Biomedical Engineering Reference
In-Depth Information
surface area, optical transparency, good biocompatibility, and relatively good conduc-
tivity. Various TiO
2
fi lms were also used to immobilize proteins or enzymes on the
electrode surface for either mechanistic study of the proteins or fabricating electro-
chemical biosensors. For example, Durrant and coworkers immobilized a range of
proteins into nanoporous TiO
2
fi lm-modifi ed electrodes and successfully used this
strategy to develop electrochemical and optical biosensors [141-142]. Luo and cow-
orkers used nanocrystalline TiO
2
fi lms on electrodes to entrap heme proteins such as
cyt
c
, Mb, and Hb, and observed the direct electrochemistry of these proteins [143].
Accumulation and electroactivity of cyt
c
in mesoporous layer-by-layer fi lms of TiO
2
and phytate at ITO electrodes was also studied [144]. Titania sol-gel matrix fi lms were
used to immobilize HRP, and with the aid of mediators, HRP-TiO
2
fi lm electrodes
were used to detect H
2
O
2
by amperometry [66, 145]. Hu's group [146] also incorpo-
rated HRP in TiO
2
nanoparticle fi lms modifi ed on electrodes to achieve the direct elec-
tron transfer of HRP. HRP-TiO
2
fi lm electrodes were fabricated by casting the mixture
of HRP solution and aqueous titania nanoparticle dispersion onto PG electrodes and
letting the solvent evaporate. The HRP incorporated in TiO
2
fi lms exhibited a pair of
well-defi ned and quasi-reversible CV peaks at about
0.35 V in pH 7.0 buffer, which
refl ected that the rate of electron exchange between the enzyme and PG electrodes was
greatly enhanced in the TiO
2
nanoparticle fi lm microenvironment. The HRP-TiO
2
fi lm
electrodes were quite stable and amenable to long-time voltammetric experiments. The
UV-Vis spectroscopy showed that the position and shape of the Soret absorption band
of HRP in TiO
2
fi lms kept nearly unchanged and were different from those of hemin
or hemin-TiO
2
fi lms, suggesting that HRP retains its native-like tertiary structure in
TiO
2
fi lms. The electrocatalytic activity of HRP embedded in TiO
2
fi lms toward O
2
and H
2
O
2
was retained.
17.2.3.2 Direct electron transfer of catalase
Catalase (cat) is also a heme enzyme, which is present in almost all aerobic organisms
[147]. Cat has a molecular weight of approximately 240 000, and is composed of four
identical subunits, each containing a single heme prosthetic group. The heme group
consists of a protoporphyrin ring and a central Fe atom, where iron is usually in the
ferric oxidation state as its stable resting state [148]. As a catalyst, cat functions either
in the catabolism of H
2
O
2
or in the peroxidatic oxidation of small molecule substrates
by H
2
O
2
[149-150]. Under normal physiological conditions, cat controls the H
2
O
2
concentration so that it does not reach toxic levels that could bring about oxidative
damage in cells. The mechanism of disproportionation of H
2
O
2
catalyzed by cat can
be expressed as [149-150]:
H
2
O
2
CatFe(III)
→
Compound I
H
2
O
(2)
H
2
O
2
Compound I
→
CatFe(III)
O
2
H
2
O
(3)
where CatFe(III) is the resting state of catalase, Compound I is a two-equivalent
oxidized form of CatFe(III) containing an oxyferryl heme (Fe
IV
O) and a porphyrin
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