There are a number of various kinds of sensors which can be used as essential components in different designs for machine olfaction systems.

Electronic Nose (or eNose) sensors belong to five categories [1]: conductivity sensors, piezoelectric sensors, Metal Oxide Field Effect Transistors (MOSFETs), optical sensors, which employing spectrometry-based sensing methods.

Conductivity sensors could be made up of metal oxide and polymer elements, each of which exhibit a modification of resistance when subjected to Volatile Organic Compounds (VOCs). In this particular report only Metal Oxide Semi-conductor (MOS), Conducting Polymer (CP) and Quartz Crystal Microbalance (QCM) will be examined, since they are well researched, documented and established as important element for various types of machine olfaction devices. The application, in which the proposed device will likely be trained to analyse, will greatly influence deciding on a load cell.

The response in the sensor is really a two part process. The vapour pressure in the analyte usually dictates the number of molecules exist inside the gas phase and consequently what percentage of them will be at the sensor(s). Once the gas-phase molecules are at the sensor(s), these molecules need in order to interact with the sensor(s) in order to create a response.

Sensors types used in any machine olfaction device may be mass transducers e.g. QMB “Quartz microbalance” or chemoresistors i.e. based on metal- oxide or conducting polymers. In some cases, arrays could have both of the aforementioned 2 kinds of sensors [4].

Metal-Oxide Semiconductors. These miniature load cell were originally created in Japan within the 1960s and found in “gas alarm” devices. Metal oxide semiconductors (MOS) happen to be used more extensively in electronic nose instruments and therefore are widely accessible commercially.

MOS are made from a ceramic element heated by a heating wire and coated by a semiconducting film. They could sense gases by monitoring alterations in the conductance during the interaction of a chemically sensitive material with molecules that should be detected inside the gas phase. Away from many MOS, the content which has been experimented using the most is tin dioxide (SnO2) – this is because of its stability and sensitivity at lower temperatures. Various kinds of MOS might include oxides of tin, zinc, titanium, tungsten, and iridium, doped with a noble metal catalyst such as platinum or palladium.

MOS are subdivided into 2 types: Thick Film and Thin Film. Limitation of Thick Film MOS: Less sensitive (poor selectivity), it require a longer time to stabilize, higher power consumption. This type of MOS is simpler to create and thus, are less expensive to purchase. Limitation of Thin Film MOS: unstable, hard to produce and thus, higher priced to get. On the other hand, it offers much higher sensitivity, and much lower power consumption than the thick film MOS device.

Manufacturing process. Polycrystalline is the most common porous material used for thick film sensors. It is almost always prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is ready in an aqueous solution, that is added ammonia (NH3). This precipitates tin tetra hydroxide that is dried and calcined at 500 – 1000°C to produce tin dioxide (SnO2). This can be later ground and combined with dopands (usually metal chlorides) then heated to recoup the pure metal as being a powder. Just for screen printing, a paste is created up from the powder. Finally, in a layer of few hundred microns, the paste will be left to cool (e.g. on the alumina tube or plain substrate).

Sensing Mechanism. Change of “conductance” within the MOS is the basic principle from the operation inside the sensor itself. A change in conductance takes place when an interaction using a gas happens, the lexnkg varying depending on the concentration of the gas itself.

Metal oxide sensors fall into two types:

n-type (zinc oxide (ZnO), tin dioxide (SnO2), titanium dioxide (TiO2) iron (III) oxide (Fe2O3). p-type nickel oxide (Ni2O3), cobalt oxide (CoO). The n type usually responds to “reducing” gases, as the p-type responds to “oxidizing” vapours.

Operation (n-type):

Because the current applied involving the two electrodes, via “the metal oxide”, oxygen within the air commence to interact with the surface and accumulate on the top of the sensor, consequently “trapping free electrons on the surface from your conduction band” [2]. This way, the electrical conductance decreases as resistance in these areas increase due to insufficient carriers (i.e. increase effectiveness against current), as you will have a “potential barriers” between the grains (particles) themselves.

Once the rotary torque sensor subjected to reducing gases (e.g. CO) then your resistance drop, since the gas usually react with the oxygen and thus, an electron will likely be released. Consequently, the discharge in the electron increase the conductivity since it will reduce “the possibility barriers” and enable the electrons to start to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from the top of the sensor, and consequently, as a result of this charge carriers will likely be produced.