Ion ISFETs

When two identical transistors, one with the cation-sensitive membrane (+) and the other with the anion-sensitive membrane (-), are operated in the feedback mode the individual responses are given by (6.68), which for the Na+ ISFET is 

Because the two ISFETs are on the same chip, and the reference electrode is common to both, subtraction of (6.72) from (6.71) yields the value of activity of sodium chloride salt with double the slope (Fig. 6.26). 

This probe has been fabricated and used for the determination of sodium chloride in perspiration, as with the test for detection of cystic fibrosis (Bezegh et al., 1988) The constant in (6.73) accounts for nonideality and differences in the two ISFETs. 

Another remarkable feature of that probe is its complete independence of the reference electrode potential and nearly perfect temperature compensation. Thus, a pseudo-reference such as silver wire, functioning only as a signal return, can be used. 

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Field effect transistor, FET an electronic device in which the conductivity of the semiconductor material is controlled by the electrical field at a particular part of its surface, known as the gate . This electrical field may be varied by attachment (and subsequent reaction) of enzymes (ENZFET) or antibodies (IM-MUNOFET) to the gate. The gate may also be sensitized to specific ions (ISFET) or other chemicals (CHEMFET). 

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