GB2290382A - Electrochemical measurement and reactions in highly-resistive solvents - Google Patents

Abstract

Methods are described that permit the formation of a direct electrochemical interface between electrochemical transducers and highly resistive organic samples. The method is based on a formed ion network on the surface of microelectrodes. Solutes or gases in contacting organic samples enter the electrochemical interface and undergo reaction at the electrode surface. The electrochemical interface permits electrochemical reactions of solutes and dissolved gases directly in highly resistive samples for electrochemical measurements and electrosynthesis of compounds. The method uses a cell of closely spaced microelectrodes covered by an ionically conducting film which contacts the highly resistive sample and which contains an organic electrolyte salt such as a tetraalkyl ammonium salt; tetrabutylammonium toluene-4-sulphonate being preferred.

Description

Direct Methods for Interfacing Electrochemical Transducers to Highly Resistive Organic Samples Abstract Methods are described that permit the formation of a direct electrochemical interface between electrochemical transducers and highly resistive organic samples. In the preferred embodiment, the electrochemical interface permits electrochemical reactions of solutes and dissolved gases directly in highly resistive organic samples in which electrochemical measurements have not previously been feasible. The method also permits electrosynthesis of compounds within highly resistive organic samples. The described method has a number of novel technological applications in organic samples that were previously considered electrochemically inaccessible. The method is based on a formed ion network on the surface of microelectrodes.Solutes or gases in contacting organic samples enter the electrochemical interface and undergo reaction at the electrode surface.

Introduction Electrochemical transducers (electrodes) can be used for the detection or measurement of a range of electroactive compounds for a number of research and industrial applications. This is not unexpected given the ease of device manufacture, low cost, wide dynamic range with good detection limits and a wealth of fundamental information on electrode reaction chemistry. A fundamental requirement for performing electrochemical reactions is an electrically conducting medium or, in other words, the presence of ionic species that migrate between the electrodes and thereby carry the cell current (the electrolyte ions).

Amongst other determinants, the efficacy of electrochemical methods for quantifying chemical species (electroanalysis) will be strongly dependent on the extent of electrical conductivity of a given sample medium. As is well known to those skilled in the art, electroanalysis is readily observed in media that are deemed sufficiently conductive. Examples include aqueous solutions; polar organic solvents such as acetonitrile, dimethylsulphoxide and ethanol. In contrast, electrochemical measurements are not presently observed in highly resistive, non-polar solvents such as hexane, decane or hexadecane. There are. however.

well known methods by which electrochemistry can be performed in some resistive polar organic media by either adding supporting electrolyte and/ or using microvoltammetric electrodes. Supporting electrolyte ís added to help increase the conductivity of the medium by increasing the number of ion carriers and/or increasing the specific charge of an ion carrier. By increasing the conductivity of the medium, the resistance is decreased, thereby improving the scope for electrochemical measurements. It is known to those in the art that microelectrodes permit electrochemistry, to an extent, in some resistive solvent media as a consequence of their small size and enhanced diffusion of solution molecules to the electrode surface. By decreasing the size of the electrode, the current (i) becomes small so that the ohmic potential loss (V=iR, Ohms law) is reduced.Although some electrolyte ions must be present in the medium to generate the electric field at the elect es and carry the cell current, the actual concentration of electrolyte may be very small. In some cases, Actrolyte addition can be omitted altogether because the medium will contain a small amount of ionic contaminants that will serve as ion carriers and form the electrical double layer at the electrodes.

Hence with microelectrodes, a higher resistance (R) can be tolerated or, in other words, the addition of electrolyte may be markedly reduced or absent. However, even with microelectrodes there is an upper working limit in highly resistive, usually non-polar sample media. Practitioners of the art acknowledge this limitation and ascribe the effect to the lack of electrolytes (deliberately added or as contaminants) capable of dissociation in these types of apolar organic media. For example, electrochemistry has not been feasible in hexane or decane because there is a lack of electrolyte materials that are capable of appreciable dissociation in these solvents. The lack of possible electrolyte ions means that the electrical conductivity will be severely restricted. Hence, the solution resistance will be so intense, that there is no scope for electrochemical measurements.The inability to perform electrochemical measurements in highly resistive media has major technological and economic implications in many industrial sectors. The non-aqueous chemical industry, the petroleum industry, the food and drink sectors (oils, fats etc.), environmental analysis in organic wastes and more recent non-aqueous biotechnological conversions are examples of industrial sectors that would benefit if electrochemical measurements were possible in highly resistive organic media.

