ELECTRODE

An electrode (1) includes a resin film (2), a metal underlayer (3), and a conductive carbon layer (4) in sequence in a thickness direction. A surface of the conductive carbon layer (4) has an arithmetic average roughness Ra of 1.50 nm or less and a skewness Rsk of 0.00 or more.

TECHNICAL FIELD

The present invention relates to an electrode.

BACKGROUND ART

There has been known an electrode including a flexible substrate, a metal layer, and a conductive carbon layer in sequence in the thickness direction (ref: for example, Patent Document 1). Patent Document 1 describes in the electrode, noise is suppressed by setting the surface roughness of the conductive carbon layer to 2.0 nm or less.

CITATION LIST

Patent Document

SUMMARY OF THE INVENTION

Problem to be Solved by the Invention

However, there is a disadvantage that signal intensity is likely to decrease in the electrode described in Patent Document 1.

The present invention provides an electrode capable of suppressing noise and suppressing a decrease in the signal intensity.

Means for Solving the Problem

The present invention (1) includes an electrode including a resin film, a metal underlayer, and a conductive carbon layer in sequence in a thickness direction, in which a surface of the conductive carbon layer has an arithmetic average roughness Ra of 1.50 nm or less and a skewness Rsk of 0.00 or more.

The present invention (2) includes the electrode described in (1), in which the conductive carbon layer contains metal, and the metal is in a proportion of 5% by mass or more and 50% by mass or less relative to the conductive carbon layer.

The present invention (3) includes the electrode described in (1) or (2), in which the metal is titanium.

The present invention (4) includes the electrode described in any one of the above-described (1) to (3), being an electrode for electrochemical measurement.

Effects of the Invention

In the electrode of the present invention, the surface of the conductive carbon layer has an arithmetic average roughness Ra of 1.50 or less and a skewness Rsk of 0.00 or more, so that it is possible to suppress noise and also suppress a decrease in the signal intensity.

DESCRIPTION OF THE EMBODIMENTS

One Embodiment

One embodiment of the electrode of the present invention will be described with reference toFIG.1.

As shown inFIG.1, an electrode1has a thickness. The electrode1has a film shape (including a sheet shape). The electrode1includes a resin film2, a metal underlayer3, and a conductive carbon layer4in sequence toward one side in the thickness direction. The electrode1preferably only includes the resin film2, the metal underlayer3, and the conductive carbon layer4.

The resin film2has a thickness. The resin film2is a substrate film in the electrode1. The material of the resin film2is a resin. Examples of the resin include a polyester resin, an olefin resin, an acetate resin, a polyether sulfone resin, a polycarbonate resin, a polyamide resin, a polyimide resin, a polyolefin resin, an acrylic resin, a polyvinyl chloride resin, a polyvinylidene chloride resin, a polystyrene resin, a polyvinyl alcohol resin, a polyarylate resin, and a polyphenylene sulfide resin. These can be used alone or in combination. For the resin, preferably, a polyester resin is used. Examples of the polyester resin include polyethylene terephthalate (PET) and polyethylene naphthalate. For the polyester resin, preferably, PET is used.

The thickness of the resin film2is not particularly limited. The thickness of the resin film2is, for example, 2 μm or more, preferably 20 μm or more, and for example, 1000 μm or less, preferably 500 μm or less.

An arithmetic average roughness Ra of one surface in the thickness direction of the resin film2is not particularly limited. The arithmetic average roughness Ra of the one surface in the thickness direction of the resin film2is, for example, 5 nm or less, preferably 1 nm or less, and for example, 0.1 nm or more, preferably 0.3 nm or more. The arithmetic average roughness Ra of the one surface in the thickness direction of the resin film2is measured in accordance with JIS B0601:2013. The arithmetic average roughness Ra of the following layers are measured in the same manner as above.

A skewness Rsk of the one surface in the thickness direction of the resin film2is not particularly limited. The skewness Rsk of the one surface in the thickness direction of the resin film2is, for example, −0.3 or more, preferably −0.2 or more, and for example, 1.5 or less, preferably 0.8 or less. The skewness Rsk of the one surface in the thickness direction of the resin film2is determined as skewness of a roughness curve in accordance with JIS B0601:2013. The skewness Rsk of the following layers are measured in the same manner as above.

