WO1999030144A1

WO1999030144A1 - Sensor devices and analytical method

Info

Publication number: WO1999030144A1
Application number: PCT/GB1998/003662
Authority: WO
Inventors: Pankaj Madganlal Vadgama; Ian Mcintyre Christie; Peter Barrington Long
Original assignee: The Victoria University Of Manchester
Priority date: 1997-12-11
Filing date: 1998-12-08
Publication date: 1999-06-17
Also published as: CA2313795A1; EP1038172A1; AU1495299A

Abstract

Sensor systems and devices with a detecting (especially ion-selective or redox) electrode and reference electrode with the detecting electrode (and preferably both electrodes) covered by a permeable barrier interfacing with samples under examination so that the main bridging contact between the electrodes to complete a measuring circuit is not through the sample. The permeable barrier encloses a liquid zone (e.g. electrolyte) at the electrodes and controls, by restricting diffusion, the passage of analytes and impurities in either direction through the barrier. The main function of the permeable barrier is to slow the rate of diffusion of analyte. More rapid identification of the analyte content in a sample can be facilitated, e.g. by 'loading' the liquid zone with analyte or impurity to control diffusion. The strongly preferred method for analyte determination is based on using measurements of the rate of change of the detecting electrode's output potential as analyte diffuses through the barrier.

Description

TITLE: SENSOR DEVICES AND ANALYTICAL METHOD.

This invention relates to sensor devices and more particularly to electrochemical sensor devices, and to analytical methods using them.

Various types of electrodes are known for use in the electrochemical analysis of samples, and one of these is the ion-selective electrode (conveniently referred to as an "ISE"), which functions on the basis of measuring the electrical potential of the ISE when in contact with the sample. This requires two electrodes - one being the ion- selective electrode (ISE) and the other being a reference electrode assembly. Such electrodes are well-known. The reference electrode assembly usually employs a liquid junction between (a) the sample and (b) the reference electrode and its associated internal electrolyte. The liquid junction maintains electrical continuity in the electrochemical cell while restricting contamination of the inner electrolyte of the reference electrode assembly by the sample.

Other potentiometric measurements are made based on redox reactions at electrodes, preferably metals such as platinum and gold. A reference electrode is required for these measurements, and both the redox and the reference electrode assemblies should contact the sample.

The known methods and devices for using detecting electrodes such as an ISE or redox electrode for detection and/or measurement purposes are all based on the simple procedure of putting both the detecting electrode and the liquid junction of the reference electrode assembly in contact with the sample, and then measuring the electrical potential between the detecting electrode and the reference electrode. Appropriate analytical conclusions are drawn from the measurements of this potential, e.g. by comparison with the potential generated when standard solutions are used. However, the known devices and systems have been found to suffer from disadvantages so that they are not entirely satisfactory in use, because the standard modes for using them require careful calibration against standards and also stabilisation before they can give accurate or reliable results .

Proposals have been made for various forms of active electrode which are covered or surrounded by a membrane, as a feature which can serve to protect the electrode from physical damage or, more commonly, to retain a controlled internal electrolyte or liquid film or to confer other selective control over how the different components from a sample can gain access to the electrode. Such control is often needed when a sample under examination contains compounds which can interfere seriously with the detection of desired analytes at the electrode - sometimes by behaving similarly to the analyte and sometimes by deactivating (fouling) the electrode so that it ceases to function properly. This selectivity can act in several ways, but with the aim of holding back an undesirable interfering component while the desired analyte can pass on towards the detecting electrode. This can be simply by molecular size or molecular charge (polarity) , but an alternative way is to incorporate into the membrane a reactive component which may either destroy the undesirable interferents or convert the desired component into another compound which is more readily able to reach the electrode and be determined there. An example is an enzyme electrode, particularly one in which glucose oxidase is used to catalyse the oxidation of glucose to form by-product hydrogen peroxide which readily passes on to the electrode.

Despite their merits for some purposes, membrane barriers are regarded as a necessary nuisance that slows up responses when rapid response and equilibrium are wanted.

As most active sensing electrodes are used in amperometric mode for detection and measuring analyte contents of samples, the conventional procedure of plotting measurements of current flow approaches zero (or at least very small current values) and this level (a "baseline") can be used for reference or as a starting point for making measurements of the analyte content. In contrast to this, ISEs operate in a very different way, as they generate the voltage to be measured and normally respond within milliseconds to seconds, and the desired baseline can be almost anywhere in terms of any measurable mV values. Therefore satisfactory measurements with such electrodes can be made very difficult by this phenomenon, termed "baseline drift."

