Patent Description:
There are currently a number of known methods for fabricating arrays. These include printing techniques such as screen printing or ink jet printing, lithographic techniques whereby the array is etched onto a surface, photolithography, direct electrodeposition (deposition of wires), patterning of carbon nanotube / nanofiber arrays and assembly techniques, for example, wires set in an epoxy resin. However, these known methods have a number of limitations. In particular, they are cumbersome to carry out and it is difficult to accurately define the arrays over a large surface area and on the millimeter to nanometer scale. Thus, the resolution of the arrays produced is often poor due largely to that lack of definition. The inability to accurately place sensor sites on such arrays causes problems as qualitative and quantitative measurement is detrimentally affected. In particular, issues of cost arise with the fabrication of nanoscale arrays as, while they can be made, control over definition and cost remain problems which cannot be easily overcome. Economy of scale is a particular issue.

The fabrication of arrays on the millimeter to nanometer scale, particularly on the micrometer to nanometer scale over large surface areas having improved accuracy of definition would be particularly valuable in the areas of sensing, electrochemistry and catalysis. Electrochemistry is the branch of chemistry that deals with the use of spontaneous chemical reactions to produce electricity, and the use of electricity to bring about non-spontaneous chemical change. In particular, it is the study of aqueous chemical reactions which occur at the interface of an electron conductor such as a metal or a semiconductor (the electrode) and an ionically conducting medium (the electrolyte) and which involve electron transfer between the electrode and the electrolyte or species in solution. Catalysis concerns the creation of a new reaction pathway with a lower activation energy, thereby allowing more reactant molecules to cross the reaction barrier and form reaction products.

In a typical electrochemical detection process it is, in general, preferable to employ an array of smaller electrodes as opposed to a single large electrode. Reasons for this include:.

Accurately defined arrays would also be valuable for use in:.

<CIT> shows a functionalised interdigitated electrodes arranged on a flat substrate with inert material positioned between the fingers of the interdigitated electrodes.

<CIT> shows a sensor comprising a substrate covered by an electrically conductive layer and a porous insulating layer. Carbon nanotubes extend from the electrically conductive layer through and out of the pores of the insulating layer. Noble metal nano particles are deposited on the exposed ends of the carbon nanotubes and are functionalised with enzyme molecules for analyte conversion.

Co-pending PCT application number<CIT>also concerns microarray structures. However, the microarrays as described in <CIT> simply include a continuous inert base substrate with functionalisable areas isolated by an inert material. The functionalisable areas are not stated to be conductively interconnected and the structures do not include at least one continuous interconnected layer, separate to the base substrate material and inert material, that allows for improved functional and structural flexibility of the microarrays formed.

It is therefore an object of the present invention to provide arrays including isolated but conductively interconnected functionalised areas and/or methods of forming such arrays.

The present invention provides a microarray structure as claimed in claim <NUM> and a method of its manufacture as claimed in claim <NUM>.

Preferably, the 3D surface layer is a metal.

Alternatively, the continuous 3D surface layer according to the claims is a carbon based conducting polymer.

Preferably, the continuous 3D surface layer is a unitary layer that covers the substrate material layer.

Preferably, the continuous 3D surface layer is cut into a plurality of isolated continuous 3D surface layer segments on the substrate material layer, each segment including a plurality of functionalisable areas, wherein each group of functionalisable areas is capable of separate functionalisation.

Optionally, the inert material is also an insulating material.

Optionally, the substrate material layer is formed from a conductive material or a non-conductive inert material which, optionally, is also an insulating material.

Optionally, the structure includes an adhesion layer between the continuous 3D surface layer and the substrate material layer.

The present invention concerns the development of arrays of various sizes for use as sensors in electrochemistry and catalysis. In particular, the present invention relates to method for fabricating arrays comprising functionalisable areas at the millimeter to nanometer (inclusive) scale. These functionalisable areas are conductive and are isolated at the surface of the array but joined below the material used to isolate them. They are functionalised to create sensor or catalytic sites for a multitude of applications. Examples of applications include the detection of enzymatic catalysed reduction or oxidation reactions (e.g. glucose oxidase), the direct detection of oxidisable species within a solution (e.g. metals, metal oxides, organic species), the detection of antibodies, DNA, cells or small molecules where an appropriate haptan has been attached to the array surface, and detecting and binding of their complimentary antigen via an associated electrochemical method including the measurement of changes in the resistance between the binding surface and a counter electrode or an electrochemical reaction. In each instance, concentration of the target analyte is related to the level of current passed through the conductive, continuous 3D, array surface.

Thus, the present invention provides a microarray structure as claimed in claim <NUM>. The microarray structure includes a substrate material layer, a continuous 3D surface layer on the substrate layer that is capable of functionalisation for use as an array and an inert material. The structure includes functionalisable areas which are part of the continuous 3D surface layer and are isolated by the inert material but are interconnected within the structure by the continuous 3D surface layer.

