Electrochemical ph sensor

Abstract

A composite working electrode and an electrochemical sensor comprising the same are disclosed, the composite working electrode comprising at least one carbon material coated with a polysulfone and at least one active redox species comprising an oxygen family atom bonded in a ring structure, wherein the ring structure is substituted with a carbon ring, and wherein a moiety containing a hydrogen atom is connected to the carbon ring such that it is configured to provide a hydrogen bonding with the bonded oxygen family atom.

Description

Background Technology

[0001] Specific examples of this application provide a composite working electrode and an electrochemical sensor including the composite working electrode, the electrochemical sensor being used for electrochemical pH sensing using a chemical / redox active substance configured to provide hydrogen bonding with a portion immobilized in a carbon ring.

[0002] In chemistry, pH is a numerical scale used to specify the acidity or alkalinity of an aqueous solution. It is approximately the negative logarithm of the base 10 of the molar concentration of hydrogen ions measured in moles per liter. More precisely, it is the negative logarithm of the base 10 of the reactivity of hydrogen ions. Solutions with a pH less than 7 are acidic, while solutions with a pH greater than 7 are alkaline. Pure water is neutral; it is neither acidic nor alkaline.

[0003] pH measurement is crucial in agronomy, medicine, biology, chemistry, agriculture, forestry, food science, environmental science, oceanography, marine research, civil engineering, chemical engineering, nutrition, water treatment, water management (including water resource management and wastewater management), and water purification, as well as many other applications.

[0004] For nearly a century, glass electrodes have been the most commonly used method for pH measurement. A glass electrode is a composite electrode that combines glass and a reference electrode into a single unit.

[0005] The combined electrode consists of the following components:

[0006] The sensing element of the electrode is a light bulb made of a specific type of glass; the internal electrode is typically a silver chloride electrode or a calomel electrode; the internal solution is typically 0.1 mol / L KCl or 1×10⁻⁶ KCl. -7 A pH 7 buffer solution of mol / L HCl;

[0007] The reference electrode is usually the same type as the internal solution of the reference electrode, typically 0.1 mol / L KCl;

[0008] The connectors to the solution under study are typically made of ceramic or capillary and quartz fiber;

[0009] The electrode body is made of non-conductive glass or plastic.

[0010] Glass electrodes cannot be used in many industries because they are fragile, require regular calibration due to reference electrode drift, and need to be stored under proper conditions.

[0011] Many chemical analysis tools are known from chemical laboratory practice. These known tools include, for example, various types of chromatographic, electrochemical, and spectroscopic analyses. In particular, potentiometry has been widely used in laboratory and groundwater quality control for water composition measurements. U.S. Patent No. 5,223,117 discloses a two-terminal voltammetric microsensor with an internal reference, which is formed using molecular self-assembly to create a system in which both a reference electrode and an indicator electrode are located on the sensor electrode. The reference molecule is described as a pH-insensitive redox system, while the indicator molecule is pH-sensitive and formed from a hydroquinone-based redox system whose potential varies with pH. Both the reference molecule layer and the indicator molecule layer are prepared by self-assembly on a gold (Au) microelectrode. In known microsensors, the pH reading is derived from the peak reading of the voltammogram.

[0012] Recently, there has been considerable work in the development of pH sensors for use in the water industry, where the concentrations of dissolved buffer solutions and / or ionic salts are low. This focus originated from the work of Compton et al., who demonstrated that monitoring pH in these systems using the classical quinone / hydroquinone voltammetry was unsuccessful. Compton determined that when the analyte solution contains little or no buffer solution and / or ionic salts, proton-coupled electrochemical processes locally disrupt the pH of the electrode solution due to the consumption or release of protons by redox processes.

[0013] To this end, the work of Lawrence et al. has shown that this problem can be alleviated by using various quinone- and phenol-based systems that provide a means for internal hydrogen bonding of the transferred protons during electrochemical processes. Dihydroxyanthraquinones and alizarins have been shown to be suitable for quinone systems, where the ketone moiety closest to the –OH group allows for electron transfer that facilitates proton coupling and provides a mechanism that follows a coordinated rather than a discoordinated reaction. Furthermore, these results indicate that the oxidation of phenolic substances containing a ketone moiety at the 2-position of the benzene ring, such as salicylaldehyde, provides an electroactive polymeric material with pH activity and the ability to measure the pH of low-buffered media such as water. For the purposes of this disclosure, the terms low buffer value / low buffer capacity are used interchangeably. A variety of derivatives, including compounds based on aldehydes, esters, and nitrogen groups, have been tested and claimed.

[0014] Furthermore, electrodes incorporating redox systems into the support material have been reported. Carbon-based materials are widely used as support materials, and therefore carbon-based electrodes are widely used in electrochemistry and are extensively studied due to their desirable chemical and physical properties, low cost and commercial availability, and applicability to chemical modification. Carbon nanotubes are now particularly prevalent in electroanalytical applications.

[0015] The structure of carbon is primarily influenced by the presence of certain surface defects, which are essential for rapid electron transfer reactions. It has been demonstrated that the presence of pentagonal or heptagonal carbon rings in the case of carbon nanotubes (CNTs) induces chemical structural changes, leading to 'bent carbon nanotubes'. However, the main type of surface defect in CNTs appears at the ends of graphene sheets, termed "edge-plane" defects. These defect sites are high-energy defects, and therefore highly chemically reactive, making them ample sites for electron transfer reactions.

[0016] Although several synthetic methods for producing carbon nanotubes have been developed, the main techniques are: arc discharge, laser evaporation, and chemical vapor deposition. Arc discharge synthesis, also known as electric arc discharge, produces the highest quality carbon nanotubes; however, while their quality is very high, they are also mixed with a large amount of amorphous carbon, making this technique difficult to scale up. In laser evaporation, a pulsed laser is emitted at high temperature and pressure towards a graphite target, which is typically placed at one end of a quartz tube in an inert environment. The shape and structure of the resulting nanotubes are easily controllable, and very little amorphous carbon is produced, but the efficiency is very low and the yield is small.