The metal underlayer3is disposed on the one surface in the thickness direction of the resin film2. Specifically, the metal underlayer3is in contact with the entire one surface in the thickness direction of the resin film2. The metal underlayer3has a thickness.

The material of the metal underlayer3is a metal. Examples of the metal include titanium, chromium, tungsten, aluminum, copper, silver, gold, molybdenum, tantalum, palladium, silicon, and alloys thereof. For the metal, preferably, titanium is used.

The thickness of the metal underlayer3is not particularly limited. The thickness of the metal underlayer3is, for example, 1 nm or more, preferably 3 nm or more, more preferably 5 nm or more, and for example, 1000 nm or less, preferably 100 nm or less, more preferably 50 nm or less.

The conductive carbon layer4is disposed on one surface in the thickness direction of the metal underlayer3. Specifically, the conductive carbon layer4is in contact with the entire one surface in the thickness direction of the metal underlayer3. The conductive carbon layer4has a thickness.

The material of the conductive carbon layer4is mainly carbon. The carbon has, for example, an sp2bond and an sp3bond. Such carbon has a graphite structure and a diamond structure.

The conductive carbon layer4can further contain a metal. When the conductive carbon layer4further contains a metal, it is possible to further suppress noise and also further suppress a decrease in the signal intensity. The metal in the conductive carbon layer4may be the same as or different from the metal in the metal underlayer3. Preferably, the metal in the conductive carbon layer4is the same as in the metal underlayer3.

Examples of the metal include titanium, chromium, tungsten, aluminum, copper, silver, gold, molybdenum, tantalum, palladium, silicon, and alloys thereof. For the metal, preferably, titanium is used. When the metal is titanium, it is possible to further enhance adhesion between the conductive carbon layer4and the metal underlying layer3.

The metal is in a proportion of, for example, 0.1% by mass or more, preferably 1% by mass or more, more preferably 5% by mass or more, and for example, 50% by mass or less, preferably 35% by mass or less, even more preferably 20% by mass or less relative to the conductive carbon layer4. When the proportion of the metal in the conductive carbon layer4is the above-described lower limit or more and the above-described upper limit or less, it is possible to further suppress noise and also further suppress a decrease in the signal intensity. The presence or absence of and the proportion of the metal in the conductive carbon layer4are determined by X-ray fluorescence measurement.

The conductive carbon layer4may further contain a rare gas. Examples of the rare gas include helium, argon, krypton, xenon, and radon. Examples of a method for analyzing the rare gas in the conductive carbon layer4include secondary ion mass spectrometry, resonance ionization mass spectrometry, and X-ray fluorescence analysis.

A surface resistance value of one surface in the thickness direction of the conductive carbon layer4is not particularly limited. The surface resistance value of the one surface in the thickness direction of the conductive carbon layer4is, for example, 1.0×104Ω/□ or less, preferably 1.0×103Ω/□ or less. The surface resistance is measured by a four-terminal method in accordance with JIS K7194.

The thickness of the conductive carbon layer4is not particularly limited. The thickness of the conductive carbon layer4is, for example, 1 nm or more, preferably 2 nm or more, more preferably 5 nm or more, and for example, 100 nm or less, preferably 70 nm or less, more preferably 50 nm or less. When the thickness of the conductive carbon layer4is the above-described upper limit or less, an arithmetic average roughness Ra to be described later in the one surface in the thickness direction of the conductive carbon layer4can be easily set within a desired range. When the thickness of the conductive carbon layer4is the above-described lower limit or more, a uniform film can be deposited in an in-plane direction. The in-plane direction is a direction orthogonal to the thickness direction. The thickness of the conductive carbon layer4is calculated by measuring X-ray reflectivity.

