GB2331370A - Polymer matrix containing electrodes for electrochemical sensors - Google Patents

Abstract

The electrode comprises a solid conductor 1 having a polymeric material 3 on a surface 9 thereof. The polymeric material comprises an essentially non-conductive polymer matrix incorporating anionic carrier molecules dispersed therein. The solid conductor is preferably metal, particularly noble metal, or carbon, particularly vitreous carbon fibres. The polymer matrix is preferably thermoplastic, and may be polyvinyl chloride, poly(vinyl chloride-co-vinyl acetate-co-vinyl alcohol), cellulose triacetate or ethyl cellulose. It is preferably soluble in a polar aprotic solvent. The anionic carrier may be selected from quaternary ammonium halides, quaternary phosphonium halides, organometallic halides of Group 4 or Group 5 metals and heterocyclic ring complexed cation salts. Many examples are given. A reference electrode 7 of silver chloride coated silver may be wrapped found the conductor 1 within the polymer matrix, and an outer, gas permeable membrane 8 applied to form an oxygen sensor.

Description

2331370 Electrochemical Sensor Elements, Methods for Their Production and

Uses Thereof as Oxygen Sensors This invention relates to electrochemical sensor elements for detecting the presence of an analyte, particularly a dissolved analyte in a liquid sample, and to methods of producing such electrochemical sensor elements.

More specifically, the invention is concerned with sensors for measuring dissolved gases, in particular the dissolved oxygen content of aqueous media. The electrochemical sensors of the present invention may be used to determine the metabolic activity of micro-organisms in applications such as the measurement of Biochemical Oxygen Demand (BOD) in water samples.

The BOD of a sample is correlated with the content of organic matter and measuring BOD is of importance in monitoring the purity of water samples (for example, samples extracted from rivers) and in assessing the degree of contamination caused by release of polluting organic matter discharged to a water course.

In one method of measuring BOD, a sample is contacted with a defined culture of microorganisms and the oxygen content of the solution measured at intervals over a defined time period. Biochemical oxidation occurs in which dissolved oxygen in the water is consumed by the action of microorganisms on sources of carbon from the polluting matter. The rate of oxidation depends on a number of factors including the temperature and the nature of the polluting substances. The BOD may be determined by measuring the difference in dissolved oxygen content of the sample liquid before and after an incubation period at a fixed temperature. Typically, the chosen conditions are an incubation period of five days and a temperature of 200C.

An example of a batch test that was implemented in water management practice around the beginning of this century was based on the recommendations of the Royal Commission on Sewage Disposal set up in 1898. In the original test a sample of raw water is sealed into a closed container and held at a suitable incubation temperature (originally 18.3 'C, but later modified to 20 OC) for 5 days. The oxygen content before and after the 5 day period is measured and the difference is taken as a measure of the biodegradable organic materials in the water.

The need to conduct measurements over a lengthy period is disadvantageous and one approach which is used to overcome this is to reduce the reaction time and extrapolate to the 5 day value. A further refinement is to artificially dose the sample with a large increase in micro-organisms of a type chosen to rapidly metabolise typical waste water nutrients, thereby reducing incubation time. This method may use a standardised cell culture, and this enables some standardisation of results allowing crosscorrelation between plants as well as enabling nonpathogenic microorganisms to be used. Essentially this procedure is still a batch measurement method even though sampling, micro-organism dosing and incubation may be carried out automatically. Timescaies may be further reduced by the use of oxygen sensors capable of measuring small differences in oxygen level reliably.

In a particularly preferred mode of operation, the BOD of a sample is measured by arranging for the sample to flow continuously past two oxygen sensors which lie on either side of a zone in which the sample is exposed to a standardised culture of one or more micro-organ isms. The extent to which the oxygen content of the sample is reduced while in contact with the culture may be correlated with the BOD of the sample.

A number of designs of electrochemical sensors for detecting the presence of dissolved oxygen have been developed. For example, one such sensor is the so-called Clark ceil-type sensor arrangement (L.C. Clark et al, J. AppL Physiol. (1953), fl, 189) in which a polymeric membrane separates the sample from the internal electrolyte in contact with the electrodes. In such an arrangement, the membrane is permeable only to gases and this has the advantage of preventing the electrode from being contaminated by fouling agents. The flow dependence is reduced in such an arrangement because the diffusion profile arising from the consumption of oxygen at the sensing electrode is predominantly set up within the membrane.

