Biology Reference
In-Depth Information
Figure 7.13.
(A) Schematic showing the electron transfer process across
the dsDNA film between the negative redox probe [Fe(CN)
6
]
3
−
/
4
−
and gold
transducersurface.Electrontransferprocessisfacilitatedbytheadditionof
metal ion that neutralizes the phosphate backbone of DNA and allows the
enhanced diffusion of the redox probe. As a result the differences in charge
transfer resistance
R
ct
before and after metal ion addition are significantly
different and are in fact affected by the presence of a single nucleotide
mismatch. (B) Nyquist plot showing the charge transfer resistance across a
matchedandamismatchedfilminabsenceandpresenceofZn
2
+
intheform
of semicircle. Inset shows the modified Randle's equivalent circuit used to
fit the data. (C) The plot showing the detection limit of the system as low
as 10 fM. Reproduced by permission from X. Li, J. S. Lee, H.-B. Kraatz,
Anal.
Chem.
2006,
78
, 6096-6101. Copyright 2008 American Chemical Society.
high potential for applications also in an array electrode format
and has allowed to detect a range of different mismatches [92, 93].
Scanning electrochemical microscopy (SECM) studies were critical
to elucidate the mechanism of this process and rationalize the
differences in
R
ct
in terms of the diffusive properties of the probe
molecules (see Fig. 7.14) [94, 95]. Using SECM, the heterogeneous
electron transfer constants were evaluated and it was shown that
in the presence of Zn
2
+
the
k
et
increases from 4.6
×
10
−
7
cm/s (no
Zn
2
+
)to5.0
×
10
−
6
cm/s (Zn
2
+
added).
BasedontheinitialSECMresults,itwaspostulatedthatitshould
be possible to evaluate differences in
R
ct
directly by SECM and
monitor the amperometric feedback current in the presence and
absenceofZn
2
+
.ThepresenceofSNPcausedanincreaseinelectron
transfer rate constant, presumably due to better penetration of
