DEVICE FOR SENSING VOLATILE MATERIALS

Abstract: A device is described for sensing volatile materials, the device comprising: a) a plurality of sensors each comprising a pair of spaced-apart electrical contacts (12) and a semi-conductor polymer extending between the contacts, wherein the polymer is such that its electrical resistance can change when exposed to a volatile material; and b) means for measuring the electrical resistance (or a parameter varying therewith) of the semi-conductor polymer of each sensor. The distance between the contacts across which the polymer extends is up to 25 mu m, or in the range of 7 to 13 mu m, more preferably 9 to 11 mu m and most preferably about 10 mu m.

Description: Device for Sensing Volatile Materials Technical Field The present invention relates to a device for detecting volatile materials and in particular the taste and aroma of foodstuffs and beverages such as beer, c Background Art Currently, there is no device commercially available for assessing the quality of a product based on its taste and aroma and this task is usually performed by tasting panels. However, tasting panels are expensive to convene, are not always able to detect the subtle changes in taste or aroma and it is not economic to monitor the quality of, for example, all batches of beer produced in a large 10 brewery using a tasting panel. The same is true of other beverages and foodstuffs not only for human consumption but also for animal consumption. It is known from International Application No.86/01599 (Persaud et al) and an article by Pelosi and Persaud ('Gas Sensors: Towards an Artificial Nose' NATO ASI Series, Vol F43, Sensors and Sensory Systems for Advanced Robots. Ed-P. Dario, Springer-Verlag Berlin Heidelberg 15 1988) to detect odours and gases by means of a device that is said to mimic the olfactory system. The present invention is concerned with a device having an array of sensors' each of which has a different response to a volatile material or a mixture of volatile materials, such as an odour, so that a "fingerprint" of the odour can be built up from the response given by the sensors in the array. Each sensor in the array is composed of a 20 pair of contacts that are bridged by an organic semi-conducting polymer such as polypyrrole that may be grown electrochemically in the presence of a counter-ion between the two contacts. In each sensor of the array the resistance across the semi-conductor polymer is measured and because the polymers in the sensors in the array are different, their resistance changes in a different way on exposure to the volatile 25 material(s). In this way, the fingerprint of the odour can be built up. For example, the polymers of certain sensors will interact with (possibly by absorption) low molecular weight materials more readily 4 than the polymers of other sensors and accordingly the polymers that '_ interact with the low molecular weight components of an odour will give a different response to those that do not interact with such low molecular weight components. Each sensor can differ from other sensors in the array as a result of the different polymer used, e.g. one polymer could be polypyrrole and another couldbe poly(N-methylpyrrole) . Alternatively, two (or more) sensors can utilise the same basic polymer (e.g. both could be polypyrrole) but their responses could differ because the polymer is grown in the presence of a different counter-ion. This difference arises because the polymer must be oxidised in order to be conducting and so, to balance the charge in the polymer, counter-ions are incorporated into the polymer, e.g. in a proportion of approximately one counter-ion to four monomer units. The nature of the counter-ion alters the resisting characteristics of the polymer when exposed to volatile material(s) . Another factor affecting the resistivity characteristics of the polymer when exposed to volatile material(s) is the solvent used when growing the polymer. The polymer used in the sensors can, for example, be chosen from: polypyrrole, poly N-methylpyrrole, poly 3-methylthiophene, polyaniline, poly-5-carboxyindole, poly-3-methyldiphenylamine, polybi hiophene, polythiophene, poly-3-thiopheneacetic acid and polyfuran. Typical counter-ions that can be used in growing the polymer of the sensors may be one or more of: tetrafluoroborate, alkyl sulphonates, tetramethylammonium chloride, chlorates and perchlorates. Solvents used in polymer growth can be or include water, ethanol or acetonitrile. Unfortunately, the polymers in the sensors tend to degrade rapidly, resulting in sensors having only a relatively short operational lifespan. The present invention provides improvements in the above system. Disclosure of the Invention In accordance with the present invention, there is provided a device for sensing volatile materials, the device comprising: (a) a plurality of sensors each comprising a pair of spaced-apart electrical contacts and a semi-conductor polymer extending between the contacts, wherein the polymer is such that its electrical