Biomedical Engineering Reference
In-Depth Information
(b) how the mass transport or heterogeneous electron transfer
kinetics occur.
In this Chapter, after a brief introduction to the theory of SECM,
recent progress in SECM studies toward DNA arrays including
simple DNA-attached electrodes will be summarized. Some of our
achievements will be included. For additional reviews on SECM
for bioimaging, the reader is guided to recent reviews.
29, 30
1.
Introduction and Principle of SECM
(
i
)
Operation of SECM for Surface Imaging
SECM is a type of scanning probe technique in which the
current detected is caused by an electrochemical reaction at the tip
(
Fig. 6
). The apparatus includes a positioning stage combined with
a bipotentiostat, which enables the tip to scan in the
x-y
plane
(raster) across a surface in micrometer steps with the electrode
potential of the tip remaining constant. For electrodes, typically a
Pt or C disk with radii of 5 to 25 μm is sealed in glass and then
polished to form an ultramicroelectrode (UME). The tip, together
with auxiliary and reference electrodes, is immersed in a solution
containing an electrolyte and a mediator. The sample to be inves-
tigated is often connected to the bipotentiostat as a second working
electrode. When polarized at the appropriate potential for the oxi-
doreduction reaction of the mediator, e.g., O +
n
e
-
ļ R at concen-
tration of
C
O
* and with diffusion coefficient of
D
O
for substance O,
a diffusion-limited current will be detected at the tip as given by:
O
i
4
nFD
C
a
(1)
T,
f
O
where
n
is the number of electrons transferred,
F
is the Faraday's
constant and
a
represents the radius of the disk electrode. When
the positioning stage brings the tip very near the substrate surface,
typically within the radius of the tip, two effects can perturb the
diffusion-limited current, depending on the substrate. If the sub-
strate is insulating, spatially hindered mass-transport of O to the tip
tends to decrease the reduction current of the mediator. In case of
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