Nanoelectrochemistry: Fundamentals and Applications in Biology and Medicine - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 12

small nanowires at the end of the sharp silica tip, Zhang's 52 group
has extended the laser pulling method, by further encasing the
pulled Pt wire tip into borosilicate glass tubing. This configuration
was then further ground and polished using a home-made continui-
ty tester to ensure that the polishing is aborted as soon as the very
end of the Pt nanowire is exposed. Using this method, they report
to have made Pt electrodes of radii 1-3 nm. 52, 53 The laser pulling
approach has also been used for Pt wires sealed in Teflon 54 tubes.
Images of several types of nanoelectrodes can be seen in Fig. 2 .
Although the above mentioned techniques for producing
nanoelectrodes are relatively simple and inexpensive, they have
inherent uncertainties in the precise shape and size of the final
electrode ( vide infra ) especially as dimensions shrink below 10 nm.
The main alternative is to use standard microfabrication
/micromachining processes. Although these microfabrication pro-
cesses require specialized equipment and are therefore more ex-
pensive, they also have considerable potential for commercializa-
tion if robust protocols for fabrication can be developed. The prin-
cipal advantages of this approach are that the geometry of the elec-
trodes is precisely and reproducibly controlled and that the size
and shape can be determined independently of indirect (mainly
electrochemical) methods. Some approaches to nanoscale elec-
trodes have included focused ion-beam (FIB) sculpting to form
ring-shaped nanoelectrodes around an AFM tip for use in SECM-
type measurements. 60-64 FIB milling was also used to sculpt an
electrophoretic paint-coated W tip to produce electrodes with di-
mensions of 100-1000 nm. 58 Other silicon-based microfabrication
techniques 65,66 have also been used to make Pt nanoelectrodes.
Carbon nanotube based nanoelectrodes have also been pre-
pared. 56,67,68 Our group used lithographically fabricated nanopores
in an insulated silicon membrane that are subsequently blocked on
one side with metal to yield electrodes with lateral dimensions
between 15-200 nm. 69 The fabrication protocol was later altered to
obtain conically shaped Au electrodes with radii down to 2 nm. 57,70
Another bottom-up approach to making nanoelectrodes relies on
depositing noble-metal nanoparticles on conducting substrates like
carbon electrodes 59 and single-walled carbon nanotubes (SWNTs).