Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – Involving enzyme or micro-organism
Patent
1993-02-08
1996-04-30
McMahon, Timothy M.
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
Involving enzyme or micro-organism
204290R, 204403, 204434, 436451, 2057895, 205792, 2057945, G01N 2704
Patent
active
055124895
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to an improved method of making an electrode (in particular a microelectrode), an electrode made by the method, an assay method for the determination of the presence of a redox species (e.g. a heavy metal) in liquid carrier media using the electrode and redox species detection equipment designed to utilise the assay method.
BACKGROUND ART
Microelectrodes provide many advantages in electro-chemistry and electroanalysis (some of which are discussed in Wightman, R. M., Anal. Chem., 1982, 54, 2532) in particular the possibility of electrolysis in resistive media or dilute solutions, and of microanalysis with improved sensitivity and precision. For these reasons a great deal of effort has been dedicated to establish a way of fabricating a regular microscopic disc array electrode. Any proposed method of fabrication must ensure that all the distinctive features of individual microelectrodes are retained, and in addition circumvent the problems associated with the measurement of extremely small currents (pA-nA), observed for single microdisc electrodes.
Many approaches have been proposed for the construction of a microdisc array electrode including photolithographic methods (see e.g. Osteryoung, J. and Hempel, T., J. Electrochem. Soc., 1986, 133, 757-760 and W. Siu and R. S. C. Cobbold, Med. & Biool. Eng., 1976, 14, 109), as well as the use of micropore membrane matrices (see e.g. Wang, J., J. Electrochem. Soc., 1988, 249,339-345, and Cheng, F., Anal. Chem., 1989, 61, 762-766). The former approach failed because of adhesion problems between the insulating photoresist layer and the metal substrate. The latter method of fabrication was self defeating because, although the size of the pore is well characterised, the pore density and distribution were undefined and the electrode ultimately irreproducible.
SUMMARY OF THE INVENTION
According to the first aspect of this invention a method of making a microelectrode comprising a layer of electrically insulating material having an array of apertures formed therein and electrically conducting material visible through the apertures, is characterised in that the apertures are formed by photo-ablation.
Suitably the layer is a sheet of flexible plastics material (suitably a polyester or other polymer film) and the apertures are formed by photo-ablation. A particularly preferred form of electrode made by the method of this invention is a microdisc array electrode in which the apertures are of the same size and shape and are distributed over the sheet in a regular array. In the field of microdisc electrodes the sheet can be of a thickness in the range 2 microns to 500 microns and the apertures are suitably of diameter (or transverse dimension if not circular) of between 5 microns and 100 microns. A hexagonal array of 10 micron diameter apertures with edge/edge separations of 100 microns between the apertures in the array has been found to be one convenient arrangement offering many useful commercial applications.
An alternative method of making the electrode involves supporting a layer of electrode material on a substrate, laying a thin polymer film onto the electrode material (e.g. by vapour polymerisation) and then drilling apertures by photo-ablation through the polymer film. The layer of electrode material is conveniently thick-film printed (to any desired pattern) on the substrate. This alternative method has a particular advantage when the electrode material includes carbon, since the photo-ablation used to form the apertures can vitrify the carbon in the areas of electrode material exposed by the photo-ablation.
As well as microdisc arrays the invention extends to microband arrays, a microband array electrode comprising a set of spaced-apart slot-shaped apertures exposing narrow strips (e.g. parallel strips) of a common electrode material.
The formation of the array of apertures by a photoablation technique has proved to be particularly convenient and typically involves photograohically creating spaced-apart ablatable areas on
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Langmuir, vol. 4, No. 3, American Chemical Society, E. S. Kirkor et al.: "A simple method for the production of controlled electroplated designs on a metal surface", pp. 578-579 (1988).
Dechiema Monogr., vol. 117, J. Thielke et al.: "Microelectrode processes in a focused laser beam", pp. 175-189 (1989).
Girault Hubert H. J.
Seddon Brian J.
Ecossensors Limited
McMahon Timothy M.
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