INTRODUCTION TO CHEMICAL SENSOR TECHNOLOGIES 9 of critical components. The concept of interposing membrane layers between the solution and the elec- trode also provided the basis for the first biosensor, invented by Clark and Lyons (1962). The first resistive gas sensor using an oxide semiconductor was reported in 1962 by Seiyama et al. and Taguchi. As is well known, the oxygen sensor which uses a stabilized zirconia electrolyte has been most important for the control of automobile emissions. Probably the first attempt to apply an electro- chemical cell with a solid electrolyte to the gas phase was made in 1963 by Goto and St. Pierre. A thin- film conductometric sensor was first introduced in the early 1970s. Perhaps the most important promise of these thin-film sensors for the development of viable chemical microsystems is their compatibility with processes for fabricating standard integrated circuits. The early 1970s also saw the development of the ion-selective field-effect transistor (ISFET). An ISFET is simply a metal­oxide­semiconductor field- effect transwistor (MOSFET) without a gate. The oxide layer of the FET is replaced by an insulating, chemically sensitive membrane. Charges from sensitive chemicals accumulate on top of this insulating membrane and are amplified through the operation of the FET. Modifications and hybrids of the ChemFET and the ISFET (such as the surface-accessible FET, or SAFET, and the suspended-gate FET, or SGFET) were also introduced into research in the 1970s. Humidity sensors have also been used popularly for various aspects of domestic life ranging from air conditioning to the protection of electronic instruments from condensation. Unlike classic hydrometers, the history of such humidity sensors is rather short. In 1976, Matsusita Electric Industrial Co. commercialized electronic ovens equipped with ceramic humidity sensors for automated cooking. Notable among recent achievements in the field of chemical sensors are the contributions of Persaud and Dodd (1992), Toko et al. (1992), and Vlasov et al. (1995), which resulted in the creation of the "electronic nose," the "taste" sensor, and the "electronic tongue" (Vlasov et al. 2005). Sensor technol- ogy has been further advanced by new discoveries in one-dimensional materials, conductive polymers, nanocomposites, and many other sensing materials. We hope that these advances will result in the com- mercialization of a variety of chemical sensors in coming years. 3. MOTIVATIONS FOR DESIGN OF CHEMICAL SENSORS Many industries have a need to use and analyze process gases (see Table 1.5). The breadth of such uses can be illustrated by listing the key market sectors that either produce, use, or analyze process gases, namely (Cleaver 2001): · · · · · · · Chemical and petrochemical Environmental, including both ambient air and stack emissions monitoring Scientific and engineering research organizations, including universities and national laboratories Medical institutions, including hospitals Food and drinks processing, which use gases such as nitrogen to enhance the shelf life of products by reducing oxidation and use carbon dioxide to carbonate soft drinks and alcoholic beverages Microelectronics, including semiconductor manufacturing and telecommunications Agriculture