Biosensors based on immobilization of biomolecules in sol-gel matrices - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 511

16.3.1.1 Biosensor applications of enzymes
The inherent features of the sol-gel matrices, such as optical transparency, high surface
area, tunable porosity, chemical and photochemical inertness, and the ability to obtain
any desired shape (monoliths, thin fi lms, powders, fi bers), enable the design of bio-
sensors [27, 85, 86]. The slow diffusion of the electroactive species inside the sol-gel
matrix causes long response times. To dissolve this limitation sol-gel matrices are doped
with metal particles, such as graphite, carbon nanotubes, palladium, etc. in the construc-
tion of electrochemical biosensors [70, 71, 87]. Very recently carbon nanotubes were
decorated with platinum (CNT-Pt) by the chemical reduction method [88]. This new
kind of CNT-Pt was intercalated with graphite to prepare a modifi ed electrode covered
with cholineoxidase-doped TEOS fi lm. The CNT-Pt-doped electrodes showed higher
catalytic activity than the CNT electrode for the reduction of hydrogen peroxide. The
linear range for cholesterol measurement was 4.0 10 6 to 1.0 10 4 M with a detec-

10 6 M. Mediators such as ferrocene and its derivatives can also be
entrapped in the sol-gel/ormosil matrices in two ways, either by direct conjugation with
sol-gel precursors [27] or conjugation with active biomolecule and then entrapment [69,
89]. In such a case the mediator is in close proximity to the active protein and trans-
ducer or electrode surface. Further coimmobilization of mediator and redox enzyme is
a highly convenient way for the development of reagentless biosensors. Sol-gel-derived
electrochemical biosensors mainly rely on two basic confi gurations: conductive ceramic
composites and electrode surface coatings.

tion limit of 1.4

16.3.1.2 Carbon-ceramic composite electrodes (CCEs)
Since the pioneering work of Lev and coworkers [90] sol-gel derived composite car-
bon electrodes have been widely used to develop various kinds of amperometric bio-
sensors. Carbon-ceramic composite electrodes (CCEs) are comprised of a dispersion
of carbon powder in organically modifi ed or non-modifi ed silica matrixes. Usually the
electrodes are prepared by mixing an appropriate amount of carbon black or graphite
powder with the sol-gel precursors. A porous, brittle composite matrix can be formed
after gelation and drying. The composite electrodes benefi t from the mechanical prop-
erties of the silicate backbone, from the electron percolation conductivity through the
interconnected carbon powder and from the ability to manipulate the physicochemical
characteristics of the matrix easily by incorporation of suitable monomer precursors or
sol-gel dopants. It is also possible to cast silica-carbon matrixes in virtually any desired
geometrical confi guration, including fl at layers spread on insulating or conductive
matrixes, monolithic disks or rods and even in the form of a miniature. The choice of
carbon powder signifi cantly affects the properties of the CCEs [91]. Wang and cowork-
ers reported that the pore size distribution could be increased by increasing the H 2 O:
Si ratio used for the preparation of the CCEs [92]. For the long-term use of enzyme
biosensors one of the hurdles is fouling and contamination of the surface during opera-
tion; however, an advantage is the use of polishable biosensors/renewable biosensors.
The renewable amperometric biosensors are commonly comprised of either carbon