Biosensors based on immobilization of biomolecules in sol-gel matrices - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 515

The mediators such as ferrocene and its derivatives, osmium complexes, quinone salts
and dyes, and ruthenium hexamine have been employed in biosensors. Some studies
have added a mediator directly to the reaction mixture [109]. Sensors in which the
mediator is coimmobilized with enzyme on electrode do not require the addition of
the mediator to the test solution thereby eliminating the many practical limitations.
The immobilized mediator catalyzes the redox reactions of biomolecules and shuttles
the electrons more effi ciently between enzyme and electrode to greatly enhance the
sensing performance. Recently a sensitive mediated biosensor has been reported, in
which glucose oxidase was immobilized in chitosan/methyltrimethoxysilane (MTOS)
sol-gel and casted on Prussian blue electrodeposited GCE [110]. The formed sol-gel/
chitosan fi lm was covered with a Nafi on coating to eliminate the effect of interfering
compounds such as ascorbic acid, uric acid, etc. The linear response of the obtained
glucose biosensor was 50 3). Tan

et al. [111] reported a cholesterol amperometric biosensor based on hybrid sol-gel
fi lm. MTOS sol-gel was doped with MWNTs, chitosan, and cholineoxidase and over-
laid on Prussian blue electrodeposited carbon glassy electrode. The MWNTs act as
both nanometer conducting wire and catalyst. Recently Ju and coworkers [69] reported
a highly conductive, biocompatible and mediator-entrapped ormosil composite matrix.
The composite matrix was porous, stable, and crack free (Fig. 16.6). The electrodes
modifi ed with ormosil composite showed well-defi ned redox peaks and the presence
of MWNTs decreased the background current and increased the peak currents (Fig.
16.7a). In the conjugated form of ferrocene monocarboxylic acid and bovine serum
albumin (FMC-BSA), the mediator was well retained inside the matrix. In batch analy-
sis the K M app and linear detection range were 6.6 mM and 0.06-18 mM, respectively
(Fig. 16.7b). The FIA-based glucose biosensor showed a wide linear range from 0.05
to 20 mM. In another study HRP was doped in ormosil composite matrix for prepara-
tion of an H 2 O 2 biosensor [112]. This H 2 O 2 biosensor exhibited a linear range of 0.02
to 4.0 mM with a detection limit of 5.0

µ M to 20 mM with a detection limit of 8 µ M (S/N

3) and a K M app value of 2.0 mM. µ M (S/N

(a) (b) (c)

FIGURE 16.6 Scanning electron microphotos of (a) ormosil, (b) MWNTs/ormosil composite, and (c)
FMC-BSA conjugate/ormosil fi lms (adapted from [69]).