[0017] Therefore, chemical vapor deposition (CVD) is the most promising method for producing carbon nanotubes. In this process, a gas, such as acetylene or ethylene, passes over a metal nanoparticle catalyst (typically iron, nickel, or molybdenum) that has been deposited on a porous substrate (e.g., silica, alumina). As the carbon atoms pass over the catalyst, they dissociate from the gas molecules, thereby rearranging themselves on the surface to form nanotubes.

[0018] Compared to CNTs produced by ARC (Archive-Cellular), CNTs produced by CVD (Chemical-Cellular-Dwelling) exhibit a greater number of edge-plane defects, which can affect their electrochemical reactivity. These highly reactive chemical sites can react with atmospheric oxygen, generating various oxo groups, such as hydroxyl and carboxyl functional groups. These functional groups can be introduced to varying degrees during CNT fabrication, and they can potentially influence the electrochemical performance of the CNTs. In certain media, these functional groups can disrupt the environment near the electrode surface compared to the bulk solution—the carboxyl group inducing localized acidic pH in a non-buffered medium is one such example.

[0019] Therefore, the erroneous surface functionalization of carbon-based materials, especially multi-walled nanotubes, poses a serious problem when attempting to construct working electrodes based on precise redox systems for electrochemical sensors. Thus, a solution capable of correcting these erroneous surface functionalizations is needed.

[0020] Therefore, there has long been a need for novel electrochemical sensors, and in particular, for new methods of pH determination that can overcome these operational problems.

[0021] In addition to measuring the pH of water, it is also necessary to measure the pH of seawater. One important reason for measuring the pH of saline water is to monitor the impact of atmospheric carbon dioxide on ocean pH.

[0022] Systems based on ion-sensitive field-effect transistors (ISFETs) offer solutions for seawater applications, but often require the deployment of salinity sensors to understand the reference potential. Optical systems are also used in seawater, but they require the deployment of optical dye bags, which need to be replaced periodically.

[0023] As part of its operational definition of pH scaling, IUPAC defines a series of buffer solutions (usually denoted by NBS or NIST) spanning a certain pH range. These solutions have relatively low ionic strengths (~0.1) compared to seawater (~0.7), and therefore are not recommended for characterizing the pH of seawater because the difference in ionic strength causes variations in electrode potential. To address this issue, a series of alternative artificial seawater-based buffer solutions have been developed. This new series resolves the problem of ionic strength differences between the sample and the buffer, and the new pH scale is called the “total scale,” often denoted as pHT. The total scale is defined using a medium containing sulfate ions. These ions undergo protonation…

[0024]

[0025] The effect of making the total scale include protons (free hydrogen ions) and hydrogen sulfate ions:

[0026] [H + ]T=[H + ]F+[HSO4 - ].

[0027] In addition to water and seawater, it is also necessary to measure the pH of low-buffer capacity solutions, such as saline solutions in the medical industry, biological solutions in the pharmaceutical industry, food and beverage-related solutions in the food and beverage industry, aquaculture solutions, and solutions in the fish farming and hydroponics industries.

[0028] This invention addresses these needs. Summary of the Invention

[0029] The following description provides some specific examples of the invention and is not intended to limit the scope, applicability, or construction of the invention. Various changes can be made to the function and arrangement of elements without departing from the scope of the invention as described herein. Some specific examples can be practiced without all specific details. For example, circuits can be shown as block diagrams so as not to obscure the specific examples with unnecessary detail. In other cases, well-known circuits, processes, algorithms, structures, and techniques can be shown without unnecessary detail to avoid obscuring the specific examples.

[0030] Referring now to specific examples, embodiments of which are illustrated in the accompanying drawings. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the subject matter herein. However, it will be apparent to those skilled in the art that the subject matter can be practiced without these specific details. In other instances, well-known methods, procedures, components, and systems have not been described in detail to avoid unnecessarily obscuring the characteristics of the specific examples. In the following description, it should be understood that the characteristics of one specific example can be combined with the characteristics of another specific example, wherein the characteristics of different specific examples are not incompatible.

[0031] It should also be understood that while the terms first, second, etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first object or step may be referred to as a second object or step, and similarly, a second object or step may be referred to as a first object or step. The first object or step and the second object or step are both objects or steps, but they should not be considered the same object or step.

[0032] The terminology used herein to describe this disclosure is for the purpose of describing particular specific instances only and is not intended to limit the subject matter. Unless the context clearly indicates otherwise, the singular forms “a / an” and “the” as used herein and in the appended claims are also intended to include the plural forms. It should also be understood that the term “and / or” as used herein refers to and covers any and all possible combinations of one or more of the associated listed items. It should be further understood that the terms “includes / including” and “comprises / comprising”, when used in this specification, specify the presence of the stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0033] Specific examples of this disclosure relate to composite working electrodes and electrochemical sensors including said working electrodes for detecting and monitoring pH. More specifically, but not limitingly, specific examples of this disclosure provide pH sensors capable of measuring the pH in solutions with low buffering capacity using redox-active chemicals / active redox substances, such as water, seawater, potassium chloride (KCl) solutions, sodium chloride (NaCl) solutions, biological media, food and beverage solutions, aquaculture solutions, etc., wherein said redox-active chemicals / active redox substances are configured to provide hydrogen bonding with portions immobilized in carbon rings.

[0034] In specific examples of this disclosure, redox-active chemical substances provide, for example, as part of a hydroxyl group, etc. connect Hydrogen atoms in the carbon ring and Replaced on the carbon ring Hydrogen bonding between oxogroup atoms, which can include oxygen, sulfur, selenium, etc.

[0035] Electrode substrates include carbon materials coated with polysulfone, carbon derivative materials coated with polysulfone, etc.

[0036] In specific examples of this disclosure, the carbon material includes graphite, carbon nanotubes, glassy carbon, and C. 60 Any one or a mixture of conductive boron-doped diamond powder or other conductive carbon materials.

[0037] For the purposes of this application, the terms “buffer capacity” and “alkalinity” are used interchangeably. The buffer capacity of a solution is defined as the number of moles of acid or base required to change the pH of the solution by one (1) pH unit, divided by the pH change and the volume of the buffer solution in liters; it is a unitless number. A buffer solution resists pH changes caused by the addition of acid or base through the consumption of the buffer solution. Solutions with low buffer capacity include: water, seawater, saline solutions; pharmaceutical solutions—which generally have low buffer capacity to prevent overwhelming the body's own buffering system; biological media, which are often aqueous / saline solutions; some food and beverage solutions, aquaculture solutions, etc. For example, many solutions containing a high percentage of water and inactive chemicals may have low buffer capacity.

