METHOD FOR MANUFACTURING pH SENSITIVE ELECTRODE, MEASURING METHOD EMPLOYING pH SENSITIVE ELECTRODE, MEASURING DEVICE, AND pH SENSITIVE ELECTRODE

Abstract

Provided are a method for manufacturing a pH sensitive electrode which suppresses drift of an electromotive force during pH measurement and / or has a fast response speed, and with which the same can be achieved easily, and a measuring method employing the pH sensitive electrode. The method for manufacturing a pH sensitive electrode according to the present invention includes: a manufacturing step for manufacturing a composite electrode comprising iridium and iridium oxide by thermal oxidation treatment; and a first application step for applying a voltage to the composite electrode so as to maintain a negative state with respect to an opposing electrode that opposes the composite electrode, in a state in which the composite electrode and the opposing electrode are immersed in a solution.

Description

Method for manufacturing pH-sensitive electrode, method for measuring pH-sensitive electrode, measuring device, and pH-sensitive electrode

[0001] The present invention relates to a method for manufacturing a pH-sensitive electrode, a measurement method for a pH-sensitive electrode, a measurement device, and a pH-sensitive electrode.

[0002] As background art of the present invention, for example, Patent Document 1 describes an indicator electrode for a pH sensor including a substrate, an electrode disposed on the substrate, a reactive layer disposed on a portion of the electrode, and a conductive layer disposed on the reactive material. Patent Document 1 describes that the reactive layer is iridium / iridium oxide or the like. Patent Document 1 also describes that before depositing (electropolymerizing) a conductive polymer, which is a conductive layer, on the reactive layer, a charge can be attached at a voltage of about −0.5 V to about 1.0 V at 50 mV / second, for example.

[0003] Special Publication No. 2017-532571

[0004] In the indicator electrode for a pH sensor described in Patent Document 1, attachment of charges to the surface of the reactive layer, i.e., activation, can be achieved by depositing a conductive layer on the surface of the reactive layer by electrolytic polymerization, for example, by improving the bond between the reactive layer and the conductive layer, and therefore a voltage ranging from about −0.5 V to about 1.0 V, including a positive range, is applied.

[0005] It is known that the electromotive force of a solution measured using a composite electrode made of iridium / iridium oxide prepared by thermal oxidation treatment drifts over time. It is also known that such composite electrodes have a slow response speed to pH changes. The inventors conducted various studies and found that applying a voltage including a positive range to an iridium / iridium oxide composite electrode does not solve the problems of the electromotive force drift and slow response speed. The electromotive force drift can be improved by immersing the composite electrode in water or an aqueous solution for several tens of days, but this treatment requires a long period of time and is not convenient.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for manufacturing a pH-sensitive electrode, a measurement method, a measurement device, and a pH-sensitive electrode that can easily achieve at least one of suppressing electromotive force drift and fast response speed during pH measurement.

[0007] The method for manufacturing a pH-sensitive electrode according to the present invention, which solves the above-mentioned problems, includes a manufacturing step of manufacturing a composite electrode made of iridium and iridium oxide by thermal oxidation treatment, and a first application step of applying a voltage to the composite electrode so as to maintain a negative state with respect to the counter electrode while the composite electrode and a counter electrode facing the composite electrode are immersed in a solution.

[0008] The present invention can provide a method for manufacturing a pH-sensitive electrode, a measurement method, a measurement device, and a pH-sensitive electrode that can easily achieve at least one of suppressing electromotive force drift during pH measurement and fast response speed.

[0009] Schematic diagram illustrating the configuration inside the chamber 2 of the measurement device 1. Ir / IrO with a diameter of 0.127 mm immediately after fabrication (immersion day 0). ｘ Graph (top) showing the pH response of a microelectrode (Comparative Example 1). Optical microscope photograph of an Ir wire with polished end faces taken from the side. Optical microscope photograph of an Ir wire with unpolished end faces taken from the side. Ir / IrO microelectrode with a diameter of 0.127 mm after immersion in pure water for 33 days. ｘ Graph showing the pH response of the microelectrode (Comparative Example 2) (upper graph). Graph showing the change in electromotive force over time after 267 days. Graph showing the change in electromotive force drift over time in a pH 4 buffer solution. Graph showing the change in electromotive force drift over time in a pH 9 buffer solution. Ir / IrO stored in water for 267 days. ｘ Graph showing the electromotive force response of the microelectrode. Ir / IrO stored in water for 267 days. ｘGraph showing pH response of microelectrode. Graph showing effect of voltage application (positive voltage application) on electromotive force drift in pH 4 buffer solution. Graph showing effect of voltage application (negative voltage application) on electromotive force drift in pH 4 buffer solution. Graph showing effect of voltage application (positive voltage application) on electromotive force drift in pH 9 buffer solution. Graph showing effect of voltage application (negative voltage application) on electromotive force drift in pH 9 buffer solution. Graph showing duration of voltage application effect in pH 4 buffer solution. Graph showing duration of voltage application effect in pH 9 buffer solution. Ir / IrO in pH 4 buffer solution. ｘ Graph showing the effect of applied voltage on electromotive force drift of a microelectrode. Ir / IrO in pH 9 buffer solution. ｘ Graph showing the effect of applied voltage on electromotive force drift of a microelectrode. Ir / IrO in pH 7 buffer solution. ｘ Graph showing the effect of voltage application on the electromotive force of a microelectrode. ｘ Graph showing the results of comparing differences in pH response of microelectrodes. Graph showing the variation in 90% response time when changing from pH 7 to pH 4. Graph showing the variation in 90% response time when changing from pH 7 to pH 9. Plot showing the results when changing from pH 7 to pH 4. Plot showing the results when changing from pH 7 to pH 9. Spectrograph showing the results of X-ray photoelectron spectroscopy (XPS) analysis of an iridium oxide film with no voltage applied. Ir / IrO when a voltage of -1.5 V was applied for 0, 5, 10, 20, and 40 minutes. ｘ Spectroscopic diagram showing the XPS spectrum of the microelectrode. ｘ 1 is a graph showing the relationship between electromotive force and pH indicated by a microelectrode.

[0010] The manufacturing method of a pH-sensitive electrode, the measurement method of a pH-sensitive electrode, the measurement device, and the pH-sensitive electrode will be described in detail below with reference to the drawings as appropriate. In the description of each embodiment, substantially the same or similar components are designated by the same reference numerals, and redundant descriptions may be omitted.

[0011] [Method for Manufacturing pH-Sensitive Electrode] The method for manufacturing a pH-sensitive electrode according to this embodiment (hereinafter, sometimes referred to as the present manufacturing method) includes a manufacturing step and a first application step.

[0012] (Manufacturing Process) In the manufacturing process, iridium (Ir) and iridium oxide (IrO) are produced by thermal oxidation treatment. ｘ The thermal oxidation treatment involves heating the target iridium under atmospheric conditions in which oxygen is present, such as in the air, to oxidize at least the surface of the target iridium. Specifically, the composite electrode is configured such that the surface of the metallic iridium is covered with iridium oxide produced by the thermal oxidation treatment.

