WO2001013103A1

WO2001013103A1 - Analytical apparatus and method

Info

Publication number: WO2001013103A1
Application number: PCT/GB2000/003108
Authority: WO
Inventors: Pankaj Madganal Vadgama; Andrew Gill
Original assignee: Iit Limited
Priority date: 1999-08-14
Filing date: 2000-08-11
Publication date: 2001-02-22
Also published as: AU6459900A

Abstract

Electrodes having a surface coating of an oxidised phenolic compound with a surface-active agent incorporated in it, preferably by forming the coating by deposition from a solution containing the surfactant. The electrodes are preferably of platinum and are useful as anodes for electrochemical detection of hydrogen peroxide, especially amperometrically. The hydrogen peroxide may be formed by action of an enzyme, usually on or near the electrode. The electrodes can be surrounded with a membrane to control access of contaminants and minimise mechanical damage to the coating. The membrane can be a permeable polymer layer, acting by porosity or selective permeability, and a combined construction of the coated electrode (as anode) with a cathode can be a convenient sealed unit. The main use intended is for determining an analyte, especially glucose through its interaction with glucose oxidase, particularly in biological environments and especially measurement of the components of blood and serum while eliminating interferents.

Description

ANALYTICAL APPARATUS AND METHOD.

This invention relates to an improved analytical method and apparatus, more particularly those useful for electro-analytical work and especially in amperometric electrodes for use in chemical and biochemical sensing, and to analytical methods using them.

It is known to use electrodes in analytical procedures to detect and measure a variety of chemical species present in a liquid medium. The procedures may involve the measurement of various properties of the medium so that the presence of particular chemical species or groups of such species may be detected or measured. It is desirable to make these measurements as accurate, quantitative and selective as possible, so that the result of the measurements can be interpreted and used with maximum reliability.

One use for such electrodes is in the analysis of blood or serum, to detect amounts of components which may be very small but also very significant for diagnostic purposes. Especially, it is often important to measure hydrogen peroxide -- which may be generated in situ as a result of the action of cellular enzymes (as present in leukocytes) or an immobilised or soluble oxidase enzyme. Oxidases are used as reagents in various biological media, and so it is often desirable to be able to detect their activity.

Unfortunately, biological fluids, especially blood and serum, contain a variety of components which interfere with these methods, and especially the electrolytic ones. These result in interference or electrode fouling, so that the measurements become inaccurate or even impossible to make. The presence of a variety of drugs, which the patient may have used, also can interfere seriously with the accuracy of the measurement. Thus, the electrodes suffer from the disadvantages of being subject to interference from a variety of chemical species other than the ones sought, so that the accuracy of the measurements often falls short of the reliability required to be clinically usef l.

It has been proposed to reduce this interference in use by lowering the polarising voltage applied to the electrode, but this is not always satisfactory because reduction of the voltage sufficiently to reduce the interference usually makes it impossible to maintain a voltage high enough to achieve the electrode reaction desired for the measurement to be made.

It has also been proposed to reduce this interference by interposing membranes between the electrode and the liquid medium (e.g. serum) being analysed, to exclude interfering components. In this, the membrane acts in a selectively permeable manner, physically holding back some components and allowing others to pass through and reach the electrode. This has not been entirely satisfactory because of the difficulty of finding a membrane material which will have the desired selectivity and also the problems of relying entirely on a fragile barrier.

In our International Patent Specification No. PCT/GB 93/01568 (International Publication No. WO.94/02629) and the corresponding European Patent No. 0672167, there is described an improved method for making an electrode having a coating of selective permeability, which comprises electrolytically oxidising a phenolic compound at the surface of the said electrode, whereby selectivity can be varied in a controlled manner, and also new electrodes having a coating of an oxidised phenolic compound on their surface. These electrode coatings give selectivity in favour of hydrogen peroxide to amperometric use as sensor .

