HK1066062B - Electrochemical cell - Google Patents

Description

Electrochemical cell

Technical Field

The present invention relates to an electrochemical cell comprising a first working electrode, a first counter electrode, a second working electrode and a second counter electrode, wherein the electrodes are spaced such that reaction products from the first counter electrode reach the first working electrode and reaction products from the first and second counter electrodes do not reach the second working electrode. The present invention also provides a method of determining the concentration of a reduced or oxidized form of a redox species using such an electrochemical cell with a higher degree of accuracy than can be obtained using an electrochemical cell having only a single working and counter electrode.

Background

In amperometric electrochemistry, the current flowing at an electrode can be used as a measure of the concentration of electroactive species that reacts electrochemically at the working electrode. In coulometry, the current flowing at the electrode can be integrated over time to give a total amount of charge passed which yields a measure of the amount of electroactive species reacted at the working electrode. The current flowing at the electrode (or the charge passed at any time) depends on the rate of transfer of the electroactive species to the working electrode. When a substantial concentration of electroactive species is located near an electrode and a potential is applied to the electrode sufficient to cause the electroactive species to electrochemically react at the electrode/solution interface, initially a higher current flows, which current decreases with time. For an isolated and substantially planar electrode, where the potential applied to the electrode is sufficient for the electroactive species to react effectively immediately upon reaching the electrode, and the transfer of the electroactive species to the working electrode is controlled by diffusion, the current follows a curve known in the art as the Cottrell equation. According to this equation, the current is inversely proportional to the square root of time. This produces a current that decays over time because the electroactive species reacting at the electrodes is gradually depleted close to the electrodes, so over time the electroactive species must be transported from further and further away in order to reach the electrodes.

If, in addition to the electrochemical reaction of the electroactive species at the electrode, the electroactive species is generated by a chemical reaction near the working electrode, the form of the current flowing at the electrode becomes complicated. Electrode reactions reduce the concentration of the electroactive species near the working electrode, while chemical reactions increase the concentration of the electroactive species in the region. The time-dependent nature of these two processes is mixed together and it is difficult to measure chemical reaction kinetics from the current (or charge) flowing at the electrodes.

Thus, in the published literature, the rate of chemical reactions is not generally measured electrochemically, except in specialized applications where specialized equipment is used. One example of such a device is known in the art as a rotating ring/disk electrode. The device is only suitable for relatively fast reaction kinetics and requires that the electrodes rotate at a known controlled rate with good characteristics of fluid dynamics.

Summary of The Invention

There is a need for an electrochemical cell and method of determining the concentration of a reduced or oxidized form of a redox species using such an electrochemical cell with a measurement accuracy that is greater than that achievable using an electrochemical cell having a single working and counter electrode. Preferred embodiments provide such electrochemical cells and methods.

In a first embodiment, there is provided a method of determining the concentration of a reduced or oxidized form of a redox species, the method comprising the steps of: providing an electrochemical cell comprising a first working electrode, a first counter electrode, a second working electrode, and a second counter electrode; selecting the spacing between the first working electrode and the first counter electrode such that reaction products from the first counter electrode reach the first working electrode; selecting the spacing between the first working electrode and the second counter electrode such that a substantial amount of the reaction products from the second counter electrode do not reach the first working electrode; selecting the spacing between the second working electrode and the second counter electrode such that a substantial amount of the reaction products from the second counter electrode do not reach the second working electrode; applying a potential difference between the first working electrode and the first counter electrode; applying a potential difference between the second working electrode and the second counter electrode; selecting the potential of the first working electrode such that the rate of electro-oxidation of the reduced form of the substance or the rate of electro-reduction of the oxidized form of the substance is diffusion controlled; selecting the potential of the second working electrode such that the rate of electro-oxidation of the reduced form of the substance or the rate of electro-reduction of the oxidized form of the substance is diffusion controlled; subtracting the current flowing between the second working electrode and the second counter electrode from the current flowing between the first working electrode and the first counter electrode, thereby obtaining a corrected current; and deriving from the corrected current a value indicative of the concentration of the reduced or oxidized form of the species.

In one aspect of the first embodiment, the surface area of the first working electrode and the surface area of the second working electrode are substantially the same.

In another aspect of the first embodiment, the surface area of the first working electrode and the surface area of the second working electrode are different, and the step of subtracting the current comprises: measuring the current flowing between the first working electrode and the first counter electrode; measuring the current flowing between the second working electrode and the second counter electrode; normalizing the current flowing between the first working electrode and the first counter electrode and the current flowing between the second working electrode and the second counter electrode with respect to the same electrode surface area, producing a normalized current flowing between the first working electrode and the first counter electrode and a normalized current flowing between the second working electrode and the second counter electrode; and subtracting the normalized current flowing between the second working electrode and the second counter electrode from the normalized current flowing between the first working electrode and the first counter electrode, thereby obtaining a corrected current.

In yet another aspect of the first embodiment, the spacing between the first working electrode and the first counter electrode is less than about 500 μm, or less than about 200 μm. The spacing between the second working electrode and the second counter electrode, or between the first working electrode and the second counter electrode, is greater than about 500 μm, or greater than about 1 mm.

In yet another aspect of the first embodiment, the redox species can be a mediator. When the redox species is a mediator, the concentration of the reduced or oxidized form of the mediator is indicative of the concentration of the analyte, wherein a measure of the diffusion coefficient of the reduced or oxidized form of the mediator is defined as a precursor to determining the concentration of the analyte.

In yet another aspect of the first embodiment, the electrochemical cell further comprises a separate reference electrode.

In yet another aspect of the first embodiment, the analyte can be glucose.

In a second embodiment, an electrochemical cell is provided, comprising: a first working electrode, a first counter electrode, a second working electrode, and a second counter electrode, the first working electrode being spaced from the first counter electrode by less than about 500 μm, the first working electrode being spaced from the second counter electrode by greater than about 500 μm, and the second working electrode being spaced from the second counter electrode by greater than about 500 μm.