As used herein, the "substrate material layer" (herein referred to as substrate material) refers to the base of the microarrays of the present invention. It may be flexible or rigid and ranging in thickness from the micrometer to millimeter scale. As will be known to a skilled person in the art, the thickness of the substrate material is primarily governed by the thickness required to ensure proper handling. Where required, the substrate material should also be optically transparent. Therefore, preferably, the substrate material is between about <NUM> micron to about <NUM> thick, or between about <NUM> micron to about <NUM> thick, or between about <NUM> micron to about <NUM> micron thick. Preferably, the substrate material is a polymer material. Alternatively, the substrate material may be a conducting material or an inert, non-conducting material. Where the substrate material layer is inert, it may also act as an insulating material. Examples of suitable flexible materials for use in the present invention include thermoplastic polyurethane, rubber, silicone rubber, and flexible epoxy. Examples of suitable rigid substrate materials for use in the present invention include glass, PMMA, PC, PS, ceramic, resin, composite materials and rigid epoxy. The substrate material may also be formed from a metal such as gold, silver, nickel or the like, as discussed in more detail below.

As used herein, "functionalisable areas" should be taken broadly to encompass those parts of the microarrays of the present invention which protrude, through an inert material or are exposed and are therefore capable of being functionalised as desired by a user. When the continuous <NUM>-D surface layer protrudes through the inert material, it is exposed above that material. The functionalisable areas can be in any shape as desired by the user and preferably form the uppermost surface or tip of a three-dimensional (3D) pillar like structure (nanometer to millimeter size) formed as part of the substrate material of the microarrays of the present invention. However, a person skilled in the art would understand that the functionalisable areas can also form the uppermost surface of a 3D rib like structure formed as part of the substrate material of the microarrays of the present invention.

Throughout the specification reference to 3D should be taken to mean a three-dimensional structure, or where required by context, a three dimensional coated structure, wherein, the three-dimensional structure is in the form of a pillar like structure or a rib like structure.

The functionalisable areas preferably range in size from the millimeter to the nanometer scale. More preferably, the functionalisable areas are between about <NUM> to about <NUM> micron in size, more preferably between about <NUM> to about <NUM> micron in size. Likewise, the spaces between individual functionalisable areas can be on the millimeter to nanometer scale.

In one embodiment of the present invention, the functionalisable areas are accurately defined areas in that they form a defined pattern on the surface of a microarray to the scale desired. This, in turn, allows a user or a computer program to pinpoint specific functionalisable areas on the surface of a microarray and make a desired measurement and allows for the functionalisation of only selected functionalisable areas on the surface of a microarray. Alternatively, the functionalisable areas are randomly arranged on the surface of a microarray of the present invention.

<FIG> shows, diagrammatically, the use of embossing techniques to shape the surface of the substrate material <NUM> into a desired 3D pattern that is accurately defined to the scale desired. <FIG> shows the use of a stamp to achieve this. First a stamp <NUM> is formed to the negative of the desired pattern (<FIG>). This pattern is shown in <FIG> as being of repeating triangles, however, this could be replaced by other options as desired by the user. The pattern does not have to be uniform. The embossing creates tips <NUM> that extend from the surface of the substrate material <NUM> and, therefore, also creates the desired spaces between those tips <NUM>. The stamp <NUM> is typically made from silicon or nickel. However, it can be formed from any suitable material that is capable of use in this manner. The stamp <NUM> is then used to emboss the substrate material <NUM> with the desired pattern (<FIG>). As will be apparent, embossing techniques are well known and a number of other options may be available for use to create an appropriate and accurately defined pattern in a desired substrate material. These could include casting, stamping (<FIG>), etching, grinding, lithography, pressure forming, vacuum forming, roll forming (<FIG>), injection moulding and laser scribing / ablation. Other suitable methods for forming an accurately defined pattern to the millimeter/nanometer scale would be known to those skilled in the art.

The 3D patterned substrate material <NUM> is then pulled away from the stamp <NUM> and is coated with a coating layer <NUM> to form a 3D coated and patterned structure <NUM> (<FIG>). The coating step forms a continuous single 3D surface over the substrate material <NUM>.

