 470 Si-O out of plane deformation ν: stretching; δ: bending; ρ: rocking; a: asymmetric; and s: symmetric Center for Interdisciplinary Research, DYPU Kolhapur 110

4.4.3. UV- Vis spectroscopy The optical properties of silica thin films were studied by recording the spectra over 400 and 1100 nm using a UV-VIS spectrophotometer as illustrated in Figure 4.7. The optical properties revealed the formation of highly transparent silica thin films. The optical properties of silica thin films are changing with a decrease in silane content. Both films show the similar behavior up to the wavelength 600 nm, after that films show a variation in transmittance. Thin film prepared by using sample A 2 shows slight increase in the transmittance about 95% in the range of wavelength 600 nm to 1100 nm as compared to thin film prepared by using sample A 1 (88 %). This behavior may be due to the less silane content in sample A 2 as compared to sample A 1. However, this increase in transmittance of thin films of sample A 2 is favorable for fabrication of biosensor due to its greater optical transparency. Figure 4.7: Transmittance spectra of silica thin film of samples A 1 and A 2 Center for Interdisciplinary Research, DYPU Kolhapur 111

4.4.4. Scanning Electron Microscopy and Elemental X-ray mapping In order to confirm the formation of the silica matrix during the sol-gel reaction, EDS mapping was used to characterize the silica distribution. Figure 4.8 represents the Si-mapping analysis of the silica sol-gel thin film. The electron beam penetrates 2-3 microns below the surface of sample to reveal the embedded silica distribution. The bright spots represent that the Si element spreads uniformly throughout the matrix. It suggests that the sol-gel reaction of TEOS and water take place throughout the channel networks of the Silica matrix [14]. Figure 4.8: (a) SEM image, and (b) Elemental X-ray mapping for the element Si of Silica sol-gel thin film 4.4.5. EDS The purity of the Silica matrix was ascertained from the EDS studies (Figure 4.9). It was indicated the presence of silicon, oxygen and carbon as the elemental composition. The carbon present in the system originated from the alcohol evolved in reaction. Center for Interdisciplinary Research, DYPU Kolhapur 112

Figure 4.9: EDS spectra of Silica sol-gel thin film 4.4.5. Field Emission Scanning Electron Microscopy FESEM micrographs of surface of silica sol-gel thin film are shown in Figure 4.10. The silica matrix is in the form of porous membrane (a), and their uniform surface is clearly visible at high magnification. At low magnification (b), cracked surface is observed, which is phenomenon of the silica sol-gel material. Figure 4.10: FESEM micrographs of Silica sol-gel thin films from (a) High magnification, and (b) Low magnification Center for Interdisciplinary Research, DYPU Kolhapur 113

4.5. Conclusion This chapter addresses the influence of silane content on properties of silica thin films. These thin films were synthesized using the sol - gel method and spin coating method was employed for the synthesis of uniform and crack free thin films. The results showed that the optical properties of silica thin films were affected by decrease in silane (TEOS) concentration. The greater optical transparency of the silica thin film prepared by using the sample A 2 makes it valuable in the application of enzyme nanobiosensor. Center for Interdisciplinary Research, DYPU Kolhapur 114

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[10] S. Singh, R. Singhal, B. D. Malhotra, (2007), Immobilization of cholesterol esterase and cholesterol oxidase onto sol-gel films for application to cholesterol biosensor, Analytica Chimica Acta, 582, 335-343 [11] Z. H. Zhou, J. M. Xue, J. Wanga, H. S. O. Chan, T. Yu, Z. X. Shen, (2002), NiFe 2 O 4 nanoparticles formed in situ in silica matrix by mechanical activation, Journal of Applied Physics, 91, 96015-6020 [12] R. F. S. Lenza, and W. L. Vasconcelos, (2001) Preparation of Silica by Sol-Gel Method Using Formamide, Materials Research, 4, 189-194 [13] S. Duhana, N. Kishore, P. Aghamkar, and S. Devi, (2010), Preparation and characterization of sol-gel derived silver-silica nanocomposite, Journal of Alloys and Compounds, 507, 101-104 [14] P. K. Leung, Q. Xu, T. S. Zhao, L. C. Zeng, (2013), Zhang Preparation of silica nanocomposite anion-exchange membranes with low vanadium-ion crossover for vanadium redox flow batteries, Electrochimica Acta, 105, 584-592 Center for Interdisciplinary Research, DYPU Kolhapur 116