The coated electrodes so described are very effective, but we have found that they can be further improved by incorporating a surface-active agent in the said coating of oxidised phenolic compound. Thus according to our invention we provide new electrodes having a coating of an oxidised phenolic compound on their surface, characterised in that the said coating of an oxidised phenolic compound has a surface- active agent incorporated therein.

We have found that the surface active agent is preferably incorporated in the coating of an oxidised phenolic compound by forming the coating of an oxidised phenolic compound in the presence of the surface active agent, so that it is incorporated into the coating as it forms.

This procedure may incorporate the surface-active agent by physical adsorption or trapping or by involvement chemically in the in the electro-oxidised film structure as it is formed, or by a combination of such modes, as we have found that the surface-active agent cannot be incorporated by simply applying the surface-active agent to a pre-formed coating (e.g. by dipping in a solution of the surface- active agent) . Thus according to our invention we also provide an improved method for making an electrode having a coating of selective permeability, which comprises electrolytically oxidising a phenolic compound at the surface of the said electrode in the presence of a surface active agent. The methods used for the formation of the coating of electrolytically oxidised phenolic compound are those more fully described in the specification of our European Patent No. 0672167, and all the details of that specification are imported and incorporated herein by reference, including details of the said methods and coatings.

The deposit on the electrode surface, though formed by electrolytic oxidation, may sometimes be referred to as a "polymerised phenol" though this term may not be entirely appropriate. It is referred to herein as an "oxidised phenol." Formation and deposition of the film of electro- oxidised phenolic compound is conveniently carried out using the phenolic compound and the surface-active agent in solution in an aqueous medium. However, it is not essential to use a wholly aqueous medium, and other media may be used - for example organic solvents or mixtures which have the appropriate conductivity to allow the electro-oxidation and film deposition to take place and can dissolve the starting components. If desired mixed media which are partly aqueous may be used, for example using a proportion of water mixed with organic solvents or components.

The electrode itself may be any electrode having the properties of an anode at which the current for peroxide, or any other species which may be sought, can be determined. It is preferably platinum metal, but it may be any other conventional form and/or material, for example gold or glassy carbon.

The phenolic compound may be substituted (for example with alkyl groups, halogens, a ino groups, carboxylic acid groups, or combinations of these) but should allow ortho- to para- radical polymerisation following generation of a radical phenol electrochemically, and preferably any substituents should be ones not causing major steric hindrance. The term phenolic compound or "phenol" is also used here to include polyhydric phenolic compounds (for example catechol) and polycyclic phenolic compounds (for example naphthols) , which may if desired be substituted, as mentioned above. Especially useful examples of phenolic compounds are phenol itself and dopamine, but other phenolic compounds, often of relatively complex structure but nevertheless still phenolic in nature, may be used if desired. Examples of these include phenol, phenol red, rosolic acid, New Fuchsin, Disperse Blue 1. Mixtures may be used if desired. Conveniently, the voltage applied to form the coating or film is in the range + 0.4 to + 0.8 volts (+400 to +800 mV) or higher if so desired, and commonly about + 0.6 volt

(600 mV) , as measured against a silver/silver chloride electrode, but voltages outside this range may be used if desired. In general, using a lower voltage requires a longer deposition time, and the properties of the deposited film may be modified or controlled by the voltage level used to form it. We have found that some phenolic compounds are less readily oxidised (e.g. rosolic acid) and for these the use of higher voltages -- e.g. up to about 900mV -- may be used to hasten the formation of the coating. We find that 650 mV is often a very convenient voltage for most purposes and at least for any phenolic compounds other than those most resistant to oxidation.

The surface active agent may be any of those known in the art. These usually contain a hydrophilic group (e.g. a long-chain hydrocarbon group) in conjunction with a hydrophilic group which may be ionic or non-ionic. For the purposes of this invention the surface-active agent may be of any nature -- i.e. it may be anionic, cationic or non- ionic, or even may have a structure combining more than one of these characteristics. Likewise, mixtures or combinations of more than one surface active agent may be used if desired.

