There are a number of different types of sensors which you can use as essential components in different designs for machine olfaction systems.
Electronic Nose (or eNose) sensors fall under five categories : conductivity sensors, piezoelectric sensors, Metal Oxide Field Effect Transistors (MOSFETs), optical sensors, and these employing spectrometry-based sensing methods.
Conductivity sensors could be made from metal oxide and polymer elements, both of which exhibit a modification of resistance when in contact with Volatile Organic Compounds (VOCs). In this report only Metal Oxide Semi-conductor (MOS), Conducting Polymer (CP) and Quartz Crystal Microbalance (QCM) is going to be examined, because they are well researched, documented and established as important element for various machine olfaction devices. The applying, where the proposed device is going to be trained on to analyse, will greatly influence deciding on a load sensor.
The response in the sensor is a two part process. The vapour pressure in the analyte usually dictates how many molecules are present in the gas phase and consequently what number of them will be in the sensor(s). When the gas-phase molecules are at the sensor(s), these molecules need to be able to interact with the sensor(s) to be able to generate a response.
Sensors types found in any machine olfaction device can 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 above two kinds of sensors .
Metal-Oxide Semiconductors. These sensors were originally manufactured in Japan inside the 1960s and utilized in “gas alarm” devices. Metal oxide semiconductors (MOS) have been used more extensively in electronic nose instruments and are easily available commercially.
MOS are created from a ceramic element heated by a heating wire and coated with a semiconducting film. They could sense gases by monitoring modifications in the conductance throughout the interaction of the chemically sensitive material with molecules that should be detected within the gas phase. From many MOS, the fabric which was experimented with all the most is tin dioxide (SnO2) – this is due to its stability and sensitivity at lower temperatures. Different types of MOS can include oxides of tin, zinc, titanium, tungsten, and iridium, doped having a noble metal catalyst including platinum or palladium.
MOS are subdivided into two types: Thick Film and Thin Film. Limitation of Thick Film MOS: Less sensitive (poor selectivity), it require an extended period to stabilize, higher power consumption. This kind of MOS is a lot easier to create and thus, are less expensive to purchase. Limitation of Thin Film MOS: unstable, hard to produce and for that reason, more expensive to buy. On the other hand, it provides greater sensitivity, and far lower power consumption than the thick film MOS device.
Manufacturing process. Polycrystalline is the most common porous material used for thick film sensors. It will always be 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 is dried and calcined at 500 – 1000°C to create tin dioxide (SnO2). This is later ground and mixed with dopands (usually metal chlorides) then heated to recuperate the pure metal being a powder. Just for screen printing, a paste is produced 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 will be the basic principle of the operation inside the compression load cell itself. A change in conductance occurs when an interaction using a gas happens, the conductance varying depending on the power of the gas itself.
Metal oxide sensors fall into 2 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, whilst the p-type responds cqjevg “oxidizing” vapours.
Since the current applied in between the two electrodes, via “the metal oxide”, oxygen inside the air start to interact with the top and accumulate on the top of the sensor, consequently “trapping free electrons on the surface from the conduction band” . This way, the electrical conductance decreases as resistance during these areas increase because of lack of carriers (i.e. increase potential to deal with current), as there will be a “potential barriers” involving the grains (particles) themselves.
When the sensor exposed to reducing gases (e.g. CO) then your resistance drop, because the gas usually react with the oxygen and therefore, an electron will be released. Consequently, the discharge in the electron boost the conductivity because it will reduce “the possible barriers” and allow the electrons to begin to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from the surface of the tension compression load cell, and consequently, due to this charge carriers will likely be produced.