Other Sol-gel Processes

To prepare hexaaluminates for ceramic applications a slightly different sol-gel process was proposed by Debsikbar.19 Ba-hexaaluminates were prepared via hydrolysis of Al di(isopropoxide) acetoacetic ester chelate and anhydrous Ba acetate obtained by reaction between BaC03 and glacial acetic acid. The substitution of Al(i-OC3H7)3 with the alkoxy ester was intended to control the chemical polymerization during gel formation. The reaction was performed in 1-butanol. The formation of the gel slowly occurred at room temperature in about 10 h. To obtain the final phase the gel precursor was dried at 70 °C for about 2 weeks, ground and calcined at 1200°C for 2 h. However no data on the morphology of the final materials were reported by the author. 

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Similarly to the above-mentioned entrapment of proteins by biomimetic routes, the sol-gel procedure is a useful method for the encapsulation of enzymes and other biological material due to the mild conditions required for the preparation of the silica networks [54,55]. The confinement of the enzyme in the pores of the silica matrix preserves its catalytic activity, since it prevents irreversible structural deformations in the biomolecule. The silica matrix may exert a protective effect against enzyme denaturation even under harsh conditions, as recently reported by Frenkel-Mullerad and Avnir [56] for physically trapped phosphatase enzymes within silica matrices (Figure 1.3). A wide number of organoalkoxy- and alkoxy-silanes have been employed for this purpose, as extensively reviewed by Gill and Ballesteros [57], and the resulting materials have been applied in the construction of optical and electrochemical biosensor devices. Optimization of the sol-gel process is required to prevent denaturation of encapsulated enzymes. Alcohol released during the... 

Interest in sol-gel processing was awakened by the work of Avnir et al. in 1990 who performed successful experiments with such enzymes as [1-glucosidasc, alkaline phosphatase, chitinase and aspartase [68]. This gave impetus to their own systematic study of the entrapment of biopolymers in a silica matrix as well as those of other teams [69-79]. The results have been summarized and discussed in numerous review articles (see, e.g., Refs. [41—43,45—49,51,80—85]). 

The hybrids generated by the sol-gel process combine the flexibility and mechanical strength of the organic constituent with the hardness, stiffness and transparency of the inorganic silica network. Hardness is often further conferred by the employment of other inorganic oxide particles (such as alumina or titania). This first started in the 1980s when... 

Ichinose, H. Senzu, and T. Kunitake, A surface sol-gel process of Ti02 and other metal oxide films with molecular precision, Chem. Mater. 9(6), 1296-1298 (1997). 

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