We have found that these problems can be overcome by using a membrane which is not selective in favour of a desired analyte, and may even slow up access of the desired analyte to the electrode. This is novel and in contrast to the known methods of using membranes, where the membrane is acts in the opposite manner and is used to impede the access of undesirable interferents without impeding the access of desired analyte. Especially, we have found that a sensor system using a permeable barrier (e.g. a membrane) in this novel way can enable an analyte to be determined very much more readily, by measuring the rate of change of output signal from the detecting electrode, which is caused by the regulated diffusion of the analyte through the membrane. Plotting the output signal from the ISE against time gives a measure of this rate generates a graph in which the slope determines the measured parameter. Any slowness in the response is no longer a nuisance, so long as the electrode system and measuring equipment can discern this rate of change, usually as the slope of the response chart or graph, and more reproducible readings can be obtained by ensuring that the membrane transport (the rate of passage of the analyte through the membrane) is the determining factor for the procedure . - A -

In our knowledge and experience, ISEs in particular have so far been used only in bare form, without a covering membrane, so the covering technique is new - especially for an ISE and, we believe, for other detecting electrodes too. It offers the further advantages that the signal from the electrode may not need to fully reach equilibrium, and of being applicable to any other electrode which functions in a non-amperometric manner.

So, we have now found that the difficulties can be overcome by covering the detecting electrode with a permeable barrier of restricted permeability which then interfaces with the sample, so that the sample itself no longer provides the sole bridging contact between the detecting electrode and reference electrode assembly to complete the measuring circuit. This permeable barrier allows the system to operate by diffusion of components, between the sample and the region within the permeable barrier, before the output signals from the electrodes are measured and such measurements are used as a basis for the determination of the composition of the sample. For this, the permeable barrier covers at least the detecting electrode, and preferably both the detecting electrode and the reference electrode assembly.

Thus according to our invention we provide a sensor system comprising a detecting electrode and a reference electrode in combination, characterised in that the detecting electrode is enclosed within a permeable barrier adapted to interface with a sample under examination, so that the sample itself no longer provides the main bridging contact between the detecting and reference electrodes to complete the measuring circuit .

Preferably, both the detecting electrode and the reference electrode are enclosed within the same permeable barrier to separate them from the sample . Thus according to our invention we also provide a sensor device comprising a detecting electrode and a reference electrode in combination, characterised in that these are both enclosed within a permeable barrier adapted to interface with a sample under examination, so that the sample itself no longer provides the main bridging contact between the detecting electrode and the reference electrode to complete the measuring circuit .

Our invention also provides an improved method for the determination of an analyte in a sample, which comprises using a sensor device with an ion-selective electrode or redox electrode and a reference electrode assembly in combination as described herein.

Thus according to our invention we also provide a method for the determination of an analyte in a sample, which comprises using a sensor system or device with a detecting electrode and a reference electrode in combination, characterised in that the detecting electrode

(and preferably also the reference electrode) is enclosed within a permeable barrier adapted to interface with a sample under examination, so that the sample itself no longer provides the main bridging contact between the detecting and reference electrodes to complete the measuring circuit, measuring the potential between the detecting electrode and the reference electrode and using this measure for determining the content of the analyte . An arrangement in which the permeable barrier encloses the detecting electrode, together with the reference electrode, is advantageous in use and gives better results.

The two types of electrode used (the detecting electrode and the reference electrode) are well-known and are amply described in the literature. Their precise form and construction are not critical but the following summary assists in describing them.

The reference electrode may be any of those known or used in the art . The preferred and most convenient form of reference electrode assembly comprises a conventional half- cell containing a silver/silver chloride (Ag/AgCl) or calomel electrode system, with its conventional filling solution (usually an electrolyte) , enclosed in a container which has a "liquid junction", "salt bridge" or "double junction" arrangement customarily intended to make contact with the sample. This "liquid junction" is commonly a porous membrane which serves to allow the necessary electrical conductivity to complete the electrical measuring circuit while restricting flow and/or diffusion of the sample or its components into the reference cell or any outward flow to contaminate the sample. The presence of a further permeable barrier between the reference electrode and the sample, as in the present invention, also provides a further safeguard against such contamination.

For ease of fabrication, forms having simple geometry, for example planar forms, are most convenient. Construction may also be simple. For example, a Ag/AgCl electrode may be overlaid with a membrane of material such as polyvinyl alcohol, which readily hydrates and (in terms of the electrode potential) the arrangement is satisfactorily stable, especially when the electrode is required for only a short measurement duration.

The detecting electrode for use in our invention may be any of those known or used in the art for detecting analytes by producing output signals representative of a component or characteristic which can provide a measure of the analyte present in a sample under examination. The preferred detecting electrode is one which is not normally used in an amperometric measuring mode, and the signal output (potential) is preferably measured by a non- amperometric method.

Especially, it may be an ion-selective electrode

("ISE") for example a conventional ion-selective electrode with an internal electrolyte and an internal reference

(e.g. Ag/AgCl) or a coated wire electrode where a base metal wire is in direct contact with a covering ion- selective membrane. The invention is applicable to a variety of alternative forms of detecting electrode, for example redox electrodes, and is not limited only to use for ISE devices.

In this specification the term "ion-selective" has been used, but it should be noted that in the art the term "ion-sensitive" is also used, and these two terms are to be regarded as interchangable for the purposes of this invention.

