JP3388572B2 - Nickel-titanium oxide alloy electrode and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an electrode capable of electrochemically detecting sugars and alcohols and a method for producing the electrode.

[0002]

2. Description of the Related Art Conventionally, carbon has been often used as an electrode of an electrochemical detector. This is because carbon is electrochemically stable and has many features such as a measurable potential range, that is, a wide potential window, and a small residual current that causes noise. In addition, carbon materials such as graphite are also inexpensive and easy to process. However, the activity against sugars and alcohols is low, and there are many problems such that an excessive potential must be applied to cause an electrochemical reaction of sugars and alcohols using carbon. In addition, noble metals such as gold and platinum are relatively stable electrochemically, so they are sometimes used as electrodes in electrochemical detectors, but like carbon, they have low detection sensitivity in detecting sugars and alcohols. However, it was difficult to apply it to the detection of trace amounts of sugar and alcohol. Alternatively, in order to increase the sensitivity, it is necessary to use a special potential application method such as a pulse method, and an additional device for that is required.

On the other hand, in the nickel electrode, nickel itself is electrochemically oxidized in an alkaline solution to become nickel hydroxide. This nickel hydroxide is known to catalytically oxidize sugars and alcohols.

Ni (OH) ₂ → NiO (OH) + e ⁻ + H ⁺ NiO (OH) + organic matter → Ni (OH) ₂

However, in the electrode of nickel alone, the oxidation current of nickel flows at a level that does not change from the signal current due to the catalytic reaction, so that a sufficient signal / noise ratio cannot be obtained, and the dynamic range of the detector is not sufficient. In this case, there is a problem that the signal current is buried in the current due to the oxidation reaction of nickel. In particular, when measuring the current by immersing the electrode in a stationary sample solution such as voltammetry and scanning the potential or snapping the potential, the charging current generated to change the potential and the redox current of nickel itself are measured. big,
This causes the detection limit to be determined.

On the other hand, in the case where the sample solution is forcibly transported to the electrode without changing the potential of the electrode such as the rotating disk electrode or the flow injection method, if all the nickel on the electrode surface is oxidized, the theoretical Since the current does not flow further, the effect of the redox current of nickel itself is less than that in the case where the potential is changed, but in reality, the current such as the reaction with impurities is difficult to identify the cause. This residual current generally increases as the surface area of the electrode increases, causing a decrease in the signal / noise ratio of the detector.

In order to solve these problems, Kwana et al. Attempted to detect sugars by liquid chromatography using a 1: 1 alloy electrode of nickel and titanium (An.
Alytical Chemistry, Volume 66, 27
75, 1994). They found that the nickel-titanium alloy electrode had improved long-term stability, but the residual current after a few minutes of application of a potential was as large as 100 nA to 1 μA and was 8-10 nA even after 2 hours. The sensitivity was 20 pA per pmol.

On the other hand, in order to reduce the noise proportional to the electrode area such as the charging current generated when the potential is scanned,
It is effective to use microelectrodes. Usually, when an electrode having a size on the order of millimeters is used and an electrochemical reaction is performed, the redox species that react are diffused perpendicularly to the electrode. Although there are non-vertical portions at the electrode edges, the amount is negligible as a whole. On the other hand, when the electrode size is in the order of microns, the diffusion at the edge portion of the electrode cannot be ignored, and conversely, the diffusion portion at the electrode surface becomes smaller. As a result, the diffusion profile changes from planar diffusion to spherical or hemispherical, and the number of molecules that diffuse per unit area increases significantly. Therefore, the signal current per unit area is significantly increased. Noise such as charging current, which is proportional to only the area of the electrode, decreases as the electrode area decreases, but the signal current does not decrease significantly due to the above-mentioned effect.
It is known that the noise ratio is significantly improved. However, there have been no cases where this effect was positively incorporated into electrochemical detection at nickel electrodes.

[0009]

As described above, the conventional microelectrodes using a noble metal have problems such as the need to apply an excessive potential, and the nickel electrode capable of reducing the applied electrode is highly sensitive to sugar and alcohol. However, there was a problem that the residual current other than the charging current and the signal was large and the signal / noise ratio was small when the detection was carried out with. Further, in the conventional nickel electrode, since the electrode area is large, it takes time for the state of the electrode surface to stabilize, and as a result, it takes time for the residual current to stabilize.

