WO1999026061A1

WO1999026061A1 - Gas sensor

Info

Publication number: WO1999026061A1
Application number: PCT/GB1998/003356
Authority: WO
Inventors: Peter Alfred Payne
Original assignee: Osmetech Plc
Priority date: 1997-11-13
Filing date: 1998-11-10
Publication date: 1999-05-27
Also published as: GB9723845D0

Abstract

There is disclosed a method for fabricating a gas sensor comprising the steps of: producing an array of spaced apart electrodes disposed on a substrate, the number of electrodes in the array being three or greater; electrochemically depositing at least one conducting organic polymer on and between said electrodes so that the electrodes are electrically connected to one another by the deposited at least one conducting organic polymer; providing sensor interrogation means for measuring one or more electrical properties of the at least one polymer; and connecting said sensor interrogation means to a single pair of electrodes.

Description

GAS SENSOR

This invention relates to gas sensors and methods for the manufacture of same, with particular reference to gas sensors which employ conducting organic polymers as the active gas sensing medium.

Gas sensors which employ conducting organic polymers (COPs), such as polypyrrole and substituted derivatives thereof, are well known (see, for example, International Publications WO 96/00384, WO 96/00383, and K C Persaud and P Pelosi in "Sensors and Sensory Systems for an Electronic Nose", pp 237-256, eds J W Gardner and P N Bartlett, 1992, Kluwer Academic Publishers, Netherlands, and references therein). Typically, a single gas sensor is produced by depositing a layer of COP between a pair of electrodes, the sensor being operated as a chemiresistor, i.e. the presence of a gas is detected by measuring variations in the dc resistance of the COP, these variations being caused by adsorption of the gas onto the COP. It is common for a plurality of sensors to be incorporated into a single gas sensing device. Each sensor has a different COP and/or a different dopant anion, and thus each sensor displays different response characteristics. The use of such arrays of COP sensors allows gases, vapours and odours to be recognised by the response "fingerprint" - the pattern of sensor responses across the array. In this manner, an impressive range of gases can be detected and identified - either individually or as components of mixtures - with good sensitivity.

It is desirable that the spacing between the electrodes is relatively large - preferably 100 μm or greater. This is because larger electrode spacings permit a greater surface area of COP to be presented to a gas, resulting in a higher threshold for saturation of the response of the sensor. However, it is difficult to achieve large electrode spacings in practice. This is because the performance of a sensor tends to diminish greatly if the electrode spacing exceeds the length of the polymeric chains, which are typically irHhe range 1 to 10 μm. The diminution in sensor performance is manifest by a large increase in electrical resistance and a worsening of mechanical properties compared to sensors in which the electrode spacing is comparable to the lengths of the polymeric chains. Thus, for example, International Publication WO 93/03355 discloses gas sensors having COP deposited on and between a pair of electrodes having a spacing in the range 7 to 25 μm.

International Publication WO 96/00383 discloses a method for producing COP gas sensor having much larger electrode spacings of 100 μm and greater, in which a bilayer of COP is employed. A first layer of a suitable COP such as polypyrrole is deposited in a vapour phase oxidative process, and this layer acts as a substrate for a second layer of COP which is deposited electrochemically. Although this method successfully produces COP gas sensors having large electrode spacings, it requires extra production steps and extra materials compared to a standard electrochemical deposition of a single layer of COP.

Slater et al ( J M Slater, J Paynter and E J Watt, Analyst, 118 (1993) 379) discloses a COP gas sensor comprising polypyrrole or various derivatives thereof deposited onto an array of eight electrodes of 5 μm width, the spacing between adjacent electrodes being 5 μm. The electrodes are connected to a multiplexer which permits selected pairs of electrodes to be interrogated by an electrometer. In this way, a matrix of responses of different pairs of electrodes is compiled as a function of response time. However, the use of single pair of electrodes for gas detection is not discussed.

The present invention overcomes the aforementioned problems and disadvantages by providing a COP gas sensor which may be manufactured by a single electrodeposition step, but which permits large gaps to be bridged by COP.

For the avoidance of doubt, the term "gas" is intended to encompass within its scope vapours or volatile species evaporating from a liquid or subliming from a solid. According to a first aspect of the invention there is provided a method for fabricating a gas sensor comprising the steps of : providing a number of spaced apart electrodes disposed on a substrate, the number of electrodes being three or greater; electrochemically depositing at least one COP on and between said number of electrodes so that the electrodes are electrically connected to one another by the deposited at least one COP; providing sensor interrogation means for measuring one or more electrical properties of the at least one polymer; and connecting said sensor interrogation means to a single pair of electrodes.

The interrogation means may be connected to the first and last electrodes in the array.

The spacings between adjacent electrodes may be less than 25 μm, preferably in the range 1 to 15 μm.

