GB2489041A

GB2489041A - Diamond microelectrode for electrochemical use

Info

Publication number: GB2489041A
Application number: GB1104572.1A
Authority: GB
Inventors: Kevin John Oliver; Stephen Charles Lynn
Original assignee: Diamond Detectors Ltd
Current assignee: Diamond Detectors Ltd
Priority date: 2011-03-18
Filing date: 2011-03-18
Publication date: 2012-09-19
Also published as: GB201104572D0

Abstract

This invention relates to a diamond microelectrode for electrochemical use, the use of amicroelectrode and a method of making a microelectrode. The microelectrode comprisesan electrically non-conductive diamond plate 2 having a first surface 4 containing at least two discrete recesses (16, 18, Fig 1b) and an opposed second surface 6. The recesses contain electrically conductive diamond material substantially flush with the first surface 4, the material providing two working electrodes 8, 10. At least two electrically conductive connection members (14, Fig 1c) extend from the first surface 4 to the second surface 6, each of the connection members having a first end (15, Fig 1c) at the first surface 4 that is electrically connected to respective ones of the working electrodes 8,10. A layer 12 of electrically non-conducting material covers the first ends of the contact members, but leaves at least part of the working electrodes uncovered.

Description

DIAMOND MICROELECTRODE

Field

The invention relates generally to a diamond microelectrode for electrochemical use, the use of a microelectrode and a method of making a microelectrode.

Background

Diamond microelectrodes for electrochemical use are used for, for example, the monitoring of electrochemical reactions in which oxidation or reduction reactions occur at an electrode. Diamond microelectrodes for electrochemical use are distinct from electrodes in electronic devices and thermionic or field emission devices. In a diamond microelectrode for electrochemical use, the microelectrode enables electrons to be transferred either from the microelectrode to an ion (or other species) in solution or from the ion (or other species) in solution to the microelectrode, thereby changing the charge state of the ion (or other species).

Electrodes in electronic devices simply allow the passage of the electrons from one solid electrical conductor to another. Electrodes in thermionic devices (such as cathode ray tubes) and field emission devices (such as cold cathodes) allow electrons to exit from a material surface into a vacuum.

Boron-doped diamond electrodes are known for use in electrochemistry. Boron-doped diamond electrodes have been developed for the destruction of waste by electrochemical means (e.g. US Patent Number 5,399,247), for chemical synthesis (e.g. Panizza et al, Electrochemical oxidation of phenol at boron-doped diamond electrode. Application to electro-organ ic synthesis and wastewater treatment', Ann. Chim., 92 (2002), 995-1006), and for chemical analysis (e.g. EP1651951).

D. J. Zhong et al, Fabrication and electrochemical characterization of boron-doped diamond interdigitated array disc electrode', Proc. IMechE. Part E: J. Process Mechanical Engineering, 221 (2007), 201-205, describes a device comprising an interdigitated array of boron-doped diamond electrodes that are deposited by chemical vapour deposition (CVD) on an undoped diamond layer, itself deposited by CVD on a silicon wafer. Both the undoped and doped diamond layers are rough and there is a height difference between the surfaces of the doped and undoped layers.

A similar structure is disclosed in W. Zhang et al (Electrochemical characteristics of an interdigitated microband electrode array of boron-doped diamond film', Collect. Czech. Chem. Commun., 74 (2009), 393-407).

In US 2008/0314744, an interdigitated microelectrode is disclosed in which the electrodes are initially screen printed as metal pastes and then their dimensions are controlled by a photolithography and etching process. In the same document the use of plated-through-holes' to connect the interdigitated electrodes to the back surface of the electrode array is described.

In W02008/059428, an interdigitated electrode array comprising a diamond substrate with conductive electrodes that are optionally conductive diamond is proposed for use as a solid state radiation detector.

Summary

Viewed from a first aspect there is provided a microelectrode for electrochemical use comprising: (a) an electrically non-conductive diamond plate having a first surface containing at least two discrete recesses and an opposed second surface; (b) electrically conductive diamond material contained within said recesses and being substantially flush with the first surface of said diamond plate, the electrically conductive diamond material in each of said at least two discrete recesses providing respective working electrodes in use; (c) at least two electrically conductive connection members that extend from the first surface to the second surface of the electrically non-conductive diamond plate, each of the connection members having a first end at the first surface of the electrically non-conductive diamond plate that is electrically connected to respective ones of the working electrodes; and (d) a layer of electrically non-conducting material covering the said first ends of the connection members, but leaving at least part of the working electrodes uncovered.

The arrangement of the electrically non-conducting material means that if the microelectrode is placed in an electrolyte, the electrolyte can contact at least part of each of the working electrodes but not the first ends of the connection members.

By "substantially flush" it is meant that the root-mean-square roughness Rq measured across any interface between a working electrode in the first surface and the first surface of the electrically non-conducting diamond plate is 50 nm or less.

The definition of Rq is well known in the art and may be found in Tribology: Friction and Wear of Engineering Materials' by I. M. Hutchings (published by Edward Arnold (London), 1992, pages 8-9). For completeness the Rq is defined as: Rq =\/JY2(x)dx where y is the deviation of the surface height from the mean height at a distance x from the origin, and L is the overall length of the profile under examination.

Methods of determining values of Rq are well known in the art and include the use of probe profilometers, non-contact (e.g. optical) profilometers and other surface probe instruments such as atomic force microscopes.

Viewed from a second aspect, there is provided a method of using a microelectrode for electrochemical use for characterising an electrochemical reaction comprising: (a) providing a microelectrode comprising (i) an electrically non-conductive diamond plate having a first surface containing at least two discrete recesses and an opposed second surface; (ii) electrically conductive diamond material contained within said recesses and being substantially flush with the first surface of said diamond plate, the electrically conductive diamond material in each of said at least two discrete recesses providing respective working electrodes in use; and (iii) at least two electrically conductive connection members that extend from the first surface to the second surface of the electrically non-conductive diamond plate, each of the connection members having a first end at the first surface of the diamond plate that is electrically connected to respective ones of the working electrodes; (b) connecting the working electrodes of the microelectrode via the connection members within an electrical circuit; (c) immersing the microelectrode in an electrolyte such that at least part of each working electrode, but not the first ends of the connection members, is exposed to the electrolyte; (d) imposing a potential difference between the working electrodes; and (e) monitoring the current flow between the working electrodes.

