JP4513093B2 - Carbon electrode for electrochemical measurement and manufacturing method thereof - Google Patents

Description

  TECHNICAL FIELD The present invention relates to an electrode for electrochemical measurement suitable for analysis of living organisms, environment, medical care, foods, etc. and analysis using a chemical sensor, and a method for producing the same, and relates to a flow injection analysis device, a high performance liquid chromatography analysis device, a capillary. For electrochemical measurement that enables high sensitivity and long-term stability by incorporating it into the liquid flow cell of various electrochemical analyzers such as electrophoretic analyzers or by using it as a detector for microanalytical chips or chemical sensors The present invention relates to an electrode and a manufacturing method thereof.

  An electrochemical measurement method using an electrochemical reaction in a solution is used for identifying a trace heavy metal in a solution, measuring a concentration, and detecting a trace component in a living body. In this electrochemical measurement method, an electrode is brought into contact with a solution to be measured, and an electric potential is applied to the solution to cause an oxidation or reduction reaction in the molecule to be measured contained in the solution, resulting in a change in current. The electrode used is capable of detecting current changes in a wide potential range, charging current that becomes a noise component during detection, and the electric charge of substituents present on the electrochemically active electrode surface. It is important to be able to measure while suppressing the influence of the current generated by the chemical reaction. This detectable potential range (potential window) depends on the electrode material, solvent, supporting electrolyte, and the like. On the other hand, the noise component of the electrode varies depending on the material and structure of the electrode and the surface area. Electrode materials used for electrochemical measurements include precious metal materials such as gold, platinum and silver, metal materials such as mercury and nickel, semiconductor materials such as tin dioxide and indium oxide, crystalline carbon, carbon fiber, and glassy carbon. Various carbon materials are used. In particular, carbon materials have high corrosion resistance and a relatively wide potential window on the redox side, so that they can detect substances at a wide range of potentials. Is widely used as a detection electrode of an apparatus for separating and detecting the same.

  On the other hand, in recent years, many microelectrodes have been used for the purpose of measuring extremely small amounts of biological samples. For example, in addition to micro-area measurement such as measurement of molecules released from cells, microelectrophoresis analysis using capillary electrophoresis that performs analysis with nano-picoliter sample volume requires the detection cell itself to be extremely small. Therefore, microelectrodes are used. Furthermore, recently, research and development of micro analysis chips that perform high-performance liquid chromatography and capillary electrophoresis using micro flow channels formed of glass or plastic have been actively conducted. A microelectrode is used as the electrode.

  As a method for producing a minute carbon electrode, a method of embedding a carbon fiber in a resin has been conventionally used. However, with this method, it is extremely difficult to obtain electrodes of the same size and characteristics with good reproducibility, in addition to being unable to obtain microelectrodes of any size and shape.

  Therefore, in recent years, a method for producing a minute carbon thin film electrode by applying a lithography technique has been proposed. In this method, after forming a carbon thin film on a substrate, a resist which is a photosensitive polymer is applied, an image mask having an electrode pattern is overlaid, exposed and developed, and the remaining resist is exposed as a mask. A microelectrode pattern is obtained by etching a carbon thin film (Non-patent Document 1). In order to produce this minute carbon thin film electrode, a carbon thin film having a uniform and good adhesion to the substrate and having conductivity that can be energized to cause an electrochemical reaction is required.

  Carbon thin films for electrochemical measurements include carbon thin films (non-patent document 2) prepared by the thermal CVD (Chemical Vapor Deposition) method of aromatic compounds and diamond thin films doped with boron (B) prepared by the plasma CVD method. (Non-Patent Document 3) has been reported. In addition, a method has also been reported in which a polymer material is formed by spin coating or an electrochemical method, and is then subjected to thermal decomposition to obtain a carbon thin film (Non-Patent Document 4).

