Page 198 - Analytical Electrochemistry 2d Ed - Jospeh Wang

6-1 ELECTROCHEMICAL BIOSENSORS 183

FIGURE 6-12 The mixed tissue (banana) carbon-paste sensor for dopamine.
PPO ˆ polyphenol oxidase. (Reproduced with permission from reference 36.)

matrix to yield a fast-responding and sensitive dopamine sensor (Figure 6-12). These
biocatalytic electrodes function in a manner similar to that of conventional enzyme
electrodes (i.e., enzymes present in the tissue or cell produce or consume a
detectable species).
Other useful sensors rely on the coupling of microorganisms and electrochemical
transducers. Changes in the respiration activity of the microorganism, induced by the
target analyte, result in decreased surface concentration of electroactive metabolites
(e.g., oxygen), which can be detected by the transducer.

6-1.2 Af®nity Biosensors

Af®nity sensors exploit selective binding of certain biomolecules (e.g., antibodies,
receptors, or polynucleotides) toward speci®c target species. The biomolecular
recognition process is governed primarily by the shape and size of the receptor
pocket and the ligand of interest (the analyte). An associative process of this kind is
governed by thermodynamic considerations (in contrast to the kinetic control
exhibited by biocatalytic systems). The high speci®city and af®nity of these
biochemical binding reactions lead to highly selective and sensitive sensing devices.
As will be shown in the following sections, electrochemical transducers are very
suitable for detecting these molecular recognition events. Such devices rely on
measuring the electrochemical signals resulting from the binding process. Another
type of interaction, the hybridization of two complementary DNA strands, can also
be probed and used for sequence-selective biosensing of DNA. Such detection of
speci®c DNA sequence, although still in its infancy, holds great promise for
diagnosing infectious or genetic diseases.

6-1.2.1 Immunosensors Most reported af®nity biosensors are based on
immunological reactions involving the shape recognition of the antigen (Ag) by
the antibody (Ab) binding site to form the antibody=antigen (AbAg) complex:

Ab ‡ Ag „ AbAg 6-13†