Viewed from a third aspect, there is provided a method of making a microelectrode for electrochemical use comprising the steps of: (a) providing an electrically non-conductive diamond plate with substantially parallel first and second surfaces; (b) forming at least two discrete recesses in the first surface; (c) forming at least two connection holes within the electrically non-conductive diamond plate that extend from the first surface of the electrically non- conductive diamond plate to the second surface of the electrically non-conductive diamond plate, each connection hole being in communication with respective ones of the recesses; (d) depositing electrically conductive diamond material in said recesses and in said connection holes, the electrically conductive diamond material in said recesses forming working electrodes and the electrically conductive diamond material in said connection holes forming connection members, each connection member having a first end at the first surface of the electrically non-conductive diamond plate, each connection member being electrically connected to respective ones of the working electrodes; and (e) depositing a layer of non-electrically conductive material so as to cover the said first ends of the connection members but so as to leave uncovered at least part of the working electrodes.

Detailed Description

The electrically non-conductive diamond plate may be rectangular when viewed in plan. Other shapes viewed in plan are possible including square, circular, oval, parallelogram-shapes, rhombus-shaped, kite-shaped, trapezoidal or any other regular or irregular shape.

The electrically non-conductive diamond plate may comprise single crystal diamond or polycrystalline diamond. The electrically non-conductive diamond plate may comprise natural or synthetic diamond. In embodiments where the electrically non-conductive diamond plate comprises synthetic diamond, the synthetic diamond may be produced by a chemical vapour deposition (CVD) process (often referred to as "CVD diamond") or a high-pressure/high-temperature (HPHT) process (often referred to as "HPHT diamond"). In an embodiment of the invention, the electrically non-conductive diamond plate may comprise polycrystalline synthetic diamond produced by a CVD process (referred to hereinafter as "polycrystalline CVD diamond material").

In an embodiment of the invention, the electrical resistivity of the electrically non-conductive diamond plate is 106 ohm.cm or greater. Alternatively the electrical resistivity may be 108 ohm.cm or greater, alternatively 1010 ohm.cm or greater, alternatively 1011 ohm.cm or greater. An advantage of using a substrate material with a high electrical resistivity is that the leakage current is reduced between the working electrodes enabling very small separations between electrodes, for example of the order of 1 pm to 100 pm.

The electrically conductive diamond material contained within the recesses may be CVD material.

The electrically conductive diamond material contained within the recesses may be made electrically conductive by any suitable means, for example by doping with, for example, boron, phosphorus, sulphur, lithium, or any other suitable element. In one embodiment the electrical conductivity of the electrically conductive diamond material is provided by doping with boron.

Generally, in embodiments where the electrically non-conductive diamond plate is single crystalline in nature, then the electrically conductive diamond material contained in the recesses will also be predominantly single crystalline in nature.

Similarly, generally in embodiments where the electrically non-conductive diamond plate is polycrystalline in nature, the electrically conductive diamond material contained in the recesses will also be polycrystalline in nature. These combinations are typical because of the relative ease of deposition of single or polycrystalline diamond material on single or polycrystalline diamond plate. Other combinations may be possible.

In certain embodiments, as described hereinafter, the diamond plate contains additional recesses, which contain electrically conductive diamond material which provides one or more counter electrodes and/or reference electrode(s) in addition to the two or more working electrodes already described. In these embodiments the electrically conductive diamond material making up any of the working electrodes, any counter electrodes, or any reference electrodes may be boron-doped CVD diamond, The concentration of boron in the electrically conductive diamond material may optionally be 1018 atoms/cm3 or greater, alternatively 1019 atoms/cm3 or greater, alternatively 1020 atoms/cm3 or greater, alternatively. 1021 atoms/cm3 or greater. The concentration of boron atoms in the electrically conductive diamond material may be 1022 atoms/cm3 or lower, alternatively 1021 atoms/cm3 or lower, alternatively 1020 atoms/cm3 or lower. The concentration of boron in the electrically conductive diamond material may be between 1018 atoms/cm3 and 1021 atoms/ cm3, alternatively may be between 1019 atoms/cm3 and 1 21 atoms/cm3.

The electrical resistivity of the electrically conductive diamond material that provides any of the working electrodes, any counter electrodes, or any reference electrodes may be about i04 ohm.cm or lower, alternatively 10 ohm.cm or lower, alternatively 5 x 101 ohm.cm or lower, alternatively 2 x 101 ohm.cm or lower, alternatively 1 x 101 ohm.cm or lower, alternatively I x 10 ohm.cm or lower, alternatively i03 ohm.cm or lower. Alternatively, the electrical resistivity of the electrically conductive diamond material that provides any of the working electrodes, any counter electrodes, or any reference electrodes may be in the range between 5 x 101 ohm.cm and 2 x 102 ohm.cm. An advantage of minimising the electrical resistivity of the material comprising the layer of electrically conductive diamond material is that as the electrical resistivity decreases the potential drop between any two points on a working electrode also decreases, so giving the microelectrode spatially more uniform performance. Whilst for some embodiments the electrical resistivities of the electrically conductive diamond material that provides the respective working electrodes are the same, this need not necessarily be the case.

The working electrodes are those electrodes between which, during use of the microelectrode, an electrochemical reaction is monitored. At any time during use at least one working electrode is an anode and at least one working electrode is a cathode. "In use" in this context means when there is a non-zero potential difference between the working electrodes.

One of the principle uses of microelectrodes is for performing cyclic voltammetry.

