Biology Reference
In-Depth Information
that of the transistor. In this configuration, the ratio
C
dl
/
C
ox
can
be largely improved by increasing the double-layer area, offering a
larger voltageshift for the measurement [4].
6.3.2
Differential OCP Measurement
Taking advantage of its high-input impedance, low-output
impedance, and miniaturization, the metal-gate FET is being used
to measure variations in the open-circuit potential that occur
upon interaction. Recently, direct OCP measurements using an
instrumentationamplifierhavebeenperformedresultinginreliable
detection of protein interactions [36].
Theopen-circuitpotentialwasmeasuredinrealtimebyusingan
ultra-lowinputbiascurrentinstrumentationamplifier,providingan
accurate differential measurement of voltage. The very high input
impedance and very low input bias current minimize the effect of
the measurement on the OCP. The gain of the amplifier was set to 1
inordertoeliminateinstabilityeffects,temperaturedrift,etc.,ofthe
external resistor needed to set a higher amplifier gain.
The functionalized gold electrode and the reference electrode
were connected to the amplifier differential inputs (see Fig. 3.6).
The amplifier output voltage, equal to the open-circuit potential for
+9V
INA116
G=1
V
0
Au
-9V
Figure 6.6.
Schematic instrumentation amplifier set-up for the open
circuit potential measurement (RE, reference electrode).
