Biology Reference
In-Depth Information
unit gain, was recorded using a potentiostat. The amplifier output
reference terminal was grounded to ensure good common-mode
rejection.
The simplest on-chip circuit that can be conceived is for differ-
ential OCP measurement, although complex electrostatic discharge
protection needs to beincorporated.
6.4 Electrochemical Impedance Spectroscopy
Many electrochemical biosensors rely on the reduction and oxi-
dation (redox) processes that occur at a functionalized electrode.
These sensors are engineered so that a biomolecular interaction
induces a change in the redox current. These amperometric
techniques rely on the measurement of output currents upon a
voltage-driven electrochemical event. The measurement of elec-
trochemical currents requires the use of a potentiostat with a
three-electrode cell arrangement since a current flowing through
the reference electrode creates an electrochemical reaction at its
surface and, consequently, alters the applied potential. The voltage
is applied through a reference electrode connected to a high-
impedance input of the potentiostat so that no current flows
through it, and the current is measured with the help of a counter
electrode.
Many standard electrochemical techniques can be used, depend-
ing on the biological system to be studied. In the presence of redox
markers in solution, modification of the electrode resulting from
biomolecularinteractionaffectstheimpedanceofthesystem,which
can be measured by using electrochemical impedance spectroscopy
(EIS). EIS is a very promising technique, in particular for the
detection of DNA hybridization.
In EIS, the impedance of the system is measured by applying a
smallacsignalandbythefrequencyscanned(typicallybetween10-
100kHzand1Hzorless).Stableimpedancespectracanbeobtained
withelectrically charged redox markers insolution. Thedata can be
fittedwithanequivalentelectricalcircuit,wherethemostimportant
components are the charge transfer resistance
R
ct
and the double
layer/biolayer capacitance
C
dl
.