However, a major disadvantage of the conventional Clark cell arrangement when applied to the measurement of BOD is the need to maintain the internal aqueous electrolyte of the sensor during both storage and use. This requirement is particularly undesirable if the sensor is to be dry packaged in a single assembly with dormant micro-organisms. Other disadvantages include complexity of construction and the difficulty of making the sensors an acceptably small size.

The use of an anionically conductive polymer membrane, which acts as both a gaseous diffusion barrier and sensor electrolyte, is disclosed in US Patent 3,703,457 and overcomes certain disadvantages of the Ciark cell arrangement. The arrangement disclosed therein involves the in situ formation of an anion exchange resin on an electrode assembly. However, the prohibitive difficulties of polymerising the anion exchange material on a very small electrode assembly and lack of commercial availability of the solvent-soluble ion exchanger referred to in the patent makes such sensors impractical.

It is the object of the present invention to provide an improved electrochemical sensor element suitable for detecting and measuring the presence of an analyte, in particular the measurement of dissolved oxygen in a liquid. A yet further object of this invention is to provide a novel method of construction of an electrochemical sensor element. A still further object of the invention is to provide such an apparatus in the form of a single unit, comprising an electrochemical sensor electrode and a reference electrode.

Accordingly, a first aspect of the invention provides an electrode assembly which utilises a membrane of essentially non-conductive polymer which is rendered anionically conductive by the addition of an anion carrier to the polymer.

According to this aspect of the present invention there is thus provided an electrochemical sensor element for detecting the presence of an analyte, said sensor comprising a solid conductor having at least a surface portion thereof provided with a layer of a polymeric material, characterised in that the polymeric material comprises an essentially nonconductive polymer matrix incorporating anionic carrier molecules dispersed therein.

The non-conductive polymer matrix of the efectrochemicai sensor is preferably formed of a thermoplastic material, which preferably has one or more of the following characteristics: (i) it has a dielectric constant of preferably e> 3, (ii) it is soluble in selected organic solvents but insoluble in water, (iii) it is capable of acting as a solid solvent for the anionic carrier, Ov) the physical dimensions thereof do not appreciably change when the polymer matrix is in contact with water for prolonged periods, (v) the polymer matrix does not hydrate to an appreciable extent when in contact with water and (vi) the polymer matrix is electrochemically inactive at the oxygen reduction potential. Preferred examples of polymers include polyvinyl chloride, cellulose triacetate, ethyl cellulose and poly(vinyl chloride-covinyl a cetate-co vinyl alcohol).

The polymeric material, including the essentially non-conductive polymer matrix incorporating anionic carrier molecules dispersed therein may conveniently be applied to the solid conductor by straight forward coating techniques, e.g., using a solution of the stated components in a suitable solvent. The solvent is desirably chosen so that (i) it is a solvent for all components of the polymeric material, i.e., non- conductive polymer, plasticiser (if used) and the anionic carrier, (ii) it is readily removed either by natural evaporation or with the application of mild heat and (iii) residual traces are electrochemically inactive at the oxygen reduction potential. Many solvents fulfil these criteria, but preferred examples include polar aprotic solvents such as tetra hydrof uran, cyclohexanone, dichloromethane, propylene carbonate and acetonitrile and mixtures thereof.

The anionic carrier may be chosen so that (i) it is capable of forming a salt with the anion(s) to be transported, (ii) it is soluble in both the polymer and its solvent and (iii) it ionises to a significant extent within the polymer and (iv) it is electrochemically inactive at the oxygen reduction potential.

Examples of anionic carrier include quaternary ammonium halides, quaternary phosphonium halides, organometallic halides of Group 4 or Group 5 metals and heterocyclic ring complexed cation salts.