If methods could be devised that permit electrochemical measurements directly in highly resistive media, there would be considerable economic and technological reward to be expected. The present invention relates to methods by which electrochemical reactions and measurements can be performed directly in contacted highly resistive, non-polar organic sample media. In particular. the invention relates to methods by which electrochemical transducers can be directly interfaced to highly resistive. non-polar organic samples for the electrochemical reaction and/ or measurement of chemical species residing in the contacting resistive sample medium.

For this invention, the term "highly resistive media" relates to highly resistive organic solvents in which electrochemistry using conventional techniques has not been possible prior to this invention. In particular, the organic solvents hexane, decane and hexadecane will be used to demonstrate said titled invention. The solvents, hexane, decane and hexadecane are particularly representative media for said titled invention given that electrochemical measurements using conventional methodology is not currently observed therein.

Prior Art The Clark oxygen electrode [1] can be used as a method for measuring dissolved gases in aqueous solution.

The Clark electrode can also find application in organic media. However, it is stressed that the Clark electrode is based on a gas permeable membrane that covers an electrochemical cell containing aqueous electrolyte solution. Only gaseous compounds that are able to pass through the membrane can undergo electrochemical reactions. Solutes from a contacting organic phase do not partition across the membrane restricting electrochemical measurement to dissolved gases. Moreover, electrochemical measurements are not carried out directly in the organic phase but rather within the aqueous solution phase behind the gas permeable membrane. In the present invention, electrochemical measurements of solutes is performed directly in organic media.

The Present Invention In this invention it is shown how the concept of directly interfacing electrodes to highly resistive non-polar organic solvents for electrochemical measurements is reduced to practice where it can be utilised for novel applications. The non-dissociation of tetra-alkyl ammonium compounds into ion components that are largely incapable of movement under an applied electric field in highly resistive, non-polar solvents can be used to novel advantage for said titled invention. This situation is in stark contrast to the conventional electrochemical utility of tetra-alkyl ammonium compounds. In the normal course of events, the dissociation of tetra-alkyl ammonium compounds into charge carrying ions in solvent is the overriding goal; in other words the primary role of tetra alkyl ammonium compounds is to serve as charge carriers in solution.In this invention, the ionic components of tetra-alkyl ammonium compounds are present as an ion network, most probably held together by electrostatic interactions between network ion components. Using a prescribed technique developed for the said titled invention, the tetra alkyl ammonium compound is used as an ion network that forms an electrochemical interface at the electrode surface and a contacting highly resistive, non-polar solvent.

Using a prescribed method, a suitable electrolyte is also present within the ion network and the resulting structure is used to modify closely spaced microelectrodes. If the resulting electrode body is then placed in a highly resistive, non-polar solvent, electrochemical measurement of species residing in the bulk of the nonpolar organic phase becomes possible. Using this method, the ion network and its associated electrolyte forms a direct electrochemical interface between the electrode and the non-polar organic phase. As shown in this invention, electrochemical accessibility to non-polar solvents permits electrochemical measurements in previously inaccessible measurement domains for a range of new devices and novel appliactions.

The method embodies a novel use of tetra-alkyl ammonium compounds and their congeners, permitting electrochemical measurements in solvents such as hexane, decane, hexadecane and other non-polar phases in which direct electrochemical measurement has not previously been achieved. An example of a tetra-alkyl ammonium salt that can be used to form an ion network for said titled invention is tetra butyl ammonium toluene-4-sulphonate. In one sense, the poor dissociation of tetra-butyl ammonium toluene-4-sulphonate and other tetra-alkyl ammonium compounds in resistive, non-polar solvents is used to advantage.

