Ormosil

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). 

Precursors are generally ORMOSILs, that is, organically modified silicates. The procedure for metal catalysts is similar to that for lipases, apart from the use of an argon atmosphere. More details of these procedures can be found in the literature [26]. 

An example of the appropriate application of organically-modified silica precursors is alkoxides with an alkyl group. When methyltrimethoxy- or methyl-triethoxysilane (Figure 3.2) was added in formulations to increase the hydro-phobicity of ORMOSILs, it resulted in a better enzymatic activity of lipases immobilized in the alkyl-modified silica than in a hydrophilic matrix fabricated by means ofTEOS alone [51,80,129-133]. Similarly, an increased stability of lipase from Candida antarctica B was observed after its immobilization in a silica matrix... 

On the basis of the sol-gel process, much more attention has been paid to silicate-containing Class II hybrids, also referred to as organically modified silicas, ormosils. In this section, the synthesis of some of the most characteristic ormosils for bone and dental applications are described. 

Incorporation of silica nanoparticles as colloidal silica in ormosils, also contributes to improvement in the mechanical... 

Fig. 12.7 Some examples of network formers and network modifiers used for the synthesis of ormosils.

Poly(oxyethylene)-Si02 ormosils have been prepared as an approach to the preparation of biologically active polymer-apatite composites. For this purpose, Yamamoto et al. [72] obtained these Class II hybrids from triethoxysilyl-terminated poly(oxyethylene) (PEG) and TEOS by using the in situ sol-gel process. After being subjected to the biomimetic process to form the bone-like apatite layer, it was found that a dense apatite layer could be prepared on the hybrid materials, indicating that the silanol groups provide effective sites for CHA nucleation and growth. 

Fig. 12.8 Electron microscopy study of a PDMS-Si02-CaO ormosil (A) Original HRTEM image of the amorphous matrix, (B) filtered HRTEM image and (C) Fourier transform pattern. Distances up to 0.53 nmfor (Si04)4-can be observed in the filtered image, indicating the presence of Ca2+ between the tetrahedra.

Aburatani, Y., Tsuru, K., Hayakawa, S. and Osaka, A. (2002) Mechanical properties and microstructure of bioactive ORMOSILs containing silica particles. Materials Science and Engineering C, 20, 195-198. 

Figure 6. Tip of the 20-pm tip of a fiber optic oxygen microsensor. The tip is coated with a ormosil-type of sol-gel doped with a ruthenium indicator for oxygen that display red luminescence. The sensor measures its decay time as a function of oxygen partial pressure.

An optical sensor for the measurement of carbon dioxide in modified atmosphere packaging (MAP) applications was developed89. It was based on the fluorescent pH indicator l-hydroxypyrene-3,6,8-trisulfonate (HPTS) immobilized in a hydrophobic organically modified (ormosil) matrix. The CO2 sensor was stable over a period of at least 7 months and its output was in excellent agreement with a standard reference method for carbon dioxide analysis. 

Internet site http //www.ea.ucla.edu/ deans/bios/jdm/ormosil.html. 

Lechna-Marczynska M., Ulatowska A., Podbielska H., Grzegorzewski B., Light scattering measurements in ormosil and PHEMA - possible new materials for contact lenses, EOS 23, 33-34, (1999). 

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