The ISE response is a potential change across a membrane or coating, and the ISE assembly includes this layer and an internal electrode, covered or enclosed by the coating or membrane material having the ion-sensitive or ion-selective properties. This coating or membrane surface interacts with the ionic components from a sample to generate a measuring voltage (EMF) and may allow preferential interaction or passage of those ions which it is desired to measure. the internal electrode may be a conventional one capable of use for measurement of electrochemical potentials such as Ag/AgCl but may also be, for example, metals such as platinum, gold, silver or copper (though others may be used if desired) as in coated wire electrodes. Examples of coatings include those containing additive components which are ion-selective in the sense of having powers for ion-exchange, ion- adsorption, complex-forming neutral compounds or chelating ions, or the like, or combinations of such properties. Examples include liquid ion exchangers, neutral carriers and plasticisers (solvents) retained physically or incorporated into a polymer layer in any combination or singly. The potential forms at the coating or membrane, or across it; the internal electrode is there simply to make a contact and, like the reference electrode, to be used to measure potential between two points either side of the membrane . Likewise, solid-state electrodes may be used without the need for an ion-selective coating if they already themselves possess the necessary ion-selective properties.

The ISE usually comprises a covering or layer (e.g. a membrane or a gel) over an internal core sensing electrode and the components imparting ion-sensitivity may be in or on the internal core sensing electrode and/or the said covering or layer. A common form of ISE may contain, when in use, an inner medium (commonly an electrolyte) , which may conveniently be in liquid or gel form, and associated contacting electrodes . The inner medium is usually aqueous but may be non-aqueous if desired, or contain a combination of aqueous and non-aqueous components.

Alternatively, it may be a solid-state electrode, for example any of those available commercially. One form of these can be used to detect chloride ions (Cl^~) . Examples of this type include ion-selective field effect transistors (conveniently referred to as "ISFET" devices) .

The detecting electrode (e.g. ISE) can be in any convenient shape. It is easy and convenient to make them in planar form, for example as a flat form of the coated wire.

It is possible also for the reference electrode to be another ISE which is under conditions which make it produce a stable EMF, in a manner comparable to a true conventional reference electrode. This can, if desired, be another ISE with an inner (and constant) stable electrolyte within the porous liquid-junction membrane.

The permeable barrier surrounding the detecting electrode - or the detecting electrode and reference electrode - may be of various materials and forms. Its main function is to enclose the detecting electrode (or the detecting electrode and reference electrode assembly) and providing the means for contact with the sample, but also it may serve to hold the two electrode systems together as a single assembly, and even provide some protection for them against damage from contact with other bodies. Also, it usually contains a zone of liquid (e.g. electrolyte) medium enclosed within the permeable barrier. As the main function of the permeable barrier is to slow the rate of diffusion of analyte, it is preferably is not selective in favour of the analyte sought . The permeable barrier may be made of any material which can provide the desired degree of permeability towards the sample or its components in addition to a sufficient degree of cohesion, strength and durability to maintain its physical integrity while the device is in contact with the sample and in use .

Conveniently, it may be simply a membrane or a gel, but it may comprise any combination of these - using one or more of either type of material (which may be the same or different) if desired. As its principal purpose is only to regulate, by diffusion, access of the sample or its components to the detecting electrode and the reference electrode assembly, its composition and form are not critically important. The range of suitable materials is therefore quite conveniently extensive, (and can be used to improve selectivity) , which is one of the advantages of our invention. Also, when the detecting electrode (e.g. ise) and the reference electrode are both under the cover of the permeable barrier, it does not matter whether or not the barrier may produce a potential of its own (e.g. a "membrane potential") because such a potential, if generated, does not alter the potential difference which we wish to use -- i.e. the potential between the detecting electrode and the reference electrode .

The diffusion is a function of the concentrations of the species (e.g. ion species) on opposite sides of the permeable barrier, and in its simplest form that is all that is required of it, as it then functions only to regulate the access of the sample or its components to the detecting electrode. This regulation can be very helpful in keeping the concentration of the components affecting the detecting electrode potential within limits which allow ease of measurement or preventing excessive amounts contacting the detecting electrode and distorting the output signal or potential from it and consequently distorting the accuracy of measuremen . The diffusion is usually inward diffusion (i.e. through the permeable barrier from the sample towards the detecting electrode) by the analyte species (e.g. ionic species) to be determined. This results in the detecting electrode being uniformly exposed to the inwardly diffusing species to be measured. It also has the advantage that the detecting electrode is less likely to be exposed to any unsuitably high concentration of the analyte species before the measurement is completed - as could be the case if the sample contains a very high ionic concentration and an ISE or redox electrode, as detecting electrode, were exposed directly to the sample. In our present method, by the time concentration levels within the permeable barrier rise sufficiently to cause problems, the measurement usually can have been completed. Furthermore, it causes the concentration of the analyte in contact with the detecting electrode to change progressively as diffusion proceeds, and this change - especially the rate of change of concentration - is an exceptionally useful basis for the determination of analyte (e.g. ion) content which we wish to make and a key advantage provided in our present invention.