The internal electrolyte is believed to be responsible for carrying the cell current within the formed ion network although we do not want to be limited to this. The tetra-alkyl ammonium salt forms the ion network and is primarily responsible for the structural basis of the electrochemical interface to non-polar solvents. The internal electrolyte (hereon referred to as electrolyte) is usually a solvent that is generally immiscible with the non-polar solvent phase in which electrochemical measurement is executed. Further ion forming compounds may also be dissolved into the electrolyte to assist the cell current. Among others, an example of an electrolyte is water. Among others, examples of ion forming compounds dissolved into the electrolyte to assist the cell current are potassium chloride. sodium phosphate and other buffer salts.

Related work A publication by Michael & Wightman [2] reported the use of a per fluorinated ion exchange membrane (Nafion) as an electrolytic matrix which allowed electrochemical measurements in resistive supercritical carbon dioxide when traces of water were present. No reference was made to direct electrochemical measurements in organic solvent media.

A recent patent [3] has shown the utility of the base material, tetra-butyl ammonium toluene-4-sulphonate used to illustrate this invention as a novel medium for performing biologically linked electrochemical reactions in the gas-phase. There are, however, distinct inventive steps in the present application over [3].

These are discussed in proceeding sections.

The direct electrochemical interface to non-polar, highly resistive environments The electrochemical interface in this invention consists of electrolyte ions that are embodied by an ion network retained at the surface of electrodes. The retention of electrolyte ions in an electrochemical interface at close proximity to electrodes requires some way of physically confining the electrolyte. Three methods are firstly discussed where existing materials are used in a novel way to achieve electrochemical compatibility to highly resistive, non-polar solvents. The examples below are based on polymeric structures.

1. The use of a polymeric material containing cationic or anionic groups bound to the polymer chain which act as counterions to small unbound and potentially mobile ions. When this form of material is placed in solvent, the coulombic attraction between potentially mobile ions and polymer bound counter-ions is decreased allowing ionic movement of the former. A candidate material in this category is Nafion. In this invention, it is shown how Nafion can be used as an electrolytic matrix in the non-polar solvent, hexane.

2. A further category of materials that find application to organic resistive samples are polymeric hydrogels.

In this case a polymer network is used to entrap a predominantly aqueous electrolyte solution at the surface of the electrode.

3. Solvating polymers possess the ability to dissolve certain salts and support ionic conductivity. An example of this category is poly(propylene oxide) with dissolved lithium salts.

Tetra-alkyl ammonium compounds that form an electrochemical interface with non-polar solvents We have discovered a further family of materials based on tetra-alkyl ammonium salts, that can be used for the formation of an electrochemical interface to highly resistive. non-polar organic media. The tetra-alkyl ammonium compounds have significant potential as electrochemical interfaces to highly resistive organic media in which direct electrochemical measurements have not been feasible before this invention. The tetraalkyl ammonium compounds possess significantly different properties to the three materials oulined in the preceding section as potential electrolytic interfaces to non-polar, highly resistive organic media.

The preferred method for the formation of the electrochemical interface from tetra-alkyl ammonium compounds is as follows. The tetra alkyl ammonium compound is dissolved in a solvent that is sufficiently polar to dissociate the compound into ions. An example is the use of tetrabutyl ammonium toluene-4sulphonate (TBATS) in water with and without added ions. When the TBATS-water solution is placed onto an electrode and left to dry for a known length of time, a material of gel-like appearance forms. The viscosity of the TBATS-water solution is increased as a result of water loss to the atmosphere. The interionic distances between TBATS co-ions is reduced though water ions and possibly some TBATS ions are still capable of ionic migration under the influence of an electric field.Further drying results in diminished ionic conductivity as a result of the decreased ionic movement as further water is lost to the atmosphere and the increased coulombic attraction between ions. The diffusivity of incorporated solutes is also reduced as the drying process continues. The effect continues until only a very small background ionic current (in the nA range) is observed. This process is seen in the invention described in [3]. However, in [3], the aim was markedly different to the present invention. In [3], the aim was to achieve gas-phase operation using a self contained ion matrix. Furthermore. [3] was related to a method for achieving bioelectrochemical reactions in the self contained ion matrix without contacting solvents for the gas-phase detection of gaseous analytes.