In one aspect of yet a second embodiment, the first working electrode and the first counter electrode and/or the second working electrode and the second counter electrode face each other or are arranged side by side.

In another aspect of the second embodiment, the first working electrode and the second working electrode have substantially corresponding areas.

In yet another aspect of the second embodiment, the electrochemical cell further comprises a separate reference electrode.

In yet another aspect of the second embodiment, the electrochemical cell can be a hollow electrochemical cell. The electrochemical cell can have an effective cell volume of less than 1.5 microliters.

In a third embodiment, there is provided an apparatus for determining the concentration of a redox species in an electrochemical cell, the apparatus comprising: an electrochemical cell having a first working electrode, a first counter electrode, a second working electrode and a second counter electrode, characterized by: the interval between the first working electrode and the first counting electrode is less than 500 mu m, the interval between the first working electrode and the second counting electrode is 500 mu m, and the interval between the second working electrode and the second counting electrode is more than 500 mu m; means for applying a potential difference between the first working electrode and the first counter electrode; and means for applying a potential difference between the second working electrode and the second counter electrode.

In one aspect of the third embodiment, the device may be a glucose meter.

In a fourth embodiment, an electrochemical cell is provided that includes a first working electrode, a first counter electrode, and a second working electrode, the first working electrode being spaced from the first counter electrode by less than about 500 μm, and the second working electrode being spaced from the first counter electrode by greater than about 500 μm.

In a fifth embodiment, there is provided a method of determining the concentration of a reduced or oxidized form of a redox species, the method comprising the steps of: providing an electrochemical cell comprising a first working electrode, a counter electrode, and a second working electrode; selecting the spacing between the first working electrode and the counter electrode such that reaction products from the counter electrode reach the first working electrode; providing a redox species, wherein at least an effective portion of the redox species in solution initially present above the second working electrode is reduced or oxidized at the second working electrode; applying a potential difference between the first working electrode and the counter electrode; selecting the potential of the first working electrode such that the rate of electro-oxidation of the reduced form of the substance or the rate of electro-reduction of the oxidized form of the substance is diffusion controlled; measuring the current flowing between the first working electrode and the counter electrode; and deriving from the current a value indicative of the concentration of the reduced or oxidized form of the species.

In one aspect of the fifth embodiment, the surface area of the first working electrode and the surface area of the second working electrode are substantially the same.

In another aspect of the fifth embodiment, the surface area of the first working electrode and the surface area of the second working electrode are substantially different.

In a sixth embodiment, there is provided a method of determining the concentration of a reduced or oxidized form of a redox species, the method comprising the steps of: providing an electrochemical cell comprising a first working electrode, a second working electrode, and a counter electrode; selecting the spacing between the first working electrode and the counter electrode such that reaction products from the counter electrode reach the first working electrode; selecting the spacing between the second working electrode and the counter electrode such that a substantial amount of the reaction products from the counter electrode do not reach the second working electrode; applying a potential difference between the second working electrode and the counter electrode, thereby fully charging the second working electrode and fully completing the surface set reaction; opening the electrical circuit between the second working electrode and the counter electrode before a reaction of the quantity of the substance at the second working electrode occurs; applying a potential difference between the first working electrode and the counter electrode; selecting the potential of the first working electrode such that the rate of electro-oxidation of the reduced form of the substance or the rate of electro-reduction of the oxidized form of the substance is diffusion controlled; measuring the current flowing between the first working electrode and the counter electrode; and deriving from the current a value indicative of the concentration of the reduced or oxidized form of the species.

In a seventh embodiment, there is provided a method of determining the concentration of a reduced or oxidized form of a redox species, the method comprising the steps of: providing an electrochemical cell comprising a first working electrode, a second working electrode, and a counter electrode; selecting the spacing between the first working electrode and the counter electrode such that reaction products from the counter electrode reach the first working electrode; selecting the spacing between the second working electrode and the counter electrode such that a substantial amount of the reaction products from the counter electrode do not reach the second working electrode; applying a potential difference between the second working electrode and the counter electrode and between the first working electrode and the counter electrode, thereby fully charging the second working electrode and the first working electrode and fully completing the surface set reaction; opening the electrical circuit between the second working electrode and the counter electrode before a reaction of the quantity of the substance at the second working electrode occurs; applying a potential difference between the first working electrode and the counter electrode; selecting the potential of the first working electrode such that the rate of electro-oxidation of the reduced form of the substance or the rate of electro-reduction of the oxidized form of the substance is diffusion controlled; measuring the current flowing between the first working electrode and the counter electrode; and deriving from the current a value indicative of the concentration of the reduced or oxidized form of the species.

Brief description of the drawings

Fig. 1 shows a cross-sectional view of an electrochemical cell 10 of a preferred embodiment having electrode surfaces in parallel and opposing configurations.

Fig. 2 shows a cross-sectional view of an electrochemical cell 50 of a preferred embodiment having electrode surfaces in a side-by-side configuration.

Description of The Preferred Embodiment

The following description and examples illustrate preferred embodiments of the invention in detail. It will be appreciated by those skilled in the art that numerous variations and modifications may be made to the invention as described herein without departing from the scope of the invention. Therefore, the description of the preferred embodiments should not be taken as limiting the scope of the invention.

When an electrochemical cell is used as a amperometric sensor for detecting and quantifying analytes, it may be desirable to be able to detect very low analyte concentrations. One of the limitations of the prior art in detecting low concentrations of analyte is the presence of an applied current that masks the current of interest. Some of these unwanted currents come from capacitive charging currents of the electrodes and electrical noise picked up from the environment. It is an object of a preferred embodiment to propose a method which minimizes the influence of these currents on the total signal and thus allows a better detection of the analyte.

As is known in the art, the electrodes in a two or three electrode electrochemical cell may be arranged such that the working electrode is isolated from the counter electrode reaction and reaction products, or such that the products of the counter electrode reaction diffuse to the working electrode where they react. The former type of electrochemical cell is well known in the prior art. The latter type of electrochemical cell is discussed in US6179979 and US 5942102.

