Biology Reference
In-Depth Information
The maximum percentage change of charge transfer resistance
upon hybridization with fully complementary target oligonu-
cleotideswasobtainedwithsamplespreparedbyco-immobilization
of oligonucleotide probes and mercaptohexanol with a DNA mole
fraction of 20%. This corresponds to a mean probe surface density
of 5.4
10
12
cm
−
2
.
×
6.4.1
PNA-Based Sensing
The electrostatic barrier to the negatively charged redox markers
changes upon DNA hybridization, causing the EIS signal. The use
of PNA probes yields much larger EIS signals upon hybridization.
Since PNA is uncharged, the potential barrier before hybridization
isnegligibleresultinginaverylowchargetransferresistancevalue;
upon hybridization with the charged DNA target, the potential
barrier is strongly felt resulting in a particularly large variation of
R
ct
.
Optimization of PNA surface density resulted in a massive
enhancement of the fractional change in
R
ct
upon hybridization,
without the use of additional biochemical amplification steps [38].
A fractional change 100-fold larger than previously reported has
been achieved. Another relevant aspect is that the optimization
of PNA surface density in a mixed PNA/MCH SAM results in a
small initial
R
ct
, controlled by the mercaptohexanol regions of
the SAM. For a given electrode area and overpotential, a smaller
R
ct
gives a greater current density. For a given sensitivity of the
detection electronics, higher current densities enable a reduction
of the minimum sensing electrode area and therefore an improved
detectionlimit.Adetectionlimitof25fmoltargetwasdemonstrated
by Keighley
et al.
[38]. This is likely to be further improved by
reduction of the electrode area and sample volume. For example,
reducing the electrode diameter from 2 mm to 100
μ
m, a 400-fold
decrease in area, would increase the initial
R
ct
to around 4 M
.A
10 mV AC overpotential would result in an approximately 2.5 nA
AC current, feasibly measured in a portable detection system. This
would allow the sample volume to be scaled to 3 nl, reducing the
detection limit to 3 amol. This shows electrochemical impedance