In the present invention. the TBATS material is not used as a self-contained ion matrix in contact with a gas-phase. While there are many methods known to those skilled in the art for interfacing electrochemical transducers to the gas-phase there are no reports of a direct electrochemical interface to highly resistive.

non-polar solvents as described in this invention. The present invention requires there to be a contacting solvent phase that is central to the novel structure-functional aspects of the electrochemical interface which further distinguishes the present invention over [3].

The action of placing the pre-formed ion network into a non-polar solvent not only permits the formation of an electrochemical interface between the electrode and the non-polar solvent but also confers other, novel properties that characterise the said titled invention.

Novel properties of the electrochemical interface based on TBATS in non-polar solvents In the present invention, the presence of a non-polar, organic solvent medium is essential for the desired functioning of the ion network and the novel material properties conferred to the electrochemical interface.

In-built functional characteristics 1. The ability to perform electrochemical measurements directly in contacting highly resistive non-polar solvents 2. The ability to perform electroanalysis in highly non-polar solvents 3. The ability to perform electrochemical measurements of compounds that are water soluble, solvent soluble or dissolved gases.

4. The ability to electrochemically measure the water content of a contacting non-polar phase.

5. The change in the structure and properties of the ion network when immersed in a non-polar solvent.

The contacted solvent causes free volume changes within the ion network. This confers a number of important and novel properties to the electrochemical interface as discussed herein: 5.1 Improved ionic conductivity of the ion network by the presence of å contacting non-polar solvent. The presence of solvent decreases coulombic attractions between the matrix and the electrolyte ions permitting mobility of the latter. Increased electrolyte ion mobility confers improved ionic conductivity. Increased ionic conductivity reduces the effect of ohmic distortion in electrochemical measurements.

;),9 The ion network confines the electrolyte to the surface of the electrodes. This is also aided by the immiscibility of the internal electrolyte with the bulk organic phase.

5.3 Owing to the free volume increase of the ion network by the presence of the contacted non-polar solvent, the internal structure of the ion network becomes more open. This allows an increase in the diffusion coefficient of a variety of different solutes residing in the matrix or diffusion of those solutes that have partitioned into the matrix from the bulk non-polar solvent.

5.4 The diffusivity of solutes within the ion-network results in a number of important advantages for electrochemical measurements. These aspects are particularly prevalent when the ion-network is formed at microelectrode transducers.

5.4.1 The increased diffusivity of solutes within the ion-network results in high rates of mass transfer at microelectrodes.

5.4.2 The manifestation of high mass transfer rates yields increased current density at the microelectrode.

The increased current density at the microelectrode yields improved sensitivity in analytical applications.

5.4.3 The increased current density also results in enhanced conversions of solutes in electrosynthetic applications in non-polar solvents.

5.4.4 Other microelectrode properties which are widely known to those in the art will also become apparent with the formed electrochemical ion-network interface. The resultant properties attained by combining the electrochemical interface with microelectrode transducers are particularly relevant for the envisaged applications.

6. Extraction of polar solutes into the electrochemical interface. Solutes with favourable partition coefficients are transferred to the organic ion network. This has important consequences for monitoring electrochemical reactions of polar analytes in non-polar environments or for electrosynthetic applications of polar compounds.

7. Biochemical reagents are able to maintain activity in the organic ion network. This is particularly the case for biochemicals that are usually unstable when in direct contact with organic media.

8. The ability to retain solutes within the ion network without leaching into the bulk solvent phase. Certain solutes tend to remain within the confines of the matrix owing to unfavourable partition coefficients to the bulk organic phase. Incorporated solutes in the ion network retain high diffusivity when a non-polar phase contacts the ion network.

9. The electrochemical interface comprising the organic ion network is overall uncharged. This permits the rapid mass transfer of solutes within the ion network. Solutes from the bulk phase are unlikely to encounter high electrostatic interactions when entering the electrochemical interface. This is important for achieving high current density at the electrode.