Among the carbon films reported so far, it has been reported that a boron-doped diamond film produced by a plasma CVD method has a wider potential window than a carbon film having a normal graphite structure. Boron is doped to make the diamond film conductive. The structure of this diamond film is crystalline and shows a sharp Raman spectrum having a peak at 1332 cm ⁻¹ . Further, it is known that a molecule having a low electron transfer rate with an electrode such as L-ascorbic acid can be distinguished from a molecule having a relatively high electron transfer rate such as dopamine by non-patent document 5). That is, with a normal carbon electrode, when L-ascorbic acid, uric acid, and catecholamines such as dopamine are measured simultaneously, the oxidation peaks of both are observed, and the two cannot be separated. On the other hand, when a boron-doped diamond thin film electrode is used, the peaks of L-ascorbic acid and uric acid, which are slow in electron transfer, shift to a high potential, and therefore, both can be separated.

Niwa et al., Anal. Chem., Vol. 66, 285-289 (1994). A. Rojo, et al., Anal. Chem., 58, 2988-91 (1986) J. Xu et al., Anal. Chem., Vol. 69, 591A-597A (1997) Niwa et al., J. Electroanal. Chem., 367, 265-69 (1994) E. Popa et al., Anal. Chem., 72, 1724-27 (2000)

  By the way, in a conventional carbon thin film electrode, for example, in the case of a carbon film produced by a thermal CVD method, it shows a potential window equivalent to that of a normal glassy carbon electrode, and the background current contributing to noise components is also similar to that of a gold or platinum thin film. Compared to thin metal thin-film electrodes, it has low characteristics, but on the other hand, it is highly hydrophobic like alkylphenol, has a high oxidation potential, and is electrochemically irreversible (after being oxidized and then converted to another compound by a chemical reaction). In the case of a molecule to be measured that changes and is not reduced and does not return to its original state (100%), there is a problem that if the measurement is continued, the product of the oxidation reaction is deposited on the electrode surface and the response current decreases. In addition, the potential window is narrower and the background current is larger than the boron-doped diamond electrode described later.

  On the other hand, in a boron-doped diamond electrode, since a normal diamond thin film is insulative, it is necessary to dope boron to impart conductivity in order to use it as a thin film for electrochemical measurement. Further, it is necessary to form a film at a high temperature of about 700 ° C. for a relatively long time at the time of production, and it is not possible to form a film on a material having poor heat resistance such as a polymer. In addition, since the prepared thin film is made of a polycrystalline material and the surface is not flat, it is necessary to devise processing an extremely small electrode by a process such as light exposure and dry etching when forming a thin film electrode. .

  Further, the method of obtaining a carbon thin film by thermally decomposing a polymer thin film has problems that a high heat treatment temperature is required depending on the material and that the adhesion of the film is poor.

  The present invention has been made in view of the above-described problems, and is versatile at the time of fabrication, can be applied to many substrates, has a wide potential window, is difficult to adsorb an electrode reaction product, and has a noise. An object of the present invention is to provide a carbon thin film electrode for high-sensitivity electrochemical detection having a low charging current and the like and having sufficient flatness when processed into an extremely small electrode, and a method for producing the same.

As a result of intensive studies, the inventors of the present application can achieve the above object by configuring the carbon electrode with carbon composed of microcrystalline domains composed of sp ² bonds and sp ³ bonds. The present inventors have found that a carbon electrode can be formed by depositing carbon on a substrate by a sputtering method typified by electron cyclotron resonance sputtering, and completed the present invention.

That is, the present invention relates to a carbon electrode for electrochemical measurement in which a molecule to be measured is detected by oxidation or reduction reaction in a solution to be measured, and the carbon electrode is composed of a microcrystalline domain composed of sp ² bonds and sp ³ bonds. A carbon electrode for electrochemical measurements is provided. The present invention also provides a method for producing the carbon electrode of the present invention, comprising depositing carbon on a substrate by electron cyclotron resonance sputtering, RF sputtering, DC sputtering, or ion beam sputtering.

