JPH0769289B2 - Microelectrodes for electrochemical analysis - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an improved microelectrode for electrochemical analysis, such as an electrode for measuring dissolved oxygen, an electrode for measuring ion concentration, an electrode for a biosensor, or a reference electrode, which has low sensitivity to flow rate and is resistant to contamination.

BACKGROUND ART Conventionally, the analysis of oxygen, sugars, amino acids, pH, etc. has been carried out by electrochemical methods using various electrodes. However, in all cases, there are problems such as electrode contamination. Below, we will discuss the prior art of various electrodes and sensors.

Dissolved oxygen measurement sensors It is important to measure dissolved oxygen in body fluids such as blood and in culture fluids in fermentation tanks. Currently, Clark-type oxygen sensors are widely used. This sensor is designed to fill the sensor container with an electrolyte such as a caustic alkaline solution, with a platinum cathode and a lead anode installed inside, and to measure the dissolved oxygen in the external (measurement solution)
The sensor is constructed with an oxygen-selective permeable membrane, such as polytetrafluoroethylene, separating the two. Oxygen molecules pass through the membrane from the solution to be measured into the sensor container and are electrochemically reduced on a platinum electrode. The current generated during this process can be measured to determine the amount of dissolved oxygen. While this method has made it easy to measure dissolved oxygen levels, it has many problems, including instability with pressurization during sterilization, tedious maintenance, difficulty in miniaturization, the ability to measure only oxygen partial pressure rather than actual dissolved oxygen concentration, inability to measure dissolved oxygen during pressurized fermentation or at the bottom of the ocean or lake, and large changes in current value due to temperature changes.

Other proposed electrodes for measuring oxygen concentration, particularly those for body fluids, include a method in which the surface of a fine metal electrode is covered with a multilayer porous membrane to create a stable contact state between the electrode surface and the solution (Japanese Patent Laid-Open No. 57-117838), and a method in which a thin wire metal electrode is inserted into a tube with an open tip so that the tip of the electrode is positioned back from the open tip of the tube (Japanese Patent Laid-Open No. 57-195436).

However, these methods have drawbacks such as electrode contamination and the possibility of the porous membrane peeling off, which can lead to iatrogenic diseases.

Glass electrodes have been widely used to measure hydrogen ion concentrations. However, they have problems such as the glass membrane being easily damaged or contaminated, and their use in alkaline solutions is limited. Furthermore, glass electrodes require an internal reference solution chamber, making them difficult to miniaturize, and there is a risk of the internal reference solution leaking out.

Recently, it has been discovered that the potential of a film formed by electropolymerization on the surface of a conductive substrate changes with the addition and removal of hydrogen ions [William R. Heineman, Anal. Chcm . 5
2 345 (1980)]. A pH sensor has also been devised that utilizes a film with reversible redox function formed on the surface of a conductive substrate, which undergoes a redox reaction accompanied by the addition and desorption of hydrogen ions (Japanese Patent Laid-Open Publication No. 194343/1986). However, when these pH sensors are used for continuous measurements in body fluids or fluorescent solutions, potential drift occurs due to contamination.

Biosensors are classified into two types: one that determines the concentration of a target substrate involved in a biochemical reaction from the current value obtained when a substance involved in the biochemical reaction directly reacts with an electrode, and one that determines the electrode-sensitive substance involved in the biochemical reaction from the membrane potential generated in a membrane that selectively responds to it. The former is called the amperometric type and uses an oxygen electrode and a hydrogen peroxide electrode, while the latter is a potentiometric type and uses an ion-selective electrode for hydrogen ions, etc., or an ammonia or carbon dioxide electrode (consisting of a type of gas-permeable membrane and a combined pH electrode). Biochemical reactions include enzymes, microorganisms, and antigen-antibody reactions.
Since the basis of the biosensor is an oxygen sensor and a pH electrode, there are problems as mentioned above and in the previous section.

Reference electrode Conventionally used reference electrodes are saturated calomel electrodes,
Because the electrodes contain an internal liquid such as silver/silver chloride electrodes,
They had the following drawbacks: 1) they were easily damaged, and 2) they could not withstand the pressure applied during sterilization.

Therefore, the present invention aims to eliminate the drawbacks of the prior art described above and to provide various electrodes and sensors using the electrodes that can stably and accurately measure dissolved oxygen, sugars, amino acids, pH, etc.