In cyclic voltammetry, the voltage applied to one working electrode relative to the voltage applied to the other working electrode is swept from zero to a positive value, back through zero to a negative value before returning to zero.

Consequently each working electrode is an anode for part of the cycle and a cathode for part of the cycle.

When a first working electrode is biased relative to a second working electrode such that anions (negatively charged ions) are attracted to the first working electrode, the first working electrode is an anode. When a first working electrode is biased relative to a second working electrode such that cations (positively charged ions) are attracted to the first working electrode, the first working electrode is a cathode.

The recesses may be any suitable shape.

For example any recess may be an elongate recess, for example a channel-shaped elongate recess. By "elongate" it is meant that the length of the recess is typically 2 or more, alternatively 5 or more, alternatively 10 or more, alternatively 20 or more times the greater than the mean width of the recess. Where a recess is an elongate recess, the length of the recess may typically be 0.5 mm or greater, alternatively 1 mm or greater, alternatively 3 mm or greater, alternatively 5 mm or greater.

The layout of the recesses typically corresponds to a pre-selected desired ultimate electrode pattern.

Where the recesses are generally channel-shaped, such channel shaped recesses may for example be generally "U"-shaped or "V"-shaped or "V"-shaped with a flat bottom in cross-section (profile). The recesses, when containing electrically conductive diamond material provide working electrodes. Recess configu rations other than channel-shaped ones that could provide a desired pre-selected working electrode configuration could be deduced by the person skilled in the art.

The depth of the recesses in the electrically non-conductive diamond plate may be 300 pm or less, alternatively 100 pm or less, alternatively 30 pm or less, alternatively 10 pm or less, alternatively 3 pm or less, alternatively 1 pm or less, alternatively 300 nm or less, alternatively 100 nm or less, alternatively 30 nm or less.

The depth of the recesses in the electrically non-conductive diamond plate may be nm or more, alternatively 30 nm or more, alternatively 100 nm or more, alternatively 300 nm or more, alternatively 1 pm or more, alternatively 3 pm or more, alternatively 10 pm or more, alternatively 30 pm or more, alternatively 100 pm or more.

The depth of the recesses in the electrically non-conductive diamond plate may be between 1 pm and 300 pm, alternatively between 1 pm and 100 pm, alternatively between 100 nm and 3 pm, alternatively between 1 pm and 100 pm, alternatively between 1 pm and 30 pm, alternatively between 1 pm and 10 pm, alternatively between 3 pm and 300 pm, alternatively between 3 pm and 100 pm, alternatively between 3 pm and 10 pm, alternatively between 10 pm and 300 pm, alternatively between 10 pm and 100 pm, alternatively between 10 pm and 30 pm.

One factor to consider in selection of the recess depth (and also its profile) is the method involved in manufacturing the microelectrode, and in particular in making, and filling the recesses, and if necessary planarising any over-filled recesses so that the electrically conductive diamond material is substantially flush with the diamond plate surface. Deeper recesses disadvantageously take longer to make and fill than shallower recesses, but the margin for error in planarising any over-filled recesses is advantageously larger for deeper recesses than for shallower recesses.

By "over-filled" it is meant that the thickness of electrically conductive diamond material that is deposited in the recess is greater than the depth of the recess. A recess having such a thickness of material is said to be "over-filled".

The width of the recess where the sides of the recess meet the first surface of the electrically non-conductive diamond plate may be about 1 pm or more, alternatively about 3 pm or more, alternatively about 10 pm or more, alternatively about 30 pm or more, alternatively about 100 pm or more. The width of the recess where the side of the recess meets the surface of the electrically non-conductive diamond plate may be 500 pm or less, alternatively 300 pm or less, alternatively 100 pm or less, alternatively 30 pm or less, alternatively 10 pm or less, alternatively 3 pm or less.

The width of a recess may or may not be greater than its depth. In an embodiment the ratio of the width of the recess where the sides of the recess meet the surface of the diamond plate, w, to the depth of the recess, d, i.e. wld is 2 or more, alternatively 5 or more, alternatively 10 or more. Considering the method used to make the microelectrode it is advantageous for the recesses to have a width that is substantially greater than the depth in embodiments in which the recesses are filled with electrically conductive diamond material using a CVD process.

The cross sectional profiles of the recesses in which the working electrodes are formed may be any shape and may vary with position and may be the same or different for different working electrodes. In an embodiment, the cross-sectional profile of one or more of the recesses is of substantially constant depth and is the same for all working electrodes. This situation has an advantage that it is easier to fill the recess uniformly. Another advantage is that avoids the need to over-fill shallower parts of a recess to ensure that deeper parts of the recess are completely filled.

In some embodiments, when considering a cross-section through a recess, for example through a channel-shaped recess, the maximum width of a recess (at any point measured in its depth) is 150% or less of the width of the recess where it meets the surface of the electrically non-conductive diamond plate. Alternatively, the maximum width of a recess is 120% or less of the width of the recess where it meets the surface of the electrically non-conductive diamond plate. Alternatively, the maximum width of a recess is 110% or less of the width of the recess where it meets the surface of the electrically non-conductive diamond plate. Alternatively, the maximum width of a recess is the same as the width of the recess where it meets the surface of the electrically non-conductive diamond plate.

In some embodiment, the cross sectional profile of a recess may have substantially straight sides. The angle that the straight sides of the recess make with surface of the diamond plate may be in the range 90°±10°, alternatively 90°±5°, alternatively 90°±3°, alternatively 90°±2°, alternatively 90°±1 0 In this case, the base of the recess may also be substantially straight and similarly at an angle of 900±100, alternatively 900±50, alternatively 90°±3°, alternatively 90°±2°, alternatively 900±1 °from the straight sides. An advantage of the angle that the sides the recess make with the surface of the diamond plate being close to 90° is that the width of the recesses, and hence separation between the working electrodes is constant with depth. This is advantageous in terms of the method of manufacture of the microelectrode, because it means that if the electrode is planarised after the conductive diamond material is deposited in the recesses then the variation in the width of the electrode at the surface after different extents of planarisation is reduced which provides tolerance in the planarisation process to achieve a particular electrode separation.

