Ion sensor and ion concentration measuring device

Abstract

This ion sensor is placed in soil in which measured ions and non-measured ions having the opposite charge to the charge of the measured ions are present, and measures the measured ions. The ion sensor comprises: a substrate; a sensitive film that includes holes, which are ionophores corresponding to the ionic radius of the measured ions; measurement electrodes that are disposed between a substrate main surface and the sensitive film to detect the potential of the sensitive film; and repulsion electrodes that are disposed near the measurement electrodes between the substrate main surface and the sensitive film, to form, on the sensitive film, a repulsive electric field that causes a force repelling the non-measured ions to act.

Description

Ion sensor and ion concentration measuring device

[0001] The present invention relates to an ion sensor and an ion concentration measuring device.

[0002] In the agricultural field, it is extremely important to adjust the concentration of each ion species and to apply fertilizer and irrigation according to the growth stage. Optimal management to supply nutrients such as potassium in just the right amount will become increasingly important in the future as resource prices continue to rise. For example, Patent Document 1 discloses an ion sensor that can detect or quantify target ions.

[0003] Ion detection methods are broadly divided into oxidation-reduction methods, which measure current, and potential detection methods, which measure potential (voltage). Current measurement methods change the concentration of the target substance in the measurement environment. In contrast, potential measurement methods can measure the concentration of the target substance without affecting the concentration of the target substance in the measurement environment.

[0004] An example of an element used in a technique for measuring potential is an ion-sensitive field effect transistor (ISFET). For example, Patent Document 2 by the present inventors discloses an apparatus for measuring ion concentration using an ion-selective field effect transistor.

[0005] Japanese Patent Publication No. 2020-153695 International Publication No. 2019 / 230917 Japanese Patent No. 6407513 Japanese Patent No. 6560560

[0006] Among the methods for measuring ion concentration by potential, there is one that uses cyclic chemicals such as crown ethers to capture specific ions. This method utilizes the fact that the vacancies formed by crown ethers selectively capture ions to be measured that match the size of the vacancies. This method then measures the potential based on the captured ions to be measured.

[0007] However, vacancies capture ions of a suitable size, regardless of the charge of the ion. Therefore, it is possible that non-measurement ions with a size close to the ionic radius of the measurement ion may be captured in the vacancy. In particular, when a charged ion is captured, ions with the opposite charge are attracted and tend to remain in the vacancy. As the number of non-measurement ions captured in the vacancy increases over time, the positive and negative charges cancel each other out, affecting the relationship between the potential at the site where the ion is trapped and the concentration of the measurement ion. As a result, it has been difficult to obtain reliable measurement results over long periods of time.

[0008] An object of the present invention is to provide an ion sensor and an ion concentration measuring device that can provide reliable measurement results over a long period of time.

[0009] One aspect of the present invention is an ion sensor that is placed in soil containing measurement ions and non-measurement ions having an opposite charge to that of the measurement ions, and that measures the measurement ions. The ion sensor includes a substrate, a sensitive membrane containing an ionophore corresponding to the ionic radius of the measurement ions, a measurement electrode that is placed between a main surface of the substrate and the sensitive membrane and that detects the potential of the sensitive membrane, and a repulsion electrode that is placed between the main surface of the substrate and the sensitive membrane and in the vicinity of the measurement electrode and that forms a repulsion electric field that applies a force to the sensitive membrane to repel the non-measurement ions.

[0010] The ion concentration measuring device uses an electrical mechanism to achieve the function of suppressing the capture of non-measurement ions in the sensitive membrane. This electrical mechanism prevents the function of suppressing the capture of non-measurement ions in the sensitive membrane from deteriorating over time. As a result, the ion concentration measuring device can continue to suppress the capture of non-measurement ions in the sensitive membrane even during measurements over a long period of time, such as several months. Therefore, the ion sensor can obtain reliable measurement results over a long period of time.

[0011] The ion sensor further includes an antifouling film provided on the main surface of the sensitive membrane and composed primarily of a phospholipid polymer. The antifouling film may allow at least the ions to be measured to pass from the main surface facing the soil to the back surface in contact with the main surface of the sensitive membrane. This configuration prevents foreign matter other than the ions to be measured from adhering to the sensitive membrane. As a result, foreign matter adhering to the main surface of the sensitive membrane does not prevent the ions to be measured from being captured by the sensitive membrane, thereby obtaining good measurement results.

[0012] The ion sensor further includes an interface modification film disposed between the measurement electrode and the sensitive film and between the repulsion electrode and the sensitive film. The affinity of the interface modification film to the sensitive film may be higher than the affinity of the measurement electrode to the sensitive film and the affinity of the repulsion electrode to the sensitive film. This configuration can prevent the sensitive film from peeling off from the measurement electrode. Furthermore, it can also prevent the sensitive film from peeling off from the repulsion electrode.

[0013] The sensitive membrane of the ion sensor includes an ion-trapping membrane portion including the main surface of the sensitive membrane and where the ionophore is present, and a non-ion-trapping membrane portion including the back surface of the sensitive membrane and where the ionophore is not present. The non-ion-trapping membrane portion may be located at least one of between the repulsion electrode and the ion-trapping membrane portion and between the measurement electrode and the ion-trapping membrane portion. This configuration allows the voltage of the ion sensor to change without delay in response to changes in the concentration of the ions to be measured in the soil.

[0014] The sensitive membrane of the ion sensor has an ion-trapping membrane portion including a main surface of the sensitive membrane where ionophores are present, and a non-ion-trapping membrane portion including a rear surface of the sensitive membrane where ionophores are not present. The non-ion-trapping membrane portion may be located between the repulsion electrode and the measurement electrode. This configuration can suppress leakage current caused by the ions to be measured coming into contact with the repulsion electrode or the measurement electrode.

[0015] Another embodiment of the present invention is an ion concentration measuring device that measures ions to be measured in soil containing ions to be measured and non-measurement ions having an opposite charge to that of the ions to be measured. The ion concentration measuring device includes an ion sensor disposed in the soil and outputting a voltage corresponding to the captured ions to be measured, a voltage measuring unit that measures the voltage output from the ion sensor, and a power supply unit that applies a predetermined voltage to the ion sensor. The ion sensor includes a substrate, a sensitive membrane containing an ionophore corresponding to the ionic radius of the ions to be measured, a measurement electrode disposed between the main surface of the substrate and the sensitive membrane and that detects the potential of the sensitive membrane, and a repulsion electrode disposed between the main surface of the substrate and the sensitive membrane and near the measurement electrode that forms a repulsive electric field, using a voltage received from the power supply unit, on the sensitive membrane on the measurement electrode, causing a force to repel non-measurement ions.

