The invention relates to apparatus comprising one or more metal electrodes such as electrochemical cells, sensor elements and the like, and more particularly to extending the shelf life of such apparatus.
Metal electrodes have proved useful in sensor elements for sensing a diverse range of biologically important molecules eg glucose, and for determining physical properties such as pH. A range of possible configurations and applications involving metal electrodes are discussed in our co-pending applications PCT/AU96/00210, PCT/AU96/00365 and PCT/AU96/00723.
A desirable attribute of all sensor elements is that they have a long shelf lifexe2x80x94that is, the sensing characteristic of the sensor element does not change significantly between manufacture and use (ie on storage).
In an electrochemical sensor element the stability of the electrode is critical to the stability of the sensor as a whole. Typically, when left to stand for long periods of time, electrodes become prone to instability in subsequent use thus limiting the useful shelf life. It is thought that such instability is caused by absorption or reaction of the metallic surface with atmospheric contaminants. It has also been observed that filling time of sensors deteriorates on prolonged storage.
It is an object of the present invention to overcome or ameliorate at least some of the above disadvantages in the prior art.
Surprisingly, the present applicant has found that by coating a metal electrode with a monolayer or multilayer of selected materials, electrode behavior can be significantly stabilised in comparison with uncoated metal electrodes without loss of the desirable sensing characteristics of the electrodes.
According to a first aspect, the invention consists in a metal electrode stabilised by a coating, said coating comprising a sulfur containing moiety in its molecular structure, said coating increasing the temporal stability of the electrode relative to a corresponding uncoated metal electrode without modifying other electrochemical properties of said metal electrode.
xe2x80x9cComprisingxe2x80x9d as herein used is used in an inclusive sense, that is to say in the sense of but not limited to xe2x80x9cincludingxe2x80x9d or xe2x80x9ccontainingxe2x80x9d. The term is not intended in an exclusive sense (xe2x80x9cconsistingxe2x80x9d of or xe2x80x9ccomposed ofxe2x80x9d).
Preferably, the sulphur-containing moiety is selected from the group comprising thiol, disulphide and SOx. Most preferably the sulphur-containing moiety is a disulphide. The sulphur-containing moiety may also be incorporated in a cyclic structure.
According to a second aspect, the invention consists in a metal electrode stabilised by a coating according to the first aspect, further comprising a hydrophilic group in its molecular structure.
Preferably, the hydrophilic group is selected from the group comprising hydroxyl, amine, carboxyl, carbonyl, oligo (ethylene oxide) chain, and zwitterionic species. Most preferably, the hydrophilic group is a zwitterionic species. The most preferred zwitterionic species comprises an amine and a carboxyl group in proximity.
According to a third aspect, the invention consists in a metal electrode stabilised by a coating according to the second aspect, further comprising a spacer between the sulphur-containing moiety and the hydrophilic group.
Preferably, in the third aspect, the spacer consists of an alkyl group or an aromatic group. It is preferable that methylene or ethylene groups be included in the spacer element.
According to a fourth aspect, the invention consists in a method of preparing a metal electrode stabilised by a coating, comprising the step of contacting a metal electrode with a substance comprising a sulphur-containing moiety in its molecular structure.
According to a fifth aspect, the invention consists in a method of preparing a metal electrode stabilised by a coating, comprising the steps of contacting a metal electrode pith a substance comprising a sulphur-containing moiety and a hydrophilic group in its molecular structure.
According to a sixth aspect, the invention consists in a method of preparing a metal electrode stabilised by a coating comprising the steps of contacting a metal electrode with a substance comprising a sulphur-containing moiety, a hydrophilic group and a spacer between the sulphur-containing moiety and the hydrophilic group in its molecular structure.
The preferred substances for use in the methods described in the fourth, fifth and sixth aspects are identical to those substances described in respect of the first, second and third aspects.
The invention also consists in a method of sensing an analyte, comprising the step of substituting the electrode in a known sensor device with a metal electrode stabilised by a coating according to the present invention, and sensing an analyte.
Various embodiments of the invention will now be described by way of example only.
It is known in the prior art that thiols form coatings on metals. Thiols have also been used to tether species such as antibodies onto metal surfaces, for instance those of gold particles, for the purposes of immobilisation etc. One would expect that such coatings would also bind contaminants to the surface.
As much electrode chemistry involves interaction at the electrode surface, it is thus surprising that coatings used to bind molecules to the metal surface can be useful in preventing contamination of the electrode surface. It is also surprising that notwithstanding the application of the coating an electrode retains desirable electrochemical properties. The procedure for preparing the metal electrode stabilised by a coating involves contacting a metal electrode with selected sulphur-containing compounds, such as thiols, disulphides and compounds of the formula SOx among others being suitable in the context of the present invention. The coatings also desirably contain a hydrophilic group which includes such species as hydroxyl, amino, carboxyl, carbonyl, oligo (ethylene oxide) chains and zwitterionic species. The latter two compounds indicate that compounds having one or more hydrophilic groups are also suitable groups for use in the present invention
Between the sulphur group, which acts to tether the molecule onto the metal surface, and the hydrophilic group, which presents a hydrophilic surface, spacers may be employed.
Compounds useful in the present invention include, but are not limited to 2-mercaptoethanol, 2-mercaptoethylamine, 3-mercaptopropionic acid, thiophene, 3-carboxythiophene, cysteine, homocysteine and cystine. Most preferably the molecule is cystine. In any of the above aspects, the D or L isomers can be used or mixtures of D and L isomers can be used, where such isomers are possible.
The compound in accordance with the invention is then applied as a monolayer or multilayer onto the surface of the electrode. It is possible to apply the compound by simply exposing the electrode to the coating material, with the coating material in either the vapour phase or in solution. The substance can be applied by dipping, spraying, painting, printing etc. After application, it is possible to wash the surface of the contacted electrode.
In a further aspect of the current invention the layer of the sulfur containing compound can optionally be overcoated with a surfactant layer. The surfactant layer can be applied after the application of the sulfur containing layer or at the same time as the sulfur containing layer, for example the sulfur containing species and the surfactant can be placed in a coating bath into which the electrode material is immersed. Due to the higher affinity of the sulfur containing species for the electrode material it will bind to the electrode surface in preference to the surfactant, leaving the surfactant in a layer over the sulfur containing layer. An example of a suitable surfactant is Triton X-100.