Page 131 - Analytical Electrochemistry 2d Ed - Jospeh Wang

116 PRACTICAL CONSIDERATIONS

catalytic chemical vapor deposition. Different carbon ®ber microstructures are
available, depending upon the manufacturing process. They can be classi®ed into
three broad categories, namely, low-, medium-, and high-modulus types. The last
type is most suitable for electrochemical studies because of its well-ordered
graphite-like structure and low porosity (30). Improved electron transfer perfor-
mance can be achieved by various electrode pretreatments, particularly ``mild'' and
``strong'' electrochemical activations, or heat treatment (31). Most electroanalytical
applications rely on ®bers of 5±20 mm diameter that provide the desired radial
diffusion. Such ®bers are typically mounted at the tip of a pulled glass capillary with
epoxy adhesive, and are used in cylindrical or disk con®gurations. Precautions
should be taken not to contaminate the carbon surface with the epoxy. The main
advantage of carbon-®ber microelectrodes is their small size (5±30 mm diameter for
commercially available ®bers), which makes, them very attractive for anodic
measurements in various microenvironmentsÐfor example, the detection of neuro-
transmitter release in the extracellular space of the brain. Nanometer-sized carbon
®bers can be prepared by etching the ®ber in ¯ame or under ion beam (e.g., Figure 4-
11). The various electroanalytical applications of carbon ®bers have been reviewed
by Edmonds (32).

FIGURE 4-11 Scanning electron image of a carbon-®ber electrode.