Page 187 - Analytical Electrochemistry 2d Ed - Jospeh Wang

172 ELECTROCHEMICAL SENSORS

biologically produce an electrical signal that relates to the concentration of an
analyte. For this purpose, a biospeci®c reagent is either immobilized or retained at a
suitable electrode, which converts the biological recognition event into a quantitative
amperometric or potentiometric response. Such biocomponent±electrode combina-
tions offer new and powerful analytical tools that are applicable to many challenging
problems. A level of sophistication and state-of-the art technology are commonly
employed to produce easy-to-use, compact, and inexpensive devices. Advances in
electrochemical biosensors are progressing in different directions. Two general
categories of electrochemical biosensors may be distinguished, depending on the
nature of the biological recognition process: biocatalytic devices (utilizing enzymes,
cells, or tissues as immobilized biocomponents) and af®nity sensors (based on
antibodies, membrane receptors, or nucleic acids).

6-1.1 Enzyme-Based Electrodes
Enzymes are proteins that catalyze chemical reactions in living systems. Such
catalysts are not only ef®cient but are also extremely selective. Hence, enzymes
combine the recognition and ampli®cation steps, as needed for many sensing
applications.
Enzyme electrodes are based on the coupling of a layer of an enzyme with an
appropriate electrode. Such electrodes combine the speci®city of the enzyme for its
substrate with the analytical power of electrochemical devices. As a result of this
coupling, enzyme electrodes have been shown to be extremely useful for monitoring
a wide variety of substrates of analytical importance in clinical, environmental, and
food samples.

6-1.1.1 Impractical and Theoretical Considerations The operation of an
enzyme electrode is illustrated in Figure 6-1. The immobilized enzyme layer is
chosen to catalyze a reaction, which generates or consumes a detectable species:

Enzyme
S ‡ C ! P ‡ C 0 6-1†

0
where S and C are the substrate and coreactant (cofactor), and P and C are the
corresponding products. The choice of the sensing electrode depends primarily upon
the enzymatic system employed. For example, amperometric probes are highly
suitable when oxidase or dehydrogenase enzymes (generating electrooxidizable
peroxide or NADH species) are employed, pH-glass electrodes are used for
enzymatic pathways that result in a change in pH, while gas (carbon dioxide)
potentiometric devices will be the choice when decarboxylase enzymes are used.
The success of the enzyme electrode depends, in part, on the immobilization of
the enzyme layer. The objective is to provide intimate contact between the enzyme
and the sensing surface while maintaining (and even improving) the enzyme
stability. Several physical and chemical schemes can thus be used to immobilize
the enzyme onto the electrode. The simplest approach is to entrap a solution of the