Disclosure of the Invention The present invention provides a microelectrode for electrochemical analysis, which is made of a carbon fiber bundle covered with a non-conductive resin, and in which the tips of the carbon fibers are retracted at one end of the fiber bundle to form micropores that are open to the outside. This configuration results in a microelectrode that is less sensitive to flow rate and less susceptible to electrode contamination.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a conceptual diagram of the microelectrode of the present invention, and Figure 2 is a conceptual diagram of a biosensor electrode, which is one specific example of the microelectrode of the present invention. Figure 3 shows the state of contamination of the microelectrode with and without micropores, as performed in Example 4.

BEST MODE FOR CARRYING OUT THE INVENTION The microelectrode of the present invention is conceptually as shown in Figure 1. Here, a plurality of carbon fibers 1 are sandwiched between a non-conductive resin 2.
The carbon fiber bundle is formed by covering the carbon fiber 1 with the non-conductive resin 2. In the figure, the upper ends of the carbon fibers 1 form micropores 3 that recede from the upper ends of the non-conductive resin 2 and are open to the outside, and the tip surfaces of the carbon fibers, which form the bottom surfaces of these micropores, become the electrode surfaces. If necessary, the electrode surface may be modified with an electrochemically catalytically active substance 4. The electrode of the present invention constructed in this manner will hereinafter be referred to as a basic electrode.

There are no particular restrictions on the carbon fibers that make up the base electrodes, but carbon fibers made from raw materials such as polyacrylonitrile, pitch, rayon, and phenolic resin, and carbon fibers produced by vapor phase growth are preferably used.

The non-conductive material is not particularly limited, but preferably used are polymeric materials such as fluorine resin, polyester resin, epoxy resin, polyphenylene oxide resin, polyphenylene sulfide resin, urethane resin, silicone resin, vinyl chloride resin, phenolic resin, etc. When the microelectrode of the present invention is to be inserted into a living body, it is preferable to use a resin with excellent antithrombogenicity.

The carbon fibers are each covered with a non-conductive resin and solidified into a single unit. When the cross section of the resulting composite is cut perpendicular to the fiber axis, each carbon fiber is present as an island component in a sea of non-conductive resin. In other words, the carbon fibers are configured so that they do not come into contact with each other. The number of carbon fibers is determined appropriately depending on the intended use of the microelectrode of the present invention, but is generally 50.
When used as a reference electrode,
More may be used.

The ratio of the cross-sectional area of the carbon fiber to the area of the cross section is usually about 3 to 60%. The diameter of the carbon fiber is 20 μm or less, and preferably 10 μm or less. The diameter of the carbon fiber is also the diameter of the micropores, and when the diameter is 20 μm or less, the so-called filter effect is exhibited.

The depth of the micropores is usually 0.5 to 500 μm, and particularly 20 to 400
The electrode surface does not need to be flat, and may be pointed like a pencil lead, or may have a downward convex shape, but in such cases the depth of the micropores is defined as the deepest position.

The electrochemical catalytic active substance that modifies the electrode surface as needed is platinum, silver, gold, iridium, phthalocyanines, etc. By forming this layer, the oxygen reduction activity of the electrode can be increased.

The basic electrode of the present invention can be made less susceptible to contamination by covering the electrode surface with a selectively permeable membrane. This membrane is preferably an electrolytically oxidatively polymerized membrane formed by electrolytic oxidation polymerization.

The basic electrode of the present invention is manufactured, for example, as follows.

First, a carbon fiber bundle is passed through a non-conductive resin containing a curing agent, and then the resin is hardened by heat treatment or other means to obtain a wire-like carbon fiber bundle covered with the non-conductive resin. This is then cut to the desired electrode length, one cross section is polished, and a lead wire is attached to the other end using silver paste. The carbon fiber on the polished side is then scraped away using the electrolytic oxidation method described below to form micropores.

The electrolytic oxidizing solution may be an acidic or alkaline aqueous solution,
An aqueous solution containing dissolved salts and alcohols such as methanol are used. The carbon fiber is connected to the anode and a metal electrode is used as the counter electrode for anodization. The voltage during oxidation is 1 to 10
The range of 0 volts is preferred. A method in which oxidation and reduction are repeatedly performed is also a preferred method.

When the electrode surface is modified with the electrochemically catalytically active substance, it can be done by a method such as plating, vacuum deposition, or sputtering.