As used herein, "continuous 3D surface layer" (herein referred to as continuous 3D surface) refers to the coating layer <NUM> which is formed from an electrically conductive material and which can be fabricated in large, continuous sheets over the polymer substrate material <NUM>. Thus, the continuous 3D surface (coating layer <NUM>) is separate from the substrate material <NUM> and will effectively be between the substrate material <NUM> and the inert material <NUM> (best seen in <FIG>). The coating layer <NUM> (continuous 3D surface) is preferably between about <NUM> to about <NUM> micron thick, more preferably between about <NUM> to about <NUM> thick, more preferably about <NUM> to about <NUM>, more preferably between about <NUM> to about <NUM> thick. Preferably, the coating layer <NUM> (continuous 3D surface) is a unitary layer that covers the substrate material <NUM>. Alternatively, the coating layer <NUM> (continuous 3D surface) is laser scribed or otherwise cut using techniques such as lithography to give a plurality of isolated continuous 3D surface layer segments on the substrate material <NUM>. Each isolated continuous 3D surface layer segment includes a plurality of functionalisable areas so that the surface of the microarray includes a plurality of groups of functionalisable areas (<FIG>). Preferably, each group of functionalisable areas is capable of separate functionalisation. Preferably the coating layer <NUM> (continuous 3D surface) is formed from an electrically conductive material, preferably it is formed from a metal. Suitable metals for use as a coating layer <NUM> in the present invention include gold, platinum, silver, nickel and copper amongst others. Alternatively, the coating layer <NUM> is formed from a carbon-based conducting polymer such as polypyrrole and polythiophene.

Application of the continuous 3D surface (coating layer <NUM>) can be achieved by a number of methods, including but not limited to sputtering, evaporation or electroless deposition. The continuous 3D surface may be used as a seed layer as will be described later herein.

The continuous 3D surface (coating layer <NUM>) typically includes an adhesion layer (not shown in <FIG>) to promote adhesion to the substrate material <NUM>. This adhesion layer therefore sits between the continuous 3D surface layer and the substrate material. Suitable adhesion materials for use in forming an adhesion layer would be known to those skilled in the art. Options would include plasma treatment of the surface to increase surface roughness, deposition of a thin layer (nanometers) of chromium or vanadium, and plasma deposited or covalently bound thiols or amines to enhance adhesion.

The inventors have found that inclusion of the continuous 3D surface in the structure of the microarrays of the present invention allows for improved functional and structural flexibility over other microarrays known in the art. In particular, the continuous 3D surface may achieve any one of a number of important roles in the present invention. For example, it protects the underlying substrate material <NUM> (<FIG>). It also promotes attachment of binding chemistry at the functionalisable areas of the microarrays of the present invention. When it is formed from a conducting material, it allows electrochemical reactions to occur at the surface of the microarray at the functionalisable areas. When also formed from a conducting material, it ensures that the functionalisable areas are conductively interconnected with each other. As used herein, "conductively interconnected" refers to electrical communication of the isolated functionalisable areas of an array with each other and with an electroanalytical device such as a voltage meter, a potentiostat, a galvanostat, an impedance analyser and any other device capable of measuring current as would be known to those skilled in the art. Where the continuous 3D surface (coating layer <NUM>) is a unitary layer covering the substrate material, it may be connected to an electroanalytical device at only one point. Alternatively, where the continuous 3D surface (coating layer <NUM>) has been laser scribed or otherwise cut into isolated continuous 3D surface layer segments, each segment may not necessarily be interconnected with other segments in the wider array structure and each may be connected to an electroanalytical device to give individual electrodes within the array. <FIG> shows such an arrangement. Reference to a "continuous 3D surface" in this context is intended to include such options (i.e. there may be a plurality of continuous 3D surfaces within the array structure).

Gold, as a choice of coating layer <NUM> (continuous 3D surface), achieves all of these roles. In some embodiments of the present invention, the coating layer <NUM> may also need to be transparent. Again, gold is capable of being transparent. A person skilled in the art will readily understand that other conductive materials (for example, silver, platinum and conducting polymers such as polypyrrole and polythiophene) will also be capable of achieving the above identified roles.

As indicated above, gold is the preferred coating material for use as a continuous 3D surface in the present invention because it is highly conductive (and therefore capable of acting as an electrode), is inert, forms a strong covalent bond with sulphur, is easy to deposit on the substrate material, has a well known chemistry and it is readily available. It is also able to withstand harsh chemical cleaning treatments which in turn ensures that the arrays of the present invention can be used more than once.

As used herein, "inert material" refers to a flexible or rigid material which physically isolates individual functionalisable areas from each other. Thus, the inert material forms an "inert surface" through which the functionalisable areas protrude, therefore exposing isolated areas of the continuous 3D surface (coating layer <NUM>) and thus allowing those areas to be functionalised as desired. In this arrangement, the array remains as a 3D array.

Suitable inert materials for use in the present invention include, but are not limited to, epoxy, spray-coatable materials such as paint, silicon dioxide, or photoresist materials such as SU-<NUM>. Epoxy or photoresist materials are typically used where flexibility is not required. The inert material may also be formed from a solid film or a monolayer of thiol terminated molecules, or a self-assembled monolayer (SAM) which are well known in the laser field. SAM's include an alkyl chain which is usually terminated by an -SH functional group at one end but may also be terminated by a variety of other functional groups, including but not limited to, -CH<NUM>, -OH,COOH, -NH<NUM>, -CN, and -CHO. The choice of functional group depends on the target species to be bound to the microarrays of the present invention. The inert material may also act as an insulating material, and may also be seen to be a filler material or an isolation layer.