The two electrode configurations differ in that: in the isolated case, the counter electrode is arranged sufficiently far from the working electrode that, when the cell is in use, the products of the electrochemical reaction on the counter electrode do not reach the working electrode. In practice, this is typically achieved by separating the working electrode from the counter electrode by at least 1 mm.

In the non-isolated configuration, the working electrode and counter electrode are disposed sufficiently close to each other such that, in use of the cell, products of the electrochemical reaction on the counter electrode can diffuse to the working electrode. These reaction products then react at the working electrode, producing a higher current than in the case of the isolated electrode. In the non-isolated configuration, the reaction on the working electrode can be depicted as a reaction coupled to the counter electrode.

Electrode arrangement

In a preferred embodiment, an isolated working electrode is combined with a working electrode coupled to a counter electrode in an electrochemical cell to improve detection of low concentrations of substances. Fig. 1 and 2 illustrate different electrode configurations in an electrochemical cell of a preferred embodiment.

Fig. 1 shows a cross-sectional view of an electrochemical cell 10 of a preferred embodiment. The exposed portions of the conductive layers 12, 14, 16, 18 serve as electrodes 32, 34, 36, 38 in the battery 10. The conductive layers 12, 14, 16, 18 are in contact with layers of resistive material 20, 22, 24, 26. One or more spacer layers (not shown) maintain the electrodes 32, 34 at a spacing of less than about 500 μm. Either electrode 32, or electrode 34, or electrode 36, or electrode 38 may be a working electrode, so long as electrodes 32 and 34 constitute one working and counting electrode pair and electrodes 36 and 38 constitute another working and counting electrode pair. The thicknesses of layer 24 and layer 26 are selected such that the separation between the nearest edges of electrode 32 and electrode 36 and between the nearest edges of electrode 34 and electrode 38 is typically greater than 500 μm, preferably greater than 1 mm. In another embodiment, the resistive material layer 22 or 24 and the conductive layer 16 or 18 it supports may be replaced by a single layer of a suitable conductive material, such as an aluminum foil layer or a conductive polymer layer (not shown). For ease of manufacturing, in some embodiments it may be desirable to completely cover one surface of one or more layers of resistive material 20, 22, 24, 26 with a conductive layer 12, 14, 16, 18. Alternatively, in other embodiments, it may be desirable to partially cover the resistive material layers 20, 22, 24, 26 with conductive layers 12, 14, 16, 18, for example, to save material costs if the electrode material contains a noble metal. For example, in the cell 10 shown in fig. 1, the conductive layer 12 may cover only the portion of the insulating layer 20 adjacent to the sample reservoir (sample reservoir) 28. The portion of the insulating layer 20 adjacent to the layer 26 is not covered. The configuration of the conductive layers 12, 14, 16, 18 and their adjacent resistive layers 20, 22, 24, 26 will be apparent to those skilled in the art.

Another electrode configuration in an electrochemical cell 50 of the preferred embodiment is shown in fig. 2. In this configuration, the electrodes 52, 54, 56, 58 are all in the same plane. Spacer layer 60 is shown in fig. 2 over electrode 52 and electrode 54. When the electrochemical cell 50 is used with the current subtraction method described below, the spacer layer 60 is preferably omitted. When spacer layer 60 is omitted, the planar diffusion to electrode 54 more closely coincides with the planar diffusion to electrode 58, resulting in more accurate current subtraction.

When the electrochemical cell 50 is used with the current amplification method described below, it is desirable to leave the spacer layer above the electrodes 52 and 54 to provide a smaller volume of space 62 and a correspondingly higher amplification factor than without the spacer layer 60. One or more spacer layers (not shown) maintain the separation of the electrodes 52, 54, 56, 58 from the layer 64, thereby providing a sample reservoir 66 in the electrochemical cell 50. The distance between the closest edges of electrodes 52 and 54 is less than 500 μm, preferably less than about 450, 400, 350, 300 or 250 μm, more preferably less than about 200, 150 or 100 μm, and most preferably less than about 90, 80, 70, 60, 50, 40, 30, 25, 20, 15, 5 or 1 μm. The distance between the closest edges of electrode 52 and electrode 58 and the distance between the closest edges of electrode 54 and electrode 58 are typically greater than about 500 μm, more preferably greater than about 550, 600, 650, 700, 750, 800, 850, 900 or 950 μm, and most preferably greater than about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 25 or 50 mm. However, any suitable spacing, including spacings less than about 500 μm, may be suitable so long as a substantial amount of the reaction products at the first electrode do not reach the second electrode. In this context, a large amount of reaction product means that the amount of reaction product is sufficient to generate an additional current at the second electrode that is too large to affect the practical use of these battery-utilizing methods to achieve the desired results. In some embodiments, it may be desirable to omit any spacer layers and layers 64, thereby providing an electrochemical cell comprising electrodes 52, 54, 56, 58 on a single layer 68 of resistive material. This embodiment may be preferred when the sample size is sufficient to immerse both layer 68 and electrodes 52, 54, 56, 58 in the sample, or there are sufficient layers of sample applied to electrodes 52, 54, 56, 58.

As will be appreciated by those skilled in the art, different electrode configurations that maintain appropriate spacing between the electrodes 52, 54, 56, 58 may be preferred in various embodiments. For example, the electrochemical cell shown in fig. 2 may be modified to have one or both of the electrodes 56 and 58 disposed on layer 64 instead of layer 68. Alternatively, one or both of the electrodes 56 and 58 may be disposed on the layer 64 or 60 rather than the layer 68. If only one of electrodes 56 and 58 is disposed on layer 68, then layers 64 and 68 or layers 60 and 68 should be close enough to maintain a spacing between the nearest edges of electrodes 52 and 54 of less than 500 μm, preferably less than about 450, 400, 350, 300, or 250 μm, more preferably less than about 200, 150, or 100 μm, and most preferably less than about 90, 80, 70, 60, 50, 40, 30, 25, 20, 15, 5, or 1 μm. In another embodiment, an additional layer (not shown) is disposed on layer 68, and one or both of electrodes 56 and 58 are disposed on the additional layer.