According to the carbon electrode of the present invention, by forming a carbon thin film composed of a microcrystalline domain composed of sp ² bonds and sp ³ bonds on a substrate, compared with a glassy carbon electrode or the like conventionally used for measurement. It is possible to suppress a decrease in electrode area due to strong adsorption on the surface of a substance to be analyzed accompanying an electrochemical reaction. Moreover, since it has electroconductivity, an electrochemical measurement can be performed without doping other elements, such as boron, like a diamond film electrode at the time of film preparation. As a result, the manufacturing process is easy.

  Therefore, the carbon thin film of the present invention can be used for general electrochemical measurement electrodes such as voltammetry and amperometry, electrochemical measurement in a flow system such as high performance liquid chromatography, flow injection analysis, capillary electrophoresis, biosensor, It is extremely useful as a thin film for electrodes used in detectors such as integrated microchips.

The carbon electrode of the present invention is composed of microcrystalline domains composed of sp ² bonds (graphite type structure) and sp ³ bonds (diamond type structure). That is, the microcrystalline domain (graphite structure) composed of sp ² bonds is composed of microcrystalline domains connected by sp ³ bonds. The size (major axis) of each microcrystalline domain is usually about 0.5 nm to 100 nm, and this can be controlled by adjusting the acceleration voltage of irradiation ions described later. This carbon electrode is characterized in that it has a high electrical conductivity and does not substantially adsorb and coat an electrochemically irreversible substance to be measured on the electrode after the electrode reaction. The electrode for electrochemical measurement has a stable and wide potential window as compared with a conventional glassy carbon or graphite electrode by including a microcrystal containing many sp ³ bonds like a diamond electrode. In addition, by including sp ² bonds, the conductivity is lower than that of graphite, but the conductivity is sufficient for electrochemical measurement, and it is not necessary to perform doping as in a diamond thin film.

The bond ratio [sp ³ / sp ² ] between the sp ³ bond and the sp ² bond is not particularly limited, but is preferably in the range of 0.05 to 1.2, and more preferably in the range of 0.6 to 0.9. When the coupling ratio is within this range, the potential window is much wider than that of the glassy carbon electrode, and a measurement object having a high oxidation potential can be detected as a clearer current signal. In this case, the potential window can be widened by ± 200 mV or more in the positive direction and the negative direction, respectively, as compared with the conventional glassy carbon electrode.

  The center electrode average roughness (Ra) of the carbon electrode of the present invention is not particularly limited, but preferably has a flat surface of ± 1 nm or less. Since it is smooth at the atomic level, for example, it is not electrochemically reversible such as histamine, phenols, NADH, and adsorption of a measurement target having a high oxidation potential can be suppressed. Further, when processing and producing an extremely small electrode by a lithography method, a finer electrode can be obtained because the surface is much smoother than a film made of a large number of microcrystals such as a boron-doped diamond film. Further, the charging current on the surface can be reduced by an order of magnitude (about 1/10) as compared with the conventional glassy carbon electrode.

  The film thickness of the electrode of the present invention is not particularly limited, but about 30 nm to 200 nm is preferable from the viewpoint of securing sufficient conductivity and producing a very small electrode.

  The carbon electrode of the present invention can be formed by depositing carbon on a substrate by electron cyclotron resonance (ECR) sputtering, RF sputtering, DC sputtering, or ion beam sputtering.

  Since the ECR sputtering method itself is well known and an apparatus therefor is commercially available, the carbon electrode of the present invention can be produced using a commercially available ECR sputtering apparatus. In this case, various substrates can be used, and examples of preferable substrate materials include silicon, gallium arsenide, indium phosphide, stainless steel, nickel, polyimide, and the like. Moreover, the carbon source used as a target will not be specifically limited if it is carbon, but a sintered carbon can be mentioned as an example of a preferable carbon source.