10. Migration effects are markedly reduced or absent with the electrochemical interface which can be a problem in electrochemical measurements in resistive organic media.

11. The ability to perform electrochemical polymerisation within the ion network resulting in a composite or mixed conductor electrochemical interface.

12. Some solutes may become ionised within the organic ion network.

13. The electrochemical interface may exist as a very thin film (micrometers thick and below) or relatively thick films in the millimetre scale.

14. The electrochemical interface may be formed onto chemically modified electrodes.

15. The electrochemical interface can be formed onto individually accessible microelectrodes or onto a common array of microelectrodes.

16. Generation-collection mode electroanalysis can be performed at microelectrodes onto which the electrochemical interface has been formed.

Novel applications of electrochemical interfaces to highly resitive organic samples 1. Sensor devices that operate in resistive organic media. Examples include sensors for monitoring compounds in organic processes as encountered in the petroleum and chemical industries.

2. Direct electrochemical detectors for normal phase liquid chromatography.

3. Sensors in the food industry, for example monitoring the quality of edible oils, butter. margarine. fats etc.

4. Sensors for the pharmaceutical industry.

5. Sensors for environmental applications such as. monitoring in organic wastes or the quality of water following a liquid-liquid extraction.

6. Sensors for the biotechnological processes such as organic phase or 2 phase bioconversions.

7. Sensors for clinical applications such as monitoring cholesterol or bilirubin.

8. Electrosynthetic applications in resistive organic media.

EXAMPLES OF ELECTROCHEMICAL INTERFACES WITH HIGHLY RESISTIVE, NON POLAR ORGANIC MEDIA 1. A method forplterfacing electrochemical transducers to highly resistive organic media using a polymer electrolyte: Nafion.

The use of a perflourinated ion exchange membrane (Nafion) as a means for interfacing electrodes to highly resistive organic media. In this example, it is shown how an existing material, Nafion, can be used for a novel application where it functions as an electrochemical interface when contacted by polar and highly resistive non-polar organic solvents.

Experimental 1. Operation in Polar Solvents. Cyclic voltammetry in pure dioxan. Figure 1 shows undistorted voltammograms in dioxan using ferrocene. The redox peaks correspond to the one electron reversible charge transfer of ferrocene at microband electrodes modified with Nafion. In the absence of Nafion in pure dioxan, highly distorted voltammograms resulted. Figure 1.

1.2 Cyclic voltammetry of a Nafion modified electrode in pure hexane resulted in undistorted peaked voltammograms of the reversible 1 electron redox reaction of ferrocene at the array of gold indicator electrodes. In the absence of Nafion, no currents were recorded. Figure 2.

1.3 Cyclic voltammetry of a Nafion modified electrode in pure decane. Redox peaks correspond to the one electron transfer of ferrocene in pure decane. Figure 3.

2. A method for interfacing electrochemical transducers with highly resistive, non-polar solvents using a tetra-alkyl ammonium salt. In this example, the tetra alkyl ammonium compound, tetra butyl ammonium toluene-4-sulphonate (TBATS), is used to form an organic ion network on the surface of microband electrodes.

2.1 A TBATS modified electrode was placed in decane containing 1,1 dimethyl ferrocene. Cyclic voltammetry revealed reversible, undistorted peaked voltammograms indicating the I electron redox reaction of the electroactive compound. The electrochemical behaviour was constant for 9 hours. Figure 4.

2.2 A new TBATS modified electrode was placed in the significantly resistive solvent, hexadecane. Cyclic voltammetry revealed stable voltammograms for the redox compound, 1,1 dimethyl ferrocene, in hexadecane. The voltammetric response was constant for at least 9 hours at room temperature. Figure 5.

These experimental results show the excellent ability of TBATS to be used as an effective interface between the electrode transducer and highly resistive organic media.