In some embodiments, the width of the recess is narrower at its base than where the sides of the recess meet the first surface of the electrically non-conductive diamond plate. For some embodiments the difference in the width may be in the range 0.1% to 25%. For example, the width of the recess at the base may be less than 100% and 99.9% or more of the width of the recess where the sides of the recess meet the first surface of the electrically non-conductive diamond plate, alternatively less than 100% and 99% or more of the width of the recess where the sides of the recess meet the first surface of the electrically non-conductive diamond plate, alternatively less than 100% and 98% or more of the width of the recess where the sides of the recess meet the surface of the electrically non-conductive diamond plate, alternatively less than 100% and 97% or more of the width of the recess where the sides of the recess meet the first surface of the electrically non-conductive diamond plate, alternatively less than 100% and 95% or more of the width of the recess where the sides of the recess meet the first surface of the electrically non-conductive diamond plate, alternatively less than 100% and 90% or more of the width of the recess where the sides of the recess meet the first surface of the electrically non-conductive diamond plate, alternatively less than 100% and 85% or more of the width of the recess where the sides of the recess meet the first surface of the electrically non-conductive diamond plate, alternatively less than 100% and 80% or more of the width of the recess where the sides of the recess meet the first surface of the electrically non-conductive diamond plate, alternatively less than 100% and 75% or more of the width of the recess where the sides of the recess meet the first surface of the electrically non-conductive diamond plate. In some instances, particularly where adjacent recesses are close to each other (e.g. closer than about 10 pm), it an advantage for the width of the recess to be narrower at its base than where the side surfaces of the recess meet the first surface of the electrically non-conductive diamond plate. This is because the material adjacent to the edge of the recess at the first surface of the electrically non-conductive diamond plate is better supported by the sloping side surfaces compared to a recess with side surfaces that are perpendicular, or close to perpendicular, to the first surface of the electrically non-conductive diamond plate causing the structure to be less weak, thereby allowing a microelectrode having a small separation between a working electrode and an adjacent working electrode to be fabricated more easily.

In some embodiments, one side of the recess may be at an angle of approximately 900 to the first surface of the electrically non-conductive diamond plate and one side of the recess may be at an angle of less than 90° to the first surface (measured within the recess) such that the width of the base of the recess is less than the width of the recess where the sides of the recess meet the first surface.

This embodiment represents a compromise between embodiments with two substantially vertical sides in which the material separating a working electrode from another working electrode may be structurally weak and embodiments with two sloping sides in which more precision is required in any planarisation process applied to remove any electrically conductive diamond material overfill, in order to achieve a particular separation between the working electrodes. This embodiment finds particular application for embodiments where the electrodes are closely spaces, for example where two working electrodes are parallel and unbranched.

The recesses may be substantially uniform in depth or vary in depth over their length. By "substantially uniform" the inventors intend that over 75% or more of the length of the recess, the depth of the recess deviates by 10% or less from the average depth of the recess. Such a measurement may be made using any suitable means; suitable means include contact and non-contact surface profilometers. An advantage of the recesses having substantially uniform depths is that the amount of electrically conductive diamond material that needs to be deposited to completely fill the recesses is substantially uniform over the whole recess.

The separation of the recesses will in general eventually constitute the separation between the working electrodes (anode and the cathode). The desired separation of the working electrodes is in some applications dependent upon the reaction that the device is intended to monitor.

The separation of the working electrodes measured at the surface of the electrically non-conductive diamond plate may be 0.5 pm or more, alternatively 1 pm or more, alternatively 10 pm or more, alternatively 30 pm or more, alternatively 100 pm or more, alternatively 300 pm or more, alternatively 1000 pm or more. The separation of the working electrodes measured at the surface of the electrically non-conductive diamond plate may be 3000 pm or less, alternatively 1000 pm or less, alternatively 300 pm or less, alternatively 100 pm or less, alternatively 30 pm or less, alternatively 10 pm or less, alternatively 3 pm or less, alternatively 1 pm or less. Typically the separation between the working electrodes is between about 0.5 pm and 300 pm. In an embodiment of the invention the separation between the working electrodes is between 3 pm and 300 pm. Alternatively invention the separation between the working electrodes is between 10 pm and 100 pm.

Smaller separations result in a higher electric field between the working electrodes and shorter times for species to reach one of the working electrodes, thereby enabling the behaviour of shorter lived species to be measured or monitored.

However there are practical difficulties associated with making the separation between the working electrodes too small, for example, the regions separating the recesses may become fragile and it may be difficult to ensure that there are no short circuits between created during the deposition of the electrically conductive diamond material and its subsequent processing. The problem of fragility may be exacerbated by the recess having side surfaces that are perpendicular to the first surface of the electrically non-conductive diamond plate and may be ameliorated by the recess having side surfaces that are not perpendicular to the first surface of the electrically non-conductive diamond plate as previously described.

For some embodiments, the separation at the surface of the diamond plate between the recesses forming the working electrodes is substantially constant. By "substantially constant" the inventors mean that for 60% or more of the length of the electrode or the maximum deviation from the mean separation is 10% or less.

The electrically conductive diamond material is substantially flush with the first surface of the electrically non-conductive diamond plate, by which we mean that the root-mean-square roughness, Rq, of the surface measured across any interface between a working electrode in the first surface and the first surface of the electrically non-conductive diamond plate is 50 nm or less. For some embodiments, the root-square-mean roughness, Rq, is even less; for example for some embodiments, the root-mean-square roughness, Rq, of the surface measured across any interface between a working electrode in the first surface and the first surface of the electrically non-conductive diamond plate is 20 nm or less.