The pair of electrodes connected to the interrogation means may be separated by greater than 100 μm, preferably greater than 200 μm.

The number of electrodes in the array may be ten or greater, preferably twenty or greater.

The spacings between adjacent electrodes summed between the pair of electrodes connected to the interrogation means may be greater than 50 μm, preferably greater than 100 μm, most preferably greater than 200 μm.

The interrogation means may measure the dc resistance between the pair- of electrodes. According to a second aspect of the invention there is provided a gas sensor comprising : an array of spaced apart electrodes disposed on a substrate, the number of electrodes in the array being three or greater; at least one COP deposited on and between said number of electrodes so that the electrodes are electrically connected to one another; and sensor interrogation means for measuring one or more electrical properties of the at least one COP, the interrogation means being connected to a single pair of electrodes.

The interrogation means may comprise means adapted to measure the dc resistance between the pair of electrodes. According to a third aspect of the invention there is provided the use of a gas sensor according to the second aspect of the invention in the detection of gases.

Gas sensors and methods for the manufacture of same will now be described with reference to the accompanying drawings, in which:-

Figure 1 shows a diagrammatic view of (a) an electrode array configured for deposition of COP and (b) the gas sensor produced by deposition of the COP;

and Figure 2 shows a diagrammatic plan view of (a) an electrode array configured in an alternative way for deposition of COP and (b) the electrode array after deposition of the COP.

Figures 1 and lb depict stages in a method for fabricating a gas sensor comprising the steps of : providing an array 10 of spaced apart electrodes 12 disposed on a substrate 14, the number of electrodes 12 in the array being three or greater; electrochemically depositing at least one COP 16 on and between said electrodes 12 so that the electrodes are electrically connected to one another by the deposited COP 16; providing sensor interrogation means 18 for measuring one or more electrical properties of the COP; and connecting said sensor interrogation means 18 to a single pair of electrodes 12a, 12b.

By performing the method, a gas sensor may be produced having a large spacing between the two electrodes interrogated by the sensor interrogation means^~18. The gas sensor is fabricated by a single electrode deposition step (if a single layer of COP is employed in the sensor). The invention solves the problem discussed previously, namely that it is usually not possible to produce useful gas sensors by electrochemically depositing COP between electrodes having spacings in excess of ca. 20 μm. The solution is to provide an array of electrodes in which adjacent electrodes are spaced apart by a distance commensurate with the properties of the COP to be deposited, and to electrochemically deposit COP on and between the electrodes in the array.

As described in more detail below, the electrochemical deposition is most conveniently performed whilst holding some or all of the electrodes in the array at the same potential, by providing suitable conductive tracks 20 which electrically connect to electrodes 12. Alternatively, it is possible to hold the electrodes at a constant current level. After the electrodeposition step is performed, most of the conductive tracks 20 are removed, so that the electrodes 12 in the array 10 are only electrically connected by the COP 16.

Usually the interrogation means is connected to the first 12a and last 12b electrodes in the array, as shown in Figure lb, thereby maximising the spacing between the pair of electrodes.

Slater et al discloses a gas sensor in which COP is deposited over eight microband electrodes of 5 μm width and 5 μm separation. After electrodeposition of COP, connection is made to the first five electrodes with a multiplexer, enabling measurement to be made between selected pairs of electrodes, i.e. in selected zones of the sensor. Indeed, the purpose of the device of Slater et al is that a matrix of responses of different pairs of electrodes is compiled as a function of response time, in order to improve discrimination between gases. There is no indication in Slater et al that measurements between a single pair of electrodes might be advantageously obtained. Furthermore, there is no indication or teaching in Slater et al that devices having larger gaps between the electrodes, and/or more electrodes in the array, might be produced in order to provide a sensor having a large gap between the interrogated electrodes. Indeed, the maximum gap between the interrogated electrodes in Slater et al is 35 μm, of which 20 μm is COP and 15 μm comprises "intermediate" electrodes.

Turning now to the present invention, the spacings between adjacent electrodes are usually less than 25 μm, preferably in the range 1 to 15 μm. The actual spacing employed depends on the particular COP used - the purpose is to use an electrode spacing across which the COP in question can be conveniently and usefully deposited.

