Biosensors based on immobilization of biomolecules in sol-gel matrices - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 510

glucose oxidase and horseradish peroxidase) [64, 65]. Brennan et al. [66] reported the
addition of sugar (sorbitol) and amino acids ( N -methylglycine) increased the thermal
stability and improved the -chymostriosin and RNAse T1 activity, because the added

osmolytes (sorbitol, N -methylglycine) altered the hydration effects, protein-silica
interaction, and pore morphology.

α

16.2.6 Improvement of biocompatibility and conductivity of sol-gels
During the gel formation, alcohol liberation will take place. The retained water content
is also low in aged sol-gel matrices. In such a condition most of the proteins are not
stable or lose their activity. Hence there is a great need to stabilize the protein mole-
cules by the addition of protein stabilizing agents or employment of biocompatible sol-
gel monomers. As discussed above several new methods have been introduced for the
reduction of the negative affect of alcohol. Some of the studies reported that the doping
of BSA, cellulose, and chitosan into the sol-gel matrices can improve the hydrophilic
nature of the matrices, so that the enzyme is stabile for quite longer time [67-69]. In
view of the fact that most of the sol-gel matrices are not conductive materials, the dop-
ing of highly conductive nanoparticles such as carbon nanotubes, palladium, graph-
ite, etc. into the sol-gel matrices/ormosil greatly enhances the conductive and catalytic
properties [69-71].

16.3 APPLICATIONS OF SOL-GEL ENTRAPPED
BIOACTIVE MOLECULES

16.3.1 Enzyme-based biosensors
Several enzymes have been immobilized in sol-gel matrices effectively and employed
in diverse applications. Urease, catalase, and adenylic acid deaminase were fi rst encap-
sulated 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 sta-
bility. 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], hemo-
globin (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].