Scanning Electrochemical Microscopy Imaging of DNA Arrays for High Throughput Analysis Applications - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 122

insulating regions, the current response at a given d differs over
dissimilar regions: over a conductor we see i T > i T , while i T < i T
is given over the insulating portions and, thus, we can differentiate
them. Figure 7 shows an example of SECM imaging of a 8 u 8
array electrode prepared on a glass substrate; each electrode was
prepared from a 50-μm-diameter evaporated gold film with 500
μm of spacing. 31 In the raster image of the tip current, the Au mi-
crodot is recognized by a rise in the current from the background
exhibited by glass, which distinctly indicates the occurrence of
positive feedback. The particular mode of feedback can be ex-
plained by the formation of a concentration cell, which promotes
the lateral charge transport and the interfacial reaction at the sur-
face; R will be oxidized to O at the sample vicinity of the tip and,
for compensation, O will be converted to R at a point distant from
the tip. 32 Each 50-μm microdot is resolved as ca. 80 μm in the
current mapping image, showing that the method possesses suffi-
cient lateral resolution for use in typical DNA array experiments.
Finer UME is acknowledged to give higher resolution and further
improvements hold promise in providing a clearer result.
In addition to the SECM experiments, using the feedback
mode it is possible to work in the generation-collection mode.
Here, if the substrate or the substrate-attached functional material
can produce any electrochemically labile material, the tip held
close to the substrate could readily detect such species by convert-
ing them into other oxidation states. The current flowing along
with the reaction is used for imaging. This type of measurement is
useful when used for studying biological entities including DNA
tests using enzyme labels. Examples of SECM imaging will be
presented in the next section.
( ii ) Approach Curve at Various Substrate Surfaces
As described, the current at the tip depends specifically on d ,
the distance from the substrate. A plot of the tip current, i T , as a
function of tip-substrate, d , is called an approach curve, which
provides information about the nature of the substrate. From the
results achieved up to now, we can obtain sets of numerical con-
stants to drive theory data. Figure 8 shows the approach curves for