  Since the RF sputtering method itself is well known and an apparatus therefor is commercially available, the carbon electrode of the present invention can be produced using a commercially available RF sputtering apparatus. In this case, as the substrate, various substrates described for the ECR sputtering method described above can be used. Moreover, the carbon source used as a target will not be specifically limited if it is carbon, However, As an example of a preferable carbon source, sintered carbon can be mentioned.

  Since the DC sputtering method itself is well known and an apparatus therefor is commercially available, the carbon electrode of the present invention can be produced using a commercially available RF sputtering apparatus. In this case, as the substrate, various substrates described for the ECR sputtering method described above can be used. Moreover, the carbon source used as a target will not be specifically limited if it is carbon, However, As an example of a preferable carbon source, sintered carbon can be mentioned.

  Since the ion beam sputtering method itself is well known and an apparatus therefor is commercially available, the carbon electrode of the present invention can be produced using a commercially available RF sputtering apparatus. In this case, as the substrate, various substrates described for the ECR sputtering method described above can be used. Moreover, the carbon source used as a target will not be specifically limited if it is carbon, However, As an example of a preferable carbon source, sintered carbon can be mentioned.

In each of the sputtering methods described above, the bond ratio of sp ³ bond and sp ² bond can be controlled by adjusting the acceleration voltage of irradiated ions. The ion acceleration voltage can be controlled by applying an RF or DC bias to the substrate and adjusting the bias voltage. In the following examples, in the ECR sputtering method, when the ion acceleration voltage was changed from 20 V to 160 V, the ratio of sp ³ bonds was the highest at 85 V. Therefore, as described in the following examples, the desired bond can be determined by examining the bond ratio between the sp ³ bond and the sp ² bond by changing the ion acceleration voltage in the ECR sputtering apparatus and the manufacturing conditions to be used by routine experiments. The ion acceleration voltage that achieves the ratio can be easily known.

  Also, by using each of the above sputtering methods, the center line average roughness (Ra) of the surface of the carbon electrode can be reduced to ± 1 nm or less. In the following examples, a center of about ± 0.1 nm is obtained by the ECR sputtering method. The line average roughness (Ra) has been achieved, and the center line average roughness (Ra) of about ± 0.1 nm has been achieved by RF sputtering. The centerline average roughness (Ra) can be measured by observing the surface with an atomic force microscope (AFM).

The ratio of sp ³ bonds to sp ² bonds can be measured by X-ray photoelectron spectroscopy (XPS). That is, the binding energy (eV) and strength (CPS) are measured by XPS, and the binding energy (eV) is plotted on the horizontal axis and the strength (CPS) is plotted on the vertical axis. The curve representing the sp ³ bond has a peak at 285.3 eV, and the curve representing the sp ² bond has a peak at 284.3 eV. By taking the ratio of these peak heights, the sp ³ bond / sp ² bond can be calculated, and the above bond ratio [sp ³ / (sp ³ + sp ² )] can be calculated from this.

  Hereinafter, the present invention will be described more specifically based on examples. However, the present invention is not limited to the following examples.

1. Production of carbon electrodes by ECR sputtering
A schematic diagram of the configuration of the ECR sputtering apparatus used in the ECR sputtering method is shown in FIG. In FIG. 1, reference numeral 1 is a main chamber, 2 is a load lock chamber, and the main chamber 1 is composed of a plasma chamber and a film forming chamber. Further, 5 is a Y-branch type microwave waveguide, 6 is a microwave introduction window, 7 is a microwave branch point, 8 is an annular first coil, and 9 is arranged in parallel with the first coil 8. An annular second coil, 10 is a substrate holder, 11 is a stopper, 12 is a gate valve, and 13 is a lead wire for applying a radio frequency (RF) bias. A silicon substrate 14 for forming the carbon thin film for electrochemical measurement of the present invention is fixed to the substrate holder 10, and a sputtering target 15 is provided in the film forming chamber 4. The central part is an area where plasma is generated.