[0013] The composite electrode does not necessarily have to be made entirely of iridium. The composite electrode may be, for example, a composite electrode having a coating of iridium and iridium oxide formed as an active layer on the surface of a wiring or substrate made of a metal other than iridium, such as gold, silver, copper, or iron. Even in this embodiment, the surface of the composite electrode is covered with a coating of iridium and iridium oxide, thereby achieving at least one of the effects of suppressing electromotive force drift during pH measurement and fast response speed. Furthermore, in this embodiment, the coating can be obtained simply by forming an Ir coating on the surface of the wiring or substrate, and then performing a thermal oxidation treatment in the manufacturing process, thereby easily obtaining a composite electrode with the above-mentioned effects. The Ir coating can be formed before the manufacturing process by, for example, physical vapor deposition (PVD) or plating on the surface of the wiring or the like.

[0014] The thermal oxidation treatment can be carried out, for example, as follows, but is not limited to this. 1. Ultrasonic cleaning of the Ir electrode with acetone for 10 minutes twice. (The furnace is started to be heated to 750°C.) 2. Ultrasonic cleaning of the Ir electrode with ethanol for 10 minutes twice. 3. Ultrasonic cleaning of the Ir electrode with ultrapure water for 10 minutes twice. 4. Wipe off the water on the surface of the Ir electrode with a Kimwipe, and after drying, place a gold boat in a ceramic container and place the Ir electrode obtained in steps 1 to 3 on top of it. 5. Place lithium carbonate (Li ２ CO ３6. After 3 hours, the electrode is cooled and removed after the temperature inside the furnace has returned to room temperature. 7. The gold boat with the electrode is placed in a beaker filled with ultrapure water, and the melted and solidified Li powder is stirred with a stirrer. ２ CO ３ Dissolve Li ２ CO ３ After dissolution, the composite electrode is removed and dried.

[0015] In the thermal oxidation treatment, the predetermined temperature is preferably, for example, 723 to 1000°C, and more preferably 750°C. In addition, in the thermal oxidation treatment, the predetermined time is, for example, 15 minutes to 4 hours, and more preferably 30 minutes to 150 minutes. Specifically, the predetermined time is preferably, for example, 15 minutes, 30 minutes, 45 minutes, 60 minutes, or 150 minutes.

[0016] It is preferable to polish the surface of the composite electrode before the thermal oxidation treatment. This can prevent or reduce defects such as cracking of the surface during the thermal oxidation treatment. Examples of polishing include, but are not limited to, polishing with sandpaper followed by polishing with diamond particles having a diameter of 1 μm. The composite electrode may be, for example, a wire-shaped electrode having a diameter of 0.1 mm to 1 mm, but is not limited thereto. For example, a rod-shaped or thin-film-shaped electrode may be used. Furthermore, the composite electrode may be one in which the coating is laminated on a substrate or wiring as an active layer for pH.

[0017] (First Application Step) In the first application step, a voltage is applied to the composite electrode that has been thermally oxidized and a counter electrode facing the composite electrode, while the composite electrode is immersed in a solution, so as to maintain a negative state with respect to the counter electrode. Tetravalent iridium (IrO) is deposited on the surface of the thermally oxidized composite electrode as iridium oxide. ２ ), but by carrying out this step, a part of it can be easily converted into trivalent iridium (Ir ２ O ３) can be used.

[0018] Specifically, the composite electrode after the first application step contains tetravalent iridium (IrO ２ ) to Ir ＩＶ and trivalent iridium (Ir ２ O ３ ) to Ir ＩＩＩ When Ir ＩＶ / Ir ＩＩＩ It is preferable that the ratio is 1.15 or less. ＩＶ / Ir ＩＩＩ By setting the ratio to 1.15 or less, it is possible to obtain at least one of the effects of suppressing electromotive force drift in pH measurement and achieving a fast response speed. The reason for obtaining such effects will be discussed in the section entitled "Examples."

[0019] When a voltage is applied to the counter electrode, a charge of the opposite polarity to the charge flowing through the composite electrode flows through the counter electrode. Any electrode that exhibits the above-described function with respect to the composite electrode can be used as the counter electrode. Examples of such counter electrodes include, but are not limited to, a platinum (Pt) electrode and a gold (Au) electrode.

[0020] The solution is preferably, for example, a buffer solution of pH 4 to pH 9, and more preferably a buffer solution of pH 7. The buffer solution is preferably, for example, a phosphate buffer solution. The voltage in the first application step is preferably, for example, -1 V to -2 V, and more preferably -1.25 V to -1.75 V. Specifically, the voltage can be, for example, -1.0 V, -1.25 V, -1.5 V, -1.75 V, -2.0 V, etc., with -1.5 V being more preferable. The voltage application time in the first application step is, for example, preferably 2 minutes to 90 minutes, and more preferably 5 minutes to 60 minutes. The application time is preferably, for example, 2 minutes, 5 minutes, 10 minutes, 20 minutes, 60 minutes, 90 minutes, etc., but is not limited to these.

[0021] By performing the above-described manufacturing steps and the first application step, this manufacturing method can manufacture a pH-sensitive electrode that exhibits at least one of the effects of suppressing electromotive force drift during pH measurement and achieving a fast response speed. Thus, this manufacturing method can manufacture such a pH-sensitive electrode simply by performing these steps. Therefore, this manufacturing method can manufacture the above-mentioned pH-sensitive electrode more easily than the conventional technique, which requires a long treatment period, such as immersing a composite electrode in water or an aqueous solution for several tens of days to improve the electrode.

[0022] Furthermore, the pH-sensitive electrode manufactured by this manufacturing method can be suitably used at pH 4 to pH 13 (i.e., it can exhibit a stable pH response). The pH-sensitive electrode manufactured by this manufacturing method exhibits a linear response at pH 4 to pH 13, and the slope of the measured electromotive force relative to pH change nearly matches the theoretical value. The pH-sensitive electrode manufactured by this manufacturing method has a response speed to pH change on the order of subseconds. These characteristics will be explained in the "Examples" section. The pH-sensitive electrode manufactured by this manufacturing method can maintain the above-mentioned effects for several weeks (e.g., about two weeks).

[0023] [pH-Sensitive Electrode Measurement Method] The pH-sensitive electrode measurement method according to this embodiment (hereinafter sometimes referred to as the present measurement method) includes a second application step in which, before measuring the pH of a target object using a composite electrode made of iridium and iridium oxide produced by thermal oxidation treatment, a voltage is applied to the composite electrode so as to maintain a negative state with respect to the counter electrode while the composite electrode and a counter electrode facing the composite electrode are immersed in a solution.

[0024] In other words, the second application step in this measurement method is performed to condition the surface of an already manufactured composite electrode (preferably a pH-sensitive electrode manufactured by the above-described present manufacturing method) before measuring the pH of the object to be measured using the composite electrode. In contrast, the first application step in the above-described present manufacturing method differs from the second application step in that it is performed during the initial manufacturing of the composite electrode.

[0025] By carrying out the second application step, the composite electrode (pH-sensitive electrode) is made of tetravalent iridium (IrO ２ ) to Ir ＩＶ and trivalent iridium (Ir ２ O ３ ) to Ir ＩＩＩ When ＩＶ / Ir ＩＩＩ The ratio can be set to 1.15 or less.

