SEMICONDUCTING ORGANIC POLYMERS

Abstract: There is disclosed a gas sensor which comprises: a pair of electrodes; one or more semiconducting organic polymers deposited between the pair of electrodes in such manner as to effect a semiconducting electrical connection between said electrodes; means for applying electric signal across the electrodes; detection means for detecting a chosen electrical property in the presence of a gas; wherein one or more of the semiconducting organic polymers is polymerised from a monomer comprising a five membered heterocyclic aromatic ring with substituent groups at the 2 and 5 positions.

Description: SEMICONDUCTING ORGANIC POLYMERS This invention relates to semiconducting organic polymers which may be used in gas sensors. It is known that certain electrochemically prepared semiconducting polymers such as polypyrrole may be employed in sensors in order to detect gases, vapours and odours . Such a sensor may comprise a pair of electrodes mounted on a substrate, with a layer of the semiconducting organic polymer deposited on and between the electrodes in order to produce an electrical connection between the electrodes. The semiconducting organic polymer may be sensitive to the presence of a gas or, more likely, to a range of gases, to the extent that adsorption of the gas onto the polymer surface affects the electrical properties of the polymer. Hence the presence of gas may be detected by monitoring, for example, the change in DC resistance of the sensor on exposure to the gas. For instance, Maisik et al (Maisik, JJ, Hooper, A and Tofield, BC) JCS Faraday Trans. 1, 1986, _3_2, 1117-26 demonstrated a polypyrrole gas sensor displaying a DC resistance which was sensitive to the presence of nitrous oxide and hydrogen sulphide. GB-2,203,553-B discloses an improved method of detection wherein various AC impedance characteristics are measured at different AC frequencies . A given semiconducting organic polymer will typically be sensitive to a range of compounds. Clearly this lack of selectivity is a major problem if one wishes to develop a sensor which is specific to a particular gas. Conversely, a sensor which employs a given semiconducting organic polymer may not be sufficiently sensitive to such a broad range of gases that it may be considered a general purpose device. A solution to these problems is a device which employs a plurality of sensors, wherein each sensor incorporates a different polymer and each polymer possesses differing gas response profiles. Thus a suite of polymers may be selected which possess broadly overlapping responses, but which are individually chemically tailored to enhance differences in response to certain molecules or classes of molecules. Often the variation of a substituent group on the monomer unit is sufficient to enable such "fine tuning" of response. A multi-sensor device detects gases and odours as a characteristic pattern of individual responses across the array of sensors. The present invention relates to a class of semiconducting organic polymers based on 2,5 substituted five membered heterocyclic aromatic ring monomer units . The substituent groups at the 2 and 5 positions are preferably aromatic rings; in particular, thienyl or thienyl derivatives appear to confer enhanced stability to the resulting polymer. Further substitution on the heteroatom of the 2,5 substituted ring and/or at the 3 position of the ring may be desirable. By judicious variation of the substituent groups, a range of semiconducting polymers for use in gas sensors of the type described above can be produced, wherein the response profile of a polymer can be "fine tuned" to a molecule or a class of molecule. According to the present invention there is provided a gas sensor which comprises a pair of electrodes, one or more semiconducting organic polymers deposited onto and between the electrodes so as to produce a semiconducting electrical connection between the electrodes, and gas sensitive transduction means, wherein one or more of the semiconducting organic polymers is polymerised from a monomer which comprises a five membered aromatic heterocyclic ring, with substituent groups occupying at least the 2 and 5 positions. The gas sensitive transduction means may comprise means for applying electric signal across the electrodes and detection means for detecting a chosen electrical property in the presence of a gas. For instance, if a DC electric signal is applied the change in polymer resistance on exposure of the sensor to a gas may be monitored; if an AC electric signal is applied, the change in an impedance