IMPROVEMENTS IN AND RELATING TO MICROELECTRODES AND AMPEROMETRIC ASSAYS

Abstract: A photo-ablation technique is used to create apertures (4) in a layer (2) of electrically insulating material and allow electrically conducting material (3) exposed through the apertures to create a microelectrode. The microelectrode can be used for assay methods and in an assay unit.

Description: IMPROVEMENTS IN AND RELATING TO MICROELECTRODES AND AMPEROMETRIC ASSAYS Technical Fie c This invention relates to an improved methoo of maκιng an electrode (in particular a microelectrode), an electrode made by the method, an assay method for the determi ation of -che presence of a redox species (e.g. a heavy metal _ in liquid carrier media using the electrode and reαox species detection equipment designed to utilise the assay metnod. Background Art Microelectrcdes provide many advantages in electro¬ chemistry and electroanalysis (some of which are discussed in Wightman, R.M., Anal. Chem. , 1982, 54, 2522) in par¬ ticular the possibility of electrolysis in resistive media or dilute solutions, and of microanalysis with improved sensitivity and precision. For these reasons a great deal of effort has been dedicated to establish a way of fabri¬ cating a regular microscopic disc array electrode. Any proposed method of fabrication must ensure that all the distinctive features of individual microelectrodes are retained, and in addition circumvent the problems assoc¬ iated with the measurement of extremely small currents (pA- nA), observed for single microdisc electrodes. Many approaches have been proposed for the construc- tion of a microdisc array electrode including photolitho¬ graphic methods (see e.g. Osteryoung, J. and He pel , T., J. Electroch ' em. Soc. , 1986, 133, 757-760 and W. Siu and R.S.C. Cobbold, Med. & Biol . Eng., 1976, 14, 109), as well as the use of icropore membrane matrices (see e.g. Wang, J., J. Electrochem. Soc, 1988, 249, 339-345, and Cheng, F., Anal. Chem., 198S, 61, 762-766). The former approach failed because of adhesion problems between the insulating photoresist layer and the metal substrate. The latter method of fabrication was self defeating because, although the size of the pore is well characterised, the pore density and distribution were undefined and the electrode ultimately irreproducible. Summary cf the Invention According to the first aspect of this invention a method of making a microelectrode comprising a layer of electrically insulating material having an array of aper¬ tures formed therein and electrically conducting material visible through the apertures, is characterised in that the apertures are formed by photo-ablation. Suitably the layer is a sheet of flexible plastics material (suitably a polyester or other polymer film) and the apertures are formed by photo-ablation. A particularly preferred form of electrode made by the method of this invention is a microdisc array electrode in which the apertures are of the same size and shape and are distri¬ buted over the sheet in a regular array. In the field of microdisc electrodes the sheet can be of a thickness in the range 2 microns to 500 microns and the apertures are suitably of diameter (or transverse dimension if not circular) of between 5 microns and 100 microns. A hexagon¬ al array cf 10 micron diameter apertures with edge/edge separations of 100 microns between the apertures in the array has been found to be one convenient arrangement offering many useful commercial applications. An alternative method of making the electrode involves supporting a layer of electrode material on a substrate, laying a thin polymer film onto the electrode material (e.g. by vapour polymerisation) and then drilling apertures by photo-ablation through the polymer film. The layer of electrode material is conveniently thick-film printed (to any desired pattern) on the substrate. This alternative method has a particular advantage when the electrode material includes carbon, since the photo-ablation used to form the apertures can vitrify the carbon in the areas of electrode material exposed by the photo-ablation. As well as microαisc arrays the invention extenαs to icroband arrays, a microband array electrode comprising a set of spaced-aoart slot-shaped apertures exposing narrow strips (e.g. parallel strips) of a common electrode material . The formation of the array of apertures by a photo- ablation technique has proved to be particularly convenient and typically involves photographically creating spaced- apart ablatable areas on the sheet interspersed by more resistant non-ablatable regions, and then removing the sheet material only over the ablatable areas using a nigh power source of radiation (e.g. from an excimer laser). Using photo-ablation, means that an exceedingly accurate array of apertures of any required size can be produced using well established techniques. The conducting material is suitably a thixotropic paste based on carbon or metallic particles (e.g. platinum or gold) applied sufficiently thickly so that where the paste is applied to a pre-drilled sheet of plastics mater¬ ial each aperture formed in