Alternatively, as a variant, the contents of the permeable barrier may be provided with a concentration of the analyte species (the analyte ion when the detector electrode is an ISE) which is higher than that in the sample to be examined, so that diffusion of the analyte species will then be outwards - away from the detecting electrode and into the sample - so resulting in a decrease of its concentration adjacent to the detecting electrode. This mode can also be used, as it is the rate of change that can be more important for the measurement purposes than the the absolute concentration itself or whether it is increasing or decreasing - especially when using an ISE.

In the simplest form of our invention no selectivity is necessary for the material of the permeable barrier but, if desired, the permeable barrier may have selective properties, especially in the way it limits diffusion. This limitation of diffusion is distinct from selectivity on the basis of other phenomena. This limitation of diffusion is distinct from an ISE membrane selectivity phenomenon. This could be important in improving the apparent selectivity of a detecting electrode, especially a redox electrode, and improved usefulness of our invention may be secured by making the barrier of a material which provides some degree of selectivity. This may then enable the device to exclude any components which could compromise the selective functioning of the detecting electrode, and so serve as a means for eliminating problematic interference with measurements being made. For this variant, a complete or high degree of selectivity may not be necessary, and even a partial discrimination against access by particular components may be sufficient to ensure satisfactorily reliable measurements - depending upon the particular application of the invention and the nature of the sample and/or the components sought to be determined. It is usual for a zone of liquid (e.g. an electrolyte medium) to be required between the detecting electrode and the reference electrode within the permeable barrier, as a "filling," though some forms of detecting electrode may be able to function without the need for any such filling electrolyte,

This electrolyte medium may be provided in a variety of ways .

(1) It may be provided in the device as made. This allows the device to be made in a form suitable for sale or storage but also for immediate use. For this form, the electrolyte can be in the form of a hydrated gel, which is both practicable and convenient .

(2) It may be added at the time it is to be used, for example by making the device in a form in which the detecting electrode (e.g. an ISE or redox electrode) is surrounded by the permeable barrier material, and then soaking it in a suitable liquid (e.g. electrolyte solution) to make it ready for use. An example of this mode is to use a thin permeable membrane (e.g. a dialysis membrane) around the electrodes. Such a membrane may comprise any conventional material, e.g. cellulose or cellulosic material as often used for dialysis membranes. If desired, the robustness of the construction or the degree of permeability can be obtained by using multiple layers of the membrane (which may be the same or different) . Using four layers of dialysis membrane can provide a very convenient form of such a device - though four is a number found to be convenient, and not an obligatory one.

(3) It may be provided by obtaining the necessary liquid from a sample itself, by permeation of water and diffusion of any required electrolyte ions from the sample through the barrier when the device and the sample are brought together.

Our preference is for the last of these, (3) , but the most readily practicable way we prefer is that marked (2) above .

The permeable barrier is adapted to interface with a sample under examination by the fact that at least the detecting electrode - and preferably both the detecting electrode and the reference electrode assembly - are enclosed within the permeable barrier. This enables all that is required of a sensor device to be included in a single unit, by carrying the detecting electrode and the reference electrode assembly upon a support which serves to hold the assembly together while insulating the detecting electrode and the reference electrode assembly from each other. Various forms of construction may employed. For example, if the detecting electrode and the reference electrode assembly are assembled upon a substantially flat insulating support, the permeable barrier may take the form of a "bubble" or "envelope" over them. This allows the sample under study to be applied to the side of the insulating support on which the electrodes are exposed, and the electrical connections to the measuring circuit. The electrical connecting leads to the electrodes will need to be properly insulated, both chemically and electrically, from the media around them so as to avoid any interference of loss of the electrode signals.

Alternatively, the electrical connections to the two electrodes can be made in the usual manner and all the leads and connections from them insulated and sealed to pass through the region within the permeable barrier. This allows the whole device to be made in a form in which the permeable barrier is more extensive and covers more than just the area required for sample to be applied and access the electrodes, so that the barrier can cover as much of the device as desired - even the whole of it - so making it easier to use by dipping into a sample.

The measuring circuit may be any of the conventional ones for electrochemical measurement, and use conventional apparatus (meters, recording devices, and the like) for detecting an EMF or potential differences, and the signals from the electrode system of our devices can be interpreted and converted to specific measurements of components by conventional methods . When the detecting electrode is an ISE or redox electrode, it may suffer interference from unknown amounts of another species, e.g. ions or molecules. This interference can be reduced or eliminated by filling the enclosed region (around the detecting electrode and within the permeable barrier) with a zone of liquid (e.g. an electrolyte) containing an appropriately high concentration of this other species (especially an ion species) liable to interfere with the desired measurements, so that it can diffuse out through the permeable barrier and thereby reduce interference from that species if present in the sample. In this way, by "loading" the filling within the permeable barrier, the concentration of the interfering species can near the detecting electrode can be kept substantially constant and its potentially troublesome effects can be reduced - and especially it can be kept effectively constant with time - so that measurements showing the rate of change of potential in the output signal due to the desired analyte can thus be distinguishable and used as the basis for determination of analyte content . The components of a sample which can be determined by the use of an ISE according to the present invention are those for which the conventional ion-selective electrodes are applicable. These include anions, e.g. sodium (Na⁺) and potassium (K⁺) , and anions, e.g. nitrate (N0₃ ^~) and fluoride (F^") and chloride Cl^", but others may be determined if desired by appropriate ion-selective electrodes. Selectivity can sometimes be improved by appropriate choice of the barrier membrane. For example, selectivity for organic anions (e.g. chloride Cl^") may be enhanced by use of an anionic barrier membrane.

