WO1999051973A1

WO1999051973A1 - Sensor devices and methods for using them

Info

Publication number: WO1999051973A1
Application number: PCT/GB1999/000985
Authority: WO
Inventors: Pankaj Madganlal Vadgama; Keith Stewart Robert Warriner
Original assignee: The Victoria University Of Manchester
Priority date: 1998-04-03
Filing date: 1999-03-30
Publication date: 1999-10-14
Also published as: GB2335985A; AU3159999A; GB9807112D0

Abstract

Copper electrodes coated on their surface with diamond-like carbon ('DLC'), and their use in electrolytic devices and procedures. The diamond-like carbon coating, usually less than 5 microns thick, enables the benefits of copper electrodes to be obtained in media less alkaline than the highly alkaline ones usually required for bare copper electrodes. The copper may be solid or carried as a surface layer upon a carrier of any desired shape or degree of conductivity or strength. The coated electrodes are especially advantageous as working electrodes in sensors and methods for electrochemical analysis and particularly for determination of analytes which undergo reaction, especially oxidation, at copper electrodes. They have especial value for electro-analysis of hydroxylated compounds, especially those relatively resistant to electrolytic oxidation (e.g. sugars and ethanol) and their operability at less alkaline pH enables convenient and easy monitoring of fermentation processes.

Description

SENSOR DEVICES AND METHODS FOR USING THEM.

This invention relates to sensor devices and methods for their use, and more particularly to improved sensor devices useful for electrolytic analytical methods for the detection and determination of aliphatic analytes, especially ethanol and carbohydrates .

It is known to make and use a variety of electrolytic sensor devices incorporating one or more electrodes to produce a signal output from which specific analytes can be detected and measured. These electrodes can act in several ways, for example by detecting such conditions as oxidation, reduction, acidity/alkalinity (pH) , electrical potential and current flow. The species which can be detected in this way include glucose, fructose, sucrose, lactose, ethanol, and many other compounds.

The detection and measurement of ethanol is of great commercial importance, as the wine and brewing industries are very extensive and taxes and duties are payable to governments on the basis of measurement of the ethanol content of fermentation products. Consequently, there is a great demand for reliable devices for monitoring the progress and efficiency of alcoholic fermentation processes by measuring the content of ethanol in them, and also, in many instances, other properties of the fermentation media, for example the reducing sugar (for example glucose) content as it is fermented into ethanol. The detection and measurement if carbohydrates (e.g. sugars) is also of widely applicable importance. This monitoring is also desirable for other process or waste liquors or effluents and also for finished or saleable products themselves.

Among the proposed sensor devices which have been proposed for carrying out such monitoring and measurement, many have contained enzymes - which act on the substrate chemical being evaluated and generates a different chemical which can be determined, thus providing means for - 2 -

determining the substrate chemical indirectly. Especially, glucose oxidase has been used because it catalyses oxidation of glucose to gluconic acid -- producing hydrogen peroxide via oxygen reduction. The hydrogen peroxide is very readily and conveniently determined electrolytically .

Existing sensor devices are not entirely reliable or durable for the more demanding industrial uses, for example continuous monitoring of the whole fermentation cycle, and suffer from the disadvantage of being unable to survive the heat sterilisation steps which are so important in the fermentation industries. These often involve temperatures as high as 140 degrees C (as in steam sterilisation) and enzymes are de-activated at such temperatures.

Therefore, current methods still rely on removing samples periodically from the fermentation process and determining the ethanol content - usually by specific density or chromatographic techniques. Clearly, this is inconvenient (as it does not give continuous measurement and may even contaminate the process media) and is slow. Therefore there is a need for an enzyme- free electrochemical sensor for aliphatic compounds such as alkanols (especially ethanol) , carbohydrates and the like which can withstand repeated steam sterilisation or rigorous chemical sterilisation, and thus can be part of the "clean in place" systems used industrially.

