Sol-gel materials

Due to the very flexible synthetic approach, imprinted layers are highly suitable for sensor measurements in complex mixtures Sensor coatings consisting of a carbonic-acid-imprinted sol-gel material e.g. incorporate oxidative degradation products from engine oil leading to a chemical lubricant sensor. 

Adsorption on solid matrices, which improves (at optimal protein/support ratios) enzyme dispersion, reduces diffusion limitations and favors substrate access to individual enzyme molecules. Immobilized lipases with excellent activity and stability were obtained by entrapping the enzymes in hydrophobic sol-gel materials [20]. Finally, in order to minimize substrate diffusion limitations and maximize enzyme dispersion, various approaches have been attempted to solubilize the biocatalysts in organic solvents. The most widespread method is the one based on the covalent linking of the amphiphilic polymer polyethylene glycol (PEG) to enzyme molecules [21]. 

Hydroxy-terminated PDMS, however, has disadvantages. The monofunctional ends limit the number of connections between the polymer (or oligomer) molecule and the glass network to two. This limitation raises the possibility that some PDMS molecules are not tied at both ends to the glass network if the polycondensation does not go to completion i.e. there may be "dangling" or loose PDMS chains in the final sol-gel material. This occurance of free ends would indeed be anticipated since the extent of reaction most likely is not 100%. Hence, the physical properties, specifically the mechanical behavior of the overall material, would be expected to suffer as a result of loose PDMS chains in the system. Disregarding this potential problem, the mechanical behavior of the sol-gel hybrids are, ultimately, influenced by the mechanical behavior of the modifying elastomer ... 

Furthermore, the same sol-gel matrices have been used in a system where acid and base catalysis occur in the same pot without quenching either catalyst [29]. In this case, the acids were either entrapped Nafion (perfluorinated resin sulfonic super acid, a3) or entrapped molybdic acid (M03-Si02, a2), while the bases were two ORMOSILs (organically modified silica sol-gel materials), one with H2N (CH2)2NH(CH2)3 groups (bi) and the other guanidine base residues (b2) (Scheme 5.12). 

The limitations of the method come from the stability of the complexes under sol gel conditions however, even biocatalysts can be suitable for entrapment in sol-gel materials.87... 

Miller, J.M., Dunn, B., Valentine, J.S. and Zink, J.I. (1996) Synthesis conditions for encapsulating cytochrome c and catalase in Si02 sol-gel materials. Journal of Non-Crystalline Solids, 202, 279-289. 

Immobilization of plant cells in hybrid sol-gel materials. Journal of Sol-Gel Science and Technology, 7, 87-97. 

Reetz, M.T. (1997) Entrapment of biocatalysts in hydrophobic sol-gel materials for use in organic chemistry. Advanced Materials, 9, 943-953. 

Ferrer, M.L., Garcia-Carvajal, Z.Y., Yuste, L., Rojo, F. and Del Monte, F. (2006) Bacteria viability in sol-gel materials revisited Cryo-SEM as... 

Preparation and physisorption characterization of D-glucose-templated mesoporous silica sol-gel materials. Chemistry of Materials, 11, 2023-2029. 

Reetz, M.T., Zonta, A., Simpelkamp, J. andKonen, W. (1996) In situ fixation of lipase-containing hydrophobic sol-gel materials on sintered glass - Highly efficient heterogeneous biocatalysts. Chemical Communications, 1397-1398. 

Yang, J.M., Lu, C.S., Hsu, Y.G. and Shih, C.H. (1997) Mechanical properties of acrylic hone cement containing PMMA-Si02 hybrid sol-gel material. Journal of Biomedical... 

Construction of the optode for optical biosensor requires immobilization of sensitive compounds in the host matrix. There are several methods enabling molecules entrapment. One can use gels, polymers, saccharose, various meshes and membranes78. In case of fiberoptic indirect sensors optode must be attached to the fiber tip. Nowadays, there are two commonly used optode host materials sol-gel materials and polymers. 

Graham A., Carlson C., Edmiston P., Development and characterization of molecularly imprinted sol-gel materials for the selective detection of DDT, Anal. Chem. 2002 74 458-467. 

D protein arrays based on biotin-streptavidin architectures are likely to be the system of choice due to their ease in handling, excellent signal-to-noise ratio and non-specific interactions. 3D surfaces based on porous gold, sol-gel materials, polymer brushes and dextran surfaces are widely used to mimic the properties of bulk solution and increase the immobilization capacity of proteins. 