[0026] The composite electrode (pH-sensitive electrode) manufactured by the first application step and the composite electrode whose surface has been adjusted by the second application step can maintain the effects described in the present manufacturing method for several weeks (e.g., about two weeks). The second application step can be performed whenever it is determined that the effects have decreased. The second application step can also be performed at an appropriate time, such as during maintenance performed every two weeks on a measurement device using a composite electrode (pH-sensitive electrode). The conditions for the second application step, such as the counter electrode, solution, voltage, and voltage application time, are the same as those for the first application step described above, and therefore will not be described here.

[0027] The measurement target can be any liquid whose pH can be measured, including, but not limited to, cell culture fluid, aqueous solutions, waste liquids, and effluents containing specific chemical substances, drinking water, beverages, solutions and suspensions containing cells or biological samples, and blood.

[0028] [Measuring Device] FIG. 1 is a schematic diagram illustrating the configuration inside a chamber 2 of a measuring device 1. As shown in FIG. 1, the measuring device 1 according to this embodiment includes a first measuring unit 3 and a second measuring unit 4 located near the center of the chamber 2. Examples of the measuring device 1 include a surface plasmon resonance (SPR) device, which measures the amount of a substance by flowing a specific chemical substance, biological sample, or cell as the measurement target. Such measuring devices have a minute space serving as the measurement region, which corresponds to the chamber 2 described above. The interior of the chamber 2 is, for example, approximately 20 mm long, approximately 2 mm wide, and approximately 0.5 mm high. The chamber 2 has an inlet 44 at one end in the longitudinal direction through which the measurement target is introduced. The chamber 2 has an outlet 45 at the other end in the longitudinal direction through which the measurement target is discharged. The measurement target is introduced into the chamber 2 through the inlet 44, passes through the second measuring unit 4 and the first measuring unit 3, and is then discharged from the chamber 2 through the outlet 45.

[0029] The first measurement unit 3 measures components of a substance contained in the measurement target. The substance contained in the measurement target may be anything that can be measured by the first measurement unit 3, and examples thereof include a variety of substances such as specific chemical substances, proteins, and antibodies. In other words, the measurement target and application of the measurement device 1 according to this embodiment are specified to some extent by the first measurement unit 3. Suitable examples of the measurement device 1 according to this embodiment include biomeasurement devices such as the SPR device described above.

[0030] The second measurement unit 4 measures the pH of the measurement target. The second measurement unit 4 includes a composite electrode 41 (pH-sensitive electrode 42) made of iridium and iridium oxide, and a counter electrode 43 facing the composite electrode 41. The measurement target and counter electrode 43 are as described above. The first measurement unit 3 and the second measurement unit 4 may be installed in the same location. It is preferable to install the first measurement unit 3 and the second measurement unit 4 as close as possible so that the pH of the measurement unit can be measured.

[0031] The composite electrode 41 has, at least on the surface thereof, tetravalent iridium (IrO２ ) to Ir ＩＶ and trivalent iridium (Ir ２ O ３ ) to Ir ＩＩＩ When ＩＶ / Ir ＩＩＩ It is preferable that the ratio is 1.15 or less, which can be obtained and maintained by manufacturing the surface by the above-mentioned manufacturing method or by adjusting the surface by the above-mentioned measurement method.

[0032] Therefore, the measurement device 1 according to this embodiment, which includes the second measurement unit 4, achieves at least one of the following effects: suppression of electromotive force drift during pH measurement and a fast response speed. Furthermore, the measurement device 1 according to this embodiment can suitably measure pH at pH 4 to pH 13 (i.e., can exhibit a stable pH response). The measurement device 1 according to this embodiment exhibits a linear response at pH 4 to pH 13, and the slope of the measured electromotive force relative to pH change nearly matches the theoretical value. The measurement device 1 according to this embodiment has a response speed to pH change on the order of subseconds. The measurement device 1 according to this embodiment can maintain the above-described effects for several weeks (e.g., about two weeks). The measurement device 1 according to this embodiment can be used simply as a pH measurement device when the first measurement unit 3 is not functioning.

[0033] Incidentally, measurement devices such as the SPR device described above have a problem in that the measurement region is a very small space (chamber 2), making it extremely difficult to measure the pH inside the chamber 2. For such measurement devices, there is a demand for real-time monitoring of the pH inside the chamber 2. Furthermore, for such measurement devices, there is a demand for in situ online monitoring, if possible. The measurement device 1 according to this embodiment has the composite electrode 41 (pH-sensitive electrode 42) configured as described above, and exhibits the various effects described above, thereby addressing these problems and demands.

[0034] [pH-Sensitive Electrode] The pH-sensitive electrode 42 according to this embodiment is a composite electrode 41 made of iridium and iridium oxide. The pH-sensitive electrode 42 according to this embodiment contains tetravalent iridium (IrO ２ ) to Ir ＩＶ and trivalent iridium (Ir ２ O ３ ) to Ir ＩＩＩ In this case, Ir ＩＶ / Ir ＩＩＩ The ratio is 1.15 or less, which can be obtained and maintained by manufacturing the material using the above-mentioned manufacturing method and adjusting the surface using the above-mentioned measurement method.

[0035] Because of the above configuration, the pH-sensitive electrode 42 according to this embodiment can achieve at least one of the effects of suppressing electromotive force drift during pH measurement and a fast response speed. Furthermore, the pH-sensitive electrode 42 according to this embodiment can suitably measure pH at pH 4 to pH 13 (i.e., can exhibit a stable pH response). The pH-sensitive electrode 42 according to this embodiment exhibits a linear response at pH 4 to pH 13, and the slope of the measured electromotive force relative to pH change nearly matches the theoretical value. The pH-sensitive electrode 42 according to this embodiment has a response speed to pH change on the order of subseconds. The pH-sensitive electrode 42 according to this embodiment can maintain the above-mentioned effects for several weeks (e.g., about two weeks).

[0036] In the pH-sensitive electrode 42 according to this embodiment, the band energy structure of iridium oxide preferably has an unfilled energy level 1 eV higher than the Fermi level. Having an unfilled energy level facilitates electron exchange via this energy level. As a result, the pH-sensitive electrode 42 according to this embodiment can more reliably suppress the electromotive force drift during pH measurement and more reliably improve the response speed.

[0037] Furthermore, the pH-sensitive electrode 42 according to this embodiment preferably has an average film thickness of 5 μm or less. In this case, the average film thickness of the iridium oxide is appropriate, making it difficult for the iridium oxide coating to peel off from the surface of the pH-sensitive electrode 42. As a result, the pH-sensitive electrode 42 can exert the above-described effects for a longer period of time.

[0038] Next, the present invention will be described more specifically with reference to examples.

[0039] (1) Comparative Examples (1-1) Comparative Examples 1 and 2 The present inventors produced Comparative Examples 1 and 2 as follows. The outline of Comparative Examples 1 and 2 is as follows. Comparative Example 1 was produced by subjecting an iridium (Ir) thin wire to an oxidation heat treatment to produce an iridium / iridium oxide (Ir / IrO ｘ In Comparative Example 2, a microelectrode, i.e., a pH-sensitive electrode, was fabricated. ｘ The microelectrode was immersed in pure water for 33 days.