Ethanol and carbohydrates are is relatively inactive electrochemically, and electro-oxidation typically requires strongly alkaline solutions, but by using platinum electrodes it is possible to detect ethanol in neutral or acidic solutions. Even so, the response signals are pH- dependent and are also weak, so they are susceptible to masking by background (capacitative) currents or "noise, " and a practical sensor is not possible to produce.

The electroactivity of aromatic compounds (e.g. phenols) is enhanced by their conjugated structure but that of aliphatic compounds is much lower because of the absence - 3 -

of elements of structure which can stabilise a free radical product or lower the activation barrier for electrochemical oxidation. Consequently, electro-oxidation proceeds slowly, if at all. The electrochemistry of aliphatic compounds can, in some cases, be improved by selection of particular electrode materials (especially transition metals) or conditions of use, but these have not proved to be entirely satisfactory. It is well known, for example, that one can use electrode substrate surface materials which have an electron structure (either empty shells or un-paired electrons in the d-orbits) which promotes the adsorption of intermediate oxidation products and thus stabilises them -- for example platinum, gold, nickel, carbon and copper. However, such strongly adsorbing materials tend also to passivate rapidly, so that they have far too short a useful life. As a result, in practice, the list of suitable electrode materials tends to be very limited. Copper has been identified as a superior material, as it provides a high degree of stability, negligible drift and electro- catalytic properties for the detection of carbohydrates and ethanol .

Examples of publications describing the advantages of copper electrodes include :-

(1) Z.L. Chen and D.B. Hibbert (1997), J. Chromatography, A, 766, 27-33.

(2) J.F.Hong and R.P. Baldwin (1997),

J. Capillary Electrophoresis, 4, 65-71. (3) P. Singhal, K.T. Kawagoe, C.N. Christian and W.G. Kuhr (1997), Anal. Chem. , 69, 1662-1668. (4) K. Kano, M. Torimura, M. Goto and T. Ueda (1994) , J. Electroanal . Chem., 372, 137-143. However, it has been found that these major advantages of copper electrodes are difficult to utilise, as a very high alkalinity is essential for reliable performance (e.g. about 0.1M sodium hydroxide) and under less alkaline conditions, even at pH 12, copper becomes unstable and only negligible electrochemistry towards analyte compounds, especially aliphatic compounds, can be observed. Thus, even when the strong alkali is replaced by a buffered solution (e.g. a phosphate buffer) at pH 12, the desired activity vanishes. In view of this, any prospect of effective use in media of much lower pH (e.g. fermentation liquors) is to be dismissed and the use of copper for making "on line" measurements in real sample matrices, for example fermentation liquors, has hitherto been very restricted.

Highly alkaline solutions are dangerous to handle and undesirable in commercial procedures, and are best avoided unless their use is essential and no convenient alternative is available. Therefore there is a need for a way for getting the benefits of copper as an electrode material while using it at more moderate pH conditions, which can enable users to avoid the requirement - hitherto essential to make it effective - for the inconvenient and dangerous solutions of strong alkali.

We have now found, surprisingly, that this can be achieved and the performance of copper without the presence of highly alkaline media can be greatly enhanced by the simple procedure of coating the copper surface with a film or layer of diamond-like carbon (conveniently referred to as "D C") . Such coated electrodes represent a very significant advance over un-modified copper electrodes, as they enable the benefits of copper electrodes to be obtained in media less alkaline than the highly alkaline ones usually required for bare copper electrodes.

Thus according to our invention we provide an improved copper electrode characterised in that it has a copper surface which carries a coating of diamond-like carbon (conveniently referred to as "DLC") .

We also provide the improvement in electrolytic - 5 -

devices containing a copper electrode, which comprises using a copper electrode having a copper surface which carries a coating of diamond-like carbon, especially in conjunction with an alkaline electrolyte. Further, we provide improved electrolytic procedures in which a liquid medium is contacted with a copper electrode, characterised in that the copper electrode has a copper surface which carries a coating of diamond-like carbon. This applies especially to those procedures wherein the liquid medium is an alkaline medium or electrolyte.