Manufacture of electrochemical cells

The electrochemical cells of certain embodiments may be arranged and designed for a single experiment. In a preferred embodiment, the electrochemical cell may be manufactured in a manner similar to that disclosed in U.S. 5942102. In one method of the preferred embodiment for making the electrochemical cell 10 shown in fig. 1, the resistive material layer 20, 22, 24 or 26 is a polyester sheet having a sputtered coating of palladium as the conductive layer 12, 14, 16 or 18, with the portions remaining exposed after manufacture forming the electrodes 32, 34, 36 or 38.

Those skilled in the art will appreciate that the conductive layers 12, 14, 16, 18 and the resistive material layers 20, 22, 24, 26 may be individually selected as desired, for example, for ease of manufacturing, to reduce material costs, or to obtain other desired characteristics or manufacturing processes of the cell 10, etc. Also, the layer of conductive material 12, 14, 16, 18 may be applied to the layer of resistive material 20, 22, 24, 26 in any suitable pattern, such as a pattern that only partially covers the resistive layer 20, 22, 24, 26.

Once the conductive material is coated or otherwise adhered to the respective resistive layer 20, 22, 24 or 26, the coated layers 40, 42 may be adhered to one another to form the electrode bearing layer 48. In the electrochemical cell of fig. 1, the coating 40 is adhered to the coating 42 with the conductive layer 16 of the resistive layer 26 adjacent the coating 42. The coating layers 44 and 46 are adhered in the same manner to form an electrode supporting layer 49.

In a preferred embodiment, the layers in the cell may be adhered with a suitable adhesive. Suitable adhesives include, for example, heat activated adhesives, pressure sensitive adhesives, heat cured adhesives, chemically cured adhesives, hot melt adhesives, hot flow cured adhesives, and the like. Pressure sensitive adhesives may be preferred for certain embodiments where it is desirable to simplify the manufacturing process. However, in other embodiments, the tackiness of the pressure sensitive adhesive may cause the manufacturing tool to become tacky or cause the product to become tacky. In these embodiments, a thermally or chemically curable adhesive is generally preferred. Preferably a heat activated or heat cured adhesive, which can be conveniently activated at the appropriate time.

In some embodiments, a hot melt adhesive may be required. A hot melt adhesive is a solvent-free thermoplastic material that is solid at room temperature and applied in a molten state to the surface to which it is adhered when cooled to a temperature below its melting point. There are various chemicals and hot melt adhesives of various melting points. The hot melt adhesive may be in various forms, such as a web, a non-woven material, a powder, a solution, or any other suitable form. For some embodiments, polyester hot melt adhesives may be preferred. These adhesives, which are commercially available from, for example, Bostik Corp. of Middleton, MA, are linear saturated polyester hot melts having melting points from 65 ℃ to 220 ℃ and are from completely amorphous to highly crystalline in nature. Polyamide (nylon) hot melt adhesives (also available from Bostik) including dimer acid and nylon type polyamide adhesives may also be preferred. Suitable hot melt adhesive chemistries include Ethylene Vinyl Acetate (EVA), polyethylene, and polypropylene.

Alternatively, in certain other embodiments, it may be desirable to bond certain layers together using lamination techniques. Suitable lamination techniques are described in co-pending application serial No. 09/694120, filed on 20/10/2000 and entitled "Laminates of asymmetric films". The layers to be superimposed are placed adjacent to each other, heated and a bond is formed between the layers. Pressure may also be applied to assist in forming the bond. The lamination method is preferably used to bond two materials that can be bonded under heat and/or pressure. Lamination may be preferred to form a bond between two suitable polymeric materials.

The position of the electrode bearing layers 48 and 49 is then fixed by the electrodes 32 and 34 facing the electrodes 36 and 38. This is typically done by adhering one or more shaped spacer layers (not shown) between the electrode bearing layers 48 and 49. The spacer layer should be shaped to provide sample reservoirs 28 and 29 between electrode bearing layers 48 and 49. The spacer layer may be a sheet of electrically resistive material with a portion removed to form sample reservoirs 28 and 29, for example, a ring portion formed in the center of the sheet or a portion removed along one edge of the sheet. The spacer layer may also comprise two or more shaped portions adjacent to each other leaving a space in between that provides access for the sample to the sample reservoirs 28 and 29 and the sample reservoirs 28 and 29 themselves. Instead of using rigid or flexible sheets, a resistive adhesive layer is selected as the spacer layer. In such an embodiment, an adhesive is applied to the electrode side of the electrode bearing layers 48 and 49, and then the other electrode bearing layer 49 or 48 is placed over the adhesive layer, with the bond being formed, for example, by pressure, curing, heat, or other suitable method.

In a preferred embodiment, the spacer layer is a sheet of resistive material with annular holes therethrough and is adhered to the electrode bearing layers 48 and 49 with an adhesive. The annular aperture is preferably centered along the edge of electrode 32 adjacent electrode 38 (or the edge of electrode 34 adjacent electrode 38). This defines a cell 10 having a cylindrical side wall closed at one end by an electrode bearing layer 48 and on the other by an electrode bearing layer 49. The assembly has slots for sample to enter the cell 10 or to be drawn in by siphoning or capillary action and for air to escape. The electrode layers 32, 34, 36, 38 are connected to appropriate leads or components so that potentials can be applied and currents can be measured.

In another preferred embodiment, the spacer layer is formed by applying a pattern of adhesive on one or both of the electrode bearing layers 48 and 49. This method may be preferable when a simple manufacturing process is required and material costs are reduced.