[0038] One issue with electrochemical sensors is the ability to perform electrochemical measurements in the absence of buffer solutions and / or similar substances that can facilitate proton transfer reactions—i.e., low-buffer-capacity solutions. Low-buffer-capacity solutions are broadly categorized into two distinct types. The first type includes low-electrolyte media / solutions, such as pure water, drinking water, and raw water. The second type includes high-ionic-strength solution media that are naturally buffered but cannot withstand localized pH changes in conditions where proton transfer is inconvenient; examples include seawater, sodium chloride solutions, hard water, potassium chloride solutions, many pharmaceutical solutions, and organic solutions.

[0039] Buffer solutions are often used to test and calibrate pH sensors, providing a stable pH value due to buffering. The buffer concentration in such solutions can be less than 0.25 moles, less than 0.2 moles, less than 0.15 moles, less than 0.1 moles, or even about 0.05 or 0.01 moles or less. Reference systems, such as silver-silver chloride and calomel reference systems, use reference solutions such as sodium chloride (AgCl) and potassium chloride (KCl) as low-buffered solutions. For example, the reference solution may contain less than about 0.1 or even 0.01 moles of buffer.

[0040] In some specific embodiments of this disclosure, a working electrode for an electrochemical pH sensor is provided, comprising an active redox substance. The working electrode is configured to generate a pH / hydrogen ion concentration-sensitive redox response in low-buffered-capacity solutions such as water, seawater, etc. In specific embodiments of this disclosure, the active redox substance comprises oxogroup atoms bonded in a ring structure by ether bonds. The ring structure in which the oxogroup atoms are bonded is replaced by a carbocyclic ring, and the hydrogen atoms attached to the carbocyclic ring are configured to provide hydrogen bonding with the bonded oxogroup atoms.

[0041] In some specific examples of this disclosure, the ring structure and / or carbocyclic ring may include electron-withdrawing or electron-donating groups. Electron-withdrawing groups attract electrons away from the reaction center. Examples of electron-withdrawing groups include: halogens (F, Cl); nitriles (CN); carbonyl groups (RCOR'); nitro groups (NO2), etc. Electron-donating groups release electrons to the reaction center. Examples of electron-donating groups include: alkyl groups; alcohol groups; amino groups, etc.

[0042] In some specific examples of this disclosure, a set of derivatives / active redox substances are provided for electrochemically determining pH in non-buffered media, wherein the derivatives / active redox substances are based on hydrogen bonding formed through a ring structure containing substituted oxalic members.

[0043] In some specific instances, the ring structure containing the substituted oxalic member is bonded to / substituted with phenol, and hydrogen bonding occurs between the substituted oxalic member and the phenol proton. In some specific instances, the ring structure may include a 5- or 6-membered ring.

[0044] In some specific examples of electrochemical pH sensors, according to specific embodiments of this application, redox-active pH-sensing molecules (active redox substances) are encapsulated in polymers to provide enhanced stability and reproducible pH responses in a wide variety of low-buffered media. This polymeric material can facilitate proton transfer between the redox-active substance and the analyte medium, thereby contributing to the stabilization of hydrogen-bonded intermediates.

[0045] Previous electrochemical pH sensors configured for operation in low buffer capacity used conjugated or carbonyl oxygen to provide hydrogen bonding with phenol protons. Unexpectedly, the applicant discovered that substituted oxo atoms in the carbon ring have greater degrees of freedom of movement, such as free rotational ability, compared to conjugated or carbonyl oxygen atoms, and this degree of freedom of movement provides better hydrogen bonding than previous pH chemistries. This improved hydrogen bonding provides the basis for pH sensors for measuring pH in low buffer capacity solutions, where the pH sensor provides a more stable, reproducible, and definitive / clearer output than previous systems.

[0046] In some specific examples of this disclosure, the oxygen atom forming the hydrogen bond can be ether-bonded, and therefore, can rotate freely relative to the carbon atom where the bond is intended to occur. This free rotation has been found to provide optimal hydrogen bonding with the phenol proton. In some specific examples of this disclosure, hydrogen bond formation can be facilitated by the fact that the ether bond forms part of a six-membered ring, wherein the carbonyl moiety is at position 4 relative to the ether oxygen, thereby providing a degree of spatial confinement to the positioning of the ring relative to the benzene ring. In some specific examples, the ether bond can form part of a five-membered ring.

[0047] It has been found that groups can be added to a carbocyclic ring containing a chalcogenide member and / or phenol to alter the hydrogen bonding between the substituted chalcogenide member and the phenol proton. In some specific examples of this disclosure, the carbocyclic ring containing the substituted chalcogenide member and / or phenol includes an active moiety for providing polymerization.

[0048] In some specific examples, the working electrode comprising an active redox substance can be part of an electrochemical sensor such as a voltammetric pH sensor, wherein the active redox substance comprises a substance configured to form hydrogen bonds through a five (5) or six (6) membered ring containing a substituted oxalan member. In these specific examples, the working electrode comprises a redox-active material capable of undergoing electron and proton transfer.

[0049] By applying a potential to the working electrode, the resulting measurement potential (peak potential, half-wave potential, onset potential, etc.) in response to the applied potential provides a measure of the pH of the solution in contact with the working electrode. According to a specific embodiment of this disclosure, the measurement potential as a function of the hydrogen ion concentration in the sensing solution approximates an active redox substance. As previously stated, according to a specific embodiment of this disclosure, the active redox substance is configured to produce a pH-dependent response, wherein the solution is a low-buffer capacity solution.

[0050] In testing, the applicant has found that, according to specific examples of this disclosure, active redox substances, including those configured to form hydrogen bonds through a five (5) or six (6) membered ring containing a substituted oxalo group, can generate a redox current / potential corresponding to the pH of a solution having a very low buffer capacity of 0.05 or less molar buffer, or less than 0.01 molar buffer to a higher buffer capacity solution, approximately 0.2 molar buffer. While the active redox substances according to specific examples of this application can generate a redox potential corresponding to the pH of the solution they contact, the redox active substances are configured to generate a redox active potential corresponding to the pH of a solution having a very low molar buffer, such as 0.01 or less molar buffer.