We find that the coated copper electrodes of our invention are especially advantageous as working electrodes in sensors and methods for electrochemical analysis and particularly for determination of analytes which undergo reaction, especially oxidation, at copper electrodes.

Thus according to our invention we also provide an improved sensor device, useful in electrolytic analysis procedures, which comprises a working electrode having a copper surface which carries a coating of diamond-like carbon .

Further, according to our invention we also provide a method for electrolytic analysis of a medium to determine an analyte component therein, which comprises contacting the said medium with a sensor device or electrode system as defined above, i.e. a sensor device or electrode system which comprises a working electrode having a copper surface which carries a coating of diamond-like carbon.

The medium containing the analyte sought is usually an electrolyte and preferably a liquid. The medium may be the sample itself, as provided for examination, but if it does not already contain the analyte then conventional means may be used to get the analyte (or the sample containing it) into solution in it . The method may be carried out by using the electrode or sensor in conventional manner and using it to obtain a - 6 -

signal output from it which can be measured and used as a measure of analyte content .

The electrodes and methods of the present invention are useful for detection and determination of a variety of organic compounds as analyte substrates to be subjected to electrolytic action at the said electrodes and so are capable of being detected electrochemically . Thus the invention may be applied to any compounds which could benefit from the known advantages of a copper electrode surface but especially to those which have, until now, been unable to do so because of the known disadvantages of copper, for example the requirement for highly alkaline conditions as indicated above.

Compounds which can be substrates to be detected and determined by the electrodes and methods of our invention include in particular those compounds which have oxygen in their chemical structure, notably hydroxy or hydroxylated compounds, and especially those which tend to be relatively resistant to electrolytic oxidation. Such substrate compounds may be aliphatic, cyclo-aliphatic or aromatic in nature (or any combination of these types) , and may be saturated or unsaturated, provided they contain the oxygen- hydroxy or hydroxylated element . Preferred examples of such compounds include carbohydrates and sugars (for example glucose) and alkanols (aliphatic alcohols) , and especially ethanol .

Usually, the DLC-coated copper electrodes of our invention will be used as anodes .

Diamond-like carbon is already well-known in itself and described in the art, and commonly referred to as "DLC". A variety of publications describe it, and a convenient summary is contained in our International Patent Application No. PCT/GB 93/00982 (publication No. WO.93/24828) . DLC is a form of amorphous carbon or a hydrocarbon polymer with properties approaching those of diamond rather - 7 -

than those of other hydrocarbon polymers . Various names have been used for it, for example "diamond-like hydrocarbon" (DLHC) and "diamond-like carbon" (DLC) , but the term "DLC" appears to be comprehensive and the most common. It possesses properties attributable to a tetrahedral molecular structure of the carbon atoms in it, similar to that of diamond but with some hydrogen atoms attached. It has been described in the art as being a designation for "dense amorphous hydrocarbon polymers with properties that differ markedly from those of other hydrocarbon polymers, but which in many respects resemble diamond" [J.C. Angus, EMRS Symposia Proc . , 12, 179 (1987)].

The formation and application of the diamond-like carbon (DLC) to the membrane material as coatings or films for the purposes of the present invention may be carried out by methods known in the art . It is usually formed by decomposition of carbon-containing compounds in gaseous or vaporised form (particularly hydrocarbon gases) induced by radiation or electrical fields. Thus, it may be prepared from hydrocarbon precursor gases (e.g. propane, butane or acetylene) by glow-discharge deposition, by laser-induced chemical vapour decomposition, by a dual-ion beam technique, or by introduction of the hydrocarbon gases directly into a saddle-field source. A saddle-field source is a source of ions produced by a collision between gas atoms excited by thermionic emission, and this method is preferred because it allows heat- sensitive materials to be coated by a beam that is uncharged -- so facilitating the coating of insulating or non-conductive materials.

Its properties can vary according to the particular raw materials used and its mode of formation. It can also be made in other ways, for example by sputtering solid carbon, as an alternative to dissociating hydrocarbon gases.

