Biosensors based on immobilization of biomolecules in sol-gel matrices - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 518

to 3 months. The renewable or screen-printed sensors were highly convenient for the
fi led applications and low cost. In order to circumvent limitations of microbial adhe-
sion and growth on the surface, recently nitric oxide (NO) releasing sol-gel particles
have been reported [122]. Nitric oxide is a potential antimicrobial agent [123]; if it
releases slowly from the matrix adhesion the growth of microbes can be controlled.
NO-releasing glucose biosensors prepared by doping diazeniumdiolate-modifi ed sol-
gel particles in a polyurethane membrane exhibited high sensitivity (mention details),
reproducibility, and fast response up to 18 days [122].
16.3.2 Photoactive proteins-based biosensors
Photoactive proteins bR and phycoerythrin (PE) retain their optical activity when encap-
sulated within sol-gel glasses, with enhanced stability against photodegradation [124].
Bacteriorhodopsin is another naturally occurring transmembrane protein that converts
light energy into metabolic energy. The absorption of light initiates a photocycle in
the bR molecule, which accompanies the transportation of protons. The characteristics
and effects of this photocycle make it a potentially useful material for development as
an optically sensitive fi lm that is self-developing and erasable. The encapsulation of
light sensitive proteins in transparent matrices is of interest because of the potential
application to photovoltaic devices, photoimaging, molecular computing, and chemi-
cal sensing. Wu et al. [84] and Weetall [125, 126] entrapped bR in wet sol-gel glasses.
The D96N mutant bR retained its activity in a dried sol-gel glass. Encapsulation of
photochromatic proteins in transparent fi lms can be employed in optoelectronic
sol-gel devices, which seems to be technically feasible. Phycobiliproteins are biomolecu-
lar assemblies located on the outer thylakoid membranes of marine algae. Although
PE was stable under ambient light it denatured at intense light illumination. The
sol-gel-entrapped PE retained its conformation, exhibited improved photodegradation
capacity and was more stable than in solution [124]. Such stabilized photoactive pro-
teins could be used in potential applications in biomolecular sensing, imaging, and
information processing and storage. The bioluminescence from aequorin, a biolumi-
nescent protein found in the jellyfi sh Aequorea sp., is specifi cally triggered by the
presence of calcium ions. Based on this triggering effect of calcium ion, the concentra-
tion of calcium ion has been determined using aequorin immobilized in a porous sol-
gel matrix [127].
16.3.3 Immunosensors
Compared with enzymes fewer reports are available on immobilization of antibody
(Ab) in sol-gels and their applications in immunosensing. Immobilization of Abs on
a solid support was fi rst reported in 1967 [128] and the technology has widespread
application in affi nity chromatography and other areas. However, the major problem
associated with covalent immobilization of antibody on solid surface is partial loss of
biological activity due to the random orientation of the asymmetric macromolecules,