Page 196 - Analytical Electrochemistry 2d Ed - Jospeh Wang

6-1 ELECTROCHEMICAL BIOSENSORS 181

FIGURE 6-10 Reagentless ethanol bioelectrode.

NADH. Immobilized redox mediators, such as the phenoxazine Meldola Blue or
phenothiazine compounds, have been particularly useful for this purpose (20) (see
also Figure 4-12). Such mediation should be useful for many other dehydrogenase-
based biosensors. High sensitivity and speed are indicated from the ¯ow-injection
response of Figure 3-21. The challenges of NADH detection and the development of
dehydrogenase biosensors have been reviewed (21). Alcohol biosensing can also be
accomplished in the presence of alcohol oxidase, based on measurements of the
liberated peroxide product.

6-1.1.2.3 Urea Electrodes The physiologically important substrate urea can be
sensed based on the following urease-catalyzed reaction:
urease
‡
NH CONH ‡ 2H O ‡ H ‡ ! 2NH ‡ HCO 6-12†
2 2 2 4 3
The sensor is an ammonium ion-selective electrode surrounded by a gel impregnated
with the enzyme urease (Figure 6-11) (22). The generated ammonium ions are
detected after 30±60 s to reach a steady-state potential. Alternately, the changes in
the proton concentration can be probed with glass pH or other pH-sensitive
electrodes. As expected for potentiometric probes, the potential is a linear function
of the logarithm of the urea concentration in the sample solution.

Enzyme electrodes for other substrates of analytical signi®cance have been
developed. Representative examples are listed in Table 6-1. Further advances in
enzyme technology, and particularly the isolation of new and more stable enzymes,
should enhance the development of new biocatalytic sensors. New opportunities
(particularly assays of new environments or monitoring of hydrophobic analytes)
derive from the ®nding that enzymes can maintain their biocatalytic activity in
organic solvents (31,32).

6.1.1.2.4 Toxin (Enzyme Inhibition) Biosensors Enzyme affectors (inhibitors
and activators) that in¯uence the rate of biocatalytic reactions can also be measured.
Sensing probes for organophosphate and carbamate pesticides, for the respiratory