Further description of diamond-like carbon (DLC) , - 8 -

including its constitution, nature and properties, and the variations in its form which can be made, and modes for its preparation, are to be found for example in the following published references (among others) :- (a) "Diamond-Like Carbon Applied to Bio-Engineering Materials;" A.C. Evans, J. Franks and P.J. Revell, of Ion Tech Ltd., 2 Park Street, Teddington, TW11 OLT, United Kingdom; Medical Device Technology, May 1991, pages 26 to 29. (b) "Preparation and Properties of Diamondlike Carbon Films;" J. Franks; J . Vac . Sci . Technol . Vol. A, No .3 , May/June 1989, pages 2307-2310;

(c) "Biocompatibility of Diamond-like Carbon Coating;" L.A. Thomson, F.C. Law, J. Franks and N. Rushton; Biomaterials, Vol.12, January 1991 (pages 37-40);

(d) "Categorization of Dense Hydrocarbon Films;" J.C. Angus; E.M.R.S. Symposium Proc . , 1987, Vol. 17, page 179; Amorphous Hydrogenated Carbon Films, XVII . June 2-5 1987, Edited by P.Koide & P. Oelhafen. (e) "Properties of Ion Beam Produced Diamondlike Carbon Films;" M.J. Mirtech; E.M.R.S. Symposium Proc, 1987, Vol. 17, page 377;

(f) "Diamond-like Carbon - Properties and Applications;"

J. Franks, K. Enke and A. Richardt; Metals & Materials (the Journal of the Institute of Metals) ; and

(g) U.S. Patent No. 4490229; M.J.Mirtich, J.S.Sorey & B.A.Banks .

The convenient source of the carbon is a hydrocarbon gas or vapour, especially one which is readily decomposed by an electric field or discharge. A very convenient source gas is acetylene, though others may be used if desired. Individual hydrocarbons (or mixtures thereof) may be used, and diluent gases may be added if desired. The decomposition/deposition procedure may be carried out at pressures at atmospheric or above or below atmospheric, as found most suitable for particular instances. The - 9 -

thickness of the DLC coating or deposit may be varied according to the particular requirements for its use, depending upon such factors as the nature (physical and chemical) of the material upon which the DLC is deposited, and its porosity or permeability, and the particular characteristics appropriate to its intended use. The coating is conveniently carried out at a rate which allows the deposit to adhere to the membrane material and form a coating of the desired thickness - preferably also evenly coated so as to cover substantially all the surface without leaving any areas too thinly covered or even un-covered. When using acetylene as a source, for example, the deposition may be carried out at a rate of up to 0.5 yum per hour, though higher or lower rates may be used if desired. For the present invention, the coating may be made of a thickness which may be varied according to the particular requirements desired for the performance of the sensor and the system to be analysed. Thus, the thickness of the coating or deposit may be in the range 0.01 to 5 yum, but thicker or thinner coatings may be used if desired. A typical and convenient coating deposit is one approximately 0.1 /urn thick, but this is not necessarily the optimum for all purposes . The thickness in any particular case will depend upon such factors as the nature (physical and chemical) of the material upon which the DLC is deposited, its porosity or permeability, and particular characteristics appropriate to the intended use of the sensor. The coating is conveniently carried out at a rate which allows the deposit to adhere to the copper sufficiently to form a coating of the desired thickness - preferably evenly coated so as to cover substantially all the surface without leaving any areas too thinly covered or even un-covered.

It is not essential for the DLC coating to cover the entire surface of the copper, but as bare copper is ineffective it is sensible for the DLC to coat as much as - 10 -

is practicable of the operative electrode surface.