[0016] The ion concentration measuring device includes the above-described ion sensor, and therefore, the ion concentration measuring device can obtain reliable measurement results over a long period of time.

[0017] According to the present invention, an ion sensor and an ion concentration measuring device are provided that are capable of obtaining reliable measurement results over a long period of time.

[0018] FIG. 1 is a schematic diagram showing an ion concentration measuring device of the first embodiment placed in soil. FIG. 2 is a perspective view showing a measurement electrode and a repulsion electrode of the ion sensor. FIG. 3 is an enlarged cross-sectional view of the ion sensor. FIG. 4 is a diagram for explaining the function of an antifouling film provided in the ion sensor. FIG. 5(a) is a graph showing the results of a first analysis example. FIG. 5(b) is a graph showing the results of a second analysis example. FIG. 6 is a graph showing the distribution of electric field strength obtained in the first and second analysis examples. FIGS. 7(a), 7(b), and 7(c) are graphs showing the results of an experimental example. FIG. 8 is a graph for comparing the characteristics of an ion concentration measuring device of a comparative example with the characteristics of the ion concentration measuring device of the first embodiment. FIG. 9 is an enlarged cross-sectional view of the ion sensor of the second embodiment. FIG. 10 is a diagram for explaining a first additional effect and a second additional effect achieved by the ion sensor of the second embodiment. FIG. 11 is an enlarged cross-sectional view of the ion sensor of Modification 1. Fig. 12 is a schematic diagram showing the configuration of a multimodal type ion sensor of Modification 2. Fig. 13 is an enlarged view of a cross section of an ion concentration measurement device of a comparative example. Figs. 14(a) and 14(b) are diagrams for explaining the characteristics of the ion concentration measurement device of the comparative example.

[0019] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicated explanations will be omitted.

[0020] First Embodiment [Ion Concentration Measuring Device] The ion concentration measuring device 10 shown in FIG. 1 is placed in, for example, soil 100, which is the measurement target. The ion concentration measuring device 10 obtains the concentration of ions to be measured. The water 100W contained in the soil 100 includes ions to be measured 102 and ions not to be measured 103. Examples of the ions to be measured 102 include potassium ions, calcium ions, nitrogen ions, ammonium ions, chloride ions, sodium ions, and phosphate ions. The ions not to be measured 103 are ions not selected as the ions to be measured 102. To more specifically define the ions not to be measured 103 in this embodiment, the ions not to be measured 103 refer to ions having a charge opposite to that of the ions to be measured. For example, if the ions to be measured 102 are potassium ions having a positive charge, all ions having a negative charge can be defined as the ions not to be measured 103. For example, chloride ions can be exemplified as ions having a negative charge. In the following description, the ions to be measured 102 are assumed to be potassium ions having a positive charge, and in the drawings, "K" is used. ＋ The non-measurement ions 103 are all ions with a negative charge, and are shown in the drawing as "X － " is shown in the diagram.

[0021] The ion concentration measuring device 10 includes an ion sensor 1 and a sensor unit 2. The sensor unit 2 includes a reference electrode 21, a voltmeter 22 (voltage measuring unit), a power supply 23 (power supply unit), and a computer 24.

[0022] The reference electrode 21 is embedded in the soil 100. The reference electrode 21 receives a DC voltage from a power source 23. The reference electrode 21 sets the potential of the region where the ion sensor 1 is placed to a predetermined value.

[0023] The ion sensor 1 is embedded in soil 100. The ion sensor 1 has an output end 18 and an input end 19. One end of a cable connected to a voltmeter 22 is detachably connected to the output end 18. One end of a cable connected to a power source 23 is detachably connected to the input end 19. A used ion sensor 1 can be removed from the end of the cable. A new ion sensor 1 can be attached to the end of the cable.

[0024] The ion sensor 1 can capture the ions 102 to be measured and can also release the captured ions 102 to be measured depending on the concentration of the ions 102 to be measured contained in the soil 100. As the number of captured ions 102 to be measured increases, the ion sensor 1 appears to have an increased positive charge. In other words, the ion sensor 1 appears to be positively charged. As a result, the ion sensor 1 generates a predetermined potential difference with respect to the reference potential GND. The ion sensor 1 generates a voltage corresponding to the concentration of the ions 102 to be measured. There is a correlation between the number of captured ions 102 to be measured and the voltage generated by the ion sensor 1. Specifically, as the number of captured ions 102 to be measured increases, the absolute value of the voltage (potential difference) generated by the ion sensor 1 increases. As the number of captured ions 102 to be measured decreases, the absolute value of the voltage (potential difference) generated by the ion sensor 1 decreases. Therefore, by obtaining the voltage generated by the ion sensor 1, the concentration of the ions 102 to be measured contained in the soil 100 can be determined.

[0025] A voltmeter 22 is connected to the output terminal 18 of the ion sensor 1. The voltage generated by the ion sensor 1 is measured by the voltmeter 22. The voltmeter 22 outputs the voltage to the computer 24. The computer 24 obtains the concentration of the ions 102 to be measured using the value (data D1) received from the voltmeter 22. For example, the computer 24 holds information indicating the relationship between the concentration of the ions 102 and the voltage, and converts the value (data D1) received from the voltmeter 22 into the concentration of the ions 102 to be measured. The information indicating the relationship between the voltage and the concentration of the ions 102 to be measured may be, for example, a function in which the voltage is an independent variable and the concentration of the ions 102 is a dependent variable. The information indicating the relationship between the voltage and the concentration of the ions 102 to be measured may be a table in which the voltage and the concentration of the ions 102 to be measured are associated.

[0026] The computer 24 provides the power supply 23 with a control signal D2 for the power supply 23. The computer 24 outputs the control signal D2 that determines the polarity of the DC voltage output by the power supply 23, depending on whether the charge of the ions 102 to be measured is positive or negative. The control signal D2 may include information that determines the magnitude (absolute value) of the DC voltage output by the power supply 23.

[0027] Assume that non-measurement ions 103 are trapped in the ion sensor 1. When measurement ions 102 with a positive charge are trapped in the ion sensor 1, the ion sensor 1 appears to be positively charged. When non-measurement ions 103 with a negative potential are trapped in the ion sensor 1 in which measurement ions 102 with a potential are trapped, the potential is neutralized. This causes a disturbance in the relationship between the number of measurement ions 102 trapped in the ion sensor 1 and the voltage generated by the ion sensor 1.

