Page 139 - Analytical Electrochemistry 2d Ed - Jospeh Wang

124 PRACTICAL CONSIDERATIONS

the protection from foulants present in the serum sample. Such size-exclusion
(sieving) properties are attributed to the morphology of electropolymerized ®lms
(Figure 4-19). Similarly, Na®on-coated microelectrodes are often used for in-vivo
monitoring of cationic neurotransmitters (e.g., dopamine) in the presence of
otherwise interfering ascorbic acid (64). Such anionic interference is excluded
from the surface through electrostatic repulsion with the negatively charged
sulfonated groups (Figure 4-20). Examples of these and other discriminative ®lms
are given in Table 4-2.

4-5.3.6 Conducting Polymers Electronically conducting polymers (such as
polypyrrole, polythiophene, and polyaniline) have attracted considerable attention
due to their ability to switch reversibly between the positively charged conductive
state and a neutral, essentially insulating, form and to incorporate and expel anionic
species (from and to the surrounding solution), upon oxidation or reduction:
0

P ‡ A ! P A ‡ e 4-12†

‡

where P and A represent the polymer and the ``dopant'' anion, respectively. The
latter serves to maintain the electrical neutrality, that is, to counterbalance the
positive charge of the polymer backbone. The electrical conductivity of these ®lms,
which originates from the electronic structure of their polymeric backbone (i.e.,
electron hopping involving the delocalized p electrons), thus varies with the applied
potential. The structure of common conducting polymers, and their conductivity
ranges (from the undoped to doped states) are displayed in Figure 4-21. The redox

FIGURE 4-18 Permselective coatings: ¯ow injection response of a poly(1,2-diaminoben-
zene)-coated electrode to the following: a, hydrogen peroxide (1 mM); b, ascorbic acid (1 mM);
c, uric acid (1 mM); d, L-cysteine (1 mM); and e, control human serum. (Reproduced with
permission from reference 63.)