Examples of such surface-active agents include many which are commercially available under trade names or otherwise known in the art, for example :- (a) Anionic (usually sulphonated or sulphated compounds, as free acid or salts), for example :- taurocholic acid. This compound is (2- [3-alpha, 7- alpha, 12-alpha trihydroxy-24-oxo-5-beta-cholan-24-yl] ethane sulphonic acid, (derived from taurine and cholic acid) and is available from Sigma, catalogue number T4009. sodium dodecylsulphonate. dodecylbenzenesulphonic acid.

(b) Cationic (usually quaternised amino compounds) , for example : - methyltrialkylammonium chloride ("MTAC") . This is available as Adogen 464, Aldrich catalogue number 85,657-6 (the alkyl being C8 to CIO) . cetyltrimethylammonium bromide .

(c) Non- ionic (usually hydroxy or amino compounds condensed with various proportions of ethylene oxide and/or propylene oxide to form condensates containing poly-oxyalkylene chains), for example :-

Pluronic 68. This is a commercially available product derived from a polyethylene oxide/polypropylene oxide condensate (Sigma catalogue, number P7061) .

Triton X100. This is a commercially available product which is stated in the supplier's list to be a condensate of 4-octylphenol with approximately 10 molecular proportions of ethylene oxide, in which the octyl substituent is 1, 1, 3, 3-tetramethylbutyl .

(d) Both anionic and cationic (zwitterion type) : N-dodecyl-N, -dimethyl-3-ammonio-l-propanesulphonate .

Mixtures of two or more surfactants may be used if desired, and these may be of the same or different types. Indeed, we find that a combination of anionic and cationic properties is advantageous as this gives superior results . The two properties may be combined together in the same molecule, as in the zwitterion type (mentioned above) , but may be obtained by using a mixture of both cationic and anionic types of surfactant together, for example by using both methyltrialkylammonium chloride and taurocholic acid.

For incorporation in the electro-oxidised phenolic film, the surface active agent is preferably included as a component in the liquid medium (solution) containing the phenolic compound used for the formation of the electro- oxidised coating by deposition. This makes it more easy for the surface active agent to become incorporated in the film as it is formed. The surface active agent may be present at the start, before electrolytic oxidation begins, or may be added after the oxidation has started and some of the coating has commenced to form.

The surface-active agent is preferably not left in contact with the bare surface of the electrode material (e.g. platinum) for long before the electro-oxidation is started, to minimise the possibility of it being adsorbed first on to the electrode surface and thereby affecting the desired formation and/or adhesion of the electro-oxidised coating. Any extensive adsorption of the surface-active agent on the surface of the electrode (e.g. platinum) can impede formation of the electro-oxidised phenolic coating, and it is desirable to start with a well-cleaned surface. Exposure to the surface-active agent does not spoil the surface immediately and we have found that exposure for periods of 10 to 15 minutes (and even longer) usually can be tolerated without adverse effects, but it is a sensible precaution to avoid any unnecessary or prolonged exposure to the solution of phenolic compound and surface-active agent before the electro-oxidation starts. Likewise, high concentrations of the surface-active agent in the solutions used for forming the coating can also impede the formation of the electro-oxidised phenolic coating, and so are best avoided by keeping the concentration of surface-active agent as low as is practicable - i.e. enough to perform the effect of modifying the coating without having excessive amounts beyond this. Fortunately, the adverse effect is sometimes avoided by the limited solubility of the surface- active agent in the solution used. Therefore, it is in general most convenient for the surface-active agent to be included in the solution used for forming the coating at the start provided these possible causes of adverse effects are borne in mind. The proportion of the surface active agent used in forming the coating in this way may vary, depending upon such factors as the particular surface active agent and phenolic compound used, and the optimum in any particular case may be determined by simple trial.