[0028] Therefore, the ion sensor 1 has a function of suppressing the capture of the non-measurement ions 103. The ion sensor 1 generates an electrical repulsive force against the non-measurement ions 103. More specifically, the ion sensor 1 generates a repulsive electric field F (see FIG. 3 ) that applies a repulsive force to the non-measurement ions 103 having a negative potential. The ion sensor 1 generates the repulsive electric field F in response to the voltage received from the power supply 23 via the input terminal 19.

[0029] [Ion Sensor] As described above, the ion sensor 1 has the output terminal 18 and the input terminal 19. Furthermore, the ion sensor 1 has a substrate 11, a measurement electrode 12, a repulsion electrode 13, a sensitive film 14, and an antifouling film 15.

[0030] FIG. 2 is a perspective view of the ion sensor 1 shown in FIG. 1 . In FIG. 2 , the sensitive film 14 and the antifouling film 15 are omitted in order to clearly show the measurement electrode 12 and the repulsion electrode 13. The substrate 11 is the base of the ion sensor 1. The substrate 11 may be a substrate on which the measurement electrode 12 and the repulsion electrode 13 can be formed by etching. An example of the substrate 11 is a glass epoxy substrate. An output terminal 18, an input terminal 19, the measurement electrode 12, and the repulsion electrode 13 are provided on the substrate main surface 11a. The output terminal 18 and the input terminal 19 are pads made of a metal material such as copper. The measurement electrode 12 and the repulsion electrode 13 are also circuit patterns made of a metal material such as copper. One end of the measurement electrode 12 is connected to the output terminal 18. One end of the repulsion electrode 13 is connected to the input terminal 19.

[0031] The measurement electrode 12 includes a measurement main pattern 121 and multiple measurement comb patterns 122. The measurement main pattern 121 and the measurement comb pattern 122 are physically and electrically integrated parts. The measurement main pattern 121 extends in the X direction. The output end 18 is connected to a first end of the measurement main pattern 121. Multiple measurement comb patterns 122 are connected to a second end of the measurement main pattern 121. In other words, the measurement electrode 12 has a structure known as a comb electrode. The measurement comb pattern 122 extends in the Y direction, which is perpendicular to the X direction in which the measurement main pattern 121 extends. The width H122 (see FIG. 3 ) of the measurement comb pattern 122 is, for example, 0.05 mm or more and 0.2 mm or less. The base end side of the measurement comb pattern 122 is connected to the measurement main pattern 121. The tip side of the measurement comb pattern 122 faces the exclusion main pattern 131. The tip surface of the measurement comb tooth pattern 122 faces the main exclusion pattern 131 but is spaced a predetermined distance from the main exclusion pattern 131 .

[0032] Like the measurement electrode 12, the repulsion electrode 13 is a so-called comb electrode. The repulsion electrode 13 includes a main repulsion pattern 131 and a plurality of repulsion comb patterns 132. The input terminal 19 is connected to a first end of the main repulsion pattern 131. The plurality of repulsion comb patterns 132 are connected to a second end of the main repulsion pattern 131. The repulsion comb patterns 132 are arranged between the measurement comb patterns 122. In other words, a pair of adjacent repulsion comb patterns 132 sandwich one measurement comb pattern 122 in the Y direction. In other words, the measurement comb patterns 122 and the repulsion comb patterns 132 are arranged alternately in the Y direction. The repulsion comb patterns 132 also have the same dimensions as the measurement comb patterns 122.

[0033] Fig. 3 is an enlarged view of a cross section of the ion sensor 1. Fig. 3 shows an electrical connection configuration of the measurement comb pattern 122, the exclusion comb pattern 132, the reference electrode 21, the voltmeter 22, and the power supply 23 using a simple circuit diagram.

[0034] As shown in FIG. 3 , measurement comb tooth patterns 122 and exclusion comb tooth patterns 132 are alternately arranged on the substrate main surface 11a. The measurement comb tooth pattern 122 has a measurement pattern main surface 122a, a measurement pattern back surface 122b, and a measurement pattern side surface 122c. The measurement pattern main surface 122a faces the soil 100. The measurement pattern back surface 122b is in contact with the substrate main surface 11a. The measurement pattern side surface 122c faces the exclusion pattern side surface 132c. The exclusion comb tooth pattern 132 has a similar configuration. That is, the exclusion comb tooth pattern 132 has an exclusion pattern main surface 132a, a exclusion pattern back surface 132b, and an exclusion pattern side surface 132c. The exclusion pattern main surface 132a faces the soil 100. Rejection pattern back surface 132b contacts substrate main surface 11a. Rejection pattern side surface 132c faces measurement pattern side surface 122c.

[0035] An interface modification film 16 is formed on the measurement pattern main surface 122a, the measurement pattern side surface 122c, the exclusion pattern main surface 132a, and the exclusion pattern side surface 132c. In other words, the measurement pattern main surface 122a, the measurement pattern side surface 122c, the exclusion pattern main surface 132a, and the exclusion pattern side surface 132c are in contact with the interface modification film 16. The measurement pattern main surface 122a, the measurement pattern side surface 122c, the exclusion pattern main surface 132a, and the exclusion pattern side surface 132c are not in direct contact with the sensitive film 14.

[0036] The interface modification film 16 easily hydrophilizes the measurement pattern main surface 122a, the measurement pattern side surface 122c, the exclusion pattern main surface 132a, and the exclusion pattern side surface 132c. The interface modification film 16 is a surface modification polymer developed by the present inventors (Japanese Patent No. 6560560). The interface modification film 16 is a block polymer that includes a hydrophilic portion that imparts hydrophilicity to the material surface, a hydrophobic portion that enhances film-forming properties, and an adhesive portion that can bond to the material via a covalent bond.

[0037] The interface-modified membrane 16 is a polymer obtained by copolymerizing first to third monomer units, which is a so-called terpolymer. The first monomer unit includes a copolymer having a zwitterionic structure and / or an ether structure in the side chain and forming a block segment. In other words, the first monomer unit corresponds to a so-called block polymer in that it forms a block segment.

[0038] The second monomer unit has a hydrophobic group containing a Si atom in a side chain. The third monomer unit has a trialkoxysilyl group in a side chain. The second monomer unit and the third monomer unit do not need to form a block segment. The second monomer unit and the third monomer unit may be bonded randomly. The second monomer unit and the third monomer unit may be bonded in a manner having some regularity or periodicity.

[0039] An example of the copolymer contained in the interface modified film 16 is a polymer having a structure shown in the following formula: Note that the structure of the copolymer contained in the interface modified film 16 is not limited to that shown in the following formula.