the non-conducting sheet is substantially filled with conducting particles. A pre-drilled coated sheet can be bonded, conducting side down, on a substrate of sufficient thickness to give the required rigidity to the sandwich thus produced. The alternative thick film method which drills apertures through to the electrode material already has a suitable support substrate. The substrate can be of electrically conducting or non-conducting material, and can incorporate suitable electrical conductors for applying potentials to and/or leading currents to/from the one or more conducting areas exposed in the apertures. From the above, it will be appreciated that the fabri- cation method proposed in tms first aspect of the inven¬ tion is essentially a three dimensional thick film process. The procedure circumvents all the past difficul¬ ties including the aforementioned, and is based on pπn- ciples of photo-machining of polymer films by laser pnoto- ablation to produce matrices of well defined aperture size, distribution, density anc geometry. The process can fabricate array patterns in support matrices of thickness 2μm to 500μm, and the apertures can be filled with the required conducting particles. The filled sheet can then be mounted by encapsulation on an inert substrate e.g. by a heat sealing process. A microelectrode made oy tne method of the invention represents a further aspect of this invention. Initial trials have suggesteo that such electrodes have useful applications in electroanalysis, high resolution electro¬ chemistry, pathogen testing techniques, antibody or enzyme reactions, and bioelectrochemical assay methods. The following list details the important characteris- tics of an electrode made by the method of the invention which underline its use in electroanalysis and application to chemical sensors. It should be appreciated that the method of fabrication here discussed is uniquely controll¬ able in terms of device specification and this has not been found to be true for earlier attempts at fabrication. 1. Uniformity of electrode shapes within an array, size may range from 5 μm to 50 μm in diameter. A reproducible manufacturing method produces identical array electrodes and is essential for reliable analytical application. The small aperture size produces an amperometric signal with improved analytical properties such as detectabil- ity, sensitivity and precision even in the presence of dissolved oxygen in the analyte. 2. Variable array size, from a few apertures (say six) to many thousands. The number of apertures within tne array will αepend on application: current amp ification increases linearly with the number of isolated electrodes. 3. Arrays may be patterned: e.g. hexagonal, square etc. 4. The spacing between conducting areas can be varied at will. The ability to set the aperture separation is a very advantageous feature. For example, the separation can be made very small (a few microns) for the mom or- ing of catalytic current. 5. Arrays of electrodes can be made from different conσuct- ing materials (e.g. platinum, gold, carbon etc). 6. The composition of electrode material can be varied (e.g.ligandε and complexing agents can be incorpcrated in the conducting material to add chemical selectivity to an electroanalysis to be performed with the electrode array) . 7. Apertures or groups of apertures within an array can be individually addressed to a set electric potential and therefore the electrode array could be used in multicom- ponent determination simultaneously. 8. The electrode design is ideally suited to chemical immobilisation of reactants (enzyme or antibody) onto the insulating area by covalent bonding hence allowing direct application to biosensor and biochemical assay technology. A particularly promising application involves the use of an electrode made by the method of this invention for redox species analysis using Voltam etry. Accurate determination of redox species such as pesticides or heavy metals in water is important to tne monitoring ana control of environmental pollution. The type of information required for comprehensive environment studies not only includes a single sampled measurement cf a specific redox species (e.g. metal j at a given place and time, but should include extensive distriDution anG fluc¬ tuation data on major pollutants in order to ascertain origin. The techniques presently employed by consultant analysts for tne determination of heavy metals in water samples include the laboratory based techniques. Atomic Spectroscopy (Absorption and Emission) and Polarσgrapny (Pulse and Derivative). These conventional analytical methods are by nature timely and consequently expensive, relying on sampling procedures of a given water supply. This invention also relates to an analytical method and device which need not be laooratory bound. In one embodiment, the device is a multi-heavy metal ion detector directed essentially at in-the-fielc measurement which may be used as a portable hand-held device or form part of a remote sensing network. An analytical method based en this aspect of the invention is electrochemically based