Resistive materials that may be preferred as spacer layers, as support layers for electrode layers, or suitable in other layers of the cell include, for example, the following various materials: polyesters, polystyrene, polycarbonate, polyolefins, polyethylene terephthalate, glass, ceramics, mixtures and/or combinations thereof, and the like. Examples of resistive adhesives suitable for use as the spacer layer include, but are not limited to, polyacrylates, polymethacrylates, polyurethanes, and sulfonated polyesters.

In embodiments where the spacer layer is a sheet of resistive material with a portion removed to form sample reservoirs 28 and 29, an electrode bearing layer 48 or 49 is mounted on one side of the sheet of resistive material and extends over the aperture to form the end wall. The electrode bearing layer 48 or 49 may be adhered to the spacer sheet with, for example, an adhesive. A plurality of spacer sheets may be bonded to each other to form a spacer layer conforming to the stepped surfaces of the electrode bearing layers 48 and 49. A deformable adhesive may also be used as a spacer layer to conform the adhesive to the shape of the electrode bearing layers 48 and 49. In a preferred embodiment, the overall shape of the combined sample reservoirs 28 and 29 is circular, but other shapes, such as square, rectangular, polygonal, elliptical, ellipsoidal, irregular, or other shapes, may be used in some embodiments.

A second electrode bearing layer 48 or 49 is then mounted on the other side of the spacer layer, also extending over the aperture to form a second end wall. Electrodes 32 and 34 are generally spaced less than about 500 μm apart, preferably less than about 450, 400, 350, 300 or 250 μm apart, more preferably less than about 200, 150 or 100 μm apart, and most preferably less than about 90, 80, 70, 60, 50, 40, 30, 25, 20, 15, 5 or 1 μm apart. A second hole or channel is then provided for the entry of liquid into the cell 10. Such channels may be formed by the following method: a kerf is formed along the edge of the device and extends to the electrode bearing layers 48 and 49 and the hole. The electrode bearing layers 48 and 49 are provided with connection lines which can connect the electrodes into a measuring circuit.

Those skilled in the art will appreciate that the above-described techniques for manufacturing the electrochemical cell shown in fig. 1 may be modified to manufacture the electrochemical cell shown in fig. 2.

The chemicals used in the cell, such as redox reagents, lysing agents, buffers, inert salts, and other substances, may be supported on the electrodes or walls of the cell, on one or more separate supports contained within the cell, or may be self-supporting. If the chemical is to be supported on the cell's electrodes or walls, the chemical may be applied using coating techniques known in the art, such as ink jet printing, screen printing, photolithography, ultrasonic spraying, slot coating, gravure printing, and the like. Suitable discrete supports include, but are not limited to, meshes, non-woven sheets, fibrous fillers, macroporous membranes, and sintered powders. The chemicals used in the battery may be supported on or contained in the support.

In a preferred embodiment, the preferred materials in the cell, as well as the materials comprising the cell, are suitable for mass production, but the cell itself is designed for a single experiment and is disposed of after use. A disposable cell is one that is inexpensive to produce and is economically acceptable for use only in a single test. Disposable cells are cells that are conveniently used only in a single test, i.e., after a single use, the cells need to be processed by washing and/or reloading the cells with reagents to make them suitable for later use.

By economically acceptable in this context is meant that the visible value of the test results to the user is equal to or greater than about the cost of purchasing and using the battery, which is added to the cost of providing the battery to the user plus a premium. For many applications, batteries with low material costs and simple manufacturing processes may be preferred. For example, the electrode material of the battery may be inexpensive, such as carbon, or the material may be used in an extremely small amount, and thus an expensive material may be selected. Screen printing carbon or silver ink is a process suitable for forming electrodes from less expensive materials. However, if it is desired to use electrode materials such as platinum, palladium, gold or iridium, it is advantageous to use more material efficient methods such as sputtering or evaporative vapor coating, since very thin films can be formed by these methods. The substrate material of the disposable battery is also preferably inexpensive. Examples of such inexpensive materials are polymers, such as polyvinyl chloride, polyimide, polyester, coated paper and cardboard.

The cell assembly method is preferably suitable for mass production. These methods include the manufacture of a plurality of cells on a card sheet, the singulation of the card sheet into individual battery strips after the main assembly steps, and conveyor belt (web) manufacture, i.e. the manufacture of the cells on a continuous conveyor belt, followed by the singulation into individual battery strips. The card sheet process is most suitable when tight spatial alignment of multiple parts is required for manufacturing and/or when rigid battery substrate materials are selected. The belt process is most suitable when the lower belt alignment of the parts is less critical, and a flexible belt may be selected.

Disposable batteries are preferably conveniently used only once so that the user does not attempt to reuse the battery and inaccurate test results may be obtained. The battery may be described in instructions for use of the battery for only one use. Preferably, in embodiments requiring a single use, the battery can be made difficult or impossible to reuse after a single use. For example, a reagent that is washed away or consumed on the first test and thus does not function on the second test may be included in the battery. Alternatively, the test signal may be checked to indicate that the reagents in the cell have reacted, e.g., an initial signal that is too high, and that the test has failed. Another method includes providing a means for breaking an electrical connection in the battery after a first test of the battery is completed.

Electrode for electrochemical cell

In a preferred embodiment in which the electrochemical cell detects the presence and/or amount of analyte in a sample, or a substance indicative of the presence and/or amount of analyte, at least one of the electrodes of the cell is a working electrode. When the potential of the working electrode is indicative of the analyte (e.g., in a potentiometric sensor), there is a second electrode that serves as a reference electrode to provide a reference potential.

In the case of a amperometric sensor, the current of the working electrode is indicative of the amount of analyte, e.g., glucose, and preferably at least one further electrode is used as a counter electrode to complete the circuit. This second electrode may also be used as a reference electrode. Alternatively, a separate electrode may be used to perform the function of the reference electrode.

