WO1994017399A1

WO1994017399A1 - Long-life miniaturizable reference electrode

Info

Publication number: WO1994017399A1
Application number: PCT/DE1994/000081
Authority: WO
Inventors: Christa Dumschat
Original assignee: Institut Für Chemo- Und Biosensorik
Priority date: 1993-01-28
Filing date: 1994-01-27
Publication date: 1994-08-04
Also published as: DE4302322A1; DE4302322C2

Abstract

The development of miniaturized chemical semiconductor sensors necessitates the development of a miniaturized reference electrode. a new long-life reference electrode is miniaturizable and has an increased service life in comparison with state-of-the-art arrangements. This reference electrode consists of a perchlorate-sensitive or potassium-sensitive membrane in contact with a reference electrolyte chamber in which is located solid perchlorate in an aqueous solution or in a gel. The perchlorate saturation concentration is set in the solution or in the gel, at least at the proximity of the membrane. A potential constancy is thus achieved. An electrolyte contact exists between the reference electrolyte chamber and the measurement solution. This arrangement allows the reference electrode to be integrated on the sensor chip.

Description

Durable miniaturizable reference electrode

The invention relates to a long-life miniaturizable reference electrode for generating a constant reference or reference potential in any solution. This miniaturizable reference electrode can preferably be used in ion-selective potentiometry when using miniaturized measuring electrodes. However, it can also be used as a macroscopic reference electrode in combination with a macroscopic measuring electrode. Frequent fields of application are environmental analysis and clinical chemistry.

It is known that in ion-selective potentiometry, an electrochemical measuring chain is set up with the aid of an ion-selective measuring electrode and a potential-constant reference electrode. A galvanic voltage which is dependent on the activity of the measuring ion is established at the measuring electrode. Such ion-selective electrodes are commercially available for a number of ions and are widely used (K. Cammann, Working with ion-selective electrodes, Springer-Verlag Berlin Heidelberg New York 1977). The galvanic voltage is not accessible for direct measurement, since two phases cannot be connected to a measuring device without adding a new phase boundary with new galvanic voltages. Therefore, constant reference electrodes are required for the measurement in addition to the measuring electrode.

The primary reference electrode, whose potential difference compared to the measurement solution is set to zero by definition, is the standard hydrogen electrode. A platinum-plated platinum sheet is immersed in a solution with a pH of 0 and is flushed with hydrogen gas (101.3 kPa). It is generally used as a reference for electrodes with aqueous electrolytes. The standard hydrogen electrode, however, is hardly suitable for daily measurement practice because of its complicated handling. Here the easier-to-handle electrodes of the 2nd type have prevailed. They are made of a metal with a thin layer of one of them poorly soluble salts is coated. The most commonly used electrodes of this type are the silver / silver chloride and calomel electrodes. They can be symbolized by the following basic structure:

Ag / AgCl, KCl or Hg / Hg ₂ Cl ₂ , KCl.

The potential balance is based on the following equilibria:

Ag <=> Ag ⁺ + e "

Ag ⁺ + CI ^" => AgCl.

The actually potential determining silver ion activity is determined based on the solubility balance by the chloride ion activity. Inserting it into the Nernst equation for the Ag / Ag ⁺ electrode gives the following relationship:

ΦgpΦgl ° - ^{RT F ln a} Cl "

with: φgi = standard electrode potential

R = general gas constant T = absolute temperature

F = Faraday constant ^a Cl " ⁼ chloride ion activity.

A constant chloride ion activity is a prerequisite for a constant electrode potential. Since the chloride ion activity in the measuring solution is almost never constant, a current key is inserted between the measuring solution and the electrode, which is usually filled with concentrated KC1 as the reference electrolyte. This keeps the chloride ion activity constant. Reference electrolyte and measurement solution are marked by a Connected diaphragm, which makes an electrolyte contact but largely prevents the mixing of the two solutions. A Electrolyte contact leads to the formation of a diffusion potential which should be as low and constant as possible during the measurement. Its constancy decisively determines the measuring accuracy of the entire measuring chain. These conditions are met by electrolytes whose cations and anions have similar ion mobilities (eg KC1) in high concentrations. Such electrodes are well suited for the construction of electrodes in connection with ion-selective macro electrodes. In some applications, for example in solutions containing sulfide or in solutions containing redox systems, the potential of these solution components changes (by interfering with the solubility balance or redox potentials on silver).

In recent years, the rapid development of microelectronics has led to the development of miniaturized potentiometric sensors. The ion-selective field effect transistor (ISFET) should be mentioned in particular. Such sensors have a number of advantages over conventional ion-selective electrodes such as the possibility of inexpensive mass production, the miniaturization and the possibility of integration.