[0040] (1-2) Reagents and Materials In this study, the prepared IrO ｘ Ultrapure water was used for conditioning the electrodes. pH 4, pH 7, and pH 9 standard solutions (pH standard solution set 101-S, Horiba, Ltd., Japan) and homemade pH 7 buffer solution NPBS and pH 13 KOH solution HPCS were used for measuring the pH response. ｘ The fabrication of the Li-Iridium wire (diameter 1mm and 0.127mm, purity 99.9%, Nilaco Co., Ltd., Japan) ２ CO ３ The powder (anhydrous powder, 99.9% purity, Wako Pure Chemical Industries, Japan) was placed in an electric furnace, and Li ２ CO ３ was melted and fabricated by thermal oxidation.

[0041] (1-3) Ir / IrO ｘFabrication of microelectrodes: Iridium wires (Ir wires) with diameters of 1 mm and 0.127 mm were cut with nippers and the end faces were polished. Approximately 20 mm long Ir wires were sandwiched between pieces of wood (cypress) and fixed with screws, and polished with sandpaper and 1 μm diameter diamond particles.

[0042] Thermal oxidation of the Ir wire was carried out using an electric furnace according to the following protocol. 1. The Ir wire was ultrasonically cleaned in acetone for 10 minutes twice. (The furnace was heated to 750°C.) 2. The Ir wire was ultrasonically cleaned in ethanol for 10 minutes twice. 3. The Ir wire was ultrasonically cleaned in ultrapure water for 10 minutes twice. 4. The water on the surface of the Ir wire was wiped off with a Kimwipe, and after drying, a gold boat was placed in a ceramic container, and the Ir wire obtained in steps 1 to 3 was placed on top of it. 5. Lithium carbonate (Li ２ CO ３ 6. After 3 hours, the Ir wire was cooled and the temperature in the furnace returned to room temperature before being removed. 7. The gold boat containing the thermally oxidized Ir wire was placed in a beaker filled with ultrapure water, and the melted and solidified Li was stirred with a stirrer. ２ CO ３ 8. Li ２ CO ３ After dissolving, the composite electrode (Ir / IrO ｘ The wire was taken out and dried. ｘ A microelectrode was fabricated.

[0043] (1-4) pH response evaluation system Fabricated Ir / IrO ｘ The pH response of the microelectrode was evaluated in a batch mode. ｘ The microelectrode and the reference electrode were connected to an electrometer (Keithley, Model 6417), and the time change in electromotive force was measured using custom-made measurement software.

[0044] (1-5) Results and Considerations for Comparative Examples 1 and 2 The present inventors have investigated the results of the thermal oxidation of Ir / IrO2 particles with diameters of 1 mm and 0.127 mm by using molten lithium carbonate at 750°C for 150 minutes. ｘ The microelectrode was stored immersed in ultrapure water, and the pH response was measured over time.

[0045] Figure 2 shows the Ir / IrO crystal with a diameter of 0.127 mm immediately after fabrication (immersion day 0). ｘ 2 is a graph (upper figure (upper diagram in horizontally written Figure 2)) showing the pH response of the microelectrode (Comparative Example 1). In the figure, the horizontal axis represents time (sec.) and the vertical axis represents electromotive force (V vs. Ag / AgCl). The graph at the bottom left of the figure shows the pH response of four Ir / IrO microelectrodes prepared under the same conditions. ｘ 1 is a graph showing the magnitude of electromotive force at a microelectrode versus pH. In the figure, the horizontal axis represents pH, and the vertical axis represents electromotive force (V vs. Ag / AgCl). The figure also shows the calibration lines (y = -0.0571x + 0.7852, y = -0.058x + 0.7635, y = -0.0574x + 0.785, y = -0.0542x + 0.714) and the coefficient of determination (R ２ = 0.9987, R ２ = 0.9976, R ２ = 0.9979, R ２ = 0.9993) is also shown. Figure 3 is an optical microscope photograph of an Ir wire with polished end faces taken from the side. Figure 4 is an optical microscope photograph of an Ir wire with unpolished end faces taken from the side. Figures 3 and 4 show that end face processing improves the flatness of the cut surface. For pH response, the end face processed surface was used as a pH electrode. In Figure 2, the electromotive force changes when the pH of the solution is changed from 7 to 4 to 7 to 9 to 7. It can be seen that the electromotive force drifts when immersed in solutions of different pHs and requires time to stabilize. In particular, it can be seen that the electromotive force drift is large when immersed in an HPCS solution with a pH of 13.

[0046] The inventors have prepared two Ir / IrO ｘThe microelectrode was continuously immersed in ultrapure water, and the time-dependent change in pH response was measured in the same manner. Figure 5 shows the time-dependent change in pH response of an Ir / IrO microelectrode with a diameter of 0.127 mm after 33 days of immersion in pure water. ｘ 5 is a graph (upper figure (the upper diagram in the horizontally written upper part of FIG. 5)) showing the pH response of the microelectrode (Comparative Example 2). In the figure, the horizontal axis indicates time (sec.), and the vertical axis indicates electromotive force (V vs. Ag / AgCl). The graph at the bottom left of the figure shows the pH response of two Ir / IrO microelectrodes prepared under the same conditions. ｘ 1 is a graph showing the magnitude of electromotive force at a microelectrode versus pH. In the figure, the horizontal axis represents pH, and the vertical axis represents electromotive force (V vs. Ag / AgCl). The figure also shows the calibration lines (y = -0.0597x + 0.8123, y = -0.0593x + 0.7948) and coefficients of determination (R ２ = 0.9973, R ２ = 0.9974) is also shown.

[0047] As shown in Figure 5, after 33 days of immersion in pure water, the electromotive force drift was sufficiently small, and a stable pH response was obtained. ｘ Similar results were obtained for the microelectrode both immediately after fabrication and after 33 days of immersion in pure water (not shown).

[0048] (1-6) Ir / IrO ｘ Long-term stability of the microelectrodes. ｘ Two microelectrodes were continuously immersed in ultrapure water for storage, and their pH response was measured intermittently to evaluate their long-term stability. One of the microelectrodes was continuously evaluated for over 100 days.

[0049] Figure 6 is a graph showing the change in electromotive force over time 267 days after fabrication. In the figure, the horizontal axis shows MilliQ soaking time (number of days immersed in ultrapure water) (days), and the vertical axis shows electromotive force (mV vs. Ag / AgCl). Figure 7 is a graph showing the change in electromotive force drift over time in a pH 4 buffer solution. In the figure, the horizontal axis shows MilliQ soaking time (number of days immersed in ultrapure water) (days), and the vertical axis shows drift (mV / min). Figure 8 is a graph showing the change in electromotive force drift over time in a pH 9 buffer solution. In the figure, the horizontal axis represents MilliQ soaking time (days of immersion in ultrapure water) and the vertical axis represents drift (mV / min). ｘ 10 is a graph showing the electromotive force response of a microelectrode. In the figure, the horizontal axis represents time (sec.) and the vertical axis represents electromotive force (V vs. Ag / AgCl). FIG. 10 shows the electromotive force response of an Ir / IrO microelectrode stored in water for 267 days. ｘ 7A and 7B are graphs showing the pH response of a microelectrode. In the figure, the horizontal axis represents pH, and the vertical axis represents electromotive force (V vs. Ag / AgCl). In Figures 7 and 8, the drift amount on the vertical axis is defined as the difference in electromotive force measured 2 minutes and 7 minutes after the electrode immersion solution was changed from a pH 7 buffer solution to a solution with a different pH, divided by the elapsed time of 5 minutes, i.e., the slope of the electromotive force difference. The drift amount in the other graphs is also defined in the same way.

