Recent developments in electrochemical immunoassays and immunosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 115

is achieved by the molecular recognition of target analytes (usually the antigens) by anti-
bodies to form a stable complex on the surface of an immunoassay system or an immu-
nosensor [1]. On the other hand, sensitivity depends on several factors including the use
of high affi nity analyte-specifi c antibody(ies), their orientation after being immobilized
on the immunoassay or immunosensor surface and the appropriate detection system for
measuring the analytical signal [1]. Electrochemical detection overcomes problems asso-
ciated with other modes of detection of immunoassays and immunosensors. For example,
the short half-life of radioactive agents, concerns of health hazards, and disposal prob-
lems are frequently raised in radioimmunoassays, while limited sensitivity in the analysis
of colored or turbid samples is achieved in immunoassays coupled with optical detec-
tion. In contrast, electrochemical immunoassays and immunosensors enable fast, simple,
and economical detection that is free of these problems. Furthermore, electrochemis-
try is an interfacial process in which the relevant reactions take place at the electrode-
solution interface, rather than in bulk solution. Therefore, in conjunction with
developments in micro- and nanoelectrochemical sensors, electrochemistry offers an
added bonus of detecting analytes in very small volumes [2]. In this chapter, we will
regard an electrochemical immunoassay as a solid phase system in which an antibody-
antigen reaction takes place but the corresponding electrochemical detection is carried
out elsewhere. However, an electrochemical immunosensor is a stand-alone device with
the immunoreaction and electrochemical detection occurring within the same device.
There are already several excellent reviews on electrochemical immunoassays
available in the literature [2-7]. In this chapter, we will focus our discussions based
on work primarily reported post-2002. Many aspects of earlier immunoassays have
already been reported in reviews quoted above. We will begin here with some basic
background information including antibody structure and the antibody-antigen inter-
action, which are the crucial components in the development of all immunoassays and
immunosensors after adopting a particular immunoassay format. Following this, spe-
cifi c discussions on recent developments in electrochemical immunoassays and immu-
nosensors will be presented so that non-experts can readily gain an understanding and
appreciation of the signifi cance of work in the area.
5.2 THE ANTIBODY-ANTIGEN INTERACTION
Immunoassays and immunosensors are analytical systems that both use the
remarkable specifi city provided by the molecular recognition of an antigen by anti-
bodies. Antibodies are a family of glycoproteins known as immunoglobulins (Ig).
There are generally fi ve distinct classes of glycoproteins (IgA, IgG, IgM, IgD, and
IgE) with IgG being the most abundant class (approximately 70%) and most often
used in immunoanalytical techniques [8]. As depicted in Fig. 5.1, IgG is a “Y”-shaped
molecule based upon two distinct types of polypeptide chains. The molecular weight
of the smaller (light) chain is approximately 25 000 Da, while that of the larger (heavy)
chain is approximately 50 000 Da. In each IgG molecule, there are two light and two
heavy chains held together by disulfi de linkages. Both heavy and light chains are