Biomedical Engineering Reference
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(a) (b) (c)
Figure 6. Schematic diagram of SECM apparatus. The inset compares the principle
of SECM, showing hemispherical diffusion of the mediator ion to disk-shaped tip
located far from the substrate (a), blocking of the mass-transport process by ap-
proaching of the tip to an insulating substrate (b), and positive feedback when the
exhausted mediator ions are regenerated by redox reaction occurring at a conductive
substrate surface (c).
a conductive substrate, on the other hand, if a particular electro-
chemical condition is attained, the substrate can regenerate O,
which is further re-reduced at the tip, giving rise to the reduction
current of the mediator. This type of operating control, in which
the tip generates and simultaneously detects the mediator in rela-
tion to its oxidation state, is termed feedback mode imaging. In the
two extreme conditions, the term of negative feedback has been
adopted for the former case, whereas in the latter case an en-
hancement of the Faradaic current is termed positive feedback .
If the tip is rastered above the substrate, the surface topogra-
phy that relates to changes in the tip-substrate distance d , can be
imaged by recording the changing current vs. tip position in the
x-y plane. With a substrate that consists both of conductive and
 
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