There are a number of different types of sensors which can be utilized as essential parts in numerous 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, and those employing spectrometry-based sensing methods.

Conductivity sensors might be made up of metal oxide and polymer elements, each of which exhibit a change in resistance when in contact with Volatile Organic Compounds (VOCs). Within this report only Metal Oxide Semi-conductor (MOS), Conducting Polymer (CP) and Quartz Crystal Microbalance (QCM) will likely be examined, because they are well researched, documented and established as vital element for various types of machine olfaction devices. The application, where proposed device will be trained onto analyse, will greatly influence the option of 3 axis force sensor.

The response from the sensor is a two part process. The vapour pressure in the analyte usually dictates the amount of molecules can be found inside the gas phase and consequently what percentage of them will be at the sensor(s). If the gas-phase molecules are in the sensor(s), these molecules need to be able to interact with the sensor(s) to be able to produce a response.

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

Metal-Oxide Semiconductors. These sensors were originally produced in Japan inside the 1960s and used in “gas alarm” devices. Metal oxide semiconductors (MOS) have already been used more extensively in electronic nose instruments and are widely available commercially.

MOS are made from a ceramic element heated with a heating wire and coated by way of a semiconducting film. They are able to sense gases by monitoring alterations in the conductance throughout the interaction of any chemically sensitive material with molecules that need to be detected within the gas phase. From many MOS, the material that has been experimented with the most is tin dioxide (SnO2) – this is due to its stability and sensitivity at lower temperatures. Various kinds of MOS might include oxides of tin, zinc, titanium, tungsten, and iridium, doped having a noble metal catalyst including 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 kind of micro load cell is simpler to create and thus, are less expensive to get. Limitation of Thin Film MOS: unstable, challenging to produce and therefore, more expensive to purchase. On the other hand, it offers greater sensitivity, and far lower power consumption compared to the thick film MOS device.

Manufacturing process. Polycrystalline is easily the most common porous materials used for thick film sensors. It is usually prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is ready inside an aqueous solution, that is added ammonia (NH3). This precipitates tin tetra hydroxide which can be dried and calcined at 500 – 1000°C to create tin dioxide (SnO2). This can be later ground and blended with dopands (usually metal chlorides) and then heated to recoup the pure metal as 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 a alumina tube or plain substrate).

Sensing Mechanism. Change of “conductance” inside the MOS is the basic principle of the operation within the sensor itself. A change in conductance takes place when an interaction using a gas happens, the conductance 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, while the p-type responds to “oxidizing” vapours.

Operation (n-type):

As the current applied in between the two electrodes, via “the metal oxide”, oxygen within the air commence to react with the top and accumulate on the top of the sensor, consequently “trapping free electrons on rocdlr surface through the conduction band” [2]. In this way, the electrical conductance decreases as resistance in these areas increase because of absence of carriers (i.e. increase resistance to current), as you will see a “potential barriers” between the grains (particles) themselves.

When the load cell in contact with reducing gases (e.g. CO) then your resistance drop, because the gas usually react with the oxygen and therefore, an electron is going to be released. Consequently, the release in the electron raise the conductivity as it will reduce “the possibility barriers” and let the electrons to begin to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from the top of the sensor, and consequently, because of this charge carriers will likely be produced.