| There are a number of different types of sensors | | | | (SnO2). This is later ground and mixed with dopands |
| which can be used as essential components in | | | | (usually metal chlorides) and then heated to recover |
| different designs for machine olfaction systems. | | | | the pure metal as a powder. |
| 1. Electrochemical sensors. | | | | For the purpose of screen printing, a paste is made |
| 2. Metal oxide semiconductors. | | | | up from the powder. |
| 3. Schottky diode-based sensors. | | | | Finally, in a layer of few hundred microns, the paste |
| 4. Calorimetric sensors. | | | | will be left to cool (e.g. on a alumina tube or plain |
| 5. Quartz crystal microbalances. | | | | substrate).b. Sensing Mechanism |
| 6. Optical sensors. | | | | Change of "conductance" in the MOS is the basic |
| Electronic Nose (or eNose) sensors fall into five | | | | principle of the operation in the sensor itself. A |
| categories [1]: conductivity sensors, piezoelectric | | | | change in conductance takes place when an |
| sensors, Metal Oxide Field Effect Transistors | | | | interaction with a gas happens, the conductance |
| (MOSFETs), optical sensors, and these employing | | | | varying depending on the concentration of the gas |
| spectrometry-based sensing methods. | | | | itself. |
| Conductivity sensors may be composed of metal | | | | Metal oxide sensors fall into two types [2]: |
| oxide and polymer elements, both of which exhibit a | | | | |
| change in resistance when exposed to Volatile | | | | 1. n-type (zinc oxide (ZnO), tin dioxide (SnO2), |
| Organic Compounds (VOCs) [1]. | | | | titanium dioxide (TiO2) iron (III) oxide (Fe2O3). |
| In this report only Metal Oxide Semi-conductor | | | | 2. p-type (nickel oxide (Ni2O3), cobalt oxide (CoO). |
| (MOS), Conducting Polymer (CP) and Quartz Crystal | | | | The n type usually responds to "reducing" gases, |
| Microbalance (QCM) will be examined, as they are | | | | while the p-type responds to "oxidizing" vapours. |
| well researched, documented and established as | | | | Operation (n-type) [2]: |
| important element for various types of machine | | | | As the current applied between the two electrodes, |
| olfaction devices. The application, where the | | | | via "the metal oxide", oxygen in the air start to react |
| proposed device will be trained on to analyse, will | | | | with the surface and accumulate on the surface of |
| greatly influence the choice of sensor. | | | | the sensor, consequently "trapping free electrons on |
| The response of the sensor is a two part process | | | | the surface from the conduction band" [2]. In this |
| [3]: | | | | way, the electrical conductance decreases as |
| | | | resistance in these areas increase due to lack of |
| 1. The vapour pressure of the analyte usually dictates | | | | carriers (i.e. increase resistance to current), as there |
| how many molecules are present in the gas phase | | | | will be a "potential barriers" between the grains |
| and consequently how many of them will be at the | | | | (particles) themselves. |
| sensor(s). | | | | When the sensor exposed to reducing gases (e.g. |
| 2. When the gas-phase molecules are at the | | | | CO) then the resistance drop, as the gas usually |
| sensor(s), these molecules need to be able to react | | | | react with the oxygen and therefore, an electron will |
| with the sensor(s) in order to produce a response. | | | | be released. Consequently, the release of the |
| Sensors types used in any machine olfaction device | | | | electron increase the conductivity as it will reduce |
| can be mass transducers e.g. QMB "Quartz | | | | "the potential barriers" and let the electrons to start |
| microbalance" or chemoresistors i.e. based on metal- | | | | to flow [2]. |
| oxide or conducting polymers. In some cases, arrays | | | | Operation (p-type): |
| may contain both of the above two types of | | | | Oxidising gases (e.g. O2, NO2) usually remove |
| sensors [4]. | | | | electrons from the surface of the sensor, and |
| Metal-Oxide Semiconductors | | | | consequently, as a result of this charge carriers will |
| These sensors were originally produced in Japan in | | | | be produced.c. Limitation of MOS sensors [4] |
| the 1960s and used in "gas alarm" devices. | | | | 1. Poor Selectivity - In particular when a thick film |
| Metal oxide semiconductors (MOS) have been used | | | | MOS device is used. The poor selectivity can be |
| more extensively in electronic nose instruments and | | | | reduced by the deposition of a suitable catalyst layer |
| are widely available commercially [1]. | | | | of noble metals like Pd, Pt, Au and Ag. |
| MOS are made of a ceramic element heated by a | | | | 2. MOS need high temperatures (around 300°c) |
| heating wire and coated by a semiconducting film. | | | | to operate efficiently; this result higher power |
| They can sense gases by monitoring changes in the | | | | consumption. |
| conductance during the interaction of a chemically | | | | 3. Sensitive to humidity and to compounds such as |
| sensitive material with molecules that need to be | | | | ethanol and CO2.d. Advantages [4] |
| detected in the gas phase. Out of many MOS, the | | | | 1. Widely available in a variety of types and |
| material which has been experimented with the most | | | | sensitivities. |
| is tin dioxide (SnO2) - this is because of its stability | | | | 2. Very sensitive to a number of organic vapours (e.g. |
| and sensitivity at lower temperatures. Different | | | | oil). |
| types of MOS may include oxides of tin, zinc, | | | | 3. Fast response, usually less than 10 seconds. |
| titanium, tungsten, and iridium, doped with a noble | | | | Altawell |
| metal catalyst such as platinum or palladium. | | | | © Altawell 2008 |
| MOS are subdivided into two types [4]: Thick Film | | | | References |
| and Thin Film | | | | [1] Nagle, H. T., Schiffman, S. S., Gutierrez-Osuna, |
| Limitation of Thick Film MOS: Less sensitive (poor | | | | R.(1998) "The How and Why of |
| selectivity), it require a longer time to stabilize, higher | | | | Electronic Noses" IEEE Spectrum September 1998, |
| power consumption. This type of MOS is easier to | | | | Volume 35, Number 9, pp. 22-34. |
| produce and therefore, cost less to purchase. | | | | [2] Arshak K., Moore E., Lyons G.M., Harris J., Clifford |
| Limitation of Thin Film MOS: unstable, difficult to | | | | S "A review of gassensors employed in |
| produce and therefore, more expensive to purchase. | | | | electronicnose applications". (2004). |
| On the other hand, it has much higher sensitivity, and | | | | [3] Hurst, W. J., (1999) "Electronic Noses & |
| much lower power consumption than the thick film | | | | Sensory Array Based Systems". |
| MOS device [5].a. Manufacturing process [5] | | | | Technomic Publishing Company, ISBN No. |
| Polycrystalline is the most common porous material | | | | 1-56676-780-6. |
| used for thick film sensors. It is usually prepared in a | | | | [4] Sberveglieri D., (1999) "Metal-Oxide |
| "sol-gel" process [5]: | | | | Semicondictors" ASTEQ Technologies for sensors |
| Tin tetrachloride (SnCl4) is prepared in an aqueous | | | | 1999 |
| solution, to which is added ammonia (NH3). This | | | | [5] Nose Office (2003) "NOSE II - The Second |
| precipitates tin tetra hydroxide which is dried and | | | | Network on Artificial Olfactory Sensing". |
| calcined at 500 - 1000°C to produce tin dioxide | | | | |