In this ECR sputtering apparatus, the mean free process and kinetic energy of electrons in the main chamber 1 are increased by the action of the microwave and the magnetic field. As a result, the ionization efficiency of the neutral gas due to electron impact is increased, and is about 0.01 Pa. Even under a low gas condition, a high-density ECR plasma p of 10 ¹² cm ⁻³ level can be generated. The ECR plasma p does not require an external electric field to extract plasma, and ions are automatically extracted.

  Next, a method for forming a carbon thin film on the silicon substrate 14 using this ECR sputtering apparatus will be described. First, a 2-inch silicon substrate (no oxide film) 14 was set on the substrate holder 10 of the ECR sputtering apparatus. In this ECR sputtering apparatus, since the cylindrical target 15 is provided in the middle of the plasma p flow, a voltage is applied to this target. Ions collide with the inner peripheral surface of the target 15 to cause sputtering. For this reason, a thin film is formed on the silicon substrate 4 placed perpendicular to the plasma p flow.

Here, argon (Ar) was used as the sputtering gas, sintered carbon (C) was used as the target 15, and the gas pressure of argon (Ar) was 6 × 10 ⁻³ to 1 × 10 ⁻¹ Pa. The substrate temperature was 200 ° C. In this ECR sputtering apparatus, desired power can be obtained by adjusting gas pressure and microwave power. Here, the ion current density was set to ^two types of 0.8, 3.6, and 7.9 mA / cm ² . Moreover, the ion acceleration voltage was changed from 20 to 160 V by controlling the RF power bias with respect to this ion current density. A DC voltage of 500 V was applied to the target.

  The film thickness of the carbon thin film obtained with a bias of 85 V was 40 nm. In this method, damage to the substrate during sputtering is small. Unprecedented film formation characteristics such as high-quality film formation at a relatively low temperature and promotion of diffusion of sputtered atoms on the substrate can be obtained.

2. Characteristics of the electrode After the carbon thin film was prepared, the surface of this thin film was observed with an atomic force microscope (AFM), and as a result, it was found that the surface was smooth on the atomic order with irregularities of about ± 0.1 nm. When this electrode is measured by XPS, a shoulder-like peak based on the sp ³ bond is observed in the vicinity of the peak due to the sp ² bond (FIG. 2). This peak is a peak of the sp ³ bond when the ion acceleration voltage is 85V. Was found to be maximal (FIG. 3). When the ion acceleration voltage was 85 V, the ratio of sp ³ bonds to sp ² bonds (sp ³ / sp ² ) was 0.85 as shown in FIG. On the other hand, in a commercially available glassy carbon film or a graphitic carbon thin film produced by a thermal CVD method (produced by the method described in Non-Patent Document 1), only a peak due to sp ² bonds was observed. Further, when the formed carbon thin film is observed with an electron microscope, a microcrystalline domain composed of sp ² bonds (graphite structure) and a microcrystalline domain composed of sp ³ bonds (diamond structure) are mixed. It was confirmed that The size of each microcrystalline domain in the carbon thin film electrode produced at an ion acceleration voltage of 85 V was about 0.5 nm to 5 nm.

  As XPS, a commercially available XPS device, Kratos AXIS Ultra, was used, and the X-ray source was Al Kα: 1846.6 eV. The binding energy (eV) and strength (CPS) were measured, and the binding energy (eV) was plotted on the horizontal axis and the strength (CPS) was plotted on the vertical axis (FIG. 2).