[0010]

In order to solve such a problem, the nickel-titanium oxide alloy electrode according to the present invention is a working electrode of a detector utilizing an electrochemical reaction, and the surface thereof is a titanium oxide surface with nickel fine particles. Are dispersed.

In the method for producing a nickel-titanium oxide alloy electrode according to the present invention, an alloy of nickel and titanium is processed into a desired shape, the alloy is connected as a working electrode in a three-electrode type electrochemical cell, and water is used. It is characterized in that a potential is applied in a sodium oxide aqueous solution to electrochemically oxidize titanium on the surface.

Further, according to the second method for producing a nickel-titanium oxide alloy electrode according to the present invention, nickel and titanium are simultaneously vapor-deposited or sputtered on the substrate to deposit the alloy,
Characterized by processing into a desired shape, connecting the alloy as a working electrode in a three-electrode electrochemical cell, and applying a potential in an aqueous sodium hydroxide solution to electrochemically oxidize titanium on the surface. .

In order to solve the above problems, in the present invention, as shown in FIG. 1, a nickel electrode 1 capable of electrochemical catalysis of sugar or alcohol is miniaturized and surrounded by an insulator 2. An electrochemical detector that is an assembly of the microelectrodes 1 was devised. In the detector, by using nickel as an electrode, sugar and alcohol can be efficiently electrochemically oxidized. Further, since nickel is a minute electrode, it is possible to improve the signal / noise ratio. Further, by assembling the microelectrodes, it is possible to increase the absolute value of the signal current, which was small with one microelectrode. The nickel-titanium oxide alloy electrode according to the present invention has a structure in which the nickel microelectrode 1 is surrounded by titanium oxide 2. Titanium oxide behaves almost as an insulator in the absence of ultraviolet rays. realizable.

Further, as a method of producing the electrode of the present invention,
After making an electrode with an alloy of nickel and titanium, connect the alloy as a working electrode in a three-electrode type electrochemical cell, and apply a potential in an aqueous solution of sodium hydroxide to electrochemically oxidize titanium on the surface. As a result, a structure in which minute nickel is surrounded by an insulator is realized. Further, by using an alloy of nickel and titanium as a starting material, nickel can be dispersed in the alloy on the atomic order, and an extremely fine nickel electrode can be realized.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below.

In the present invention, a stencil 3 having an electrode shape is superposed on a substrate whose surface or whole surface is insulative, such as a silicon wafer with an oxide film or a nitride film, a quartz substrate, an aluminum oxide substrate, or a glass substrate. A nickel-titanium alloy thin film is formed on the substrate by setting it on a substrate holder of a thin film forming device such as a vacuum vapor deposition device or a sputtering device, and simultaneously depositing or sputtering nickel and titanium. At this time, in the case of vapor deposition, the temperature of the sample boat can be controlled, the distance between the vapor deposition source and the substrate can be changed, and a screen mesh can be placed above the vapor deposition source to change the size of the target in the case of sputtering. ,
The alloy ratio of nickel and titanium is changed by changing the distance from the target to the substrate or placing a screen mesh on the upper part of the target. In the case of sputtering, not only co-sputtering in which two targets are used at the same time, but, for example, by placing a titanium chip on a nickel target, there is one target, but the mixing ratio is changed by changing the amount of titanium to be placed. It is also possible.

When the stencil is not used, a nickel-titanium alloy thin film may be first formed on the entire surface of the substrate and used as it is, or may be appropriately cut and used. Also,
A nickel-titanium alloy thin film electrode having a desired shape can be obtained by forming a fine pattern by lithography using a resist, performing wet etching with mixed acid, or milling with argon. Further, first, an electrode pattern is formed on the substrate, and a nickel-
It is also possible to obtain an electrode having a desired shape by a lift-off method in which a titanium thin film is deposited and the resist is peeled off. on the other hand,
Machine a nickel-titanium alloy into a suitable shape,
It is also possible to embed it in a plastic material such as el-F or glass to form an electrode, but if an alloy of nickel-titanium is to be cast, an alloy having a desired mixing ratio cannot be obtained. Ratio of compounds, eg Ni
It is very difficult to manufacture except for electrodes such as Ti and NiTi ₂ .

