Biomedical Engineering Reference
In-Depth Information
into close proximity while maintaining a parallel orientation. Ad-
vances in microfabrication techniques have allowed the electrode
spacing to be reduced to about 50 nm, and in the future will most
likely be reduced even further.
As an alternative to TLCs, interdigitated electrodes (IDEs) can
also be employed to study systems that involve redox cycling. As
the name suggests, IDEs consist of two comb-shaped electrodes
with the teeth of the two electrodes arranged in an interdigitated
fashion, similar to a set of gears.
IDEs, also known as interdigitated arrays or band-array elec-
trodes, have been fabricated and analyzed since the 1980s.
78-82
They became popular as the availability of microfabrication tech-
niques increased. IDEs are much simpler to create using microfab-
rication techniques than TLCs. Since both electrodes in the IDE
geometry are in the same plane, they can be created using a single
lithographic step. The sensitivity of IDEs is generally determined
by how small the spacing between electrodes can be made, which
is determined by fabrication limitations.
IDEs are most often used as conductometric sensors
83-85
but
they are also utilized in analytical
86-88
and bioanalytical
89-91
am-
perometric detection strategies. Recent fabrication improvements
have rekindled interest in the technology. With electron-beam li-
thography becoming more widely available, there has been a move
toward nanoscale electrode widths as well as spacing between
electrodes. Several groups have demonstrated sub-micrometer
wide electrodes and spacing
92-95
, and novel geometries and designs
have been analyzed using simulations.
96-99
By reducing the space between electrodes, the chance that a
molecule residing in the space between electrodes will undergo
redox cycling increases. IDEs usually generate lower cycling cur-
rents than TLCs because they are not as efficient in trapping mole-
cules between their electrodes. One way to increase the average
time that a molecule spends between the electrodes of an IDE de-
vice is to purposely make the electrodes thicker.
92
This idea was
taken one step further by Dam et al., who etched trenches in the
substrate before patterning metal, so that the electrodes face each
other.
100
Although TLCs and IDEs have proven to be powerful tools in
the study of electrochemical phenomena, their principal limitation
is that the electrode spacing is fixed. By contrast, Scanning Elec-
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