Page 143 - Analytical Electrochemistry 2d Ed - Jospeh Wang

128 PRACTICAL CONSIDERATIONS

peaks re¯ect the passage of the carbonate anions [which are capable of penetrating
into the ®lm, equation (4-12)] over the surface. These and other analytical
opportunities have been reviewed by Ivaska (75), Wallace (76), and Bidan (72).
The widespread use of polypyrrole is attributed to its electropolymerization from
aqueous media at neutral pH (which allows the entrapment of a wide range of
dopants). Other ®lms are more limited in this regard. For example, thiophene is
soluble only in organic solvents, and aniline in acidic media. The electropolymer-
ization growth can also lead to nonconducting, self-limiting ®lms, which are often
used as permselective=protective layers (Section 4-5.3.5) or for the physical
entrapment of biomolecules (Chapter 6).

4-5.4 Microelectrodes
Miniaturization is a growing trend in the ®eld of analytical chemistry. The
miniaturization of working electrodes not only has obvious practical advantages,
but also opens some fundamentally new possibilities (77±79). The term ``micro-
electrode'' is reserved here for electrodes with at least one dimension not greater
than 25 mm.
Such dimensions offer obvious analytical advantages, including the exploration
of microscopic domains, measurements of local concentration pro®les, detection in
micro¯ow systems or within very narrow electrophoresis capillaries, and analysis of
very small (microliter) sample volumes. Particularly fascinating are recent studies
aimed at time-resolved probing of dynamic processes (e.g., secretion of chemical
messengers) in single cells (80), the in-vivo monitoring of neurochemical events
(e.g., stimulated dopamine release), and the use of nanoscopic electrode tips for
single molecule detection (81) or high-resolution spatial characterization of surfaces
(see Section 2-3). Figure 4-23 illustrates the use of a carbon-®ber microelectrode for
measuring the vesicular release of dopamine following cellular stimulation.

FIGURE 4-23 Experimental setup for monitoring dopamine release by exocytosis, from a
cell body. The microelectrode and glass capillary (containing the chemical stimulant) are
micromanipulated up to the cell body. (Reproduced with permission from reference 82.)