JP2002189016A - Thiol concentration measuring method and sensor used for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method, capable of specifically and easily measuring the concentration of clinically or sitologically important thiol, in a short time. SOLUTION: The property of thiol, that it is electrochemically and specifically oxidized by a diamond electrode, is utilized. In other words, the diamond electrode with conductive diamond and a counter electrode are prepared, and a voltage for causing oxidation reaction on the diamond electrode is impressed between the diamond electrode and the counter electrode to measure a current value under the voltage. Through the use of a previously prepared calibration curve, the concentration of thiol in a test sample is computed from the current value obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring thiol concentration using a diamond electrode, a sensor used for the method, and an apparatus therefor.

2. Description of the Related Art Diamond is originally an insulating material having a resistivity of about 10 ¹³ Ωcm, but acquires conductivity by doping a trace amount of impurities. Various applications are expected for this conductive diamond. One of them is utilization as an electrode for electrochemical use. When diamond made conductive is used as an electrode for electrochemical use, it has the excellent features of having a wide potential window and extremely low background current. Furthermore, it also has features that it is physically and chemically stable and has excellent durability. Electrodes having conductive diamond (preferably a thin film thereof) have been commonly referred to as diamond electrodes.

[0003] Pioneering research on diamond electrodes was carried out by Iw.
(Iwaki etal., Nuclear Instru
ments and Methods, 209-210, 1129 (1983)). They are,
The purpose of this study was to study the properties of single-crystal diamond with surface conductivity by implanting argon or nitrogen ions as an electrically conductive material. At the same time, a cyclic voltammogram in the electrolyte solution was also shown. Subsequently, the properties of polycrystalline diamond synthesized by gas phase using hot filaments have been reported (Pleskov et al., J. Electroana
l. Chem., 228, 19 (1993)).

[0004] Some of the present inventors have previously reported the reduction of nitrogen oxides using a diamond electrode synthesized in the vapor phase (Tenne et al., J. Electroanal. Chem., 347,
409 (1993)). In this study, a p-type semiconductor diamond into which boron was introduced as a dopant was used as an electrode. After that, as a diamond electrode, the use of a p-type semiconductor using boron as a dopant or a metal-like conductive diamond having higher conductivity has become mainstream. 199
In the 0's, research on diamond electrodes was conducted by several groups, and since 1995, plasma CVD, which can obtain diamond thin films with a larger area than gas phase synthesis, has been performed.
Research on diamond electrodes obtained using (PCVD) equipment has also been found in the electrochemical field.

On the other hand, thiols such as cysteine, homocysteine, and glutathione have various physiological functions in animals and plants. If the amount of thiol in a living body or in a sample derived from a living body or in a food can be easily known, it is extremely significant in medicine and pharmacy or food production.

[0006]

SUMMARY OF THE INVENTION The present inventors have now found that a diamond electrode is specifically sensitive to thiol and its abundance can be quantified from the current value at its oxidation potential. The present invention is based on this finding.

Therefore, an object of the present invention is to provide a method capable of easily and specifically measuring the concentration of thiol in a short time.

It is another object of the present invention to provide a sensor used in the above method and an apparatus therefor.

According to a first aspect of the present invention, there is provided a method for measuring the concentration of thiol in a test sample, the method comprising: a diamond electrode having conductive diamond;
Prepare a counter electrode, the diamond electrode, the counter electrode is brought into contact with a test sample, the diamond electrode,
Between the counter electrode, a voltage at which an oxidation reaction occurs on the diamond electrode is applied, a current value under the voltage is measured, and the concentration of the thiol in the test sample is calculated from the obtained current value. That is,

According to another aspect of the present invention, there is provided a sensor for use in the above-mentioned measuring method, wherein the sensor comprises a diamond electrode having conductive diamond.

Further, according to another aspect of the present invention, there is provided an apparatus for performing the above-mentioned measuring method, the apparatus comprising:
A diamond electrode having conductive diamond, a counter electrode, a means for contacting the diamond electrode and the counter electrode with a test sample, and an oxidation reaction on the diamond electrode between the diamond electrode and the counter electrode. And a means for measuring a current value under the applied voltage, and a means for calculating the concentration of the thiol in the test sample from the obtained current value. .

