ION-SENSITIVE FIELD-EFFECT TRANSISTOR (ISFET)­BASED CHEMICAL SENSORS 263 20. ISFET APPLICATIONS AND MARKET ISFET pH sensors/meters are marketed by several companies, but ISFET-based chemical and biosensors have still to carve their niche in the marketplace. Many devices are still in the research laboratory stage. ISFET-based chemical sensors, however, have great potential in diversified applications, a few of which are mentioned below. 20.1. WATER ANALYSIS AND ENVIRONMENTAL MONITORING Superficial and groundwaters often contain heavy-metal ions such as Hg 2+ , Cu 2+ , Pb 2+ , Cd 2+ , etc., as toxic species. To date, quality control of water is accomplished by reliable but very expensive analytical measurements carried out in the laboratory. High-reliability, sensitive, and low-cost online monitoring systems must be developed. Potentiometric chemical sensors based on ISFET devices or all-solid-state microelectrodes will be useful components in these systems. Acid rain is a global environmental issue. The important chemical species in acid rain are nitrates and sulfates, because the acidification is caused by nitrogen oxides and sulfur oxides exhausted from mobile and stationary sources. Nitrate and sulfate ISFETs are useful for this field. Silver(I)-selective CHEMFETs can have interesting applications in fields such as environmental monitoring, where they can be used in microflow titrations for salinity measurements. Bacterial growth lowers the pH of water, providing a useful indication of risk. 20.2. DIAGNOSTIC AND HEALTH-CARE APPLICATIONS The ISFET-based biosensors have the proper configuration for in situ monitoring of various chemical processes such as in vivo medical diagnosis and many other fields to which conventional medical analyti- cal apparatus is diffi cult to apply, often requiring tedious, cumbersome procedures (Lee et al. 2000). Diabetes milletus is a gigantic killer disease, and an ISFET glucose sensor can provide diagnostic information. The serum uric acid concentration is an important index for clinical diagnosis of gout, leukemia toxemia of pregnancy, and severe renal impairment. Urinary stone formation has evolved to a widespread disease during recent years. Urinary stones grow from little crystals, mostly composed of calcium oxalate, which are formed in human kidneys. The early diagnosis of the risk for urinary stone formation in patients can be determined by the Bonn Risk Index method based on the potentiometric detection of the Ca 2+ concentration and an optical determination of the triggered crystallization of cal- cium oxalate in unprocessed urine. An ISFET calcium-ion sensor has a potential market in this area. Acetylcholine (ACh) is a chemical transmitter in the peripheral and central nervous systems of several organisms including humans. Acetylcholine neurotransmission plays a critical role in human beings during processes such as behavioral activity, arousal, attention, learning, memory, etc. An abnor- mally low supply of acetylcholine is linked with Alzheimer's disease, which ranks fourth for mortality among adults. Therefore, the sensing of acetylcholine concentration is vital and of considerable interest. Acetylcholine measurement using an ISFET biosensor is promising.