Depending on the inert material to be used in the present invention, its application may involve spin-coating the coating layer <NUM> of the microarray to a known thickness. Where this method of application is employed, the inert material is then cross-linked under ultra-violet light and individual functional areas are exposed by etching back the inert material by reactive ion etching. Numerous alternative methods for applying the inert layer would be known to those skilled in the art and include, but are not limited to, spray-coating followed by physical removal of the inert material from areas to be functionalised (for example, by wiping the tips), spray-coating a dilute coating material onto the coating layer <NUM> which upon application will flow off the tips and into the valleys of the 3D array, and dip coating a SAM monolayer followed by physical removal of the SAM on the tips.

The microarray of the present invention is functionalised to be a micro-electrode sensor array (as indicated above) and/or a microcatalyst array.

The present invention provides an accurately defined and functionalised array, including a continuous 3D surface layer, formed from an intermediate structure <NUM>. Again, individual functionalisable areas of the array are separated by a layer of the inert material to give an inert surface through which the functionalisable areas protrude and the individual functionalisable areas are conductively interconnected by the continuous 3D surface. The intermediate structure forms the base of the array.

Optionally, the intermediate structure <NUM> will include an adhesion layer between the coating layer <NUM> and the substrate material <NUM>.

<FIG> shows an intermediate structure <NUM>. <FIG> shows a <NUM> micron gold coated patterned substrate material while <FIG> shows a <NUM> micro gold coated patterned substrate material. Both are "intermediate" structures <NUM>.

The intermediate structure <NUM> can be fabricated separately to the arrays in large continuous sheets thus providing economies of scale to the user. These large continuous sheets of coated and patterned material include accurately defined 3D patterns on the millimeter to the nanometer (inclusive) scale. A method (not claimed) for the formation of the intermediate structure involves the steps of:.

Preferably, the coating layer <NUM> covers substantially all of the patterned area of the substrate material to form a continuous 3D surface layer.

Optionally, the method includes the step of adding an adhesion layer between the substrate material <NUM> and the coating layer <NUM>.

The substrate material of the intermediate structure <NUM> (as depicted in <FIG>) is preferably formed from an inert polymer material. However, as indicated above, there are a number of other suitable flexible and non-flexible materials which may be used including thermoplastic polyurethane, rubber, silicon rubber, epoxy, PMMA, PC, PS, ceramic, resin and composite materials. Suitable techniques for placing an accurately defined 3D pattern at the millimeter to nanometer scale on the surface of the substrate material are described above and include embossing, casting, stamping, etching, grinding, lithography, pressure forming, vacuum forming, roll forming, injection moulding and laser scribing / ablation techniques.

The intermediate structure <NUM> can be used to form arrays of the present invention. Thereby, individual spaces <NUM> between functionalisable areas (depicted in the form of tips) <NUM> in the 3D pattern on intermediate structure <NUM> (<FIG>) are filled with an inert material <NUM>. When used in this manner, the inert material essentially acts as a filler material or an isolation layer (<FIG>) to give an inert surface <NUM> through which the functionalisable areas or tips <NUM> of the intermediate structure <NUM> protrude such that they are exposed above the inert material. <FIG> depicts the use of a solid film as the inert material <NUM>. The functionalisable areas or tips <NUM> are thus isolated from each other and are capable of being functionalised as desired. Thus, once functionalised, they become functional areas (e.g. sensor sites) in an array form. <FIG> shows a diagrammatic top view of the functionalisable array of <FIG>. The functionalisable areas or tips <NUM> remain connected to each other by the coating layer <NUM> (continuous 3D surface) of the intermediate structure <NUM>. However, not all of the coating layer <NUM> of intermediate structure <NUM> needs to be covered by the inert material <NUM>. Those areas of the coating layer <NUM> which are not covered are then available for use in making electrical connection to the tips <NUM>.

The present invention provides a method for the formation of the microarray structure according to claim <NUM>, the method as claimed in claim <NUM>. The method includes the steps of taking the intermediate structure <NUM> and filling individual spaces <NUM> between the tips <NUM> of the 3D pattern on the intermediate structure <NUM> with an inert material <NUM> to give an inert surface <NUM> through which the tips <NUM> protrude, said tips <NUM> forming functionalisable areas. Thus an array including a continuous 3D surface layer with isolated but conductively interconnected, functionalisable areas in an accurately defined pattern is formed.