The sample may be obtained and prepared in any conventional manner, but is preferably a liquid. If solids or samples which are not completely liquid are to examined, it may be necessary to add water or other aqueous solvent media to them to ensure that the components in them are put into a suitable state for measurement .

Usually, all that is required is that the analyte (e.g. ionic analyte) to be detected and measured should be in solution in the sample so that it is able to diffuse through the permeable barrier. The analyte may be present initially in the sample under examination as such (and therefore can be determined directly) , but if desired it may be generated in situ, for example by enzyme or chemical action (e.g. titration) and this may enable measurements of some analytes to be made indirectly. Such indirect measurement can be useful as means for making one analyte into another which diffuses more readily through the permeable barrier or be detected at the detecting electrode

(for example, when using an ISE, by converting a non-ionic analyte into an ionic one) , but it can also be used to assist in reducing interference by components which could otherwise behave similarly to the analyte that is to be measured.

Redox measurements can also be made both directly and indirectly, and may be made in these ways using our invention. Examples of the latter are the "quinhydrone" sensors for ph and potentiometric titration systems, and other arrangements are possible. reagents or enzymes may be added to the sample in any form or immobilised or retailed above the covering permeable barrier (membrane) in liquid or dry form, or dried in a gel or liquid layer below it, for reaction above or below the permeable barrier (membrane) .

The sensor devices of this invention may be used in substantially the same way as an ordinary detecting electrode, by contacting the permeable barrier over the detecting electrode with the sample (if, necessary, prepared for this in the manner described above) . This may be done by applying it to the sensor device or, more conveniently, by dipping the sensor device into the sample. Of course, some forms of construction may be better adapted for particular modes of contacting with the sample, but the choice can easily be made to suit the particular situation and the user's preferences.

In use, advantages which can be secured by use of an ISE or redox electrode in the sensor devices of the present invention include : - (1) the ISE (or redox electrode) and the reference electrode, being combined, make the device very much more easy and convenient to use.

(2) potential problems of contamination of the reference electrode are reduced or eliminated.

(3) the variety of analytes which can be measured is wide because a considerable range of standard ion-selective

(e.g. ion-exchange) materials are available.

(4) selectivity of component measurement can be enhanced by use of a perm-selective layer in the permeable barrier.

(5) small sample volumes can be examined and their contents determined, particularly when using a flat form of device .

(6) no calibration of the device is required to counteract for any baseline drift . This is a key advantage so far as practical use is concerned, as in "single shot" use.

(7) the device is simple enough to be made disposable, for a "use once, no rinsing" procedure.

(8) in its flat form, the device is of a very similar format to amperometric planar sensors, i.e. they can use different circuitry but the same fabrication and user presentation techniques, and so can offer opportunities for "mixed technique" multi-analyte sensor strips . (9) adaptable to use any ISE system.

(10) advantageous for the examination of samples containing the analyte at high concentrations which would not allow the ISE to function satisfactorily if brought into direct contact with the ISE. Similar advantages - except of course those which are peculiar to only an ISE - are found when using other types of detecting electrode in place of an ISE, particularly a redox electrode .

Applications for which the devices of the invention may be used include medical and clinical use, especially as a disposable sensor - which reduces risk of cross- contamination between sample or subjects; checks on levels of fertiliser components in soils, rivers, plant materials and the like; checks on levels of components (which may be considered to be desirable ones or may be any considered as contaminants or undesirable) in foods, waters, industrial liquid and effluents and the like. This is especially useful for the determination of ionic components or contaminants, by use of an ISE.

The device is most advantageous for single use in a constant sample, and after that use can be discarded. After making a single measurement, i.e. not continuous monitoring, the device can be "reconditioned" to some extent so that it can be used again, but this can be slow and not worthwhile. If it is to be re-used, the advantage of calibration avoidance is effectively lost .

The devices of our invention may be made in a variety of forms and shapes, to suit the particular needs of a user. Thus, the device may be a single one - which can then be made conveniently small and inexpensively, and be most simple to use. Other forms include combinations or arrays containing more than one of our sensor devices, which may be constructed to obtain an enhanced output signal for easier measurement or for special uses.