The quaternary ammonium halides may have the general formula R2 4 2 3 4 R R'R N + X- wherein W, R, R and R ' which may be the same or different, each represents an alkyl, aryl, aralkyl or alkaryl group having from 1 to 20 carbon atoms and X- is a halide, for example, chloride, bromide or iodide. Examples of suitable alkyl groups include branched or straight chain Cl to C2. hydrocarbyl groups including lower Cl to C4 alkyl groups such as methyl, ethyl, n-propyl, iso-propyl, n-butyi, iso-butyl and tertbutyl and higher alkyl groups such as hexyl and dodecyi.

Typical aralkyl groups include benzyl and typical aryl groups include phenyl and naphthyl.

The alkyl, aryi, aralkyl or alkaryl groups referred to may be unsubstituted, or substituted by one or more substituents selected from halo (i.e. fluoro, chloro, bromo, or iodo), nitro, hydroxy, alkoxy, carboxy, carboalkoxy or oxo groups. The alkoxy groups referred to preferably have from 1 to 4 carbon atoms.

Other examples of quaternary ammonium halides include compounds of the general formula WIR 2 R 3 R 4 N + X- wherein one, two or three of the substituents R', R2, R 3 and R 4 are as defined above and the remaining group or groups is or are derived from alkaloids or offier nitrogencontaining molecules of biological origin.

The quaternary phosphonium halides may have the general formula WIR 2 R 3 R4 p+ X- wherein R', R 2, R 3, R 4 and X are as defined above.

The organometalfic halides which may be used as anion carriers according to the invention are chosen so that the compounds are stable in the presence of moisture. Suitable classes of organometallic halides which may be used as anion carriers according to the invention include (i) organometallic halides of Group 4 and Group 5 metals and (ii) heterocyclic ring complexed cation salts.

Examples of organometallic halides of Group 4 and Group 5 metals include those having the formula (R),,M(X-), wherein M is a Group 4 or Group 5 metal such as germanium, antimony and, preferably tin; each R, which may be the same or different, represents and alkyl, aryf, aralkyl or alkaryl group having from 1 to 20 carbon atoms; and X is defined as above.

Examples of heterocyclic ring complexed cation salts include those having the formula IRM' (X-), wherein R is a heterocyclic ring molecule capable of forming a stable complex with a central metal ion M, where M is a metal, e.g. a transition metal or a main group metal, such as manganese or cobalt, which is capable of forming such a complex. Other examples of such metals are well known in the art. Typical heterocyclic ring molecules are polyfunctional ligands in which electron donor atoms (e.g. N, 0 or S) are oriented towards the central cavity of the molecule so that the complexed metal interacts with a plurality of the electron donor atoms. Examples of suitable R groups include porphins, phthalocya nines, corrins, crown ethers and cryptates.

Typical examples of anionic carriers include: quaternary ammonium halides, such as benzyitributyl ammonium chloride, tridodecyl methyl ammonium chloride, benzethonium chloride and N-benzylcinchonidinium chloride; quaternary phosphonium halides, such as tetraphenylphosphonium bromide and (carbomethoxymethyl)triphenylphosphonium bromide; and organometailicchlorides including trioctyltin chloride, aquocyano[heptakis(2-phenylethyi)]Co(lil) cobyrinate chloride (a lipophilic derivative of vitamin B12) and 5, 10, 15, 20-tetraphenyl-21 H, 231-1-porphin manganese (111) chloride.

Preferably, in the present invention, the anion carrier is loaded to saturation in the polymer to allow sufficient ionic conductivity for it to function in an amperometric sensor. In a typical formulation, a final concentration of from 2 to 15%, preferably from 5 to 15% of anion carrier is employed. Typically a concentration of around 10% w/w of anion carrier in the solid membrane is used.

A theoretical review of such anion carriers in the potentiometric mode was given by W. Simon et al., Heiv. Chim. Acta, (1990), 13, 148 1, however this review contains no suggestion of the use of these carriers in the manufacture of electrochemical sensors as defined herein and the concentrations referred to above are considerably higher than those used in potentiometric ion selective electrode applications.

According to a preferred embodiment, the invention provides an electrochemical sensor element which comprises a micro-electrode which is coated at one end with a thin membrane layer, the latter comprising of an essentially nonconductive polymer matrix incorporating an anionic carrier.