Some non-silica sol-gel materials have also been developed to immobilize bioactive molecules for the construction of biosensors and to synthesize new catalysts for the functional devices. Liu et al. [33] proved that alumina sol-gel was a suitable matrix to improve the immobilization of tyrosinase for detection of trace phenols. Titania is another kind of non-silica material easily obtained from the sol-gel process [34, 35], Luckarift et al. [36] introduced a new method for enzyme immobilization in a bio-mimetic silica support. In this biosilicification process precipitation was catalyzed by the R5 peptide, the repeat unit of the silaffin, which was identified from the diatom Cylindrotheca fusiformis. During the enzyme immobilization in biosilicification the reaction mixture consisted of silicic acid (hydrolyzed tetramethyl orthosilicate) and R5 peptide and enzyme. In the process of precipitation the reaction enzyme was entrapped and nm-sized biosilica-immobilized spheres were formed. Carturan et al. [11] developed a biosil method for the encapsulation of plant and animal cells. 

H. Frenkel-Mullerad and D. Avnir, Sol-gel materials as efficient enzyme protectors preserving the activity of phosphatases under extreme pH conditions. J. Am. Chem. Soc. 127, 8077-8081 (2005). 

D.T. Nguyen, M. Smit, B. Dunn, and J.I. Zink, Stabilization of creatine kinase encapsulated in silicate sol-gel materials and unusual temperature effects on its activity. Chem. Mater. 14, 4300-4308 (2002). 

M.T. Reetz, A. Zonta, and J. Simpelkamp, Efficient heterogeneous biocatalysts by entrapment of lipases in hydrophobic sol-gel materials. Angew. Chem. Int. Ed. 34, 301-303 (1995). 

It is evident that the silica support influences the catalytic performance and it is important to understand the details of the processes involved. For the sol-gel material it was shown by 31P NMR spectroscopy that the immobilised cationic complex completely transforms to the neutral rhodium-hydride species under a CO/H2 atmosphere (Scheme 3.3). On dried silica, however, this conversion might not be complete since the dried support is more acidic [32], It is therefore very likely that the neutral and cationic rhodium complexes co-exist on the silica support. 31P NMR measurements on homogeneous rhodium complexes have shown that a simple protonation indeed converts the neutral rhodium hydride species into the cationic complex. 

When in 1994 the US and European patent offices recognized that the doped sol-gel technology had been invented by David Avnir and colleagues, granting the Hebrew University of Jerusalem a series of patents covering the generic methodology for the preparation of sol-gel materials, and their use in various applications, the time was ripe for the foundation of the first sol-gel chemical companies. 

Recent bio-applications of sol-gel materials, J. Mater. Chem., 2006, 16, 1013). These materials deserve a thorough, consistent treatment that would exceed the scope of the present text. 

Figure 1.2 September 2007 cover of a leading international chemistry journal dedicated to sol-gel materials shows mesostructured silicas derived from the cooperative assembly of soluble sol-gel precursors and organic surfactant molecules. (Reproduced from acs.org, with permission.)...

In general, the structure of sol gel materials evolves sequentially as the product of successive and/or simultaneous hydrolysis and condensation and their reverse reactions (esterification and depolymerization). Thus, in principle, by chemical control of the mechanisms and kinetics of these reactions, namely the catalytic conditions, it is possible to tailor the structure (and properties) of the gels over a wide range. For example, stable silica xerogels of tailored particle dimensions, pore morphology, density and porosity, from relatively... 

Along with hydrophobicity, large amounts of both water (to promote hydrolysis) and methanol employed as co-solvent in the catalyst preparation (to promote homogeneity) are needed to ensure optimal reactivity, showing the number of experimental parameters of the sol-gel synthesis which can be controlled independently to optimize the performance of the resulting catalyst. Finally, in contrast to zeolites and other crystalline porous materials, amorphous sol-gel materials show a distribution of porosity which does not restrict the scope of application of sol gel catalysts to substrates under a threshold molecular size. 

D. Avnir, Organic Chemistry within Ceramic Matrixes Doped Sol Gel Materials, Acc. Chem. Res., 1995, 28, 328. 

Figure 6.2 A bifurcated optical fibre is used to carry light to a probe. The hose tip consists of a thin layer of hydrophobic sol-gel material doped with a ruthenium complex which senses oxygen and reveals its content. (Image courtesy of Ocean Optics.)...

D. Avnir, L. C. Klein, D. Levy, U. Schubert and A. B. Wojcik, Organo-silica sol-gel materials, in Organic Silicon Compounds, ed. Z. Rappoport and Y. Apeloig, John Wiley Sons, 1998, vol. 2. 

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