and relies on the properties of a microelectrode according to this invention. According to this aspect of the invention an assay method for a target redox species analysis in a liquid carrier comprises moistening an absorbent medium containing a dry reactant for the species with the liquid carrier, the absorbent medium being sandwiched between two electrodes, one of which is a microelectrode according to this inven¬ tion, the moistening process being monitored by measuring the conductance between the two electrodes and the con¬ centration of the target redox species being determined by measuring the current flowing between the two electrodes when applying a varying potential therebetween. Suitably a programmed voltage scan ramp or step formed) is used for the concentration measurement. The target redox species could be, inter alia, a crug, a bioproduct or a heavy metal ion. According to a further aspect of the analytical method aspect of the invention an assay method for a trace redox species analysis in a liquid carrier comprises moistening an absorbent medium containing a dry reactant for tne species with the liquid carrier, the absorbent medium being sandwiched between two electrodes, one of which is a microelectrode according to this invention, applying a first potential across the electrodes to cause any species drawn into the medium and reacted with the reactant therein to migrate ions onto the conductive areas of the micro¬ electrode, applying a second potential of opposite polar¬ ity across the electrodes to strip the said ions from the microelectrode and determining the nature of the "stripped" ion from a knowledge of the redox potential requirec tc effect stripping. A particularly useful application of the assay method of this invention for trace redox species is in the determination of which heavy metal ions are present in a sample of water, the dry reactant in the absorbent medium being a mercury salt. The method of analysis in this case is an adaption of anodic stripping voltammetry. In its equipment aspect the invention comprises an assay unit which includes a pad of dry absorbent material incorporating the required dry form redox reagent sand- wiched between a microelectrode according to this invention and a counter electrode, the sandwich being mounted on a support means that includes a contact to at least one of the electrodes. Suitably the support means is a non-conducting .e.g. glass) plate having two contacts, one leading to eacn of the different electrodes. In the case of an assay αevice for heavy metal detection, the redox reagent can oe a mercury salt and where testing on waters with very low electrical conductivity is in mind, a readily lonisable salt (such as sodium chloride). An assay device in accordance with this aspect of the invention can be used as a disposable component whicn is connected to (e.g. plugged into) a suitable potentiostat. A fully portable testing equipment is available in this manner, with testing simply requiring a wetting of the absorbent pad of a fresh device and subsequent application of the appropriate potentials to the electrodes using the potentiostat to carry out the required Voltammetry or Anodic Stripping Voltammetry. Brief Description of Drawings The various aspects of this invention will now be further described, by way of example, with reference tc the accompanying drawings, in which: Figure 1 is a much enlarged sectional view cf part of a microdisc array electrode according to the invention, Figure 2 is a plan view from above of the part shown in Figure 1 , Figure 3 is a much enlarged sectional side view of part of a microband array electrode according to the invention, Figure 4 is a plan view from above of the part shown in Figure 3, Figure 5 is a schematic view of laser equipment for forming an array of apertures in a component of a micro¬ electrode, Figure 6 is a siαe elevational view of an assay device in accordance with a further aspect of the invention. Figure 7 is a schematic sectional view througn part of the device of Figure 6 with the vertical scale exaggeratec for ease of viewing, and Figure 8 is a graph showing the applied potential for a trace element detected with a device such as that snown in Figures 6 and 7 by stripping analysis. Description of Preferred Embodiments The microdisc array electrode partially shown in Figures 1 and 2 comprises a substrate 1 on which a pet— forated sheet 2 of non-conducting material coated on one side with a layer 3 of conducting material is applied. The perforations are regularly spaced equally sized apertures A drilled by photo-ablation and forming an array over the entire effective area of the microσisc array electrode. In a typical case the substrate 1 would be a sheet of polyvinylchloride of a thickness of 500 microns, the sheet 2 a polyester sheet of a thickness of 100 microns and the layer 3 a cured carbon paste of a cured thickness of 500 microns. The array shown in Figure 2 has circular discs of 10 micron diameter in a 100 micron spaced hexagon pattern. Other geometries such as those leading to microband elec¬ trodes are also envisaged. Since