Biomedical Engineering Reference
In-Depth Information
3.0 × 10
5
2.5 × 10
5
ds-DNA
2.0 × 10
5
1.5 × 10
5
4 base
mismatched
1.0 × 10
5
ss-DNA
5.0 × 10
4
0.0
0
1 × 10
5
2 × 10
5
3 × 10
5
4 × 10
5
5 × 10
5
6 × 10
5
7 × 10
5
Z
re
(Ohm)
FIGURE 4.21
Nyquist plot of the nanodiamond films with 1 mM Fe(CN
6
)
3-
/
4-
(From Nebel et al.,
Br. J. Soc. Interface
, 4, 439, 2007.
With permission.) [133].
A recent report on BDD verifies that material is qualified to be the next generation of
DNA electrode, for its widely electrochemical potential window both in aqueous and non-
aqueous media, relative low double-layer capacitance, as well as insensitivity to dissolved
oxygen [139]. In 2005, Gu et al. [131] investigated polyaniline/polyacrylate (PANI/PAA)-
modified BDD electrode in DNA hybridization applications. The BDD was fabricated on
MWCVD with methanol and boron oxide mixture gas. Then, the PANI/PAA-modified
BDD electrodes were dipped into a mixture of the NH
2
-ssDNA (2
μ
M in 0.1 M Na
3
PO
4
buffer) solution with 20 mg/mL of
N
-ethyl-
N
→
-[3-(dimethylamino)-propyl], and the EIS
Nyquist plot for the polymer-modified BDD and DNA-modified BDD was measured by
the potentiostat at a given open-circuit voltage, from 100 kHz down to 0.1 Hz, with AC
amplitude of 10 mV.
Figure 4.22a shows the Nyquist plot of probe-immobilized BDD electrodes measured
at different potential (open-circuit potential vs. Ag/AgCl) in phosphate-buffered solution
(pH 7.4). The characteristic potential dependence of the BDD electrode could be obtained at
the high-frequency range (
f
> 10 kHz), which was a region dominated by the space charge
region of diamond. After the immobilization of the probe DNA on the polymer-modified
BDD electrode, the diameter derived from the Nyquist plot semicircle (in Figure 4.22b)
increases significantly comparing with the naked BDD and the PANI/PAA-modified
BDD.
Figure 4.23 shows the Nyquist plot of DNA-immobilized PANI/PAA-modified BDD
electrode, before (solid squares) and after (solid triangles) exposure to fully complemen-
tary target, with one-base mismatch (hollow circles), after denature (hollow triangles) and
renature (hollow squares) with the fully complementary target, measured at -1.0 V open-
circuit potential vs. Ag/AgCl.
At the frequency range between 10 and 100 Hz, the impedance modulus decreased from
13 to 6 k
Ω
. The author explained that phenomenon was due to the effect of DNA hybrid-
ization, which reduced the electron-transfer resistance on the electrode, clearly manifested
as a much smaller diameter of the semicircle in the Nyquist plot after DNA hybridiza-
tion. The BDD electrodes/electrolyte interface was divided into three physical regions, the
bulk electrolyte solution, the molecular layer (including the DNA and the polymer com-
posite thin film), and its associated double layer, as well as the space-charge layer in the
BDD electrode based on equivalent circuit elements [142]. The schematic equivalent circuit
