Aspects of recent development of immunosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 234

electrostatic interactions, van der Waals force, and hydrogen bonding. The antigen-
antibody reaction is reversible and, owing to the relative weakness of the forces holding
the antibody and antigen together, the complex formed would dissociate in dependence
upon the reaction environment (e.g. pH and ion strength). The strength of the bind-
ing of an antibody to an antigen could be characterized by its affi nity constant ( K ),
which is of the order between 5

10 12 L mol 1 . The high affi nity and
specifi city of this antigen-antibody binding reaction defi nes the unique immunosensor
characteristics.
The general immunosensor design consists of three individual parts in close contact:
a biological recognition element, a physicochemical transducer, and an electronic part.
Antibodies or antibody derivatives (antigens or haptens) usually serve as the biologi-
cal recognition elements, which are either integrated within or intimately associated
with a physicochemical transducer. This recognition reaction defi nes the high selectiv-
ity and sensitivity of the transducer device. The electronic part is used to amplify and
digitalize the physicochemical output signal from the transducer devices such as elec-
trochemical (potentiometric, conductometric, capacitative, impedance, amperometric),
optical (fl uorescence, luminescence, refractive index), and microgravimetric devices.
Gizeli and Lowe [3] suggested that an ideal immunosensor design should possess the
following specifi cations: the ability to detect and quantify the antigens (antibodies), the
capacity to transform the binding event without externally added reagents, the ability
to repeat the measurement on the same device, and the capacity to detect the specifi c
binding of the antigens (antibodies) in real samples. All of these specifi cations have
been the main issues to pursue in developing immunosensors applied in various fi elds.

10 4 and 1

9.1.2 Main performance characteristics of immunosensors in
clinical analysis
As an important branch of immunoassay techniques, immunosensors possess all essen-
tial performance characteristics of immunoassays. They show high selectivity, sensitiv-
ity, reversibility and effi cient reagent usage. At the same time, the immunosensors are
generally simple to operate, and easy to realize automation, digitization, and miniatur-
ization. They may bypass some inherent problems of traditional analytical methods.
Therefore, immunosensors have been the subject of expanding interest in the immuno-
chemical studies with enormous potential in clinical diagnosis [1-2, 4], environmental
analysis [5-6], and biological process monitoring [7]. As for the medical diagnosis of
some diseases, herein considerable efforts have been devoted to the development of
precise, rapid, sensitive, and selective immunosensors by measurement of the markers
or pathogenic microorganisms responsible for the diseases, such as proteins, enzymes,
viruses, bacteria, and hormones [1, 8-9]. Chagas' disease, an American trypanosomia-
sis caused by the hemofl agellate Trypanosoma cruzi , is an example. An amperomet-
ric immunosensor has been recently proposed to probe the presence of antibodies
against T. cruzi in blood donors, and to follow the antibody decay during treatment
of chagasic patients with the available drugs [10]. Yuan et al. reported a novel poten-
tiometric immunosensor for detection of hepatitis B surface antigen by immobilizing