[0051] In some specific examples, the electrode potential applied to the working electrode can be scanned linearly, stepwise, or using pulse techniques, and the current recorded. In some specific examples, the electrochemical sensor may include a reference electrode containing an inactive redox substance. For example, the solution contacting the working electrode and the reference electrode may include a solution with low buffer capacity, and the inactive redox substance may include: quinone / benzoquinone, phenol-based polymers, anthraquinone, naphthoquinone, bare carbon, carbon with an active surface, etc. The low buffer capacity solution may include water, saline solution, etc., and the reference electrode may include acidic active redox substances, basic redox substances, anthraquinone, anthraquinone derivatives, quinones, quinone derivatives, carbon substrates with a small number of redox active centers derived thereon, etc.

[0052] In some specific instances, the oxogroup atoms of active redox substances include oxygen atoms, sulfur atoms, or selenium atoms.

[0053] In some specific instances, the hydrogen atom is part of the hydroxyl group attached to the carbon ring.

[0054] In some specific instances, the carbon ring includes phenol.

[0055] Specific examples of this disclosure provide a new set of derivatives / active redox substances for electrochemically determining pH. More specifically, but not limitingly, these derivatives / active redox substances can be used in pH sensors for use in low-electrolyte media consistent with conditions present in drinking water, source water, etc., and / or in high-ionic-strength media that are naturally buffered but cannot resist local pH changes under conditions where proton transfer is inconvenient. The active redox substances of this disclosure are configured to promote proton exchange at the surface of the sensing electrode of a pH sensor system, enabling the sensor to measure the pH of low-buffered-capacity solutions (e.g., solutions containing less than about 0.2 moles of buffer, less than 0.05 moles of buffer, or less than 0.01 moles of buffer, etc.) such as water, seawater, saline solutions, pharmaceutical solutions, reference solutions, biological media, aquaculture solutions, etc.

[0056] In specific examples of this disclosure, a pH sensor comprising a novel set of derivatives / active redox substances for electrochemically determining pH in unbuffered / low ionic strength media is provided. The derivatives / active redox substances are configured to form hydrogen bonds through the interaction of a five- or six-membered ring containing oxogroup atoms with hydrogen. For example, in some specific examples, a carbon ring containing oxogroup members such as oxygen or sulfur atoms is bonded to phenol, and hydrogen bonds occur between the oxygen or sulfur atoms and the phenol protons.

[0057] In specific examples of this disclosure, the oxalo group member atom forming the hydrogen bond is free to rotate relative to the phenol or hydroquinone moiety. It is free to rotate because it is not conjugated relative to the active redox substance and it remains within the ether bond.

[0058] One embodiment of a member of the derivatives / active redox substances group comprises a 2'-hydroxyflavanone, which is a compound of the following formula:

[0059]

[0060] Other examples of redox-active substances include dihydroxyanthraquinone, dihydroxynaphthoquinone, alizarin and its derivatives, as well as salicylaldehyde, 2-hydroxybenzyl alcohol, 2-hydroxyacetone, 2,3-dihydroxybenzaldehyde and 2,5-dihydroxybenzaldehyde.

[0061] In these specific instances, the oxygen atoms that form hydrogen bonds are bonded by ether bonds and are therefore free to rotate relative to the α-carbon.

[0062] This free rotation allows for optimal hydrogen bonding with phenol protons, unlike previous redox structures where the carbonyl moiety is effectively fixed in place. In some specific examples of this disclosure, hydrogen bonding is further facilitated by the fact that the ether bond forms part of a five- or six-membered ring, with the carbonyl moiety at position 4 relative to the ether oxygen, thereby providing a degree of spatial confinement to the positioning of the ring relative to the benzene ring.

[0063] However, as those skilled in the art will understand, the illustrated embodiments show structures with properties that can also be provided by other chemicals, including substituted oxogroup members in a carbocyclic ring and hydrogen atoms coupled to another carbocyclic ring, wherein the two rings are coupled together to allow hydrogen bonding with the substituted oxogroup member. Therefore, it is not possible to provide an exhaustive list of chemicals that produce the aforementioned effects. In fact, molecules can be bonded within polymeric network structures. To date, the applicant has tested numerous phenolic and quinone structures in which freely rotating oxogroup atoms are located, as described, and found that these chemicals provide identifiable redox outputs that vary with pH in low-buffered-capacity solutions—most of which were conducted in water, seawater, and biological media. Significantly improved stability and response of the tested structures have been found on structures including conjugated oxygen atoms or oxygen atoms in carbonyl structures.

[0064] Unexpectedly, the applicant has discovered that the working electrode of an electrochemical pH sensor, comprising the aforementioned redox substances and carbon materials, configured to measure the pH of low-buffered-capacity solutions—such as water, seawater, etc.—cannot provide accurate pH measurements. The applicant has found that this inaccuracy is attributed to the carbon materials disturbing the pH of the low-buffered-capacity solution near the working electrode. The applicant has further discovered that this effect of the carbon materials can be removed by coating the working electrode comprising the redox substances and carbon materials, or at least the active surface of the working electrode, with a polymer coating. The applicant has found that the polymer coating prevents and minimizes pH disturbance near the working electrode caused by the carbon materials.

[0065] In a specific example of this disclosure, the working electrode comprises at least one carbon material coated with polysulfone.

[0066] Polysulfone is a collective noun for polymers whose repeating units are linked by sulfone groups according to the following formula:

[0067]

[0068] Known polysulfones can include the following general structures I to IV.

[0069]

[0070] R represents alkyl or aryl, especially phenyl.

[0071] In some specific examples of this disclosure, the polysulfone includes structure (II), where R is phenyl.

[0072] In some specific examples of this disclosure, the working electrode includes at least one coated carbon material, said coated carbon material being coated with polyphenylene sulfide, polyether ether ketone, polyamide derivatives, polyester and / or any polymer material having high chemical stability.

[0073] In some specific examples of this disclosure, the carbon material is not coated with a fluorinated polymer or functionalized with fluorine chemicals.