A form of interesting applicability is that in which several individual sensor devices of our invention are mounted together and the internal zone of liquid (e.g. electrolyte) of each of these, within the permeable barrier, is pre-loaded with different concentrations of the analyte to be sought and measured. By contacting all the sensor devices with the sample, the flux of any interfering species (ions or molecules, or the like) will be be constant for all of them, but the fluxes of the desired analyte will be different and their differential behaviour will then be a function of the concentration of the desired analyte in the sample - and so enable interference effects to be reduced or eliminated. Another variant of this is an array of a number of our sensor devices, each with its own internal electrolyte loaded with different pre-determined (and known) concentrations of the analyte sought . When such an array is contacted with the sample, the different sensors will give different responses - but for the particular sensor in which the loaded concentration of analyte (e.g. analyte ion) equals that in the sample there will be no diffusion through the permeable barrier and no potential change with time will be observed. This can reduce the need for detailed measurements to be made, as the sensor showing "no change" ("nil diffusion") can be distinguished easily and quickly and will indicate the analyte concentration immediately. A measurement may be made by using signals of more than one sensor element with appropriate signal processing methods.

An especial feature in using our invention is in the way in which the measurements are made and interpreted. As exemplified by the use of an ISE, an ion to be determined diffuses through the permeable barrier and this progressive diffusion gives a continually changing response from the ISE/reference electrode combination. We have found that, using a sensor device of the present invention, the rate of change of response is most useful as an indication of the analyte content, and that it is possible to obtain more reliable measures of the analyte concentration by determining this . When measurements (by observation and recording) of the output potential of the electrode system are made, for example by being plotted, these show the rate of change of potential and can be used as an indication or measure of the analyte content . The slope of the output , as measurements proceed and are thus plotted, is independent if any baseline EMF and also of the particular reference electrode used. The responses are usually and conveniently measured in mV/minute, and are plotted as the rate of change of potential against the concentration or, preferably, against the log concentration. For this mode of using the measurements, the absolute value of the ISE/reference or redox/reference potential is not important so long as the reference keeps stable during the short time required for measurement, the problems previously caused by long term drift are minimised, and the need for calibrations for use are rendered substantially unnecessary. Though the baseline EMF of ISE systems may drift about, this slope of the response plot is much more stable. This is in contrast to the usual way in which an ISE is used, which involves waiting long enough for the ISE to be at equilibrium. Similar effects are seen with redox systems .

Sometimes, during initial stages of setting up a sensor device of our invention for use, it may be found that there can be some initial abnormality (a "spike" or surge) in the response, but this is only brief and can be allowed to pass prior to making measurements to determine the slope of the plot as indicated above. The detailed description given herein, though written with principal reference to an ISE as the detecting electrode, should not be taken as meaning that the invention is only applicable to the use of an ISE. Emphasis has been put on an ISE because the invention is seen as being especially suited to dealing with the problems of using an ISE, but the description should be read as being applicable to any other detecting electrode which may be used in place of the ISE, particularly redox electrodes . The principle of operation of the invention lies in the use of an enclosing permeable barrier to provide a zone of liquid (usually an electrolyte) in contact with the detecting electrode - and possibly with both the detecting electrode and reference electrode - so that this zone of liquid acts as an intermediate phase which can act to moderate the extreme conditions which may which may be present in a sample under examination. The membrane regulates the passage of components (e.g. ions) - whether analyte components or not - between the sample and the zone adjacent to the detecting electrode, in either direction. This serves to improve the ability of the electrode system (detector electrode and reference electrode) to cope with a wider variety of samples and give greater ease and accuracy of measurement than is practicable when the electrode system is exposed directly to the sample under examination. More particularly, it enables rate measurements to be made as detected species cross the permeable barrier (membrane) and alter the concentration in the inner liquid region.

The invention is illustrated but not limited by the following Example and accompanying drawings, which are schematic and not drawn to scale.

EXAMPLE 1.

Figures 1 and 2 represent illustrations of forms of sensor constructed according to the present invention, and are schematic drawings, in transverse section and not to scale.

In Figure 1, a planar sheet of ceramic material of approximately 0.5 mm thickness and 1.5 cm by 3.0 cm in area (1) serves as an insulating support and carries, upon one of its planar surfaces, two electrodes -- (A) an ion- selective electrode comprising a thick metallic film (2) of platinum deposited from a platinum-containing ink or paint and coated with an ion-selective material (3) , and (B) a standard reference electrode (4) comprising a film of silver coated with silver chloride, surrounded by an aqueous solution (5) of potassium chloride (concentration in the range 0.5 to 3.5 M) (5) and enclosed within a porous layer (6) to serve as the required liquid junction in use. The two electrodes (A) and (B) are totally enclosed by a permeable barrier layer or membrane (8) which also makes sealing contact with the sheet of insulating support material (1) all around the area containing both electrodes. The space between this enclosing membrane (8) and the two electrodes (A) and (B) is filled with an aqueous solution (7) of sodium chloride. This completes the electrolyte-filled permeable barrier as the enclosure for the pair of electrodes.

On the other side of the planar support sheet (l) , remote from the two electrodes (A) and (B) , electrical leads are fitted (10 and 11 respectively) to provide electrical connection to the electrodes (A) and (B) (more specifically, to the conducting films (2) and (4) . These leads (10 and 11) are sealed into the sheet (l) to prevent leakage of liquid past them, and are insulated and provided with means for connection to a voltage measuring device V (not shown) .

