Biosensors based on direct electron transfer of protein - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 546

reaction of cat and for biomolecular attachment [160-163], Wang [164] accomplished
the direct electrochemistry of cat at a gold electrode modifi ed with single-wall carbon
nanotubes (SWNTs). A pair of well-defi ned redox peaks was obtained for cat with the
reduction peak potential at 0.414 V and a peak potential separation of 32 mV at pH

5.9. Both refl ectance FT-IR spectra and the dependence of the reduction peak current
on the scan rate revealed that cat adsorbed onto the SWNTs surfaces. The redox wave
corresponds to the Fe(III)/Fe(II) redox center of the heme group of the cat adsorb-
ate. Compared to other types of carbonaceous electrode materials (e.g. graphite and
carbon soot), the electron-transfer rate of cat redox reaction was greatly enhanced at
the SWNTs-modifi ed electrode. The catalytic activity of cat adsorbate at the SWNTs
appeared to be retained, as the addition of H 2 O 2 produced a characteristic catalytic
redox wave. The facile electron-transfer reaction of cat could be attributed to the
unique properties of SWNTs (e.g. the excellent electrical conductivity of SWNTs, the
enhanced surface area arising from the high aspect ratio of the nanotubes, and the ame-
nability of SWNTs for the attachment of biomolecules).

17.2.3.3 Direct electron transfer of GOD
Glucose oxidase (GOD) is a typical fl avin enzyme with fl avin adenine dinucleotide
(FAD) as redox prosthetic group. Its biological function is to catalyze glucose to form
gluconolaction, while the enzyme itself is turned from GOD(FAD) to GOD(FADH 2 ).
The GOD molecule is a structurally rigid glycoprotein with a molecular weight of
152 000-186 000 Da, and consists of two identical polypeptide chains, each contain-
ing a FAD redox center. Since the FAD moiety is deeply embedded within a protec-
tive protein shell, well-defi ned direct electrochemical behavior of GOD is rather
diffi cult. In the literature, only a few examples of quasi-reversible voltammograms for
direct electron transfer between the GOD-active site and the electrode surface were
reported. Ianniello [165] reported the direct electron transfer of adsorbed GOD at a
graphite electrode and a cyanuric chloride-modifi ed graphite electrode using differen-
tial pulse voltammetry. The direct electrochemistry of GOD, immobilized at a self-
assembled monolayer of 3,3-dithiobis-sulfosuccinimidyl propionate, was obtained by
Jiang [166]. Direct electron transfer of GOD realized by CNTs was also studied by var-
ious groups in recent years [167-169]. Wang [170] found that the direct electrochemis-
try of GOD could be conducted at SWNT-modifi ed Au electrodes. A reversible redox
wave could be observed with the cathodic peak potential at

0.465 V and the separation
of the peak potentials of 23 mV at pH 7.0. The peak potential was pH dependent and
shifted to the cathodic direction by 48 mV per unit of pH. The cathodic peak current
was found to be proportional to the scan rate, suggesting that the redox wave was
given rise by the GOD adsorbate at the SWNTs. The specifi c enzyme activity of the
GOD adsorbates at the SWNTs was found to be retained, suggesting that SWNT-
modifi ed electrodes covered with redox-active enzymes could retain the enzyme
activities and provide an attractive route for the development of biosensors and
nanobiosensors.