Biosensors based on immobilization of biomolecules in sol-gel matrices - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 519

sometimes steric hindrance caused by the neighboring antibody molecules [129].
The Abs encapsulated in sol-gel-derived glasses can interact with target molecules,
with a same degree of specifi city as in solution, and the signal can be detected using an
appropriate sensing scheme [17]. Wang et al. [130] encapsulated fi rst antifl uorescein
antibodies in TMOS sol-gel. The entrapped antibodies retained activity signifi cantly
and could bind with fl uorescein molecules, which led to the decrease in the fl uores-
cence. The entrapped antibodies stored at 4 C in water was stable up to 4 weeks. In

another report [131] antifl uorescin antibody was entrapped in a sandwich confi gura-
tion, in aerosol-generated sol-gel-derived thin fi lms (0.62 0.05 µ m). The fl uorescent

hapten 5-(and 6-)carboxy-4 5 dimethylfl uorescein (Me 2 F) was employed to determine

the accessibility and viability of the entrapped antifl uorescein antibody. The antibody
was effi cient in recognition and binding of the Me 2 F up to 13 weeks when stored in
0.1 M pH 8 PBS at 4 C [83]. The addition of PEG into the sol-gel matrix improved

the binding activity of the encapsulated antibody. The binding effi ciency of sol-gel-
entrapped antigentamicin antibody was improved from 42% to 95% in the presence
of PEG and prevented the shrinking of the sol-gel glass matrix and denaturation of
the immobilized antibody. This improved binding activity was probably due to the
high diffusion rate of the gentamicin [132]. Hence while designing the matrices for
immobilization of biomolecules, it is imperative to choose dimensions of the support
matrices so as to minimize diffusional resistance and make the entire population of
encapsulated biomolecules participate in the reaction.
Like other immunoassays sol-gel-based immunosensors can also entrap either anti-
gen or antibody. Sol-gel-encapsulated anti-TNT antibody has been used as a detector
for TNT [133]. The entrapped antibody is more stable than that immobilized through
surface attachment. It is able to differentiate TNT and analog trinitrobenzene (TNB),
and can be employed in different immunoassay formats such as competitive and dis-
placement assays. The aged gels exhibit signifi cantly faster response than xerogels
due to the larger pore diameter. Tofi ño et al. [134] developed a fl ow-through fl uoroim-
munosensor for the determination of isoproturon. The encapsulated antibodies could
completely retain their activity and exhibited good performance in competitive immu-
noassay. The antigentamicin Mab entrapped in mesoporous TMOS so-gel monolith has
been employed for the development of fl ow-injection fl uorescence immunoassay for
the quantitative analysis of the gentamicn [132]. The entrapped antigentamicin MAbs
were able to recognize antibiotic up to 200 ng ml 1 in the serum with a working range
of 250-5000 ng ml 1 . The immunoreactor column was stable up to one month at room
temperature and 3 months at 4

C. Anticortisol antibody has been encapsulated in opti-
cally transparent sol-gel silica matrices for cortisol detection of 1-100

g dl 1 with
competitive immunoassays [17]. The antibody-doped thin fi lm showed good accessi-
bility for antigen and signifi cant reduction in assay time. Wang et al. [64] reported a sol-
gel-derived thick fi lm amperometric immunosensor by encapsulation of antigen (RIgG)
in TEOS sol-gels. This thick fi lm immunosensor showed very fast response (20 s) due
to the effective contact between electrode surface and reaction centers though the
doped graphite powder, retained antigenic properties of the sol-gel-entrapped antigen,
and effective binding effi ciency between antigen and anti-IgG-HRP conjugate.

µ