Biology Reference
In-Depth Information
The relationship
V
G
=−
Q
DNA
/
C
ox
,where
Q
DNA
is the
charge of the DNA SAM and
C
ox
is the gate insulator capacitance
has been proposed in the literature [32]. This relationship is of
the same form as that of a shift in flat-band potential due to a
fixed oxide charge
Q
f
located at the Si-SiO
2
interface in a MOSFET.
However,theDNAchargeisinsteadlocatedatthemetal-solutionor
insulator-solutioninterface.Therefore,toreturnthesemiconductor
to the state it would be in the absence of the DNA charge requires
charging the double-layer capacitance. Approximating the double-
layer capacitance as constant, the relationship should be
=
V
G
−
/
C
dl
.
Other authors equate
V
G
to the change in electrochemical
double-layer surface potential resulting from the change in surface
charge, calculated using the Grahame equation [33]. The solution
and semiconductor are coupled by the electric field in the oxide,
E
SiO
2
.If
V
G
is adjusted to operate the FET at constant current,
E
SiO
2
and the potential drop across the semiconductor and oxide remain
constant, and the only changes in the system occur in the double
layer. Therefore,
V
G
is equal to the change in potential across
the double layer, and no consideration of semiconductor physics is
necessary [7].
If the biomolecular probe is immobilized onto a metal electrode,
such as the metal gate of a MOSFET, a contact can be made to this
electrode and the open-circuit potential
E
OC
measured against the
reference electrode. Since
V
G
=
ϕ
solid state
−
E
OC
,where
ϕ
solid state
is
the constant potential difference between the FET source and the
solid-solution interface,
E
OC
corresponds to the shift of the
I
-
V
characteristics from those measured by direct connection between
the gate and source or back contact. Therefore, the FET is simply
being used to measure the change in open-circuit potential, taking
advantage of its high-input impedance, low-output impedance, and
small size.
A one-dimensional model for electrolyte-insulator-metal-
oxide-semiconductor and electrolyte-insulator-semiconductor
structures modified with a charged membrane has been presented
[34]. It was shown that the largest sensitivity occurs at low
electrolyte concentrations, and that the signal from hybridization
is expected to be smaller than that from probe immobilization,
Q
DNA
