GB2221761A

GB2221761A - Sensor

Info

Publication number: GB2221761A
Application number: GB8918030A
Authority: GB
Inventors: C P Jones
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1988-08-08
Filing date: 1989-08-07
Publication date: 1990-02-14
Also published as: GB8818802D0; GB8918030D0

Abstract

A gas sensor for sensing methane includes a first electrode, a second electrode and an organic polymer in contact with the first and second electrodes, said organic polymer being doped such that the organic polymer is sensitive to methane. The organic polymer may be, for example, polypyrrole doped with species comprising C1<->,Br<-> or BF4<-> ions. Other polymers may be polythiophene or polyfuran. The doped polymer may be formed electrochemically from an electrolyte solution containing a suitable precursor for the polymer and the appropriate ions.

Description

Sensor The present invention relates to a gas sensor, a method of preparation of a gas sensor and a method of using a gas sensor.

The invention provides in one aspect a gas sensor for sensing methane which includes a first electrode, a second electrode and an organic polymer in contact with the first electrode and the -second electrode, said organic polymer being doped such that the organic polymer is sensitive to methane.

It is to be understood that "sensitive to methane" in this Specification means that a property of the organic polymer is sensitive to the presence of methane. For example, the organic polymer may be sensitive in respect of one or more electrical properties and the gas sensor may include means for detecting a change in one or more electrical properties. Thus, in one particular example the electrical resistance of the polymer may be influenced by the presence of methane and means for measuring electrical resistance may be provided.

The organic polymer may be, for example, doped with ions (e.g. C1-, Br or BF4 ions). Thus, for example, the organic polymer may be polypyrrole doped with ions (e.g. C1-, Br or BF4 ions).

The invention provides in another aspect a method for the preparation of a gas sensor which includes the step of forming doped organic polymer said organic polymer being doped such that the organic polymer is sensitive to methane.

Preferably the organic polymer is formed so as to be in contact with a first electrode and a second electrode.

The first electrode and the second electrode may, for example, be supported by a substrate (e.g. an alumina substrate).

Preferably the organic polymer is formed, as a doped organic polymer, electrochemically.

Examples of doped organic polymers which may be prepared electrochemically are polypyrrole doped with ions, polythiophene doped with ions, and polyfuran doped with ions.

By way of example doped organic polymers may be formed electrochemically from an electrolyte solution containing a suitable precursor for the polymer (e.g. pyrrole in the case of forming a polypyrrole polymer) and ions with which it is desired to dope the polymer (e.g. C1-, Br- or BF4).

Examples of suitable electrolytes for forming polymers in accordance with the present invention are background electrolytes (for example 0.1M KC1, or LiBF4, or Na p-methylsulphonate) containing a suitable precursor for the polymer.

By way of further example if desired the doped polymer may be treated to exchange some of the doping ions therein by exchange with ions from an aqueous solution.

For example, polypyrrole doped with BF4 may be formed electrochemically from a solution containing pyrrole and lithium tetrafluoroborate at room temperature, so as to give doped polypyrrole in contact with two electrodes. If desired the doped polypyrrole may then be treated to exchange some of the tetrafluoroborate ions therein by exchange with ions from an aqueous solution (e.g. a very dilute solution of ions such as is provided by tap water or an aqueous solution).

In one embodiment of the present invention a pair of gold electrodes supported on an insulating substrate is immersed in aqueous solution containing pyrrole and lithium tetrafluoroborate and a voltage is applied to the electrodes thereby to produce a polypyrrole film between and in contact with the pair of gold electrodes.

Electrodes may be formed by any suitable method examples of which are sputtering and screen printing.

Electrodes may be, for example, inter-digitated.

The insulating substrate may be, for example, alumina (e.g. an alumina tile).

The invention provides in a further aspect a method for sensing methane which includes the step of contacting methane with a gas sensor for sensing methane, said gas sensor including a first electrode, a second electrode and an organic polymer in contact with the first electrode and the second electrode, said organic polymer being doped such that the polymer is sensitive to methane.

Exposure of a gas sensor for sensing methane, comprising an alumina tile supporting a pair of interdigitated gold electrodes bridged by a polymer film of doped polypyrrole, to methane gives an increase in resistance between the electrode pair.

Response to methane is substantially instantaneous at room temperature. Reversal is also very rapid; initial resistance values are reached within seconds of exposure to high concentrations (e.g. approximately 100 p.p.m.) of gas.

A gas sensor for sensing methane in accordance with the present invention may be operated at room temperature.