Then, the alloy electrode is dipped in an alkaline solution such as an aqueous solution of sodium hydroxide, and when an oxidizing potential is applied to the silver / silver chloride reference electrode by a potentiostat, titanium on the surface is oxidized, and normally an insulating material is used. It becomes titanium oxide that can be handled as a body. The potential to be applied may be any as long as titanium is oxidized, but normally, about 0.4 V sufficiently oxidizes. Further, at that time, the potential may be kept constant to oxidize for several minutes, or the potential may be changed. For example, when the cyclic voltammogram is measured between 0 V and 0.4 V at a scanning speed of 100 mV / sec, a large current due to the oxidation of titanium is observed in the first sweep to the oxidation side, but after the second time, , Titanium oxide becomes electrochemically inactive, and only the response due to the redox of nickel is observed. In this state, the nickel electrodes dispersed in titanium oxide behave as a microelectrode array. That is, when sugar such as glucose is added to the electrolytic solution, the sugar is oxidized by the oxidized nickel, and the reduced nickel is immediately electrochemically oxidized to oxidize the sugar again.

Such a reaction occurs even with an electrode of nickel alone, but in the case of an alloy electrode, since nickel is the minute electrode 1 as shown in FIG. 1, the sugar molecules diffuse spherically with respect to the nickel electrode. Therefore, the reaction amount per unit area becomes extremely large as compared with the planar diffusion in the case of the nickel electrode 1 of nickel alone as shown in FIG. As a result, only the signal current is increased and the noise proportional to the electrode area is reduced, so that the signal / noise ratio is significantly improved. In FIG. 5, 2 is titanium oxide.

The particle size of such nickel fine particles is preferably 10 μm or less in this microphone. If it exceeds 10 μm, the effect of the microelectrode may not be sufficiently exhibited.

The proportion of nickel in the nickel-titanium oxide alloy electrode is preferably less than 55 atm%. This is because if it exceeds 55 atm%, it becomes difficult to form a microelectrode and the microelectrode effect cannot be sufficiently exhibited.

When actually used as an electrochemical detector, the concentration of the sample can be determined by immersing it in a sample solution and measuring voltammetry. It can also be used as an electrochemical detector for flow injection analysis or liquid chromatography.

[0023]

Example 1 Hereinafter, a method for producing a nickel-titanium oxide alloy electrode and the results of the implementation will be described with reference to the drawings. First, a stencil 3 having a hollowed electrode pattern P shown in FIG. 2 was placed on a quartz glass substrate having a diameter of 3 inches, and the stencil 3 was mounted on a substrate holder of a sputtering deposition apparatus (manufactured by Nippon Seed Research Institute). An alloy of titanium and nickel 1: 1 (diameter: 125) while rotating the substrate with an argon gas pressure of 5 mTorr, a flow rate of 31 SCCM, and an RF power of 500 W.
mm) was used as the target to perform titanium-nickel sputter deposition. A titanium-nickel alloy having a thickness of 100 nm could be deposited on the quartz glass substrate by sputtering for 10 minutes.

As a result of ICP analysis, the contents of the deposited alloy thin film were nickel 53.3% and titanium 46.7%. The substrate was divided into chips, the lead wires were connected to the pads 5 with solder, the portions other than the electrodes 4 were covered with epoxy resin, and the working electrode terminals of the potentiostat were connected. Then, the electrode was immersed in a 0.1 M aqueous solution of north sodium hydroxide together with a counter electrode made of platinum and a silver / silver chloride reference electrode, and the counter electrode and the reference electrode were connected to a potentiostat, and then ± 0. A potential of 4 V was applied at a sweep rate of 100 mV / sec. Figure 3 shows the obtained cyclic voltammogram.
Shown in. A large oxidation current is observed in the first potential sweep, and the titanium on the surface is oxidized, but since the titanium oxide becomes an insulator after the second time, that part does not act as an electrode, only the redox of nickel. Came to be observed.