The solid conductor of the electrochemical sensor according to the invention is preferably formed of a conductive metal or carbon, e.g. a noble metal such as gold, silver, platinum, iridium, palladium, rhodium or vitreous carbon fibres. Thus the solid conductor may be in the form of a micro-electrode constructed from a metal wire or vitreous carbon fibre. The metal wire is preferably formed from a noble metal, such as platinum, gold or silver.

The invention according to a further aspect thereof provides a novel manufacturing process for producing electrochemical sensors. In this regard, the invention overcomes a problem associated with the use of polymeric gas-permeable membranes of the kind described by L.C. Clark et aL (that in cells of conventional dimensions, the sensor elements are large, having diameters of around 0.5 mm to 5 mm). Thus even when saturated with anionic carrier material, the membranes of the type proposed by Clark etaL still possess a high electrical impedance, which gives rise to an unacceptably large potential drop across the membrane.

The invention according to this further aspect provides, a means of overcoming this problem by enabling the production of membrane-associated electrochemical sensors of much smaller dimensions (i.e. so-called microelectrodes). One of the advantages of using a micro-electrode is that the potential drop across the membrane is reduced by virtue of the lower operating current. Additionally, the use of a micro-electrode according to the invention restricts the oxygen diffusion profile significantly, thus enabling the use of extremely thin membranes without compromising the flow independence properties of thesystem. As a typical example, membranes of the order of a few microns thick may be employed in such an arrangement.

Thus the present invention provides electrochemical sensor element as defined above, wherein the solid conductor is in the form of an elongated element having a transverse dimension in the range 10 pm to 250 pm, preferably in the range 10 pm to 150 pm, most preferably 10 urn to 50 pm. The use in electrochemical sensor elements of a solid conductor in the form of an elongated element of such a small size allows the applied layer of polymeric material also to be of reduced thickness. Thus the electrochemical sensor element may be provided with a polymeric material comprising an essentially non-conductive polymer matrix incorporating anionic carrier molecules dispersed therein, of substantially reduced thickness compared to the membranes suggested by Clark etal. Typical membrane thicknesses employed for Clark cells are 300pm whereas the membranes used in the present invention are from 1 to 15 pm, preferably 5 to 12 pm, thick. At the lower end of this range the membranes tend to be rather weak and prone to pinholing while thicknesses of greater than 15 pm tends to increase the membrane impedance without any further advantage with respect to flow independence or strength. Preferably membranes of about 10 pm are used.

Such electrochemical sensor elements may be formed, according to the invention, by applying to at least a surface portion of the solid conductor, a -g- solution comprising a solvent, an essentially non-conductive polymer, an anion carrier and optionally a plasticiser, and allowing the solvent to evaporate so as to form a coating comprising the essentially nonconductive polymer matrix incorporating anionic carrier molecules dispersed therein.

The invention further provides apparatus for detecting the presence of oxygen in a liquid comprising an electrochemical sensor element as defined previously and an external reference electrode, said sensor element and reference electrode being electrically insulated from one another.

Such apparatus may conveniently consist of an electrochemical sensor element as defined previously and a reference electrode, wherein the electrochemical sensor element includes a solid conductor in the form of an elongated element of having a transverse dimension in the range 10 pm to 250 pm, more preferably 10 pm to 150 pm, most preferably 10 pm to 50 pm. The reference electrode comprises an insulated wire having a surface portion of insulation removed thereof, said exposed surface portion being coated with a layer of silver chloride and wrapped around the exterior surface of the elongated element, both the sensor electrode and the silver chloride-coated portion of the reference electrode being coated with polymeric material comprising of an essentially nonconductive polymer, an anionic carrier and optionally a plasticiser as described above.

A further embodiment of the invention comprises the above electrochemical sensor and reference electrode assembly, but which is further provided with a polymeric coating as described above but omitting the anionic carrier material, thus forming a gas permeable membrane comprising of an essentially non-conductive polymer and optionally, a plasticiser. By this means, a non-conductive over-layer is provided which protects the electrode from contamination by anionic fouling agents such as sulfide or arsenate, which could be transported by any crosssensitivity of the anionic carrier.

