Biomedical Engineering Reference
In-Depth Information
As standard use, immobilization of the probe DNA was made
through normal 5'-thiol modification chemistry. A Fe(CN)
6
3-/4-
redox pair was chosen for the mediator and SECM experiments
were made in the presence of MB. Initially, the approach curve
was measured with the electrode potential of the DNA array con-
trolled. They found that, with the electrode potential cathodically
polarized, the feedback behavior of the Fe(CN)
6
3-/4-
turned posi-
tive even though it was initially in the negative feedback mode.
This was due to the cathodic atmosphere providing the reduced
form of MB (leucomethylene blue, LB), which could reduce the
mediator ion for the positive feedback functions at the tip, Eqs.
(6)-(8).
SECM tip (0.5 V):
Fe(CN)
6
4-
-
e
-
ĺ Fe(CN)
6
3-
(6)
MB
+
+ 2
e
-
+ H
+
ĺ LB
DNA surface (-0.4V):
(7)
Overall: 2 Fe(CN)
6
3-
+ LB ĺ 2 Fe(CN)
6
4-
+ MB
+
+ H
+
(8)
SECM experiments were achieved with varying visualization
conditions from the negative to the positive feedback mode. The
DNA microarray was imaged at a detection level of 14 fmol per
spot for the particular dsDNA consisting of 15 base pairs. More
importantly, the feasibility of detecting base pair mismatches was
demonstrated. Later, Barton's group independently reported a sim-
ilar type of SECM experiment using Nile Blue as the mediator.
46
Previously, we reported the surface-grafting immobilization of
capture-probe (CP) DNA sequences on carbon electrodes coated
with poly(1,4-benzoquinone), PQ, that were oxidatively polymer-
ized using peroxidase.
47
The DNA-polymer conjugate film was
found to be electrochemically responsive, allowing the label-free
detection of hybridization. Furthermore, the potentially electro-
conductive nature of the covalently linked, redox-active conjugate
molecule was expected to be suitable for the robust, posi-
tive-feedback detection in SECM. From these viewpoints, we
adopted the surface modification, as a DNA array model, towards
a carbon fiber (CF) electrode that was 33 μm in diameter and ex-
amined the material by SECM characterization.
48
The CP DNA
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