In general, we find that an important factor is the extent to which the surface active agent forms micelles (and is therefore present as such) during formation of an electro-oxidised coating. Therefore we prefer that the concentration of the surface active agent in the liquid medium (solution) used for the electro-deposition of the coating is kept below the critical micelle concentration

("CMC") of the surface active agent. This minimises any possible effects which tend to hinder the deposition process . The CMC can be expected to vary according to the surface active agent used, and possibly also other conditions, but in general we prefer to use the surface active agent in low concentration - below 0.1% (by volume for liquid surface active agents or by weight for solid surface active agents), and especially 0.05% or even less. Higher concentrations may be used if desired, subject to reasonable trial to optimise them to suit the particular components use and properties desired in the final coating, but this is usually not necessary as the smaller amounts indicated give satisfactory results. Preferably the amount of surface-active agent used should not lead to appreciable micelle formation.

The method of our invention may be used to produce new electrodes which are useful for the analysis in a biological environment, for example in biological fluids and under biological conditions. Especially, it provides electrodes which are useful for the measurement of the components of blood and serum while eliminating inter erents . By controlled film deposition, it is possible to form an array of cross-reacting sensors which enable the user to "pattern recognise" some analytes .

In use, the electrode may be immersed in a sample of the fluid (e.g. blood) and then linked with a suitable cathode 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. The method can be carried out using conventional electrodes, for example as in an oxygen cell, but with the anode as the platinum metal and the cathode as the silver/silver chloride electrode (which also acts as a pseudo-reference electrode) . A three-electrode cell of working/reference/counter electrodes can also be employed if desired for the coating procedure using the phenolic compound .

Operating in the amperometric mode, the use of the electrode as anode in a liquid medium as electrolyte displays a high degree of selectivity to hydrogen peroxide and is exceptionally free from interference from the presence of a number of compounds which normally are prone to interfere considerably, especially ascorbate and paracetamol (acetaminophen) . Such selectivity is superior to that obtainable using the conventional cellulose acetate membranes, as in oxidase-based bio-sensors, and also has the added advantage of giving superior results even when compared with those obtained using electrodes with the electro-oxidised phenol coatings without the surfactant incorporated in them.

To improve the selectivity attainable, and also to protect the coated electrode from possible damage, e.g. attrition or erosion which could break the continuity of the coating, we prefer to surround the electrode with a membrane to avoid mechanical damage to the coating. This membrane may be composed of any permeable polymer layer, for example a dialysis membrane or a polycarbonate with pores, depending upon the application intended.

According to our invention we also provide a new composite electrode device comprising as anode an electrode having a coating of an oxidised phenolic compound on its surface (as more fully described above) in conjunction with a second electrode as cathode, with the two electrodes being held together in an assembly which prevents direct electrical contact between them.

In one construction, a cathode surrounds the anode, and is spaced from it by a non-conductive spacer element which also secures a membrane in place around the anode and the cathode in a sealed relationship which provides a means for retaining an appropriate electrolyte between the membrane and the anode and also provides some added physical protection of the anode against surface damage.

The electrode can also be associated with an enzyme, so that the electrode can extend its usefulness to the detection and measurement of an analyte compound which does not itself produce a suitable response at the electrode but can be converted, for example by enzymic action, into another compound which can produce a more suitable response at the electrode. For example, hydrogen peroxide generated by enzymic action from a substrate compound can be used to provide an indirect measure of that substrate compound. An especially useful example of this is the determination of glucose, using glucose oxidase as the enzyme. The enzymic reaction can be at or near the electrode surface, so the enzyme may be incorporated on or near the electrode or the apparatus in which it is used or it may be further away from the electrode.

