Sol-gel derived materials

The design of bioeompatible (blood compatible) potentiometric ion sensors was described in this chapter. Sensing membranes fabricated by crosslinked poly(dimethylsiloxane) (silicone rubber) and sol gel-derived materials are excellent for potentiometric ion sensors. Their sensor membrane properties are comparable to conventional plasticized-PVC membranes, and their thrombogenic properties are superior to the PVC-based membranes. Specifically, membranes modified chemically by neutral carriers and anion excluders are very promising, because the toxicity is alleviated drastically. The sensor properties are still excellent in spite of the chemical bonding of neutral carriers on membranes. 

Orthosilicic acid, Si(OH)4, could be an appropriate precursor, but it cannot be stored in monomeric form at reasonable concentrations in aqueous solutions. It enters easily into polycondensation reactions (1) that result in its polymerization. As a monomer, the orthosilicic add exists in aqueous solutions at a concentration of less than 100 ppm [18]. This is too small to fabricate sol-gel derived materials, although diatoms and sponges have the property of concentrating silica from seawater containing only a few mg per liter [60]. Sol-gel processing in the laboratory can be performed with a rather concentrated solution of orthosilicic add. This requires freshly prepared add the procedure is time consuming which restrids its widespread use. 

Of great importance is the volume shrinkage (syneresis) of the synthesized gel after its fabrication. The phenomenon is typical of the sol-gel derived materials fabricated through the two stages (Scheme 3.1, Figure 3.3). By using TEOS, one may... 

Various materials can be produced by sol-gel methods monoliths, thin films, fibers, powders can. Controlling of the physical and chemical parameters of production process yields materials with precisely tailored parameters such as mechanical strength, transparency, size and distribution of the pores network7. The sol-gel derived materials may provide excellent matrices for a variety of organic and inorganic compounds. 

Flora K., Brennan J., Comparison of formats for the development of fiber-optic biosensors utilizing sol-gel derived materials entrapping fluorescently-labelled protein, Analyst 1999 124 1455-1462. 

W. Jin and J.D. Brenan, Properties and applications of proteins encapsulated within sol-gel derived materials. Anal. Chim. Acta 461, 1—36 (2002). 

Finally, it is worth noting that the mullite precursors shown above can be made at the kilogram scale in a matter of hours, whereas sol-gel derived materials can require up to one month to obtain atomically mixed gels. To date, no attempts have been made to form transition metal precursors via the OOPS or the Kemmitt and Milestone processes. However, the following section suggests that there are good reasons to explore this approach. 

Harmer et al.196 used 1,1,2,2-tetrafluoroethanesulfonic acid in the alkylation of para-xylene with 1-dodecene. The silica-embedded catalyst prepared by the sol-gel method showed much higher activity than the neat acid (almost complete conversion in 15 min at 100°C over the sol-gel-derived material versus 10% conversion, using the same molar amounts of acid). Practically no leaching was detected and the catalyst could be recycled with a slight decrease in conversion. It is in sharp contrast with silica-supported triflic acid, which showed much lower activity due to the loss of volatile triflic acid. 

Figure 20.3 Digital photographs of sol-gel-derived materials monoliths, powders, and films. (See color insert.)...

While technically simpler than the covalent approach, the self-assembly approach creates more heterogeneous sites and also requires templates with specific functional groups.8 Since sol-gel chemistry is aqueous based, H-bonding interactions are significantly weaker compared to the conventional organic polymerization methods. Often, hydrophobic effects and ionic interactions are utilized. A number of other examples of the noncovalent approach to imprinting in sol-gel-derived materials are provided in recent reviews.5 17 In the sections below, the focus will be on some of the newer aspects of small molecule imprinting in silica that involve the use of chiral templates... 

The focus of this chapter is on electron-transfer processes occmring in porous media, with particular emphasis on zeolitic, mesoporous materials and sol-gel-derived materials. The definition of different classes of porous solids as specified by lUPAC is based on the pore diameter ... 