each aperture is filled with the conducting paste, each filled aperture forms a conducting area surrounded by a non-conducting region. All exposed conducting areas (or discs) can be connected to the same layer 3 but it will be appreciated that where end uses reαuire different potentials to be aDcliec tc cifferent discs or cifferent grouDs of discs. the layer 2 car. oe appropriately σivideo and connectec tc its own suppl lεaα. Tne supply eaα has not been snown in Figures 1 and 2 out sucr, a leac can oe seen at 11 in Figure 5. Figures 3 anc 4 correspond tc Figures 1 anσ 2 but snow a microband electrode the apertures 4 now peing narrow slits. Tne micro-array electrode shown m Figure 3 was made by tmck-film printing conαuctmc electrode material 2 onto an electrically insulating substrate 1 n the required pattern. A layer 2 of polymer film is tnen vapour polymerised over the printed material 2 anc suose- αuent.y Grilled using laser photo-ab ation tc oroαuce tne apertures 4. With carbon in the material 2 vitrified carbon is formed in tne apertures 4 giving the electrode high durability and good electrical properties. Photo-ablation of the required aperture pattern is schematically illustrated in Figure 5 where 30 indicates a high power laser source (e.g. a Lambda Physih LPX 2051 unit), 21 a reducing lens and 2 the plastics film requiring to be drilled. The film 2 can have an apertured mask printed thereon wnereby only the film exposed througn tne apertures in the mask is available for ablation or a mask 32 (e.g. of metal) can be located in the optical path anywhere between the laser source 31 and the film 2. A metal mask contacting the film 2 can also be used. The minimum size of the apertures 4 that can be produced in a plastics film are expected to be 1 to 2 microns but they can be as large as desired. The film 2 could be poly-para-xylylene (Parylene N) or other CH materials (such as polystyrene or polyethylene for example) . As examples of suitable laser sources may be mentioneα a KrF laser (wavelength 248 nm) of fluence in the range 0.2 to 0.4 J/cm" and (depending on the polymer film 2 being used) ArF (wavelength 1S3 nm) and XeC (wavelength 208 nm) lasers of fluences in the range 0.1 to 1.0 J/cm . For contact imaging using a metal mask 32 in contact with a Parylene N film of 2 microns thickness, a KrF laser of 300 mJ output (a Questek 2440 unit) can be used generat¬ ing a fluence at the mask of 0.3 J/cm" over an area cf 1 cm'". Using 25 shots of the source 3C, an array cf 15 micron diameter apertures 4 on 100 micron hole centres was accurately formed in the film 2. For projection imaging, a metal mask 32 with an array of 45 micron diameter holes on 300 micron hole centres was located in the beam from a 600 mJ KrF laser and projected down onto a 2 micron thick Parylene N film with a three times reduction of image in the lens 31. The fluence at the mask was 0.044 J/cm 2 and at the film it was 0.4 J/cm2. Again 25 shots of the source 30 were required to dril the accurate array of apertures 4 in the film 2. The device shown in Figures 6 and 7 comprises a rectangular substrate 10 of non-conducting material on which a microelectrode 12 has been provided. Adhered over the electrode 12 is a pad 13 of absorbent material and overlying the pad 13 is a counter electrode 14. The electrode 14 can be applied by printing and will have an area larger than the sum of the apertures of the electrode 13. Conducting leads 11 and 15 connect contact areas 16 and 17 to the two electrodes 12 and 14. Conveniently, the areas 16 and 17 are positioned so that slipping the substrate 10 into a holder (shown dotted at 20) which is connected to an electronic unit 21 by a cable 22, automatically places the device in circuit with the electronic equipment in the unit 21. The unit 21 can comprise a miniatured microprocessor witn control poten¬ tiostat incorporating a sensitive ππcroammeter. For testing for the presence of heavy metals .e.g. Pb, Cd, Fe, Zn) in water, the pad 13 will include some par- tides of common salt and mercurous chloride (say 105& w/w of NaCl and 5% w/w of Hg^Cl^) or other αried reactants necessary to provide conductivity and oxidised mercury for an electroplating operation. The micro-conducting areas of the electrode 12 will be carbon and the electrode 14 will be of si 1 er/si1ver chloride. Dipping the pad 13 into a sample of the water to be tested, will draw water into the pad 13 by capillarity action where it will dissolve the salt and redox reagent forming a conductive solution between the electrodes 12 and 14. For voltammetric analysis, current is passeα with electrode 12 as working electrode and electrode 14 as a controlled voltage secondary electrode. The electrochemi¬ cal reaction on electrode 12, i.e. oxidation or reduction of the target redox species is followed by classical microelectrode electroanalytical methodology. For stripping analysis, current is passed with elec¬ trode 14 as anode to plate mercury and any heavy metal present onto the micro-conductive areas of the electrode 12. Plating current is maintained by hemispherical mass transport inherent to the