[0012]

DETAILED DESCRIPTION OF THE INVENTION Test Compound According to the method of the present invention, the concentration of thiol in a sample solution can be selectively known. We have:
As described above, on the conductive diamond electrode,
It was confirmed that the thiol was specifically oxidized electrochemically, and that the current generated between the conductive electrode and the counter electrode was directly proportional to the concentration of thiol in the system. . As a result, it was possible to quantitatively detect thiol in the test sample.

[0013] The test sample may be blood, a body fluid or the like derived from a living body which is thought to contain thiol, or a food or a diluted solution or suspension of the food.

Specific examples of thiols that can be measured by the method according to the present invention include (L-) cysteine, homocysteine, glutathione, metaionine, glutamine cysteine, N-acetylcysteine, carbocysteine and the like.

[0015] In particular, cysteine, especially L-cysteine, is abundant in natural proteins, and it is extremely significant that its concentration can be measured in a short time and simply because of its existence and various functions. .

The diamond electrode and its production diamond are inherently excellent insulators. However, by adding a Group 3 or Group 5 impurity, the semiconductor or metal becomes conductive. In the present invention, diamond having a semiconductor-metal-like conductive property is used as an electrode.

Examples of the substance added to impart such a conductive property include elements belonging to Groups 3 and 5 as described above, more preferably boron, nitrogen and phosphorus, and most preferably boron Or nitrogen. The amount of the substance added for imparting this conductivity may be appropriately determined within a range in which the conductivity can be imparted to diamond, and for example, a conductivity of about 1 × 10 ⁻² to 10 ⁻⁶ Ωcm is provided. It is preferable to add in an amount. Generally, the amount of the substance added to impart the conductivity is controlled by the amount added in the process of producing the conductive diamond.

The diamond electrode according to the present invention uses the conductive diamond as an electrode. This diamond electrode had an extremely interesting property that when an electrochemical reaction was performed in water, oxygen was not generated by water electrolysis as an oxidation reaction, and the thiol oxidation reaction was specifically caused. This characteristic is considered to be exhibited regardless of the surface state of the conductive diamond.
That is, a hydrogen atom was bonded to a terminal carbon atom on the surface of an untreated (as grown) diamond thin film manufactured using hydrogen gas as a carrier by a chemical vapor deposition method as described later. Although it is usually in a state, it is considered that a thiol oxidation reaction can also be specifically caused by conductive diamond in which this terminal hydrogen atom is replaced by another atom.

Although it is possible to use the conductive diamond itself as an electrode instead of supporting the substrate, according to a preferred embodiment of the present invention, a conductive diamond thin film is formed on the substrate, It is preferable to connect a conducting wire to the electrode to form an electrode. As the base material, Si (for example, single crystal silicon), Mo, W, Nb, Ti, Fe, Au, Ni, Co,
Al ₂ O ₃ , SiC, Si ₃ N ₄ , ZrO ₂ , MgO, graphite, single crystal diamond, cBN, quartz glass, etc., and particularly single crystal silicon, Mo, W, Nb, Ti, Si
C, the use of single crystal diamond is preferred.

The electrode of this embodiment will be further described with reference to FIG. FIG. 1A is a cross-sectional view of a diamond electrode 1, which comprises a conductive diamond thin film 3 formed on a base material 2, and a conductive wire 5 on the conductive diamond thin film 3. For example, they are connected via a gold coating 4. FIG. 1B is a perspective view of the diamond electrode 1, comprising a conductive diamond thin film 3 formed on a substrate 2, and a gold coating 4 for using the conductive diamond thin film 3 as an electrode. Through the conductor 5
Is connected.

The thickness of the conductive diamond thin film is not particularly limited, but is preferably about 1 to 100 μm.
More preferably, it is about 5 to 50 μm.

According to a further preferred embodiment of the present invention, the diamond electrode according to the present invention can take the form of a microelectrode. The concept of a microelectrode is already known. In the present invention, a diamond electrode in the form of a microelectrode is formed by sharply cutting the end of a fine wire such as Pt, further sharpening the end by electrolytic polishing, and then forming a conductive surface on the end surface. Means a structure in which a thin film of conductive diamond is formed.

According to a preferred embodiment of the present invention, the conductive diamond thin film is preferably produced by a chemical vapor deposition method. The chemical vapor deposition method is a method of synthesizing a substance by chemically reacting a gaseous raw material in a gas phase.
al Vapor Deposition) method. This method is widely used in semiconductor manufacturing processes,
The present invention is also applicable to the production of the conductive diamond thin film with appropriate modification.

