Biomedical Engineering Reference
In-Depth Information
the initial growing solution and the deposition potentials [11, 12]. In this
way, pH stability of PB i lm seems to be dependent also on the dif erent
modes of deposition of the PB layer. For this reason, the solution pH is
a critical point not only during deposition, but also during its applica-
tions in real samples. h e reason for this behavior has been ascribed to the
strong interaction between the Fe
3+
from PB and hydroxide ions (OH
−
)
which form Fe(OH)
3
at pH higher than 6.4, thus leading to the solubili-
zation of the PB i lm [11, 15]. For the second factor, the applied poten-
tial should not be lower than 0.2 V
vs
SCE, where ferricyanide ions are
intensively reduced (
vide infra
). Dif erent strategies have been described
in order to obtain stable PB i lms, such as galvanostatic or cyclic voltam-
metric methods, chemical deposition or PB microparticle synthesis. In
this context, cyclic voltammetric methods and a heat-treatment step are
commonly used to activate and stabilize the PB i lm, respectively [11].
12.3
Prussian Blue: Hydrogen Peroxide
Electrocatalysis
Although H
2
O
2
is electrochemically ambivalent in that it can be either
oxidized to molecular oxygen or reduced to hydroxide ions, depending
on the applied potential used [21], the former (anodic) mode of electro-
activity has been by far the more common approach for the detection of
enzyme-generated H
2
O
2
in i rst-generation biosensors [22]. However, an
intrinsic problem associated with the relatively high applied anodic poten-
tials needed to oxidize H
2
O
2
ei ciently on most electrode materials (0.4
to 0.7 V
vs
SCE [23, 24]) is that many substances, including ascorbic and
uric acids, in the biosensor target medium (blood, fat, neural tissues,
etc
.)
also oxidize at these potentials, thus interfering with the biosensor signal.
In recent decades, one strategy being explored to limit this interference is
the use of PB and its analogues to electrocatalytically reduce H
2
O
2
at mild
applied potentials (~0 V
vs
SCE).
In 1984, Itaya
et al.
[7] showed that the reduced form of PB (Prussian
White, PW; see
Figure 12.4
) displayed catalytic activity for the reduction
of O
2
and H
2
O
2
. h e zeolite structure of PB, with its small channel diam-
eter (see Section 12.2.2), allows the dif usion of low molecular weight mol-
ecules (such as O
2
and H
2
O
2
) through the crystal structure [11]. Nowadays
its electrochemical behavior is well understood with cyclic voltammo-
grams (CVs) of PB-modii ed electrodes showing two quasi-reversible
redox couples [11, 25] (
Figure 12.4
). h e i rst peak pair corresponds to
the interconversion of PW and PB forms, and the second pair from PB to
