Page 210 - Analytical Electrochemistry 2d Ed - Jospeh Wang

6-3 SOLID-STATE DEVICES 195

the laboratory to be transported to the sample. A range of assays have already been
adapted to the microchip format. These possibilities have led to the concepts of
mTAS (micro-total analytical systems) and ``Lab-on-a-Chip'', which are hot topics of
analytical chemistry. The extremely small dimensions of electrochemical detectors,
coupled with their remarkable sensitivity and compatibility with microfabrication
technologies, make them very suitable for adaptation for ``Lab-on-a-Chip'' analytical
microsystems. Microsystems relying on electrosmotic ¯ow obviate the needs for
pumps or valves but require proper attention to the decoupling of the detector
potential from the high voltage used to control the micro¯uidics. Such precise ¯uid
control is accomplished by regulating the applied potentials at the terminus of each
channel of the microchip (81). The channel networks of these chips include mixing
tees and cross intersections for mixing reagents and injecting samples with high
reproducibility (e.g., Figure 6-22). Particularly powerful is the use of electrochemical
detectors for monitoring on-chip electric-®eld-driven separations (see Section 3-6).
Large-scale sensor fabrication can be accomplished not only by lithographic
techniques but also using modern screen-printing (thick-®lm) processes (82,83). The
screen-printing technology relies on printing patterns of conductors and insulators
onto the surface of planar (plastic or ceramic) substrates. Various conducting and
insulating ink materials are available for this task. The screen-printing process
involves several steps (as illustrated in Figure 6-23 for the fabrication of carbon
electrodes), including placement of the ink onto a patterned screen or stencil,
followed by forcing it through the screen with the aid of a squeegee, and
drying=curing the printed patterns. Such a process yields mass-producible (uniform

FIGURE 6-22 Schematic of a microchip system for enzymatic assays. The channels
terminate at reservoirs containing the indicated solutions. (Reproduced with permission
from reference 81.)