Detection of charged macromolecules by means of fi eld-effect devices (FEDs): possibilities and limitations - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 194

case of DNA brushes (polyelectrolyte brushes consist of charged macromolecules
densely end-grafted to the surface, see Fig. 7.3b), the capacitance of the brush (i.e. the
diffuse layer capacitance inside the DNA layer at the underlying layer-brush interface)
depends among other things on the DNA coverage, the interfacial potential as well
as on the ionic strength inside the brush, which is much higher than that of the bulk
solution [55].
As can be seen in Fig. 7.4, the C-V curve of the functionalized EIS structure is also
shifted along the voltage axis with respect to the C-V curve of the original (unmodi-
fi ed) EIS structure. The direction of the shift of the C-V curves along the voltage axis
in Fig. 7.4 corresponds to an additional negative charging of the gate-insulator surface.
This indicates that the molecular layer may also induce an interfacial potential change
(change in fl at-band voltage V fb ) at the electrolyte side and/or gate-insulator side of

the molecular layer, in series to the applied gate voltage V G .
Such a simultaneous shift of the C-V curve along both the capacitance and volt-
age axis makes the C-V measurements more interesting and informative than static
DC measurements with the transistor structure. If these two effects are independent,
the capacitance change and fl at-band voltage change induced by adsorption or bind-
ing of charged macromolecules can be obtained from one and the same measurement.
However, it should be noted that in the case of the presence of charges in the gate insu-
lator and surface and interface states (see Eq. (2) for the fl at-band voltage), changes in
the fl at-band voltage can also be caused due to the series capacitance, thus coinciding
with the effect of modulation of the fl at-band voltage induced by the molecular charge.
In the following sections, we will discuss the origin of possible mechanisms of fl at-
band voltage changes induced by charged macromolecules, mainly focusing on a direct
electrostatic detection of charged macromolecules by their intrinsic molecular charge,
and the mechanism that utilizes the DNA hybridization-induced charge redistribution
within the intermolecular spaces.

∆

7.3 DIRECT ELECTROSTATIC DNA DETECTION BY ITS
INTRINSIC MOLECULAR CHARGE

Since FEDs are surface-charge measuring devices detecting the charge in a capacitive
way, they are principally able to measure the charge of adsorbed macromolecules such
as DNA or the charge change due to a hybridization event. The electric fi eld in the gate
insulator depends, among other parameters, on the net surface charge at the electrolyte-
insulator interface. Any charge changes at the insulator surface will result in an equal
change in the charge density of opposite sign in the semiconductor space-charge region.
Since DNA molecules are polyanions with negative charges at their phosphate back-
bone, it can be expected that during the event of hybridization of ssDNA molecules
with their complementary strands (cDNA), the charge associated with the target mole-
cule effectively changes the charge applied to the gate and thus modulates the fl at-band
voltage and capacitance of the EIS sensor as well as the threshold voltage and the drain
current of the FED.