Sensors - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 21

2.7 Chemical Sensors

Chemical sensors detect the presence or concentration of particular chemical elements
or compounds in a given sample. A chemical sensor usually consists of a chemi-
cally sensitive film or a membrane and a transducer.
A chemical process occurring in or on a chemically sensitive film or membrane
causes a signal to be generated at the transducer. Examples of mechanisms commonly
employed include host-guest binding, catalytic reactions, or a redox process.
Chemical sensors have a vast variety of applications ranging form medical diag-
nostics and nutritional sciences, through security to automotive industry (Fig. 2.2 ).
Based on the principal of operation, several types of chemical sensors can be
identified.

2.7.1 Interdigital Transducer Sensors

Interdigital transducers using capitative measurement are often used in chemical
sensors. Sensitive layer is used as the dielectric between two electrodes. The dielec-
tric properties of the sensitive layer are changed when it interacts with certain
substances, affecting the capacitance between the two electrodes.

2.7.2 Conductivity Sensors

In these sensors the sensitive layer is used as a conductor of electricity. Interactions
with certain chemicals (e.g., absorption of gasses) modify the conductivity of this
layer. There are two types of sensing layers: Metal Oxide and Conducting Polymers.

Fig. 2.2 Structure of a chemical sensor. Legend: CS - Chemical substance, SL - Sensitive layer,
TD - Transducer, EL - Electronics. Explanation: Chemical substance reacts with the chemical
layer. Reaction causes a signal to be generated at the transducer. The signal is then processed by
electronics and converted into a format suitable for further processing