3. Electrode performance Next, the carbon thin-film electrode of the present invention prepared in 1 above was used as a working electrode, and immersed in a phosphate buffer solution (pH 7) together with a silver-silver chloride reference electrode and a platinum wire used as a counter electrode. The electrode was connected to a potentiostat (CHI802), and a potential of 700 mV was applied to the reference electrode. After applying the potential, the current value decreases as shown in FIG. The current value measured after 10 minutes was about 3 nA. On the other hand, when the same measurement was performed using a glassy carbon electrode having the same electrode area, a current of about 50 nA was observed. Since the experimental system does not include active species that oxidize on the electrode, this result shows the difference in the actual electrode surface area or the difference in the reactivity of the electrode surface. It was confirmed that it is low and suitable for high-sensitivity measurement. Moreover, when cyclic voltammogram measurement was performed after deoxygenation in a 0.1 M sulfuric acid solution, it was found that the carbon electrode of the present invention had a wider potential of about 400 mV on the oxidation side than the glassy carbon electrode. Next, measurement was performed by adding 100 μM histamine to a pH 7 phosphate buffer solution. As a result, a clearer peak was observed in the carbon thin film electrode of the present invention than in the glassy carbon electrode, and the potential window was clearly wide. It was confirmed.

Moreover, cyclic voltammetry was performed using the carbon thin film electrode of the present invention prepared in 1 above or a glassy carbon (GC) electrode for comparison, and the charging current was measured. The measurement conditions were a sweep rate of 100 mV / sec, a sweep range of 0 to 2 V (initial potential 0 V), a phosphate buffer (pH 7.0), an electrode area of 0.071 cm ² (diameter 3 mm), no deaeration, and no stirring. . The results are shown in FIG. As shown in FIG. 5, in the carbon electrode of the present invention, the charging current is one digit smaller than that of the conventional GC electrode.

1. Production of carbon electrodes by RF sputtering
A carbon film was formed by RF sputtering. The substrate was a silicon substrate, and the target was a sintered target. The argon gas pressure was 1 Pa and the RF power was 500 W. An RF bias was applied to the substrate, and the self-bias voltage was controlled by the applied RF power.

2. Electrode characteristics By the self-bias voltage, the sp ² and sp ³ components were controllable in the same manner as in ECR sputtering. When the self-bias was −100 V, sp ³ was maximized, and the coupling ratio [sp ³ / sp ² ] at this time was 0.4. The flatness (centerline average roughness) of the electrode was 1 nm. The coupling ratio [sp ³ / sp ² ] and the flatness of the electrodes were measured in the same manner as in Example 1.

FIG. 6 is a block diagram showing an example of a flow injection analyzer (electrochemical detection) including the carbon electrode of the present invention, and detects the carbon thin film electrode of the present invention in a liquid flow cell for flow injection analysis. This is an example of incorporating as a container. In FIG. 6, reference numeral 21 is a carbon thin film electrode, 22 is a thin layer type flow cell (fluid flow cell for flow injection analysis), 23 is a pump, 24 is a reservoir, 25 is an injector for injecting a sample, and 26 is a potentiometer. Ostat (manufactured by BAS, LC4C), 27 is a recorder, 28 is a silver / silver chloride reference electrode, and 29 is a stainless steel tube serving as a counter electrode.

  Next, a method for performing flow injection analysis using this flow injection analyzer will be described. First, the carbon thin film electrode 21 was produced by ECR sputtering by the same method as in the first example. For comparison, a commercially available glassy carbon electrode (diameter 6 mm) was used. Next, the carbon thin film electrode 21 of the present invention was covered with a Teflon (registered trademark) tape other than the disk electrode portion having a diameter of 6 mm, and then set on the thin layer type flow cell 22 to assemble a flow injection analyzer. Similarly, the glassy carbon electrode was set in order to the thin layer type flow cell, and the flow injection analyzer was assembled.

  Next, a phosphate buffer solution (pH 7) was fed by a pump at a flow rate of 200 μl / min. A potential of 900 mV was applied to the silver / silver chloride reference electrode, and a phosphate buffer solution containing 100 μM nonylphenol was injected from the injector. The result is shown in FIG. In the carbon thin film electrode of the present invention, even when the sample was injected several times, the magnitude of the oxidation current did not change at all, but when the glassy carbon electrode was used, the current peak was clearly reduced. This result shows that in the carbon thin film electrode of the present invention, unlike the commercially available glassy carbon electrode, phenol does not cover the electrode after oxidation.

