Most chemical sensor applications have been based on a broad background of measurement principles and chemical reactivity developed through research in analytical and other branches of chemistry. Many fundamental ideas, devices, and materials have been adapted from other sciences and technologies (Murray et al., 1989):

• Inexpensive optical fibers from the communications industry have been applied in spectroscopically based direct-reading sensors and near-field microscopy (Betzig et al, 1991).

• Lithographic patterning technology widely used in the manufacture of modern microelectronics has been exploited to fabricate miniaturized electro-chemical devices such as interdigitated array electrodes, microelectrodes, and chemically sensitive field-effect transistors and to form patterned electrodes on surface acoustic wave (SAW) devices. ⁶ (Kepley et al., 1992; Ricco and Martin, 1992; Martin et al., 1990).

• Materials research has resulted in advanced piezoelectric materials that are employed as micro-positioners; these materials have enabled new forms of microscopy, like scanning electrochemical, scanning tunnelling, and atomic force microscopy. The availability of these micropositioners is also critical for chemical sensing on an extremely small dimensional scale (Snyder and White, 1992).

• Ultrasensitive light detection using charge coupled devices, which were developed for astronomy, is under active consideration for detection of laser-induced fluorescence from extremely small populations of molecules.

Research in the above areas involves applying new technologies to analytical chemistry and chemical sensor research. It is intrinsically multi-disciplinary, with contributions from analytical chemists, materials scientists, electrical engineers, and professionals in other fields. At the initial stages of research, the interest is generally focused on exploring and proving the principles by which a new technology can be applied to measure a chemical substance. Open access to specialized equipment and facilities, such as those required for lithographic patterning, can be crucially important to foster interest and progress as applications to specific practical analytical and chemical sensing measurements start to appear.

As previously mentioned, the most important materials-related opportunities to improve direct-reading chemical sensors involve the choice of materials employed to elicit stable selectivity of interaction with the target analyte. Table 6-4 summarizes materials needs for direct-reading chemical sensors. Nearly all the requirements are presented in terms of material functionality rather than material type (e.g., ceramic, polymer, semiconductor) to avoid inappropriate assumptions based on existing solutions.

Limitations of the existing chemistry or technology can become apparent at any stage during sensor development. Table 6-5 summarizes some key materials challenges for various chemical sensor technologies. The most frequent materials limitation for chemical sensors probably relates to the chemistry required to fashion an adequately selective response to the target analyte. Considerable potential exists to enhance the selectivity of direct-reading chemical sensors by the use of novel materials. One strategy to address this is the development of miniaturized high-speed separations-based sensors. These have the potential for avoiding difficulties in molecular selectivity but present major challenges in improving detector sensitivity.

Miniaturized total analytical systems are a relatively new area of research in analytical chemistry, but their development could greatly supplement the capabilities of existing direct reading sensors. The numerous materials issues in designing and fabricating low-cost, miniaturized separations-based analytical systems include:

• coatings and films with improved properties for enhanced sensor performance (e.g., chemical selectivity, chromatographic efficiency, stability under electric field gradients, electrocatalysis efficiency);

• materials that enhance detector sensitivity and increase performance range (fiber optics);