Page 193 - Analytical Electrochemistry 2d Ed - Jospeh Wang

178 ELECTROCHEMICAL SENSORS

FIGURE 6-5 ``Second-generation'' enzyme electrode: sequence of events that occur in a
mediated system. (Reproduced with permission from reference 12.)

the mediator (Figure 6-7). The screen-printing technology used for mass-scale
production of this and similar biosensors is discussed in Section 6-3. Other classes
of promising mediators for glucose oxidase are quinone derivatives, ruthenium
complexes, ferricyanide, phenothiazine compounds, and organic conducting salts
(particularly tetrathiafulvalene-tetracyanoquinodimethane, TTF-TCNQ). An elegant
nondiffusional route for establishing electrical communication between GOx and the
electrode is to ``wire'' the enzyme to the surface with a long polymer having a dense
array of electron relays [e.g., osmium(bipyridyl) bound to poly(vinylpyridine);
Figure 6-8 (14)]. Such a polymeric chain is ¯exible enough to fold along the
enzyme structure. The resulting three-dimensional redox-polymer/enzyme network
offers high current outputs and stabilizes the mediator to the surface. An even more
elegant possibility is chemical modi®cation of the enzyme with the redox-active
mediator (15). Glucose electrodes of extremely ef®cient electrical communication
with the electrode can be generated by the enzyme reconstitution process (16). For
this purpose, the ¯avin active center of GOx is removed to allow positioning of the
electron-mediating ferrocene unit prior to the reconstitution of the enzyme (Figure
6-9). The challenges in establishing electrical communication between redox
enzymes and electrode surfaces have been reviewed (2,17). Ultimately, these and
similar developments would lead to invasive (needle-type) and noninvasive devices
for continuous real-time monitoring of glucose (18). Such probes would offer a tight
control of diabetes, in connection with an alarm detecting hypo- or hyperglycemia or
a future closed-loop insulin-releasing system (i.e., arti®cial pancreas).

6-1.1.2.2 Ethanol Electrodes The reliable sensing of ethanol is of great
signi®cance in various disciplines. The enzymatic reaction of ethanol with the
‡
cofactor nicotinamide±adenine dinucleotide NAD †, in the presence of alcohol
dehydrogenase (ADH)
ADH
C H OH ‡ NAD ‡ ! C H O ‡ NADH 6-10†
2 5 2 5