micro-array geometry for long enough (typically 2 or 3 minutes) to plate a detectable amount of the heavy metal onto the conducting areas. By virtue of the fact that the microelectrodes are cheap enough to be disposed of after a single use, "memory" problems, such as can occur with laboratory-based equip¬ ment, will be avoided since after use for one analysis, the electrode will be discarded. The polarity of the applied potential is now reversed so that electrode 12 serves as anode and the polarity is increased as a plot is taken of current against voltage. One example of such a plot is shown in Figure 8 which snows the stripping of lead cations from mercury coated ππcrc- conductive areas. It will be appreciated, therefore, that the invention makes possible an assay device which comes in two sections; a sensing unit consisting of a disposable or reusable film cartridge driven by a pocket size computerised potentiostat which comprises the control and signal handling unit. The output of the instrument is a current which is proportional to the concentration of the trace metal ion at a potential specific to the ion itself. The electroanal sis method described hereabove can be easily adapted to enzyme chemistry or/and immunochemistry for use as a biosensor. Indeed, enzymes or/and antibodies can be covalently immobilized on the perforated polymer sheet 2 in Figure 1 and electron carrier species cften called mediators can be detected on the microelectrode array in a very efficient manner due to the close proximity between the electrode and the immobilised species leading to feedback diffusion enhancement. It should be stressed that the sensitivity and ac¬ curacy of the method and apparatus described herein, are at least comparable to the equivalent laboratory-based methods and equipment.

Claims: CLAIMS 1. A method of making a microelectrode comprising a layer of electrically insulating material having an array of apertures formed therein and electrically conducting material visible through the apertures, characterised m that the apertures are formed by photo-ablation. 2. A method according to claim 1, characterised in that the insulating layer is a plastics film and the conducting material is applied to one side of the film after the apertures have been formed therein. 3. A method according to claim 1, characterised in that the insulating layer is applied over conducting material supported on an electrically insulating substrate and the apertures are then formed in the insulating layer to expose the conducting material. 4. A microelectrode made by the method of claim 1, characterised in that the layer of insulating material has a thickness in the range of 2 to 500 microns and each aperture has a maximum transverse dimension of between 5 and 100 microns. 5. A method for a target redox species analysis in a liquid carrier comprising moistening an absorbent medium containing a dry reactant for the species with the liquid carrier, the absorbent medium being sandwiched between two electrodes, characterised in that one of the electrodes is a microelectrode according to claim 1, the moistening process being monitored by measuring the conductance between the two electrodes and the concentration of the target redox species being determined by measuring the current flowing between the two electrodes when applying a varying potential therebetween. 6. A method according to claim 5, characterised in that a programmed voltage scan is used for the concentra¬ tion measurement. 7. An assay method for a trace redox species analysis in a liquid carrier comprising moistening an absorbent medium containing a dry reactant for the species with the liquid carrier, the absorbent medium being sandwiched between two electrodes, characterised in that one of the electrodes is a microelectrode according to claim 1, and in that a first potential is applied across the electrodes to cause any species drawn into the medium and reacted with the reactant therein to migrate ions onto the conductive areas of the microelectrode, applying a second potential of opposite polarity across the electrodes to strip the said ions from the microelectrode and determining the nature of the stripped ion from a knowledge of the redox potential required to effect stripping. 8. An assay unit, characterised in that it includes a pad of dry absorbent material sandwiched between a micro- electrode as claimed in claim 1 and a counter electrode, the absorbent material incorporating a dry-form redox reagent and the sandwich being mounted on a support means that includes a contact to at least one of the electrodes. 9. An assay unit as claimed in claim 8, characterised in that the support means is a non-conducting plate having two contacts, one leading to each of the different electrodes. 10. An assay unit as claimed in claim 9, charac¬ terised in that the redox reagent is a mercury salt.

Inventors: GIRAULT HUBERT HUGUES JACQUES, SEDDON BRIAN JEFFREY, GIRAULT, HUBERT, HUGUES, JACQUES, SEDDON, BRIAN, JEFFREY
Applicants: ECOSSENSORS LTD, ECOSSENSORS LIMITED
Application year: 1990
Main classification: G01N27
IPC codes: G01N27, G01N33