The micropores of the electrodes described below, including the basic electrodes described above, may be filled with a gel-like substance, such as agar, gelatin, or polyacrylamide gel.

Next, various electrodes using the basic electrode of the present invention will be described in more detail.

Dissolved Oxygen Measurement Electrode: While a basic electrode can be used as is for measuring dissolved oxygen, better results can be obtained by forming a permselective membrane on the electrode surface of the basic electrode by electropolymerization. Electropolymerization is most conveniently performed by adding a supporting electrolyte, such as sodium perchlorate or sodium sulfate, to an organic solvent such as acetonitrile or water, adding the monomer compound to be electropolymerized, and conducting constant-potential electrolysis at 1 to 1.5 volts. The monomer compound used here is not particularly limited as long as it is non-redox-responsive and produces a permselective membrane that prevents electrode fouling. Examples of monomer compounds include nitrogen-containing aromatic compounds or hydroxyaromatic compounds, such as 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfide, phenol, aniline, or phenol and aniline derivatives. Depending on the polymerization conditions, a redox-responsive membrane may be formed. For example, when aniline is used as the monomer, a redox-responsive membrane will form if it is not polymerized under neutral or alkaline conditions.
Furthermore, repeating polymerization as a polymerization method is
This results in a more complete membrane coverage and increased permselectivity.

When this electrode is used to prepare an oxygen sensor, for example, a silver/silver chloride counter electrode is combined with it. This electrode can also be used to measure dissolved electrochemically reactive substances other than dissolved oxygen by changing the selective permeability of the permselective membrane.

Ion electrode: An ion electrode is an electrode in which the surface of a basic electrode is covered with an electrolytic polymerized film that responds to hydrogen ions. Examples of hydrogen ion-responsive films include 2,6-dimethylphenol, 4,4'
Examples of the polymerizable film include electrolytic oxidation polymer films of 1,2-diaminobiphenol, 1,2-diaminobenzene, 1-aminopyrene, 1,8-diaminonaphthalene, and 1,5-diaminonaphthalene.

To measure the hydrogen ion concentration in an aqueous solution using this ion electrode, the ion electrode is immersed in the measurement solution together with a reference electrode such as a saturated calomel electrode, and the voltage between these electrodes is read.

Biosensor The electrode of a biosensor is an electrode in which enzymes or microorganisms 5 are immobilized by some method on the side of pores made of a non-conductive material 2, or the pores are covered with a membrane 6 on which enzymes or microorganisms are immobilized, as shown in Figure 2.

As the enzyme, for potentiometric electrodes, urease, penicillinase, glucose oxidase, lipase, etc. can be used, and for amperometric electrodes, various oxidase enzymes such as glucose oxidase, lactate oxidase, L-amino acid oxidase, xanthione oxidase, urate oxidase, pyruvate oxidase, lactate oxidase, etc. can be used.

The immobilization membrane 6 may be made of polyvinyl chloride, polyacrylamide, acetyl cellulose, cellulose triacetate, cross-linked polyvinyl alcohol, etc. Instead of a membrane, nylon fiber or a mesh of fibers may also be used.

The enzyme or microorganism is immobilized by being adsorbed or entrapped in the immobilization membrane, by being covalently bonded to the immobilization membrane, or by being cross-linked with glutaraldehyde.

To immobilize enzymes or microorganisms on the side surfaces of the pores of a non-conductive material, for example, an epoxy resin is used as the non-conductive material and the enzyme is directly bonded to the functional groups of the resin, or a substance having an amino group is bonded to the side surfaces of the pores and then the enzyme is bonded onto the substance.

To make a biosensor, for example, an enzyme-immobilized electrode is combined with a silver/silver chloride counter electrode.

The reference electrode consists of silver and silver halide bonded to the base electrode. Chlorine and bromine are preferred halogens. As mentioned above, the micropores are preferably filled with a gel-like substance, and it is especially preferred that the gel-like substance contain a salt containing halide ions.

The present invention will be described in more detail below with reference to examples.

Examples 1, 2, 3 and Comparative Example 1: A bundle of 1000 carbon fibers ("Torayca T-3001K", diameter 7 μm)
The fibers were then impregnated with epoxy resin containing a curing agent. The impregnated fiber bundle was then heated and hardened while stretched, yielding a wire-like composite material with a diameter of approximately 0.3 mm. The sides of the bundle were completely insulated with epoxy resin, and the wire was then cut. One end was polished using conventional methods, and a lead wire was attached to the other end using silver paste to form an electrode.