2.3 Enhanced mass transfer through the TBATS interface in highly resistive organic media An insulated microband array electrode was modified with TBATS and placed in hexadecane containing 1,1' dimethyl ferrocene. At a scan rate of 0.005 V/s vs. CC+QRE, the cyclic voltammogram yielded a predominantly non-peaked current-voltage curve that indicated rapid diffusion of the redox compound through the TBATS matrix to the electrode surface. This behaviour was consistent over 9 hours in hexadecane. Figure 6.

2.4 Entrapment of highly mobile redox species in the TBATS interface in highly resistive, non-polar organic media.

Two TBATS/ potassium hexacyanoferrate (II) modified electrodes were prepared and placed in pure decane and hexadecane respectively. Cyclic voltammetry in both solvents yielded non-peaked voltammograms over 9 hours indicating predominantly non-planar diffusion of the water soluble redox compound to the electrode.

These results show that the TBATS material was significantly open when contacted by a highly resistive non-polar solvent permitting the rapid diffusion of solutes to the electrode. Figures 7 and 8.

An additional feature is that water soluble compounds while significantly mobile are strongly retained within the TBATS matrix. In other words, there is no leaching into the solvent phase. This is a major advantageous feature since the requirement for selective membranes is excluded.

2.5 Complex electrochemical reactions in TBATS matrix in highly resistive organic media In this example, it is shown how complex catalytic reactions can be coupled to redox compounds within the confines of the TBATS matrix in highly resistive organic media. In this example, the product of a catalytic biological redox catalytic reaction is used to demonstrate the electron transfer coupling to simple electroactive solutes.

A TBATS-polyphenol oxidase-potassium hexacyanoferrate (II) modified electrode was placed in decane.

Phenol was added so that the final cell concentration per addition was 250 uM. The biological redox catalyst, polyphenol oxidase, oxidised phenol in a two step reaction to catechol and then to the quinone product. The quinone species then underwent reduction following its reaction with potassium hexacyanoferrate (II). Potassium hexacyanoferrate (II) became oxidised to the potassium hexacyanoferrate (III) compound which diffused to the electrode and undenvent electrochemical reduction to regenerate the active form of the redox compound. Figure 9.

Experimental Methods Materials and Instrumentation All solvents were HPLC grade and stored over molecular sieves for at least 3 days prior to use; ferrocene and derivatives; alumina (0.3)1 diameter particle size) and Nafion in the perfluoroinated powder form suspended in a solution of lower alcohols were obtained from Aldrich (Gillingham, UK).

Tetrabutylammonium toluene-4-sulphonate was obtained from Fluka (Glossop, UK) and used as supplied.

Phenol was obtained from Sigma (Poole, UK). Polyphenol oxidase (PPO) [E.C. 1.1.3.13] (610 U / mg solid) was obtained as a lyophilised preparation from Sigma. An aerosol modified silicone conformal coating (RS Components. UK (stock no. 567-682) was used to insulate electrodes. All water was double distilled. Sodium phosphate buffer (0.1 M, pH 7.0) was used with polyphenol oxidase. Unless otherwise stated, all experiments were performed at room temperature (18 OC). The interdigitated gold microband array electrodes consisted of two separate gold electrodes on a single ST-quartz substrate. Each electrode formed an array of 50 microbands of 15,pom width and 5000 um length. The interdigitated configuration of the two electrodes resulted in 15pm spacing between adjacent electrodes. All electrochemical experiments were performed on an Auto lab Electrochemical Analyser (Ecochemie, Utrecht, Netherlands).

Electrochemical cells consisted of variously sized glass beakers.

Methods Insulation of microband electrodes Electrical insulation of the macro-regions of the interdigitated microband electrodes was necessary to confer the properties of enhanced diffusion to the microband elements and to decrease the iR potential drop.