resistance can change when exposed to a volatile material, and • (b) means for measuring the electrical resistance (or a parameter varying therewith) of the semi-conductor polymer of each sensor, wherein the typical distance between the contacts across which the polymer extends is up to 25 μm, advantageously in the range of 7 to 13 μm, more preferably 9 to 11 μm and most preferably about 10 μm. We have found that by providing a spacing between the contacts of 10 μm ± 3 μm (as opposed to a spacing of about 100 μm in prior art sensors) the polymer does not degrade as rapidly and so the life-span. of the sensors in the device of the present invention is extended. A further advantage of the present invention is that the response time of the sensors is shortened as compared to the prior art. The polymers of the sensor are preferably grown electrochemically by placing the contacts of each sensor in a solution containing a monomer precursor of the polymer and a counter-ion and establishing a potential difference between the contacts and a reference electrode and ■ wherein a counter electrode is also provided and the potential between the contacts and the reference electrode is controlled by a potentiostatic circuit that maintains the potential of the two contacts at a substantially fixed or cyclically scanned potential with respect to the reference electrode and wherein the potentiostatic circuit causes a current to flow between the contacts and the counter electrode. The reference electrode may be a standard calomel electrode or an Ag/AgCl electrode. A potential of approximately 1 volt should be applied between the two contacts (typical value of IV) and the counter electrode in order to grow the polymer. The advantage of this aspect of the present invention is that the growth of the polymer can be achieved in a more reproducible manner than is the case when a potential is simply applied between the contacts and a counter electrode and so the sensor thus formed will give more reproducible results. The polymer is preferably (but not necessarily) grown on a substrate, which is most preferably an alumina substrate or a fused quartz substrate; other suitable substrate materials may be envisaged by persons skilled in the art. It is possible to regulate the amount of polymer grown by terminating the polymer growth after a predetermined period of time or when a predetermined amount of current has passed. Alternatively, a small potential difference can be maintained between the two contacts of the sensor or a small amplitude a.c. signal can be applied across the two contacts of the sensor to monitor the growth of the semi- conductor polymer using a bipotentiostatic circuit; polymer growth may be terminated when the resistance of the semi-conductor polymer during its growth reaches a threshold value. After the polymer has been grown, a potential difference may be maintained between the contacts and a reference electrode which is lower than that at which substantial polymer growth occurs. We have found that maintaining the polymer at a potential lower than the growth potential allows equilibration of the counter-ion within the polymer. It is also possible, according to this aspect of the present invention, to change the counter-ion in the polymer by removing the polymer from the solution in which it was grown and dipping it into a further solution containing a different counter-ion and maintaining the polymer at the said potential lower than the polymer growth potential until the substitute counter-ion has replaced the original counter-ions in the polymer. The contacts and the polymer are preferably supported on an alumina substrate which provides excellent adhesion between the polymer and the substrate. In accordance with a preferred aspect, the sensors may each be made by depositing a metal (preferably gold) onto a substrate (preferably onto an alumina substrate) to form the said contacts, depositing a photoresist mask over the electrodes, which mask exposes a locus corresponding to the desired position of the polymer (the locus of the polymer being a limited area between the contacts together with parts overlying the two contacts that enable electrical contact to be established between the contacts and the polymer), dipping the substrate into a solution of a monomer precursor of the polymer and applying a potential difference between the contacts and a reference electrode to cause the monomer precursor to polymerise between the contacts in the said locus. The photoresist, which will be of a low bake type and electrochemically inert, preferably covers the whole of the contacts except for the locus of the polymer and a further area allowing the contacts to be connected to external detecting and controlling circuit. The photoresist can remain in place to protect the contacts in use and thus improve the ruggedness of the sensor, which prevents damage to the ς contacts and in particular prevents the contacts from exfoliating from the substrate. Brief Description of Drawings The present invention will now be described