Alternatively the root-mean-square roughness, Rq, of the surface measured across any interface between a working electrode and the electrically non-conductive diamond plate is 10 nm or less, alternatively 5 nm or less, alternatively 2 nm or less. Low values of Rq measured across the interface are advantageous as this minimises the disruption in flow of electrolyte across the surface between the working electrodes giving more accurate results.

The thickness of electrically non-conductive material used to cover the first ends of the connection members may be chosen according to the application. For some applications, they are advantageously thick enough to ensure that the layer is continuous and sufficiently robust, but not so thick as to disrupt the flow of fluid (e.g. an electrolyte) across the microelectrode when the microelectrode is in use.

In an embodiment, the electrically non-conductive material that covers the first ends of the contact members may have a thickness of 10 pm or less, alternatively 3 pm or less, alternatively, the thickness is 1 pm or less, alternatively 300 nm or less, alternatively 100 nm or less, alternatively 30 nm or less, alternatively 10 nm or less.

In one embodiment, the electrically non-conductive material that covers the first ends of the contact members may have a thickness of 3 nm or more, alternatively nm or more, alternatively 30 nm or more, alternatively 100 nm or more, alternatively 300 nm or more, alternatively 1 pm or more, alternatively 3 pm or more.

The electrically non-conductive material that covers the first ends of the contact members may be any suitable material. In one embodiment the electrically non-conductive material that covers the first ends of the contact members comprises electrically non-conductive diamond-like carbon (DLC). An advantage of using electrically non-conductive diamond-like carbon for the covering material is in terms of the method of manufacture of the microelectrode, in that it easily applied, for example, as a thin layer and in a defined location. Another advantage of using electrically non-conductive DLC is in terms of the use of the microelectrode is that electrically non-conductive DLC is chemically compatible with the diamond material of the diamond plate. A further advantage is that where the connection members comprise electrically conductive diamond material, electrically non-conductive DLC is chemically compatible with the diamond material of the connection members and also has a similar level chemical resistivity. Thus when the covering material is DLC, the nature of the covering material does not significantly restrict the nature of the chemical environments in which the microelectrode can be used. Also the use of DLC is advantageous as it adheres well to diamond, being chemically very similar and its thermal expansion coefficient is also very similar to that of diamond such that there is a low possibility of delamination of the DLC layer from the underlying material. An example of a method for applying a DLC layer that is suitable for use in the current invention is disclosed in W02008/099220. The compatibilities of DLC to diamond material of the plate and in some embodiments the connection members are particularly applicable to CVD diamond material which is used in some embodiments as the conductive diamond material in the recesses making up the working electrodes, and in the connection holes making up the connection members.

The recesses may be formed, for example, by a plasma etching process.

Inductively-coupled plasma etching is suitable for performing the plasma etching.

In one embodiment the inductively-coupled plasma etching process utilises a gas mixture comprising argon and a halogen-containing gas. In a particular embodiment the halogen-containing gas is chlorine; the use of such a process and gas mixture for the etching of diamond surfaces is disclosed in W02008/09051 1.

As another example the recesses may be formed by a laser processing technique.

An example of a laser processing technique that is capable of ablating diamond and is therefore suitable for forming the recesses of the invention is a deep ultra-violet (UV) argon-fluorine (Ar-F) excimer laser, which produces deep UV radiation at a wavelength of 193 nm.

Connection holes in the diamond plate that are in communication with the recesses can have any suitable geometry. In a simple form a connection hole consists of a hole of approximately cylindrical or truncated cone shape extending from the first surface to the second surface of the electrically non-conductive diamond plate.

Where the cross section of the connection hole is approximately circular, the diameter may be between about 10 pm and 1 mm. Where the connection hole has an approximately truncated cone shape, the taper of the cone may be typically between 0.1° and 20°. As an example, the connection hole may be approximately cylindrical or truncated cone shaped for its entire extent or a combination of the cylindrical for part of its extent or truncated cone for part of its extent. Other geometry connection holes are also envisaged, for example those of rectilinear profile or any other regular or irregular profile.

Connection holes may be formed by any suitable process. In one embodiment, connection holes are be formed by a laser drilling process using laser operating at a wavelength of approximately 1 pm (for example Nd:YAG).

Methods of use of microelectrodes have been described hereinbefore in the summary of the invention. In certain embodiments of method of use the microelectrode may have any one or more of the features that have been described hereinbefore.

Methods of making a microelectrode have been described hereinbefore in the summary of the invention. In certain embodiments methods involve depositing electrically conductive material so that it not only lies in the recesses but also extends over at least part of the surface of the electrically non-conductive diamond plate around the recesses. The deposition step may also involve overfilling the recesses so that the electrically conductive material extends above the level of the electrically non-conductive diamond plate above the recess. In these cases the method may involve planarising the surface after electrically conductive diamond material has been deposited. Planarising may be so as to make the surface of the working electrode flush with the electrically non-conductive diamond plate on either side of the recesses. Planarisation may also generally expose the non conductive diamond plate on either side of the recess. Planarisation may in some embodiments be to the original first surface of the electrically non-conductive diamond plate. Planarisation may in other embodiments be further, and be to a depth below the original first surface of the electrically non-conductive diamond plate, i.e. into the thickness of the electrically non-conductive diamond plate.

The planarisation process may be performed by any suitable means. Suitable processes include conventional lapidary processes such as lapping with diamond grit and diamond polishing using a diamond-loaded resin-bonded wheel.

The electrically conductive diamond material is generally laid down to provide the working electrodes in a desired electrode pattern. The planarisation process may remove sufficient depth of material to expose that electrode pattern, but not so much that the desired electrode pattern is partially or completely removed. Thus careful measurement of the depth of material initially present, the initial thickness of the electrically non-conductive diamond plate is desirable. It is advantageous for the first and second surfaces of the initial electrically non-conductive diamond plate to be substantially parallel in order to facilitate precise and accurate planarisation.