characteristic, such as the capacitance, may be detected at a specific AC frequency. The heterocyclic aromatic ring may also be substituted at the heteroatom. There may be a substituent group at the 3 position. This 3-substitution may be in conjunction with the substitution at the heteroatom to produce a 1,2,3,5 substituted five membered heterocyclic aromatic ring monomer. Alternatively, the 3-substitution may be instead of a 1-substitution on the heteroatom to produce a 2,3,5 substituted five membered heterocyclic aromatic ring monomer. In any of the aforementioned cases, the heterocyclic aromatic ring monomer which contains substituent groups occupying at least the' 2 and 5 positions may be pyrrole. The substituents at the 2 and 5 positions may be aromatic rings and, in particular, may be thienyl or derivatives thereof. Any of the polymers described above may be poly¬ merised electrochemically from a solution containing the monomer and a counter-ion. This counter-ion may be BF~, PF ~ , C10~, CgH-^SO " , Fe(CN)^ " or O^Cg^SO " . A gas sensor in accordance with the invention will now be described with reference to the accompanying drawings, in which : Figure 1 shows a plan view of a silicon chip carrier; Figure 2 shows the view from below a chip carrier; Figure 3 shows the synthesis of N-substituted 2,5-Di-(2-thienyl)-pyrrole; and Figure 4 shows the electrochemical polymer¬ isation process. Figure 1 shows one embodiment of a gas sensor based on a modified 40 pin silicon chip carrier 10 (Hybritek 40 L CC), wherein the gold pins 12 of the carrier are patterned onto a ceramic substrate 14. Adjacent pins 14a and 16a act as electrodes, and a layer of semiconducting organic polymer 18 is deposited so that there is a semiconducting electrical connection between the electrodes. The electrodes are connected to plugs 14b and 16b, located on the underside of the chip carrier. Leads are attached to the plugs 14b and 16b in order to apply a DC potential across the electrodes and the resistance of this electrical circuit is measured by known means (see, for example, BA Gregory; "An Introduction to Electrical Instrumentation and Measurement Systems", 1982, MacMillen). When the sensor is exposed to a gas to which the polymer is sensitive, the presence of the gas is detected by a variation in the DC resistance of the circuit. The semiconducting organic polymer is polymerised from a monomer which comprises a five membered aromatic heterocyclic ring, the ring having at least two substituent groups wherein one substituent group is appended at the 2 position and another substituent group is appended at the 5 position. Suitable heterocyclic rings are pyrrole, thiophene and furan. In order to produce a polymer which displays responses to certain classes of molecules which are close to those responses desired, it is often necessary to effect further substitution of the monomer at the heteroatom. The heteroatom substituent group is not limited in scope, and may be, for instance, alkyl, acyl or aryl. Similarly, it may be desirable to employ a polymer wherein the monomer is substituted at the 3 position on the heterocyclic ring. The 3-substitution may be in addition to, or instead of, the substitution at the heteroatom and the nature of the substituent group is not limited in scope. A preferred choice as the five membered heterocyclic aromatic ring is pyrrole, and preferred substituents at the 2 and 5 positions are further aromatic rings. The reason for the latter preference is that the polymerisation of unsubstituted pyrrole proceeds almost exclusively via linkage at the 2 and 5 positions, and therefore it is possible to inhibit polymerisation if the substituents at these positions do not themselves participate in the polymerisation process. The aromatic ring substituents may themselves be derivatised. A class of monomer which has proved particularly useful is N-substituted 2,5-Di (2-thienyl)-pyrrole. Polymers based on this monomer unit have proved to be particularly stable when employed in gas sensors. The synthesis of 2,5-Di-(2-thienyl)-pyrrole and N-substituted derivatives thereof is shown in Figure 3. 3-dimethylamino-l-(2-thienyl)-propanone hydro- chloride (4) is produced in 89% yield by refluxing for 16 hours a mixture of 2-acetylthiophene (1), para- formaldehyde (2), dimethylamine hydrochloride (3) and concentrated hydrochloric acid in ethanol. The product is isolated, treated with 35% aqueous ammonia and extracted with ether to yield 3-dimethylamino-l- (2-thienyl)-propanone (5). A solution of 2-thiophene carboxaldehyde (6) in DMF is added under nitrogen to a suspension of sodium cyanide in DMF. The mixture is stirred and the 3-dimethylamino-l-(2-thienyl)-propanone (5) is added slowly. The mixture is allowed to stand overnight and the l,4-Di-(2-thienyl)-l,4-butanedione product (7) is extracted with dichloromethane in 64% yield. 