According to our invention we also provide an improved method for the detection and measurement of hydrogen peroxide or any other desired species, which comprises adding a sample material under examination to a liquid medium and then detecting by electrolytic analysis (especially amperometric analysis) the amount of hydrogen peroxide (or other desired species, as the case may be) present as such or indirectly formed (e.g. from an enzymic reaction) using an electrode having a coating of selective permeability made by electrolytically oxidising a phenolic compound at the surface of the said electrode according to the present invention.

This can be used to measure the amount of analyte (hydrogen peroxide, etc.), however it may be present. Thus it may be present as such or generated in situ, for example by enzymic action, especially an enzymic reaction at or near the electrode surface. Use of an enzyme allows the substrate on which it acts (e.g. glucose) to become the analyte even though it is not detected directly as such at the electrode .

When the hydrogen peroxide to be measured is generated by enzymic action, the enzymes used (usually oxidases) may be any of those known in the art, and they may be located anywhere in the system which is found to be convenient, provided they are not detrimental to the function of the

"oxidised phenol" coating. Thus the enzyme may be free

(e.g. in solution) or immobilised, and may even be made part of an electrode or sensor assembly. Such enzymes, methods for immobilising them, and forms of construction for using them with or as part of an electrode or sensor system are well known in the art .

The sample may especially be a biological one, for example a biological fluid and especially blood or serum. The liquid medium, which acts as the electrolyte in the cell containing an electrode of the present invention, may be any in which the desired action of the enzyme can take place and in which the components are soluble. Thus, it need not necessarily be aqueous, and organic solvents (as such or as mixtures with each other and/or water) may be used provided that they do not interfere with the desired electrochemical processes and the deposition of the film on the anode. Preferably, the electrolyte is an aqueous buffer solution (most conveniently a phosphate buffer solution) which maintains the pH of the solution and added sample at a desired level. This pH is not critical in the broad sense, as it depends principally on the pH needs of the enzyme for its activity. An example is thus approximately pH 7.4 (the pH of blood) .

The proportion of the hydrogen peroxide or peroxide precursor in it is preferably in the range 10^~5 to 10 m/1 of the liquid medium, though larger or smaller proportions may be used if desired. Water-miscible organic solvents may be present if they do not interfere with the essential oxidation or reduction processes involved. The procedure and apparatus to be used for carrying out the amperometric electrolytic analysis, and for displaying and/or recording the resulting measurements, may be any of those which are known or conventional for the purpose . According to our invention we also provide an improved electrode system for use in amperometric analysis which comprises an electrode as defined above, i.e. an electrode having a coating of selective permeability comprising a film of an oxidised phenolic compound on its surface. The sample of the blood or serum for examination may be obtained by standard methods . The quantity of the blood or serum added to the liquid medium should be such as to provide a quantity in the range 10 ³ to 10 m/1 of the liquid medium. The procedure can be much improved by surrounding the anode with a membrane which serves to reduce the access of undesired materials to the anode surface. This membrane may act by virtue of its effectiveness as a dialysis membrane (i.e. by impeding passage of larger molecules) or as a perm-selective membrane (which controls the passage of molecules or ions according to their properties other than just physical size) . These function to reduce the fouling of the electrode and increase accuracy. such a membrane may be for example composed of conventional materials, especially a polycarbonate, cellulose acetate, nitro- cellulose, or the like.

A convenient example is an outer polycarbonate membrane of porosity 0.03 to 0.05 ^m, which allowed the phenolic compound to pass through to the platinum surface. This (a) prevented any damage or scratching of the surface films and (b) allowed slower, more controlled deposition of the phenolic compounds.

The physical state of the membrane may be controlled in terms of such features as thickness, pore size, and any other feature which may have an effect on it permeability so as to control diffusion to the electrode surface to prevent excess of solute exposure. The membrane and/or anode may be prepared for use in the analytical process of the invention by soaking it, when it is in place around the anode, in a solution corresponding to the electrolyte medium before the blood/serum sample is added.