The arithmetic average roughness Ra of the one surface in the thickness direction of the conductive carbon layer4is 1.50 nm or less. The one surface in the thickness direction of the conductive carbon layer4in this embodiment is synonymous with the surface of the conductive carbon layer4. The arithmetic average roughness Ra of the one surface in the thickness direction of the conductive carbon layer4is preferably 1.25 nm or less, more preferably 1.00 nm or less, even more preferably 0.75 nm or less, particularly preferably 0.70 nm or less. When the arithmetic average roughness Ra of the one surface in the thickness direction of the conductive carbon layer4exceeds the above-described upper limit, noise cannot be suppressed. Specifically, when cyclic voltammetry is performed using the electrode1, a capacitance value becomes extremely high. This increases noise. That is, in the present embodiment, since the arithmetic average roughness Ra of the one surface in the thickness direction of the conductive carbon layer4is 1.50 nm or less, noise can be suppressed. For example, in the present embodiment, the capacitance value measured by cyclic voltammetry can be lowered, and therefore, the suppression of noise as described above is demonstrated (see the Examples section below). To set the arithmetic average roughness Ra of the one surface in the thickness direction of the conductive carbon layer4in the above-described range, for example, the thickness of the conductive carbon layer4is adjusted.

On the other hand, the lower limit of the arithmetic average roughness Ra of the one surface in the thickness direction of the conductive carbon layer4is not particularly limited. The lower limit of the arithmetic average roughness Ra of the one surface in the thickness direction of the conductive carbon layer4is, for example, 0.01 nm, preferably 0.10 nm.

The one surface in the thickness direction of the conductive carbon layer4has a skewness Rsk of 0.00 or more. The skewness Rsk of the one surface in the thickness direction of the conductive carbon layer4is preferably 0.15 or more, more preferably 0.20 or more. When the skewness Rsk of the one surface in the thickness direction of the conductive carbon layer4is less than the above-described lower limit, the signal intensity decreases. Specifically, when cyclic voltammetry is performed using the electrode1, an oxidation-reduction potential difference ΔEp increases. This decreases the signal intensity. In other words, in the present embodiment, since the skewness Rsk of the one surface in the thickness direction of the conductive carbon layer4is 0.00 or more, it is possible to suppress a decrease in the signal intensity. This demonstrates that a decrease in the signal intensity is suppressed as described above, because the oxidation-reduction potential difference ΔEp (specifically, ferricyanide activity value) by cyclic voltammetry is low (see the Examples section below). To set the skewness Rsk of the one surface in the thickness direction of the conductive carbon layer4in the above-described range, for example, type of sputtering method, electric power applied to a target, pressure during sputtering, and/or thickness of the conductive carbon layer4are adjusted.

The one surface in the thickness direction of the conductive carbon layer4having a skewness Rsk of 0.00 or more indicates that the skewness Rsk is 0 or a positive integer. Then, protrusions in the one surface in the thickness direction of the conductive carbon layer4are steep and sparse. More specifically, spaces between the protrusions become wider, which in turn increases a proportion occupied by recessed portions. This allows a large amount of measurement target substance to be held in the recessed portions, and the measurement target substance can be measured with good sensitivity, so that it is presumed that a decrease in the signal intensity can be suppressed.

On the other hand, the upper limit of the skewness Rsk of the one surface in the thickness direction of the conductive carbon layer4is not particularly limited. The upper limit of the skewness Rsk of the one surface in the thickness direction of the conductive carbon layer4is, for example, 1.50.

Next, a method for producing the electrode1will be described. First, the resin film2is prepared. Then, the metal underlayer3and the conductive carbon layer4are formed in sequence on one side in the thickness direction of the resin film2.

Examples of the method for forming the metal underlayer3include a dry method and a wet method. Preferably, a dry method is used. Examples of the dry method include a physical vapor deposition (PVD) method and a chemical vapor deposition (CVD) method. For the dry method, preferably, a PVD method is used. Examples of the PVD method include a sputtering method, a vacuum deposition method, a laser vapor deposition method, and an ion plating method (arc vapor deposition method). For the PVD method, preferably, a sputtering method is used. The sputtering method is not particularly limited. Examples of the sputtering method include unbalanced magnetron sputtering (UBM sputtering), high power impulse magnetron sputtering, electron cyclotron resonance sputtering, RF sputtering, DC sputtering (DC magnetron sputtering), DC pulse sputtering, and ion beam sputtering.

