GB2217461A

GB2217461A - Making microelectrode assembly

Info

Publication number: GB2217461A
Application number: GB8905515A
Authority: GB
Inventors: David Edward Williams; Christopher Peter Jones; Murr Nabil El
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1988-03-10
Filing date: 1989-03-10
Publication date: 1989-10-25
Also published as: GB8905515D0

Abstract

A microelectrode assembly is made by screen process printing a layer of electrode material 2, of thickness characteristic of a microelectrode, (1 micrometre of less), onto a substrate 1. An insulating material 3 is provided e.g. by screen process printing, to overlay at least part of the layer of microelectrode material, and an edge of the layer is exposed to constitute an active microelectrode surface. The electrode material is preferably gold but may be platinum, silver, silver- palladium or copper, the substrate a ceramic and the insulating material a dielectric, glass or polymer. A biologically active material such as an enzyme may be bonded to the insulating material and a counter electrode 4 may be formed on the back of the substrate. <IMAGE>

Description

Electrodes This invention relates to microelectrode assemblies and their manufacture.

Microelectrodes are electrodes having at least one dimension in the micrometre scale, for example less than 100 micrometres, such as about a micrometre or less. A microelectrode may have an exposed surface having a ratio of length to width which is so large that it can be operated so that substantially only two dimensional diffusion will occur. The small size of microelectrodes gives them a number of advantages over electrodes of macroscopic dimensions when they are used in electroanalytical chemistry: they have higher sensitivity; they are useable in liquids of high electrical resistance; they are not sensitive to the effects of flow or stirring in the analyte. One form of microelectrode is described in GB-A-1 104 704.

The present invention is concerned with making rugged and reliable microelectrodes in a simple way. Thus, the invention provides a method of making a microelectrode assembly comprising the steps of (i) screen process printing a layer of electrode material onto a substrate, the thickness of the layer being of a dimension characteristic of a microelectrode; (ii) provided an insulating material (preferably by screen proc-ess printing) to overlay at least part of the layer of the electrode material; and (iii)exposing an edge of the layer of electrode material to constitute an active surface of the microelectrode.

The generally known process of screen process printing, alternatively known as screen printing or silk-screen printing, is thus used to carry out step (i) and optionally step (ii). The process is a mesh-stencil process in which an ink, as a precursor for the desired electrode material or insulating material, is squeezed through the open parts of a mesh onto the area of the substrate defined by a stencil, followed by firing, if necessary, to cause the layer to adhere.

In step (ii), part of the layer of the electrode material may remain uncovered by the overlay of insulating material to enable electrical connection to be made therewith. Examples of ways of carrying out st-ep (ii) other than screen process printing are spin casting, doctor blading and solvent casting.

Step (iii) need not necessarily be performed as a separate step but may be performed in step (i) by printing the layer of electrode material to extend to an edge of the substrate. Alternatively, substrates treated according to steps (i) and (ii) may be butted together, or a substrate treated according to steps (i) and (ii) may be cut or an edge thereof ground off or otherwise removed.

The substrate may be of a ceramic material, for example an alumina tile, or of a plastics material, or of a glass provided it is compatible with the layers and treatment applied thereto. The substrate may be provided with a counter electrode, e.g. on a surface remote from a surface treated by steps (i) and (ii), to give a complete assembly for use in a device. Such a counter electrode may, for example be of Pt or Ag.

In a particular embodiment of the invention, the substrate may comprise a temporary support material such as release paper onto which an insulating material has been screen process printed. Steps (i) and (ii) may then be carried out on said insulating material and the temporary support removed before-firing and step (iii) are carried out.

In the above-mentioned particular embodiment and also in the invention generally, the steps thereof may be carried out sequentially more than once in order to create a sequence of microelectrodes, e.g. in parallel, carried between the initial substrate and successive overlays of insulating material.

The materials used in the present invention must, of course, be mutually compatible and, subject to this requirement, the electrode material may be of any metal that can be screen process printed. Examples of such a metal are gold, platinum, silver, silver-palladium and copper. Examples of insulating material that can be used are dielectrics, glazes, and polymeric materials.

Gold can be screen process printed onto an alumina substrate using a commercially available ink based on an organometallic compound to give a layer that is about one micrometre thick. Such a dimension makes the resulting microelectrode very suitable for many applications in electroanalytical chemistry. However, conventional gold inks based on colloidal gold with fritting agents give thicker layers when screen process printed, for example 20 micrometres thick. The resulting microelectrode works in certain applications, but not always as effectively as the microelectrode made as above with a thinner gold layer.

Inks are commercially available for applying dielectric or glazed overlays by screen process printing.

The above-mentioned inks require firing in accordance with the manufacturer's recommendations to ensure adhesion of the layer or overlay applied.

Since the microelectrodes assemblies prepared by the present invention need only consist of a ceramic, a metal and a glass, i.e. adhesive need not be present, they can be made to be very rugged and resistant to attack by chemicals. They can easily be sterilised and cleaned thereby permitting them to be used in, for example, food testing and biotechnology. In the latter aspect, it has also been found that the insulating material provides a surface to which a biologically active material such as an enzyme can be bonded. A microelectrode in which an enzyme is so bonded can be used to detect electrochemically the product of a reaction of an analyte catalysed by the enzyme. For example, such a microelectrode where the enzyme is glucose oxidase may be used to analyse glucose, the substance detected being oxygen or hydrogen peroxide.

In operation, glucose diffuses towards the glucose oxidase which catalyses a reaction resulting in hydrogen peroxide, whose concentration is proportional to that of the glucose.