The reasons for this effect of DLC on the copper electrode is not fully understood, but clearly it makes a great difference to the behaviour of the copper electrode. The electrode may be made of solid copper or it may be made up of a layer or surface of copper carried upon another material - for example nickel or aluminium. The copper used to form the electrode, especially at its surface, is preferably a substantially pure form of copper (at least 99% pure) for good conductivity, especially at its surface, but may if desired be alloyed with another metal provided that any other component present does not interfere with the activity of the copper. The key feature is the presence of copper atoms at the surface in juxtaposition to the diamond-like carbon, and the absence of any other metals available for competing electrochemical reactions at the same polarisation voltages used for the copper .

The electrode may be in any conventional form, for example sheet or wire, or as a coating deposited or carried upon a conventional substrate or support of any desired shape or degree of conductivity or strength.

This method of analysis according to the present invention is especially applicable to the monitoring, measurement and assessment of media in which ethanol is the analyte substrate. These are, especially, media in which ethanol is being formed or produced -- e.g. fermentation media, in which ethanol is formed by alcoholic fermentation of sugars -- and for monitoring and controlling the progress of fermentation processes. Thus, it is especially useful for analysis of the media used in the course of making alcoholic beverages, as well as the "finished" alcoholic products, e.g. alcoholic beverages for which fermentation is not being continued before sale, storage or later treatment. The operability of the coated copper electrodes at pH values nearer to neutrality (at or near - 11 -

neutrality) makes this more practicable, easy and convenient, as the damaging effects of high alkalinity can be avoided. It is also applicable to other alcoholic media, e.g. distilled or fortified spirits, whether intended for use as beverages or not, and for the study of waste materials which may contain or give rise to ethanol.

The pH of a sample liquid contacted with copper electrodes in a sensor device normally has a great effect on the utility when the copper is not coated with DLC, to the extent that bare copper requires a strong alkalinity (for example 0.1M NaOH) .

In contrast, the coating of DLC overcomes this dependency upon pH and a DLC-coated copper electrode enables the sensor to be stabilised and have a much lower pH dependency than bare un-coated electrodes, allowing use at near-neutral pH values in relatively weakly buffered solutions - even as low as pH 8.

The principal advantage of our sensors is that they can be used in conditions of variable pH or over a range of pH values without having to use highly alkaline media, and they can be subjected to heat without their performance being destroyed.

Diamond-like carbon coatings have the advantages of a high degree of chemical inertness and compatibility with a wide variety of media.

An especial advantage of the sensor of the present invention is that it permits the reliable analysis of samples for their content of the desired analyte (e.g. ethanol or glucose) concentrations at convenient pH levels at or near neutrality (e.g. at pH levels down to pH 8) and avoids the previous need for very high alkalinity. This was not possible with the known forms of copper-containing sensor. Of course, the value of the invention is not restricted to being solely applicable to the analysis of samples for their ethanol or glucose content or to fermentation liquors, and it may be applied to analysis of - 12 -

other media containing quite high concentrations of glucose or other electrochemically active species (as indicated herein) at levels which commonly present considerable problems for analysis . Such other media include a variety of a media of organic or non-organic origin, for example chemical process liquids, and the like.

The DLC-coated copper electrode may also be used in a variety of positions within apparatus, for example as a detector at the end of columns on process or analytical equipment for purposes of monitoring or control of operation. Also, the DLC-coated copper electrodes of our invention may be used as a replacement for copper electrodes or the like on devices which at present rely upon the use of highly alkaline electrolyte to make them function -- e.g. in fuel cells -- so that hazards and other disadvantages of the highly alkaline electrolytes can be avoided and less caustic electrolytes can be used.

It is a notable point (though one which is not fully understood yet) that the layer of DLC on the surface of the copper electrodes can be removed readily by simply wiping with a tissue. This suggests that, even though no strong linkage between the DLC and the copper occurs, nevertheless the mere presence of the DLC is sufficient to alter the properties of the underlying copper electrode in such a significant and substantial way as we have now found.

The preferred mode of electrolytic analysis is that known as Differential Pulsed Voltametry ("DPV") . In this, a "staircase pulsed" potential is applied to the coated copper electrode (i.e. the voltage applied is increased in a series of pulses or steps of increasingly higher voltage, each step being maintained steady for a predetermined time before the next increase) and the generated current flow is measured before and after each pulse.