2,5,-Di-(2-thienyl)-pyrrole (DTP) can be produced in 80% yield by refluxing (7) for 12 hours with ammonium acetate in a mixture of acetic acid and acetic anhydride. The acid acts as a catalyst and the anhydride as a water scavenger which drives the equilibrium reaction towards the desired product. An N-substituted derivative of 2,5-DTP (9) may be produced by using an appropriate primary amine instead of ammonium acetate. The yield of (9) is often comparable to that of 2,5-DTP but does exhibit some dependence on the substituent group R; in particular, if R is bulky the yield may decrease substantially. Where appropriate, yields may be improved by the use of a benzene/acetic acid solvent system thus allowing the azeotropic removal of water from the reaction mixture by attaching a Dean-Stark trap. Alternatively, a stronger acid catalyst such as titanium (IV) chloride can be used. In order to produce the polymer in its conducting form an electrochemical polymerisation process is employed. The polymerisation may be carried out by electrolytic oxidation of the monomer in an electrochemical cell. Figure 4 shows the electrolytic oxidation of 2,5-Di-(2-thienyl)pyrrole in an electrochemical cell 40. The chip carrier 10 is connected, at 14b and 16b, to the anode 42 of the cell. The cell also comprises a cathode 44, a standard calomel reference electrode 46 and is flushed with nitrogen through ports 48. The anode is at 1.3 V with respect to the reference electrode 46. The electrolyte comprises 0.01 M 2,5-Di-(2-thienyl)pyrrole and 0.1 M tetraethylammonium p-toluenesulphonate in a 99% acetonitrile/1% water medium. The further substituted variants are typically polymerised under similar conditions : the monomer concentration is typically between 0.01 - 0.1 M, and the solvent mixture is typically as described above, although a 50 % dichloromethane, 49.5% acetonitrile, 0.5% water mixture is sometimes employed. In solution the tetraethylammonium p-toluenesulphonate yields the tosylate anion, which is incorporated into the polymer film during polymerisation as a counter-ion to ensure overall electrical neutrality in the polymer. Other counter- ions may be employed including, for example, BFT, PF " , C10~, CgH-^SO or Fe(CN)g . Variation of the counter-ion is another means by which the response characteristics of a polymer may be moderated.

Claims: CLAIMS 1. A gas sensor which comprises : a pair of electrodes; one or more semiconducting organic polymers deposited onto and between the pair of electrodes in such manner as to effect a semiconducting electrical connection between said electrodes; gas sensitive transduction means; wherein one or more of the semiconducting organic polymers is polymerised from a monomer comprising a five membered heterocyclic aromatic ring with substituent groups at the 2 and 5 positions. 2. A gas sensor according to claim 1, wherein the gas sensitive transduction means comprise means for applying electric signal across the electrodes and detection means for detecting a chosen electrical property in the presence of a gas . 3. A gas sensor according to claim 1 or claim 2, wherein the heterocyclic ring with substituent groups at the 2 and 5 positions is also substituted at the hetero atom. 4. A gas sensor according to any of the previous claims, wherein the heterocyclic ring is substituted at the 3 position. 5. A gas sensor according to any one of the previous claims, wherein the substituted heterocyclic ring is pyrrole. 6. A gas sensor according to claim 5, wherein the substituent groups at the 2 and 5 positions are aromatic rings. 7. A gas sensor according to claim 6, wherein the substituent groups at the 2 and 5 positions are thienyl or thienyl derivatives. 8. A gas sensor according to any one of claims 1 to 7, wherein the five membered aromatic heterocyclic ring monomer is polymerised electrochemically from a solution containing the monomer and a counter-ion. 9. A gas sensor according to claim 8, wherein the counter-ion is one of the following: BFT, PF7, CIOT, CgH 17 S0~, Fe(CN)g~ or CH- j CgH^SO "

Inventors: PELOSI PAOLO, PERSAUD KRISHNA CHANDRA, PELOSI, PAOLO, PERSAUD, KRISHNA, CHANDRA
Applicants: AROMASCAN PLC, PELOSI PAOLO, PERSAUD KRISHNA CHANDRA, PELOSI, PAOLO, PERSAUD, KRISHNA, CHANDRA
Application year: 1995
Main classification: G01N27
IPC codes: G01N27