The array formed includes an intermediate structure <NUM> formed from a substrate material <NUM> (which is inert) and 3D patterned to a millimeter to nanometer scale, a coating layer <NUM> over at least part of the patterned area to form a continuous 3D surface, and a layer of inert material <NUM> which is layered over the continuous 3D surface and fills spaces <NUM> between functionalisable areas or tips <NUM> in the 3D pattern on the intermediate structure <NUM> to give an inert surface <NUM> from which the functionalisable areas or tips <NUM> of the 3D pattern protrude such that they are exposed above the inert surface. As indicated above, the functionalisable areas or tips <NUM> are isolated by the inert material <NUM> but are continuously interconnected via the coating layer <NUM> (continuous 3D surface) which is present over at least part of the 3D patterned area (preferably substantially all of the patterned area).

As shown in <FIG>, electrically isolated groups of arrays can also be formed using the above methods in combination with a process of laser scribing, wherein the coating layer <NUM> (continuous 3D surface) between individual functionalisable areas or tips <NUM> is etched out (as depicted by <NUM> in <FIG>, <FIG>and <FIG>). This allows a single sensor chip to have individually addressable areas which could include a counter electrode(s), a reference electrode(s), a redox electrode(s) and working electrode(s). There is no restriction to the shape of the lasered lines. However, it is preferable that the width of the lasered lines is between about <NUM> to about <NUM> micron. The individually addressable areas or isolated micro-electrode arrays can be configured in a number of ways, two examples of which are shown in <FIG>. In <FIG>, the micro-electrode array includes two working electrodes 21a and 21b, separated by a counter electrode <NUM>, and a reference electrode <NUM>. In <FIG>, the micro-electrode array includes three working electrodes (21a, 21b and 21c), a counter electrode <NUM>, a reference electrode <NUM> and a redox electrode <NUM>, wherein the counter electrode separates working electrodes 21a and 21b and the reference electrode <NUM> and redox electrode <NUM> together separate working electrodes 21b and 21c. The isolated micro-electrode arrays may also be arranged such that each functions as a working electrode <NUM>.

The ability to create accurately defined arrays at the millimeter to nanometer scale has been an issue in the array field for some time. The small sizes at issue, particularly in the nanometer scale, present particular problems when seeking to obtain accurate quantitative and/or qualitative analyses. The present invention provides an economic approach to the creation of such arrays.

As is clear from <FIG> and as discussed above, the isolated functionalisable areas (formed by tips <NUM>) in the array are interconnected below the inert material <NUM> via a continuous 3D surface (i.e. coating layer <NUM>). Where the continuous 3D surface is formed from a conductive material (for example, gold), the isolated functionalisable areas are conductively interconnected with each other as discussed above and therefore act as interconnected but isolated conductive islands. As indicated above, the continuous 3D surface can be laser scribed or otherwise cut into individual sections such that individually isolated blocks of functionalisable areas are formed within a wider array structure. The use of a conductive material allows the arrays to be functionalised to form micro-electrode arrays as discussed above. Therefore, the entire array can act as a single micro-electrode. Alternatively, the array can include multiple individual electrodes where the continuous 3D surface has been cut into isolated blocks. The interconnection also allows efficiencies of charge functionalisation of the isolated sites in the micro-electrode array. Micro-electrode arrays can be of a variety of types as shown in <FIG>, including:.

When functioning as a micro-electrode array, the continuous 3D surface is connected to an electroanalytical device, electrical contact is made with an electrolytic solution and current is allowed to flow through the solution. Target species in the electrolytic solution bind to the functionalised areas of the microarray and therefore aid or impede current flow. In this way, the target species are "sensed" by the micro-electrode array. Capture agents that are specific to the target species can also be appended to the functionalisable areas of the micro-electrode array to aid in this interaction. Individual micro-electrode arrays may also be used as counter-electrodes to each other, whereby a current is passed between individual functionalised areas on each and the current is measured.

The electrical communication achieved by use of a conducting continuous 3D surface (coating layer <NUM>) also allows the arrays of the present invention to provide insight into the redox environment of a sample passing over the surface of the array. For example, the arrays can be used to ascertain whether the redox environment of the sample is oxidative or reductive (therefore allowing for the establishment of the likes of anti-oxidant response elements), or whether there are peroxides present or radicals present.

The use of techniques such as laser scribing or lithography to cut the continuous 3D surface (coating layer <NUM>) into individual isolated blocks or areas also imparts on the microarrays of the present invention the ability to function as multiplexing arrays, wherein simultaneous testing or measurement of multiple analytes or biomarkers can be conducted (<FIG>). Such a system could be used to detect known biomarkers relevant to a specific disease, organ or system. This also allows the user to isolate a known number of sensor sites for different purposes.

The isolated functionalisable areas (identified at <NUM>/<NUM> in <FIG>) are used in a number of array applications. For example, they are functionalised to act as catalysts in a variety of micro reactions, or to act as sensors for various target biomolecules or compounds of interest.