in further detail with reference to the accompanying drawings, in which: Figure 1 shows a mask for use in making a sensor in accordance 10 with the present invention; Figure 2 shows a partly made sensor in accordance with the present invention; Figure 3 shows a further mask for use in making a sensor in accordance with the present invention; Figure 4 shows a partly made sensor in accordance with the present invention and 5 Figure 5 shows a third mask for use in making a sensor in accordance with the present invention. Figure 6 is a schematic drawing of three sensors for use in the device of the present invention. Best Modes for Carrying out the Invention A 0.5 inch (1.25 cm) square tile made of alumina is used as a substrate for a sensor and the sensor was built up on it as follows: 0 (1) A layer of gold 500 Angstrom thick was vapour-deposited over one side of the square tile. (2) Dust particles are removed from the gold-coated substrate by washing it with xylene. (3) A photoresist (Shipley positive TF-16) was applied to the substrate by spinning for 15 seconds. 5 (4) The substrate covered by the resist was soft-baked in an oven for 20 minutes at 90°C. (5) A mask 12 (shown in Figure 1 where the shaded area are translucent) was placed against the resist and the resist was exposed to ultra-violet light through the mask for 4 minutes. (6) The unexposed area of the photoresist was removed using Shipley MF-319 developer for 1 minute. (7) The gold-coated substrate was washed in distilled water and hard-baked in an oven for 1 hour at 115°C to render the resist insoluble to an etchant used in step (8) below. (8) At the area where the resist was removed, the gold was etched using a gold-etchant; the etching time was 1 minute. (9) The remaining photoresist was removed using acetone and the substrate was washed in distilled water. Thus, referring to Figure 2, the substrate 10 bears a gold image of mask 12 and this image forms three pairs of contacts 14 each pair of contacts being part of a different sensor. The contacts of each pair are separated by a gap 15 to 10 μm in width. (10) A brass mask (shown in Figure 3 where the holes cut in the mask are shaded) was placed against the substrate and chromium was vapour-deposited on top of the gold contacts to form contact pads for forming electrical connections to a monitoring circuit. The substrate, contacts and contact pads 16 are shown in Figure 4. Alternatively, the contact pads may be deposited after the polymer growth. (11) Xylene was spun over the surface of the substrate to remove any dust particles. (12) A Shipley 1813 photoresist was spun over the surface of the substrate for 15 seconds. (13) The substrate bearing the contacts and the contact pads was soft-baked in an oven for 5 minutes at 90°C. (14) The photoresist was exposed to ultra-violet light through a mask 18 (shown in Figure 5, the shaded areas being opaque) for 12 minutes. (15) The photoresist was developed using equal parts of Shipley 351 developer diluted with water; the developing time was 1 minute. (16) The substrate was then washed in distilled water and hard- baked at 180°C for 1 hour; this temperature caused the photoresist to become insoluble in the various solutions used in the electrochemical growth of the polymer. (17) The masked substrate was cleaned by cycling in 1 molar sulphuric acid between -0.3 and 1.8 volts at 100 millivolts/second for 3 cycles. After cleaning, the substrate was stored in pure water until required. (18) The masked substrate was placed in a monomer solution (details of the composition of which are given below) and both contacts c of the device are connected to a potentiostatic circuit that maintains the potential of the contacts substantially with respect to a calomel reference electrode; a counter electrode is also provided. (19) The potential between the contacts and a reference electrode is stepped or cycled to a predetermined working voltage to begin growth. After the desired time, the potential is stepped back to 0 V. If after viewing the device under the microscope or by measuring the 10 resistance of the polymer it becomes apparent that the gap between the contacts has not been crossed, then the electrode can be returned to the working solution and further growth performed. (20) The polymer is maintained in the monomer solution at a potential less than the growth potential for a sufficient time to ensure the concentration of counter-ions in the polymer has equilibrated. 15 (21) The device is washed in the solvent used to make the growth solution. This removes any surface debris and prevents any drying marks due to electrolyte deposition. (22).The conductivity of the polymer was then tested. Figure 6 shows the tile bearing three sensors. For ease of illustration, -the top photoresist, which covers the whole of the tile 10 except for the contact pads 16 and the areas 20 in which the polymer 20 is deposited, has been omitted. In use, the resistivity of the polymer 20 between each pair of contacts 12 is measured by passing a current through the polymer by means of the contacts. The resistivity of each sensor is analysed using known pattern-recognition techniques to give an indication of the volatile material(s) present in a sample to which the sensors are exposed. 