Biomedical Engineering Reference
In-Depth Information
higher than of oxygen reduction. At more positive potentials the selectivity of Prussian
blue in H
2
O
2
reduction is even higher.
Stability of the Prussian blue-based hydrogen peroxide transducer is a crucial point
commonly raised as an objection against its practical applications. Indeed Prussian white
(the redox state of Prussian blue at 0.0 V) is thermodynamically unstable on electrode
surfaces. In addition, hydroxyl ions being the products of hydrogen peroxide reduction
in neutral media [113] are able to solubilize the inorganic polycrystal. However, through
continuous efforts in improving the crystalline structure of the deposited Prussian blue
and its additional post-treatment, excellent operational stability, which even exceeds the
stability of the known H
2
O
2
transducers, has been achieved [114, 115].
The kinetics of hydrogen peroxide reduction catalyzed by Prussian blue has been
investigated [12, 113]. In neutral media the reaction scheme of H
2
O
2
reduction has
been found to be the following:
k
cat
.
HO
2
e
⎯→
⎯⎯⎯
2
OH
(3)
22
The electrochemical rate constants for hydrogen peroxide reduction have been found
to be dependent on the amount of Prussian blue deposited, confi rming that H
2
O
2
pen-
etrates the fi lms, and the inner layers of the polycrystal take part in the catalysis. For
4-6 nmol cm
2
of Prussian blue the electrochemical rate constant exceeds 0.01 cm
s
1
[12], which corresponds to the bi-molecular rate constant of k
cat
10
3
L
mol
1
s
1
[114]. The rate constant of hydrogen peroxide reduction by ferrocyanide cat-
alyzed by enzyme peroxidase was 2
3
10
4
L mol
1
s
1
[116]. Thus, the activity of the
natural enzyme peroxidase is of a similar order of magnitude as the catalytic activity
of our Prussian blue-based electrocatalyst. Due to the high catalytic activity and selec-
tivity, which are comparable with biocatalysis, we were able to denote the specially
deposited Prussian blue as an
artifi cial peroxidase
[114, 117].
It is important to compare the catalytic properties of Prussian blue with known
hydrogen peroxide transducers. Table 13.2 presents the catalytic parameters, which are
of major importance for analytical chemistry: selectivity and catalytic activity. It is seen
that platinum, which is still considered as the universal transducer, possesses rather low
catalytic activity in both H
2
O
2
oxidation and reduction. Moreover, it is nearly impos-
sible to measure hydrogen peroxide by its reduction on platinum, because the rate of
oxygen reduction is ten times higher. The situation is drastically improved in case of
enzyme peroxidase electrodes. However, the absolute records of both catalytic activity
TABLE 13.2
Electrocatalytic properties of the low-potential H
2
O
2
transducers
Transducer
Selectivity:
j
H
2
O
2
/
j
H
2
O
Electrochemical constant, cm/s
10
6
Pt
0.1
4
10
3
peroxidase electrodes
30-40
1
10
2
Prussian blue
400-600
1
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