When a microarray of the present invention is used as a micro-electrode array (and therefore includes a conductive continuous 3D surface layer), the array is functionalised by attaching a capture agent that is specific for the target analyte. Examples of suitable capture agents include small molecules, antibodies, and single stranded DNA. Other capture agents would be known to those skilled in the art. There are numerous methods for attaching the capture agent to an array. Methods of attachment typically include initial attachment of a linker molecule with a terminal carboxyl or amino group, onto which the capture agent is bound using standard binding methods, as would be known to those skilled in the art and are also discussed in related co-pending PCT application number<CIT>. Suitable applications for the use of a micro-electrode array of the present invention include detection of small molecule biomarkers, proteins, DNA/RNA and organisms.

Electrical connection to a micro-electrode array of the present invention is typically achieved by attaching a clip or pressing a conductor (conductive paste, wire, ribbon) against the part of the electrode which is not in contact with the solution to be passed over the surface of the micro-electrode array.

When a microarray of the present invention is used as a microcatalyst array, it is essential that suitable binding chemistry is attached to the functionalisable areas of the array. The attachment of suitable binding chemistry is achieved in a number of ways including, electrochemical deposition of the binding chemistry where a conductive continuous 3D surface layer is used, and exposure of the functionalisable areas to suitable functional groups. Suitable functional groups may include a metal catalyst (for example, platinum or palladium), DNA, and conducting polymers such as polypyrrole and polythiophene. The different surfaces of the microcatalyst arrays so created react with the species in solution to a greater or lesser extent. The combination of responses allows the solution to be characterised electrochemically.

A series of computer modelling experiments were carried out on a single functionalisable area or tip of two individual micro-electrode, wherein the two micro-electrode arrays 25a and 25b were precisely aligned over each other as shown in <FIG>. Thus, each micro-electrode array 25a and 25b was acting as a counter-electrode to the other.

The aim of these experiments was to calculate the impedance profile for different shapes and sizes of condensed droplets formed in between the tips. This was dependant on the concentration of the buffer solution and the size and shape of the droplet formed between the electrodes. The total impedance between two electrodes is the sum of the impedance at the electrode-electrolyte interface and the impedance of the electrolyte solution. In order to measure the impedance changes due to changes in the geometry of the system, the total impedance of the system was simulated by solving the modified Laplace's equation for two different geometries. The interface impedances were assumed to remain constant for different geometries and the interface reactions were not considered in the model.

An AC potential of <NUM> V was applied between the upper and lower electrodes and total impedance was calculated from the current distribution for a frequency range of <NUM> to 1x106 KHz. The buffer was considered as a solution with conductivity of <NUM>/m and a relative permittivity of <NUM>. The results are illustrated in <FIG> which model three parameters including the distance between the electrodes, the area of the electrodes, and the volume of the electrolyte (as either a droplet between the tips or as a solution that completely covers the tips). In summary, the results showed that sensitivity is inversely related to both the distance between the electrodes and the electrode area, but not significantly affected by the volume of the electrolyte at the micron scale. In <FIG>, two different tip geometries are shown. In <FIG> droplet formation is shown while <FIG> shows a chamber filled with an electrolytic solution. Impedance results are shown by the potential distribution (<FIG> and <FIG>). The arrows indicate current density vectors in the 2D and 3D domain.

<FIG> show various impedance measurements, while <FIG> shows frequency vs. impedance for two different geometries.

For bio-sensing applications, the electrodes were assumed to be functionalized with capture agents and then the impedance was measured before and after the capture. The change in the impedance was primarily due to changes at the electrode interface. The equivalent circuit model of the interface can be given by the Randle's circuit (neglecting the Warburg element due to diffusion of ions at the interface). The total equivalent circuit of the system with the interface and the solution impedance is illustrated in <FIG>, and the domain equations and boundary conditions are shown in <FIG>.

Variations in double layer capacitance (the ability of a body to store an electric charge) are measured using Non-faradaic electrochemical impedance spectroscopy (EIS). This involves neglecting any changes due to redox reactions and measuring the capacitance changes due to changes in the double layer thickness. In order to determine the total impedance change of the system due to changes in double layer thickness, the model was simulated for various double layer thickness (Ddl) (<FIG>, <FIG>). For all the cases charge transfer resistance (Rct) equates to <NUM> ohm. The results indicated that the droplet is only slightly more sensitive than using a completely submerged sensor tip.

The second way to detect the changes at the interface is by measuring the redox reaction at the interface. When there is a change in the interface due to biological capture agents, the rate at which the redox reaction takes place changes. This changes the current at the interface, which consequently changes the Rct of the system. The Rct values vary for various different interfaces. Impedance changes of the system were simulated for various Rct. Results are illustrated in <FIG> and <FIG> and show that a droplet provides slightly better sensitivity at lower frequencies.