FIG. 8 is a schematic view showing a fine carbon thin film array electrode using the carbon electrode of the present invention. 31 is a fine disk electrode, and 32 is a photoreactive polymer (resist) film. The insulating film 33 used is a pad portion for connecting the electrode and the potentiostat.

  First, the manufacturing method of this micro carbon thin film array electrode is demonstrated. First, a carbon thin film electrode was produced by ECR sputtering by the same method as in Example 1. Next, a positive photoresist (PMMA) was applied on the carbon thin film to a thickness of about 0.3 μm by spin coating. After that, an electron beam exposure apparatus exposes a circular pattern with a diameter of 200 nm and a 2 mm square area with a sense of 20 μm, and then develops and dissolves the resist in the exposed portion, thereby forming a micro disk array electrode in which a large number of 200 nm disk electrodes are arranged. Was made. For comparison, the same pattern was formed using a carbon thin film prepared by thermal CVD and a boron-doped diamond thin film. The graphite thin film electrode was able to form the same size, but the diamond thin film was 400 nm. Only the above pattern can be formed with high accuracy, which indicates that the flatness of the carbon thin film electrode of the present invention is superior to that of the diamond thin film.

  After the pattern formation, the micro carbon thin film array electrode was set in the same flow cell as in the embodiment 3. For comparison, a carbon thin film prepared by thermal CVD was converted into a micro carbon thin film array electrode by the same method. In this example, instead of the pump used in FIG. 6, two syringe pumps were used, one containing a phosphate buffer solution (pH 7) and the other containing 100 μM histamine phosphate buffer solution. The characteristics were evaluated by alternately feeding with a syringe pump. The liquid feeding speed was 20 μl / min, and a fixed potential of 1.2 V was applied to the electrode with respect to the silver / silver chloride reference electrode. In the micro carbon thin film array electrode fabricated from the carbon thin film electrode of the present invention, when the phosphate buffer solution is changed to a phosphate buffer solution containing histamine, the current value decreases little after the current increase due to histamine oxidation is observed. . On the other hand, in a micro carbon thin film array electrode fabricated from a carbon thin film fabricated by thermal CVD, histamine was continuously introduced, and once the oxidation current increased, the current value decreased rapidly. This result shows that, in the carbon thin film electrode of the present invention, the oxidation reaction product does not cover the electrode surface even when used for detection of a measurement object having a high oxidation potential.

Measurement of Oxidation Reaction FIG. 9 shows another oxidation reaction of cyclic AMP (cyclic adenosine monophosphate, hereinafter referred to as cAMP) using the carbon thin film electrode of the present invention prepared in Example 1. cAMP has an oxidation potential at a high potential and is known to be a biologically very important nucleic acid derivative (intracellular second messenger). In the experiment, carbon thin film electrode was used as working electrode, silver silver chloride reference electrode and platinum wire used as counter electrode were immersed in phosphate buffer solution (pH7.4), and each electrode was potentiostat (CHI 660A) The voltage was swept from 0 mV to 2300 mV with respect to the reference electrode by a cyclic voltammetry method and observed. As a result, an oxidation potential of cAMP was observed at 1731 mV with respect to the silver-silver chloride reference electrode. In addition, for other nucleic acid derivatives (cGMP; cyclic guanosine monophosphate, cCMP; cyclic cytidine monophosphate, cTMP; cyclic thymidine monophosphate), 1.284 mV, 1.871 mV, 1. A clear oxidation peak current could be observed at 846 mV. This is not observable with a glassy carbon electrode having a high background current, so the usefulness of high potential oxidation of the carbon thin film electrode according to the present invention was clearly recognized.