In use, a liquid sample to be examined (9) is put into contact with the surrounding membrane (8) . Conveniently, this is done by simple dipping the assembly (constructed as described above) into the sample liquid (9) . The insulation covering on the connecting leads (10 and 11) ensures that there is no electrical short-circuit occurring between them. Alternatively, the assembly is laid horizontally, with the electrodes, membranes, etc. uppermost, and the sample is then applied on top of the outer membrane (8) .

This construction allows electrolyte contact at each of the membranes (3) and (8) and also the bridging part (7) , to allow the completion of an electrically conducting circuit between electrode (A) and (B) , which avoids direct exposure of electrode coverings (3 ( and (6) to the sample (9) itself.

Measurement of the potential between the two electrodes (A) and (B) is made by an appropriate meter, usually an ISE meter, typically a voltmeter with a single high impedance input for the ISE. In Figure 2, which represents a transverse section of part of a long strip of a ceramic base (1) coated with a pair of metallic stripes (12) and (13) . Stripe (12) is of gold and serves as a conductor for the ISE part, and stripe (3) is of metallic silver coated with silver chloride and serves as the reference electrode. Over the base (11) is a layer of insulating material (14) , made by casting a solution of un-plasticised PVC in tetrahydrofuran over the stripe-coated ceramic base and allowing the solvent to evaporate off . A small "window" in the deposit of PVC is left so that the stripes (12) and (13) are exposed and not covered by the PVC (14) . Then, the area over and around the gold stripe (12) is coated with a solution of PVC, a plasticiser, and an ion-carrier to form a coating (15) . Then finally the whole area which is not covered by the PVC

(14) is covered with polyvinyl alcohol, which forms a permeable layer (16) which is sealed on to the surrounding

PVC (14) and completely covers the electrodes. Electrical connections are made (by means not shown, but conveniently comprising the ends of stripes (12) and (13) protruding from under the PVC layer (14) beyond the "well" or "window" filled by the layers (15) and (16) . For use, the sample is then contacted with the of the polyvinyl alcohol layer (16) , and the potential difference between electrodes (12) and (13) is measured.

In this form of construction, basically electrolyte and membrane are combined.

For these electrodes, the un-plasticised PVC used has a molecular weight of 100,000 to 200,000 and is dissolved in tetrahydrofuran. This is used to form the PVC layers.

To make the ISE coatings (3) and (15) , the solution of un-plasticised PVC in tetrahydrofuran is used, with addition of tri-caprylyl methyl ammonium chloride as plasticiser and as ion carrier for chloride or with di- octyl phthalate as plasticiser and valinomycin as ion- carrier for potassium. The layers of metal are deposited from metal- containing paints, in conventional manner, and the various coatings of polymer-based material are applied by dip- coating the ceramic strip in the solutions, masking areas which are not to be coated and cutting out parts of the applied coatings where a "well" or "window" is to be formed.

In place of the gold, using platinum and copper as alternative metals gives substantially the same results . Comparable results are also obtained by direct or indirect use of a redox electrode system instead of an ISE.

Claims

CLAIMS : -

1. Sensor system or device comprising a detecting electrode and a reference electrode in combination, characterised in that the detecting electrode (preferably together with the reference electrode) is enclosed within a permeable barrier adapted to interface with a sample under examination, so that the sample itself no longer provides the main bridging contact between the detecting and reference electrodes to complete the measuring circuit .

2. Sensor system or device as claimed in Claim 1 wherein the detecting electrode is one which operates non- amperometrically, especially an ion-selective electrode ("ISE") or a redox electrode.

3. Sensor system or device as claimed in Claim 3 wherein the ion-selective electrode (ISE) is a solid-state electrode, for example an ion-selective field effect transistor (conveniently referred to as an "ISFET" device) .

4. Sensor system or device as claimed in any of Claims 1 to 3 wherein the permeable barrier interfacing with the sample provides a zone of liquid (which may be an electrolyte) in contact with the detecting electrode - or with both the detecting electrode and reference electrode - so that this zone of liquid acts as an intermediate phase which can act to moderate the extreme conditions which may be present in a sample under examination and/or regulate the passage of components (whether analyte components or not) between the sample and the zone of liquid adjacent to the detecting electrode, in either direction.

5. Sensor system or device as claimed in any of Claims l to 4 wherein the zone of liquid is provided in the device as made, for example in the form of a hydrated gel.

6. Sensor system or device as claimed in any of Claims l to 5 wherein the zone of liquid is added at the time the sensor is to be used, for example by soaking in an appropriate liquid or solution to make it ready for use, or is provided by obtaining the necessary liquid from a sample itself, by permeation or diffusion from the sample through the permeable barrier when the device and a sample are brought together.

7. Sensor system or device as claimed in any of Claims l to 8 wherein the permeable barrier is a material which readily hydrates, for example poly vinyl alcohol.

8. Sensor system or device as claimed in any of Claims 1 to 7 wherein the permeable barrier has selective properties in the way it limits diffusion, which is distinct from any selectivity phenomenon due to a membrane in the detecting or reference electrodes .

