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1. The substrate is transported to the surface of the sensor;
2. The substrate diffuses the membrane to the immobilized microorganisms;
3. A reaction occurs at the organism;
4. The products formed in the reaction are transported through the membrane into the
surface of detector;
5. The product is measured by the detector.
Here, an example of a microbial sensor is given to demonstrate the detecting course of
microbial sensor including ammonia (NH
3
) and nitrogen dioxide (NO
2
) sensors that utilize the
nitrifying bacteria as the biological sensing component. A NH
3
biosensor can be constructed
on the base of nitrifying bacteria that uses ammonia (NH
3
) as a source of energy and oxidizes
ammonia as follows:
Figure 16. Sensing principle of a NO
2
microbial biosensor
N H
3
+ 1.5O
2
=
Nitrosomonas
N O
2
+ H
2
O + H
+
Biomedical Sensor, Device and Measurement Systems
http://dx.doi.org/10.5772/59941
199

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This oxidation process proceeds at high rate, and the amount of oxygen consumed by the
immobilized bacteria can be measured directly by a polarographic oxygen electrode placed
behind the bacteria.
Nitric oxide (NO) and NO
2
are two principal pollution gases of nitrogen in the atmosphere.
The principle of a NO
2
biosensor is shown in figure 16. When a sample of NO
2
gas diffuses
through the gas-permeable membrane, it is oxidized by the nitrosomonas bacteria as follows:
2N O
2
+ O
2
=
Nitrosomonas
N O
3
Similar to an ammonia biosensor, the consumption of oxygen O
2
around the membrane is
determined by an electrochemical oxygen electrode.