  FIG. 10 shows the results of NADH (nicotine adenine dinucleotide) measurement using the carbon thin film electrode and glassy carbon electrode of the present invention, which were performed in the same manner as the histamine measurement described in Example 1-3. As the number of measurements increased, the oxidation peak current value ○ 3 (Mull 3) of the glassy carbon electrode decreased and the oxidation potential ○ 4 (Mull 4) shifted to a higher potential. This indicates that the product formed on the electrode surface is adsorbed in an electrochemically inactive state and inhibits subsequent reactions. In contrast, the carbon thin film electrode showed almost no fluctuation due to the number of measurements in the oxidation peak current 1 (maru 1) and the oxidation potential 2 (mal 2). This indicates that the adsorption of the produced product is effectively suppressed as compared with the glassy carbon electrode.

In the Example of this invention, it is a block diagram which shows the ECR sputtering apparatus used when producing a carbon thin film electrode. In the Example of this invention, the XPS C1s spectrum of the carbon thin film electrode produced by the ion accelerating voltage 20V or 85V by ECR sputtering method is shown. It is a diagram illustrating an acceleration voltage dependence of the sp ³ / sp ² ratio of carbon thin film electrode prepared by ECR sputtering in the embodiment of the present invention. It is a figure which shows the measurement result which compared the time change of the electric current at the time of the oxidation potential application of the carbon thin film electrode produced by the ECR sputtering method in the Example of this invention with the commercially available glassy carbon electrode. It is a figure which shows the result of having performed cyclic voltammetry using the carbon thin film electrode of this invention or the glassy carbon (GC) electrode for a comparison, and measuring the charging current. It is a block diagram which shows an example of the flow injection analyzer using the carbon electrode of this invention. It is a figure which shows the measurement result of the paranonyl phenol measured using the flow injection analyzer using the carbon electrode of this invention produced in the Example of this invention. It is the schematic which showed the structure of the micro carbon thin film array electrode using the carbon electrode of this invention. In the Example of this invention, it is a measurement result of cyclic AMPs measured using the carbon thin film electrode produced by the ECR sputtering method. In the Example of this invention, it is a figure which shows the time-dependent change of the peak value and potential of a NADH cyclic voltammogram measured using the carbon thin film electrode or the glassy carbon electrode produced by the ECR sputtering method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Main chamber 2 Load lock chamber 3 Plasma chamber 4 Film formation chamber 5 Y branch type micro waveguide 6 Microwave introduction window 7 Microwave branch point 8 First coil 9 Second coil 10 Substrate holder 11 Stopper 12 Gate valve 13 Lead wire 14 Silicon substrate (no oxide film)
15 target 21 silicon substrate on which the carbon thin film of the present invention is formed 22 thin layer type flow cell (fluid flow cell for flow injection analysis)
23 Pump 24 Reservoir 25 Injector for sample injection 26 Potentiostat (BAS, LC4C)
27 Recorder 28 Silver / silver chloride reference electrode 29 Stainless steel tube as counter electrode 31 Minute disk electrode 32 Insulating film using photoreactive polymer (resist) film 33 Pad part for connecting electrode and potentiostat

Claims (5)

A carbon electrode for electrochemical measurement in which a molecule to be measured is detected by oxidation or reduction reaction in a solution to be measured, wherein the carbon electrode is composed of a microcrystalline domain composed of sp ² bonds and sp ³ bonds. Carbon electrode for electrochemical measurements. The electrode according to claim 1, wherein the sp ³ bond to sp ² bond ratio [sp ³ / sp ² ] is in the range of 0.05 to 1.2.   The electrode according to claim 1, wherein the center line average roughness (Ra) has a flat surface of ± 1 nm or less.   The method for producing a carbon electrode according to claim 1, comprising forming a carbon film on the substrate by electron cyclotron resonance sputtering, RF sputtering, DC sputtering, or ion beam sputtering. The method according to claim 4, comprising adjusting a binding ratio [sp ³ / sp ² ] of sp ³ bonds to sp ² bonds by adjusting an acceleration voltage of ions irradiated to the substrate.

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