As indicated the membrane layer of the eiectrochemical sensor element of the invention comprises a high molecular weight polymer, an anion carrier and preferably a plasticiser. It is preferred that the anion carrier is chosen so that the resultant solid membrane is capable of transporting anions which are expected to be present within the test solution. Typically, anion carriers having significant cross-sensitivity to hydroxide ions are preferred, in order that these ions are conveyed away from the sensing electrode and can contribute to the ion current of the sensor. Most anion carriers known in the art fulfill this criterion. However, a chloride ion carrier is particularly preferred for convenience and offers the advantage that the embodiment of the invention described above can be realised in which both the sensing and reference electrodes can be contained within a single membrane. As indicated typical anion carriers suitable for use in the apparatus of the invention include, but are not limited to, quaternary ammonium halides, quaternary phosphonium halides, organometallic haiides of Group 4 or Group 5 metals and heterocyclic ring complexed cation salts. A particularly preferred anion carrier for incorporation into the membrane is benzyitributylammonium chloride.

Electrochemical sensor elements according to the invention allow the manufacture, in the form of a single unit of a convenient electrochemical device for the measurement of dissolved oxygen in a liquid. This electrochemical device provides a suitable apparatus for the measurement of oxygen required for the determination of the Biochemical Oxygen Demand of a liquid.

Three embodiments of electrochemical sensor elements in accordance with the invention will now be described by way of example with particular reference to the accompanying drawings of which:

Figure 1 represents a first embodiment of an electrochemical sensor apparatus of the invention shown in vertical cross section in its basic mode of operation i.e. used in conjunction with an external reference electrode.

Figure 2 shows in vertical cross section, a second embodiment of a electrochemical sensor apparatus of the invention wherein the reference electrode is coiled around a portion of the electrochemical sensor electrode to form a single assembly.

Figure 3 shows in vertical cross section, of a third embodiment of the invention based on the electrochemical sensor and reference electrode assembly shown in Figure 2, but further coated with a layer of essentially nonconductive polymeric material.

Referring to Figure 1, the electrochemical sensor element of the invention comprises a solid conductor 1, which is typically formed of a platinum wire which is coated with a layer of insulating material 2 on the cylindrical surface. The insulating material is chosen so that (i) it remains firmly adherent to the wire even under prolonged periods of immersion with the cut end exposed and there must be no penetration of electrolyte between the wire and the insulation, (5) the insulating qualities should not be compromised by hydration during prolonged periods of immersion and (iii) the material should be sufficiently robust to withstand the mechanical operations involved when facing off and polishing the exposed end without cracking, splitting or chipping away from the metal. Preferred insulating material for platinum wire is glass. Alternatively, various polymeric and enamel insulation may be used. For silver, gold and carbon wire, the preferred insulating materials are polymers such as polyurethane, polyimide, polyester and epoxy.

The end of the insulated wire is cross sectioned to expose a circular region of bare platinum as the active electrode surface 9 which is 25 lim in diameter. The end portion of the insulated wire, including the exposed region of bare platinum is provided with a layer of a polymeric material 3 applied according to the method described in the following Example.

The layer of polymeric material comprises high molecular weight polyvinyl chloride, an anionic carrier benzyltributylammonium chloride and as plasticiser bis(2-ethyihexyl)adipate.

In operation the electrochemical sensor apparatus is immersed in the test solution 4 along with an external reference electrode 5. The external reference electrode is of conventional construction and examples of such reference electrodes are well known in the art.

Figure 2 represents the second embodiment of the invention in which the electrochemical sensor apparatus of the invention comprising a solid conductor 1 formed of platinum wire coated with a layer of insulating material 2, which is preferably formed from glass, on the cylindrical surface and is cross sectioned to expose a circular region of bare platinum as the active electrode surface. The reference electrode, which forms part of the assembly, comprises an insulated wire 6, which is formed from silver in order that it can perform the function of a reference electrode when coated with silver chloride, having a surface portion of insulation removed thereof, said exposed surface portion being coated with a layer of silver chloride 7 and wrapped around the exterior surface of the elongated sensor element with both the sensor electrode and the silver chloride-coated portion of the reference electrode being coated with the polymeric membrane comprising high molecular weight polyvinyl chloride, an anionic carrier benzyitributylammonium chloride, and as plasticiser bis(2-ethyihexyi)adipate according to the method described in Example 1.

