Recent developments in electrochemical immunoassays and immunosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 135

differ in both magnitude and phase, the resultant impedance, Z , is usually represented by
a vector with real ( Z Re ) and imaginary ( Z Im ) components as a function of frequency ( ω ):

Z( ω ) jZ

(3)

Re Im

Very often, the electrode-solution interface can be represented by an equivalent circuit,
as shown in Fig. 5.10, where R S denotes the ohmic resistance of the electrolyte solu-
tion, C dl , the double layer capacitance, R ct the charge (or electron) transfer resistance
that exists if a redox probe is present in the electrolyte solution, and Z W the Warburg
impedance arising from the diffusion of redox probe ions from the bulk electrolyte to
the electrode interface. Note that both R S and Z W represent bulk properties and are not
expected to be affected by an immunocomplex structure on an electrode surface. On
the other hand, C dl and R ct depend on the dielectric and insulating properties of the
electrode-electrolyte solution interface. For example, for an electrode surface immo-
bilized with an immunocomplex, the double layer capacitance would consist of a con-
stant capacitance of the bare electrode ( C bare ) and a variable capacitance arising from
the immunocomplex structure ( C immun ), expressed as in Eq. (4).

1 1 1

(4)

CC C

dl bare immun

As the immunocomplex structure is generally electroinactive, its coverage on the
electrode surface will decrease the double layer capacitance and retard the interfa-
cial electron transfer kinetics of a redox probe present in the electrolyte solution. In
this case, R ct can be expressed as the sum of the electron transfer resistance of the
bare electrode ( R bare ) and that of the electrode immobilized with an immunocomplex
( R immun ):

R ct R bare R immun (5)

There are several ways to present the Faradaic impedance data obtained at an
electrode immobilized with an immunocomplex in the presence of a redox probe.
For example, Z Im is plotted vs Z Re as a function of decreasing frequency to obtain a

C dl

R s

R ct

Z w

FIGURE 5.10 An equivalent circuit representing the interfacial features of an electrochemical immuno-
sensor in the presence of a redox probe.