Next, a sodium hydroxide solution having different glucose concentrations was prepared, and cyclic voltammetry was measured under the same conditions as above. FIG. 4 is a plot of the concentration of glucose and the current value generated by the oxidation of glucose, and a linear response was obtained in a wide concentration range. When an electrode made only of nickel is used, it is difficult to separate the redox current of nickel itself and the signal current of glucose, especially in the low concentration region, and the lower limit of measurement is mol / mol.
It was liter. The current associated with the redox of nickel itself in the absence of glucose was 0.25 μA, which was smaller than 1 μA in the electrode of nickel alone.

[0026]

Example 2 In Example 1, titanium and nickel 1:
Sputtering was carried out by placing 5 mm square titanium on the alloy target of No. 1 or 5 mm square nickel on the titanium target. Then, the composition ratio of titanium and nickel was measured by ICP in the same manner as in Example 1, and titanium was oxidized in a sodium hydroxide solution. Next, cyclic voltammetry was measured in an aqueous solution of sodium hydroxide alone and a 1 mM glucose solution. As a result, it was found that the composition ratio can be changed depending on the amount of the mounted chips, and the response changes depending on the composition ratio. Also, a large current value was observed due to the addition of glucose. The results are summarized in Table 1.

[0027]

[0028]

[Example 3] A quartz substrate having a diameter of 3 inches was mounted on a substrate holder of a sputtering deposition apparatus (manufactured by Japan Seed Research Institute), and 5 m was placed on an alloy target of titanium and nickel 1: 1.
Fifty m-square titanium chips were placed and sputtered under the same conditions as in Example 1 to form a nickel-titanium alloy thin film on the entire surface of the quartz substrate. Subsequently, a silicon oxide film was deposited on the alloy by using quartz as a target without breaking the vacuum. Argon gas pressure 0.67 Pa, flow rate 31 SCCM,
Sputter deposition was performed at a substrate temperature of 300 ° C. while rotating the substrate with an RF power of 500 W. A 100 nm thick oxide film could be deposited on the substrate by sputtering for 15 minutes.
Next, a photoresist (manufactured by Tokyo Ohka: TSMR-
V3) is applied by spin coating (rotation speed: 4000 rp)
m, time: 40 seconds, film thickness: 0.8 μm), after baking for 90 seconds on a hot plate at 90 ° C., the electrode pattern was set to 2 using a mask aligner (Canon: PLA-501).
UV irradiation was performed for 0 seconds. Next, a resist developer (Tokyo Ohka: NMD-W) was spray-developed for 90 seconds, rinsed with pure water for 2 minutes, and then spin-dried at 1500 rpm for 3 minutes. Next, the substrate was placed in a reactive ion etching device (DEM451 manufactured by Anelva), and C ₂ F ₆ gas was used for pressure: 0.25 Pa, flow rate: 25 SCCM, RF power: 150 W for 10 minutes to form an oxide film. Etching was performed to expose the electrode portion and the pad portion of the alloy. Next, the lead wire was connected to the pad portion with solder and covered with an epoxy resin.

As in Example 1, the electrode was immersed in an aqueous sodium hydroxide solution and connected to a potentiostat to oxidize titanium. When an experiment was conducted under the same conditions as in Example 2, an oxidation peak of 128 nA was observed at a potential of 0.4 V only with an aqueous sodium hydroxide solution, and a current of 11.9 μA was observed in a 1 mM glucose solution.

[0030]

[Example 4] Each electrode prepared in Example 2 was incorporated into a cell for flow injection analysis (manufactured by BS Co., Ltd.), a liquid feed pump (PM-70 manufactured by BS Co., Ltd.), an injection valve (manufactured by Rheodyne), and current detection. A vessel (manufactured by BS Co., Ltd .: LC4C) was connected. To the nickel-titanium oxide electrode, 0.4 V was applied to a silver / silver chloride reference electrode, and a 0.1 M sodium hydroxide aqueous solution was used as an eluent at a flow rate of 100 μL / min to flow at 10 μM.
The response was measured when 5 μL of glucose was injected.
The results are shown in Table 2.

