Page 130 - Analytical Electrochemistry 2d Ed - Jospeh Wang

4-5 WORKING ELECTRODES 115

FIGURE 4-10 The open-pore structure of reticulated vitreous carbon.

4-5.2.2.2 Carbon-Paste Electrodes Carbon-paste electrodes, which use
graphite powder mixed with various water-immiscible organic binders (pasting
liquids), offer an easily renewable and modi®ed surface, low cost, and very low
background current contributions (26±28). A wide choice of pasting liquids is
possible, but practical considerations of low volatility, purity, and economy narrow
the choice to a few. These include Nujol (mineral oil), paraf®n oil, silicone grease,
and bromonaphthalene. The ®rst appears to perform the best. The paste composition
strongly affects the electrode reactivity, with increase in the pasting-liquid content
decreasing electron-transfer rates, as well as the background-current contributions
(28). In the absence of pasting liquid, the dry graphite electrode yields very rapid
electron-transfer rates (approaching those of metallic surfaces). Despite their
growing popularity, the exact behavior of carbon-paste electrodes is not fully
understood. It is possible that some of the electrochemistry observed at these
electrodes involves permeation of the pasting liquid layer by the electroactive
species (i.e., solvent extraction). Carbon paste represents a convenient matrix for
the incorporation of appropriate modifying moieties (29). The modi®er is simply
mixed together with the graphite=binder paste (with no need to devise individualized
attachment schemes for each modi®er). Enzyme-containing carbon pastes have been
used as fast-responding reagentless biosensors (see Chapter 6). A disadvantage of
carbon pastes is the tendency of the organic binder to dissolve in solutions
containing an appreciable fraction of organic solvent.

4-5.2.2.3 Carbon-Fiber Electrodes The growing interest in ultramicroelec-
trodes (Section 4-5.4) has led to widespread use of carbon ®bers in electroanalysis.
Such materials are produced, mainly in connection with the preparation of high-
strength composites, by high-temperature pyrolysis of polymer textiles or via