[0040] The interface modification film 16, which is a surface modifier, can provide a stable and uniform surface coating by ensuring that adhesive portions, etc., formed by the third monomer unit are evenly present on the material surface without agglomeration, and that the hydrophilic block segments formed by the first monomer unit are dispersed on the material surface.

[0041] A sensitive film 14 is also disposed on the substrate main surface 11a. The thickness of the sensitive film 14 is greater than the height of the measurement comb tooth pattern 122 and the repulsion comb tooth pattern 132. Therefore, the measurement comb tooth pattern 122 and the repulsion comb tooth pattern 132 are embedded in the sensitive film 14. The sensitive film 14 has a sensitive film main surface 14a and a sensitive film back surface 14b. The sensitive film main surface 14a faces the soil 100 side. An antifouling film 15 is in contact with the sensitive film main surface 14a. The sensitive film back surface 14b faces the substrate 11 side. The sensitive film back surface 14b includes a substrate contact surface 14b1 in contact with the substrate main surface 11a, a measurement main surface contact surface 14b2 facing the measurement pattern main surface 122a, a measurement side surface contact surface 14b3 facing the measurement pattern side surface 122c, an exclusion main surface contact surface 14b4 facing the exclusion pattern main surface 132a, and an exclusion side surface contact surface 14b5 facing the exclusion pattern side surface 132c.

[0042] The sensitive film 14 captures the ions 102 to be measured. The sensitive film 14 contains a plurality of pores H corresponding to the ionic radii of the ions to be measured. The pores H may be formed by, for example, crown ether (18-crown-6). The pores H exist in every region of the sensitive film 14.

[0043] In addition to the measurement ions 102 and non-measurement ions 103, various microscopic objects exist in the soil 100. As shown in FIG. 4 , these microscopic objects, distinct from the measurement ions 102 and non-measurement ions 103, are referred to as "foreign objects" in the following description. Examples of the foreign objects 104 include proteins, cells, and bacteria. When such foreign objects 104 come into contact with the sensitive membrane main surface 14a, they block the pores H. As a result, the measurement ions 102 are no longer captured in the pores H. Therefore, the ion sensor 1 has an antifouling film 15 provided on the sensitive membrane main surface 14a. The antifouling film 15 allows the measurement ions 102 to reach the sensitive membrane main surface 14a but prevents the foreign objects 104 from reaching the sensitive membrane main surface 14a. In other words, the antifouling film 15, which is a phospholipid polymer coating, prevents the penetration of large substances such as proteins and bacteria and allows only small ions to pass through. As a result, the antifouling film 15 prevents the sensitive film main surface 14a from being soiled.

[0044] The antifouling film 15 is a surface modification polymer developed by the present inventors, and is a phospholipid polymer that mimics a cell membrane (Japanese Patent No. 6407513). The antifouling film 15 can prevent foreign matter 104, such as proteins and cells, from adhering to the sensitive membrane main surface 14a.

[0045] The antifouling film 15, which is a coating composition, contains, for example, a surface-modifying polymer of an inorganic or organic material and a solvent. This surface-modifying polymer is composed of first to third monomer units. The main chain formed by the first to third monomer units is a terpolymer with a methacrylic acid skeleton. For example, the first monomer unit has a phosphorylcholine group in its side chain. The second monomer unit has an alkylsilyl group or an alkylsilyloxysilyl group in its side chain as a hydrophobic group for physical adsorption to the surface of the material. The third monomer unit has an alkyloxysilyl group in its side chain as a reactive group for crosslinking the surface-modifying polymers and for chemical adsorption to the surface of the material.

[0046] For example, the hydrophobic group for physical adsorption to the surface of the above-mentioned material may have a structure represented by the following formula: where R1 represents a linear or branched alkylene group having 1 to 5 carbon atoms, and each R2, which may be the same or different, represents a linear or branched alkyl group having 1 to 5 carbon atoms.

[0047] The surface modifying polymer may have a structure represented by the following formula:

[0048] Next, the electrical configuration of the ion concentration measuring device 10 will be described with reference to Fig. 3. Note that the circuit configuration shown in Fig. 3 is an example, and the present invention is not limited to this circuit configuration. The ion concentration measuring device 10 only needs to be able to generate a repulsive force that repels the non-measurement ions 103. The specific circuit configuration of the ion concentration measuring device 10 can be selected appropriately depending on the situation in which the ion concentration measuring device 10 is to be applied.

[0049] 3, the measurement comb pattern 122 is connected to the measurement main pattern 121. The output terminal 18 to which the measurement main pattern 121 is connected is connected to the positive electrode of a voltmeter 22. The negative electrode of the voltmeter 22 is connected to the reference potential GND.

[0050] The repulsive comb pattern 132 is connected to the repulsive main pattern 131. The input terminal 19 to which the repulsive main pattern 131 is connected is connected to the negative electrode of the power source 23. The negative electrode of the power source 23 is connected to the reference potential GND. The positive electrode of the power source 23 is connected to the reference electrode 21.

[0051] According to the above circuit configuration, the voltmeter 22 outputs the potential of the measurement comb tooth pattern 122 relative to the reference potential GND. Since the negative electrode of the power supply 23 and the repulsion comb tooth pattern 132 are also connected to the reference potential GND, it can be said that the voltmeter 22 outputs the potential of the measurement comb tooth pattern 122 relative to the potential of the negative electrode of the power supply 23. It can also be said that the voltmeter 22 outputs the potential of the measurement comb tooth pattern 122 relative to the potential of the repulsion comb tooth pattern 132.

[0052] According to the above circuit configuration, a potential difference occurs between the reference electrode 21 and the repulsion comb pattern 132, which is the same as the potential difference between the positive and negative electrodes of the power supply 23. In the example shown in Fig. 3, the potential of the repulsion comb pattern 132 is lower than the potential of the reference electrode 21. As a result, a repulsion electric field F is generated around the repulsion comb pattern 132, which generates a repulsive force on the non-measurement ions 103 having a negative potential.

[0053] [Analysis Examples and Experimental Examples] Hereinafter, the results of analysis examples and experimental examples relating to the ion concentration measuring device 10 will be described.