5 The following solutions of monomers have been used successfully to grow polymers between the contacts: 1) Monomer concentration : 0.1 M pyrrole Counter- ion concentration 0.1 M tetraethyl- ammonium tetrafluoroborate Solvent Water Growth method Potential step 0 to 0.8 V Growth time 3 minutes The polypyrrole produced was smooth, strong and black. 2) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M lithium triflate Solvent Water Growth method Potential step 0-0.9 V Growth time 4 minutes The polymer had to be held at a potential below the growth potential (0.9V) to allow equilibration of the counter -ions in the polymer otherwise the polymer was non- conductive . 3) Monomer concentration 0.1 M N-methylpyrrole Counter-ion concentration 0.1 M sodium salt of dodecylbenzene sulphonic acid Solvent Water Growth method Potential step 0 - (07.-0.9) V Growth time at least 16 minutes 4) Monomer concentration 0.1 M 3-methylthiophene Counter-ion concentration 0.1 M tetraethylammoniu tetrafluoroborate Solvent Acetonitrile Growth method Potential step 0 - 1.65 V Growth time .3 minutes 5) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M tetraethylammmonium toluene-sulphonate Solvent Water Growth method Potential step 0 0.8 V Growth time 3 minutes 6) Monomer concentration 0.1 M 3-me hyldiphenyl- amine Counter-ion concentration 0.1 M LiC10« Solvent Acetonitrile Growth method Potential step 0 - 1.0 V Growth time 2 minutes 7) Monomer concentration 0.05 M 2,2' -bithiophene Counter-ion concentration 0.1 M tetraethyl- ammonium tetrafluoro¬ borate Solvent Acetonitrile Growth method Potential step 0 - 1.65 V Growth time . 3 minutes 8) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M octanesulphonic acid Solvent Water Growth method Potential step 0 - 0.8 V Growth time 2 minutes The films are dense, black and smooth with good adhesion. DC resistance is typically 50 Ω. 9) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M hexanesulphonic acid Solvent Water Growth method Potential step 0 - 0.8 V Growth time 2 minutes The films are dense, black and smooth with good adhesion. DC resistance is typically 50 Ω. 10) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.001 M anthraquinone-2- sulphonic acid Solvent Water Growth method Potential step 0 - 1.15 V Growth time 2 minutes 11) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M butanesulphonic acid Solvent Water Growth method Potential step 0 - 0.85 V Growth time 2 minutes 12) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M pentanesulphonic acid Solvent Water Growth method Potential step 0 - 0.85 V Growth time 2 minutes 13) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M heptanesulphonic acid Solvent Water Growth method Potential step 0 - 0.85 V Growth time 2 minutes 14) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M decanesulphonic acid Solvent Water Growth method Potential step 0 - 0.85 V Growth time 2 minutes 15) Monomer concentration 0.44 M aniline Counter-ion concentration 0.5 M NaHS0 4 ,Ph 1 Solvent Water Growth method Potential step 0 - 0.9 V Growth time 2 minutes 16) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M p- toluene sulphonic acid Solvent Ethanol Growth method Potential step 0 - 1.2 V Growth time 2 minutes 17) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M tetraethylammonium tetrafluoroborate Solvent Acetonitrile Growth method Potential step 0 - 1.1 V Growth time 2 minutes Sensors having different semi-conductive polymers and/or having polymers growth in the presence of different counter-ions or in the presence of different solvents can be combined into an array. hen the array of sensors is exposed to a volatile vapour, the change in the conductivity of the polymer of each sensor is measured and the response given by the sensors in the array will depend on the nature of the volatile materials, different volatile materials generally giving different response patterns. These different response patterns can be recognised using known pattern recognition techniques to give a response giving an indication of the nature of the volatile materials in the vapour. An array of eleven sensors that we have used is as follows: Sensor No. Type of polymer/counter-ion/solvent 0 Polypyrrole/butanesulphonate/water 1 Polypyrrole/butanesulphonate/water 2 Polypyrrole/toluenesulphonate/ethanol 3 Polypyrrole/toluenesulphonate/ethanol 4 Polypyrrole/tetraethyleneammonium toluene- sulphonate/water Polypyrrole/tetraethyleneammonium toluene- sulphonate/water Poly-3-methylthiophene/tetraethyleneammonium- tetrafluoroborate/acetonitrile 7 Polypyrrole/propanesulphonate/water 8 Polypyrrole/propanesulphonate/water 9 Polyaniline/bisulphate/water 10 Polyaniline/bisulphate/water This array has been able to distinguish between different brands of lager beer and between two samples of the same beer that had been stored differently.

