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Figure 15. Sensing principle of glucose sensor
Biocatalytic enzyme-based sensors generally consist of an electrochemical gas-sensitive
converter or an ion-selective electrode with an enzyme immobilized in or on a membrane that
serve as the biological mediator. The analyte diffuses from the bulk sample solution into the
biocatalytic layer where an enzymatic reaction takes place. The electroactive product that is
formed (or consumed) is usually detected by an ion-selective electrode. A membrane separates
the basic sensor from the enzyme if a new gas is produced (such as CO
2
or NH
3
) or consumed
(such as O
2
). Although the concentration of the bulk substrate drops continuously, the rate of
consumption is usually negligible. The decrease is detected only when the test volume is very
small or when the area of enzyme membrane is large enough. Thus this electrochemical
analysis is nondestructive, and the sample is reused. Measurements are usually performed at
a constant pH and temperature either in a stirred medium solution or in a flow through
solution. In order to control biochemical process including some enzyme sensors, a number
Advances in Bioengineering
198

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of microbial sensors have been continuously developed and applied to various environment,
agriculture, food and pharmaceutical.
Microbial sensors typically involve the assimilation of organic compounds by microorganisms,
followed by a change in respiration activity(metabolism) or the production of specific electro‐
chemically active metabolites, such as CO
2
, H
2,
or NH
3
, that are secreted by the microorganism.
A microbial sensor is composed of immobilized microorganisms that serve as specific recog‐
nition elements and an electrochemical or optical sensing device that is used to convert the
biochemical signal into electronic signal that can be processed. The operation of a microbial
sensor can be described by the following five-step process: