Page 209 - Analytical Electrochemistry 2d Ed - Jospeh Wang

194 ELECTROCHEMICAL SENSORS

The thin-®lm lithographic approach can be used for producing small (micro-
meter) dimension electrodes on silicon wafers (e.g. Figure 4-24) This technology
couples various processes (based on electronic integrated circuit manufacturing),
including vapor deposition of a thin metal ®lm, its coverage with a UV-sensitive
photoresist, photolithographic patterning (using an appropriate mask and UV
radiation), removal of the exposed photopolymerized soluble zone of the photoresist
with a developer, and chemical or plasma etching (78). Three-electrode micro-
systems can thus be readily prepared on a planar silicon wafer. This lithographic
approach represents an attractive route for the production of sensor arrays. For
example Figure 6-21 displays a multiple-analyte sensor array, commonly used for
point-of-care clinical assays of small blood droplets (79).
Microfabricated electrodes can be integrated with other silicon microstructures
(including micropumps, microchannels, mixing chambers, or valves) to produce
complete miniaturized analytical systems (80). Due to their ¯uid manipulation
capability such micromachined systems hold great promise for performing all the
steps of a chemical or biological assay (including the sample preparatory steps,
analytical reactions, separation and detection) on a single microchip platform. Such
on-chip integration of the sample manipulations offers greatly improved ef®ciency
with respect to sample size, reagent=solvent consumption, and response time, and
should ®nd use in numerous analytical applications. Eventually this would enable

FIGURE 6-21 A silicon-based sensor array for monitoring various blood electrolytes,
gases, and metabolites. (Courtesy of i-STAT Co.)