reach a very high power density when operated under more biologically relevant
conditions (~0.9 mW cm
À2 at 10 mM glucose). The high performance of this
EBC was enabled by an elaborate manufacturing method in which 8 nm-sized
Au NPs were layer-by-layer (LBL) assembled with small organic linkers onto
cotton fibers to produce a highly porous metallic cotton fiber electrode material
with extremely high conductivity (>2.1 Â 10
4 S cm
À1
).
Interestingly, this engineered metallic cotton fiber material was found to be a
highly efficient oxygen reduction catalyst by itself, which enabled the final EBC
assembly to have an “enzyme-free” cathode. On the anode side, these metallic cotton
fibers were used as a support material to which a GOx enzyme was immobilized.
To improve the electrical communication between the enzyme and the Au NPs
at the anode, the authors employed tris-(2-aminoethyl)amide (TREN) as a small
molecule adhesive. By both binding covalently to the Au NPs and interacting
electrostatically with the enzyme, TREN was able to bring both of the components
into a very close proximity, which greatly improved the DET rate. Additionally,
TREN was also found to have a beneficial effect on the overall operational stability
of the electrode, and the entire device was found to maintain more than half of
its initial power density over 35 days. This study by the groups of Lee and Cho
provided another excellent example of how the design of new nanomaterials
with high internal surface areas and good superconducting properties can drive
EBC development forward. The same authors subsequently used this LBL assembly
technique to prepare a related EBC based on a carbon fiber electrode material [96].
Although this device also showed a relatively high performance, it still proved to
be less efficient than the EBC described in their first study.
As can be seen from the examples presented so far, Au is evidently the main
transition metal of choice when it comes to the doping of electrode materials
with different metal NPs. As indicated by the great body of work in this field,
Au NPs appear to be superior to other metal NPs concerning the ability to promote
electron transfer processes. However, it should be noted that also some other
transition metal NPs have been studied for this purpose. Platinum, a transition
metal mainly employed in non-enzymatic fuel cells [97–99], has also been used
to generate nanostructured materials for bioelectrode applications [100–102].
For example, Tel-Vared and coworkers very recently reported on a complete
EBC setup, which featured a mesoporous carbon-based hybrid anode with Pt
nanoclusters and GOx NPs, paired with a BOx-based cathode [103]. Although
the described DET device reached a modest maximal power output and current
density (45 μW cm
À2 and 105 μA cm
À2 when operated at glucose concentrations
of 60 mM), this study is interesting from a synthetic perspective since it outlined
a new paradigm for assembling bioelectronic elements, which could also be
interesting to apply to Au-based systems. Here, the reduction of the metal precursor
PtCl 5
¯ was carried out by the immobilized GOx which had already been confined
into the mesopores of the carbonaceous matrix on beforehand. With this synthetic
approach, the authors managed to form the Pt nanoclusters in a very close proximity
to the FAD redox active center of the GOx, which allowed for a good contact
surface between the two catalytic components. This close wiring of the GOx and
Nanocatalysis Meets Biology
259
conditions (~0.9 mW cm
À2 at 10 mM glucose). The high performance of this
EBC was enabled by an elaborate manufacturing method in which 8 nm-sized
Au NPs were layer-by-layer (LBL) assembled with small organic linkers onto
cotton fibers to produce a highly porous metallic cotton fiber electrode material
with extremely high conductivity (>2.1 Â 10
4 S cm
À1
).
Interestingly, this engineered metallic cotton fiber material was found to be a
highly efficient oxygen reduction catalyst by itself, which enabled the final EBC
assembly to have an “enzyme-free” cathode. On the anode side, these metallic cotton
fibers were used as a support material to which a GOx enzyme was immobilized.
To improve the electrical communication between the enzyme and the Au NPs
at the anode, the authors employed tris-(2-aminoethyl)amide (TREN) as a small
molecule adhesive. By both binding covalently to the Au NPs and interacting
electrostatically with the enzyme, TREN was able to bring both of the components
into a very close proximity, which greatly improved the DET rate. Additionally,
TREN was also found to have a beneficial effect on the overall operational stability
of the electrode, and the entire device was found to maintain more than half of
its initial power density over 35 days. This study by the groups of Lee and Cho
provided another excellent example of how the design of new nanomaterials
with high internal surface areas and good superconducting properties can drive
EBC development forward. The same authors subsequently used this LBL assembly
technique to prepare a related EBC based on a carbon fiber electrode material [96].
Although this device also showed a relatively high performance, it still proved to
be less efficient than the EBC described in their first study.
As can be seen from the examples presented so far, Au is evidently the main
transition metal of choice when it comes to the doping of electrode materials
with different metal NPs. As indicated by the great body of work in this field,
Au NPs appear to be superior to other metal NPs concerning the ability to promote
electron transfer processes. However, it should be noted that also some other
transition metal NPs have been studied for this purpose. Platinum, a transition
metal mainly employed in non-enzymatic fuel cells [97–99], has also been used
to generate nanostructured materials for bioelectrode applications [100–102].
For example, Tel-Vared and coworkers very recently reported on a complete
EBC setup, which featured a mesoporous carbon-based hybrid anode with Pt
nanoclusters and GOx NPs, paired with a BOx-based cathode [103]. Although
the described DET device reached a modest maximal power output and current
density (45 μW cm
À2 and 105 μA cm
À2 when operated at glucose concentrations
of 60 mM), this study is interesting from a synthetic perspective since it outlined
a new paradigm for assembling bioelectronic elements, which could also be
interesting to apply to Au-based systems. Here, the reduction of the metal precursor
PtCl 5
¯ was carried out by the immobilized GOx which had already been confined
into the mesopores of the carbonaceous matrix on beforehand. With this synthetic
approach, the authors managed to form the Pt nanoclusters in a very close proximity
to the FAD redox active center of the GOx, which allowed for a good contact
surface between the two catalytic components. This close wiring of the GOx and
Nanocatalysis Meets Biology
259