[0031]

When an electrode made of only nickel was used, the base current was 20 nA and the noise current was as high as 50 pA, whereas the signal current was 20 nA. Further, when an electrode made of only nickel was used, it took about 6 hours for the base current to stabilize. On the other hand, as the nickel content decreases, the time until the base current stabilizes decreases, and it took about 2 hours in the case of Ni: Ti = 1: 1. The sample settled in a stable state in about 20 minutes, and the sample with 10 Ni chips settled in a stable state in about 5 minutes.

[0033]

Example 5 In Example 4, an ion chromatography column for sugar analysis (CarboPac PA manufactured by Dionex Co.) was connected after the injector, and a sample containing a plurality of sugars was injected. The electrodes are Ni: Ti = 1:
The target material of No. 1 had 75 Ti chips and was sputtered. The concentration of each sample was 10 μM, and the injection amount was 5 μL. Other conditions were the same as in Example 4. As a result, each sample was separated by the column, and the peak current values shown in Table 3 were obtained.

[0034]

[0035]

Example 6 In Example 4, a target material of Ni: Ti = 1: 1 was prepared by sputtering with 75 Ti chips, and the response to various alcohols was examined under the conditions of Example 4. . As a result, the results shown in Table 4 were obtained.

[0036]

[0037]

The electrode has a structure in which titanium microelectrodes are surrounded by titanium oxide, and the nickel electrodes act as a group of microelectrodes. For this reason,
While the current density of the signal becomes high, the current proportional to the area such as the charging current can be made relatively small. Further, since nickel is used to oxidize sugars and alcohols electrochemically, the oxidation reaction can proceed very efficiently without applying an excessive potential. Therefore, noise can be reduced and the signal / noise ratio can be significantly improved. As a result, it has become possible to measure sugars and alcohols, which have been difficult to measure in the past, easily and in a short time without performing treatment such as concentration.

Further, if the electrode is used, it is possible to greatly reduce the time from the application of the potential to the stabilization of the base current to the extent that measurement can be performed. Further, when a large amount of sample is to be continuously measured by flow injection analysis or the like, the base current returns to a steady state in a short time, so the measurement interval can be made very small. As a result of the above effects, it is possible to improve the overall throughput, which greatly contributes to the improvement of these analysis methods.

[Brief description of drawings]

FIG. 1 is a diagram showing a diffusion profile in a microelectrode.

FIG. 2 is a plan view showing the shape of a stencil when a nickel-titanium oxide alloy electrode is produced by sputtering.

FIG. 3 is a diagram showing a cyclic voltammogram of a nickel-titanium oxide electrode.

FIG. 4 is a diagram showing the relationship between glucose concentration and signal current.

FIG. 5 is a diagram showing a diffusion profile in a nickel electrode of nickel alone.

[Explanation of symbols]

1 Nickel electrode 2 Titanium oxide 3 stencils 4 electrodes 5 pad 3

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor, Noriyuki Watanabe 1-6-6 Matsugaoka, Hatoyama-cho, Hiki-gun, Saitama (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 27/26- 27/49 JISC file (JOIS)

Claims (5)

(57) [Claims] 1. A nickel-titanium oxide alloy electrode, characterized in that in a working electrode of a detector utilizing an electrochemical reaction, nickel fine particles are dispersed on the surface of titanium oxide. 2. The nickel-titanium oxide alloy electrode according to claim 1, containing nickel fine particles having a size of 10 μm or less. 3. The nickel-titanium oxide alloy electrode according to claim 1, wherein the content of nickel is less than 55 atm%. 4. A nickel-titanium alloy is processed into a desired shape, the alloy is connected as a working electrode in a three-electrode electrochemical cell, and a potential is applied in a sodium hydroxide aqueous solution to remove titanium on the surface. A method for producing a nickel-titanium oxide alloy electrode, which comprises electrochemically oxidizing the electrode. 5. Nickel and titanium are simultaneously vapor-deposited or sputtered on a substrate to deposit an alloy, the alloy is processed into a desired shape, the alloy is connected as a working electrode in a three-electrode electrochemical cell, and sodium hydroxide is used. A method for producing a nickel-titanium oxide alloy electrode, which comprises applying a potential in an aqueous solution to electrochemically oxidize titanium on the surface.