The reason for this is complex as the electrochemistry of aliphatic compounds at copper electrodes is very complex and involves a sequence of reactions which lead ultimately - 13 -

to the formation of formic acid and carbon dioxide. The performance of the copper surface is critically dependent upon the copper at the electrode surface; the reason for this is not entirely clear but appears to depend on the copper undergoing transition through its various oxidation states, i.e. Cu(I), Cu(II) and Cu(III), and, because of this, "ramp" potential techniques (which use increasing applied voltages) -- e.g. Voltametry -- are favoured over static potential techniques such as amperometry. Indeed, in amperometric mode copper electrodes give high background

(capacitive) currents and are readily passivated by adsorption of the oxidation products derived from the carbohydrate/ethanol . The DPV technique is one way in which this problem can be overcome . The benefit of using this DPV technique is that it enables the background capacitative current to be minimised and the oxidation transition stages of copper to occur. Both these factors contribute to high stability and electro-catalytic oxidation for carbohydrate/ethanol detection.

For our DLC-coated copper electrodes, the value of DPV alone can be limited in practice as it can give poor peak resolution in graphs plotting current against potential . In ordinary DPV the "peak height" is used to represent the response and a significant part of the response is neglected (both sides of the peak) . We find that the technique of chronocoulometry has the advantage of overcoming this. In chronocoulometry, the values of current over a range of potentials are integrated, as for example when a "potential ramp" is applied, and this enhances the peak by measuring more of its total bulk than just its maximum value.

The voltages applied to the electrode in this procedure can be varied over a considerable range, and may be varied according to the substrate compound being detected or determined. Thus the applied voltage is usually in the - 14 -

range - 0.9 volts to + 0.9 volts, though voltages outside this range may be used if considered appropriate, and a convenient range is that from - 0.2 volts to + 0.4 volts. The applied voltage may be operated at substantially constant level or in incrementally increasing steps e.g. by 10 mV at a time with an amplitude of 25 mV. The voltages mentioned here are made with reference to a standard silver/silver chloride (Ag/AgCl) electrode.

In use, the electrode of our invention can be used to carry out the method of our invention by immersion

(together with an associated cathode) in a predetermined volume of a buffer solution to be analysed, and applying a polarising voltage so that the measurements can be made and compared before and after the addition of the blood or serum sample under test. The procedure may also be calibrated by use of solutions containing known amounts of the substances sought, and its accuracy this checked and confirmed.

The sample under examination may be stirred or un- stirred, as desired or convenient.

The electrolytic procedure for use of the sensors of our invention may be carried out over a considerable range of temperatures, e.g. in the range 20 to 40 degrees C. It is usually important that the temperature used for calibration is within approximately 4 degrees C. of the assay temperature.

For calibration, an isotonic or other other buffer may be used, but it is preferable to use one which has an ionic strength similar to that of the sample to be examined. The medium is commonly aqueous, but need not necessarily be so, and an organic solvent may be used if desired (as such, or in admixture with each other and/or water) provided it is an electrolyte and dissolves any desired reagents, but is not medically relevant to the assay carried out.

For this purpose, the electrode may be immersed in a - 15 -

sample of the fluid (e.g. fermentation liquor) and then linked with a suitable reference electrode (for example a silver electrode or a calomel electrode) in conventional manner. Measurement of the voltage, current and the like may be taken and the measurements taken and recorded as desired, intermittently or continuously. For this, conventional apparatus may be used.

Samples of the media for examination may be obtained by standard methods. The quantity of sample material should be sufficient to cover the electrode and the current measured at a fixed time or after a stable response has been achieved. Likewise, samples of other media may be obtained in any convenient manner and brought into contact with the sensor of the present invention for the purpose of component detection. Alternatively, the sensor may be dipped into the sample liquid or immersed in the sample liquid so that the copper electrode makes good contact with the sample .