The angular separation between the normals to the first surface and the second surface of the electrically non-conductive diamond plate may be 3° or less, alternatively 2° or less, alternatively 1 ° or less, alternatively 0.8° or less, alternatively 0.5° or less, alternatively 0.2° or less, alternatively 0.1° or less, alternatively 0.05° or less. As another example, the variation in thickness of the initial electrically non-conductive diamond plate 2 may be 52 pm/mm or less, alternatively 35 pm/mm or less, alternatively 17 pm/mm or less, alternatively 14 pm/mm or less, alternatively 9 pm/mm or less, alternatively 3.5 pm/mm or less, alternatively 1.7 pm/mm or less, alternatively 0.9 pm/mm or less.

In embodiments of method according to the invention, subsequent to the formation of the at least two recesses in the electrically non-conductive diamond plate, the electrically non-conductive diamond plate may be cleaned to remove any debris from the recess preparation process. An example of a suitable cleaning process is a mixture of hot concentrated sulphuric acid and potassium nitrate.

In embodiments of method according to the invention, subsequent to the formation of connection holes by laser drilling, any debris may be removed by a cleaning process, for example using a mixture of hot concentrated sulphuric acid and potassium nitrate.

In certain embodiments of method the electrically conductive diamond material is deposited into one or more recesses using a CVD process. In certain embodiments the method involves at least partially filling the connecting holes with electrically conductive material other than electrically conductive diamond material.

In other embodiments of method according to the invention, the recesses, connection holes, electrically conductive diamond material and electrically non conductive material and other features have the characteristics set out above with respect to the description of the microelectrode and its features.

Description of the Drawings

For a better understanding of the present invention and to show how the same may be carried into effect, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 a is a perspective view which shows a microelectrode for electrochemical use of the invention having two working electrodes arranged in an interdigitated manner; Figure 1 b is a cross section through the microelectrode of figure 1 a along AA'; Figure Ic is a cross section through the microelectrode of figure Ia along BB'; Figure 2 is a view showing the microelectrode figure 1 connected to an external circuit and immersed in an electrolyte; Figures 3a-3g are a sequence of cross sectional views showing in schematic outline the steps for the fabrication of the microelectrode of figure 1; Figure 4 is a plan view of another microelectrode for electrochemical use according to the invention having a similar design to that of the embodiment of figure 1 but in addition having a counter electrode and a reference electrode; and Figure 5 are schematic plan views showing examples of electrode patterns comprising two unbranched electrodes that may be used in the microelectrode of figures 1 and 4.

Detailed Description of Certain Embodiments

Embodiments of the invention are described with reference to the figures.

Figures 1 a to c show an embodiment of microelectrode I. The microelectrode I for electrochemical use comprises a planar, rectangular, electrically non-conductive polycrystalline CVD diamond plate 2, having a first surface 4 and a second surface 6 that is substantially parallel to the first surface.

Two discrete recesses 16, 18 (fig ib) are arranged in the first surface, 2, in the pattern of the working electrodes (also referred to herein as the "electrode pattern"). The recesses are approximately 15 pm deep, approximately 50 pm in width and are separated from each other by approximately 100 pm. The side surfaces of the recesses are at an angle of 90° to the first surface. The recesses contain electrically conductive diamond material and this material thereby forms two working electrodes 8 and 10.

The microelectrode I contains two connection members 14 within the diamond plate 2 that extend from the first surface 4 to the second surface 6. Each connection member 14 has a first end 15 at the first surface 4 of the diamond plate 2 and a second end 15' at the second surface 6 of the diamond plate 2. The function of the two connection members 14 is to connect the two working electrodes 8, 10 in the first surface 4 to the second surface 6. In this embodiment, the first end 15 of each connection member 14 is substantially flush with the first surface 4 of the diamond plate 2.

The first end 15 of each connection member 14 is covered with a layer 12 of electrically non-conductive diamond-like carbon material, having a thickness of approximately 200 nm. This means that when the microelectrode 1 is, in use, immersed in an electrolyte, the electrolyte can contact with at least pad of each of the working electrodes 8, 10 but cannot contact the first ends 15 of the connection members 14.

The electrically non-conductive material 12 that covers the first ends 15 of the contact members 14 has the function of substantially preventing the first ends 15 from coming into contact with electrolyte when the microelectrode I is in use. By preventing such contact, the first ends 15 will not participate in any electrochemical reactions, thereby improving the accuracy of the measurements made by the microelectrode.

The microelectrode 1 of fig I may be used for monitoring electrochemical reactions. Such an embodiment is shown schematically in figure 2. In fig 2, microelectrode I is immersed in an electrolyte 42 and connected by means of electrical connections 44 to a suitable external circuit 48 that enables a potential difference to be imposed between the working electrodes 8, 10 (not shown in fig 2) and also the current that flows between the working electrodes to be monitored.

This enables, for example, electrochemical oxidation and reduction reactions to be monitored.

A method of making the microelectrode 1 of fig 1 is described making reference to fig 3. Any or all of the optional features described with reference to fig 3 may be used herein in any combination.

The first step of the method shown in fig 3a comprises providing an electrically non-conductive diamond plate 50 having a first surface 52 and an opposed second surface 54. The first surface 52 and second surface 54 are substantially parallel.

In step 56 at least two separate recesses 58, 60 are formed in the first surface 52.

The recesses are approximately 15 pm deep, approximately 50 pm wide when measured at the first surface 52 and the side surfaces of the recess are at an angle of 90° to the first surface 52. The at least two separate recesses 58, 60 are formed such that they are in a preselected electrode pattern. The recessed plate is shown in fig 3b.

In step 62 at least two connection holes 64 are formed within the diamond plate 50, the connection holes 64 extending from the first surface 52 to the second surface 54 of the diamond plate 50; each connection hole 64 being in communication with respective ones of the recesses 58, 60. The plate at this stage is shown in fig 3c.