In the sputtering method, for example, a sputtering gas and a target are used. The sputtering gas contains a rare gas. Examples of the rare gas include helium, argon, krypton, xenon, and radon. The target is made of the above-described metal.

The pressure during sputtering is, for example, 0.01 Pa or more, and for example, 10 Pa or less.

For the method for forming the conductive carbon layer4, the same method as the above-described method for forming the metal underlayer3is used. For the method for forming the conductive carbon layer4, preferably a dry method is used, more preferably a PVD method is used, even more preferably a sputtering method is used, particularly preferably a high power impulse magnetron sputtering method and a DC pulse sputtering method are used.

For the target, for example, carbon is used, preferably sintered carbon is used. When the conductive carbon layer4contains metal, carbon and metal are used as the target. Specifically, a first target made of carbon (preferably, sintered carbon) and a second target made of metal (preferably, titanium) are used. For example, each of the first target and the second target are disposed in one film deposition chamber independently of the other.

When the first target and the second target are each disposed in a film deposition chamber, a ratio of electric power to be applied thereto is controlled. This allows adjustment of the proportion of the metal contained in the conductive carbon layer4.

In the DC pulse sputtering method, the pulse width generally refers to a discharge stop period. The pulse width is, for example, 0.5 μs or more, and for example, 1 ms or less. In the DC pulse sputtering method, the frequency is, for example, 10 kHz or more, and for example, 500 kHz or less.

In the high power impulse magnetron sputtering method, the pulse width generally refers to a discharge duration. The pulse width is, for example, 10 μs or more, and for example, 3 ms or less. In the high power impulse magnetron sputtering method, the frequency is, for example, 50 Hz or more, and for example, 3 kHz or less.

In this manner, the electrode1including the resin film2, the metal underlayer3, and the conductive carbon layer4in sequence is produced.

Function and Effect of One Embodiment

In this electrode1, the one surface in the thickness direction of the conductive carbon layer4has an arithmetic average roughness Ra of 1.5 nm or less and a skewness Rsk of 0.0 or more. Therefore, it is possible to suppress noise and also suppress a decrease in the signal intensity.

When the conductive carbon layer4further contains metal and the metal is in a proportion of 5% by mass or more and 50% by mass or less relative to the conductive carbon layer4, it is possible to further suppress noise and also further suppress a decrease in the signal intensity.

When the metal is titanium, it is possible to further enhance adhesion between the conductive carbon layer4and the metal underlying layer3.

The use of the electrode1is not particularly limited. Examples of the use of the electrode1include electrodes for electrochemical measurement. Specifically, the electrode1is provided in an electrochemical measurement system as a working electrode. In this case, the electrode1is used as an electrode for electrochemical measurement. Using the electrochemical measurement system, for example, cyclic voltammetry is performed. Examples of the use of the electrochemical measurement system include a blood glucose level sensor. The blood glucose level sensor measures blood glucose levels in blood.

Modified Examples

In the following modified examples, the same reference numerals are provided for members and steps corresponding to each of those in one embodiment described above, and their detailed description is omitted. Further, the modified examples can achieve the same function and effect as that of one embodiment unless otherwise specified. Furthermore, one embodiment and the modified example thereof can be appropriately used in combination.

Though not shown, a modified electrode1can further include the metal underlayer3and the conductive carbon layer4disposed in sequence toward the other side in the thickness direction of the resin film2. In this modified electrode1, the conductive carbon layer4, the metal underlayer3, the resin film2, the metal underlayer3, and the conductive carbon layer4are disposed in sequence toward one side in the thickness direction. The other surface in the thickness direction of the conductive carbon layer4disposed on the other side in the thickness direction of the resin film2has an arithmetic average roughness Ra of 1.5 nm or less and a skewness Rsk of 0.0 or more. In this modified example, each of the one surface and the other surface in the thickness direction of the conductive carbon layer4is one example of the surface of the conductive carbon layer4.

