Sol-gel system

Mechanistic studies are particularly needed for the hydrolysis and polymerization reactions that occur in sol-gel processing. Currently, little is known about these reactions, even in simple systems. A short list of needs includes such rudimentary data as the kinetics of hydrolysis and polymerization of single alkoxide sol-gel systems and identification of the species present at various stages of gel polymerization. A study of the kinetics of hydrolysis and polymerization of double alkoxide sol-gel systems might lead to the production of more homogeneous ceramics by sol-gel routes. Another major area for exploration is the chemistry of sol-gel systems that might lead to nonoxide ceramics. 

Coffman, P. R. Barlingay, C. K. Gupta, A. Dey, S. K. 1996. Structure evolution in the Pb0-Zr02-Ti02 sol-gel system Part II—pyrolysis of acid and base-catalyzed bulk and thin film gels. J. Sol-Gel Sci. Tech. 6 83-106. 

R. Abu-Reziq, J. Blum and D. Avnir, Three-Phase Microemulsion/ Sol-Gel System for Aqueous Catalysis with Hydrophobic Chemicals, Chem. Eur. J., 2004, 10, 958. 

Mai, C. and Militz, H. (2004a). Modification of wood with silicon compounds. Inorganic silicon compounds and sol-gel systems a review. Wood Science and Technology, 37(5), 339-348. 

Nakanishi, K. and Soga, N., Phase-separation in silica sol-gel system containing polyacryhc-acid. 1. Gel formation behavior and effect of solvent composition, Journal of Non-Crystalline Solids 139(1), 1-13, 1992. 

Thus the presence of a large sulfate content in a sol-gel system with titania precursors stimulates the formation of a short-range arrangement with trace of anatase. This contrasts with the effect of sulfate on morphology of sol-gel zirconia where the opposite trend was observed.4-5 As expected, all calcined samples proved to be partly crystalline. Diffraction spectra from all calcined samples appeared to be quite similar and showed the presence of very small crystallites of anatase phase (cf. Fig. 5). 

High-resolution Si NMR spectroscopy was used to study the hydrolysis and condensation kinetics of monomeric and dimeric species in the silicate sol-gel system. Peak assignments for the kinetics experiments were determined by comparing add-catalyzed reaction solutions prepared with limited amounts of water with the synthetically prepared dimeric, trimeric, and tetrameric species. Si NMR peaks were assigned for 5 of the 10 possible dimeric species. The temporal evolution of hydrolysis and condensation products has been compared with a kinetic model developed in our laboratory, and rate constants have been determined. The results indicate that the water-producing condensation of dimeric species is approximately 5 times slower than the water-producing condensation of the monomeric species. The alcohol-producing condensation of dimeric species is comparable with that of monomeric species. 

The response of the sensor peaked within 4 min and the calibration curve is applicable within the range of glucose concentrations in normal human serum. Another glucose sensor using sol-gel systems was based upon glucose oxidase in conjunction with a mediator compound (ferrocene or hexacyanoferrate) that transferred an electron to the enzyme [211], This system can be represented as... 

In these electrodes, the silica sol-gel is mixed with graphite or carbon black powder. At a critical concentration of carbon within the sol-gel (beyond the percolation threshold), there becomes a continuous path(s) of conductive carbon throughout the system. The sol-gel system used was a methyl silicate network (by using methyltrimethoxysilane as the sol-gel precursor, the resulting network has exposed SiCH3 groups), which imparts hydrophobic properties to the electrode... 

Sol-gel materials have found most promise in exploiting enzyme-based redox chemistry. The larger porosity and the very simple methods of entrapment have facilitated the use of enzyme sol-gel systems. Lack of a regular pore structure makes the architecture of molecular assemblies difficult with sol gel materials,... 

We have published a demonstration of this strategy for an isopropanol-based aluminum-titanate sol-gel system.The seleeted addition of ethyl acetate was effective at preventing striation formation for the aluminum-titanium recipe. More... 

When constructing biosensors, which are to be used continuously in vivo or in situ, maintaining sensor efficiency while increasing sensor lifetime are major issues to be addressed. Researchers have attempted various methods to prevent enzyme inactivation and maintain a high density of redox mediators at the sensor surface. Use of hydrogels, sol-gel systems, PEI and carbon paste matrices to stabilize enzymes and redox polymers was mentioned in previous sections. Another alternative is to use conductive polymers such as polypyrrole [123-127], polythiophene [78,79] or polyaniline [128] to immobilize enzymes and mediators through either covalent bonding or entrapment in the polymer matrix. Application to various enzyme biosensors has been tested. 

Novel materials are thus needed to improve the mechanical and chemical stability of the sensor for practical applications in various conditions and, on the other hand, to improve the immobilization scheme in order to ensure sensor stability and the spatial control of biomolectdes. The most important materials for chemical and biochemical sensors include organic polymers, sol-gel systems, semiconductors and other various conducting composites. This chapter reviews the state-of-the-art biosensing materials and addresses the limitations of existing ones. 

During the last decade the number of application of MIP-based sensors has increased dramatically. The high selectivity and affinity of MIPs for target analytes make them ideal recognition elements in the development of sensors. Capacitive (Panasyuk etal., 2001), conductimetric (Piletsky et al., 1995), field effect (Lahav et al., 2004), amper-ometric (Kritz and Mosbach, 1995), and voltammetric (Pizzariello et al., 2001), electrochemical transduction systems have been used. Sensors based on conductimetric transduction have been developed by Piletsky et al. (1995) for the analysis of herbicides. A system using a TiC>2 sol-gel system, and with a linear range of 0.01-0.50 mg L-1 for atrazine, without interference of simazine, and chloroaromatic acids has been described by Lahav et al. (2004). 

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