Materials suitable for use as working, counter and reference electrodes are compatible with any reagents and substances present in the device. The compatible materials do not substantially chemically react with other materials in the cell. Examples of such suitable materials include, but are not limited to, carbon and an organic binder, platinum, palladium, carbon, indium oxide, tin oxide, mixed indium/tin oxide, gold, silver, iridium and mixtures thereof. These materials may be formed into the electrode structure by any suitable method, such as sputtering, vapor coating, screen printing, thermal evaporation, gravure printing, slot coating, or photolithographic techniques. In a preferred embodiment, the material is sputtered or screen printed to form the electrode structure.

Examples of preferred materials for use as a reference electrode include, but are not limited to, metal/metal salt systems such as silver in contact with silver chloride, silver bromide or silver iodide and mercury in contact with mercurous chloride or mercurous sulfate. The metal may be deposited by any suitable method and then contacted with a suitable metal salt. Suitable methods include, for example, electrolysis or chemical oxidation in a suitable salt solution. Such a metal/metal salt system may provide better potential control in potentiometric methods than single metal component systems. In a preferred embodiment, a metal/metal salt electrode system is selected as the sole reference electrode in the amperometric sensor.

Dissolving agent

In certain embodiments, it may be desirable to include one or more lysing agents in the electrochemical cell. Suitable lysing agents include detergents (ionic and non-ionic), proteolytic enzymes, and (lipase) enzymes. Suitable ionic detergents include, for example, sodium lauryl sulfate, and cetyltrimethyl ammonium bromide. Examples of proteolytic enzymes include, without limitation, trypsin, chymotrypsin, pepsin, papain, and pronase e (prosese e), a very active enzyme with a wide range of properties. Nonionic surfactants suitable for use include, for example, ethoxylated octylphenols (ethoxyoctylphenols) including the trinitrotoluene X (Triton X) series available from Rohm & Haas of Philadelphiz, Pennsylvania. In a preferred embodiment, saponins (i.e., plant glucose that foams in water) are selected as the solubilizing agent.

Redox reagent

Redox reagents may also be included in the electrochemical cell in preferred embodiments. Redox reagents for use in electrochemical cells for measuring glucose in blood include reagents that (re) oxidize reduced forms of enzymes that selectively oxidize glucose. Examples of suitable enzymes include, but are not limited to, glucose oxidase dehydrogenase, PQQ dependent glucose dehydrogenase, and NAD dependent glucose dehydrogenase. Examples of redox reagents suitable for use in assaying glucose include, but are not limited to, salts of ferricyanide, dichromate, vanadium oxide, permanganate, and electroactive metal organic complexes. Organic redox reagents such as dichlorophenolindophenol and quinones are also suitable. In a preferred embodiment, the redox reagent for analyzing glucose is ferricyanide.

Buffering agent

Optionally, a buffer may also be present in the electrochemical cell in dry form along with the redox reagent. If present, the buffer is present in an amount such that the resulting pH is suitable for adjusting the oxidation potential of the redox reagent to a level that will oxidize, for example, glucose without oxidizing other substances that are not desired to be detected. The buffer is present in a sufficient amount to substantially maintain the pH of the sample at the desired level during the test. Examples of suitable buffers include phosphates, carbonates, alkali metal salts of mellitic acid and alkali metal salts of citric acid. The choice of buffer may depend on the desired pH, among other factors. The buffer should be selected to be non-reactive with the redox reagent.

Inert salt

Preferred inert salts for use in the various embodiments include salts that decompose to form ions in the sample to be analyzed but do not react with any redox reagents or other materials in the sample or in the cell, including the cell electrodes. Examples of suitable inert salts include, but are not limited to, alkali chlorides, nitrates, sulfates, and phosphates.

Other substances present in the cell

In addition to redox reagents and buffers, other materials may be present in the cell. These materials include, for example, viscosity enhancers and low molecular weight polymers. Hydrophilic substances may also be included in the cell, such as polyethylene glycol, polyacrylic acid, plasma substitutes, and hydrophilic substances such as Rohm&The commercial name sold by Haas Company of Philadelphia, Pennsylvania is Triton^TMOr ICI Americas Inc. of Wilmington, Delaware under the trade name Tween^TMThe surfactant of (1). These substances can enhance the filling ratio of the battery, provide more stable measurement results, and prevent evaporation in a small-capacity sample.

Circuit arrangement

The conductive layer is preferably connected to a circuit, such as a meter, that applies a voltage between the electrodes and measures the resulting current. Any suitable means for connecting the conductive layer to the circuit may be used, including but not limited to a tongue plug, a set of connecting pins leading down from the top of the battery strip or up from the bottom of the battery strip, etc. The attachment area is not shown in fig. 1. Suitable meters include a power source, circuitry to apply a control voltage or current, a microprocessor control device, a computer, or data storage device, a display device, an audible alarm device, or other devices or components known in the art. The meter can also be connected to a computer or data storage device. For example, a typical meter may be a hand-held device powered by a battery and controlled by a built-in microprocessor, and containing circuitry for applying a predetermined voltage or current between, for example, strip electrode connecting pins, as well as circuitry such as an analog-to-digital converter. In the described embodiment, the analog signal from the battery strip may be converted to a digital signal that may be analyzed and stored by the microprocessor. The meter also contains a display (e.g., a liquid crystal display) and appropriate associated circuitry for displaying the test results to the user. In another embodiment, the meter may also contain dedicated circuitry, such as potential application and signal acquisition circuitry. Such specialized circuitry may be combined into a single module that may be connected to a general purpose computing device such as a laptop computer or other type of computer. In such embodiments, the generic device performs control, analysis, data storage and/or display functions. Such an embodiment may allow for the production of a less expensive meter because a general purpose computing device may be selected to perform a number of functions, which may not be accounted for within the cost of the electrochemical measurement system. In either embodiment of the meter, the meter or general purpose computing device can communicate with an external device (e.g., a local computer network or the Internet) to facilitate distribution of test results to the user and to provide system upgrades.