EXAMPLES

Next, the present invention will be further described based on Examples and Comparative Examples shown below. The present invention is however not limited by these Examples and Comparative Examples. The specific numerical values in mixing ratio (content ratio), property value, and parameter used in the following description can be replaced with upper limit values (numerical values defined as “or less” or “below”) or lower limit values (numerical values defined as “or more” or “above”) of corresponding numerical values in mixing ratio (content ratio), property value, and parameter described in the above-described “DESCRIPTION OF THE EMBODIMENTS”.

A 50 μm-thick resin film2made of polyethylene terephthalate (PET) was prepared. One surface in the thickness direction of the resin film2had an arithmetic average roughness Ra of 0.6 nm. The one surface in the thickness direction of the resin film2had a skewness Rsk of −0.11.

Then, a metal underlayer3made of titanium was formed on the one surface in the thickness direction of the resin film2by a DC sputtering method. The metal underlayer3had a thickness of 7 nm. Conditions of the DC sputtering method are as follows.

Thereafter, a conductive carbon layer4was formed on one surface in the thickness direction of the metal underlayer3by a DC pulse sputtering method. Conditions of the DC pulse sputtering method will be described below.

<Conditions of DC Pulse Sputtering Method>

The conductive carbon layer4contained 10% by mass of titanium. The content of the titanium was determined by X-ray fluorescence measurement. However, in the case of X-ray fluorescence measurement, the resulting peak intensities of titanium were derived from both the titanium contained in the conductive carbon layer4and the titanium used for the metal underlying layer3. For this reason, a peak intensity of a sample on which a film of only the titanium of the metal underlying layer3was previously deposited under the same conditions as in the sputtering method of the conductive carbon layer4was subtracted from the peak of the conductive carbon layer4, to thereby calculate the content of the titanium contained only in the conductive carbon layer4. The thickness of the conductive carbon layer4was 10 nm. A method for measuring the thickness of the conductive carbon layer4will be described later.

In this manner, an electrode1including the resin film2, the metal underlayer3, and the conductive carbon layer4was produced.

The same treatment as in Example 1 was performed to produce the electrode1. However, in the formation of the conductive carbon layer4, the DC pulse sputtering method was changed to a high power impulse magnetron sputtering method. The average electric power applied to the first target was set to 150 W, and electric power was not applied to the second target. During the application to the first target, the pulse width was changed to 30 vs, and the frequency was changed to 210 Hz. The pressure in the sputtering chamber was changed to 1.0 Pa. The conductive carbon layer4did not contain titanium and was substantially made of carbon. The thickness of the conductive carbon layer4was 35 nm.

The same treatment as in Example 1 was performed to produce the electrode1. However, in the formation of the conductive carbon layer4, electric power was not applied to the second target. The conductive carbon layer4did not contain titanium and was substantially made of carbon.

Comparative Example 1

The same treatment as in Example 1 was performed to produce the electrode1. However, in the formation of the conductive carbon layer4, electric power was not applied to the second target. The conductive carbon layer4did not contain titanium and was substantially made of carbon. The thickness of the conductive carbon layer4was 100 nm.

Comparative Example 2

The same treatment as in Example 1 was performed to produce the electrode1. However, in the formation of the conductive carbon layer4, the DC pulse sputtering method was changed to a high power impulse magnetron sputtering method. The electric power applied to the first target was set to 150 W, and electric power was not applied to the second target. During the application to the first target, the pulse width was changed to 30 vs, and the frequency was changed to 210 Hz. The pressure in the sputtering chamber was changed to 0.6 Pa. The conductive carbon layer4did not contain titanium and was substantially made of carbon. The thickness of the conductive carbon layer4was 35 nm.

Comparative Example 3

The same treatment as in Example 3 was performed to produce the electrode1. However, the material of the resin film2was changed to cycloolefin resin (COP).

The following items were measured. The results are shown in Table 1.