The chemical vapor synthesis of diamond is carried out by using a mixture of a carbon-containing gas such as methane and hydrogen as a source gas, exciting the source gas by an excitation source, and supplying and depositing it on a substrate.

Examples of the excitation source include a hot filament, microwave, high frequency, DC glow discharge, DC arc discharge, and combustion flame. It is also possible to adjust the nucleation density by combining a plurality of them, and to increase the area and uniformity.

[0026] As a raw material, many types that contain a carbon, decomposition by the excitation source is excited, C, activated carbon, such as _{C 2,} and _{CH, CH 2, CH 3,} C 2 H 2
For example, compounds that generate hydrocarbon radicals can be used. Preferred specific examples include CH ₄ as a gas,
C ₂ H ₂ , C ₂ H ₄ , C ₁₀ H ₁₆ , CO, CF ₄ , CH ₃ OH, C ₂ H ₅ OH, (CH ₃ ) ₂ C as a liquid
O, and solids such as graphite and fullerene.

In the vapor phase synthesis method, the addition of a substance that imparts conductivity to diamond is performed, for example, by placing a disk of the additive substance in the system, exciting the carbon material in the same manner as the carbon source material, and adding the additive substance to the carbon vapor. It can be carried out by a method of adding an additive to a carbon source in advance, introducing the additive into the system together with the carbon source, exciting by an excitation source, and introducing the additive into the carbon gas phase. According to a preferred embodiment of the present invention, the latter method is preferred. In particular, when a liquid such as acetone or methanol is used as the carbon source, boron oxide (B
_The method of dissolving ₂ O ₃ ) as a boron source is preferable because the control of the boron concentration is easy and simple.
For example, in the case of adding boron to a carbon source in a gas phase synthesis method, the amount is generally about 10 to 12,000 ppm, and preferably about 1,000 to 10,000 ppm.

According to a preferred embodiment of the present invention, the production of the conductive diamond thin film is preferably performed by a plasma enhanced chemical vapor deposition method. This plasma-enhanced chemical vapor synthesis method has a large activation energy to cause a chemical reaction,
It has the advantage that the reaction is fast. Further, according to this method, a chemical species that does not exist unless it is thermodynamically high is generated, and the reaction can be performed at a low temperature. There have been several reports on the production of conductive diamond thin films by the plasma chemical vapor deposition method, including some of the present inventors (for example, Yano et al., J. Electrochem. Soc., 145 (1998)). 1
870), it is preferred to carry out the method described in these reports.

Measurement Method and Apparatus Therefor In the present invention, the conductive diamond electrochemically oxidizes thiol specifically, the conductive diamond is used as the working electrode, and the current value generated between the conductive diamond and the counter electrode is Utilizing the property of being directly proportional to the thiol concentration in the system, thiol in the test sample is quantitatively detected. Since the measurement is performed by knowing the current value, the measurement time is short and simple, and the method according to the present invention is extremely advantageous in that the thiol concentration can be easily known in a short time.

The diamond electrode has the possibility of oxidizing compounds other than thiol, but is very specifically sensitive to thiol. Therefore, the measurement method according to the present invention
This enables specific measurement of only thiol. If a mixture of thiols or a substance that can react with the diamond electrode is present in the test sample or is likely to be present, pretreatment such as liquid chromatography is performed to make the thiol respond to the diamond electrode. Needless to say, it is preferable to apply the measurement method according to the present invention separately from the other substances contained therein.

Furthermore, since the oxidized voltage of the thiol is different for each type, that is, the peak voltage described later is different, it is possible to distinguish a plurality of thiols by controlling the applied voltage.

According to the measuring method of the present invention, thiol can be measured in a wide concentration range from about 100 nmol / l to a saturated concentration.

In the method for measuring the concentration of thiol according to the present invention, the electrochemical system for quantifying thiol may be a general electrochemical system except that conductive diamond is used as a working electrode.

That is, in the method for measuring the concentration of thiol according to the present invention, conductive diamond is used as a working electrode,
The sample is brought into contact with the test sample together with the counter electrode, a voltage causing an oxidation reaction on the diamond electrode is applied between the two electrodes, and the current value under this voltage is measured. Then, the thiol concentration in the test sample is calculated from the obtained current value. As described above, the current value generated between the conductive diamond and the counter electrode due to oxidation of the thiol is directly proportional to the thiol concentration in the system. Therefore, once the relationship between the current value and the thiol concentration at a certain voltage value is determined, the thiol concentration in the test sample corresponding to the obtained current value can be easily known from the relationship. That is, in a preferred embodiment of the present invention, a calibration curve of the thiol concentration and the current value is prepared in advance, and the calibration curve is compared with the obtained current value to thereby determine the thiol concentration. You can know.

