Biomedical Engineering Reference
In-Depth Information
rates of mass transport enable fundamental aspects of electron
transfer kinetics to be investigated.
By the late 1980s, electrodes with effective dimensions of 1-
10 Pm were being reliably and reproducibly made, and in some
instances electrodes of 100 nm sizes were also reported.
22
Various
approaches to make UMEs have been reviewed by Arrigan
23
and
Zoski.
24
Since then, several methods have been developed to further
shrink the sizes of electrodes into the sub-micrometer regime.
Most of the electrodes have geometries that can be generally clas-
sified as spherical, disk, band or cone. The earliest instance of a
truly nanoscale electrode was a Au band electrode made by White
and co-workers.
25
They deposited Pt and Au films, 2 to 50 nm
thick, onto molecularly smooth cleaved mica. The top side of this
metal layer was then coated with an epoxy insulating layer to cre-
ate an epoxy-metal-mica sandwich structure. This structure was
then ground flat perpendicular to the metal film with silicon car-
bide paper, to eventually expose a rectangular band-shaped elec-
trode. The width of this band is simply determined by the thick-
ness of the metal deposited. Because of the variable nature of the
cutting procedure employed, the actual geometry of this band-
shaped geometry was described as “a roughened surface that is
broken at numerous positions along the length of the electrode”.
However, it was the first instance of an electrode geometry where
at least one dimension of the electrode was shorter than the Debye
length and approached the dimensions of the redox-active mole-
cule being probed.
Advances in making disk and cone-shaped nanoelectrodes
with lateral dimensions of a few nanometers were occasioned by
the burgeoning interest in making very small tips for the purposes
of imaging by scanning tunneling microscopy (STM). The key
first step was to decrease the size of the active electrode area to
that below the dimensions of the commercially available metal
wire. The main procedure employed was electrochemical etching,
as used commonly for STM tips.
26-29
A vast majority of disk or
cone-shaped nanoelectrodes happen to be Pt electrodes, because
other metals like Au, Ag or Cu either dissolve rapidly or form thin
insulating oxide layers during the etching process, which introduc-
es enormous complications especially when dealing with nanome-
ter dimensions.
30
Typically, the metal wire is suspended in an
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