[0050] As shown in Figures 7 and 8, the electromotive force drift was large for approximately 20 days after the start of the evaluation, resulting in a slope sensitivity response greater than the theoretical value (59.2 mV / pH). Furthermore, as shown in Figure 6, the electromotive force change was also large. As shown in Figures 7 and 8, the electromotive force drift decreased after approximately 30 days, and the slope sensitivity remained above 55 mV / pH. Furthermore, as shown in Figure 6, while a gradual decrease in the electromotive force was observed, the change was small, settling at approximately 300 mV vs. Ag / AgCl. In this study, a 0.1 M KCl solution was used as the internal electrolyte of the reference electrode, resulting in a shift in the electromotive force vs. Ag / AgCl of several tens of mV compared to reference electrodes using commonly used 3 M KCl or saturated KCl solutions.

[0051] As shown in FIG. 9, the Ir / IrO measured on the 267th day after the start of immersion in ultrapure water ｘ The potential response of the microelectrode was good, and the electromotive force drift that occurred when the buffer solution was changed was extremely small. As shown in Figure 10, the Ir / IrO ｘ The pH response of the microelectrode was good, and the coefficient of determination (R ２ ) was 1. The Ir / IrO ｘ The slope sensitivity of the microelectrode was 58.4 mV / pH, close to the theoretical value (59.2 mV / pH). Furthermore, the electromotive force drift in pH 4 and pH 9 buffer solutions was −0.094 mV / min and −0.142 mV / min, respectively.

[0052] From the above, Ir / IrO produced by thermal oxidation at 750 °C for 150 minutes ｘ When the microelectrode was stored immersed in ultrapure water, it showed good pH response for more than 267 days, demonstrating excellent long-term stability. In the examples described below, the goal is to achieve electromotive force drift at the levels shown in Figures 5 and 9.

[0053] (2) Examples (2-1) Reduction of electromotive force drift due to voltage application Conditioning in water reduces the electromotive force drift of Ir / IrO ｘIt was shown that the potential stability of the microelectrode was improved. However, it took a long time of several weeks to reach a sufficiently stable potential. In order to achieve sufficient potential stability in a shorter time, the inventors have investigated the effects of immersion in hydrochloric acid and the addition of sodium peroxide (Na ２ O ２ We tried various methods, including thermal oxidation with a catalyst, thermal oxidation without a catalyst, and analysis of the remaining lithium carbonate. None of these methods were able to improve the initial electromotive force drift. ｘ The method of applying a voltage to a microelectrode was found to have a dramatic effect of improving electromotive force drift. In this study, Ir / IrO ｘ We developed a method to immerse a microelectrode and a Pt electrode and apply a DC voltage for a certain period of time, and optimized the voltage application conditions.

[0054] (2-2) Polarity Dependence of Voltage Application The present inventors carried out thermal oxidation at 750°C for 150 minutes using an Ir wire with a diameter of 0.127 mm and lithium carbonate as a catalyst. After that, the present inventors dissolved the lithium carbonate in ultrapure water, visually confirmed its removal, and then determined the Ir / IrO ｘ A microelectrode was fabricated (see 1. to 8. above). The inventors applied a voltage of +1 V or -1 V to the resulting electrode relative to a platinum counter electrode for 10 or 60 minutes, and measured the pH response. The inventors applied a voltage of ±1 V, and the next day, they applied +1.5 V to the electrode to which +1 V had been applied, and -1.5 V to the electrode to which -1 V had been applied, to evaluate the pH response. When the applied potential was measured using a reference electrode, -1.5 V was measured as -1.36 V vs. Ag / AgCl. Figures 11A, 11B, 12A, and 12B show the pH response of Ir / IrO when the pH 7 buffer solution was changed to pH 4 and pH 9 buffer solutions, respectively. ｘ The figure shows the results of comparing the magnitude of electromotive force drift of the microelectrode before and after voltage application.

[0055] Figure 11A is a graph showing the effect of voltage application (positive voltage application) on electromotive force drift in a pH 4 buffer solution. Figure 11B is a graph showing the effect of voltage application (negative voltage application) on electromotive force drift in a pH 4 buffer solution. Figure 12A is a graph showing the effect of voltage application (positive voltage application) on electromotive force drift in a pH 9 buffer solution. Figure 12B is a graph showing the effect of voltage application (negative voltage application) on electromotive force drift in a pH 9 buffer solution. In each of these figures, the vertical axis represents Drift (mV / min).

[0056] As shown in Figures 11B and 12B, it was found that in both pH 4 and pH 9 buffer solutions, the electromotive force drift was dramatically reduced when a voltage of -1.5 V was applied for 10 minutes and 60 minutes. On the other hand, as shown in Figures 11A and 12A, no effect of reducing the electromotive force drift was observed when a positive voltage was applied. Furthermore, as shown in Figure 12B, a slight tendency for the electromotive force drift in the pH 9 buffer solution to be reduced when a voltage of -1.0 V was applied was observed. However, the effect was limited.

[0057] The inventors investigated whether the drift reduction effect caused by applying a voltage of −1.5 V was transient or persistent. ｘ After applying a voltage of −1.5 V to the microelectrode, the pH response was measured while the microelectrode was stored in water, and the time-dependent change in electromotive force drift was measured. The results are shown in Figures 13 and 14.

[0058] Figure 13 is a graph showing the sustainability of the effect of voltage application in a buffer solution of pH 4. Figure 14 is a graph showing the sustainability of the effect of voltage application in a buffer solution of pH 9. In each of these figures, the horizontal axis represents MilliQ soaking time (number of days immersed in ultrapure water) (days), and the vertical axis represents drift (mV / min).

[0059] As shown in Figures 13 and 14, it was found that the electromotive force drift reduced by applying a voltage of -1.5 V was maintained for at least about two weeks in both the pH 4 and pH 9 buffer solutions. ｘIt was found that applying a voltage of -1.5 V to the microelectrode with a Pt electrode as the counter electrode dramatically reduced the electromotive force drift, and this effect lasted for more than two weeks.

[0060] (2-3) Dependence on the magnitude of applied voltage In order to quantitatively express the above qualitative explanation, the inventors compared the electromotive force drift in buffer solutions of pH 4 and pH 9, and the electromotive force in a buffer solution of pH 7 before and after voltage application treatment. Figures 15, 16, and 17 show the results, respectively.

[0061] FIG. 15 shows the Ir / IrO ｘ 16 is a graph showing the effect of applied voltage on the electromotive force drift of a microelectrode. ｘ 17 shows the effect of voltage application on the electromotive force drift of a microelectrode. In each of these figures, the vertical axis represents Drift (mV / min). ｘ 1 is a graph showing the effect of voltage application on the electromotive force of a microelectrode, in which the vertical axis represents electromotive force (mV vs. Ag / AgCl).