[0074] In some specific examples of this disclosure, the carbon materials do not contain any free carboxyl groups.

[0075] In some specific examples of this disclosure, carbon materials include graphite, carbon nanotubes, glassy carbon, and C. 60 Any one or a mixture of conductive boron-doped diamond powder or other conductive carbon materials.

[0076] In some specific examples of this disclosure, carbon materials include graphite and / or carbon nanotubes.

[0077] In some specific examples of this disclosure, carbon materials include carbon nanotubes.

[0078] In some specific examples of this disclosure, carbon materials include multi-walled carbon nanotubes.

[0079] In some specific examples of this disclosure, the carbon material comprises carbon nanotubes consisting of more than 90 wt.% carbon. Unavoidable impurities such as metals (ions) may be present in amounts preferably less than 10 wt.%.

[0080] In some specific examples of this disclosure, the carbon material includes carbon nanotubes with lengths of 3 μm to 12 μm or 5 μm to 9 μm.

[0081] In some specific examples of this disclosure, the carbon material includes carbon nanotubes with diameters ranging from 75 nm to 200 nm or from 110 nm to 170 nm.

[0082] In some specific examples of this disclosure, the carbon material includes carbon nanotubes with lengths of 3 μm to 12 μm or 5 μm to 9 μm and diameters of 75 nm to 200 nm or 110 nm to 170 nm.

[0083] In some specific examples of this disclosure, the carbon material includes carbon nanotubes with a length of 5 μm to 9 μm and a diameter of 110 nm to 170 nm.

[0084] In some specific examples of this disclosure, the ratio of polysulfone to carbon material is from 1:3 to 6:3.

[0085] In some specific examples of this disclosure, the ratio of polysulfone to carbon material is from 3:3 to 6:3.

[0086] In some specific examples of this disclosure, the ratio of polysulfone to carbon material is 1:3, 2:3, 3:3, 4:3, or 6:3.

[0087] A 3:3 or 6:3 ratio of polysulfone to carbon material indicates high sensitivity, accuracy, reproducibility, and lifespan properties.

[0088] In some specific examples, a voltammetric signal is applied to the working electrode to determine the pH of the solution. In some specific examples of this disclosure, a reference electrode having an inactive redox substance can be used as a reference potential for the working electrode. In some specific examples, the voltammetric signal between the working electrode and the reference electrode can be scanned. In some specific examples, the voltammetric signal between the working electrode and the counter electrode can be scanned, and the voltage and / or current between the counter electrode and the reference electrode can be measured.

[0089] In some specific embodiments of this disclosure, the working electrode includes an electrode substrate / conductive electrode material coupled to an active redox substance. The active redox substance can be immobilized on the electrode substrate. In some specific embodiments, this immobilization may include: casting the active redox substance onto the electrode substrate using a solvent; screen-printing the active redox substance onto the electrode substrate; mixing the active redox substance with a conductive powder, etc., and containing the mixture in cavities, etc., within the electrode substrate; producing a paste of the active redox substance and a conductive material, and distributing the paste in cavities or on the surface of the electrode substrate; covalently bonding the active redox substance to the electrode substrate; chemically and / or physically treating the electrode surface; and so on.

[0090] In some specific embodiments of this disclosure, the reference electrode may include a chemical substance (an inactive redox substance) configured to set the pH of the test solution. For example, the chemical substance may include inherently acidic or basic chemical structures, components, etc., such as the chemical substance may include an acid or base and / or include an acidic or basic component. In some specific embodiments, the reference electrode may include a redox substance configured such that the redox potential generated by the redox substance does not change with variations in the measurement solution, such as ion concentration, pH, etc.

[0091] In specific examples of this disclosure, the active redox substance includes a redox substance configured to undergo reduction / oxidation when an electronic signal is applied to it, wherein the active redox substance is sensitive to the presence of hydrogen ions.

[0092] In some specific instances, the active redox material of the working electrode is configured to provide an oxidation or reduction potential corresponding to the pH of the solution in contact with the working electrode, wherein the solution is a low-buffer capacity solution with a molar buffer value of less than 0.2, 0.1, 0.05, 0.02 and / or 0.01.

[0093] In some specific instances, the active redox material of the working electrode is encapsulated in a polymer. The encapsulated polymer can be configured to facilitate the transfer of protons from the working electrode / active redox material to the bulk solution.

[0094] In some specific instances, active redox substances are screen-printed onto conductive substrates or dispersed / deposited onto conductive substrates via solvent evaporation.

[0095] In some specific instances, the working electrode is part of an electrochemical sensor used to measure the pH in a low-buffer capacity solution, wherein the electrochemical sensor may include a reference electrode and a counter electrode.

[0096] In some specific instances, according to specific embodiments of this disclosure, the sensor system includes an electrochemical sensor, which may include multiple working electrodes, wherein the working electrodes may include multiple regions on a working electrode comprising an active redox substance.

[0097] In some specific instances, the reference electrode may include an inactive redox substance that is insensitive to pH.

[0098] In other specific examples, the reference electrode may include an active redox reference substance that is pH sensitive and sets the pH of the local environment of the low-buffer capacity solution to be close to that of the reference electrode.

[0099] In some specific instances, the electrochemical sensor may include a potentiostat configured to scan the voltammetric signal between the working electrode and the counter electrode to provide information on the oxidation / reduction of active redox substances on the working electrode.

[0100] In some specific instances, the active redox material on the working electrode comprises a monomeric material, and the potential of the working electrode is oxidatively scanned or maintained at a significantly oxidizing potential to induce the oxidation of the monomeric material, thereby forming an electroactive dimer, trimer, and / or polymer of the active redox material. Attached Figure Description

[0101] Figure 1Square wave voltammograms (SWV) and calibration plots are shown in IUPAC standard buffer and CO2-introduced seawater using A) old batch, A-1) additional data set of old batch, and B) new batch of multi-walled nanotubes in 2'-hydroxyflavanone-carbon composites.

[0102] Figure 2 Square wave voltammograms (SWV) and calibration plots of 2'-hydroxyflavanone-carbon composites using carboxylated nanotubes in IUPAC standard buffer and CO2-introduced seawater are shown.