In the present invention, it is preferable to use platinum, carbon, stainless steel, gold, diamond, SnO ₂ or the like as the counter electrode.

In the present invention, the voltage applied between the working electrode, the diamond electrode, and the counter electrode is not limited as long as a thiol oxidation reaction occurs on the diamond electrode. From the viewpoint, this applied voltage is preferably a voltage that gives a peak current of oxidation of the thiol compound. Here, the peak current can be determined as a voltage giving a maximum current value by, for example, cyclic voltammetry. Further, according to another preferred embodiment of the present invention, the voltage giving the maximum current value can be obtained by a rotating electrode method or a micro electrode method. The rotating electrode method or the micro electrode method is advantageous in that the possibility of a measurement error due to measurement conditions and the like can be further eliminated.

Further, according to a preferred embodiment of the present invention,
It is preferable from the viewpoint of measurement accuracy that the reference electrode is brought into contact with the test sample and the absolute value of a voltage at which an oxidation reaction occurs on the diamond electrode is controlled between the diamond electrode and the counter electrode. As the reference electrode, a known electrode can be used, and a standard hydrogen electrode, a silver-silver chloride electrode, a mercury-mercury chloride electrode, a hydrogen-palladium electrode, or the like can be used.

In the measuring method according to the present invention, the electrochemical system is the same as that of the first embodiment except that conductive diamond is used as a working electrode.
Although a general electrochemical system can be used, according to a preferred embodiment of the present invention, a predetermined solution is caused to flow through the system as a carrier at a constant flow rate, and a test sample is injected into the carrier solution and measured. And the measurement by a flow injection method using a flow cell is preferable.

The outline of the flow injection method using the flow cell is as shown in FIG. The carrier solution is injected into the flow cell 21 from the carrier solution reservoir 6 by the pump 7. Pump 7 and flow cell 21
A test sample injection port 8 is provided between the two, and a test sample can be injected into the carrier solution. The carrier solution that has passed through the flow cell 21 is collected in the waste liquid reservoir 9.

The basic structure of the flow cell 21 is as shown in FIG. In the flow cell 21, the diamond electrode 1, the counter electrode 22, and the reference electrode 23 are exposed in a channel 24 through which the carrier solution and the test sample flow,
It is configured to be able to contact the test sample. The diamond electrode 21 basically has the structure shown in FIG. 1, and the diamond thin film 3 in FIG. 1 is exposed in the channel 24 and comes into contact with the carrier solution and the test sample. The carrier solution enters through the inlet 25 of the flow channel 24, flows as indicated by the arrow in the drawing, and reaches the outlet 26. Between the conductive wire A of the diamond electrode 1 and the conductive wire B of the counter electrode 22, a voltage for causing oxidation of thiol on the diamond electrode 1, preferably a voltage for generating a peak current, is applied.

The measurement is performed as follows. First, only a carrier solution into which a test sample is not injected is caused to flow, so-called background current is made as small as possible and stabilized. Diamond electrodes are characterized by a low background current. Next, a test sample is injected from the test sample inlet 8. This injection may be performed continuously, but may be performed at a time in such an amount that the peak current can be measured. When thiol is contained in the test sample,
The thiol is oxidized on the diamond electrode 1, and the current value accompanying the oxidation reaction can be measured. From this current value, the thiol concentration is known.

In the measurement method according to the present invention, it is preferable that the pH of the test sample is fixed to an arbitrary specific value before the measurement. For some thiols, the pH at which the peak current is applied does not have a large pH dependence, but the pH range of the test sample is preferably about 5 to 12, more preferably 7 to 9.

According to another preferred embodiment of the present invention, it is preferable that the diamond electrode 1 in the flow cell 21 is in the form of a micro electrode. According to another preferred embodiment of the present invention, the diamond electrode 1 may be a rotating electrode.

As described above, according to another embodiment of the present invention, there is provided a method for measuring the concentration of thiol. According to another embodiment of the present invention, a method for measuring the concentration of thiol comprising a diamond electrode having conductive diamond. A sensor for measurement is provided.