Obtaining electrochemical measurements

The electrochemical cell shown in fig. 1 or fig. 2 may be used to improve the detection of an analyte. For ease of illustration, however, reference will now be made to the method of the preferred embodiment of the electrochemical cell shown in fig. 1, in which electrodes 34 and 38 are designated as working electrodes and electrodes 36 and 32 are designated as counter electrodes. The analyte herein may be the actual substance of interest in the sample or a product of a chemical reaction with the substance of interest. Electrodes 32 and 34 are spaced so closely that the products of the chemical reaction at electrode 32 diffuse to electrode 34 and react during the test. The spacing is generally less than about 500 μm, preferably less than about 450, 400, 350, 300 or 250 μm, more preferably less than about 200, 150 or 100 μm, and most preferably less than about 90, 80, 70, 60, 50, 40, 30, 25, 20, 15, 5 or 1 μm. And electrodes 36 and 38 are spaced far enough apart that the products of the chemical reaction at electrode 36 do not reach electrode 38 during the test. The spacing is typically greater than about 500 μm, more preferably greater than about 550, 600, 650, 700, 750, 800, 850, 900 or 950 μm, and most preferably greater than about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 25 or 50 mm. In any event, the gap between electrodes 36 and 38 is generally greater than about the gap between electrodes 32 and 34.

When the solution containing the analyte fills spaces 29 and 28, a potential is applied between electrode 32 and electrode 34 by the first external circuit and a potential is applied between electrode 36 and electrode 38 by the second external circuit. The polarity of these potentials is such that the analyte reacts at the electrodes 34 and 38, and the magnitude of the potentials is such that the rate of the electrochemical reaction is limited by the rate of mass transfer of the analyte to the electrodes 34 or 38. Due to the constant application of potential, the products of the chemical reaction at electrode 32 diffuse to electrode 34 and react, but a significant amount of the products of the reaction at electrode 36 do not have time to reach electrode 38. Subtracting the current flowing between electrode 36 and electrode 38 from the current flowing between electrode 32 and electrode 34 yields a current versus time signal that is related only to the reaction of the products of the chemical reaction at electrode 32 at electrode 34. To accurately obtain this current, electrode 34 and electrode 38 need to have the same area, or their respective currents normalized by their respective working electrode areas before the currents are subtracted.

The determination of the current due solely to the reaction of the counter electrode product on the working electrode has the advantage that the effect of other extraneous currents is eliminated. These extraneous currents include currents generated by reactions at the electrode surface groups, currents generated by oxidation or reduction of adsorbed species, and electrode discharge currents, i.e., currents that flow to polarize the bipolar electrode/solution layer to the potential applied to the interface by an external circuit. These currents flow for a short time many times, limiting the time during which the analyte-dependent current can be reliably obtained to a very short time. By eliminating the effects of extraneous currents using this method, the current signal can be used to more reliably obtain information about the analyte in a shorter period of time. It is desirable to utilize a shorter time for the current signal because it is possible to place electrodes 32 and 34 closer together than would otherwise be practical. By placing electrodes 32 and 34 closer together, the reaction products on electrodes 32 reach electrodes 34 more quickly and in greater quantities. This increases the current signal and shortens the time to monitor the current to obtain information about the desired analyte.

In the method of a preferred embodiment, electrodes 32 and 34 form an electrical circuit with a power source that applies an appropriate potential between electrodes 32 and 34. A second electrical circuit is formed between electrodes 36 and 38 and a power source, separate from the first electrical circuit, so that the current flowing between electrodes 32 and 34 and the current flowing between electrodes 36 and 38 can be measured, respectively. Alternatively, instead of measuring the currents separately, the two currents are subtracted electronically and the resulting subtracted current is measured.

In a second method of the preferred embodiment, an electrode configuration can be utilized to effectively amplify the current signal generated by the reaction of the analyte. In this method, electrode 32 is used as both electrode 34 and electrode 38 as a counter electrode during at least a portion of the test. A reagent containing an electrochemically reversible mediator that preferably chemically reacts with the analyte of interest and produces a reacted mediator that is capable of electrochemically reacting at electrode 34 and is electrochemically generated from the mediator at electrode 32 is dried or deposited between electrode 32 and electrode 34. The reagent deposited in space 28 may contain a mediator or, where the analyte is capable of reacting directly at electrode 38, the reagent may be mediator-free.

In the test, a potential is applied to cause the analyte and/or mediator that has chemically reacted with the analyte to electrochemically react at electrodes 34 and 38. In this method, the counter electrode used to complete the circuit for both electrodes 34 and 38 is electrode 32. The electrons collected from the reaction with the analyte or reacted mediator at electrode 38 result in an equal amount of reacted mediator being produced at 32. The reacted mediator then travels to the electrode 34 where it reacts to revert to mediator. In this way, the current generated by the analyte or reacted mediator in the solution volume in space 28 can be used to generate a corresponding amount of reacted mediator in the solution volume in space 29, thereby effectively concentrating the analyte-related substance from space 28 to space 29, resulting in an enhanced current signal from the analyte. Due to the diffusion distances involved, the reacted mediator in the space 29 stays substantially in the space 29 during the test. To ensure this is the case, it is preferred that the length of space 29 be greater than the distance between electrodes 36 and 38. Thus, only a small portion of the material in space 29 diffuses into space 28 during the time that mediator diffuses from electrode 36 to electrode 38.

As an example of the method, if the area of electrode 38 is 10 times that of electrode 34 and the thickness of space 28 is 10 times that of space 29, the concentration of the reacted mediator in space 29 can be as high as 101 times (the concentration) that of the case of using only electrode 32 and electrode 34. Thus, in this example, the limit of detection of the analyte may be reduced by a factor of 101. For example, if the concentration of analyte or reacted mediator in the solution filling spaces 29 and 28 is initially X, after substantially all of the analyte or reacted mediator in the solution above electrode 38 has electrochemically reacted at electrode 38, a corresponding number of moles of analyte or reacted mediator have been generated in space 29. Since in this example the volume of solution above electrode 38 is 100 times the volume of space 29, the concentration of reacted mediator in space 29 is now X + 100X, i.e. the original amount in space 29 plus 100 times the original amount (resulting from the reaction at electrode 38). It should be noted that it is not necessary to react all of the analyte or reacted mediator in solution above the electrodes 38 when using the present method. In some cases, for example, where a shorter test time is required at the expense of some signal amplification, only a portion of the analyte or reacted mediator in the solution above the electrode 38 needs to be reacted, so long as the portion of the substance reacted is sufficient to achieve useful signal amplification.