[0054] As an analysis example, the strength of the electric field formed by the repulsion comb tooth pattern 132 was evaluated. Specifically, the relationship between the gap P between the measurement comb tooth pattern 122 and the repulsion comb tooth pattern 132 and the strength of the repulsion electric field F was evaluated. FIG. 5A is a contour diagram showing the results of the first analysis example. The conditions for the first analysis example were as follows: Width H122 of the measurement comb tooth pattern 122: 0.2 mm Width H132 of the repulsion comb tooth pattern 132: 0.2 mm Gap P between the measurement comb tooth pattern 122 and the repulsion comb tooth pattern 132: 0.3 mm Voltage Vref applied to the reference electrode: 0.5 V Voltage Vg applied to the repulsion comb tooth pattern 132: 0.3 V FIG. 5B is a contour diagram showing the results of the second analysis example. The conditions for the second analysis example were as follows. Width H122 of measurement comb tooth pattern 122: 0.05 mm Width H132 of exclusion comb tooth pattern 132: 0.05 mm Gap P between measurement comb tooth pattern 122 and exclusion comb tooth pattern 132: 0.05 mm Voltage Vref applied to reference electrode: 0.5 V Voltage Vg applied to exclusion comb tooth pattern 132: 0.3 V

[0055] Comparing the contour diagrams of FIG. 5( a) and FIG. 5(b) , it was confirmed that in the second analysis example, in which the gap P between the measurement comb tooth pattern 122 and the exclusion comb tooth pattern 132 is narrow, a strong electric field is formed in the region where the sensitive film 14 is disposed. The numerical values ​​constituting each contour diagram were extracted and graphed (see FIG. 6). In FIG. 6, the horizontal axis represents the position in the X direction, and the vertical axis represents the electric field strength. Graph G61 shows the distribution of the electric field strength in the first analysis example. Graph G62 shows the distribution of the electric field strength in the second analysis example. Comparing graphs G61 and G62, it was confirmed that the electric field strength generated under the conditions of the second analysis example is 2.25 times the electric field strength generated under the conditions of the first analysis example.

[0056] The results of the above analysis example show the possibility of adjusting the strength of the repulsion electric field F by adjusting the gap P between the measurement comb tooth pattern 122 and the repulsion comb tooth pattern 132. It has become clear that the narrower the gap P between the measurement comb tooth pattern 122 and the repulsion comb tooth pattern 132, the higher the ability to repel the non-measurement ions 103.

[0057] As an experimental example, an ion sensor 1 was fabricated to confirm the relationship between the concentration of the ions 102 to be measured and the voltage generated by the ion sensor 1. The conditions for the fabricated ion sensor 1 are as follows. The conditions for the experimental example are also shown below. Width H122 of the measurement comb tooth pattern 122: 0.05 mm Width H132 of the repulsion comb tooth pattern 132: 0.05 mm Gap P between the measurement comb tooth pattern 122 and the repulsion comb tooth pattern 132: 0.05 mm Voltage Vref applied to the reference electrode: 0.5 V Voltage Vg applied to the repulsion comb tooth pattern 132: 0.3 V

[0058] In the experimental example, a solution simulating soil 100 was prepared. The initial concentration of the ions 102 (potassium ions) in the solution was 1 mM (mol / L). After a predetermined time (250 seconds) had elapsed, the concentration of the ions 102 was adjusted to 10 mM. After a further predetermined time (250 seconds) had elapsed, the concentration of the ions 102 was adjusted to 100 mM.

[0059] Graph G71 in FIG. 7( a) shows the time history of the current flowing through the measurement electrode 12. In graph G71, graph G71a shows the current value when the potassium ion concentration is 1 mM. Graph G71b shows the current value when the potassium ion concentration is 10 mM. Graph G71c shows the current value when the potassium ion concentration is 100 mM. Graph G72 in FIG. 7( b) shows the time history of the current flowing through the repulsion electrode 13. In graph G72, graph G72a shows the current value when the potassium ion concentration is 1 mM. Graph G72b shows the current value when the potassium ion concentration is 10 mM. Graph G72c shows the current value when the potassium ion concentration is 100 mM. Graph G73 in FIG. 7( c) shows the time history of the voltage generated by the ion sensor 1. The horizontal axis shows time, and the vertical axis shows the voltage value. In the graph G73, the graph G73a represents the current value when the potassium ion concentration is 1 mM, the graph G73b represents the current value when the potassium ion concentration is 10 mM, and the graph G73c represents the current value when the potassium ion concentration is 100 mM.

[0060] 7C, two results can be seen. First, when the concentration of the ions 102 to be measured was constant, the value of the voltage generated by the ion sensor 1 was also constant. This tendency was not affected by the concentration of the ions 102 to be measured. Furthermore, it was confirmed that the voltage generated by the ion sensor 1 increased as the concentration of the ions 102 to be measured increased. It was confirmed that the ion sensor 1 of the embodiment can generate a voltage whose magnitude corresponds to the concentration of the ions 102 to be measured.

[0061] <Operations and Effects> First, the problems of the ion concentration measuring device of the comparative example will be pointed out, followed by a description of the operations and effects of the ion concentration measuring device 10 of the first embodiment that solves these problems.

[0062] 13 is a diagram showing an ion concentration measuring device 5 of a comparative example. The ion concentration measuring device 5 of the comparative example also has a sensitive membrane 513 including pores H. The ion concentration measuring device 5 of the comparative example is similar to the ion concentration measuring device 10 of the first embodiment in that it outputs a voltage resulting from trapped ions 102 to be measured. The ion concentration measuring device 5 of the comparative example is similar to the ion concentration measuring device 10 of the first embodiment in that it has a function of rejecting ions 103 not to be measured. However, the ion concentration measuring device 5 of the comparative example differs from the ion concentration measuring device 10 of the first embodiment in the specific configuration that performs the function of rejecting ions 103 not to be measured.

[0063] The ion concentration measuring device 5 of the comparative example includes an ion sensor 51, a reference electrode 52, a voltmeter 53, and a power supply 54. The ion sensor 51 further includes a substrate 511, an electrode layer 512, and a sensitive film 513. The voltmeter 53 is connected to the electrode layer 512.

[0064] The sensitive membrane 513 of the ion sensor 51 of the comparative example contains an anion excluding agent A. The anion excluding agent A can make the potential of the sensitive membrane 513 lower than the water content 100 W of the soil 100 (negatively charged state). As a result, the non-measurement ions 103 having a negative potential are prevented from being captured by the vacancies H.