The polished section was immersed in a 2 mmol/L aqueous solution of sulfuric acid and anodized using a platinum wire as the counter electrode under the conditions described below. The resulting center was used to examine the effect on the current due to the flow reduction of the solution. Specifically, saline was placed in a flask equipped with a magnetic stirrer, and the sensor described above was attached as the working electrode and a silver/silver chloride electrode as the counter electrode. The flask was then exposed to air at room temperature until oxygen was saturated.

A negative voltage of 0.6 volts was applied to the working electrode, and the reduction current was measured. The current flowing when the magnetic stirrer was rotated as fast as possible and when it was left stationary was measured, and the ratio was calculated.

In Example 1, oxidation was performed at 1.75 V for 15 minutes, followed by reduction at -1.2 V for 1 minute, and this cycle was repeated 35 times. In Example 2, oxidation was performed by passing a current of 0.03 mA for 80 minutes, followed by reduction in saline at -0.7 V for 10 minutes.

In Example 3, electrolytic oxidation was carried out at 0.5 milliamperes for 9 minutes, followed by reduction in physiological saline at -0.6 volts for 10 minutes.
0 μm, Example 2 was 150 μm, and Example 3 was 240 μm.

As a comparative example, the measured value using an electrode before the oxidation etching treatment is shown.

The results are shown in Table 1.

Example 4 A wire-like composite material having a diameter of about 0.3 mm was obtained in the same manner as in Example 1 using a bundle of 1000 carbon fibers ("Torayca T-300", diameter 7 μm).

This was cut, one cross section was polished in the usual way, and the other end was attached to a lead wire using silver paste to prepare an electrode.
The polished portion was immersed in a 2 millimolar aqueous sulfuric acid solution, and electrolytic oxidation was carried out at 0.5 milliamperes for 9 minutes using a platinum wire as the counter electrode, followed by electrolytic reduction at 0.6 volts for 20 minutes.

Observation under an electron microscope revealed that the depth of the micropores in the carbon fiber was approximately 100 μm, and the electrode surface had a sharp tip, like the tip of a sharpened pencil lead.

On the microporous electrode obtained above, the following method is performed:
A platinum thin film was formed. Specifically, the microporous electrode was immersed in an aqueous solution containing 0.037 mol/L chloroplatinic acid, 0.134 mol/L ammonium phosphate, and 0.704 mol/L sodium phosphate, and a current of 8 millicoulombs was passed through the platinum counter electrode. The resulting electrode was thoroughly washed with water. Observation under an electron microscope revealed that a platinum thin film of approximately 0.6 μm thick was uniformly deposited on the electrode.

Figure 3 shows the progression of electrode fouling when an electrode before platinum plating (called carbon in the figure) and an electrode after platinum plating (called platinum in the figure) are left in LB medium (37°C). The carbon electrode is shown by a dotted line, and the platinum electrode by a solid line. There are two lines for each electrode; the lower line is the value when there are no micropores, and the higher line is approximately
This is the value when 100 μm micropores are open. In all cases, the presence of micropores results in a lower oxygen reduction current drop rate (micropore effect). This is thought to be because the micropores act as a filter, making it difficult for electrode contaminants to pass through.

In the platinum-plated electrode, the oxygen reduction current dropped by about 5% within 24 hours and then remained constant.

A thin film of polyaniline was formed on the platinum-plated electrode by electrolytic oxidation polymerization as described in detail below.

Specifically, a platinum-plated microporous electrode was immersed in an acetonitrile solution containing 0.01 mol/L aniline, 0.1 mol/L sodium perchlorate, and 0.02 mol/L pyridine, and a positive potential of 1.2 volts was applied to polymerize the electrode until no current flowed. Observation under an electron microscope confirmed the formation of a thin polymer film on the platinum.

The resulting electrode was sterilized (120°C, 20 minutes) and then left in LB medium at 37°C for 10 days. The oxygen reduction current did not decrease at all. Furthermore, no abnormalities were observed even after 10 repeated sterilization treatments.

Example 5 An electrode was prepared in the same manner as in Example 1, except that the diameter of the carbon fiber was 5 μm.