The computer modelling experiments showed that the smaller the dimensions of the tips, and the gaps between the tips, the greater the sensitivity of the sensor array.

The design was also simplified to arrange working and counter electrodes side-by-side via inter-digitation patterning, thus enabling the different electrodes of an electrochemical set up (for example, working electrodes, counter electrode and reference electrode) to all be placed on the same micro-electrode array sensor chip. The electrochemical interactions of side-by-side electrode protrusions were modelled and similar results were observed to that of the micro-electrode array set up shown in <FIG>. Therefore, in summary, impedance measurements relate to the distance between working and counter electrodes and the diameter of the working electrode (<FIG>).

The above description describes formation of arrays of the present invention having isolated functionalisable areas or tips. In <FIG> a thin layer of an inert material <NUM> has been deposited over a continuous 3D metal surface to sit between individual tips of a 3D patterned substrate material <NUM>. The functionalisation occurs at the tips of the 3D pattern of the microarray.

The following describes the development of microarrays according to the present invention.

<FIG> shows a photograph of <NUM> micron tips embossed into PMMA and which are evenly spaced at <NUM> micron intervals and are <NUM> microns in height. PMMA (an amorphous polymer) is a preferred substrate material for use in the present invention as it is easily processed and gives highly defined three-dimensional substrate surfaces.

The process for fabricating the sensor (<FIG>) includes the following steps:.

A thin layer of chromium (<NUM> to <NUM> in thickness) was deposited onto the PMMA substrate to act as an adhesion layer. Vanadium can also be used in place of chromium as an adhesion layer. Likewise, amine and thiol chemicals are known to promote adhesion of gold to a surface.

Gold was then sputtered onto the PMMA substrate to give the electrode with a continuous 3D gold surface depicted in <FIG>. The electrode was cleaned electrochemically by holding it at <NUM>. 65V vs. Ag/AgCI for <NUM> in <NUM> H<NUM>SO<NUM>, and then cycling between 0V to <NUM> V. <FIG> shows a typical CV, and a stable gold oxidation and reduction peak at <NUM> V and <NUM>. 9V respectively, thus providing support that gold had been deposited onto the PMMA substrate. The gold coating layer <NUM> so formed was between about <NUM> to about <NUM> thick. Gold tracks were then defined into the gold by laser scribing. An example of inter-digitated tracks <NUM> is shown in <FIG> (and is depicted as <NUM> in <FIG>, <FIG> and <FIG>). The lasered pattern electrically isolates individual areas of the microarray from each other, resulting in the formation of more than one electrode in a single microarray.

Three separate methods were used to deposit the inert material onto the continuous 3D gold surface so as to act as an isolation layer between the tips:.

All three methods resulted in the gold tips protruding out of the inert material.

A <NUM> micron thick layer of SU-<NUM> was applied to a <NUM> wafer of gold coated substrate which had been previously laser scribed into <NUM> inter-digitated sensor chips (<FIG>). The SU-<NUM> was then cross-linked under ultra violet light and controllably etched by reactive ion etching. This was found to give very good control of the thickness of the SU-<NUM> polymer layer and also very clean gold tips. <FIG> shows two adjacent tips. The tip shown on the right hand side has bare gold, while the tip shown on the left hand side has had a carboxylated polyterthiophene layer electrochemically deposited onto the gold. <FIG> shows adjacent tracks of an inter-digitated array in which alternating tracks have carboxylated polytherthiophene deposited onto the tips.

Self-assembled monolayer's (SAM) are well known in the art and typically form spontaneously on a substrate by chemisorption of "head groups" onto the substrate followed by slow organisation of "tail groups".

Terthiophene substituted with an alkyl alcohol and HS(CHz)sOH are examples of suitable hydroxylated self-assembled monolayer's for use in the present invention.

The continuous 3D gold surface of micro-electrodes were subjected to cyclic voltammetry in Potassium ferricyanide (K<NUM>FeCN<NUM>) solution. The CV showed a standard ferricyanide oxidation and reduction peak at <NUM> V and <NUM> V respectively. The electrodes were then immersed in solution containing <NUM> SAM-OH in <NUM>:<NUM> ethanol/water solution. The electrodes were then taken out periodically and washed with water and subjected to cyclic voltammograms in <NUM> Potassium ferricyanide solution with KCI supporting electrolyte (<FIG>). The CV showed gradual disappearance of the ferricyanide peak as the length of time the electrodes were immersed in the SAM solution was increased indicating the gradual adsorption of SAM onto the continuous 3D gold surface of the electrodes.

