Biomedical Engineering Reference
In-Depth Information
aqueous alkali solution and a voltage (AC or DC) is applied. The
metal etches away at the air-water interface, leaving behind a
sharpened tip. Electrodes sharpened in this way include Pt-Ir,
28, 31,
32
Pt,
33, 34
Ag
35
and even carbon.
36
The next step involves coating
the sharpened electrode with an insulating material, except at the
apex of the tip, thus leaving behind a very tiny exposed area. Vari-
ous insulating materials have been used such as apiezon wax,
31, 32,
34,37
poly(D-methylstyrene),
28
polyimide,
38
Teflon
39
or, most com-
monly, electrodeposited electrophoretic paint.
35, 36, 40-45
In the latter
case, after cathodic electrodeposition, the electrode is heat-cured to
provide good insulation. During heat curing the paint shrinks, thus
leaving the apex of the tip exposed. However, it is difficult to pre-
cisely control the curing step and one easily ends up with an elec-
trode of ill-defined size or geometry.
Other approaches have included coating with glass.
28,33,46,47
A
simple extension
48
of the above mentioned technique involves
starting with a sharpened Pt wire and sealing it in a glass capillary.
Heating to 1000qC ensures that the glass melts thereby sealing the
Pt wire. Once the Pt is sealed, the capillary is polished until the
very end of the Pt tip is exposed. Although it is very difficult to
controllably make electrodes of a pre-determined dimension, this
process was reported to be an inexpensive and quick way of pro-
ducing electrodes with radii in the 100 nm-50 ȝm range. Elec-
trodes with radii as small as 10 nm have been reported using this
method. Following a procedure developed by Abruña,
46
a relative-
ly more controlled and reproducible approach to making nanome-
ter sized Pt electrodes involves the use of a laser puller using Pt
wires sandwiched within borosilicate capillaries.
49-51
Briefly, a
metal wire encased in a glass sheath is heated to a temperature
which is close to the melting point of the metal but at the same
time ensuring that the glass capillary is stable with respect to bend-
ing. By using a laser puller, both the glass capillary and the metal
are rapidly heated. This ensures a tight sealing of the metal in the
glass and when both are pulled simultaneously, a significant de-
crease in the diameter of the sealed Pt wire ensues and thus the
formation of two Pt nanoelectrodes. As a consequence of the rapid
scission of the capillary, the resulting surfaces are often not flat
and suitable polishing procedures are required afterwards. More
recently, after discovering with high-resolution transmission elec-
tron microscopy (HR-TEM) that the Pt wire is pulled into ultra-
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