Sensor Used in E-nose - The Electronic Nose Artificial Olfaction Technology - page 152

6.6 Vapor/Odor Sensor Classification (E-nose Sensor
Classification)
1. Optical
Its basic principles not based on electrical changes in resistance, potential,
current, or frequency, but the modulation of light properties is measured. Optical
instruments are more complex but offer a variety of different measuring possi-
bilities. Different operation modes were developed and are deployed using changes
in adsorbance, fluorescence, optical layer thickness, and polarization.
(a) Flourscence This type of chemo sensor consists of optical fibers deposited
with fluorescent indicator Nile Red dye in polymer matrices of varying
polarity, hydrophobicity, pore size, elasticity, and swelling tendency to create
unique sensing regions that interact differently with vapor molecules [ 15 ].
(b) Absorption Moisture-related energy absorption of near-infrared light can be
used for measuring the moisture content of solids, liquids, and gases. Optical
light absorption sensor sends light into the material of interest (medium) and
makes use of the accurately measured absorption of light to determine the
composition of medium.
(c) Reflective A reflective optical sensor consists of a housing containing both a
light source and a detector. The source radiates light to outwards and it is
reflected back if an object passes in front of the sensor. The reflected light is
sensed by the detector.
2. Thermal Sensor
Pellistors or calorimetric sensors (subtype of thermal sensor) have not been
generally in use for E-nose systems. The basic design of these sensors is a catalytic
surface; platinum and palladium are most common with a heater to maintain the
sensors operating temperature as well as a temperature probe. The temperature
changes detected during sensing are related to the type of gas and gas concentration
of the combustible analyte. Pellistors, thermistors, and thermopiles are types of
thermal sensors, which measure heat changes due to chemical reactions combustion,
adsorption, and enzymatic reactions/combustion. Any chemical reaction or even
absorption/desorption process releases or adsorbs from its surroundings a certain
quantity of heat. Figure 6.8 shows the type of sensor (pellistor) and odor sensor
(thermal type) used as one of bridge elements for analysis of molecules elements.
3. Electrochemical
In the area of sensor technology, the word ''chemical sensor'' is defined as a group
of sensors that are basically different to other sensors, i.e., thermal, magnetic,
optical, and mechanical sensors. Devices that convert a chemical state into an
electronic signal are chemical or biochemical sensors. The detection method for
electrochemical biosensors involved measurement of current, voltage, conductance,
capacitance, and impedance. Chemiresistive, potentiometric, and amperometric are
this type of sensors.