Biomedical Engineering Reference
In-Depth Information
trochemical Microscopy (SECM; the acronym also applies to the
instrument) combines the best features of TLCs and IDEs as well
as scanning probe techniques. Since its introduction in the late 80's,
it has emerged as a powerful and versatile technique to probe a
wide variety of chemical and biochemical phenomena.
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Briefly,
the technique employs the use of a UME or nanoelectrode, which
is positioned close to a substrate. The faradaic current through the
UME, as it is moved normal to or laterally across the substrate,
yields information about the substrate and the intervening medium.
The technique can also be used in the chronoamperometric mode,
where the current is measured as a function of time at a fixed UME
position. The substrate can be conducting, semiconducting or insu-
lating. It can be solid or liquid. The instrument can be operated in
several modes contingent on the nature of the substrate and the
problem at hand. These modes of operation together with details of
instrumentation have been summarized in a monograph.
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The use of electrodes of nanometer dimensions has further ex-
tended the capabilities of SECM. Besides offering higher spatial
resolution in imaging experiments, they also enable higher mass-
transport rates in the steady-state regime, thus allowing kinetic
investigations of systems with very rapid electron-transfer rates. A
recent review of SECM lays special emphasis on the use of na-
nometer-sized tips.
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The biological applications of SECM have
also been reviewed recently.
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3. ChallengesofCharacterization
While the fashioning of nanoscale electrodes is becoming more
common and widespread, the precise characterization of the shape
and size of these electrodes continues to remain a daunting chal-
lenge. As discussed previously, the issues of shape and size and
their influence on the shape of the resulting voltammogram are
intimately connected. If one knows the geometry of the electrode
then one can, in principle, readily determine the size of the elec-
trode from the steady-state currents. A key challenge, therefore, is
the determination of the exact shape of the electrode. For mi-
crometer sized electrodes, optical and electron microscopy can be
used to assess the geometry. However, as dimensions shrink into
the nanometer regime (and especially below 50 nm), it becomes
increasingly difficult to image electrodes using electron microsco-
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