In step 68, electrically conductive diamond material 70 is deposited in recesses 58, and in connection holes 64. The electrically conductive diamond material 70 is deposited by a CVD technique over the whole of the first surface 52 of the diamond plate 50 such that the whole surface is covered with electrically conductive diamond material to approximately the same depth and the connection holes 64, 66 are partially filled with electrically conductive diamond material. In CVD processes, the material is built up as a layer, the thickness of which increases with the duration of the deposition process. The layer of material that is deposited substantially conforms to the profile of the surface upon which it is being deposited.

The plate with electrically conductive diamond deposited on its surface is shown fig 3d.

In step 72 the upper surface (in the orientation shown) is planarised by removing deposited diamond material 70 down to the depth of the first surface 52 of the electrically non-conductive diamond plate 50, or further, thereby providing discrete exposed electrodes 74, 76 surrounded by electrically non-conductive diamond material on the planarised surface 92. The planarised surface 92 may correspond to the first surface 52 of the plate 50 or be lower (in the orientation shown) than that surface. Because the side surfaces of the recesses are at an angle of 90° to the first surface 52, the process can tolerate the planarised surface 92 being lower than the first surface 52 without the separation between the electrodes 77 changing. During the same process step, by planarising the material above the at least two at least partially filled connection holes, at least two connection members 86 are formed, one connection member being formed per each of said connection holes. In the embodiment shown (fig 3e) the at least two connection holes are not completely filled with electrically conductive diamond material and an unfilled volume (exemplified by 84) remains within each of the connection holes 64.

The planarisation process of step 72 may be performed by any suitable means.

Suitable processes include conventional lapidary processes such as lapping with diamond grit and diamond polishing using a diamond-loaded resin-bonded wheel.

Non-abrasive processes, for example plasma processes or laser processes, may also be used for planarisation.

In step 78 a layer of electrically non-conductive material 80 is deposited on the first ends 88 of the connection members 86 so as to prevent contact between the connection members and the electrolyte in which the microelectrode is placed when it is in use. The layer of electrically non-conductive material is a layer of diamond-like carbon material having a thickness of approximately 200 nm. The diamond-like carbon layer may be deposited by any suitable method; a suitable method is described in W02008/099220. The layer of electrically non-conductive material 80 may extend beyond the first ends 88 of the connection members 86, but at least part of each working electrode 74, 76 is not covered by the layer of electrically non-conductive material. The microelectrode at this stage is shown in fig 3f.

In an embodiment, optional step 82 involves completely or partially filling unfilled volume 84 with electrically conductive material 90 ("backfilling"), which may or may not be electrically conductive diamond material. The connection member may thus comprise a portion 70a that is filled with electrically conductive diamond and a portion 90 that is with filled an electrically conductive material that is not electrically conductive diamond. The material that is used to fill or to partially fill unfilled volume 84 may be a material is, for example, silver-loaded epoxy resin. Other examples of materials include solders, brazes, other metal-loaded polymer compositions or other electrically conductive materials.

The electrical connection to the connection member on the second surface of the diamond plate may be covered or encapsulated by an electrically non conductive diamond material such that when the microelectrode is in use, access by the electrolyte to any conductive material that forms part of the electrical connection to the connection member is not possible. The microelectrode at this stage is shown in fig 3g.

Instead of directly backfilling with electrically conductive material 90, part or all of the internal surface of the unfilled volume 84, in particular that part of the surface that comprises electrically conductive diamond may be coated with one or more metal layers (not shown) to facilitate the formation of electrical connections to a suitable external circuit (for example by soldering). Such metal layers may be applied by sputtering and an example of a suitable combination of layers is a Ti base layer, a Pt barrier layer and an Au bonding layer. The electrical connection may be encapsulated such that when the microelectrode is in use, access by the electrolyte to any conductive material that forms part of the electrical connection is not possible.

In the embodiment shown in fig 4, there is present one reference electrode and one counter electrode in the first surface. Referring to figure 4, a microelectrode 99 comprises two working electrodes 108, 110 a reference electrode 100, a counter electrode 104, all in a first surface of an electrically non-conductive diamond plate 101. Reference electrode 100 is connected to the second surface (not visible) of microelectrode 99 by means of connection member 102 extending through the electrically non-conductive diamond plate 101. The first end of the connection member 102 is covered with a layer of electrically non-conductive material 103 such that when the microelectrode is immersed in an electrolyte, the electrolyte can contact at least part of reference electrode 100 but not the connection member 102. Counter electrode 104 is connected to the second surface (not visible) of microelectrode 99 by means of connection member 106 extending through the electrically non-conductive diamond plate 101. The first end of the connection member 106 is covered with a layer of electrically non-conductive material 107 such that when the microelectrode is immersed in an electrolyte, the electrolyte can contact with at least part of any counter electrodelo4 but not the connection member 106.

The electrode pattern of the microelectrodes 1, 99 of figs I and 4 respectively comprise working electrodes 8, 10 and 108, 110 that are branched and interdigitated with respect to each other. Interdigitated electrodes are advantageous because they minimise the potential drop over the length of the electrodes and hence the magnitude of the electric field between two electrodes is more uniform.

Alternatively, the electrode pattern may comprise two or more unbranched working electrodes. Two more unbranched working electrodes may be arranged parallel to each other. Non-limiting examples of arrangements of unbranched working electrodes are shown in figure 5. Referring to figure 5, in an embodiment the working electrodes 120 may be straight and parallel, one end of each working electrode terminating at a connection member 122. Referring to figure 5, in an embodiment the working electrodes, 130 and 132, may consist of a number of straight segments, one end of each electrode terminating at a connection member.

Referring to figure 5, in an embodiment the working electrodes may consist of a number of straight segments, 140, 142, connected to curved segments, 144, 146, one end of each working electrode terminating at a connection member.

Where the electrodes are unbranched, the electrodes may be straight, curved or any combination of straight sections and curved sections as long the electrodes are without branches and the separation between the adjacent edges of the electrodes is substantially constant. The use of unbranched electrodes is advantageous because the capacitance of the system is minimised.

