Biomedical Engineering Reference
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voltammetric measurements (~1000 V/s). Knowing the surface
coverage by way of measurements on bigger electrodes of known
geometry, the area of the nanoelectrode can be calculated. If the
limiting current for dissolved redox species agrees with that ex-
pected for the assumed geometry, then assumption of this geome-
try is deemed correct. Interestingly, they observed that only 50%
of the electrodes prepared by the electrophoretic polymer-coating
procedure were quasi-hemispherical, the remaining being recessed
slightly below the polymer coating. This also points to another
challenge of characterization of nanoelectrodes via indirect elec-
trochemical means. Because of large variations in the size and
shapes of electrodes fabricated using fairly uniform processes, the
need arises to characterize each electrode independently, which
complicates routine usage. In addition, indirect means of charac-
terizing the electrodes often render the electrode unfit for further
use.
Besides uncertainties in the geometry and size of the electrode
itself, there may exist defects in the insulating layer, which may
further complicate measurements. For instance, it has been ob-
served that heat curing of electrophoretic paint around metal elec-
trodes often leaves conductive pinholes in the insulating layer,
rendering it ineffective for neurophysiological measurements.
58
Several coatings of paint are required to completely eliminate pin-
holes, which often makes it difficult to obtain an electrode with a
conductive tip. Similar problems may exist for wax and glass-
coated electrodes. In the earliest example of detection of single
molecules with SECM using wax-coated nanoelectrodes, it was
proposed that the observed slow fluctuations in the current trace
was due to small channels formed in the insulating wax layer
around the nanoelectrode tip that were connected to the small res-
ervoirs or bulk solutions not in contact with the tip. When the re-
dox-active molecules enter these channels, no current is seen until
they traverse back out into the tip volume. This suggests that even
slight uncertainty in the robustness of the insulator can have a sig-
nificant effect, especially for sensitive measurements. Similarly, in
the case of glass, Mirkin and coworkers
221
have recently observed
that the effective surface area of a Pt nanoelectrode was much
larger than the geometric surface area due to surface diffusion of
adsorbed redox species at the Pt/glass interface. They also found
that surface hydrogen formed on Pt/glass interface
spills over
into
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