GB2237390A

GB2237390A - Liquid microelectrode

Info

Publication number: GB2237390A
Application number: GB9022953A
Authority: GB
Inventors: David Jorge Schiffffrin; Vincent John Cunnane
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1989-10-28
Filing date: 1990-10-22
Publication date: 1991-05-01
Also published as: GB9022953D0; GB8924338D0

Abstract

A liquid microelectrode comprises an insulator (eg glass) 1 having a microcavity 4 and an electrode 2 with active surface 6 communicating with liquid in the microcavity 4. The internal surfaces 5 of the microcavity can be hydrophobic, and the external surfaces of the insulator surrounding the mouth of the microcavity hydrophilic, in which case the microcavity is filled with organic liquid and in use the microelectrode is inserted into an aqueous liquid phase. Alternatively, the characteristics are reversed (eg internal surface 6-hydrophilic: external surface-hydrophobic), in which case the microcavity is filled with an aqueous liquid and in use the microelectrode is inserted into an organic liquid phase. In both cases a liquid-liquid interface is formed at the mouth of the microcavity. <IMAGE>

Description

Electrodes This invention relates to liquid microelectrodes 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. Their small size 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.

The present invention is concerned with a liquid microelectrode having an interface between an organic liquid and an aqueous liquid that is in the micrometre scale or dimension.

In one aspect, the invention provides a liquid microelectrode comprising an insulator having a microcavity defined therein by internal surfaces thereof and by an opening there into adjacent external surfaces thereof, either said internal surfaces being hydrophobic and said external surfaces hydrophilic or said internal surfaces being hydrophilic and said external surfaces hydrophobic; an electrical conductor as a first electrode corunicating with the microcavity; a reference electrode communicating with the microcavity;; and a liquid contained in the microcavity, the liquid being either (a) organic when the internal sufaces are hydrophobic and the external surfaces hydrophilic and being for contacting, at the opening, with an aqueous liquid phase to generate a liquid-liquid interface, or (b) aqueous when the internal surfaces are hydrophilic and the external surfaces hydrophobic and being for contacting, at the opening, with an organic liquid phase to generate a liquid-liquid interface.

In another aspect, the invention provides a method of making a liquid microelectrode which comprises the steps of (i) forming a microcavity by removing a portion of a conductor sealed in an insulator so that the microcavity is defined by internal surfaces of the insulator and by an opening thereinto adjacent external surfaces thereof; (ii) providing the insulator with a hydrophobic coating either before step (i) is carried out to coat said external surfaces only, or after step (i) is carried out to coat both said external and internal surfaces; iii) forming a reference electrode communicating with the microcavity; and (iv) (a) where the coating has been provided after step (i) has been carried out, making the external surfaces hydrophilic, and providing an organic liquid phase in the microcavity for contacting, at the opening, with an aqueous liquid phase to generate a liquid-liquid interface, or (b) where the coating has been provided before step (i) has been carried out, providing an aqueous liquid phase in the microcavity for contacting, at the opening, with an organic liquid phase to generate a liquid-liquid interface.

"Microcavity" means a cavity having a dimension small enough for liquid to be drawn there into by capillary action.

Liquid microelectrodes of the invention have the advantages, when included in electrolytic cells, of being useable in a restricted space because of their small size, and, because of the small size of the liquid-liquid interface, of enabling errors associated with the electrical resistance of test solutions to be greatly reduced. Such errors are known to be a considerable problem in the use of liquid-liquid interfaces as both amperonetric sensors and as capacitance sensors on the macro scale where the resistance has to be compensated electronically, which is complicated and can lead to instability in the measurement circuit.

Applications of the liquid microelectrodes of the invention include the following: as ion-selective electrodes in which the ion to be measured is distributed across the liquid interface and the resulting potential difference is measured; in amperometric sensors based on ion transfer across the liquid interface (e.g. for potassium), in which the current flow due to ion transfer is measured; in devices in which sensing is achieved by modulation of ion transfer across the liquid interface (e.g. gated ion transport); and n devices in which sensing is achieved by a change in capacitance of the liquid interface.

A specific construction of microelectrode embodying the invention and a method of manufacture will now be described by way of example with reference to the drawings filed herewith in which Fig 1 is a schematic section showing a glass rod carrying a wire sealed therein; Fig 2 is a schematic section of a first form of liquid microelectrode made from a glass rod of Fig 1; and Fig 3 is a schematic section of a second form of liquid microelectrode made from a glass rod of Fig 1.

The same reference numerals will be used for the same or similar functioning components.