[0103] Figure 3 Square wave voltammograms (SWV) and calibration plots of a 2'-hydroxyflavanone electrode based on polysulfone-coated nanotubes in IUPAC standard buffer and CO2-introduced seawater are shown.

[0104] Figure 4 The peak size obtained from the SWV of the pH sensor based on polysulfone-coated nanotubes is shown relative to the amount of polysulfone in the formulation.

[0105] Figure 5 The lifetime of the polysulfone-coated nanotube pH electrode is shown by (A) continuous SWV testing and (B) peak potential and (C) peak current analysis between scans. Detailed Implementation

[0106] Example

[0107] The invention will now be further illustrated with several embodiments. These embodiments should not be construed as limiting the invention in any way.

[0108] Experimental setup:

[0109] Reagents:

[0110] All chemicals were purchased from Sigma-Aldrich and used directly without further purification (unless otherwise specified). Standard IUPAC buffer solutions (pH 4, 7, 9) were prepared as follows: pH 4.07, 0.05 M potassium hydrogen phthalate; pH 6.86, 0.025 M potassium dihydrogen phosphate and disodium hydrogen phosphate; pH 9.23, 0.05 M sodium tetraborate in deionized water (Hexeal, UK). 0.1 M KCl was added to all buffers as a supporting electrolyte.

[0111] The seawater H2Ocean Natural Reef Salt was purchased from Maidenhead Aquatics (UK), with 1 kg of this salt dissolved in 25 L of water. For seawater calibration, different concentrations of CO2 were introduced into the continuously stirred seawater solution, and the corresponding pH values ​​were measured using a standard glass electrode.

[0112] Device:

[0113] Electrochemical measurements were performed using an Ana Pot potentiostat (Zimmer & Peacock, UK) with a standard three-electrode configuration: a carbon composite electrode as the working electrode, a carbon counter, and an Ag / AgCl (BASi, USA) electrode as the reference electrode. All square wave voltammetry (SWV) measurements were performed using the following parameters: frequency = 100 Hz, step potential = 1 mV, amplitude = 20 mV, without pretreatment.

[0114] Absolute pH measurements were performed using a standard glass electrode (Sensorex, California, USA). The pH meter was calibrated before each measurement using Reagecon buffer (Reagecon Diagnostics Ltd., Ireland) at pH 4.01 ± 0.01, pH 7.00 ± 0.01, and pH 10.01 ± 0.01. pH measurements were performed on each freshly prepared solution. All experiments were conducted at 20 ± 1 °C.

[0115] Preparation of composite electrodes:

[0116] The composite electrode is composed of 2'-hydroxyflavanone, The coating consists of a perfluorinated resin solution and multi-walled carbon nanotubes (outer diameter 110–170 nm × L 5–9 μm and carbon-based greater than 90%), with or without the above-described general formula II polysulfone coating, wherein R is phenyl (in bead form, molecular weight 22,000 g / mol), the coating having been previously dissolved in dichloromethane (DCM). RX771C / HY1300 epoxy resin (purchased from Robnor ResinLab, Ltd.) is used as a binder. COOH-functionalized graphitized multi-walled carbon nanotubes (outer diameter 30–50 nm, purchased from Cheap Tubes, USA) are used instead of multi-walled nanotubes for some electrodes.

[0117] The above-described polysulfone of general formula II, wherein R is phenyl, is dissolved in DCM for 15 minutes using an ultrasonic bath. Once completely dissolved, multi-walled carbon nanotubes, previously ground in a mortar until the powder is fine, are added to the dissolved solution in a known weight ratio. The mixture is dried at room temperature. The flavanone compound is dissolved in... For example, Q. Tang, Z. Shan, L. Wang, X. Qin, K. Zhu, J. Tian, ​​X. Liu, “Nafion-coated sulfurecarbon electrode for high performance lithiumesulfur batteries”, Journal of Power Resources, 246(2014), 253-259.

[0118] Once polysulfone-CNT and flavanone- Once the mixtures are completely dry, grind them separately in a mortar for 5 minutes until the mixtures are fine enough to be uniformly mixed. Then, combine the two mixtures and mix thoroughly. Once the final mixture is completely homogeneous, carefully mix it with the epoxy resin to form a carbon epoxy paste.

[0119] Next, the resulting mixture is loaded into PEEK. TM The electrode is manufactured in a groove (5mm in length and 1mm in diameter). Electrical connections are made using a brass rod (4cm in length and 2mm in diameter). The electrode is cured at 125°C for 1 hour to produce a solid carbon composite electrode.

[0120] result

[0121] As is well known, based on coating The 2'-hydroxyflavanone carbon nanotube composite electrode is suitable for pH sensing in both buffered and non-buffered media.

[0122] In short, the discovery that the resulting electropolymerized 2'-hydroxyflavanone electrode responds precisely to pH indicates excellent sensitivity. Nonfunctionalized multi-walled carbon nanotubes provide the desired chemical and physical properties and facilitate electron transfer reactions, resulting in pH-sensitive composites, such as... Figure 1 As presented in A, the stated Figure 1 A shows the square wave voltammogram (SWV) and calibration of a carbon composite electrode in CO2-aerated seawater, and as shown... Figure 1 As presented in the additional data set in A-1, the Figure 1 A-1 also shows the square wave voltammogram (SWV) and calibration of the carbon composite electrode in CO2-aerated seawater. As expected, the sensitivity in the unbuffered medium overlaps with the sensitivity of the response obtained in the IUPAC medium, as can be seen from the peak potential plot as a function of pH.

[0123] The experiment was then repeated using a previously unopened bottle of carbon nanotubes in the composite electrode. Although the supplier's batch number was the same and the carbon nanotubes were not treated before the electrode was manufactured, inconsistent responses were observed in the unbuffered medium. Figure 1 B details the corresponding square wave voltammetry and peak potential plots as a function of pH recorded in seawater experiments and experiments using standard IUPAC buffer with different CO2 concentrations.

