Chemical Sensors

The three main types of chemical sensors—ion selective electrodes, chemically sensitive field effect transistors, and metal oxide gas sensors—will be discussed.

Ion-Selective Electrodes

Ion-selective electrodes (ISEs) are electrochemical devices (11) whose voltage output, at virtually zero current, is directly related to the concentration of some ionic species around the electrode. They have been in use since the early 1930s and are discussed here since they form the basis of many advanced chemical and biological sensors. The ISEs have a membrane that selectively allows only certain ions to pass through. This selective migration of ions from a higher to a lower concentration results in an uneven charge distribution across the membrane, with a consequential building up of a potential across the membrane in a direction which opposes further movement of the ions, and an equilibrium is reached. At equilibrium, the resulting potential would exactly balance the net ionic flow in each direction, and no further net flow of charge would occur. In a system in which no current can flow, the total number of ions required to develop this opposing potential is extremely small compared to usual solution concentrations. This potential difference developed across the membrane is of the same form as that described for redox electrodes and may be written as:

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