Biology Reference
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assuming a doubling of the membrane charge density. Proto-
nation/deprotonation of surface sites significantly reduces the
magnitude of the variation of the surface potential with respect
to the bulk electrolyte by effectively pinning the insulator surface
potential. At 10 mM salt concentration, the difference in potential
upon hybridization saturates with increasing probe density, so
increasing the probe density above 1
10 12 cm 2 is not expected
to further increase the shift upon hybridization. At these high probe
densities, a
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19 mV shift is calculated for full hybridization on an
uncharged surface, 6 times greater than the
3 mV change with
an amphoteric Al 2 O 3 surface. At salt concentrations of 10 mM or
greater, where the thickness of the charged layer is significantly
greater than the Debye length, for uncharged surfaces the Donnan
potential was shown to give a good approximation of the double-
layer potential.
Finite element modeling of DNA functionalized electrodes was
applied to calculate the interfacial potential, and used to identify
conditions for maximum potential change with target hybridization
[35]. Using different models such as the Donnan potential model
[34] and numerical solution of the Poisson-Boltzmann equation
for a three-dimensional model, the authors estimate a maximum
potential variation of 17 mV for 100% hybridization e ciency at
theoptimizedDNAprobedensityof3 × 10 12 cm 2 evenatlowionic
strength.
Even though larger shifts have been reported in the literature,
the simulations give a good insight on the variation of the signal
with probe density and ionic strength. The signal decreases rapidly
at probe densities lower than 1
10 12 cm 2 , while increasing the
probe density above the optimal value has little effect due to the
reduction of hybridization e ciency. Decreasing the ionic strength
on the other hand, has little effect on the signal at high probe
densities butincreases the signalat low probe densities.
The value of the interfacial potential with ssDNA is significantly
larger than the change in potential resulting from hybridization.
In addition, decreasing the ionic strength significantly increases
the potential but not the variation in potential upon hybridization.
If uncharged PNA probes are used instead of DNA probes, the
interfacial potential before hybridization is expected to be approx-
imately zero, independent of ionic strength. Therefore, significantly
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