Biomedical Engineering Reference
In-Depth Information
bye length becomes comparable to the size of the electrode and the
diffusion region. We then discuss finite particle number fluctua-
tions in the vicinity of the nanoelectrodes. We conclude the Sec-
tion with a discussion on quantum-dot electrodes and charging and
finite-size effects seen in these systems. We move in the next Sec-
tion to the
single-molecule
limit and discuss the recent advances in
the electrochemistry of freely-diffusing single molecules as well as
electrochemical scanning probe techniques used in the investiga-
tions of immobilized biomolecules. We conclude the chapter with
a brief survey of the applications of nanoelectrodes in biosensors
and biological systems.
II. THECLASSICALREGIME
We begin our survey of nanoscale electrochemical measurements
by focusing on cases in which no new effects appear upon
downscaling, but rather where the formalisms that apply on the
micrometer scale or higher are sufficient to explain observations.
There are basically two different ways in which nanometer-scale
dimensions can be introduced into electrochemical systems.
First, it is possible to scale down the size of the working elec-
trode such that its lateral dimensions are nanometric. These exper-
iments are essentially equivalent to those performed at ultramicro-
electrodes, except that the smaller dimensions of the electrodes
yield further enhancements in performance. These advantages in-
clude the ability to achieve a true steady-state in extremely short
times, as well as the possibility of reaching higher current densities.
The latter is particularly relevant for measuring fast heterogeneous
kinetics.
Second, it is possible to create situations in which two or more
electrodes or surfaces (each of which may or may not be nanomet-
ric in size) are separated by nanometer-scale distances. This has
been most famously exploited in thin-layer cells and in the so-
called positive feedback mode of scanning electrochemical mi-
croscopy (SECM), in which oxidation and reduction taking place
at separate, closely spaced electrodes lead to enhanced mass
transport.
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