The polished part was immersed in a 2 mmol sulfuric acid solution, and electrolytic oxidation was performed at a current of 0.2 milliamperes using a platinum wire as the counter electrode, resulting in a reduction of approximately 100 μm of the carbon fiber. This was followed by electrolytic reduction at −0.7 volts for 20 minutes.

Similarly, electrodes were fabricated using bundles of 100 carbon fibers each having a diameter of 20 μm and 30 μm, with the carbon fibers shaved down to about 100 μm.

A silver/silver chloride electrode was used as the counter electrode in the LB medium solution, and a voltage of -0.7 volts was applied to these electrodes. After 24 hours, the rate of decrease in the oxygen reduction current was determined. The results are shown in Table 2. The rate of decrease in the oxygen reduction current was smaller for smaller pores.

Example 6: Electrodes were prepared in the same manner as in Example 4, except that phenol, 2,5-dimethylphenol, and 4,4'-diamino-diphenyl ether were used instead of aniline. No decrease in the oxygen reduction current was observed even after leaving the electrodes in LB medium at 37°C for 10 days.

Example 7: Bundle of 3000 carbon fibers ("Torayca T-300", diameter 7 μm)
The carbon fiber composite was inserted into a 35 cm long polyethylene tube (inner diameter 2 mm), and one end of the carbon fiber was fixed outside the tube. Epoxy resin (Toray Hisol) containing a curing agent was slowly injected through the opening holding the carbon fiber. The mixture was post-cured at approximately 60°C for 1 hour and 30 minutes. The cured carbon fiber composite was removed from the tube and cut into 10 cm lengths. It was then heat-treated in an electric furnace. A lead wire was connected to one end of the carbon fiber composite with silver paste, and the sides of this wire were completely insulated with epoxy resin. The other end was polished using conventional methods. The electrode was anodized in 2 mM sulfuric acid with a platinum plate as the counter electrode to remove a portion of the carbon fiber. Anodization was performed at a constant current of 1 mA for 10 and 20 minutes, respectively. After 10 minutes of anodization, the micropore depth was 90 μm, and after 20 minutes, it was 140 μm. The dissolved oxygen reduction current was measured by applying a voltage of -0.6 V to this electrode in physiological saline using a silver/silver chloride counter electrode, and the current flowing when the solution was stirred was divided by the current flowing when the solution was left standing. The values after 10 and 20 minutes of anodization were 1.57 and 1.34, respectively.

Example 8 A composite material of carbon fiber and epoxy resin was prepared in the same manner as in Example 1, except that the number of carbon fibers was 3,000. This carbon fiber composite material was anodized in a 2 mmolar aqueous sulfuric acid solution at a constant current (1 mA) using a platinum plate as the counter electrode, thereby removing approximately 200 μm of the carbon fiber. Next, as in Example 4, 12 millicoulombs of electricity were passed over the carbon fiber to perform platinum plating. The temperature dependence of the oxygen reduction current value of this electrode was examined, and it was found to be 0.8%/°C or less. The temperature change rate of a Clark electrode-type oxygen sensor was 4
This is a smaller value than the 5%/℃ ("Optimal Measurement and Control of Fermentation Processes", San-Ensu Forum, p. 214).

The tip of this electrode is covered with 8 wt% polyvinyl chloride (Kanto Chemical Co., Ltd.).
After immersing it in a solution (dimethylformamide), it was soaked in methanol for 3 hours. After that, it was thoroughly washed with ion-exchanged water and then immersed in 5 mg of glucose oxidase (Nagase).
The specimen was immersed in 20 cc of phosphate buffer (0.066 M) containing HCl for 10 hours.
After thoroughly washing with ion-exchanged water,
A saturated calomel electrode was used as the reference electrode and a platinum wire as the counter electrode in a 6M aqueous solution. A potential of -0.6 V (vs. SCE) was applied to the electrodes. The relationship between the glucose concentration and the decrease in the oxygen reduction current value at this time showed a good linear relationship. It took about 60 seconds for the current to reach a steady state. The rate of change in the decrease in current due to changes in the temperature of the solution was examined and found to be 1.4%/
It was °C.

When the potential applied to this oxygen electrode was set to +0.6 V (vs. SCE), the relationship between the glucose concentration and the oxidation current value (hydrogen peroxide oxidation current value) was determined, and a good linear relationship was observed.