The invention will now be described by way of example as follows. Reference will be made to the accompanying schematic drawings wherein Fig 1 is a plan view of a first form of microelectrode assembly; Fig 2 is an end elevation of Fig 1 in the direction of the arrow A; Fig 3 is an end view of a second form of micro electrode assembly; Fig 4 is an end view of a first form of microelectrode assembly; and Fig 5 is an end view of a second form of microelectrode assembly.

EXAMPLE 1 An alumina tile (2 cm x 1 cm) was screen process printed with a commercially available gold ink based on an organometallic compound, to give, after firing, a L-shaped gold layer of about one micrometre thickness and from 1 to 8 mm width extending to the edge of the tile. A commercially available dielectric ink was then applied by screen process printing to give, after firing, a coating that partly overlay the gold layer. A counter electrode was applied to the underside of the tile and electrical connections were fixed to the part of the gold layer not overlaid by the dielectric and to the counter electrode.

The resulting microelectrode assemblies is shown in Figs 1 and 2 where the reference numerals thereon indicate components mentioned above as follows: 1 alumina tile 2 gold layer 3 dielectric coating 4 counter electrode A microelectrode prepared as above was found to have the following properties: sensitivity - for a given concentration of analyte, the current per unit electrode area was 100 times greater-than that of a 4 mm diameter gold disc; it could be used in pure water; and repeatability of manufacture - individual microelectrodes differed by less than 10% in the current given by a given concentration of analyte.

EXAMPLE 2 The procedure of applying a gold layer and a dielectric coating to an alumina tile was carried out as described in Example 1 and then repeated, also as described in Example 1, using the first dielectric coating as a substrate. The resulting microelectrode assembly was as shown in Fig 3 where the reference numerals have the same significance as stated in Example 1 and the numeral 2' indicates the second gold layer and the numeral 3' the second dielectric coating.

EXAMPLE 3 An alumina tile was screen process printed with a metallic layer of about one micrometre thickness. A first dielectric coating was then applied by screen process printing to overlie the metallic layer.

A second dielectric coating of a different material from the first dielectric coating and of a material which does not react with enzyme coupling agents was applied as a protective coating to the first dielectric coating.

The edges of the coatings and layers were exposed at the edge of the tile by cutting the tile, and the end thereby created successively treated with enzyme coupling reagents and enzyme. The resulting microelectrode assembly is shown in Figure 4 where the reference numerals thereon indicate components mentioned above as follows: 1 alumina tile substrate 2 metallic layer (as microelectrode) 3 first dielectric coating 3 second dielectric coating The enzyme coating is not shown.

EXAMPLE 4 The procedure of Example 3 was repeated with the exception that a second metallic layer was interposed between the first and second dielectric coatings. The resulting microelectrode assembly is shown in Figure 5 where the same reference numerals as shown in Figure 4 are used and the second metallic layer, as a second microelectrode, is given the reference numeral 21. Again, the enzyme coating is not shown.

In operation of the assembly shown in Figure 5, the microelectrodes 2 and 2/ can be at the same or different potentials, to detect the same or different products, or so that one microelectrode 2 or 2J detects a reaction product and the other microelectrode 2 or 2' provides another reactant which, together with the analyte used, gives the product detected on the first microelectrode 2 or 2/

Claims

Claims: 1. A method of making a microelectrode assembly comprising the steps of: (i) screen process printing a layer of electrode material onto a substrate, the thickness of the layer being of a dimension characteristic of a microelectrode; (ii) -providing an insulating material to overlay at least part of the layer of the microelectrode material; and (iii) exposing an edge of the layer of microelectrode material to constitute an active surface of the microelectrode.
2. A method according to claim 1 wherein step (ii) is carried out by screen process printing.
3. A method according to either of the preceding claims wherein part of the electrode material remains uncovered by the overlay of insulating material to enable electrical connection to be made therewith.
4. A method according to any of the preceding claims wherein step (iii) is-not performed as a separate step but is performed in step (i) by printing the layer of electrode material to extend to an edge of the substrate so that an edge of the electrode material is inherently exposed.
5. A method according to any of claims 1 to 3 wherein step (iii) is carried out by butting together two substrates each treated according to steps (i) and (ii).
6. A method according to any of claims 1 to 3 wherein step (iii) is carried out by removing material from a substrate treated according to-steps (i) and (ii).
7. A method according to any of the preceding claims wherein the assembly is provided with a couunter electrode.
8. A method according to any of the preceding claims wherein the substrate comprises a temporary support material onto which insulating material has been screen process printed, said temporary support material being removed before firing and step (iii) are carried out.
9. A method according to any of the preceding claims wherein steps (i) and (ii) are carried out sequentially more than once to produce an assembly comprising more than one layer of microelectrode material.
10. A method according to any of the preceding claims wherein the substrate is a ceramic material.
11. A method according to claim 10 wherein the ceramic material is alumina.
12. A method according to any of the preceding claims wherein the electrode material is gold.
13. A method according to any of claims 1 to 11 wherein the electrode material is platinum, silver, silverpalladium or copper.
14. A method according to any of the preceding claims wherein the insulating material is a glass, a dielectric or a polymer.
15. A method according to any of the preceding claims wherein a plurality of layers of electrode material are provided.
16. A method according to any of the preceding claims comprising the further step of bonding a biologically active material to the insulating material.
17. A method according to claim 16 wherein the biologically active material is an enzyme.
18. A method according to claim 17 wherein the enzyme is glucose oxidase.
19. A method of making a microelectrode assembly substantially as described herein with reference to any of the examples.