The pair of electrodes connected to the interrogation means are typically separated by greater than 100 μm, preferably greater than 200 μm. This implies that the number of electrodes in the array is ten or greater, preferably twenty or greater. In terms of sensor response, a more meaningful quantity than the separation between the pair of electrodes connected to the interrogation means may be obtained by subtracting the sum of the widths of the intermediate electrodes from the separation between the pair of electrodes connected to the interrogation means. This approach provides the length of COP between the interrogated electrodes which is not supported by an electrode. Consider, for example, an array comprising ten electrodes of width 10 μm, the separation between adjacent electrodes being 20 μm, and the first and the tenth electrode being connected to the interrogation means. The separation between the interrogated pair of electrodes of 260 μm, whilst the spacings between adjacent electrodes, summed between the pair of electrodes connected to the interrogation means, is 180 μm. In contrast to Slater et al, in which this quantity does not exceed 20 μm, in the present invention the spacings between adjacent electrodes, summed between the pair of electrodes connected to the interrogation means, may be greater than 50 μm, preferably greater than 100 μm, most preferably greater than 200 μm. The electrochemical deposition of COP is performed using techniques well known in the art. In Figure la, the array of electrodes is connected for electrochemical deposition in an interdigitated fashion. A counter electrode is required, although it is possible to hold the two sets of electrodes in the interdigitated array at different potentials with respect to the counter electrode (see, for example, J. Phys. Chem 95 (1991) 9042). It is possible in addition to use a reference electrode. Another embodiment is shown in Figure 2a, where the conductive tracks 20 are deposited so as to electrically connect all electrodes 12 in the array. The array functions as a working electrode during electrode position of COP. Suitable auxiliary electrodes (not shown) are employed, and again it is possible to use a reference electrode. After the electrodeposition, most of the conductive tracks 20 are removed to produce the configuration shown in Figure 2b, in which a single pair of conductive tracks 20a, 20b remain, permitting connection of a single pair of electrodes 12a, 12b with the interrogation means (not shown).

Numerous substrates would suggest themselves to one skilled in the art. Examples of substrates include silicon, glass and ceramic materials.

The fabrication of the electrodes and conductive tracks is conveniently performed using photolithographic techniques.

Conveniently, the interrogation means comprises means adapted to measure the dc resistance between the pair of electrodes. It is also possible to employ interrogation means which apply and measure quantities related to a time varying electrical signal, such as an ac signal or a pseudo random binary signal (see EP 0 286 307 and International Publications WO 97/19349 and WO 97/18467).

Claims

1. A method for fabricating a gas sensor comprising the steps of : producing an array of spaced apart electrodes disposed on a substrate, the number of electrodes in the array being three or greater; electrochemically depositing at least one conducting organic polymer on and between said electrodes so that the electrodes are electrically connected to one another by the deposited at least one conducting organic polymer; providing sensor interrogation means for measuring one or more electrical properties of the at one least polymer; and connecting said sensor interrogation means to a single pair of electrodes.

2. A method according to claim 1 in which the interrogation means is connected to first and last electrodes in the array.

3. A method according to claim 1 or claim 2 in which the spacings between adjacent electrodes are less than 25 ╬╝m, preferably in the range 1 to 15 ╬╝m.

4. A method according to any of claims 1 to 3 in which the pair of electrodes connected to the interrogation means are separated by greater than 100 ╬╝m, preferably greater than 200 ╬╝m.

5. A method according to any of claims 1 to 4 in which the number of electrodes in the array is ten or greater, preferably twenty or greater.

6. A method according to any previous claims in which the spacings between adjacent electrodes summed between the pair of electrodes connected to the interrogation means is greater than 50 ╬╝m, preferably greater than 100 ╬╝m, most preferably greater than 200 ╬╝m.

7. A method according to any of the previous claims in which the interrogation means measures the dc resistance between the pair of electrodes.

8. A gas sensor comprising : an array of spaced apart electrodes disposed on a substrate, the number of electrodes in the array being three or greater; at least one conducting organic polymer deposited on and between said number of electrodes so that the electrodes are electrically connected to one another; and sensor interrogation means for measuring one or more electrical properties of the at least one polymer, the interrogation means being connected to a single pair of electrodes.

9. A gas sensor according to claim 8 in which the interrogation means is connected to the first and last electrodes in the array.

10. A gas sensor according to claim 8 or claim 9 in which the spacings between adjacent electrodes is less than 25 ╬╝m, preferably in the range 1 to 15 ╬╝m.

11. A gas sensor according to any of claims 8 to 10 in which the pair of electrodes connected to the interrogation means are separated by greater than 100 ╬╝m, preferably greater than 200 ╬╝m.

12. A gas sensor according to any of claims 8 to 11 in which the number of electrodes in the array is ten or greater, preferably twenty or greater.

13. A gas sensor according to any one of claims 8 to 12 in which the spacings between adjacent electrodes summed between the pair of electrodes connected to the interrogation means is greater than 50 ╬╝m, preferably greater than 100 ╬╝m, most preferably greater than 200 ╬╝m.

14. A gas sensor according to any one of claims 8 to 13 in which the interrogation means comprises means adapted to measure the dc resistance between the pair of electrodes.

15. The use of a gas sensor according to any one of claims 8 to 14 in the detection of gases.