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4 Characteristic Properties of Redox Enzymes as Electrocatalysts
4.2 Surface-Area Effect and Curvature Effects (or Cage
Effect) on Mesoporous Electrodes
There are huge numbers of papers that verify the importance of porous structures
to detect or magnify DET-type catalytic current [1]. Ketjen Black with the primary
particles of a diameter of ~40 nm was found to be an effective material for DET-type
bioelectrocatalysis of histamine dehydrogenase from Nocardioides simplex [2], while
aggregated gold nanoparticles or porous gold electrodes were effective for DETtype bioelectrocatalysis of FDH [3–5]. The control of the porous structure of carbon
cryogel was found to be effective to increase the DET-type catalytic current density of
FDH [6]. An MgO-templated hierarchical mesoporous carbon electrode was effective
for DET-reaction of BOD [7], HRP showed DET-type bioelectrocatalysis at a goldnanoparticle-modified electrode [8]. Several cases are presented in the reference [1].
Such mesoporous materials are frequently utilized not only for DET-type reactions but for MET-type reactions, and also for inorganic catalyst-based electrode
reactions. The major reason to use mesoporous materials appears to be an increase
in the effective surface area against the projective one. We completely agree with
the opinion for the case of MET-type reactions and inorganic catalyst-based reactions, because electrochemically communicating substances in those systems have
low-molecular mass and the orientation is of no importance.
Some suitable control of the porous structure of electrodes was shown to be
effective to increase the catalytic current density [7]. However, situations in the case
of DET-type reactions appear to be contrastive to those of MET-type and inorganic
catalyst-based reactions. Although DET-type bioelectrocatalysis of HRP and H 2 ase
(Desulfovibrio vulgaris Miyazaki F) was not shown on planar electrodes, it proceeded
very clearly on suitably tuned porous electrodes [8–10]. For BOD, DET-type catalytic
waves were shown to be quite small on a planar electrode [11], but can easily be
detected at suitable porous electrodes [8]; the ratio of the catalytic current versus nonFaradaic current was larger at porous electrodes than at planar electrodes. In addition,
it was found that aggregated gold nanoparticles played a vital role as scaffolds for
DET-type reactions [8, 12–14].
Figure 4.1 shows a typical example of the relationship between the DET-type
catalytic current density (j) by BOD-catalyzed O 2 -reduction as a function of the
ratio of electrochemically effective surface area of porous electrodes versus a planar
electrode (A/A plane ), in which porous electrodes were prepared from gold nanoparticles on a planar surface [14] or by anodization of the gold surface [13], and A/A plane
values were evaluated from non-Faradaic current ratio. The j value increases with
A/A plane with some saturation characteristics. The increasing property seems to be
explained simply due to an increase in A/A plane (as expected in MET-reactions), while
the saturation effect seems to be ascribed to the mass transfer inhibition occurring
in multilayer mesoporous electrodes or to the situation that enzyme solutions could
not become widespread into the deep part of the porous structure.
An important feature was observed in the region with small values of A/A plane , as
given in the inset of Fig. 4.1. In such a small A/A plane region, the relationship between
4 Characteristic Properties of Redox Enzymes as Electrocatalysts
4.2 Surface-Area Effect and Curvature Effects (or Cage
Effect) on Mesoporous Electrodes
There are huge numbers of papers that verify the importance of porous structures
to detect or magnify DET-type catalytic current [1]. Ketjen Black with the primary
particles of a diameter of ~40 nm was found to be an effective material for DET-type
bioelectrocatalysis of histamine dehydrogenase from Nocardioides simplex [2], while
aggregated gold nanoparticles or porous gold electrodes were effective for DETtype bioelectrocatalysis of FDH [3–5]. The control of the porous structure of carbon
cryogel was found to be effective to increase the DET-type catalytic current density of
FDH [6]. An MgO-templated hierarchical mesoporous carbon electrode was effective
for DET-reaction of BOD [7], HRP showed DET-type bioelectrocatalysis at a goldnanoparticle-modified electrode [8]. Several cases are presented in the reference [1].
Such mesoporous materials are frequently utilized not only for DET-type reactions but for MET-type reactions, and also for inorganic catalyst-based electrode
reactions. The major reason to use mesoporous materials appears to be an increase
in the effective surface area against the projective one. We completely agree with
the opinion for the case of MET-type reactions and inorganic catalyst-based reactions, because electrochemically communicating substances in those systems have
low-molecular mass and the orientation is of no importance.
Some suitable control of the porous structure of electrodes was shown to be
effective to increase the catalytic current density [7]. However, situations in the case
of DET-type reactions appear to be contrastive to those of MET-type and inorganic
catalyst-based reactions. Although DET-type bioelectrocatalysis of HRP and H 2 ase
(Desulfovibrio vulgaris Miyazaki F) was not shown on planar electrodes, it proceeded
very clearly on suitably tuned porous electrodes [8–10]. For BOD, DET-type catalytic
waves were shown to be quite small on a planar electrode [11], but can easily be
detected at suitable porous electrodes [8]; the ratio of the catalytic current versus nonFaradaic current was larger at porous electrodes than at planar electrodes. In addition,
it was found that aggregated gold nanoparticles played a vital role as scaffolds for
DET-type reactions [8, 12–14].
Figure 4.1 shows a typical example of the relationship between the DET-type
catalytic current density (j) by BOD-catalyzed O 2 -reduction as a function of the
ratio of electrochemically effective surface area of porous electrodes versus a planar
electrode (A/A plane ), in which porous electrodes were prepared from gold nanoparticles on a planar surface [14] or by anodization of the gold surface [13], and A/A plane
values were evaluated from non-Faradaic current ratio. The j value increases with
A/A plane with some saturation characteristics. The increasing property seems to be
explained simply due to an increase in A/A plane (as expected in MET-reactions), while
the saturation effect seems to be ascribed to the mass transfer inhibition occurring
in multilayer mesoporous electrodes or to the situation that enzyme solutions could
not become widespread into the deep part of the porous structure.
An important feature was observed in the region with small values of A/A plane , as
given in the inset of Fig. 4.1. In such a small A/A plane region, the relationship between