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of the electrode's material depends on the phenomenon that is monitored.
If it is the ionic transport that is under scrutiny, then any metal electrode can
be used. On the contrary, if one wants to monitor a specific electrochemical
reaction at the interface, then a careful choice has to be made from the previ-
ous techniques (i.e. potentiometry, voltammetry, and coulometry).
However, FRA measurement is viable only for a stable and reversible
system in equilibrium, as the system's linearity and stability must be ensured.
Due to the relatively long data-acquisition time of single-sine FRA method,
validity of the method can be questioned for nonstationary systems. In an
effort to reduce the measurement time, investigators have proposed meth-
ods in which no frequency scanning is employed. These methods use a
broadband perturbing signal (e.g. white noise) and a fast Fourier transform
algorithm to compute the impedance.
19,20
This approach has been refined
over years, which now allows the exploration of complex problems encoun-
tered during studies on biosensors and other electrochemical systems.
21,22
The working range for an impedance analyzer is between a few micro-
hertz for some equipment to hundreds of megahertz for some others. Polar-
ization range is of the order of tens of volts. The accuracy of impedance
measurement is typically around 0.1% and impedance dynamic is between
a few milliohms to hundreds of megaohms. An advantage of the electro-
chemical impedance method compared with voltammetry or potentiome-
try is that labels are no longer necessary, thus simplifying sensor preparation.
However, the detection limits of electrochemical impedance spectroscopy
are still poor compared with traditional methods.
6.6. DIELECTROPHORESIS
Dielectrophoresis is an electrokinetic phenomenon acting on polar-
izable particles in an inhomogeneous electric field. In 1971, H.A. Pohl
23
manipulated yeasts (
Saccharomyces cerevisiae
) thanks to a nonuniform electric
field induced by the alternative current polarization of two metallic elec-
trodes. A few years before, he established the relationship between the force
acting on the yeasts and the frequency of the polarization, the dielectric
properties of the particle and surrounding medium, and the amplitude and
gradient of the electric field (
Eqn (6.3)
).
F
dep
=2
πr
3
ε
m
CM
(
ω
) ∇
E
2
(6.3)
CM(
ω
)=(
ε
p
*
−
ε
m
*
)/(
ε
p
*
+2
ε
m
*
)
(6.4)
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