Insulated regions were all the macro sized electrode areas except for a small section at the top of the dual gold electrodes where contact with the potentiostat connectors was made. To achieve good electrical insulation, a spray-on sealant was used (RS Components. UK). Firstly the electrodes were thoroughly cleaned by successive sonication in water and iso-propanol (x3) for 1 minute. After drying. an adhesive paper rectangle that covered the entire microarray section was cut out and placed onto the electrode surface A second rectangular strip covered the top sections of the macro sized electrode region where contact with the potentiostat connectors would be made. Ensuring no microbands were visible (under a magnifying lens), the sealant was spray deposited (8 passes, at an approximate distance of 0.3 meters) onto the electrode surface.The covered electrode was then left to heat cure in a drying cabinet at 72 OC for 3 days. Following this period, masked regions were etched off using water and a cotton bud. The resulting electrode was then rinsed in distilled water, polished with an alumina-water slurry, dried and inspected under the light microscope for adequate insulation. The insulation material under magnification gave a rough edge to the microarray sections. If successful (ie. all intended macro-regions covered with sealant), cyclic voltammetry in a 0.05 M solution of potassium hexacyanoferrate (II) was then performed with the electrode. The qualitative shape of the voltammograms at low scan rates determined the success of electrical insulation. If cyclic voltammetric scans revealed non-peaked voltammograms ( < 0.05 V/s), then insulation was considered successful.In all experiments with the microband electrodes, one set of arrays were used as working electrodes while the other set served as combined counter and quasi-reference electrodes (CC+QRE). In aqueous solution (0.05 M KCI). the potential difference between the gold microband electrodes and a saturated calomel electrode (SCE) was measured at 0.138 V.

Nafion modified electrodes in organic media Microband electrodes were modified with Nafion using the following method: A 0.83 wt % Nafion solution in iso-propanol was sonicated for 5 minutes. A clean, non-insulated microband electrode was vertically suspended and dip-coated into the Nafion solution for a period of 15 minutes. The electrode was immersed to the extent that all the microbands were completely submerged. Following this period, the electrode was raised and vertically suspended in air for a further 5 minute period at room temperature. The electrode was then vertically suspended in a drying cabinet at 72 OC for 30 minutes. Following a cooling period of 5 minutes on the bench. the modified electrode was readv for use.

A newly modified electrode was then placed in either dioxan, hexane or decane with no added electrolyte or buffer. The electron mediator, ferrocene, was dissolved in each solvent (0.0125M!. Cyclic voltammetry was then carried out within these solvent systems to investigate the electrochemical properties with the polymer film.

Organic ion network modified microband electrodes Ion networks of tetrabutylammonium toluene-4-sulphonate (TBATS) were prepared by sonicating a solution of the material (500 mg in either 3 cm3 sodium phosphate buffer or 0.1 M KCI). A 0.005 cm3 volume solution of TBATS was then deposited onto the surface of the insulated or non-insulated microband array electrodes, followed by drying (at 18 OC) usually for 15 minutes. Cyclic voltammetry at various scan rates was then performed in different pure solvents with a new modified electrode in the presence of 1,1 DMF (0.0125 M). Similar procedures were used for other ionic gels based on other tetra alkyl ammonium salts.

In some experiments, potassium hexacyanoferrate (II) (0.05 M) was dissolved into the TBATS solution prior to casting.

To demonstrate complex chemical reactions within the organic ion matrix the following method was used: polyphenol oxidase was co-dissolved with 0.05 M potassium hexacyanoferrate (II) into the TBATS solution (10 my / cm 3) and then dried usually for 20 minutes before immersion in solvent.

References 1. Clark, L.C. Jr & Lyons, C. (1962). Electrode systems for monitoring in cardiovascular surgery. Annals of the New York Academy of Science 102, 29-35 2. Michael, A.C. & Wightman, R.M. (1989). Voltammetry in supercritical carbon dioxide at platinum electrodes coated with perflourinated ion-exchange membrane. Analytical Chemistry 61, 2193-2200.

3. Patent application GB 9218376, 1993.

Figure Captions Figure 1. Succession of cyclic voltammogram s using Nafion modified microband electrode in dioxan (0.0125 M ferrocene). Lower case letters refer to time of measurement after electrode immersion in solvent: (a) 60s; (b) 120 s; (c) 180 s. Scan rate 0.2 V/s vs. CC+QRE.