Further, according to another aspect of the present invention, there is provided an apparatus for measuring the concentration of thiol in a test sample.
The basic configuration of this device is as shown in FIG. FIG.
2, the power supply / ammeter 31 includes a lead A connected to the diamond electrode 1, a lead B connected to the counter electrode 22, and a reference electrode 2 from the flow cell shown in FIG.
3 is connected. The power supply / ammeter 31 serves as a means for applying a voltage at which an oxidation reaction occurs on the diamond electrode between the diamond electrode and the counter electrode, and a means for measuring a current value under the applied voltage. That is, the power / ammeter 31 applies a voltage between the diamond electrode 1 and the counter electrode 22 to cause oxidation of thiol on the diamond electrode 1,
Preferably, a voltage for generating a peak current is applied, and a current value accompanying the oxidation reaction of thiol on the diamond electrode 1 is measured. This current value is sent to the current value comparison / concentration calculation device 32. The calibration curve data 33 is sent to the device 32, and the power supply / ammeter 3
The current value sent from 1 is compared with the calibration curve data to calculate the thiol concentration. That is, a means for calculating the concentration of the thiol in the test sample from the obtained current value is provided. The obtained thiol concentration is displayed on the display device 34.

[0046]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Preparation of Diamond Electrode A diamond thin film was prepared using a microwave CV manufactured by ASTeX.
Microwave plasma assisted CVD using D film forming equipment
It was prepared by the method. Specifically, it is as follows.

First, the surface of a silicon substrate (Si (100)) was polished with 0.5 μm diamond powder,
It was set on the substrate holder of the VD film forming apparatus. A mixture of acetone and methanol (mixing ratio 9: 1 (volume ratio)) was used as a carbon source. Further, the mixture was added with boron oxide (B
₂ O ₃ ) was dissolved in an amount to give a boron / carbon ratio of 10,000 ppm. Pure hydrogen gas was passed through the mixture of carbon sources to replace the dissolved gas. On the other hand, pure hydrogen gas is flowed into the chamber at a flow rate of 532 cc / min.
× torr (115 × 133.322 Pa). next,
2.45 GHz microwave is injected into the chamber,
Discharge was performed to adjust the output to 5 KW. After confirming that the apparatus was stable, a pure hydrogen gas as a carrier gas was flowed at a flow rate of 15 cc / min into the carbon source mixture, and introduced into the chamber to form a film. Film formation speed is 1-4 μm
/ Hour. Film formation was performed until the thickness became about 30 μm. Although the substrate was not heated, it was observed that the temperature was about 850-950 ° C. in the steady state.

When the Raman spectrum of the obtained diamond thin film was taken, only a single peak was observed at 1333 cm ^-1 . Further, the electric conductivity is about 10 ⁻³ Ωcm, and as a result of measuring the cyclic voltammogram in 0.5 M sulfuric acid, −1.25 to +2.3 V (vs. SH)
E), it was confirmed to have a wide potential window.

[0050]Voltammetry measurement Voltammetric measurements were performed in a glass cell on a Hoku
to Denko HA-502 potentiostat, Hokuto Denko H
B-11 Function Generator and Riken Dens
The test was performed at room temperature using a hi x-y recorder. Above
Diamond having diamond thin film obtained in this way
The working electrode (exposed area 0.07cm) ²) As for
A platinum foil was used as a counter electrode. As a reference electrode
A saturated calomel electrode (SCE) was used.

Experimental Example 1 : Relationship between peak current value and concentration (a) L-cysteine / hydrochloride / monohydrate was determined from Wako, and 1-1000 μm was obtained using Milli-Q water (Millipore).
An aqueous solution was prepared in a concentration range of mol / l. Incidentally, the 0.5MKHCO ₃ used as the electrolyte, pH was measured using a conventional glass electrode.

(B) The pH of all aqueous solutions was set to 9 and the potential sweep rate was set to a value of 50 to 20 mVs- ¹ . Voltammetric measurement was performed to determine the value of the peak current. The result was as shown in FIG. When the relationship between the obtained peak current value and the concentration was examined, a linear relationship passing through the origin was obtained. The calibration curve obtained by the least square method in the range of 1 to 200 μM: concentration I = α × cysteine concentration M is as follows.

[0053] Concentration ([mu] M) potential sweep rate ^{mVs -1 α (μA / μM)} 1-10 50 0.020 10-100 20 0.012 100-200 20 0.013

(C) Measurement was performed five times for solutions with several cysteine concentrations, and the reproducibility of the peak current value was determined as follows:
It was confirmed as the deviation (%) of the peak current value. The results were as follows.

