Electrochemical Detection - Waterborne Pathogens: Detection Methods and Applications - page 153

of particle movement depends upon the relative polarizability of the par-
ticle and the suspending medium. Electrorotation utilizes a set of electrodes,
around which the electric field is cycled, to create a central area in which
particles undergo rotation. Each particle type exhibits a near-unique profile
of particle rotation rate against applied electric field for given environmen-
tal conditions.
The technique offers, potentially, selectivity according to the size, dielec-
tric properties, and surface electrical charge of the particle. If we consider
a biological cell at rest and without electric field, its dielectric properties
and surface charge depend on its viability and maybe some other physi-
ological parameters. These cytoplasm and surface charges are counterbal-
anced by ions of the surrounding medium. 27 When applying the electric
field, the spatial distributions of charges in the cytoplasm, in the surround-
ing medium and on the membrane surface, are modified according to the
dielectric properties of the system. This reorganization induces a dipole on
the particle, which interacts with the nonuniform electrical field. Although
it is quite simple to characterize the dielectric properties of the surrounding
medium, the characterization of the dielectric properties (i.e. conductivity
and permittivity) of the particle is rather complex. Literature shows very
different values depending on the experimental arrangement. A key param-
eter for colloidal particles is the surface conductance, K s , which appears in
the following equation giving the conductivity of the particle ( Eqn (6.5) ).
σ p = σ v +2 K s / r (6.5)
where σ v is the conductivity of the bulk and r is the radius of the particle.
The surface conductivity cannot be ignored for a particle radius smaller
than 10 µm. Three methods have been reported to measure the surface con-
ductance. The first method measures the frequency of the polarization for
which the CM factor is nul. 28 The accuracy depends on the capability to
distinguish between dielectrophoretic and Brownian motion. The second
method was proposed by Morganti 29 who used electrorotation to extract
the surface conductance from the electrorotation rate spectrum. The third
technique was introduced very recently by Honegger, which measures the
particle velocity in pure dielectrophoretic conditions. 30
The techniques used for the study and exploitation of dielectrophoresis
have made much progress in the recent years. 31 There have been significant
advances in electrode design, the construction of microfluidic chambers,
and methods to monitor the collection of particles at electrodes depend-
ing on their dielectric properties. In total, more than 2000 scientific papers