Alternatively, to further reduce electrical noise, after a desired portion (typically substantially all) of the analyte or reacted mediator has electrochemically reacted at electrode 38, the circuit between electrode 32 and electrode 38 may be broken and a potential applied only between electrode 32 and electrode 34. The current flowing between electrodes 32 and 34 is then monitored to determine the concentration of reacted mediator in space 29, which is related in a known manner to the original analyte concentration. This step reduces the electrical noise in determining the concentration because the noise generated by the electrodes 38 is eliminated. The time for the circuit to open between electrode 32 and electrode 38 can be determined, for example, by setting a threshold current between electrode 32 and electrode 38 below which the circuit opens. It should be noted that in this method of measuring current, the second counter electrode 36 is not required and may be omitted.

Another optional solution for reducing electrical noise due to electrode discharges or other extraneous currents is: in the electrode discharge stage immediately after the potential is applied between the electrode 36 and the electrode 38, the electrode 36 is used as a counter electrode for the electrode 38. When electrode 38 is polarized to the correct potential, the counter electrode of electrode 38 can be switched to electrode 32. The time to switch the counter electrode can be determined as such a fixed time: i.e., when the electrode discharge and surface set reactions are known to have substantially ended but a significant amount of analyte or reacted mediator has not yet reached electrode 38. If electrodes 36 and 38 have substantially equal areas, the discharge current does not result in any significant amount of additional reacted mediator being formed as a result of the discharge process.

Another optional solution for reducing electrical noise due to electrode discharges or other extraneous currents is: in the electrode discharge phase immediately after the potential is applied between the electrode 34 and the electrode 38, the electrode 36 is used as a counter electrode for the electrodes 34 and 38. After electrodes 34 and 38 are polarized to the correct potential and electrochemical reaction occurs between some or all of the surface of any dried reagent layer adjacent electrode 34 and the adsorbed groups or reacted mediator, the counter electrode of electrodes 34 and 38 can be converted to electrode 36. As described above, the time for switching the counter electrode may be a fixed time. The process can reduce or eliminate the effects of extraneous reactive mediators or other electrochemically reactive species. In both of these optional methods, there is a second counter electrode 36.

In the above method, it is necessary to have an electrochemically inert soluble salt present in the solution filling the cell at a concentration significantly higher than the analyte, either produced by the sample itself or by reagents deposited in the cell. When electrode 32 is used as a counter electrode for electrode 38, the inert salt serves to carry the current in solution between space 29 and space 28 to reduce the loss of reacted mediator from space 29 due to electromigration.

The above description provides several methods and materials of the present invention. The invention is amenable to modification in process and materials, as are fabrication methods and apparatus. Such modifications will become apparent to those skilled in the art after careful consideration of the disclosure and practice of the invention as set forth herein. Therefore, the present invention should not be limited to the specific embodiments set forth herein, but should cover all modifications and substitutions within the true scope and spirit of the present invention as embodied by the following claims.

Claims (14)

1. An electrochemical cell comprising a first working electrode, a first counter electrode, a second working electrode and a second counter electrode, said first working electrode being spaced from said first counter electrode by less than 500 μm, said first working electrode being spaced from said second counter electrode by more than 500 μm, and said second working electrode being spaced from said second counter electrode by more than 500 μm.

2. The electrochemical cell of claim 1, wherein: the first working electrode and the first counter electrode face each other.

3. The electrochemical cell of claim 1, wherein: the first working electrode and the first counter electrode are arranged side by side.

4. The electrochemical cell of claim 1, wherein: the second working electrode and the second counter electrode face each other.

5. The electrochemical cell of claim 1, wherein: the second working electrode and the second counter electrode are arranged side by side.

6. The electrochemical cell of claim 1, wherein: the first working electrode, the first counter electrode, the second working electrode, and the second counter electrode are arranged side by side.

7. The electrochemical cell of claim 1, wherein: the first working electrode and the second working electrode have substantially corresponding areas.

8. The electrochemical cell of claim 1, further comprising a separate reference electrode.

9. The electrochemical cell of claim 1, wherein: the electrochemical cell is a hollow electrochemical cell.

10. The electrochemical cell of claim 9, wherein said electrochemical cell has an effective cell volume of less than 1.5 microliters.

11. A device for determining the concentration of a redox species in an electrochemical cell, the device comprising:

an electrochemical cell having a first working electrode, a first counter electrode, a second working electrode and a second counter electrode, wherein said first working electrode is spaced from said first counter electrode by less than 500 μm, said first working electrode is spaced from said second counter electrode by more than 500 μm, and said second working electrode is spaced from said second counter electrode by more than 500 μm;

a circuit capable of applying a potential difference between the first working electrode and the first counter electrode; and

a circuit capable of applying a potential difference between the second working electrode and the second counter electrode.

12. The device of claim 11, wherein said device is a glucose meter.

13. A device for determining the concentration of a redox species in an electrochemical cell, the device comprising:

an electrochemical cell having a first working electrode, a first counter electrode, a second working electrode and a second counter electrode, wherein said first working electrode is spaced from said first counter electrode by less than 500 μm, said first working electrode is spaced from said second counter electrode by more than 500 μm, and said second working electrode is spaced from said second counter electrode by more than 500 μm;

means for applying a potential difference between the first working electrode and the first counter electrode; and

means for applying a potential difference between the second working electrode and the second counter electrode.

14. The device of claim 13 wherein said device is a glucose meter.