[0124] Comparing the current-voltage responses of two potentially identical electrodes reveals a stark contrast between them. Figure 1 A neutralization Figure 1 In the additional data set presented in A-1, a clearly defined reduction wave was observed, exhibiting a linear response to pH; however, Figure 1 The data presented in B show a broad reduction band that does not exhibit a clear trend in solution pH changes. This is confirmed in the inset, compared to the Nernstian response as a function of pH observed in IUPAC and seawater / CO2 solutions. Figure 1 The data presented in A and in Figure 1 The additional data set presented in A-1 is different. Figure 1 The results in B indicate that this is not the case. The electrode follows a Nernst response in IUPAC buffer solution, but shows no trend in low-buffered seawater / CO2 solution. Analysis of the baseline current shows that this increases significantly in the case of electrodes produced by newly generated carbon nanotubes, thus confirming that new batches affect the electrochemical response.

[0125] In the latter case, sensing behavior is perturbed in low-buffered media, which may be attributed to the presence of carboxyl functional groups on the electrode surface (see below). In non-buffered media, these carboxyl functional groups can set the pH of the solution near the electrode surface to the pKa of the surface functional groups. Carboxyl-based graphenes have previously been reported to exhibit two acidic pKas: 4 and 6.25. Figure 1 The peak potential obtained by extrapolating the IUPAC trend line data shown in B to seawater indicates that, for the seawater sampled at pH 8.08, the pH near the electrode is 6.82. This is consistent with literature data and suggests that the new batch of CNTs has a higher degree of carboxylation sites compared to previous batches.

[0126] In another experiment, COOH-functionalized multi-walled nanotubes replaced the non-functionalized multi-walled nanotubes in the carbon composite formulation. As expected, the resulting seawater potential became 0.299 V after calibration with IUPAC standard buffer. Figure 2 This results in a higher-than-expected peak potential (see...). Figure 2(Illustration). Therefore, it can be confirmed that the hydrophilicity conferred by the carboxyl functional group is affecting the system in an electrochemical manner.

[0127] This issue can be critical when developing sensors for low-buffered media, where the underlying carbon substrate may interfere with the local environment. To address this, CNTs are coated with a polymer, effectively counteracting the interaction between the functional groups and the aqueous phase. Next, an electrode is fabricated using polysulfone-coated CNTs, and the square-wave voltammetry generated by the electrode after immersion in seawater to increase CO2 concentration is illustrated in the figure. Figure 3 In the middle. Reassuringly, the voltaic response and Figure 1 The volt-ampere response shown in B is consistent with that obtained in IUPAC buffer medium. Furthermore, the peak potential response as a function of pH overlaps with the peak potential response obtained in IUPAC buffer medium. Figure 3 (Illustration), the IUPAC buffer medium is pH sensitive to 56.5 mV / pH unit. The seawater calibration plot is based on the average of four electrodes, and the standard deviation is shown by error bars. Furthermore, the baseline current of the nanotubes to be coated is compared with... Figure 1 The results were compared to those in the previous section, revealing the difference in current generated by the coated electrodes compared to... Figure 1 A and Figure 1 The currents shown in the supplementary data set in A-1 are similar to those in the uncoated batches, where the carboxylic acid groups were exposed. Figure 1 B) significantly increases the capacitive charging current of the electrodes compared to the coated system.

[0128] Furthermore, carboxylated CNTs are coated with polysulfone of the above general formula II, wherein R is phenyl (according to MO, average M n The number of beads (approximately 22,000) confirmed the surface functional group effect, and the voltammetric response was consistent with that obtained from coated non-carboxylated CNTs (not shown). (Using...) Figure 3 The calibration line shown in the illustration is similar to the IUPAC standard buffer calibration line, with the peak potential in seawater providing a pH of 7.95; using a glass electrode, it provides a seawater pH of 7.96. This confirms that the coating can effectively counteract the functional groups of CNTs in seawater, thereby avoiding disturbance to the pH of the medium near the electrode surface.

[0129] The polysulfone-CNT ratio was investigated to evaluate the effect of the coating on the pH sensing system and determine the optimal ratio. Table 1 compares the pH sensitivity and linearity as the polysulfone:CNT ratio in seawater / CO2 solutions based on four electrodes increases compared to the response in the buffer medium. Under optimal conditions, the seawater / CO2 sensitivity will match the sensitivity to the buffer concentration.

[0130] At low polysulfone concentrations, a 1:3 PSU-CNT ratio resulted in a slight improvement in the sensor's pH sensitivity, indicating partial polysulfone coverage of the nanotubes. However, the sensitivity did not match that of the buffer solution, meaning that functional groups present on the CNT surface still influence the local pH environment. Increasing the polysulfone concentration to a 2:3 ratio significantly improved seawater calibration, and the sensitivity was consistent with that of the IUPAC standard buffer and seawater. This is consistent with polysulfone providing full CNT coverage and effectively counteracting the surface function of the CNTs. Further increasing the polysulfone concentration to a 3:3 polysulfone-CNT ratio showed excellent sensitivity and reproducibility on all electrodes. Similar to the 2:3 ratio, the pH trend in seawater solutions followed the IUPAC standard pH trend. However, when the ratio was increased to 4:3, enhanced sensitivity was observed for both IUPAC buffer and seawater calibration; however, reproducibility decreased between electrodes. Additionally, excess polysulfone was found to affect electrode storage conditions. Dehydration of the polysulfone layer was observed under dry storage conditions. Although the electrodes were submerged in seawater again and contained water, the peak potential changed compared to the newly polished electrodes with a 3:3 ratio.

[0131] Table 1: Comparison of sensitivity and intercept of different amounts of polysulfone in nanotubes for flavanone pH sensors based on 4, 7, and 9 IUPAC standard buffers and seawater calibration using CO2.

[0132]

[0133] Figure 4 The peak current of the electropolymerized layer as a function of the polysulfone-CNT ratio is detailed. Increasing the polysulfone concentration to 3:3 revealed an increase in peak current consistent with the lifetime data presented in Table 2. At higher concentrations, the peak current decreases with the sensor's lifetime. All these results indicate that a 3:3 polysulfone-CNT ratio is the optimal formulation. Finally, it should be noted that the electrode conductivity is completely unaffected by the polysulfone concentration, and all electrodes exhibit a conductivity of approximately 80 Ω.