When using miniaturized measuring electrodes of this type, miniaturization of the reference electrode is necessary in order to obtain a miniaturized measuring system. Attempts were first made to miniaturize the Ag / AgCl reference electrode (R. Smith, DC Scott, IEEE Trans. Biomed. Eng., BME 33 (1986) 83 and A. Van den Berg, A. Griesel, HH Van den Vlekkert , NF De Rooij Sensors and Actuators B 1 (1990) 425-4323). There are limits to the miniaturization of conventional reference electrodes based on electrodes of the second type. Miniaturization of the reference electrolyte space is particularly problematic. A reduction in the volume leads to a reduction in the service life, since the low but always ongoing interdiffusion of measuring and reference electrolytes leads to changes in concentration in the Reference electrolyte leads, the smaller the reference electrolyte volume. A reduction in mixing by reducing the contact area is only possible to a limited extent. 90 If one uses .a very dense diaphragm occur within this dense

Diaphragm zone unstable diffusion potentials that interfere with the measurement. Very small openings lead to the risk of clogging of the electrolyte contact. Deposition of solid KC1 in the reference electrolyte space leads to an increase in the service life. Due to the easy solubility of the KC1, this reservoir will soon be 95 used up. Changes in the reference electrolyte concentration lead to a change in the

Reference electrode potential and thus to measurement errors. Furthermore, disruptive substances reach the reference electrode surface much faster and in higher concentrations than in macroscopic arrangements. Interfering substances are, for example, redox systems from the sample matrix, which imprint their redox potential on the electron-conducting inner metal dissipation element, complexing agents for Ag ⁺ ions or ions that form compounds which are less soluble than mercury or silver with mercury or silver.

For this reason, attempts have been made to develop miniaturizable reference electrodes without reference electrolyte. On the one hand, the surface was tried

105 of a pH-sensitive ISFET to be modified in such a way that the pH sensitivity is suppressed. Calculations (A. Van den Berg, P. Bergveld, DN Reinhoudt, EJR Sudhölter, Sensors and Actuators 8 (1985) 129-148) had shown that a drastic reduction in the number of hydroxyl groups on the sensor surface can greatly reduce the pH sensitivity . An attempt was made to do this by coupling

110 different organosilicon compounds to the hydroxyl groups of

To reach the sensor surface. However, it was found that it was not possible to block the very high proportion of hydroxyl groups required for a period of time sufficient for the practical use of the sensor, so that this type of reference electrode has so far not found any practical application.

115 Furthermore, attempts have been made to use insensitive polymer membranes as the reference electrode surface. "Ion blocking polymers" (T.Matsuo, H.Nakajima, Sensors and Actuators 5 (1984) 293-305) and "ion unblocking polymers" (doctoral thesis, Peter van der 120 Wal, University of Twente 1991) were investigated. A sufficiently stable potential could not be achieved in both cases. This is also understandable, since neither arrangement is based on a stable potential-forming process.

Furthermore, a reference electrode based on a fluoride-sensitive electrode 125 was described (F. Lindsat, W. Moritz, Poster Eurosensors 92, San Sebastian). For this purpose, solid CaF ₂ was deposited as a porous layer over a fluoride-sensitive membrane. There was a polymer membrane on top that inhibited diffusion. This creates a constant fluoride ion activity in the vicinity of the sensitive membrane. The big disadvantage of this arrangement is the strong influence on the fluoride ion activity by the 130 solution composition. The fluoride ion activity is strongly influenced by the

Calcium ion activity affected. Furthermore, the low ion concentration in the reference electrolyte space leads to strongly concentration-dependent diffusion potentials.

So far, none of the solutions developed has met the requirements for a 135 miniaturizable reference electrode. Even in macroscopic applications, there are a number of interfering substances in all reference electrodes described so far. Reference electrodes without reference electrolyte neither theoretically nor practically meet the requirements for constant potential and are therefore unsuitable so far. Miniaturized reference electrodes of the second type with reference electrolyte have only a short lifespan as a miniaturized version 140, and there are a large number of substances which are usually present in measuring solutions and which disrupt the Ag / AgCl system when they dissolve into the small reference electrolyte space. These disturbances also occur in macroscopic applications, albeit at a much slower rate. 145 The invention is therefore based on the object of developing a miniaturizable reference electrode which, as a miniaturized version, is distinguished from the known solutions by an extended service life and in all sizes (miniaturized and macroscopic) by a substantial reduction in the amount of disruptive substances that a perchlorate sensitive or

150 potassium-sensitive membrane (membrane at the interface with a solution of which a potential depends on the activity of perchlorate ions or potassium ions in the solution) is in contact with an electrolyte solution or an electrolyte solution stabilized by a gel (reference electrolyte). This electrolytic solution contains the perchloration. It is located together with a salt which is deposited in solid form

155 Perchloric acid, the saturation concentration of which is at least close to the membrane, in a room (reference electrolyte room). There is at least one electrolyte contact to the measurement solution from this reference electrolyte compartment.

