Nanoelectrochemistry: Fundamentals and Applications in Biology and Medicine - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 21

been to measure fast electrode kinetics of various redox species in
solution. Besides enabling the measurement of fast heterogeneous
electron-transfer kinetics, nanoelectrodes also offer the opportunity
to test the applicability of classical transport theories in explaining
observed voltammetric behavior. In particular, in the regime where
the diffusion length of the redox active molecules (which depends
on the electrode geometry and size) becomes comparable to the
Debye length, it is expected that the electric field in the vicinity of
the electrode will affect mass transport. We will return to this issue
in a later Section. For now we focus on the regime where mass
transport is assumed to be entirely diffusive in nature.
The most common way of measuring heterogeneous rate con-
stants from steady state voltammograms has been to fit Eq. (6) to
the entire voltammogram and extract the relevant kinetic parame-
ters ( k 0 and Į) if E 1/2 and the geometry and size of the electrode are
independently known. This approach avoids the common pitfall of
inadequate compensation of the iR drop that often affects data ob-
tained with fast-scan voltammetry at conventional UMEs. 110, 111
The experimental voltammograms can be obtained through either
the conventional two- or three-electrode set up with the nanoelec-
trode serving as the working electrode, or in an SECM experiment
in the positive feedback mode with the nanoelectrode positioned
close to a conducting substrate a distance d away. The latter ap-
proach allows independently tuning the mass-transport rate in two
different ways: by changing the radius r of the nanoelectrode or by
changing the tip-substrate distance. Using this approach with Pt
nanoelectrodes (3.7-400 nm), Sun and Mirkin 55 were able to tune
the mass-transport rate across two orders of magnitude and deter-
mine kinetic parameters for the oxidation of Ferrocene-methanol
(Fc-CH 2 OH) in aqueous solution, Ferrocene (Fc) in acetonitrile,
and the reduction of Hexammineruthenium(III) chloride
(Ru(NH 3 ) 6 3+ ) in aqueous solution and 7,7,8,8-tetracyanoquino-
dimethane (TCNQ) in acetonitrile. Interestingly, they found that
there is little correlation between the electrochemical rate con-
stants, k 0 , and the homogeneous self-exchange rate constants, k ex ,
for these redox couples, counter to the predictions of the Marcus
theory for electron transfer.
Another approach to analysis of quasi-reversible steady-state
voltammograms was developed by Mirkin and Bard. 112 This meth-
od relies on the measurement of experimental parameters, ( E 1/4 -