[0062] As shown in FIGS. 15 and 16, Ir / IrO ｘ When a voltage between -1.0 V and -2.0 V was applied to the microelectrode and it was stored in ultrapure water for one day, the electromotive force drift of the electrode applied with a voltage of -1.0 V and -1.25 V increased. The magnitude of the increase was greater for the electrode applied with a voltage of -1.0 V than for the electrode applied with a voltage of -1.25 V. Therefore, it was found that the electromotive force drift suppression effect depends on the magnitude of the applied voltage, and that the electromotive force drift suppression effect is small when the applied voltage is less than -1.25 V. As shown in Figures 15 and 16, when a voltage of -1.5 V was applied for a second time for 10 minutes, the electromotive force drift suppression effect lasted for at least three days.

[0063] As shown in FIG. ｘThe electromotive force of the microelectrodes decreased with voltage application, but increased after one day of storage in ultrapure water. The smaller the applied voltage, the smaller the decrease in electromotive force, and a tendency for a high electromotive force to be maintained was observed. After the second application of a voltage of -1.5 V, a gradual increase in the electromotive force of all electrodes was observed.

[0064] (2-4) Voltage application time dependency The inventors compared (i) a case where a voltage was applied continuously for 20 minutes at −1.5 V, and (ii) a case where a voltage was applied at −1.5 V for 10 minutes, then left to stand for 5 minutes, and then a voltage was applied again at −1.5 V for 10 minutes. ｘ 18 is a graph showing the results of comparing the pH response of microelectrodes. The upper graph in the figure (the upper graph in the horizontal column of FIG. 18) shows the pH response of Ir / IrO before voltage application (as thermally oxidized (As thermally oxidized)). ｘ The graph at the bottom of the figure (the bottom graph in horizontally written Fig. 18) shows the pH response of the Ir / IrO microelectrode after the thermal oxidation treatment and voltage application by the above two methods (i) and (ii). ｘ In each of these graphs, the horizontal axis represents time (sec.), and the vertical axis represents electromotive force (V vs. Ag / AgCl).

[0065] As shown in Figure 18, in both methods (i) and (ii), the electromotive force drift was reduced after voltage application, and a stable potential response was obtained. This effect was sustained for 6 days after voltage application. ｘ From the perspective of minimizing stress and damage to the microelectrode, a short voltage application time is preferable. Therefore, it was concluded that a voltage application time of 10 minutes is preferable. In this study, from the perspective of reducing electromotive force drift, a voltage application of -1.5 V for 10 minutes was determined to be the optimal condition for the time being. It is believed that further optimization can be achieved by analyzing the mechanism of electromotive force drift reduction due to negative voltage application, taking into account response speed, long-term stability, and other aspects.

[0066] (2-5) Response speed improvement by shortening the oxidation time ｘ The inventors considered that it is important to minimize the diffusion of ions on the surface of the film and to efficiently exchange buffer solutions of different pH levels on the surface in order to speed up the response. ｘ IrO by shortening the oxidation time in the hope of flattening the film ｘ The inventors investigated the thinning of the Ir / IrO film by thermal oxidation at 750°C for 150 or 60 minutes. ｘ The 90% response time of the microelectrodes was compared when no voltage was applied and when a voltage of -1.5 V was applied for 10 minutes. The 90% response time refers to the time required for the difference in electromotive force before and after exchanging solutions with different pHs to reach 90%. Figure 19 is a graph showing the variation in 90% response time when changing the pH from 7 to 4. Figure 20 is a graph showing the variation in 90% response time when changing the pH from 7 to 9. In both graphs, the vertical axis represents the 90% response time (seconds).

[0067] As shown in FIGS. 19 and 20, the oxidation time was shortened to 60 minutes, and IrO ｘ It was found that thinning the membrane shortened the response time and reduced the variation. The 90% response times when the pH was changed from 7 to 4 and 9 were on the order of subseconds, from 0.3 to 0.4 seconds and from 0.5 to 0.6 seconds, respectively. This indicates that the Ir / IrO ｘ It was comparable to that of microelectrodes.

[0068] (2-6) Dependence of response speed on oxidation time Next, the inventors of the present invention have investigated the oxidation time dependence of response speed by adding Ir / IrO ｘ The 90% response time was calculated from the change in potential response when the microelectrode was immersed, and the effect of negative voltage application on the response speed (oxidation time dependence of response speed) was examined. In this study, Ir / IrO was treated at a thermal oxidation temperature of 750 °C for a thermal oxidation time of 15, 30, 45, 60, or 150 minutes. ｘA voltage of -1.5 V was applied to the microelectrode for 10 minutes (indicated as "with application" in Figures 21 and 22), and the 90% response time was determined. Specifically, the average value of the stabilized electromotive force in each of the pH 4, pH 7, and pH 9 buffer solutions was calculated from the real-time response waveforms of 50 counts / sec, and the time required to reach 90% of the potential change was defined as the 90% response time, which was used as an index of the response speed.

[0069] Fig. 21 is a plot diagram showing the results when the pH was changed from 7 to 4. Fig. 22 is a plot diagram showing the results when the pH was changed from 7 to 9. In both of these graphs, the vertical axis indicates the 90% response time [seconds]. Here, Figs. 21 and 22 show the results of an Ir / IrO sample whose surface has been sufficiently conditioned by long-term storage in ultrapure water. ｘ The results for the microelectrode are shown as Au / Ir for reference. The results for the case where thermal oxidation was performed for 150 minutes but no voltage was applied (shown as "no voltage applied" in Figures 21 and 22) are also shown in the same figures.

[0070] As shown in Figures 21 and 22, even when voltage was applied (indicated as "with application" in Figures 21 and 22), the speed at which the response reached 90% was the slowest under the longest condition of 150 minutes among those investigated, and a tendency for the response speed to gradually increase under conditions shorter than 60 minutes was observed.

[0071] (2-7) Consideration of the mechanism of electromotive force stabilization by applying -1.5 V The present inventors have investigated the mechanism of electromotive force stabilization by applying -1.5 V to Ir / IrO ｘ The microelectrode was immersed in ultrapure water, and pH response measurements were performed intermittently to examine the change in pH characteristics over time. ｘ The drift of the electromotive force of Ir / IrO decreases over a period of several weeks to several months, and the electromotive force approaches a constant value. ｘ Immediately after the formation of IrO ｘ The film is in a non-equilibrium state, and the relaxation phenomenon with a large time constant of several weeks to several months causes the Ir / IrO ｘ It is believed that the microelectrode system approaches an equilibrium state. ｘWhen a voltage of −1.5 V is applied to the microelectrode in a phosphate buffer solution of pH 7 with a Pt electrode as the counter electrode, this relaxation phenomenon is thought to proceed rapidly, and an equilibrium state is reached in a short time.

[0072] 23 is a spectrum diagram showing the results of X-ray photoelectron spectroscopy (XPS) of an iridium oxide film with no applied voltage, where the horizontal axis represents binding energy (eV) and the vertical axis represents intensity (cps).