Example 9 A carbon fiber composite material was prepared in the same manner as in Example 8, and the carbon fiber was shaved to about 200 μm. This was immediately placed in 20 cc of phosphate buffer (0.066 M) containing 10 mg of glucose oxidase.
It was then thoroughly washed with ion-exchanged water.
When the relationship between the glucose concentration and the decrease in the oxygen reduction current value was determined in the same manner as in Example 6, a straight line passing through the origin was obtained.
The rate of change in the current decrease with respect to the solution temperature was 1%/°C.

Example 10: Bundle of 1000 carbon fibers ("Torayca T-300", diameter 7 μm)
A carbon fiber composite material was produced in the same manner as in Example 1 using the above.

This composite material was electrolytically oxidized in 2 mM sulfuric acid at a constant current (0.34 mA) to etch approximately 200 μm. This was then electrochemically oxidized in an acetonitrile solution containing 2,6-dimethylphenol (20 mM) and sodium perchlorate (0.2 M) at a potential of 1.5 V (vs. SCE) using a platinum wire as the counter electrode and a saturated calomel electrode as the reference electrode.
The electrode was electrolytically oxidized for 10 minutes. The potential in various pH solutions was measured using this electrode, and a good linear relationship was obtained with a slope of -56 mV/pH. When the electrode was transferred from a pH 2 solution to a pH 10 solution, it responded within 1 minute.

Comparative Example 2 A pH electrode prepared in the same manner as in Example 10, except that it was not etched, and a pH electrode prepared in Example 10 were examined for changes in potential over time in a phosphate buffer solution containing yeast extract (0.5 wt%) and bactotryptone (0.3 wt%). The etched electrode gave a stable potential, whereas the unetched electrode showed a potential deviation of 50 mV.

Example 11 A milled electrode was prepared in the same manner as in Example 10, except that the etching depth was 100 μm. This was then platinum-plated by passing a quantity of 4 millicoulombs of electricity through it as in Example 4.

The electrode was then placed in a 0.2M solution containing 1,2-diaminobenzene (20 mM).
In a phosphate buffer solution (pH = 7), a platinum wire was used as the counter electrode and a saturated calomel electrode was used as the reference electrode at a potential of 1.0 V (vs. SCE).
The electrode was electrolytically oxidized for 10 minutes. The potential in various pH solutions was measured using this electrode, and the value was −53 mV between pH 4 and pH 9.
A good linear relationship with a slope of /pH was obtained.

INDUSTRIAL APPLICABILITY The microelectrode of the present invention is useful as a variety of electrodes for electrochemical analysis, and is particularly suitable as an electrode for measuring dissolved oxygen, an electrode for measuring ion concentration, an electrode for biosensors, and a reference electrode.

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Claims (10)

[Claims] [Claim 1] A microelectrode for electrochemical analysis, comprising a bundle of carbon fibers covered with a non-conductive resin, the tips of the carbon fibers retracting at one end of the bundle to form micropores open to the outside. 2. The microelectrode according to claim 1, wherein the diameter of the carbon fiber is 20 μm or less. 3. The microelectrode according to claim 1, wherein the depth of the micropores is 0.5 to 500 μm. 4. A microelectrode according to claim 1, wherein the non-conductive resin is a polymeric material selected from the group consisting of fluorine resin, polyester resin, epoxy resin, polyphenylene oxide resin, polyphenylene sulfide resin, urethane resin, silicone resin, vinyl chloride resin and phenolic resin. 5. The microelectrode according to claim 1, wherein the surface of the retracted carbon fiber tip is covered with a selectively permeable membrane. 6. The microelectrode according to claim 5, wherein the selectively permeable membrane is an electrolytic oxidation polymer membrane. 7. The microelectrode according to claim 6 for measuring dissolved oxygen, wherein the electrolytic oxidation polymer film has no redox response. 8. The microelectrode according to claim 6 for measuring ion concentrations, wherein the electrolytic oxidation polymer film responds to hydrogen ions. Claim 9: Oxygen or microorganisms are immobilized near the retracted tip of the carbon fiber according to claims 1, 2 and 3.
Item 10. A microelectrode according to item 1, 4, 5, 6, 7 or 8. 10. The microelectrode according to claim 1, for use as a reference electrode, wherein the surface of the retracted carbon fiber tip is covered with silver, which is further covered with silver halide.