[Measurement of Thickness of Conductive Carbon Layer4]

Using an X-ray reflectivity method as the measurement principle, the thickness of the conductive carbon layer4was calculated by measuring an X-ray reflectivity with a powder X-ray diffractometer (“RINT-2200”, manufactured by Rigaku Corporation) under the following <measurement conditions> and then analyzing the obtained measurement data with an analytics software (“GXRR3”, manufactured by Rigaku Corporation). For the analysis, a three-layer model including the resin film2made of PET, the metal underlayer3made of titanium, and the conductive carbon layer4made of titanium was adopted under the following <analysis conditions>. The targeted thickness of the metal underlayer3, the arithmetic average roughness Ra of 0.5 nm, and the density of 4.51 g/cm3were input as initial values. The targeted thickness of the conductive carbon layer4, the arithmetic average roughness Ra of 0.5 nm, and the density of 1.95 g/cm3were input as initial values. Thereafter, least square fitting with the measured values was performed, thereby analyzing the thickness of the conductive carbon layer4.

[Measurement of Arithmetic Average Roughness Ra and Skewness Rsk of Conductive Carbon Layer4]

Using an atomic force microscope (AFM manufactured by Bruker Japan K.K., trade name: MultiMode 8), the shape of the one surface in the thickness direction of the conductive carbon layer4was observed in accordance with JIS R1683:2014. From the observation results, the arithmetic average roughness Ra and the skewness Rsk of the one surface in the thickness direction of the conductive carbon layer4were determined as defined in JIS B0601:2013.

(Suppression of Decrease in Signal Intensity)

An insulating tape was adhered to the one surface in the thickness direction of the conductive carbon layer4. The insulating tape had a hole with a diameter of 2 mm Therefore, the electrode had an area of 0.0314 cm2. Thus, the electrode was fabricated as a working electrode. The working electrode was inserted into a potassium chloride solution in which K4[Fe(CN)6] was dissolved, and was connected to a potentiostat (pocketSTAT, manufactured by IVIUM). The solution had a potassium chloride concentration of 1.0 mol/L. The solution had a K4[Fe(CN)6] concentration of 1.0 mol/L. In the same manner as above, a reference electrode (Ag/AgCl) and a counter electrode (Pt) were inserted into the potassium chloride solution and then connected to the potentiostat. Thereafter, cyclic voltammetry was performed in a potential range of from −0.1 V to 0.5 V at a scan rate of 0.1 V/sec. The oxidation-reduction potential difference ΔEp was acquired as a ferricyanide activity value.

The ferricyanide activity value was applied to the following criteria and the signal intensity of the electrode1was evaluated.◯: The ferricyanide activity value was less than 150 mV.×: The ferricyanide activity value was 150 mV or more.

The lower the ferricyanide activity value is, the more the decrease in the signal intensity of the electrode1is suppressed.

An insulating tape was adhered to the one surface in the thickness direction of the conductive carbon layer4. The insulating tape had a hole with a diameter of 2 mm Therefore, the electrode had an area of 0.0314 cm2. Thus, the electrode was fabricated as a working electrode. The working electrode was inserted into a 1.0 mol/L potassium chloride solution, and was connected to a potentiostat (pocketSTAT, manufactured by IVIUM). In the same manner as above, a reference electrode (Ag/AgCl) and a counter electrode (Pt) were also inserted into the potassium chloride solution and then connected to the potentiostat. Then, cyclic voltammetry was performed in a potential range of from 0 V to 0.5 V at a scan rate of 0.01 V/sec. The capacitance value was determined by substituting into the following equation.

Capacitance value=(sum of absolute values of two current values at 0.25V)[A]/2×0.01[V/sec]/0.0314[cm2]

The unit of the capacitance value is [A]/[V/sec]/[cm2], which is the same as [F/cm2].

The capacitance value was applied to the following criteria and the suppression of noise in the electrode1was evaluated.◯: The capacitance value was less than 15 μF/cm2.×: The capacitance value was 15 μF/cm2or more.

The lower the capacitance value is, the more the noise in the electrode1is suppressed.

INDUSTRIAL APPLICABILITY

The electrode is used for electrochemical measurement.

DESCRIPTION OF REFERENCE NUMERALS