[0065] The ion sensor 51 of the comparative example can fully perform the function of negatively charging the potential of the water 100 W relative to the potential of the sensitive membrane 513 at the beginning of measurement. Therefore, as shown in FIG. 14A, it is possible to suppress the capture of non-measurement ions 103 in the pores H. Over time, the anion rejector A loses its function of negatively charging the potential of the water 100 W relative to the potential of the sensitive membrane 513. As a result, it loses its function of excluding non-measurement ions 103, and as shown in FIG. 14B, the non-measurement ions 103 become more likely to be captured in the pores H. In other words, the detection sensitivity of the ion sensor 51 of the comparative example may deteriorate. The ion sensor 51 of the comparative example may also be affected by the concentration of the non-measurement ions 103. Therefore, the ion sensor 51 having the sensitive membrane 513 containing the anion rejector A is not suitable for long-term measurements, such as measurements over several months, and requires calibration each time it is used.

[0066] In contrast, the ion sensor 1 of the first embodiment includes a substrate 11, a sensitive membrane 14 containing vacancies H which are ionophores corresponding to the ionic radius of the ions 102 to be measured, a measurement electrode 12 which is disposed between the main surface 11 a of the substrate and the sensitive membrane 14 and which detects the potential of the sensitive membrane 14, and a repulsion electrode 13 which is disposed between the main surface 11 a of the substrate and the sensitive membrane 14 and in the vicinity of the measurement electrode 12 and which forms a repulsion electric field F on the sensitive membrane 14 that applies a force to repel non-measurement ions 103.

[0067] The ion sensor 1 and ion concentration measuring device 10 of the first embodiment use an electrical mechanism to achieve the function of making the potential of water 100 W negatively charged with respect to the potential of the sensitive membrane 14. With this mechanism, the function of making the potential of water 100 W negatively charged with respect to the potential of the sensitive membrane 14 does not deteriorate over time, and is never lost. As a result, the ion concentration measuring device 10 of the first embodiment can continue to exclude the capture of non-measurement ions 103 even during long-term measurements, such as several months. Therefore, the ion sensor 1 and the ion concentration measuring device 10 equipped with the ion sensor 1 can obtain reliable measurement results over a long period of time.

[0068] The effects of the ion concentration measuring device 10 of the first embodiment have been confirmed through experiments. FIG. 8 shows the change in sensitivity of the ion sensor 1 over time. The horizontal axis represents the number of tests. Continuous measurement for a predetermined period of time is counted as one test, and the measurement was repeated multiple times (four times in the example of FIG. 8). For example, "Trail-1" represents the result of the first measurement. "Trail-2" represents the result of the second measurement. "Trail-3" represents the result of the third measurement. "Trail-4" represents the result of the fourth measurement. The vertical axis represents sensitivity. Graph G81 represents the experimental results of the ion concentration measuring device 10 of the first embodiment. Graph G82 represents the experimental results of the ion concentration measuring device 5 of the comparative example. As shown in graph G82, it was found that the sensitivity of the ion concentration measuring device 5 of the comparative example decreased with repeated testing. As described above, the decrease in sensitivity is thought to be due to the inactivation of the anion rejector A. In contrast, as shown in graph G81, it was found that the sensitivity of the ion concentration measuring device 10 of the first embodiment did not decrease even when the test was repeated, and maintained a constant value. The result that the sensitivity was maintained showed that the function of rejecting the non-measurement ions 103 was not lost by the repeated test, but was maintained.

[0069] The ion sensor 1 of the first embodiment further includes an antifouling film 15 provided on the sensitive membrane main surface 14a and composed primarily of a phospholipid polymer. The antifouling film 15 allows at least the ions to be measured to pass from the antifouling film main surface 15a to the antifouling film back surface 15b that contacts the sensitive membrane main surface 14a. This configuration prevents foreign matter 104 other than the ions to be measured 102 from adhering to the sensitive membrane 14. As a result, the foreign matter 104 adhering to the sensitive membrane main surface 14a does not prevent the ions to be measured 102 from being captured by the sensitive membrane 14, thereby obtaining good measurement results.

[0070] The ion sensor 1 of the first embodiment further includes an interface modification film 16 disposed between the measurement electrode 12 and the sensitive film 14 and between the repulsion electrode 13 and the sensitive film 14. The affinity of the interface modification film 16 to the sensitive film 14 is higher than the affinity of the measurement electrode 12 to the sensitive film 14 and the affinity of the repulsion electrode 13 to the sensitive film 14. This configuration can prevent the sensitive film 14 from peeling off from the measurement electrode 12. Furthermore, it can also prevent the sensitive film 14 from peeling off from the repulsion electrode 13.

[0071] Second Embodiment An ion concentration measuring device 10A according to a second embodiment can obtain more reliable measurement results over a long period of time. Fig. 9 shows the ion concentration measuring device 10A according to the second embodiment. The ion concentration measuring device 10A according to the second embodiment differs from the first embodiment in that the sensitive membrane 14A of the ion sensor 1A is different.

[0072] In the sensitive membrane 14A of the second embodiment, pores H are present only in the vicinity of the sensitive membrane main surface 14a. In other words, in the sensitive membrane 14A of the second embodiment, pores H are not present in the vicinity of the measurement comb tooth pattern 122. Similarly, in the sensitive membrane 14A of the second embodiment, pores H are not present in the vicinity of the exclusion comb tooth pattern 132 either.

[0073] More specifically, the sensitive membrane 14A of the second embodiment includes an ion-trapping membrane portion 141 that includes pores H and a non-ion-trapping membrane portion 142 that does not include pores H. The ion-trapping membrane portion 141 and the non-ion-trapping membrane portion 142 may be formed as physically separate layers. The ion-trapping membrane portion 141 has a trapping layer main surface 141a and a trapping layer back surface 141b. The trapping layer main surface 141a is the sensitive membrane main surface 14a described above. The trapping layer back surface 141b is in contact with the non-ion-trapping membrane portion 142. The non-ion-trapping membrane portion 142 has a non-trapping layer main surface 142a and a non-trapping layer back surface 142b. The non-trapping layer main surface 142a is in contact with the trapping layer back surface 141b. The non-trapping layer back surface 142b is the sensitive membrane back surface 14b described above.

[0074] That is, in the second embodiment, the measurement comb pattern 122 and the repulsion comb pattern 132 are covered with the ion non-trapping film portion 142 that does not have the pores H.

[0075] The ion sensor 1A of the second embodiment exhibits two additional effects in addition to the effects of the ion sensor 1 of the first embodiment.