In use, the polarised electrodes (or a composite structure comprising both anode and cathode in a single assembly) may be immersed in a predetermined volume of the buffer solution to which can be added the sample (for example a sample of blood or serum) under test, so that the amperometric measurements can be made and compared before and after the addition of the sample under test . The procedure may also be calibrated by use of solutions containing known amounts of any components sought or believed to interfere, so that the results can be properly standardised and the apparatus calibrated.

Conventional apparatus may be used, for the cell, electrodes and the measurement and recording of the current-time relationships for the samples under test. Measurements may be made continuously or intermittently, as desired. The advantages of the invention are that measurements can be made with reduced interference from other components of blood and serum, which can be eliminated or at least greatly reduced to a level at which they no longer interfere with the measurement to a degree which renders the measurements unreliable or misleading under clinical conditions .

Use of poly (phenol) coatings prepared by direct electropolymerisation on the electrode surface, modified through inclusion of various surfactants during polymerisation, provides a means to improve sensor interface stability.

Success of incorporation has been determined on the basis that current flow at the electrode has decayed rapidly upon addition of phenol monomer in the presence of surfactant to below the original baseline response without the surfactant . Response of the sensor to the various interferent species (ascorbic acid, acetaminophen, catechol, uric acid, all of which are invariably present in clinical samples) is greatly reduced and in some cases negated. The action of whole blood on the response of the sensor to the various test compounds has been investigated and we have found that the use of mixed cationic/anionic species gives the greatest improvement in selectivity for peroxide over the other species.

In summary, the modification of the surface characteristics of poly (phenols) through surfactant incorporation permits specific tailoring of conditions to match the requirements of the system under investigation. This may for example be due to charge-charge interactions, but also could be due to some other functional group that could be introduced to the sensor surface. For example, variations can be made not only to the phenolic compound which is electro-oxidised and the thickness of the coating formed from it, but also the surface-active agent (or the components and proportions in a combination of such surface-active agents) can be varied to modify the properties of the coated surface. Also, as the ionic properties of the modified coating can be varied, an opportunity is available to cope with varying properties (e.g. the pH) of the media in which the coated electrode is used for analytical or monitoring purposes.

All this, coupled with the fact that poly (phenols) also provide reproducible and uniform coatings on the electrode surface, permits modification of three- dimensional structures that would be difficult -- if not impossible -- to achieve by other means. Therefore, in the manufacture of complex electrode geometries it becomes possible to apply uniform, ultra-thin membrane barrier layers, which confer both biocompatibility and selectivity and to a considerable extent reduce the need to provide any protecting or selective membrane covering.

Throughout this description, the term "surface active agent" has been used, but if desired the term "surfactant" may be regarded as equally satisfactory.

Claims

What we claim is :

1. Electrodes having a coating of an oxidised phenolic compound on their surface, characterised in that the said coating of an oxidised phenolic compound has a surface- active agent incorporated therein.

2. Electrodes as claimed in Claim 1 wherein the surface active agent is incorporated in the coating of an oxidised phenolic compound by forming the coating of an oxidised phenolic compound in the presence of the surface active agent, so that it is incorporated into the coating as it forms.

3. Electrodes as claimed in Claim 1 or Claim 2 wherein the coating of electrolytically oxidised phenolic compound are those more fully described in the specification of European Patent No. 0672167.

4. Electrodes as claimed in any of Claims 1 to 3 wherein the electrode has the properties of an anode at which the current for peroxide or any other species which may be sought may be determined.

5. Electrodes as claimed in any of Claims 1 to 4 which are gold or glassy carbon or, preferably, platinum metal.

6. Electrodes as claimed in any of Claims 1 to 5 wherein the oxidise phenol coating is derived from a phenolic compound, which may be substituted, that allows ortho- to para- radical polymerisation following generation of a radical phenol electrochemically .

