in medical implants intended for long-term use. Besides challenges associated
with biocompatibility and sterilization, the full potential of EBCs is primarily held
back by their unsatisfactory operational stabilities, which typically ranges from a
few hours to months (this can be compared to sealed batteries with lifetimes of
about 5–8 years) [57]. On the other hand, if EBCs would be made more durable
than batteries, they would certainly constitute a much more attractive alternative to
batteries since they are easier to miniaturize and they would theoretically never
run out of their source of energy. Despite the limitations and challenges associated
with the use of EBCs for medicinal applications, the field of bioelectrocatalysis
still represents an interesting research area to review in this chapter, since it has
contributed significantly to the design of new nanometal-enzyme hybrids.
3.1 Biofuel Cells Based on the Interfacing of Metal
Nanoparticles and Enzymes
In contrast to conventional fuel cells that make use of different metal catalysts for
the conversion of chemical energy into electricity, EBCs instead rely on redox-active
enzymes as the key catalytic components. This latter approach is preferable for use
in implantable devices since enzyme catalysts operate efficiently in physiological
conditions (i.e., low temperatures and at near-neutral pH) [71]. Moreover, enzymes
are highly selective for organic substrates present in living organisms. For these
reasons, EBCs are especially suited for incorporation into implantable or microscale
biomedical devices. However, the major challenge for constructing efficient
EBCs is to ensure for an efficient electron transfer between the enzymes and the
electrode surface. It is known that electron transfer rate decreases rapidly as the
distance between the enzymes’ active sites and the electrode surface increases.
Given the facts that enzymes are large molecules with catalytically active sites
often confined in a small pocket, EBC design is challenging from both a chemistry
and engineering point of view. As a result, EBCs generally exhibit power outputs
(10 μW–1 mW cm
À2 ) that are much lower than those of conventional fuel cells [57].
However, it should be emphasized that such low power outputs could still be
sufficient to power many different sorts of microscale devices, and thus EBCs
should be evaluated based on their long-term stability rather than on how high
power densities they can reach.
As shown in Fig. 1, EBCs can be divided into two main groups depending on how
the electrons are being shuttled between the enzyme and the electrode: (1) mediated
electron transfer (MET) devices where different electron transfer mediators are
used as relays in the electron transfer (this includes inorganic complexes of Fe
[72, 73], Os [74, 75], and Ru [76, 77]; organic compounds such as quinones
[78, 79], phenothiazines [80, 81], and viologens [82, 83]; or more recently also
polyoxometalate nanoclusters [84]) and (2) direct electron transfer (DET) devices
where the electrons are being directly transported between the enzyme components
and the electrodes. Although MET generally exhibits higher power densities
Nanocatalysis Meets Biology
255
with biocompatibility and sterilization, the full potential of EBCs is primarily held
back by their unsatisfactory operational stabilities, which typically ranges from a
few hours to months (this can be compared to sealed batteries with lifetimes of
about 5–8 years) [57]. On the other hand, if EBCs would be made more durable
than batteries, they would certainly constitute a much more attractive alternative to
batteries since they are easier to miniaturize and they would theoretically never
run out of their source of energy. Despite the limitations and challenges associated
with the use of EBCs for medicinal applications, the field of bioelectrocatalysis
still represents an interesting research area to review in this chapter, since it has
contributed significantly to the design of new nanometal-enzyme hybrids.
3.1 Biofuel Cells Based on the Interfacing of Metal
Nanoparticles and Enzymes
In contrast to conventional fuel cells that make use of different metal catalysts for
the conversion of chemical energy into electricity, EBCs instead rely on redox-active
enzymes as the key catalytic components. This latter approach is preferable for use
in implantable devices since enzyme catalysts operate efficiently in physiological
conditions (i.e., low temperatures and at near-neutral pH) [71]. Moreover, enzymes
are highly selective for organic substrates present in living organisms. For these
reasons, EBCs are especially suited for incorporation into implantable or microscale
biomedical devices. However, the major challenge for constructing efficient
EBCs is to ensure for an efficient electron transfer between the enzymes and the
electrode surface. It is known that electron transfer rate decreases rapidly as the
distance between the enzymes’ active sites and the electrode surface increases.
Given the facts that enzymes are large molecules with catalytically active sites
often confined in a small pocket, EBC design is challenging from both a chemistry
and engineering point of view. As a result, EBCs generally exhibit power outputs
(10 μW–1 mW cm
À2 ) that are much lower than those of conventional fuel cells [57].
However, it should be emphasized that such low power outputs could still be
sufficient to power many different sorts of microscale devices, and thus EBCs
should be evaluated based on their long-term stability rather than on how high
power densities they can reach.
As shown in Fig. 1, EBCs can be divided into two main groups depending on how
the electrons are being shuttled between the enzyme and the electrode: (1) mediated
electron transfer (MET) devices where different electron transfer mediators are
used as relays in the electron transfer (this includes inorganic complexes of Fe
[72, 73], Os [74, 75], and Ru [76, 77]; organic compounds such as quinones
[78, 79], phenothiazines [80, 81], and viologens [82, 83]; or more recently also
polyoxometalate nanoclusters [84]) and (2) direct electron transfer (DET) devices
where the electrons are being directly transported between the enzyme components
and the electrodes. Although MET generally exhibits higher power densities
Nanocatalysis Meets Biology
255