The arrangement represented in Figure 2 thus forms a compact electrochemical sensor apparatus comprising the sensor and reference electrodes in a single assembly.

Figure 3 represents an alternative embodiment of the invention in which the apparatus represented in Figure 2 is further coated with the polymeric membrane as described above, but omitting the anionic carrier material, thus forming a gas permeable membrane comprising of high molecular weight polyvinyl chloride and as plasticiser bis(2- ethyihexyi)adipate. By this means, a non-conductive over-layer 8 is provided, which protects the electrode from contamination by anionic fouling agents.

1 The following Examples illustrate the technique used to apply the polymeric membrane to the electrochemical sensor element of Figure 1 or to the electrochemical sensor assemblies of Figures 2 and 3.

Example 1

Polyvinyl chloride (high molecular weight, 750 mg), benzyitributylammonium chloride (240 mg), bis(2-ethyihexyi)adipate (1.6 g) were dissolved in freshly distilled tetrahydrofuran (30 mi). The solution was decanted from any undissolved quaternary ammonium salt and allowed to evaporate. An electrode coating solution was prepared immediately prior to use by dissolving 100 mg of the evaporated film in 1 mi freshly distilled tetra hyd rofu ran. The micro-electrode or micro- electrode plus reference assembly were dipped five times allowing 15 minutes between dippings and followed by 24 hours drying before use. This procedure gave membranes of approximately 10 microns thickness.

The second Example illustrates the technique used to apply the nonconductive polymeric membrane over-coat to the electrochemical sensor element of Figure 3.

Example 2

Polyvinyl chloride (high molecular weight, 750 mg) and bis(2ethylhexyl)adipate (1.6 g) were dissolved in freshly distilled tetrahydrofuran (30 mi). A coating solution was prepared immediately prior to use by dissolving 100 mg of the evaporated film in 1 mi freshly distilled tetrahydrofuran. The microelectrode plus reference electrode assembly was dipped five times allowing 15 minutes between dippings and followed by 24 hours drying before use. This procedure gave polymeric membrane over-coats of approximately 10 microns thickness.

Claims (17)

1. An electrochemical sensor element for detecting the presence of an analyte, said sensor comprising a solid conductor having at least a surface portion thereof provided with a layer of a polymeric material, characterised in that the polymeric material comprises an essentially nonconductive polymer matrix incorporating anionic carrier molecules dispersed therein.
2. An electrochemical sensor according to Claim 1 wherein the solid conductor is formed of a conductive metal or carbon.
3. 3.

   An electrochemical sensor according to Claim 2 wherein the microelectrode is formed from a noble metal or vitreous carbon fibres.
4. 4. An electrochemical sensor according to any preceding claim wherein the non-conductive polymer matrix is formed of a thermoplastic material.
5. An electrochemical sensor according to any of Claims 1 to 3 wherein the non-conductive polymer matrix is formed of polyvinyl chloride, poly(vinyl chloride-co-vinyl acetate-co-vinyl alcohol), cellulose triacetate or ethyl cellulose.
6. 6. An electrochemical sensor according to Claim 4 wherein the thermoplastic material is polyvinyl chloride.
7. 7. An electrochemical sensor according to any preceding claim wherein the thermoplastic material is soluble in polar aprotic solvents such as tetrahydrofuran, cyclohexanone, dichloromethane, propylene carbonate and acetonitrile.
8. An electrochemical sensor element as claimed in any preceding claim wherein the anionic carrier is selected from quaternary ammonium halides, quaternary phosphonium halides, organometallic halides of Group 4 or Group 5 metals and heterocyclic ring complexed cation salts.
9. A electrochemical sensor element as claimed in any preceding claim wherein the anionic carrier is selected from the following:

