ABSTRACT
Microbial sensors, a combination of immobilised microorganisms and physical transducers, bring together the specificity of biological systems and the high sensitivity of physical measurement. They transform the biochemical information of a substrate into a physically quantifiable, preferably electrical, signal which can then be amplified electronically. The transducer can be potentiometric (pH electrodes, ion selective electrodes) or amperometric (oxygen electrode, optoelectronic detectors, thermistors or field effect transistors) electrodes. The choice of transducer is determined by the metabolic process selected for the measurement. The mechanism underlying microbial sensors can be explained as follows. The substance or mixture to be determined is taken up by the microorganism, metabolised within the cell, and the metabolites are excreted. Oxygen is required under aerobic conditions. The metabolites and the oxygen concentration can be measured amperometrically (oxygen) or potentiometrically (pH, , CO2, H2S). By far the most commonly used transducer in microbial sensors is the amperometric oxygen electrode.