[0076] A first additional effect is that the sensor can respond to changes in the concentration of the measurement ions 102 in the water at 100 W without causing a significant delay. As shown in FIG. 10 , the measurement ions 102 trapped in the vacancies H near the sensitive film main surface 14 a sequentially move through the vacancies H toward the measurement comb pattern 122 or the repulsion comb pattern 132. In other words, the measurement ions 102 move toward the back side (substrate 11 side) of the sensitive film 14. Assume that the concentration of the measurement ions 102 in the water at 100 W has decreased. In this case, the measurement ions 102 trapped in the sensitive film 14 move toward the water at 100 W. In other words, the movement direction of the measurement ions 102 is determined based on the difference between the concentration in the water at 100 W and the concentration in the sensitive film 14. When the measurement ions 102 move from the sensitive film 14 toward the water 100 W, if the measurement ions 102 are located on the back side (substrate 11 side) of the sensitive film 14, it takes a certain amount of time for them to move to the water 100 W. This movement time causes a delay in the voltage change of the ion sensor 1 in response to a change in the concentration of the water 100 W.

[0077] To suppress this delay, it is sufficient to prevent the measurement ions 102 from moving to the inner side of the sensitive membrane 14. Therefore, the ion sensor 1A of the second embodiment employs a configuration in which pores H are present only near the main surface 14a of the sensitive membrane. With this configuration, the measurement ions 102 captured in the ion-trapping membrane 141 can quickly move to the water 100W. In other words, a change in the concentration of the water 100W can quickly cause a change in the voltage of the ion sensor 1A.

[0078] A second additional effect is to suppress leakage current. As shown in FIG. 10 , assume that vacancies H also exist near the repulsion comb pattern 132. Measurement ions 102 move to these nearby vacancies H. As a result, the measurement ions 102 come into contact with the repulsion comb pattern 132. When the measurement ions 102 come into contact with the repulsion comb pattern 132, an oxidation-reduction reaction occurs. If the measurement ions 102 are potassium ions, electrons E possessed by the potassium ions move to the repulsion comb pattern 132 and flow as a current. This is the leakage current L. The leakage current L flowing through the repulsion comb pattern 132 affects the potential for the repulsion electric field F. The leakage current L may destabilize the repulsion electric field F. Furthermore, the leakage current L may also occur in the measurement comb pattern 122. The leakage current L flowing through the measurement comb pattern 122 may affect the potential measured by the voltmeter 22.

[0079] In order to suppress the generation of leakage current L, it is sufficient to prevent the measurement ions 102 from moving near the measurement comb tooth pattern 122 and the repulsion comb tooth pattern 132. Therefore, the ion sensor 1A of the second embodiment employs a configuration in which vacancies H are present only near the sensitive membrane main surface 14a. With this configuration, the measurement ions 102 are only captured by the ion-trapping film portion 141, and do not move to the ion-non-trapping film portion 142 in which the measurement comb tooth pattern 122 and the repulsion comb tooth pattern 132 are embedded. Therefore, no oxidation-reduction reaction occurs when the measurement ions 102 come into contact with the measurement comb tooth pattern 122 and the repulsion comb tooth pattern 132. As a result, the generation of leakage current L can be suppressed.

[0080] In short, the sensitive membrane 14A of the ion sensor 1A of the second embodiment has an ion-trapping membrane portion 141 that includes the sensitive membrane main surface 14a and in which ionophore vacancies H are present, and a non-ion-trapping membrane portion 142 that includes the sensitive membrane back surface 14b and in which no ionophore vacancies H are present. The non-ion-trapping membrane portion 142 is located between the repulsion comb pattern 132 and the ion-trapping membrane portion 141, and between the measurement comb pattern 122 and the ion-trapping membrane portion 141. With this configuration, it is possible to generate a voltage change in the ion sensor 1A without delaying a change in the concentration of the measurement ions 102 in the soil 100.

[0081] In the sensitive membrane 14A of the ion sensor 1A of the second embodiment, the ion non-trapping membrane portion 142 is located between the repulsion comb pattern 132 and the measurement comb pattern 122. This configuration can suppress the occurrence of leakage current L caused by the measurement ions 102 coming into contact with the repulsion comb pattern 132 or the measurement comb pattern 122.

[0082] Although examples of the ion sensor and the ion concentration measuring device have been described above, the ion sensor and the ion concentration measuring device are not limited to the above examples and may be implemented in various forms.

[0083] <Modification 1> The structure of the repulsion electrode and the measurement electrode is not limited to the comb-tooth pattern shown in the embodiment. For example, a pattern that can form a repulsion electric field in the sensitive membrane can be adopted as the repulsion electrode structure. In the comb-tooth pattern shown in the embodiment, the repulsion electrode and the measurement electrode have the same shape. However, the shapes of the repulsion electrode and the measurement electrode do not need to be the same. Therefore, an ion concentration measurement device including a repulsion electrode and a measurement electrode with a structure different from that of the embodiment will be exemplified as Modification 1. FIG. 11 shows an ion concentration measurement device 10B of Modification 1. The ion concentration measurement device 10B of Modification 1 differs from the first embodiment in the configuration of the measurement electrode.

[0084] 11 , the ion sensor 1B has a substrate 11, a measurement electrode layer 12B, and a repulsion electrode 13. The measurement electrode layer 12B covers the substrate main surface 11a. In other words, the measurement electrode layer 12B of Modification 1 is not a comb-teeth electrode like the measurement electrode 12 of the first embodiment. The measurement electrode layer 12B has an electrode layer main surface 12Ba and an electrode layer back surface 12Bb. The electrode layer back surface 12Bb is in contact with the substrate main surface 11a. An insulating layer 12E is provided on the electrode layer main surface 12Ba, and a repulsion comb-teeth pattern 132 of the repulsion electrode 13 is provided on the insulating layer 12E.

[0085] The ion sensor 1B of the first modified example having the above-described configuration includes a portion where the substrate 11, the measurement electrode layer 12B, the insulating layer 12E, the repulsion comb pattern 132, the interface modification film 16, and the sensitive film 14 are laminated in this order. Furthermore, the ion sensor 1B of the first modified example having the above-described configuration also includes a portion where the substrate 11, the measurement electrode layer 12B, and the sensitive film 14 are laminated in this order.

[0086] The ion sensor 1B of the first modification can generate a repulsive electric field F that repels the non-measurement ions 103 by applying a voltage to the repulsive comb pattern 132. Therefore, the ion sensor 1B of the first modification can obtain reliable measurement results over a long period of time, similar to the ion sensor 1 of the first embodiment.

[0087] <Modification 2> In the first embodiment, the ion sensor 1 measures a single measurement ion 102. For example, as shown in Fig. 12, the ion sensor 1C may measure a plurality of types of ions. The ion sensor 1C of modification 2 is a so-called multimodal sensor.