   quaternary ammonium halides having the general formula represented 2 3 4 2 3 4 by R' R R R N + X-, wherein R', R, R and R, which may be the same or different, each represents an alkyl, aryl, aralkyl or alkaryl group having from 1 to 20 carbon atoms each of which may be unsubstituted or substituted by one or more substituents selected from halo (i.e. fluoro, chloro, bromo or iodo), nitro, hydroxy, alkoxy, carboxy, carboalkoxy or oxo groups and X- is a halide; quaternary ammonium halides having the general formula R2 3 4 2 R R R N + X-, wherein one, two or three of the substituents W, R ' R 3 and R 4 are as defined above and the remaining group or groups is or are derived from alkaloids or other nitrogen-containing molecules of biological origin; quarternary phosphonium halides having the general formula WIR 2 R 3 R 4p+ X-, wherein R', R 2, R 3 and R 4 and X are as defined above; organometallic halides of Group 4 and Group 5 metals having the general formula (R),,M (Xl., wherein M is a Group 4 or Group 5 metal such as germanium, antimony, and preferably tin; each R, which may be the same or different, represents and alkyl, aryi, aralkyl or aikaryl group having from 1 to 20 carbon atoms; and X is defined as above; or heterocyclic ring complexed cation salts having the general formula IRM' (Xl, wherein R is a heterocyclic ring molecule capable of forming a stable complex with a central metal ion M, where M is a metal, e.g. a transition metal or a main group metal, such as manganese or cobalt, which is capable of forming such a complex; R is a polyfunctional ligand, such as porphins, phthalocyanines, corrins, crown ethers and cryptates, in which electron donor atoms (e.g. N, 0 or S) are oriented towards the central cavity of the molecule so that the complexed metal interacts with a plurality of the electron donor atoms.
10. 10. An electrochemical sensor element as claimed in any preceding claim wherein the anionic carrier is selected from benzyltributyl ammonium chloride, trid odecyl methyl ammonium chloride, benzethonium chloride, Nbenzylcinchonidinium chloride, tetraphenylphosphonium bromide, (carbomethoxymethyi)triphenylphosphonium bromide, trioctyltin chloride, aquocyano[heptakis(2-phenylethyl)]Co(lil) cobyrinate chloride and 5,10,15, 20-tetrapheny]-21H, 231-1-porphin manganese (111) chloride.
11. An electrochemical sensor element as claimed in Claim 8 wherein the anionic carrier is benzyitributylammonium chloride.
12. An electrochemical sensor element as claimed in any preceding claim wherein the solid conductor is in the form of an elongated element of having a transverse dimension in the range 10 pm to 250pm, preferably 10 pm to 150 pm and most preferably 10 pm to 50 pm.
13. 13. A method of forming an electrochemical sensor element, which method comprises applying to at least a surface portion of a solid conductor, a solution comprising a solvent, an essentially non-conductive polymer, an anion carrier and optionally a plasticiser, and allowing the solvent to evaporate so as to form a coating comprising the essentially nonconductive polymer matrix incorporating anionic carrier molecules dispersed therein.
14. 14. A method according to Claim 11 wherein the solid conductor, the essentially non-conductive polymer and anion carrier are as defined in any of Claims 2 to 11.
15. 15. An apparatus for detecting the presence of oxygen in a liquid comprising an electrochemical sensor element according to any of Claims 1 to 12 and an external reference electrode, said sensor element and reference electrode being electrically insulated from one another.
16. 16. An apparatus for detecting the presence of oxygen in a liquid according to Claim 15 comprising an electrochemical sensor element according to any of Claims 1 to 12 and a reference electrode, wherein the electrochemical sensor element includes a solid conductor in the form of an elongated element of having a transverse dimension in the range 1 Opm to 250pm, the reference electrode comprising an insulated wire formed from silver and having a surface portion of insulation removed thereof, said exposed surface portion being coated with a layer of silver chloride and wrapped around the exterior surface of the elongated element, both the sensor electrode and the silver chloride-coated portion of the reference electrode being coated with polymeric material as defined in any of Claims 4 to 11.
17. 17. An apparatus according to Claim 15 or Claim 16 wherein the sensor assembly is further coated with a polymeric material comprising an essentially non-conductive gas permeable polymeric material and optionally, a plasticiser to provide a non-conductive over-layer.