Concentration (μM) Potential sweep rate (mVs- ¹ ) Average peak current value (μA) Deviation (%) 750 0.14 3.5 40 20 0.49 1.4 120 20 1.54 0.6

Experimental Example 2 : Relationship between peak current value and pH For 1 mM cysteine solution, under various pH conditions,
Anodic voltammography was recorded at a sweep rate of 10 mVs ^-1 . The change in the half-peak potential with pH was as shown in FIG. When the pH is 8.6 or higher, the half-peak potential is almost constant, indicating that the half-peak potential does not depend on pH in an alkaline solution.

[Brief description of the drawings]

FIG. 1 is a view showing a structure of a diamond electrode;
1A is a cross-sectional view of a diamond electrode 1. This electrode is a conductive diamond thin film 3 formed on a substrate 2. FIG.
Consists of (B) is a perspective view of the diamond electrode 1, which comprises a conductive diamond thin film 3 formed on a substrate 2.

FIG. 2 is a diagram showing a basic structure of a flow cell used for a measurement method according to the present invention.

FIG. 3 is a diagram showing a basic configuration of a measuring device according to the present invention.

FIG. 4 is a calibration curve of cysteine concentration and peak current value.

FIG. 5 is a diagram showing a change in half peak potential of a 1 mM cysteine solution depending on pH.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Nicolae, Spatar 77208, Romania, Bucharest 6, Independentai, Espreh, 202 (72) Inventor Brus, Bengata, Salad 6-21-4-407 Hongo, Bunkyo-ku, Tokyo (72) Inventors Tata, Narasinga, Lao 6-21-4-407 Hongo, Bunkyo-ku, Tokyo

Claims (12)

[Claims] 1. A method for measuring the concentration of thiol, comprising: preparing a diamond electrode having conductive diamond, and a counter electrode; bringing the diamond electrode and the counter electrode into contact with a test sample; Applying a voltage at which an oxidation reaction occurs on the diamond electrode to the counter electrode, measuring a current value under the voltage, and calculating a thiol concentration in the test sample from the obtained current value. A method comprising: 2. The method according to claim 1, wherein the diamond electrode has a diamond thin film made conductive by mixing a Group 3 or 5 element. 3. The method according to claim 1, wherein the diamond electrode comprises boron, nitrogen,
The method according to claim 1, comprising a diamond thin film made conductive by mixing one or more elements selected from the group consisting of phosphorus and phosphorus. 4. The method according to claim 1, wherein the thiol is cysteine, homocysteine, glutathione, glutathione, metaionine, glutamine cysteine, N-acetylcysteine, or carbocysteine. 5. The step of calculating the concentration of the thiol in the test sample from the obtained current value includes the step of calculating a calibration curve of the thiol concentration and the current value prepared in advance and the obtained current value. The method according to claim 1, wherein the method is performed by comparing. 6. The method according to claim 1, wherein a voltage at which an oxidation reaction occurs on the diamond electrode between the diamond electrode and the counter electrode is a voltage that gives a peak current. the method of. 7. The method further comprises bringing a reference electrode into contact with a test sample, and controlling an absolute value of a voltage at which an oxidation reaction occurs on the diamond electrode between the diamond electrode and the counter electrode. The method according to any one of claims 1 to 6. 8. A sensor for measuring the concentration of thiol, comprising a diamond electrode having conductive diamond. 9. The sensor according to claim 8, wherein the diamond electrode has a diamond thin film made conductive by mixing boron. 10. The thiol is cysteine, homocysteine, glutathione, glutathione, metaionine,
10. The sensor according to claim 8, wherein the sensor is glutamine cysteine, N-acetylcysteine, or carbocysteine. 11. An apparatus for measuring the concentration of thiol in a test sample, comprising: a diamond electrode having conductive diamond; a counter electrode; a means for bringing the diamond electrode and the counter electrode into contact with the test sample; A means for applying a voltage at which an oxidation reaction occurs on the diamond electrode between the diamond electrode and the counter electrode; a means for measuring a current value under the applied voltage; Means for calculating the concentration of the thiol in the sample. 12. A reference electrode, further comprising: a means for bringing the reference electrode into contact with a test sample; and a voltage for generating an oxidation reaction on the diamond electrode between the diamond electrode and the counter electrode. 12. The apparatus of claim 11, comprising means for controlling an absolute value.