Thus, a sensor in accordance with the present invention may be used substantially to avoid disadvantages of methane sensing devices which operate at temperatures much greater than ambient temperature, which disadvantages include potential hazards associated with explosion risks and the requirment for providing relatively large power supplies needed to maintain temperatures above ambient temperature.

Polymer thickness may be contrclled by the time of growing electrochemically. Thin polymer films can give rise to relatively rapid response at low methane concentrations.

Doping levels may also be controlled relatively readily electrochemically.

It will be appreciated that in order for the necessary electrochemical processes to occur during electropolymerisation anions (e.g. C1--, Br or BF4) from the electrolyte solution must diffuse into the polymer in order to maintain charge balance and in this way the polymer becomes doped with these anions.

The invention will now be further described, by way of example only, with reference to the Examples and to the accompanying drawings in which: Figure 1 shows a sensor in accordance with the present invention; and Figure 2 shows the response to methane of a sensor in accordance with the present invention.

Referring now to Figure 1 there is shown a sensor in accordance with the present invention which sensor has an insulating substrate 1 comprising an alumina tile upon which are supported interdigitated electrodes 2 and 3.

The electrodes 2 and 3 are in contact with an organic polymer 4, represented by the cross-hatched area, said polymer 4 being sensitive to methane.

In operation the sensor may be exposed to methane and the response of the organic polymer 4 thereto is detected by suitable apparatus connected, by means of conductors (not shown), to the electrodes 2 and 3.

Example 1 Interdigitated gold electrodes were formed by screen printing onto an alumina tile in an arrangement of the type shown in Figure 1.

The tile and electrodes were placed in an electrolyte solution such that the gaps between the electrodes were immersed. The electrolyte solution contained pyrrole (0.05 M) in a background electrolyte of 0.1 M KC1.

Conductors were attached to the electrodes and a power supply was used to apply a potential so that one electrode was held at approximately 1 volt positive relative to the other electrode. (Thus one electrode acted as an anode and the other electrode acted as a cathode).

A current of approximately 5mA was passed between the electrodes such that pyrrole was electropolymerised and, as a result, a film of polypyrrole was grown from the anode to the cathode.

When the gaps between the electrodes had been bridged by polypyrrole film the power supply was disconnected from the electrodes.

Example 2 1 A sensor prepared as in Example 1 was tested by exposing it to alternate pulses of methane and of air.

Suitable apparatus connected to the electrodes by conductors was used to monitor changes in resistance.

Resistance changes of the sensor are shown in Figure 2.

Claims

1 A gas sensor for sensing methane including a first electrode, a second electrode and an organic polymer in contact with the first and second electrode, said organic polymer being doped such that the organic polymer is sensitive to methane.

2 A gas sensor as claimed in Claim 1 wherein the organic polymer is doped with ions.

3 A gas sensor as claimed in Claim 2 wherein the ions comprise chloride ions, bromide ions or tetrafluroborate ions.

4 A gas sensor as claimed in any one of the preceding Claims wherein the doped organic polymer is doped polypyrrole, doped polythiophene or doped polyfuran.

5 A gas sensor as claimed in any one of the preceding Claims wherein the doped organic polymer is prepared electrochemically.

6 A method for the preparation of a gas sensor which includes the step of forming doped organic polymer said organic polymer being doped such that the organic polymer is sensitive to methane.

7 A method as claimed in Claim 6 wherein the organic polymer is formed so as to be in contact with a first electrode and a second electrode.

8 A method as claimed in Claim 7 wherein the first electrode and the second electrode is supported by a substrate.

9 A method as claimed in one of Claims 6 to 8 wherein a doped organic polymer is formed electrochemically from an electrolyte solution containing a suitable precursor for the polymer and ions with which it is desired to dope the polymer.

10 A method as claimed in Claim 10 wherein the electrolyte solution contains KC1, LiBF4 or Na pmethylsulphonate and a precursor for the polymer.

11 A method for sensing methane which includes the step of contacting methane with a gas sensor for sensing methane, said gas sensor including a first electrode, a second electrode and an organic polymer in contact with the first electrode and the second electrode, said organic polymer being doped such that the polymer is sensitive to methane.

12 A sensor substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

13 A sensor substantially as hereinbefore described with reference to Example 1.

14 A method for the preparation of a gas sensor substantially as hereinbefore described with reference to Example 1.

15 A method for sensing methane substantially as hereinbefore described with reference to Example 2.