EXAMPLE 1 (referring to Figs 1 and 2) Referring to Fig 1, an insulator in the form of a borosilicate glass rod 1 has a silver wire 2 (ex Goodfellow Metals; 99.9% pure; 10 micrometres diameter) sealed therein. The outer surface of the glass rod 1 was then siliconized by dipping into a solution of dimethyldichlorosilane in carbon tetrachloride (5% volume/volume) for 30 minutes to provide it with a hydrophobic coating 3 as shown in Fig 2. The so-treated rod 1 was next contacted with concentrated nitric acid for about 20 seconds thereby to etch the silver wire 2 to a depth of about 10 micronetres and generate a microcavity 4 in the glass rod 1 defined by internal surfaces 5 of the rod 1 as shown in Fig 2. The rod 1 was dipped into deionised water to remove excess acid. The internal surfaces 5 are hydrophilic as opposed to the outer surface carrying the hydrophobic coating 3.

The rod 1 as shown in Fig 2 was contacted with nearly saturated aqueous potassium chloride solution which was trapped and held in the microcavity 4 because of its hydrophilic internal surfaces 5. Silver chloride was deposited therein to form a reference electrode 6. After washing and dipping in deionised water, the rod 1 was placed in aqueous potassium chloride solution (1 x l03M) for several minutes to fill the microcavity 4.

In preparation for use and testing, the resulting microelectrode was incorporated into a V-cell containing a denser organic liquid (1 x l03M tetraphenylarsonium tetraphenylborate in 1,2-dichloroethane), aqueous KC1 (lmM) in one limb of cell and a less dense organic liquid (1 x l03M tetraphenylarsonium chloride in 1,2-dichloroethane) in the other limb forming a Nernst-Donnon interface with the denser organic liquid. The microelectrode was positioned in the limb containing aqueous KC1 thereby to generate a liquid-liquid interface between the aqueous liquid in the microcavity 4 and the denser organic liquid.

The cell also included a Ag/AgCl counter electrode positioned in the other limb and in contact with the less dense organic liquid.

EX IPLE 2 (referring to Figs 1 and 3) An insulator as shown in Fig 1 was contacted with nitric acid to etch the silver wire 2 to a depth of about 10 micrometres and then washed with water as described in Example 1. A microcavity 4 was thereby generated. The rod 1 was then siliconized as described in Example 1 to provide both its external surface and internal surfaces 5 with a hydrophobic coating 3. The tip of the rod 1 was then lightly ground with light emery paper and lightly buffed on a polishing cloth to remove the hydrophobic coating 3 from its outer surface and render the outer surface hydrophilic. The product is shown in Fig 3.

The product was allowed to stand in an organic liquid (1 x 103M tetraphenylarsonium tetraphenylborate) for several minutes which was trapped and held in the microcavity 4 because of its hydrophobic internal surfaces 3.

In preparation for use and testing, the resulting microelectrode was positioned in one limb of a V-cell, which cell contained aqueous KC1 (1 mM), to generate a liquid-liquid interface at the microelectrode between the organic liquid in the microcavity 4 and the aqueous KC1, the cell also including a Ag/AgCl counter electrode positioned in the other limb thereof and in contact with the aqueous KOl.

Claims

1. A liquid microelectrode comprising an insulator having a microcavity defined therein by internal surfaces thereof and by an opening thereinto adjacent external surfaces thereof, either said internal surfaces being hydrophobic and said external surfaces hydrophilic or said internal surfaces being hydrophilic and said external surfaces hydrophobic; an electrical conductor as a first electrode communicating with the microcavity; a reference electrode communicating with the microcavity; and a liquid contained in the microcavity being either (a) organic when the internal surfaces are hydrophobic and the external surfaces hydrophilic and being for contacting, at the opening, with an aqueous liquid phase to generate a liquid-liquid interface, or (b) aqueous when the internal surfaces are hydrophilic and the external surfaces hydrophobic and being for contacting, at the opening, with an organic liquid phase to generate a liquid-liquid interface.

2. A method of making a liquid microelectrode which comprises the steps of (i) forming a microcavity by removing a portion of a conductor sealed in an insulator so that the microcavity is defined by internal surfaces of the insulator and by an opening thereinto adjacent external surfaces thereof.

(ii) providing the insulator with a hydrophobic coating either before step (i) is carried out to coat said external surfaces only, or after step (i) is carried out to coat both said external and internal surfaces; (iii) forming a reference electrode communicating with the microcavity; and (iv) (a) where the coating has been provided after step (i) has been carried out, making the external surfaces hydrophilic, and providing an organic liquid phase in the microcavity for contacting, at the opening, with an aqueous liquid phase to generate a liquid-liquid interface, or (b) where the coating has been provided before step (i) has been carried out, providing an aqueous liquid phase in the microcavity for contacting, at the opening, with an organic liquid phase to generate a liquid-liquid interface.

3. A liquid microelectrode substantially as hereinbefore described with reference to and illustrated in Figure 2 or Figure 3 of the drawings filed herewith.

4. A method of making a liquid microelectrode substantially as hereinbefore described with reference to Figures 1 and 2 or Figures 1 and 3 of the drawings filed herewith.