7. Electrodes as claimed in any of Claims 1 to 6 wherein the oxidised phenolic coating is formed electrolytically, preferably at a voltage in the range + 0.4 to + 0.8 volts (+400 to +800 mV) and especially about + 0.6 volt (600 mV) as measured against a silver/silver chloride electrode .

8. Electrodes as claimed in Claim 7 wherein the electro- oxidised coating has been made by including a surface active agent as a component in the liquid medium (usually a solution) containing the phenolic compound used for its formation by deposition.

9. Electrodes as claimed in any of Claims 1 to 8 wherein the coated electrode is surrounded with a membrane which serves to reduce the access of undesired materials to the anode surface and minimise mechanical damage to the coating.

10. Electrodes as claimed in Claim 9 wherein the membrane is composed of a permeable polymer layer, for example a dialysis membrane or a polycarbonate with pores, or a material acting by virtue of its effectiveness as a dialysis membrane (i.e. by impeding passage of larger molecules) or as a perm-selective membrane (which controls the passage of molecules or ions according to their properties other than just physical size) .

11. Electrodes as claimed in any of Claims 1 to 10 wherein the electrode coated with the oxidised phenolic coating (and especially with a surrounding membrane) is in conjunction with a second electrode as cathode, with the two electrodes being held together in an assembly which prevents direct electrical contact between them.

12. Electrodes as claimed in any of Claims 1 to 11 wherein a cathode surrounds the coated anode, and is spaced from it by a non-conductive spacer element which also secures a membrane in place around the anode and the cathode in a sealed relationship which provides a means for retaining an appropriate electrolyte between the membrane and the anode and also provides some added physical protection of the anode against surface damage .

13. Electrodes having a coating of an oxidised phenolic compound incorporating a surface-active agent, substantially as described.

14. Method for making an electrode as claimed in any of

Claims 1 to 13 which comprises forming an oxidised phenolic coating on the electrode surface by electrolytically oxidising a phenolic compound at the surface of the said electrode in the presence of a surface active agent, preferably at a voltage in the range + 0.4 to + 0.8 volts (+400 to +800 mV) and especially about + 0.6 volt (600 mV) as measured against a silver/silver chloride electrode.

15. Method as claimed in Claim 14 wherein the electro- oxidised coating is made by forming the coating in the presence of the surface active agent, especially by forming it from the phenolic compound by deposition from a liquid medium (usually a solution) of the phenolic compound in which the surface active agent is included as a component .

16. Method as claimed in Claim 14 or Claim 15 wherein the concentration of the surface active agent in the liquid medium (solution) used for the electro-deposition of the coating is kept below the critical micelle concentration ("CMC") of the surface active agent, for example in concentration below 0.1% (by volume for liquid surface active agents or by weight for solid surface active agents) .

17. Method as claimed in any of Claims 14 to 16 wherein the surface active agent component possesses both cationic and anionic characteristics, either together in the same molecule or by use of separate anionic and cationic surface-active agents together.

18. Method for making a coated electrode substantially as described.

19. Use of an electrode claimed in any of Claims 1 to 13 for the purpose of determining or monitoring an analyte .

20. Use of an electrode, as claimed in Claim 19, wherein the analyte is glucose.

21. Use of an electrode, as claimed in any of Claims 18 to

20, for carrying out analysis in a biological environment, for example in biological fluids and under biological conditions, especially the measurement of the components of blood and serum while eliminating interferents .

22. Use of an electrode, as claimed in any of Claims 19 to 21, for the detection and measurement of hydrogen peroxide - present as such or generated in situ, for example by enzymic action, especially an enzymic reaction at or near the electrode surface.

23. Use of an electrode, as claimed in Claim 22, wherein the hydrogen peroxide is generated by enzymic action from glucose, especially using glucose oxidase as the enzyme.

24. Use of an electrode, as claimed in any of Claims 19 to 23, wherein the measurements are made amperometrically .

25. Use of an electrode for the purpose of determining or monitoring an analyte, especially glucose, substantially as described.