Claims: CLAIMS 1. A device for sensing volatile materials, the device comprising: (a) a plurality of sensors each comprising a pair of spaced-apart electrical contacts and a semi-conductor polymer extending between the contacts, wherein the polymer is such that its electrical resistance s can change when exposed to a volatile material, and (b) means for measuring the electrical resistance (or a parameter varying therewith or functionally related thereto) of the semi¬ conductor polymer of each sensor, wherein (preferably) in that the distance between the contacts in each sensor across which the polymer extends is up to 25 μm, more preferably 7 to 13 μm, 9 to 11 μm and most preferably about 10 μm. 10 2 A device as claimed in claim 1, wherein the contacts and the polymer of each sensor are supported on a substrate, e.g. an alumina or fused quartz substrate. 3 A device as claimed in claim 1, which includes a photoresist which covers each sensor except for the locus of the polymer. 4 A method of making a device according to claim 1, which comprises growing the polymer of each sensor electroche ically by placing the 15 contacts of each sensor in a solution containing a monomer precursor of the polymer and a counter-ion and establishing a potential difference between the contacts and a counter electrode, thereby causing the monomer to. polymerise between contacts. 5 A method as claimed in claim 4, wherein a reference electrode is also provided and wherein a substantially constant potential or a cyclically scanned potential is maintained between the contacts and the 20 reference electrode by means of an electrical circuit during polymerisation of the monomer, which circuit causes a current to flow between the contacts and the counter electrode. 6 A method as claimed in claim 4, wherein a constant, alternating or periodic potential difference is imposed between the contacts and wherein the polymer growth is terminated when the resistance of the 25 polymer during its growth reaches a threshold value. 7 A method as claimed in claim 4, wherein, after the polymer has been grown, a potential difference is maintained between the contacts and the counter electrode, which potential is lower than that at which substantial polymer growth occurs, thereby equilibrating the concentration of the counter-ion within the polymer. 8 A method as claimed in claim 4, which further comprises, prior to growing the polymer, depositing the contacts of each sensor onto a substrate, depositing a photoresist mask over the contacts, which mask leaves exposed a limited area between the contacts corresponding to the desired position of the polymer and wherein the polymer is grown in the said limited area. 9 A method as claimed in claim 4, wherein the polymer of at least one of the sensors is grown form a monomer electrolyte selected form the following: a) Monomer concentration : 0.1 M pyrrole Counter-ion concentration : 0.1 M tetraethyl- ammonium tetrafluoroborate Solvent : Water b) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M lithium triflate Solvent Water c) Monomer concentration 0.1 M N-methylpyrrole Counter-ion concentration 0.1 M sodium salt of dodecylbenzene sulphonic acid Solvent Water d) Monomer concentration 0.1 M 3-methylthiophene Counter-ion concentration 0.1 M tetraethylammonium tetrafluoroborate Solvent Acetonitrile e) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M tetraethylammmonium toluene-sulphonate Solvent Water f) Monomer concentration 0.1 M 3-methyldiphenyl- aπαne Counter- ion concentration 0.1 M LiCIO* Solvent Acetonitrile g) Monomer concentration 0.05 M 2 , 2 ' -bithiophene Counter-ion concentration 0.1 M tetraethylammonium tetrafluoroborate Solvent Acetonitrile h) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M octanesulphonic acid Solvent Water i) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M hexanesulphonic acid Solvent Water j ) Monomer concentration 0.1 M pyrrole Counter- ion concentration 0.001 M anthraquinone-2- sulphonic acid Solvent Water k) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M butanesulphonic acid Solvent Water 1) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M pentanesulphonic acid Solvent Water m) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M heptanesulphonic acid Solvent Water n) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M decanesulphonic acid Solvent Water o) • Monomer concentration 0.44 M aniline Counter-ion concentration 0.5 M NaHS0 4 ,Ph 1 Solvent Water p) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M p-toluene sulphonic acid Solvent Ethanol q) Monomer concentration 0.1 M pyrrole Counter-ion concentration 0.1 M tetraethylammonium tetrafluoroborate Solvent Acetonitrile

Inventors: BARTLETT PHILIP N, GARDINER JULIAN W, BARTLETT, PHILIP, N., GARDINER, JULIAN, W.
Applicants: BASS BREWERS LTD, NEOTRONICS LTD, BASS BREWERS LIMITED, NEOTRONICS LIMITED
Application year: 1992
Main classification: G01N27
IPC codes: C08G61, C08G73, C25B3, G01N27, G01N33