This results in a reduction in the number of interfering substances compared to 160 electrodes of the second type (both miniaturized and macroscopic designs) and an extension of the service life compared to all previously known miniaturizable reference electrodes. The potential constancy of this new reference electrode is achieved by keeping the activity of the potential-determining perchlorate constant. This happens because the solution always contains the saturation activity of the perchlorate in a solid form of a salt of perchloric acid.

The extension of the service life compared to miniaturized conventional electrodes of the second type is achieved by using a reference electrolyte with a much lower solubility than the reference electrodes of the second type. If this reference electrolyte is deposited in solid form in the reference electrolyte space, the activity of the potential-determining ion 170 remains constant until the reference electrolyte has completely dissolved. This requires a much higher volume of liquid than the reference electrolytes used in conventional electrodes of the second type. With the same construction of the reference electrolyte space, the service life is considerably increased. A particularly advantageous variant of this The new reference electrode is a perchlorate sensitive membrane in combination with a solid

175 potassium perchlorate in the liquid space, in which the saturation concentration of the perchlorate then forms. The solubility with 0.12 mol / l of water is significantly lower than the solubility of KC1 with 4.61 mol / l, so that a substantial extension of the service life can be expected. On the other hand, the solubility is large enough to achieve sufficient conductivity in the liquid space and the formation of highly concentration-dependent ones

To prevent 180 diffusion potentials. Furthermore, the ion mobilities of K ⁺ and Clθ4 "ions are similar, which also leads to low diffusion potentials. The interference is reduced by using the very selective perchlorate membrane. In the presence of 0, 12 mol perchlorate / l there are hardly any interferences the sample matrix to be expected. Possible interference ions such as Cl "and NO3 ^" would have to be in the

185 measuring solution in several tens of orders of magnitude higher concentrations than the perchlorate in order to interfere, which is impossible. The K ⁺ ion interferes only at concentrations above 0.12 mol / l, by changing the equilibrium activity of perchlorate according to the solubility product, which are extremely rare to be expected. The perchlorate itself is almost never to be expected in higher concentrations in measurement solutions.

190

The reference electrolyte space can be formed by cavities of any geometry in any material or only by the gel itself, which can still be partially covered by a polymer layer. Geometries with small openings are preferred in order to mix the reference electrolyte and

195 measuring solution. In the case of silicon sensors, the reference electrolyte space can preferably be formed by etching structures, e.g. formed by pyramid-shaped microcontainments with small openings to the solution side etched through the entire wafer.

An electrolyte contact must be made between the measuring solution and the reference electrolyte compartment

200, ie the interface between the two liquids must be designed in such a way that ions can be transferred between the reference electrolyte space and the measurement solution. This electrolyte contact can be made 205 through a small opening, a diaphragm, a cut, the gel / water interface or other arrangements known to those skilled in the art. It is also possible to switch one or more current keys which contain one or more further electrolytes between the reference electrolyte and the measurement solution.

At the interface between two electrolyte solutions of different composition, the diffusion of ions from one solution into the other occurs as a result of the

210 different diff sion speed of the ions to a voltage, which is referred to as diffusion potential. This diffusion potential can be reduced by adding suitable electrolytes which have opposite ion mobility differences to that of the cation of the reference electrolyte and of the ClO4 "ion (for example, if KCIO4 is used as the reference electrolyte, it decreases by adding solid Ca ₂ SO4

215 (whose saturation concentration is established in the reference electrolyte space) after the

Hendersen's equation calculated difference potential against water from -16 to - 8.5 mV).

To close the measuring circuit, the perchlorate-sensitive membrane must be seen from the rear

220 can be contacted.

As an internal lead system for contacting the ion-sensitive membrane on the side opposite to the liquid space, any design option of a fixed contact known from potentiometry, e.g. Silver or an ISFET, or a liquid contact can be used.

225 Further details and features of the invention result from the following exemplary embodiments of the invention with reference to the drawing.

FIG. 1: 230 shows an embodiment of the invention with a pyramidal microcontainment Figure 2: an embodiment of the invention in a polymer tip

235 Figure 3: a calibration curve

Figure 4: an embodiment of the invention on an ISFET

240

Figure 5: an embodiment of the invention in a vitreous

245 FIG. 1 shows a miniaturized reference electrode which has been integrated into a pyramid-shaped microcontainment 7 produced in the silicon by anisotropic etching techniques. The pointed opening of the containment 7 is in contact with the measurement solution 4 and thus forms the electrolyte contact 3. In the lower part of the containment 7, solid KCIO4 was deposited in an aqueous gel in which at least in the vicinity of the membrane

250 saturation concentration of the KCIO4. The part of the microcontainment in which the gel and the solid KCIO4 are located forms the reference electrolyte space 1. The perchlorate-sensitive membrane 2 was applied over this gel. The electrical contact (conductor) 5 to the carrier was made using a silver conductive adhesive.

