There are a number of several types of sensors which you can use as essential parts in different designs for machine olfaction systems.
Electronic Nose (or eNose) sensors fall into 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 up of metal oxide and polymer elements, both of which exhibit a modification of resistance when exposed to 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, since they are well researched, documented and established as important element for various types of machine olfaction devices. The applying, where the proposed device is going to be trained to analyse, will greatly influence deciding on a weight sensor.
The response of the sensor is a two part process. The vapour pressure from the analyte usually dictates the amount of molecules can be found within the gas phase and consequently what percentage of them is going to be on the sensor(s). If the gas-phase molecules are at the sensor(s), these molecules need so that you can react with the sensor(s) to be able to produce a response.
Sensors types utilized 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. Sometimes, arrays could have both of the aforementioned 2 kinds of sensors .
Metal-Oxide Semiconductors. These sensors were originally produced in Japan within 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 accessible commercially.
MOS are made of a ceramic element heated by way of a heating wire and coated by a semiconducting film. They could sense gases by monitoring changes in the conductance throughout the interaction of a chemically sensitive material with molecules that should be detected in the gas phase. 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. Several types of MOS might include oxides of tin, zinc, titanium, tungsten, and iridium, doped using a noble metal catalyst such as 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 a longer period to stabilize, higher power consumption. This kind of MOS is easier to create and therefore, cost less to get. Limitation of Thin Film MOS: unstable, challenging to produce and for that reason, more costly to get. On the contrary, it offers greater sensitivity, and far lower power consumption compared to thick film MOS device.
Manufacturing process. Polycrystalline is regarded as the common porous materials used for thick film sensors. It is usually prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is prepared inside an aqueous solution, which is added ammonia (NH3). This precipitates tin tetra hydroxide which is dried and calcined at 500 – 1000°C to generate tin dioxide (SnO2). This is later ground and mixed with dopands (usually metal chlorides) then heated to recoup the pure metal as a powder. Just for screen printing, a paste is made up through the powder. Finally, in a layer of few hundred microns, the paste is going to 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 in the operation within the button load cell itself. A modification of 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 cqjevg “oxidizing” vapours.
As the current applied between the two electrodes, via “the metal oxide”, oxygen inside the air commence to react with the outer lining and accumulate on the surface of the sensor, consequently “trapping free electrons on the surface from the conduction band” . This way, the electrical conductance decreases as resistance in these areas increase due to lack of carriers (i.e. increase resistance to current), as there will be a “potential barriers” between the grains (particles) themselves.
When the sensor exposed to reducing gases (e.g. CO) then the resistance drop, since the gas usually interact with the oxygen and thus, an electron is going to be released. Consequently, the production of the electron increase the conductivity since it will reduce “the possible barriers” and allow the electrons to start to flow . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from your surface of the tension load cell, and consequently, due to this charge carriers will be produced.