There are a number of different types of sensors which can be used as essential components in different designs for machine olfaction systems.

1. Electrochemical sensors.

2. Metal oxide semiconductors.

3. Schottky diode-based sensors.

4. Calorimetric sensors.

5. Quartz crystal microbalances.

6. Optical sensors.

Electronic Nose (or eNose) sensors fall into five categories [1]: conductivity sensors, piezoelectric sensors, Metal Oxide Field Effect Transistors (MOSFETs), optical sensors, and these employing spectrometry-based sensing methods.

Conductivity sensors may be composed of metal oxide and polymer elements, both of which exhibit a change in resistance when exposed to Volatile Organic Compounds (VOCs) [1].

In this report only Metal Oxide Semi-conductor (MOS), Conducting Polymer (CP) and Quartz Crystal Microbalance (QCM) will be examined, as they are well researched, documented and established as important element for various types of machine olfaction devices. The application, where the proposed device will be trained on to analyse, will greatly influence the choice of sensor.

The response of the sensor is a two part process [3]:

1.The vapour pressure of the analyte usually dictates how many molecules are present in the gas phase and consequently how many of them will be at the sensor(s).

2. When the gas-phase molecules are at the sensor(s), these molecules need to be able to react with the sensor(s) in order to produce a response. 

Sensors types used in any machine olfaction device can be mass transducers e.g. QMB "Quartz microbalance" or chemoresistors i.e. based on metal- oxide or conducting polymers. In some cases, arrays may contain both of the above two types of sensors [4].

Metal-Oxide Semiconductors

These sensors were originally produced in Japan in the 1960s and used in "gas alarm" devices.

Metal oxide semiconductors (MOS) have been used more extensively in electronic nose instruments and are widely available commercially [1].

MOS are made of a ceramic element heated by a heating wire and coated by a semiconducting film. They can sense gases by monitoring changes in the conductance during the interaction of a chemically sensitive material with molecules that need to be detected in the gas phase. Out of many MOS, the material which has been experimented with the most is tin dioxide (SnO2) - this is because of its stability and sensitivity at lower temperatures. Different types of MOS may include oxides of tin, zinc, titanium, tungsten, and iridium, doped with a noble metal catalyst such as platinum or palladium.

MOS are subdivided into two types [4]: 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 type of MOS is easier to produce and therefore, cost less to purchase.

Limitation of Thin Film MOS: unstable, difficult to produce and therefore, more expensive to purchase. On the other hand, it has much higher sensitivity, and much lower power consumption than the thick film MOS device [5].

a. Manufacturing process [5]

Polycrystalline is the most common porous material used for thick film sensors. It is usually prepared in a "sol-gel" process [5]:

Tin tetrachloride (SnCl4) is prepared in an aqueous solution, to 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 is later ground and mixed with dopands (usually metal chlorides) and then heated to recover the pure metal as a powder.

For the purpose of screen printing, a paste is made 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).

b. Sensing Mechanism

Change of "conductance" in the MOS is the basic principle of the operation in the sensor itself. A change in conductance takes place when an interaction with a gas happens, the conductance varying depending on the concentration of the gas itself.

Metal oxide sensors fall into two types [2]:

 1. n-type (zinc oxide (ZnO), tin dioxide (SnO2), titanium dioxide (TiO2)iron (III) oxide (Fe2O3).

 2. 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) [2]:

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 surface of the sensor, consequently "trapping free electrons on the surface from the conduction band" [2]. In 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, as the gas usually react with the oxygen and therefore, an electron will be released. Consequently, the release of the electron increase the conductivity as it will reduce "the potential barriers" and let the electrons to start to flow [2].

Operation (p-type):

Oxidising gases (e.g. O2, NO2) usually remove electrons from the surface of the sensor, and consequently, as a result of this charge carriers will be produced.

c. Limitation of MOS sensors [4]

1. Poor Selectivity - In particular when a thick film MOS device is used. The poor selectivity can be reduced by the deposition of a suitable catalyst layer of noble metals like Pd, Pt, Au and Ag.

2. MOS need high temperatures (around 300°c) to operate efficiently; this result higher power consumption.

3. Sensitive to humidity and to compounds such as ethanol and CO2.

d. Advantages [4]

1. Widely available in a variety of types and sensitivities.

2. Very sensitive to a number of organic vapours (e.g. oil).

3. Fast response, usually less than 10 seconds.

References

[1] Nagle, H. T., Schiffman, S. S., Gutierrez-Osuna, R.(1998) "The How and Why of  Electronic Noses" IEEE Spectrum September 1998, Volume 35, Number 9, pp. 22-34.

[2] Arshak K., Moore E., Lyons G.M., Harris J., Clifford S "A review of gas sensors employed in electronicnose applications". (2004).

[3] Hurst, W. J., (1999) "Electronic Noses & Sensory Array Based Systems". Technomic Publishing Company, ISBN No. 1-56676-780-6.

[4] Sberveglieri D., (1999) "Metal-Oxide Semicondictors" ASTEQ Technologies for sensors 1999

[5] Nose Office (2003) "NOSE II - The Second Network on Artificial Olfactory Sensing".

Najib Altawell

© Altawell 2008