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 under five categories [1]: conductivity sensors, piezoelectric sensors, Metal Oxide Field Effect Transistors (MOSFETs), optical sensors, and those employing spectrometry-based sensing methods.

Conductivity sensors may be made from metal oxide and polymer elements, each of which exhibit a modification of 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) is going to be examined, because they are well researched, documented and established as vital element for various machine olfaction devices. The applying, in which the proposed device is going to be trained on to analyse, will greatly influence the choice of weight sensor.

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

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

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

MOS are created from a ceramic element heated by way of a heating wire and coated by a semiconducting film. They are able to sense gases by monitoring changes in the conductance throughout the interaction of the chemically sensitive material with molecules that ought to be detected in the gas phase. Away from many MOS, the material which was experimented with all the most is tin dioxide (SnO2) – this is because of its stability and sensitivity at lower temperatures. Different types 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 period to stabilize, higher power consumption. This sort of MOS is easier to generate and for that reason, are less expensive to buy. Limitation of Thin Film MOS: unstable, hard to produce and for that reason, higher priced to purchase. On the contrary, it provides much higher sensitivity, and far lower power consumption compared to thick film MOS device.

Manufacturing process. Polycrystalline is the most common porous material used for thick film sensors. It is usually prepared in a “sol-gel” process: Tin tetrachloride (SnCl4) is prepared within an aqueous solution, which is added ammonia (NH3). This precipitates tin tetra hydroxide which 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) and then heated to recuperate the pure metal as being a powder. For the purpose of screen printing, a paste is made up from your 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” within the MOS will be the basic principle of the operation inside the miniature load cell itself. A change in conductance happens when an interaction using a gas happens, the conductance varying depending on the power 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, whilst the p-type responds cqjevg “oxidizing” vapours.

Operation (n-type):

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

Once the sensor subjected to reducing gases (e.g. CO) then the resistance drop, because the gas usually interact with the oxygen and thus, an electron is going to be released. Consequently, the discharge from the electron raise the conductivity because it will reduce “the potential barriers” and allow the electrons to begin to circulate . Operation (p-type): Oxidising gases (e.g. O2, NO2) usually remove electrons from the top of the tension compression load cell, and consequently, because of this charge carriers will be produced.