While this invention has been particularly shown and described with particular reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

Claims 1. A microelectrode for electrochemical use comprising: (a) an electrically non-conductive diamond plate having a first surface containing at least two discrete recesses and an opposed second surface; (b) electrically conductive diamond material contained within said recesses and being substantially flush with the first surface of said diamond plate, the electrically conductive diamond in each of said at least two discrete recesses each providing respective working electrodes in use; (c) at least two electrically conductive connection members that extend from the first surface to the second surface of the electrically non-conductive diamond plate, each of the connection members having a first end at the first surface of the electrically non-conductive diamond plate that is electrically connected to respective ones of the working electrodes; and (d) a layer of electrically non-conducting material covering the said first ends of the contact members, but leaving at least part of the working electrodes uncovered.
2. A microelectrode for electrochemical use according to claim 1 wherein electrically non-conductive diamond plate comprises synthetic polycrystalline diamond material synthesised using a chemical vapour deposition process.
3. A microelectrode for electrochemical use according to claim 1 or claim 2 wherein the electrically conductive diamond material is boron-doped diamond material.
4. A microelectrode for electrochemical use according to any preceding claim wherein the depth of the recesses is 300 pm or less.
5. A microelectrode for electrochemical use according to any preceding claim wherein the width the recess at the first surface is 500 pm or less.
6. A microelectrode for electrochemical use according to any preceding claim wherein the width of the recess is at least twice the depth of the recess.
7. A microelectrode for electrochemical use according to any preceding claim wherein the width of the recess at the first surface of the working electrodes is 1 pm or more.
8. A microelectrode for electrochemical use according to any preceding claim wherein the separation at the first surface between two working electrodes is 3000 pm or less.
9. A microelectrode for electrochemical use according to any preceding claim wherein the root-mean-square roughness, Rq, of the surface measured across any interface between either an anode or a cathode and the non-conductive diamond plate is 20 nm or less.
10. A microelectrode for electrochemical use according to any preceding claim wherein the thickness of the layer of electrically non-conducting material covering the said first ends of the contact members is 10 pm or less.
11. A microelectrode for electrochemical use according to any preceding claim wherein the thickness of the layer of electrically non-conducting material covering the said first ends of the contact members is 3 nm or more.
12. A microelectrode for electrochemical use according to any preceding claim wherein the electrically non-conducting material covering the said first ends of the contact members is diamond-like carbon.
13. A method of using a microelectrode for electrochemical use for characterising an electrochemical reaction comprising: (a) providing a microelectrode comprising (i) an electrically non-conductive diamond plate having a first surface containing at least two discrete recesses and an opposed second surface; (ii) electrically conductive diamond material contained within said recesses and being substantially flush with the first surface of said diamond plate, the electrically conductive diamond material in each of said at least two discrete recesses providing respective working electrodes in use; and (iii) at least two electrically conductive connection members that extend from the first surface to the second surface of the electrically non-conductive diamond plate, each of the connection members having a first end at the first surface of the diamond plate that is connected to respective ones of the working electrodes; (b) connecting the working electrodes of the microelectrode via the connection members within an electrical circuit; (c) immersing the microelectrode in an electrolyte such that at least part of each working electrode, but not the first ends of the connection members, is exposed to the electrolyte; (d) imposing a potential difference between the working electrodes; and (e) monitoring the current flow between the working electrodes.
14. A method of making a microelectrode for electrochemical use comprising the steps of: (a) providing an electrically non-conductive diamond plate with substantially parallel first and second surfaces; (b) forming at least two discrete recesses in the first surface; (c) forming at least two connection holes within the electrically non-conductive diamond plate that extend from the first surface of the electrically non-conductive diamond plate to the second surface of the electrically non-conductive diamond plate, each connection hole being in communication with respective ones of the recesses; (d) depositing electrically conductive diamond material in said recesses and in said connection holes, the electrically conductive diamond material in said recesses forming working electrodes and the electrically conductive diamond material in said connection holes forming connection members, each connection member having a first end at the first surface of the electrically non-conductive diamond plate, each connection member being electrically connected to respective ones of the working electrodes; and (e) depositing a layer of non-electrically conductive material so as to cover the said first ends of the connection members but so as to leave uncovered at least part of the working electrodes.
15. A method according to claim 14 wherein the step of depositing electrically conductive diamond material in the recesses also comprises depositing some electrically conductive diamond material over at least part of the first surface of the electrically non-conductive diamond plate.
16. A method according to claim 14 or 15 wherein, after the deposition of conductive diamond material, the method comprises the step of planarising the deposited electrically conductive diamond material and the electrically non-conductive diamond plate that is on either side thereof so as to expose the working electrodes and the surface of the electrically non-conductive diamond plate on either side of the working electrodes.
17. A method according to any of claims 14 to 16 wherein the depth of the recesses is 20 pm or less.
18. A method according to any of claims 14 to 17 wherein the width of the recesses is 50 pm or less.
19. A method according to any of claims 14 to 18 wherein the width of the recess is at least twice the depth of the recess.
20. A method according to any of claims 14 to 19 wherein the electrically conductive diamond material deposited into the one or more recesses is deposited using a chemical vapour deposition (CVD) process.
21. A method according to any of claims 14 to 20 wherein the electrically conductive diamond material is boron doped diamond material.
22. A method according to any of claims 14 to 21 wherein the at least two connection holes are partially filled with electrically conductive material other than electrically conductive diamond material.
23. A method according to of claims 14 to 22 wherein the electrically non-conductive material is electrically non-conductive diamond-like carbon.
24. A method according to claim 23 wherein the thickness of the layer of electrically non-conductive diamond-like carbon is 10 nm or greater.
25. A method according to claim 23 or claim 24 wherein the thickness of the layer of electrically non-conductive material is 2 pm or less.
26. A microelectrode, a method of using a microelectrode or a method making a microelectrode substantially as hereinbefore described with reference to the accompanying drawings.