9. Sensor system or device as claimed in any of Claims l to 8 wherein the detecting electrode comprises a covering or layer (e.g. a membrane or a gel) over a core electrode and the components imparting ion-sensitivity may be in or on the core electrode and/or the said covering or layer, or wherein the detecting electrode contains, when in use, an inner medium which may be liquid or gel (usually an electrolyte or other liquid) and associated contacting electrodes .

10. Sensor system or device as claimed in any of Claims l to 9 wherein the reference electrode is also an ion- selective electrode which, under the conditions of use, produces a stable EMF.

11. Sensor system or device as claimed in any of Claims 1 to 10 wherein the diffusion of analyte (especially analyte ion) to be determined is inward diffusion, i.e. through the permeable barrier from the sample towards the detecting electrode.

12. Sensor system or device as claimed in any of Claims 1 to 11 wherein the zone of liquid within the permeable barrier is provided with a concentration of analyte (especially analyte ion) which is higher than that in the sample to be examined, so that diffusion of the analyte (or analyte ion) will be outwards - away from the detecting electrode and into the sample - and result in a decrease of its concentration adjacent to the detecting electrode.

13. Sensor system or device as claimed in any of Claims 1 to 12 which is in the form of a single unit, carrying the detecting electrode and the reference electrode assembly upon a support which serves to hold the assembly together while insulating the detecting electrode and the reference electrode assembly from each other, for example by assembling the detecting electrode and reference electrode assembly upon a substantially flat insulating support with the permeable barrier in the form of a "bubble" or "envelope" over them.

14. Sensor system or device as claimed in any of Claims 1 to 13 wherein the detecting electrode has internal electrolyte and an internal reference (e.g. Ag/AgCl) or a coated wire electrode where a base metal wire is in direct contact with a covering ion-selective membrane.

15. Sensor system or device as claimed in any of Claims 1 to 14 wherein the reference electrode assembly is a conventional half-cell with its conventional filling solution (usually an electrolyte) enclosed in a container which has a "liquid junction", "salt bridge" or "double junction" arrangement customarily intended to make contact with the sample, for example a silver/silver chloride (Ag/AgCl) or calomel electrode system,

16. Sensor system or device comprising detecting and reference electrodes, especially an ion-selective electrode or a redox electrode, substantially as described.

17. Method for the determination of an analyte in a sample, which comprises using a sensor system or device with a detecting electrode and a reference electrode in combination, characterised in that the detecting electrode (preferably together with the reference electrode) is enclosed within a permeable barrier adapted to interface with a sample under examination, so that the sample itself no longer provides the main bridging contact between the detecting and reference electrodes to complete the measuring circuit, measuring the potential between the detecting electrode and the reference electrode and using this measure for determining the content of the analyte.

18. Method as claimed in Claim 17 wherein the output potential measurements of the electrode system are made, for example by being plotted, and the rate of change of the output signal potential from the detecting electrode is used as an indication of the analyte content and the basis for its determination, usually using the slope of the plot to determine the measured parameter.

19. Method as claimed in Claim 17 or Claim 18 wherein the potential is measured by a non-amperometric method.

20. Method as claimed in any of Claims 17 to 19 wherein the detecting electrode is an ion-selective electrode ("ISE") and the analyte is an ion analyte, or a redox electrode.

21. Method as claimed in any of Claims 17 to 20 wherein the enclosed region around the detecting electrode and within the permeable barrier is filled with a zone of liquid containing an appropriately high concentration of a species liable to interfere with the desired measurements so that it will swamp any effects arising from the sample and thereby reduce interference from that species if present in the sample.

22. Method as claimed in any of Claims 17 to 21 wherein there is used an array of a number of sensor systems or devices as defined therein, each having (within its permeable barrier) its own internal zone of liquid (for example electrolyte) loaded with different pre-determined and known concentrations of the analyte sought, so that when the array is contacted with a sample, the different sensors will give different responses but for the sensor in which the loaded concentration of analyte (for example analyte ion) equals that in the sample there will be no diffusion through the permeable barrier and no potential change with time will be observed, and the sensor showing "no change" ("nil diffusion") is easily and quickly distinguishable and can thereby indicate the analyte concentration immediately, and if desired the other sensor elements can be used to improve the measurement .

23. Method as claimed in any of Claims 17 to 22 wherein there are used several individual sensor devices as defined therein mounted together and the internal zone of liquid (e.g. electrolyte) of each of these is pre-loaded with different concentrations of the analyte to be measured so that, by contacting all the sensor devices with the sample, the flux of any interfering ions will be be constant for all of them, but the fluxes of the desired analyte will be different and their differential behaviour will then be a function of the concentration of the desired analyte in the sample and interference effects will be reduced.

24. Method as claimed in any of Claims 17 to 23 wherein the analyte sought is a sodium or potassium cation or a chloride, nitrate or fluoride anion.

25. Method for the electrolytic determination of an analyte (especially an ion analyte) , using a sensor system or device as claimed in any of Claims 1 to 16.

26. Method for the electrolytic determination of an analyte (especially an ion analyte) , substantially as described with reference to the foregoing Example.