Nanoelectrochemistry: Fundamentals and Applications in Biology and Medicine - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 14

71,72 We used the exposed end of a SWNT as a template for in situ
Pt electrodeposition. The SWNT itself served as a wire from the Pt
interface to macroscopic leads. Using this approach we were able
to obtain nanoparticle electrodes with radii as small as approxi-
mately 6 nm. 73

( ii ) Redox Cycling (Thin Layer Cells, IDEs and SECM)

Apart from shrinking the dimensions of individual electrodes,
another pathway for approaching nanoscale dimensions in electro-
chemistry relies on having not one, but two electrodes separated
from each other by small distances. If the electrodes are biased
such that reduction takes place at one electrode and oxidation at
the other, a steep concentration profile of reduced and oxidized
species can be set up between the two electrodes. For planar elec-
trodes separated by a distance z , the corresponding diffusion-
limited current is given by

nFADC

i (11)

lim

z

This result neglects fringing at the edges of the electrodes, a valid
approximation so long as the spacing z is much smaller than the
lateral dimensions of the electrodes. The current given by Eq. (11)
corresponds to the same limiting current density, i lim / A , as is
achieved at a disk electrode with radius R = 4 z /S, regardless of the
actual area of the planar electrode. Small enough spacings z thus
allow achieving current densities comparable to nanoelectrodes
using electrodes that are actually much larger.
This idea was first realized in the early 1960s, through the de-
velopment of thin-layer cells (TLCs). 74-77 An adjustable working
distance was offered between the two plane-parallel electrodes,
which were separated by tens of micrometers. The spacing was
regulated by having the two ends of a micrometer screw gauge
serve as the two electrodes. The screw was then used to both
clamp the electrodes together and adjust the gap size. Using this
approach, electrode spacing on the order of a few micrometers was
achieved. As in SECM, the minimum electrode spacing achievable
is ultimately limited by the ability to bring the two flat surfaces