Lastly, Sengnpta et al. ° provide some of the most detailed insight into the nature of the amorphous PZT film derived from a 2-methoxyethanol process. In their studies, the authors used EXAFS to probe the local geometry and chemistry of sol-gel-derived amorphons PZT. They found evidence for separate networks of Ti-O-Ti, Zr-O-Zr, and Pb-O-Pb, in contrast to the pictnre of sol-gel-derived materials being homogeneons on the atomic scale and the work of Arcon et al. ° ... 

Sol-gel-derived materials are popular for developing sensors.16 40"13 The physicochemical properties within these sol-gel-derived nanocomposites is an important factor in designing platforms for sensing applications.18 20 22 44-53 Factors such as polarity, microviscosity, pore size, pore wall chemistry, microscopic phase separation, partition coefficient, and solubility coefficient can dramatically alter the behavior of active dopants within a nanocomposite and may lead to undesirable properties. 

Brennan, J. D., Using intrinsic fluorescence to investigate proteins entrapped in sol-gel derived materials. Applied Spectroscopy 1999, 53(3), 106A-121A... 

Flora, K. K. Dabrowski, M. A. Musson, S. P. Brennan, J. D., The effect of preparation and aging conditions on the internal environment of sol-gel derived materials as probed by 7-azaindole and pyranine fluorescence. Canadian Journal of Chemistry 1999, 77(10), 1617-1625... 

C. Guizard, A. Julbe, A. Larbot and L. Cot, Nanostructures in sol-gel derived materials application to the elaboration of nanofiltration membranes. /. Alloys Compounds, 188 (1992) 8. 

It was reported early that alumina with H2O2 and silyl hydroperoxides, like PhsSiOOH, were active for epoxidation of substituted olefins [193]. Alumina has been studied further, and it is believed that it forms a surface hydroperoxide species by the interaction of H2O2 with surface OH groups of low acidity [194,195]. A sol-gel-derived material has good performance [196], but the catalyst deactivates slowly by an apparent structural change of the surface induced by the condensation of adjacent OH groups [197]. 

Viitala, R. et al.. Surface properties of in vitro bioactive and non-bioactive sol-gel derived materials. Biomaterials. 23, 3073, 2002. 

So far this technique has not been used for the analysis of zeolites, but rather for libraries of sol-gel derived materials [15], or beads obtained from a splil-and-pool protocol [16,17]. It makes use of different micro-XRF analyzers which focus the X-ray beam by a multi capillary system to a spot of around 50 pm diameter, as for instance in the Eagle II pprobe from Roentgenanalytik [15,16], or the ThermoNORAN Omicron system [17], One can expect that analysis of zeolites will be possible with about the same level of precision as reached for the sol-gel derived libraries. However, this is not sufficient to determine Si/AI-ralios reliably, especially since it is typically not known whether the sample is fully crystalline when analyzed by XRD in high-throughput mode. Nevertheless, some initial information on elemental composition can be achieved by micro-XRF. 

Barium hexaaluminate (BHA) has been prepared by a reverse microemulsion-mediated sol-gel method " . Several important preparation parameters were investigated in this work. Nanoparticles with excellent thermal stability could be obtained under optimal preparation conditions when compared to conventional sol-gel derived materials. This stability improvement is believed to occur since crystallization to the desired hexaaluminate phase took place at a relatively low temperature. Furthermore, this material has been tested as a catalyst for methane combustion . The light-olf temperature for 1 vol.% CH4 in air when using pure BHA as catalyst was observed to be 590 °C. This temperature could be lowered to 400 °C by depositing Ce02 on the catalyst. 

B, J. Nablo and M, H. Schoenfisch, In vitro cytotoxicity of nitric oxide-releasing sol-gel derived materials, Biomateriah., 26 [21] 4405-4415 (2005). 

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