Biology Reference
In-Depth Information
greaterpotentialchangeswithhybridizationareexpected,andthese
changes are enhanced by the use of low ionic strengths. PNA probes
also have the advantage of a PNA-DNA duplex stability that is
approximately independent of ionic strength. A much larger value
for the interfacial potential change of
−
100 mV has been calculated
for a PNA probe density of 2
10
12
cm
−
2
at low ionic strengths
[35], suggesting that PNA probes are likely to provide reliable
potentiometric DNA sensorswith low limitsof detection.
×
6.3.1.4 Equivalent electrical circuit model of functionalized FET
The impedance of a FET with the gate immersed in solution
and potential applied to a reference electrode in solution may be
represented by the equivalent circuit shown in Fig. 6.5. The circuit
consists of the silicon resistance
R
Si
, space-charge capacitance
C
SC
,
oxide capacitance
C
ox
of the FET, and the Randles equivalent circuit
for the double layer, where
Z
W
has been omitted since there are
no redox molecules in solution. In the absence of redox molecules,
R
ct
is large and
Z
imag
can be considered to result from the series
combinationof the three capacitances.
Whenthebiomolecularinteractionhappensatthesolid-solution
interface, it changes the value of
C
dl
. At fixed applied potential,
this would introduce charge redistribution between
C
dl
and
C
ox
,
where the change of potential across
C
ox
depends on the ratio of
the two capacitors,
C
dl
/
C
ox
. The value of this ratio is fixed when the
biomolecular probe is immobilized directly on the gate dielectric or
onthegateelectrodedirectlyontopofthedielectric.Inanextended
gate structure, a sensing pad is electrically connected to the gate
electrode. The area of the sensing pad can be much larger than
Figure 6.5.
Equivalent circuit for a field-effect device with gate immersed
in solution.