Figure 2. Succession of cyclic voltammograms using Nafion modified microband electrode in hexane (0.0125 M ferrocene). Lower case letters refer to time of measurement: (a) 60 s; (b) 120 s; (c) 180 s: (d) 940 s. Scan rate o.2 V/s vs. CC+QRE.

Figure 3. Cyclic voltammogram of a Nafion modified electrode in decane (0.0125 M ferrocene). Scan rate 0.2 V/s vs. CC+QRE.

Figure 4. Succession of cyclic voltammograms using TBATS modified electrode in decane (0.05 M 1,1' dimethyl ferrocene). Lower case letters refer to time of measurement: (a) 60 s; (b) 120 s; (c) 180 s; (d) 240 s. Scan rate 0.2 V/s vs. CC+QRE.

Figure 5. Cyclic voltammogram using TBATS electrode in hexadecane (0.05 M 1,11 dimethyl ferrocene). Scan rate 0.2 V/s vs. CC+QRE.

Figure 6. Cyclic voltammogram using a TBATS electrode in hexadecane (0.05 M 1,1' dimethyl ferrocene). Scan rate 0.005 V/s vs. CC+QRE.

Figure 7. Cyclic voltammogram of a TBATS-potassium hexacyanoferrate (II) electrode in decane. Scan rate 0.005 V/s vs. CC+QRE.

Figure 8. Cyclic voltammogram of a TBATS-potassium hexacyanoferrate (II) electrode in hexadecane. Scan rate 0.005 V/s vs. CC+QRE.

Figure 9. Amperometry with a TBATS-polyphenol oxidase-potassium hexacyanoferrate (II) electrode in decane. Potential of working electrode was -0.650 V vs. CC+QRE. Small arrows indicate phenol addition (final cell concentration of each addition was 250 micro molar).

Claims (13)

1. A method for interfacing electrochemical measurement systems to highly resistive, non-polar media comprising an electrochemical cell of closely spaced microelectrodes and an ionically conducting film in contact with the organic phase, thereby establishing a method of measuring an electrical response related to the concentration of a chemical analyte within the resistive solvent phase

2. A method according to claim 1 in which an organic or inorganic catalytic material is included in the ionic film

3. A method according to claim 1 and 2 in which the catalytic material is derived from a biological system or is biomimetic

4. A method according to claim 1 in which the ionic film contacts a non-polar resistive solvent phase

5. A method according to claim 1 in which the ionic film is an organic electrolyte salt

6.A method according to claim 1 in which the ionic film is of tetrabutylammonium toluene4-sulphonate

7. A method according to claim 1 in which the ionic film is one of the following preferred salts, tetrabutyl ammonium perchlorate, tetrabutyl ammonium methane sulphonate, tetrabutyl ammonium phenol borate, tetraethyl ammonium tetrafluoroborate, tetraethyl ammonium chloride and tetrabutyl ammonium iodide.

8. An electrochemical sensor comprising a cell of two or more electrodes, means for permitting access of analyte to the sensing electrode, the sensing electrode comprising a conductor, an ionic film covering the electrodes where the support comprises an organic electrolyte salt that provides an interface to the resistive solvent phase; and measuring the electrical response of the cell; the response being relatable to the concentration of the analyte in the contacting resistive solvent phase

9. A electrochemical sensor according to claim 1 for measuring the concentration of phenols in non-polar organic solvents such as decane.

10. A system for performing electrochemical reactions of compounds that are soluble in the contacting non-polar organic phase

11. A system for performing electrocatalytic reactions of compounds that are soluble in the contacting non-polar phase involving non-biological, biological or biomimetic materials

12. An ionic film according to claim 10 and 11 comprising a matrix of an organic salt, preferably tetrabutylamnonium toluene-4-sulphonate

13. An ionic film according to claim 10 and 11 comprising one of the following preferred salts, tetrabutyl ammonium perchlorate, tetrabutyl ammonium methane sulphonate, tetrabutyl ammonium phenol borate, tetraethyl ammonium tetrafluoroborate, tetraethyl ammonium chloride and tetrabutyl ammonium iodide.