[0088] In the ion sensor 1C of the second modification, multiple sensor modules 1F, 1S, and 1T are formed on a single substrate 11. Each of the sensor modules 1F, 1S, and 1T has a measurement electrode 12, a repulsion electrode 13, a sensitive film 14, and an antifouling film 15. For example, the repulsion field F of the sensor modules 1F and 1S may generate a repulsive force against negative ions, while the repulsion field F of the sensor module 1T may generate a repulsive force against positive ions. The repulsion field F only affects the inside of the sensitive film 14. Therefore, in adjacent sensor modules, the repulsion field F generated by one sensor module does not affect the repulsion field F generated by the other sensor module. Therefore, the ion sensor 1C of the second modification can include a mixture of sensor modules that use anions as the measurement ions 102 and sensor modules that use cations as the measurement ions 102.

[0089] Whether the measurement ions 102 are anions or cations can be selected by the voltage applied to the repulsion electrode 13. For example, the voltage applied to the repulsion electrode 13 when the measurement ions 102 are anions is different from the voltage applied to the repulsion electrode 13 when the measurement ions 102 are cations.

[0090] The first sensor module 1F and the second sensor module 1S measure cations. The cations are potassium ions (K ＋ ) and ammonium ion (NH ４ ＋ However, the ions are not limited to these and may be other ions classified as cations. The third sensor module 1T measures anions. Examples of anions include nitrate ions (NO ３ －) can be exemplified, but are not limited to these ions and may be other ions classified as anions. For example, the first sensor module 1F uses positively charged potassium ions 105 as measurement ions. For the first sensor module 1F, negatively charged nitrate ions 107 are non-measurement ions. For example, the second sensor module 1S uses ammonium ions 106 as measurement ions. For the second sensor module 1S, negatively charged nitrate ions 107 are non-measurement ions. For example, the third sensor module 1T uses nitrate ions 107 as measurement ions. For the third sensor module 1T, positively charged potassium ions 105 and ammonium ions 106 are non-measurement ions.

[0091] The non-measurement ions targeted by the first sensor module 1F and the second sensor module 1S have a negative charge. Therefore, a voltage is applied from the power source 23 to the repulsion electrodes 13 of the first sensor module 1F and the second sensor module 1S so that the potential of the sensitive film 14 is lower than the potential of water 100 W. Meanwhile, the first sensor module 1F and the second sensor module 1S differ in the types of measurement ions 102. The difference in the captured measurement ions 102 can be achieved by the size of the pores H in the sensitive film 14. In other words, the size of the pores H in the sensitive film 14 of the first sensor module 1F is different from the size of the pores H in the sensitive film 14 of the second sensor module 1S.

[0092] The non-measurement ions targeted by the third sensor module 1T have a positive charge. Therefore, a voltage is applied to the repulsion electrode 13 of the third sensor module 1T from the power supply 23 so that the potential of the sensitive film 14 becomes higher than the potential of water 100 W.

[0093] According to the ion sensor 1C and ion concentration measuring device 10C of the second modification, any ion can be set as a measurement ion and capture of non-measurement ions can be suppressed by appropriately selecting the size of the pores H contained in the sensitive membrane 14 and the voltage applied to the repulsion electrode 13. As a result, a plurality of reliable measurement results can be obtained over a long period of time.

[0094] 1, 1A... ion sensor, 10... ion concentration measuring device, 11... substrate, 12... measurement electrode, 13... repulsion electrode, 14... sensitive membrane, 15... antifouling membrane, 16... interface modified membrane, 21... reference electrode, 22... voltmeter (voltage measuring unit), 23... power supply (power supply unit), 100... soil, F... repulsion electric field

Claims

1. An ion sensor that is placed in soil containing measured ions and non-measured ions that have an opposite charge to that of the measured ions, for measuring the measured ions, comprising: a substrate; a sensitive membrane containing an ionophore that corresponds to the ionic radius of the measured ions; a measurement electrode that is placed between a main surface of the substrate and the sensitive membrane and detects the electric potential of the sensitive membrane; and a repulsion electrode that is placed between the main surface of the substrate and the sensitive membrane and in the vicinity of the measurement electrode, and forms a repulsion electric field that applies a force to the sensitive membrane to repel the non-measured ions.

2. The ion sensor according to claim 1, further comprising an anti-fouling film provided on a main surface of the sensitive membrane and composed primarily of a phospholipid polymer, the anti-fouling film allowing at least the ions to be measured to pass from the main surface on the soil side to a back surface in contact with the main surface of the sensitive membrane.

3. The ion sensor as described in claim 1, further comprising an interface modification film disposed between the measurement electrode and the sensitive membrane, and between the repulsion electrode and the sensitive membrane, wherein the affinity of the interface modification film to the sensitive membrane is higher than the affinity of the measurement electrode to the sensitive membrane and the affinity of the repulsion electrode to the sensitive membrane.

4. The ion sensor described in claim 1, wherein the sensitive membrane has an ion-trapping membrane portion including a main surface of the sensitive membrane and in which the ionophore is present, and a non-ion-trapping membrane portion including a rear surface of the sensitive membrane and in which the ionophore is not present, and the non-ion-trapping membrane portion is located at least either between the repulsion electrode and the ion-trapping membrane portion or between the measurement electrode and the ion-trapping membrane portion.

5. The ion sensor described in claim 1, wherein the sensitive membrane has an ion-trapping membrane portion that includes the main surface of the sensitive membrane and in which the ionophore is present, and a non-ion-trapping membrane portion that includes the rear surface of the sensitive membrane and in which the ionophore is not present, and the non-ion-trapping membrane portion is located between the repulsion electrode and the measurement electrode.

6. An ion concentration measuring device for measuring measurement ions in soil containing measurement ions and non-measurement ions having an opposite charge to that of the measurement ions, comprising: a reference electrode disposed in the soil and controlling the electric potential of the soil; an ion sensor disposed in the soil and outputting a voltage corresponding to the captured measurement ions; a voltage measuring unit that measures the voltage output from the ion sensor; and a power supply unit that applies a predetermined voltage to the ion sensor, wherein the ion sensor has: a substrate; a sensitive membrane containing an ionophore corresponding to the ionic radius of the measurement ion; a measurement electrode disposed between a main surface of the substrate and the sensitive membrane and detecting the electric potential of the sensitive membrane; and a repulsion electrode disposed between the main surface of the substrate and the sensitive membrane and in the vicinity of the measurement electrode, for forming a repulsion electric field that applies a force to the sensitive membrane on the measurement electrode to repel the non-measurement ions by the voltage received from the power supply unit.

Images