Similar procedures and conditions may be used for analysis of other media of a biological or biochemical nature, with modifications as will appear appropriate to an expert in the art having regard for the nature of the media, the components sought, and the conditions and requirements of the measurement . The sensor devices and electrodes of our invention need not necessarily be made with a single electrode element, but can if desired be made in the form of a multiplicity of separate electrode elements. Such separate elements can be made small (as in a so-called micro-array or micro-electrode array) , and may be connected electrically in any convenient way so that the signal output from them can be measured. For such arrays, the coating of DLC is especially useful, as it provides a much more viable, convenient and reliable form of electrode. An advantage of using multiple small electrode elements is that the surface of an array of them can be made to have - 16 -

various surface formations and configurations which can assist the construction and use of the device in practice. Also, such arrays can allow use of higher current densities, which can decrease background noise. For example, the surface can be made in a pitted form (i.e. a form in which the surface has a multiplicity of recesses or "pits" which are small enough to hold the coating of DLC) . These recesses or "pits" can then, if desired, be provided with means over or around their entrances (especially "guard electrodes" - conveniently in the form of small rings or conducting regions to which an appropriate voltage potential can be applied) to affect or control the entry of charged solutes into the recesses and so to the underlying electrodes. Calibration of our sensor device for use can be carried out in conventional manner, preferably by immersion of the device in samples of the medium which is to be monitored or examined (e.g. fermentation liquor) . An isotonic or other buffer may be used, but it is preferable to use one which has an ionic strength similar to to that of the media in which the device is to be used.

The invention is illustrated but not limited by the following Example, in which parts and percentages are by weight unless otherwise stated.

EXAMPLE :

COMPARATIVE DATA: USING BARE COPPER ELECTRODES.

Planar electrodes, 2.5 mm by 2 mm, of bare copper were subjected to differential pulsed voltametry in 0.1 M aqueous sodium hydroxide solution.

Measurements were made of the current flow before and after the application of each potential, and the resulting figures were plotted against the potential to which they - 17 -

related .

This gave rise to an anodic peak at about 0. IV (against a standard silver/silver chloride reference electrode) in the presence of carbohydrates (glucose, fructose, sucrose, lactose) in the solution.

When the same copper electrodes were transferred to a phosphate buffer solution containing the carbohydrate (50mM, pH 12) , no such anodic peaks were detected.

DATA OBTAINED USING DLC-COATED COPPER ELECTRODES.

Planar electrodes, each 2.5 mm by 2 mm, made of copper were coated with DLC for periods of 1.5 , 2 and 2.5 minutes

(to achieve different coating thicknesses for the DLC) , and then subjected to differential pulsed voltametry in 0.1 M aqueous sodium hydroxide solutions, as for the bare copper electrodes mentioned above.

In contrast to the bare copper electrodes, which gave a sharp anodic peak during carbohydrate oxidation (in a strongly alkaline medium) the DLC-coated ones gave very broad anodic peaks which were harder to differentiate from base line currents. This different behaviour may have been due to the DLC causing restriction of access of the carbohydrate to the underlying copper electrode surface . To overcome this behaviour, we measured the area of the peak instead just its maximum value (the peak height) .

This was done using the technique of chronocoulometry, in which the potential is scanned between two pre-selected potentials (i.e. a starting potential below that of the redox potential at which electroactivity of the analyte compound (carbohydrate) is observed and finishing at an over-potential beyond that of the redox potential for the analyte compound. The current generated during each potential step is then integrated over a selected time period. The net benefit derived from this use of chronocoulometry is that the entire redox process is taken - 18 -

into account and not just the redox peak as found with techniques such as DPV (Differential Pulsed Voltametry) .

