Biosensors based on direct electron transfer of protein - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 557

such as Mb and Hb has been linked to a number of important physiological processes.
NO is one of the products of the disproportionation reaction of NO 2 when it occurs in
acidic solution.
Cyclic voltammograms of an Mb-agarose/EPG in pH 7.0 PBS show a pair of
reversible redox peaks at approximately

0.30 V. When NO was added to the solu-

tion, an irreversible cathodic peak at 0.84 V was observed over a potential range of

1.2 V. The peak could be assigned to reduction of the NO-myoglobin adduct
(MbFe II ß NO) [233]. At the same time, the reduction peak of MbFe III widened, and
two cathodic peaks were observed at lower scan rates, suggesting NO coordinated with
MbFe II . However, the oxidation peak of MbFe II kept its original shape, indicating that
NO interacts with MbFe II more strongly than MbFe III . Both the peak potential and cur-
rent were dependent on pH. The peak potential shifted negatively with increasing pH,
suggesting a proton-coupled reaction. The peak potential was linearly dependent on
pH in the range of 3.0-10.0 with the slope value of 29 mV pH 1 , indicating that one
proton transferred in the redox reaction. In addition, large peak currents were obtained
in neutral or weakly acidic solution.
Biocompatible nanosized polyamidoamine (PAMAM) dendrimer fi lms provided a
suitable microenvironment for heme proteins to transfer electron directly with under-
lying pyrolytic graphite electrodes. The Mb-PAMAM fi lm can catalytically reduced
oxygen, hydrogen peroxide, and nitrite, indicating that the potential applicability of the
fi lm can be used to fabricate a new type of biosensor or bioreactor based on the direct
electron transfer of Mb [234].
Mb-CMC fi lm was made by casting solution of myoglobin and carboxymethyl
cellulose (CMC) on pyrolytic graphite electrode. Trichloroacetic acid (TCA), nitrite,
oxygen, and hydrogen peroxide can be catalytically reduced at the Mb-CMC fi lm elec-
trode. When TCA was added to a pH 3.0 buffer, an increase in the MbFe III reduction
peak at about

0.2 to

0.2 V was observed, accompanied by a decrease of the MbFe II oxida-
tion peak. The reduction peak current increased as the TCA concentration increased.
The catalytic reduction peak current showed a linear relationship with TCA concentra-
tion in the range of 7.5 10 3 M. Compared with the direct reduction

of TCA on Mb-free CMC fi lm at the potential more negative than

10 4

2

1.1 V, Mb-CMC
fi lms lowered the reduction overpotential of TCA by at least 0.9 V. The MbFe III in
CMC fi lms was electrochemically reduced at electrodes forming MbFe II . The elec-
trode reaction product MbFe II was then chemically oxidized by TCA and returned
to MbFe III . This formed a catalytic cycle, which presumably resulted in the reductive
dechlorination of TCA.
Nitrite can also be catalytically reduced by the Mb-CMC fi lm electrode. When
an Mb-CMC fi lm electrode was placed in a pH 5.5 buffer containing NO 2 , a new
reduction peak appeared at about 0.8 V while the original Mb Fe III /Fe II peak pair at

0.25 V was intact. This new peak increased with the concentration of NO 2 and
the catalytic reduction peak of nitrite increased linearly with nitrite concentration in
the range of 0.6-8 mM with a detection limit of 0.32 mM. Oxygen and hydrogen per-
oxide are also often detected by biosensors based on the direct electrochemistry of Mb
in the Mb-CMC fi lm electrode.

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