Biosensors based on immobilization of biomolecules in sol-gel matrices - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 512

paste or carbon-epoxy materials. Sampath and Lev [93, 94] reported a renewable GOD-
entrapped glucose biosensor by the addition of hydrophilic PEG into the hydrophobic
ormosil (present on the electrode surface). The same group [95] reported another sol-
gel-derived ferrocenyl modifi ed silicate-graphite composite electrode employed in glu-
cose biosensor design. Li et al. [96] reported MTMOS-derived carbon composite-based
renewable glucose biosensor using vinyl ferrocene as a mediator. To circumvent crack-
ing and swelling Lev et al. [97] reported some composite ceramic-carbon materials
along with surfactants, but these materials were less biocompatible and the surfactants
were detrimental to the enzyme and needed high amounts of enzyme.

16.3.1.3 Electrode surface coatings
Mainly, three approaches have been used to immobilize the enzyme on transducer or
electrode surface, single layer, bilayer, and sandwich confi gurations [69, 98]. In some
studies enzymes are covalently linked with sol-gel thin fi lms [99]. Sol-gel thin fi lms
are highly convenient for fast, large, and homogeneous electron transfer [17]. With
an increase in gel thickness the signal decays and diffusion of analytes to biomole-
cule active site becomes diffi cult; eventually these factors lead to poor response. By
employing thin fi lms various biosensors such as optical and electrochemical biosen-
sors have been reported.

16.3.1.4 Optical biosensors
Brun et al. [100] reported an optical biosensor based on the xerogel disk doped with
GOD, peroxidase, and dye for the detection of glucose. Tatsu et al. [101] prepared
tetraethyl orthosilicate-derived silica gel doped with GOD and used it as a glucose
recognition element in a fl ow-injection analytical system. Gulcev [102] reported an
optical biosensor by coimmobilizing sensitive fl uorophore and enzyme into the silica
sol-gel matrix. The changes in the enzyme reaction mixtures affect the fl uorophore
response. The coimmobilization of dextran conjugates of fl uorescein or carboxy-
seminaphtharhodafl uor-1 (SNARF-1) and enzyme showed less leaching and quantifi able
pH response of fl uorescence in reaction mixtures of lipase and urease. Another pH sen-
sitive fl uorescent biosensor was reported for the detection of acetylcholine and praoxon
[103], in which fl uorescene isothiocyanate (FITC)-Dextran conjugate and acethylcho-
linesterase (AChE) were coimmobilized in TMOS sol-gel. This biosensor showed a
linear range from 0.5 to 20 mM for achetylcholine and 30% inhibition of AChE activity
at 152 mg ml 1 of paraoxon. Hydroxyethyl carboxymethyl cellulose and TEOS hybrid
polymer could improve the stability of entrapped GOD (up to 3 years) [68]. The aging
of sol-gel matrix at 4 C resulted in reduction of shrinkage and good porosity. When

integrating this encapsulated enzyme with optical biosensor, it showed good linearity
for glucose quantifi cation between 50 and 200

M in urine. A glass capillary-based
optical biosensor for retinol was reported in which the sensing element was retinol-
binding protein (RBP) [104]. The binding events of retinoic-acid-horseradish peroxi-
dase (conjugate) and retinol to RBP were measured using a simple photomultiplier tube,

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