After a period of <NUM> minutes the current reached a steady state value showing that the electrodes were saturated with SAM. The CV of the electrode after <NUM> minutes of SAM adsorption had similar characteristics as the CV of the electrode after <NUM> hours of SAM adsorption.

Physical removal of the SAM-OH by rubbing the tips on a glass microscope slide resulted in the gold on the tips being exposed. CV in <NUM> Potassium ferricyanide solution showed the typical reduction oxidation peak at significantly reduced current. This indicated that only the gold on the tips was exposed.

The SAMs can be removed from the gold-coated substrate, allowing the substrate to be freshly coated with a new SAM. Thus, the arrays can be used a number of times without degradation of the array.

<FIG> shows the gold coated substrate material with a layer of epoxy in the valleys between the tips. The consistency of the epoxy layer provided sufficient time for the epoxy to run off the gold tips prior to cross-linking.

CV in <NUM> Potassium ferricyanide solution showed the typical reduction oxidation, and was evidence that the gold tips were uncoated.

The binding chemistry (-X, c. <FIG>) was attached to the tips of the SU-<NUM>, SAM-OH or the epoxy coated continuous 3D gold surface of the substrate. Where SU-<NUM> or an epoxy coat was employed as the inert material <NUM>, attachment of the binding chemistry was achieved by electrochemically depositing carboxylated polytherthiophene or animated polyterthiophene onto the tips (<FIG>). While <FIG> shows the controlled electrochemical deposition of different probes to conducting polymers for multiplexing of capture agents, this can also be achieved using a carboxylic SAM and altering the potential of the different working electrodes as would be apparent to those skilled in the art. Where SAM-OH was employed as the inert material <NUM>, attachment of the binding chemistry was achieved by exposing the tips to SAM-COOH (<FIG>). In each case, this resulted in the attachment of either a -COOH or -NH<NUM> group at the end of each of the tips. Use of terthiophene (or pyrrole) substituted with a carboxyl terminated side chain also allows the binding group to be added selectively at a defined tip as it can be electrochemically polymerised on those tips.

To test the selectivity of the process for attaching the binding chemistry onto the tips, <NUM> micron aminated polystyrene beads were covalently attached via the appropriate linker chemistry.

<FIG> illustrates the process for the amine functionalised tips (A). The aminated substrate was exposed to a bi-functional linker solution (<NUM> linker/<NUM> PBS), and shaken at room temperature for <NUM> minutes. After washing, the substrate was immersed into a solution containing a blue H<NUM>N-Bead solution (<NUM>µl beads suspension in <NUM> PBS), and shaken at room temperature for <NUM>. Attachment of the blue beads onto the array of tips (A) and onto a single tip (B) is shown in <FIG>. Visual or electrochemical (for example resistance, CV or impedance) techniques can be used to detect what is bound to the arrays.

Once confirmation that the attachment chemistry had been bound to the tips of the array, standard linker chemistry could be used to attach a variety of haptans including, but not limited to, antibodies, DNA and cells.

As an illustration, the following shows the use of the method to fabricate a sensor for Progesterone (P4). The steps include:.

Laser scribing was utilized to isolate individual micro-electrodes of an array to form an electrochemical version of the typical DNA or RNA microarray (<FIG>, <FIG> and <FIG>). Groups of micro-electrodes were also isolated to form smaller micro-electrode arrays within a larger array, thus constituting a platform of multiple working electrodes, reference electrodes and counter electrodes. This enabled multiplexing on a single sensor chip or array (as depicted in <FIG>).

For example, one sensor chip design was functionalised with different capture agents on each of eight working electrodes to constitute a liver panel on one chip. The antibodies used had affinity for ALT, AST, ALP, GGT, LDH, Hep A, Hep B x-antigen, and full length Hep C E2 protein on working electrodes <NUM> to <NUM>, respectively. An enzyme, glucose oxidase was tethered to working electrode <NUM> for detection of serum glucose. The tenth working electrode was a redox electrode to measure non-adhered bilirubin concentration in solution.

Claim 1:
A microarray structure including:
a three-dimensional (3D) substrate material layer (<NUM>) comprising three-dimensional (3D) structures in the form of a pillar like structure (<NUM>) or a rib like structure,
a continuous three-dimensional (3D) surface layer (<NUM>) on the 3D substrate material layer that is capable of functionalisation for use as an array, the continuous 3D surface layer formed from an electrically conductive material, and
an inert material (<NUM>);
wherein the structure includes accurately defined and functionalisable isolated areas which are millimeter to nanometer in size;
wherein the functionalisable areas are part of the continuous 3D surface layer and are isolated by the inert material but which are interconnected within the structure by the continuous 3D surface layer;
wherein the continuous 3D surface layer protrudes from the inert material such that the functionalisable areas are exposed above the inert material, and
wherein the microarray structure is functionalised to be a micro-electrode sensor array and/or a micro-catalyst array.