Machine Olfaction Device (MOD) Sensors (Part Two)

Conducting Polymer Sensors

A polymer is basically a substance made up of many repeating chemical units (or molecules). Conducting polymers, as the name indicates, are conjugated polymers, organic compounds that have an extended p-orbital system, through which electrons can freely move from one end of the polymer to the other. The most common are polyaniline (PAni) and polypyrrole (PPY) [13]. A conducting polymer film is usually used as a sensor to detect vapours/odours using the same principles as those applicable for MOS*.

Polymers can be used for many devices combining unique optical, electrical, and mechanical properties. Conducting polymers can be used for optical effects and underlying physical processes.

Conjugated polymers e.g. poly(para-phenylene, polyaniline, and poly(p-phenylenevinylene) characterized by high flexibility

Most conducting polymers can be made to transfer electrons to other materials such as Buckminsterfullerene(Carbon 60 "C60 Buckyball") [13].

Both inorganic and organic materials can be used to create LEDs (light-emitting diodes), such as InGaN (Indium Gallium Nitride) materials or cadmium selenide nanocrystals, where the physical process involves quantum-wells.

Conducting polymers are made by 'electro polymerisation' of complex organic dyes specifically derivatives of the substances polypyrole, polyaniline and polythiophene.

Depending on the exact chemical structure of the polymer, each one can be given a different conductive behaviour. In this way, a list (or database library) can be built of different types of conducting polymer (i.e. sensors) with each one testing a different type of molecular.

a. Polymer Preparation

Conducting polymer sensors are made by chemical or electrochemical polarization from monomers: "aniline, pyrolle".

The addition of dopants (any kind of conductive materials) increases the polymer conductivity as they create an accumulation of positive or negative particles, e.g. self-doped polyaniline [13].

b. Sensing Mechanism

When the analyte interacts with the sensing surface (i.e. when a molecule in the sensor interacts with another molecule in the environment) the resulting output is a detectable signal [14]. This is the basic principle of the chemical sensor.

All polymers, in general, have a similar detecting mechanism. Chemical sensors based on conjugated polymers detect a variety of analytes and have the ability to detect these at low concentrations, this is because they contain a "Chromophore", a chmical group contained within the polymer where the energy for the excitation of an electron is very low [14].

When contact is made with analyte molecules, the conductivity of the "polymer" changes. A current will be created within the sensor in proportion to the concentration of the analyte.

The next step is that the generated current will usually be detected by the signal processing circuits in the MOD. A pattern will then be generated indicating the type of element/molecules in the sample.

c. Advantages of polymer sensors

1- They work within a wide range of operating conditions (e.g. variations in average

room temperature, pressure and humidity).

2- Functional groups that interact with different classes of odorant molecules can be

built into the active material, providing a high level of sensitivities and selectivity.

3- Organic materials tend to be easier to use than inorganic oxides, as they can be

used close to ambient environmental conditions than, for example, MMOS. Also,

they are more easily modified to react with specific gaseous species than inorganic

materials.

d. Disadvantages of polymer sensors

1 - Even though polymers can be made highly sensitive (point 2 above), organic

materials in general are usually poor conductors, and hence measurement

conductivity can be difficult.

2 - Can be thermally unstable.

3 - Can be sensitive to water.

* MOS = Metal Oxide Semiconductor.

Najib Altawell

References

[13] MacDiarmid A. G.,"Polyaniline and polypyrrolw: where are we headed?"Synthetic Metals 84 (1997) 27 -34

[14] Zhou Q. and Swager T. M. "Fluorescent Chemosensors Based on Energy Migration in Conjugated Polymers: The Molecular Wire Approach to Increased Sensitivity" J. Am. Chem. Soc. 1995, 117, 12593-12602

[15] Vaefolomeev S. (1999) "Conducting Polymer Sensors"

ASTEQ Technologies for sensors 1999

[16] Finklea, H. O., lecture notes (1998 ) "Gas Phase Sensors"

Department of Chemistry West Virginia University, Morgantown, WV 26506-6045

© Altawell 2008

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Machine Olfaction Device (MOD) Sensors (Part One)

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]:

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).




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]:

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 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.

Altawell

© Altawell 2008

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".

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