[0073] As shown in Figure 23, the literature values ​​of the binding energy of iridium are Ir (metal): 60.8 eV, Ir (IV): 61.5 eV, and Ir (III): 62.4 eV. IrO with a rutile crystal structure ２ It is known that in the XPS spectrum of tetravalent iridium, the shape of the Ir(IV) 4f peak of tetravalent iridium is asymmetric, with a gentle base on the high-energy side and high intensity. However, this spectral shape cannot be perfectly fitted even with the conventional Doniach-Sunjic (DS) function. As analyzed in several papers, it can be fitted using the Ir(III) 4f peak of trivalent iridium, and tetravalent and trivalent iridium oxide (IrO) with different oxidation states can be fitted. ２ and Ir ２ O ３ The mixing ratio of trivalent iridium oxide (Ir ２ O ３ ), without assuming the presence of tetravalent iridium oxide (IrO ２Theoretical calculations have shown that even in the case of ZnO, there exists an unoccupied energy level (satellite level 1) approximately 1 eV above the Fermi level (EF) and an occupied energy level (satellite level 2) approximately 2 eV below the EF (Reference 1: Verena Pfeifer, Travis E. Jones, Juan J. Velasco Velez, Cyriac Massue, Rosa Arrigo, Detre Teschner, Frank Girgsdies, Michael Scherzer, Mark T. Greiner, Jasmin Allan, Maike Hashagen, Gisela Weinberg, Simone Piccinin, Michael Havecker, Axel Knop-Gericke and Robert Schlogl, Surf. Interface Anal., 2016, 48, 261-273).

[0074] When electrons at the Fermi level are excited to satellite level 1, a peak for satellite level 1 appears at a binding energy approximately 1 eV higher than the Ir(IV) 4f peak in the XPS spectrum. Furthermore, when electrons are excited from satellite level 2, which is occupied by electrons, to satellite level 1, a peak for satellite level 2 appears at a binding energy approximately 3 eV higher than the Ir(IV) 4f peak. Using the peaks for satellite levels 1 and 2, the Ir(IV) 4f peak can be fitted using only tetravalent iridium.

[0075] On the other hand, amorphous IrO ２ In the XPS spectrum of tetravalent iridium, the shape of the Ir(IV) 4f peak is similar to that of IrO with a rutile crystal structure. ２ It has been reported that accurate fitting to experimental data is possible by using the Ir(III) 4f peak of trivalent iridium in addition to the two satellite levels mentioned above (see Reference 1 above). ２ Ir in ２ O ３The mechanism by which IrO is formed has been explained as the generation of an Ir vacancy (vacancy) that creates a defect in the electronic state. According to theoretical calculations, the generation of an Ir vacancy creates a new energy level at the Ir 4f binding energy of 62.2 eV. This is based on Ir(III), and a satellite level is formed at a binding energy position that is approximately 1 eV higher than the Ir 4f spectrum. This results in the formation of amorphous IrO. ２ This explains the increase in intensity on the high energy side of the Ir(IV) 4f peak shape.

[0076] The above discussion is based on the experimental results of XPS and near-edge X-ray absorption fine structure (NEXAFS) and their fitting results, and describes the surface area of ​​iridium oxide (XPS: several nm, NEXAFS: up to 100 nm). ｘ The film had a granular and columnar shape, and was considered to be a film with localized crystallinity. From the results of XPS experiments, it was found that tetravalent iridium (IrO ２ ) and trivalent iridium (Ir ２ O ３ ) was suggested to be a mixed membrane.

[0077] Various models have been proposed for the pH response mechanism of iridium oxide. The following three equilibrium states of the oxidation-reduction reaction of iridium oxide related to the pH response of electromotive force are considered (Reference 2: V. T.Y. Kim, S. Yang, Fabrication method and characterization of electrodeposited and heat-treated iridium oxide films for pH sensing, Sens. Actuators B: Chem. 196 (2014) 31-38). ２ O ３ +6H ＋ +6e － ⇔2Ir + 3H ２ O ... (1) IrO ２ +4H ＋ +4e－ ⇔Ir + 2H ２ O...(2) 2IrO ２ +2H ＋ +2e － ⇔Ir ２ O ３ +H ２ O...(3)

[0078] Since the above reactions with protons are all one-electron reactions, the electromotive force E generated at the interface between iridium oxide and the sample solution can be expressed by the Nernst equation (4), which is consistent with the experimental results obtained so far. ０ -2.303 (RT / nF) pH=E ０ −0.059 pH (4) where E: oxidation-reduction potential [mV], E ０ : standard potential [mV], R: gas constant (8.314 JK －１ mol －１ ), T: absolute temperature, F: Faraday constant (96,480 Cmol －１ ), n: valence, pH: pH value.

[0079] Ir / IrO ｘ When a voltage of -1.5 V was applied to the microelectrode in a phosphate buffer solution of pH 7 with a Pt electrode as the counter electrode, Ir / IrO ｘ A reduction reaction occurs at the microelectrode. Therefore, the above formulas (1) to (3) become formulas (5) and (6). Ir ２ O ３ +6H ＋ +6e － → 2Ir + 3H ２ O ... (5) IrO ２ +4H ＋ +4e － →Ir+2H ２ O...(6) 2IrO ２ +2H ＋ +2e － →Ir ２ O ３ +H ２ O...(7)

[0080] As can be seen from the above equation, this reaction requires contact with an aqueous solution, and therefore reactions (5) to (7) are ｘThis reaction occurs only on the surface. This is also suggested by the fact that the current rapidly decays when a voltage of -1.5 V is applied.

[0081] FIG. 24 shows the Ir / IrO ｘ 1 is a spectroscopic diagram showing an XPS spectrum of a microelectrode, in which the horizontal axis represents binding energy (eV) and the vertical axis represents intensity (cps).

[0082] As shown in Figure 24, focusing on the Ir(IV) 4f7 / 2 peak (see Figure 23), the peak shape is asymmetric, the base on the high-energy side is gentler, and the intensity is higher. This is particularly noticeable for the electrode to which a voltage of -1.5 V was applied for 40 minutes, where a new shoulder peak is observed around 63 eV. Table 1 shows the results of an analysis in which the binding energy peak position is free.

[0083]

[0084] As shown in Table 1, as the time for applying a voltage of −1.5 V increases, the amount of tetravalent iridium (IrO ２ ) (Ir(IV)) decreases, and trivalent iridium (Ir ２ O ３ ) (Ir(III)) ratio increases. Before voltage application, Ir(IV) / Ir(III) was 1.23, but after applying -1.5 V for 40 minutes, Ir(IV) / Ir(III) decreased to 0.77. This is thought to be because the reaction of formula (7) progressed due to the application of -1.5 V. On the other hand, as mentioned above, the binding energy of Ir(metal) is 60.8 eV, and no peak is seen near 60.8 eV in Figure 23, and no voltage application time dependency is also seen. From this, it is thought that the reactions of (5) and (6) hardly progressed. This is because the application of -1.5 V voltage caused the Ir / IrO ｘ The surface is made of iridium oxide (IrO) because this stabilizes the electromotive force of the microelectrode. ｘIt is supported that the above is true. When the application time of a voltage of -1.5 V is 10 minutes (Ir(IV) / Ir(III) = 1.11), the effect of the present invention can be achieved, in that the composite electrode (pH-sensitive electrode) has at least one of suppression of electromotive force drift during pH measurement and a fast response speed, and can be easily obtained. Note that it is believed that the same effect can be achieved if the application time is 5 minutes or more, that is, if Ir(IV) / Ir(III) is 1.15 or less.