255 FIG. 2 shows a reference electrode which was produced in a polypropylene tip 8. In the tip there is solid KCIO4 and solid CaSO 4 in an aqueous polyhydroxyethyl methacrylate gel, so that the reference electrolyte space 9 is formed. A perchlorate-sensitive "coated wire electrode" 10 with a perchlorate-sensitive PVC matrix membrane is immersed in this gel. This electrode 10 is used as a reference electrode for pH and

260 nitrate determinations used. The room 9 is in contact with the 3 Measuring solution 4 in connection. 3 shows the calibration curve for the nitrate determination against this new reference electrode and against a conventional calomel reference electrode. The results show that the reference electrode is suitable for practical measurements.

265

FIG. 4 shows a miniaturized reference electrode on the surface of an ISFET 1 1. A perchlorate-sensitive polymer matrix membrane 12 was deposited on the gate insulator of the ISFET 1 1. In this case, the ISFET 1 1 serves as a discharge device which is connected to the membrane 12. The opening 14 provides the electrolyte contact to the measurement solution 4

270 ago. Solid KCIO4 is located above it in a gel which contains an aqueous perchlorate solution, as a result of which the reference electrolyte space 13 is formed. Except for a small opening 14, the gel was covered by a polymer layer 15.

5 shows a reference electrode in a glass body 16. The glass body 16 is connected to the measurement solution 4 by a 275 diaphragm 19. The glass body 16 contains solid KCIO4 and a saturated KC104 solution, which form the reference electrolyte space 17. A conventional perchlorate-sensitive polymer matrix membrane electrode 18 with a liquid internal drain is immersed in this solution.

Claims

Claims: 280

1. Long-life miniaturizable reference electrode for generating a constant reference or reference potential in any solution, characterized in that it consists of a perchlorate or potassium-sensitive membrane (2, 12) connected to a drainage device (5), the membrane (2, 12 ) in contact with

285 of an electrolyte solution containing perchlorate ions, which can optionally be stabilized with a gel, and which is located together with a solid salt of perchloric acid in a reference electrolyte space (1, 9, 13, 17) that has at least one electrolyte contact (3, 14, 19) to the measurement solution (4).

290 2. Reference electrode according to claim 1, characterized in that the

Abieiteinrichtung (5) and the membrane (2, 12) together form a membrane electrode (10, 18).

3. Long-life miniaturizable reference electrode according to claim 1 or 2, characterized 295 in that the reference electrolyte space (1, 9, 13, 17) by any cavities

Geometry in any material or only by the gel itself, which can still be partially covered by a polymer layer.

4. Durable miniaturizable reference electrode according to at least one of claims 300 1-3, characterized in that the reference electrolyte space (1) in a silicon sensor preferably by etching structures, e.g. is formed by pyramid-shaped microcontainments with small openings to the solution side etched through the entire wafer.

305 5. Durable miniaturizable reference electrode according to at least one of claims 1-4, characterized in that the electrolyte contact (3, 14, 19) between the measuring solution (4) and reference electrolyte space (1, 9, 13, 17) through a small opening Diaphragm, a cut, the gel / water interface or other arrangements known to those skilled in the art. 310

6. Durable miniaturizable reference electrode according to at least one of claims 1-5, characterized in that there is a current key with one or more further electrolytes between the reference electrolyte space (1, 9, 13, 17) and the measurement solution.

315 7. Long-life miniaturizable reference electrode according to at least one of claims 1-6, characterized in that the perchlorate-sensitive membrane (2, 12) is an ion-sensitive polymer matrix membrane consisting at least of a plasticizer, a polymer matrix and an active component which is the salt of a large lipophilic cation and a less lipophilic anion is used.

320

8. Durable miniaturizable reference electrode according to at least one of claims 1-7, characterized in that as the inner Abieiteinrichtung for contacting the ion-sensitive membrane (2, 12) on the opposite side of the reference electrolyte (1, 9, 13, 17) each from the Potentiometry known design option

325 a fixed contact, e.g. Silver or an ISFET, or a liquid contact, can be used.

9. Long-life miniaturizable reference electrode according to at least one of claims 1-8, characterized in that in addition to the perchlorate further electrolytes for

330 Reduction of diffusion potentials in the reference electrolyte space (1, 9, 13, 17) are included.

10. Use the reference electrode according to claims 1-9 as a reference electrode for any potentiometric measurements.

335