Aspects of recent development of immunosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 240

in electrode or membrane potential is logarithmically proportional to the specifi c ion
activity. The relationship of logarithmical proportionality constitutes the fundamental
principle of all potentiometric transducers such as the ion-selective electrodes (ISE).
The groups of biosensors are characterized as simple in preparation, robust in opera-
tion, and moderately selective in analytical performance [76-81]. Janata fi rst proposed
a potentiometric transducer for immunosensing and named it “immuno-electrode” [78],
using the immuno-electrode to detect Concanavalin A through covalent attachment to
the surface of a PVC membrane deposited on a platinum electrode. The incorporation
of ISEs, pH electrodes or gas-sensing electrodes into potentiometric immunosensors to
improve their assay sensitivity has been extensively investigated by Rechnitz and cow-
orkers, i.e. for immunochemical measurements of digoxin and human IgG [79-80].
D'Orazlo et al. reported the indirect measurements of immunoagents using ion-selec-
tive electrodes [81]. A potentiometric immunosensor based on a molecularly imprinted
polymer was prepared as a detecting element in micro total analysis systems with the
intent of providing easy clinical analysis [82]. Moreover, the ion-selective fi eld-effect
transistor (ISFET) as a semiconductor device is generally constructed by substituting
an ion-sensing membrane for the metal gate of a fi eld-effect transistor (FET) [83]. The
ISFET is able to respond to the surface potential change resulting from the specifi c
immunochemical reaction between the immobilized antibodies and the free antigens.
The pH-sensitive ISFETs, as the most widely used sensor of this type, are fabricated
with a large range of possible insulators (i.e. SiO 2 , Si 3 N 4 , and Al 2 O 3 ) and enzyme
labels (i.e. urease, peroxidase, and glucose oxidase) [84-85]. Nevertheless, only a few
examples of ISFET-based immunosensors could be found in the literature [86-88].
For example, Zayats et al. report the impedance measurements on an ISFET device
that can be used to detect antigen-antibody interactions on the gate surface [88]. In
the meantime, they performed complementary surface plasmon resonance (SPR; see
above) experiments to illustrate that the ISFET impedance measurements and the SPR
reveal comparable sensitivities.
Conductometric transducers , as the oldest electrochemical devices, seem not to enjoy
wide applications due to their poor selectivity. For example, Yagiuda et al. proposed
a conductometric immunosensor for the determination of methamphetamine (MA) in
urine [89]. The decrease in the conductivity between a pair of platinum electrodes might
result from the direct attachment of MA onto the anti-MA antibodies immobilized on
the electrode surface. The system was claimed to be a useful detection technique of MA
in comparison with a gas chromatography-mass spectrometry method.
Capacitance and impedance transducers with high sensitivity are widely employed
for various immunosensing assays [90-102]. The capacitance sensors are essentially
based on the principle that the electrolyte capacitance of an electrode depends on the
thickness and dielectric behavior of the dielectric layer on the electrode surface and
the solid/solution interface. Dijksma et al. designed an immunosensor for the direct
detection of interferon- at the attomolar level by using the AC impedance approach

[90]. The immobilization processes of antibodies (antigens) play an important role
in these immunosensors, and the sensitivity of a capacitive immunosensor increases
with the decreasing thickness of the insulating layer. Shen and coworkers fabricated

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