[0085] From the above, when a voltage of −1.5 V is applied, IrO ｘ It has been shown that the proportion of Ir(III) increases near the surface of the SiO2. Also, theoretical considerations in the literature suggest that tetravalent iridium (IrO ２ ) among trivalent iridium (Ir ２ O ３ It has been reported that the physical picture of the generation of Ir vacancies is the creation of defects in the electronic state due to Ir vacancies. This creates an unoccupied energy level approximately 1 eV above the Fermi level, causing excitation of electrons from the Fermi level or from the occupied energy level approximately 2 eV below the Fermi level, which allowed for accurate fitting of the shape of the Ir(IV) 4f peak in XPS. It is also believed that the creation of a new energy level approximately 1 eV above the Fermi level plays an important role in the smooth transfer of electrons to the valence band.

[0086] IrO ｘ When in contact with an aqueous solution and in equilibrium, IrO ｘ The Fermi level of the aqueous solution is constant. When the hydrogen ion concentration in the aqueous solution changes and the electrochemical potential changes, the Fermi level of the aqueous solution changes and a new equilibrium state is reached, causing electron transfer according to equations (1) to (3). If the electron transfer does not occur smoothly at this time, the electromotive force will not be stable and will change (drift) over time. IrO ｘ If electrons move smoothly through the energy level based on the electronic defects present near the surface of IrO, an equilibrium state can be reached quickly. ｘThe ratio of Ir(III) increases near the surface of the silicon dioxide. This increases the density of electronic defects, creating a new energy level above the Fermi level, which is thought to facilitate the smooth transfer of electrons.

[0087] IrO ｘ Immediately after formation, it is relatively IrO ２ It is thought that the ratio of Ir / IrO is high, and therefore the defect density is low, so that electron transfer is not smooth when the electrochemical potential of the aqueous solution changes, resulting in a drift in the electromotive force. ｘ When the microelectrode was stored in an aqueous solution and the potential was measured intermittently, the IrO ｘ It is thought that hydroxides are formed on the surface, and that this creates an energy level. It is also thought that the application of a voltage of -1.5 V increases the proportion of Ir(III) according to formula (7), and the defect density increases.

[0088] (2-8) Ir / IrO ｘ Relationship between electromotive force and pH of microelectrodes The present inventors have investigated the relationship between electromotive force and pH of microelectrodes using Ir / IrO ｘ Microelectrodes and Ir / IrO applied voltage after thermal oxidation treatment ｘ Using a microelectrode, the relationship between electromotive force and pH was confirmed at pH 4 to pH 14. The thermal oxidation treatment was carried out at 750°C for 60 minutes, and the applied voltage was -1.5 V for 10 minutes. ｘ 25 is a graph showing the relationship between the electromotive force of a microelectrode and pH. In the figure, the horizontal axis represents pH, and the vertical axis represents electromotive force (V vs. Ag / AgCl). As shown in FIG. 25, the Ir / IrO ｘ Not only did the microelectrode show a linear response, but the measured potentials were in agreement with the theoretical values.

[0089] The pH-sensitive electrode manufacturing method, pH-sensitive electrode measurement method, and pH-sensitive electrode according to the present invention have been described in detail above through embodiments and examples. However, the present invention is not limited to the above-described embodiments and examples and includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention and are not necessarily limited to those including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, or to add the configuration of another embodiment to the configuration of one embodiment. Furthermore, it is possible to add, delete, or replace part of the configuration of each embodiment with other configurations. These embodiments and their modifications are within the scope and spirit of the invention, and are also within the scope of the invention and its equivalents as set forth in the claims.

[0090] REFERENCE SIGNS LIST 1 Measuring device 2 Chamber 3 First measuring section 4 Second measuring section 41 Composite electrode 42 pH-sensitive electrode 43 Counter electrode 44 Inlet 45 Outlet

Claims

1. A manufacturing process for manufacturing a composite electrode composed of iridium and iridium oxide by thermal oxidation treatment, and a first application step of applying a voltage to the composite electrode so as to maintain a negative state with respect to the counter electrode in a state where the composite electrode and the counter electrode facing the composite electrode are immersed in a solution. A method for manufacturing a pH-sensitive electrode having the above steps.

2. After the first printing step, the composite electrode has, at least on the surface, iridium in iridium oxide with a valence of 4 as Ir ＩＶ and iridium with a valence of 3 as Ir ＩＩＩ , and the Ir ＩＶ / Ir ＩＩＩ ratio is 1.15 or less. The method for manufacturing a pH-sensitive electrode according to claim 1.

3. The method for manufacturing a pH-sensitive electrode according to claim 1 or claim 2, wherein the first application step is performed at a voltage of -1 V to -2 V and an application time of 2 minutes to 90 minutes.

4. A measurement method for a pH-sensitive electrode, which has a second application step of applying a voltage to the composite electrode so as to maintain a negative state with respect to the counter electrode in a state where the composite electrode composed of iridium and iridium oxide manufactured by thermal oxidation treatment and the counter electrode facing the composite electrode are immersed in a solution, before measuring the pH of the measurement target using the composite electrode.

5. After the second application step, the composite electrode has, at least on the surface, iridium in the iridium oxide with a tetravalent iridium as Ir ＩＶ and trivalent iridium as Ir ＩＩＩ , and the Ir ＩＶ / Ir ＩＩＩ ratio is 1.15 or less. The method for measuring a pH-sensitive electrode according to claim 4.

6. The measurement method for a pH-sensitive electrode according to claim 4 or claim 5, wherein the second application step is performed at a voltage of -1 V to -2 V and an application time of 2 minutes to 90 minutes.

7. A measuring device having, in the same chamber, a first measuring unit for measuring the components of substances contained in the measurement target and a second measuring unit for measuring the pH of the measurement target, wherein the second measuring unit includes a composite electrode composed of iridium and iridium oxide and a counter electrode facing the composite electrode.

8. The composite electrode is, at least on the surface, a pH-sensitive electrode in which the ratio of Ir ＩＶ to Ir ＩＩＩ is 1.15 or less, where Ir ＩＶ represents tetravalent iridium in iridium oxide and Ir ＩＩＩ represents trivalent iridium in iridium oxide, and the measuring device according to claim 7. ＩＶ is tetravalent iridium in iridium oxide and Ir ＩＩＩ is trivalent iridium in iridium oxide, and the ratio of Ir ＩＶ / Ir ＩＩＩ is 1.15 or less.

9. A composite electrode composed of iridium and iridium oxide, wherein at least on the surface, tetravalent iridium in iridium oxide is represented as Ir ＩＶ and trivalent iridium is represented as Ir ＩＩＩ , and when the Ir ＩＶ / Ir ＩＩＩ ratio is 1.15 or less, a pH-sensitive electrode.

10. The pH-sensitive electrode according to claim 9, wherein the band energy structure of the iridium oxide has an energy level that is not filled with electrons at a position 1 eV higher than the Fermi level.

11. The pH-sensitive electrode according to claim 9 or claim 10, wherein the average film thickness of the iridium oxide is 5 μm or less.