For aliphatic compound measurement (determination) the potential step was from -0.4 volts to +0.2 volts (against a Ag/AgCl reference electrode) for a duration of 10 seconds. The response recorded was obtained by subtracting the total current generated in the presence of the analyte from that obtained in its absence (i.e. for a corresponding solution from which the analyte is omitted) . By using a DLC-coated copper electrode in conjunction with chronocoulometry it was possible to detect glucose and ethanol in phosphate-buffered solution at as low a pH as 8. The responses for ethanol were found to be greater than those for glucose. 0

Claims

- 19 -CLAIMS : -

1. An improved copper electrode characterised in that it has a copper surface which carries a coating of diamond- like carbon (conveniently referred to as "DLC").

2. An electrolytic device, especially useful in conjunction with an alkaline electrolyte, containing a copper electrode which has a copper surface which carries a coating of diamond-like carbon as claimed in Claim l.

3. An electrolytic device as claimed in Claim 2 which is a sensor device, useful for electrolytic analysis procedures, which comprises a working electrode having a copper surface which carries a coating of diamond-like carbon, as claimed in Claim 1.

4. An electrode or sensor device as claimed in any of Claims 1 to 3 wherein the coating of diamond-like carbon has a thickness in the range 0.01 to 5 yum, and preferably approximately 0.1 /urn thick.

5. An electrode or sensor device as claimed in any of Claims 1 to 4 wherein the working electrode is of solid copper, for example in sheet or wire form.

6. An electrode or sensor device as claimed in any of Claims 1 to 4 wherein the copper of the working electrode comprises a layer or surface of copper carried upon another material or as a coating deposited or carried upon a conventional substrate or support, for example nickel or aluminium, for example in sheet or wire form.

7. An electrode or sensor device as claimed in any of Claims 1 to 6 wherein the copper, at least in the copper surface, is substantially pure copper, preferably at least 99% pure.

8. An electrolytic device as claimed in any of Claims 1 to 7 which is a fuel cell .

9. An electrode or electrolytic device, especially a sensor device, containing a copper electrode carrying a - 20 -

coating of diamond-like carbon substantially as described with reference to the accompanying examples .

10. An electrolytic procedure in which a medium, usually a liquid electrolyte medium, is contacted with a copper electrode, characterised in that the copper electrode has a copper surface which carries a coating of diamond-like carbon, as claimed in any of Claims 1 to 9.

11. An electrolytic procedure as claimed in Claim 10 wherein the liquid medium is an alkaline medium or electrolyte.

12. An electrolytic procedure as claimed in Claim 10 or Claim 11 wherein the copper electrode carrying a coating of diamond-like carbon is used as an anode.

13. An electrolytic procedure as claimed in any of Claims 10 to 12 which is a method for electrolytic analysis, which comprises contacting a liquid medium under examination with a sensor device or electrode system which comprises a working electrode having a copper surface which carries a coating of diamond-like carbon, as claimed in any of Claims 1 to 10, and using a signal output as a measure of analyte content .

14. A method for electrolytic analysis as claimed in Claim 13 wherein the analyte substrate compound to be detected and determined is a compound which has oxygen in its chemical structure, especially a hydroxy or hydroxylated compound .

15. A method as claimed in Claim 14 wherein the analyte substrate compound is a carbohydrate, a sugar (for example glucose) or an alkanol (aliphatic alcohol) .

16. A method as claimed in Claim 15 wherein the analyte substrate compound is ethanol .

17. A method as claimed in any of Claims 12 to 16 as used for the monitoring, measurement or assessment of a fermentation medium, an alcoholic product or an alcoholic beverage. - 21 -

18. A method as claimed in any of Claims 13 to 17 wherein the sensor is used with analyte-containing media at a pH level at or near neutrality.

19. A method as claimed in any of Claims 13 to 18 wherein the sensor is used to carry out electrolytic analysis by differential pulsed voltametry ("DPV") .

20. A method as claimed in any of Claims 13 to 19 wherein the sensor is used with an applied voltage in the range -0.9 to +0.9 volts and preferably in the range -0.2 volts to +0.4 volts, relative to a silver/silver chloride electrode .

21. A method of electrolytic analysis using an electrode which has a copper surface which carries a coating of diamond-like carbon, substantially as described, with reference to the accompanying Examples .