Gel entrapment

The most important physical methods are physical and ionic adsorption on a water-insoluble matrix, inclusion and gel entrapment, and microencapsulation with a liquid or a solid membrane. The most important chemical methods include covalent attachment to a water-insoluble matrix, cross-hnking with the use of a multifunctional, low-molecular weight reagent, and co-cross-linking with other neutral substances, for example proteins. 

Sol-gel-entrapped catalysts provide a generic method for the encapsulation of a wide variety of catalysts. Comprehensive reviews are available [59-61]. The essence ofthe concept is captured in Figure 5.6. 

The immobilization procedure performed in two stages allows one to exclude the detrimental effect of acid on the entrapped proteins [44,71,86]. It was demonstrated [87] by the example of a set of oxidases that their activity was retained if the entrapment was carried out at a pH as close to their isoelectric point (pi) as possible. Because the pi value of most enzymes is in the neutral region, the two-stage procedure favors the retention of their functionality. Therefore, the pH shift to the optimal region provides a means of extending the sol-gel entrapment to a wide range of enzymes [45,71]. 

Altstein, M., Segev, G., Aharonson, N., Ben-Aziz, O., Turniansky, A. and Avnir, D. (1998) Sol-gel-entrapped cholinesterases A microtiter plate method for monitoring anticholinesterase compounds. Journal of Agricultural and Food Chemistry, 46, 3318-3324. 

Zheng, L.L., Flora, K. and Brennan, J.D. (1998) Improving the performance of a sol-gel-entrapped metal-binding protein by maximizing protein thermal stability before entrapment. Chemistry of Materials, 10, 3974-3983. 

Butler T.M., MacCraith B.D., McDonagh C.M., Development of an extended range fibre optic pH sensor using evanescent wave absorption of sol-gel entrapped pH indicators, SPIEProc. 2508, 168-178, (1995). 

Andreou V., Clonis Y., A portable fiber-optic pesticide biosensor based on immobilized cholinesterase and sol-gel entrapped bromocresol purple for in-field use, Biosens. Bioelectr. 2002 17 61-69. 

Several enzymes have been immobilized in sol-gel matrices effectively and employed in diverse applications. Urease, catalase, and adenylic acid deaminase were first encapsulated in sol-gel matrices [72], The encapsulated urease and catalase retained partial activity but adenylic acid deaminase completely lost its activity. After three decades considerable attention has been paid again towards the bioencapsulation using sol-gel glasses. Braun et al. [73] successfully encapsulated alkaline phosphatase in silica gel, which retained its activity up to 2 months (30% of initial) with improved thermal stability. Further Shtelzer et al. [58] sequestered trypsin within a binary sol-gel-derived composite using TEOS and PEG. Ellerby et al. [74] entrapped other proteins such as cytochrome c and Mb in TEOS sol-gel. Later several proteins such as Mb [8], hemoglobin (Hb) [56], cyt c [55, 75], bacteriorhodopsin (bR) [76], lactate oxidase [77], alkaline phosphatase (AP) [78], GOD [51], HRP [79], urease [80], superoxide dismutase [8], tyrosinase [81], acetylcholinesterase [82], etc. have been immobilized into different sol-gel matrices. Hitherto some reports have described the various aspects of sol-gel entrapped biomolecules such as conformation [50, 60], dynamics [12, 83], accessibility [46], reaction kinetics [50, 54], activity [7, 84], and stability [1, 80],... 

The sol-gel-entrapped microbial cells have shown excellent tolerance to different alcohols [99], The immobilized E. coli cells followed the Michaelis-Menten equation when quantified with the (3-glucosidase activity via the hydrolysis of 4-nitrophenyl-(3-D-galactopyranosdie [142], The sol-gel matrices doped with gelatin prevented the cell lysis, which usually occurs during the initial gelation process [143], Microorganisms are now widely used in the biosorption of different pollutants and toxicants. Bacillus sphaericus JG-A12 isolated from uranium mining water has been entrapped in aqueous silica nanosol for the accumulation of copper and uranium [144], Premkumar et al. [145] immobilized recombinant luminous bacteria into TEOS sol-gel to study the effect of sol-gel conditions on the cell response (luminescence). The entrapped and free cells showed almost the same intensity of luminescence (little lower), but the entrapped cells were more stable than the free cells (4 weeks at 4°C). This kind of stable cell could be employed in biosensors in the near future. 

K. Flora and J.D. Brennan, Fluorometric detection of Ca2+ based on an induced change in the conformation of sol-gel entrapped parvalbumin. Anal. Chem. 70, 4505-4513 (1998). 

M. Altstein, G. Segev, N. Aharonson, O. Ben-Aziz, A. Turniansky, and D. Avnir, Sol-gel-entrapped cholinesterases a microtiter plate method for monitoring anti-cholinesterase compounds. J. Agric. Food Chem. 46, 3318-3324 (1998). 

Table 42.4 Enantioselective hydrogenation of itaconic acid using sol-gel-entrapped Rh complexes [51].

The R D director of SiliCyle, Dr Francois Beland, has been pivotal in guiding the development of the SiliaCat series of sol-gel entrapped catalysts. And I would like to recall here also his and Hugo St-Laurent s visit to Palermo in October 2007. 

In 1995, commenting on the physical nature of the sol-gel entrapment of molecules in porous oxides, the inventor of sol-gel doped materials (first reported in the Journal of Physical Chemistry ten years before)6 emphasized how it was really remarkable to see how many applications... 

The physical and chemical properties of the entrapped dopants are generally retained. Yet, the efficient isolation of one molecule from another and the active role played by the sol gel cage, for instance in dictating accessibility, gives place to a vast new chemistry and physics of sol gel entrapped molecules which largely encompasses and goes beyond traditional solution chemistry. 

Instead of applying synthetic methods to alter chromophore reactivity, this new way of controlling chemical reactivity involves choosing an appropriate solid micellar system (from the available multitude) and exploiting it to manipulate the chemistry of the entrapped compound. The sol-gel matrix and the micellar solubilization, in fact, have a synergetic effect. Their combination produces effects stronger and more tuneable than in solution, so that a careful selection of sol-gel entrapped surfactants allows one to induce enormous changes in the dopant properties. 

Along with these recent technologies, sol-gel entrapped catalysts made of organically modified silicates (ORMOSIL) doped with one or... 

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