Biomedical Engineering Reference
In-Depth Information
large number of catalytic crystal facets and atomic steps, and dis-
play the cumulative activity of this large heterogeneous ensemble
of catalytic sites. With typical enzymatic turnover rates between
10 and 10000 per second, the current generated per molecule will
be in the range of attoamperes to femtoamperes. For that reason,
most single-enzyme electrochemistry efforts concentrate either on
the detection of a fluorescent product, or sensing an accumulated
electroactive product with nano-SECM or scanning probe methods,
as described above, or with interdigitated electrodes. Nonetheless,
our group recently achieved direct protein film voltammetry of
<50 hydrogenase molecules immobilized on a 100x100 nm 2 gold
electrode, generating 20 fA of electrocatalytic current when reduc-
ing protons to hydrogen ( Fig. 8 ). 169
2.
Electrochemistry of Freely-Diffusing Molecules
Time-resolved electrochemical detection of a single or few freely-
diffusing molecules is a very challenging enterprise for the simple
reason that ordinarily, a redox-active molecule contributes only
one or a few electrons to the measured current with each encounter
at the electrode. While these days it is routinely possible to meas-
ure currents in the low picoamperes (pA) with commercial poten-
tiostats, and even femtoamperes (fA) with electrometers, currents
on the order of a few attoamperes (aA) present a great challenge
(an attoampere corresponds to merely 6 electrons per second!).
Redox cycling provides a means by which a single molecule
can shuttle multiple electrons between the two electrodes, thus
giving rise to a continuous, measurable current. As discussed in
Section III.2 (Eq. 13) the quantity eD/z 2 represents the contribution
of a single molecule to the current, and its magnitude increases
quadratically with decreasing electrode spacing. It is therefore
conceivable that for small enough spacing, z , even a single mole-
cule may yield a measurable current.
An implementation of this idea by Fan and Bard 37 stands as
the first example where electrochemical detection of a single mol-
ecule was reported. In this instance they used an Apiezon wax-
coated Pt-Ir nanoelectrode tip with the wax shrouding the electrode,
and used it to trap a small solution in close proximity (~ 10 nm) to
a conducting substrate (ITO). In the positive feedback mode, they
were able to amplify the current ~10 6 fold and observed stochastic
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