[0134] For lifetime testing (Table 2), square wave voltammetry was run for each electrode with a three-minute delay between scans. At low polysulfone concentrations, a 1:3 polysulfone-CNT ratio showed partial polysulfone coverage of the nanotubes, therefore lifetime testing was not performed at this ratio. Increasing the polysulfone concentration to a 2:3 ratio resulted in approximately 700 lifetime scans. In addition to improvements in sensitivity and calibration, a significant improvement of 3,000 lifetime scans was observed when the polysulfone concentration was increased to a 3:3 ratio. However, a slight decrease in lifetime was observed when the polysulfone concentration was further increased to 3:4, attributed to excessive coating amount, which was confirmed after dehydration under dry conditions.

[0135] Table 2: Comparison of lifespan of nanotubes with different amounts of polysulfone for flavanone pH sensors

[0136]

[0137] Figure 5 A details the continuous reduction square-wave voltammetric signal of the electropolymerized 3:3 ratio electrode when placed in seawater. As presented in Table 2, this shows the lifetime scans after 3,000 cycles. Figure 5 As detailed in section B, the peak potential remained constant throughout all scans, while the peak current decreased in the first 700 scans and then remained constant thereafter. Figure 5 C). Electrode calibration was then checked after 3000 scans in a seawater / CO2 solution. The results showed overlap, providing regression data of y = 55.83x + 683.89, where x and y are the pH value and peak potential, respectively. This is consistent with the results before the lifetime study, y = -54.98x + 680.61 (from Table 1), thus indicating that lifetime scanning does not affect sensor calibration.

[0138] Finally, the reproducibility of the optimal formulation (3:3 ratio) was tested on 17 electrodes prepared from different batches of CNTs. They were run and tested in IUPAC buffer and seawater solutions infused with CO2. For seawater calibration, an average sensitivity of -54.98 ± 0.96 and an average intercept of 685.5 ± 7.8 were obtained, while for IUPAC standard buffer, an average sensitivity of -55.57 ± 0.94 and an average intercept of 687.4 ± 8.9 were obtained. Compared to the uncoated system, the use of polysulfone-coated nanotubes provided a considerable improvement in stability, accuracy, and reproducibility.

[0139] Therefore, by using polysulfone as a polymer coating, the erroneous surface functionalization effect of multi-walled nanotubes was successfully corrected, resulting in an accurate flavanone-based pH sensing system in low-buffered media. The resulting sensor was found to provide a Nernst response in both buffered and non-buffered media, making it suitable for deployment as a pH sensor.

[0140] It has been demonstrated that carboxyl functional groups formed by oxygen reactions at highly reactive chemical sites on the nanotube surface induce localized acidic pH, which disrupts the pH-sensing behavior of the electrode. Successful incorporation of polysulfone effectively counteracts these surface functional groups, providing accurate pH-sensing behavior in non-buffered media. The effect of functional groups was confirmed when COOH-functionalized carbon nanotubes were added to the formulation instead of unfunctionalized nanotubes, where the same pH-disrupting behavior was observed.

[0141] Different polysulfone-CNT ratios were investigated, with 3:3 showing high sensitivity, accuracy, reproducibility, and lifespan. The resulting electrodes exhibited very precise pH sensitivity, with a linear increase in peak potential as pH decreased, achieving a pH sensitivity of 55 mV / pH unit in both buffered and seawater conditions, and a standard deviation of 0.95 mV / pH unit among the 17 electrodes.

Claims

1. A composite working electrode comprising at least one carbon material coated with a polysulfone and at least one active redox species, said redox species comprising an oxygen group atom bonded in a ring structure, wherein, The ring structure is replaced by a carbon ring, wherein the portion containing hydrogen atoms is attached to the carbon ring, such that it is configured to provide hydrogen bonding with the bonded oxalate atoms; The active redox substances include: hydroxyflavanone, dihydroxyanthraquinone, dihydroxynaphthoquinone, alizarin and its derivatives, as well as salicylaldehyde, 2-hydroxybenzyl alcohol, 2-hydroxyacetone, 2,3-dihydroxybenzaldehyde and 2,5-dihydroxybenzaldehyde.

2. The composite working electrode of claim 1, wherein, The carbon material includes any one or mixture of graphite, carbon nanotubes, glassy carbon, C 60 , electrically conductive boron-doped diamond powder or other electrically conductive carbon material.

3. The composite working electrode according to claim 1, wherein, The carbon material includes multi-walled carbon nanotubes.

4. The composite working electrode according to claim 1, wherein, The weight of the polysulfone is equal to the weight of the carbon material.

5. The composite working electrode according to claim 1, wherein, The weight ratio of polysulfone to carbon material is 1:3 to 6:

3.

6. The composite working electrode according to claim 1, wherein, The active redox substance is configured to provide an oxidation or reduction potential corresponding to the pH of the solution in contact with the working electrode, wherein the solution is a low buffer capacity solution with a molar buffer value of less than 0.

25.

7. The composite working electrode according to claim 1, wherein, The active redox substance is configured to provide an oxidation or reduction potential corresponding to the pH of the solution in contact with the working electrode, wherein the solution is a low buffer capacity solution with a molar buffer value of less than 0.

2.

8. The composite working electrode according to claim 1, wherein, The active redox substance is configured to provide an oxidation or reduction potential corresponding to the pH of the solution in contact with the working electrode, wherein the solution is a low buffer capacity solution with a molar buffer value of less than 0.

1.

9. The composite working electrode according to claim 1, wherein, The active redox substance is configured to provide an oxidation or reduction potential corresponding to the pH of the solution in contact with the working electrode, wherein the solution is a low buffer capacity solution with a molar buffer value of less than 0.

05.

10. The composite working electrode according to claim 1, wherein, The active redox substance is configured to provide an oxidation or reduction potential corresponding to the pH of the solution in contact with the working electrode, wherein the solution is a low buffer capacity solution with a molar buffer value of less than 0.

02.

11. The composite working electrode according to claim 1, wherein, The active redox substance is configured to provide an oxidation or reduction potential corresponding to the pH of the solution in contact with the working electrode, wherein the solution is a low buffer capacity solution with a molar buffer value of less than 0.

01.

12. The composite working electrode according to claim 1, wherein, The oxo group atoms include oxygen atoms, sulfur atoms, or selenium atoms.

13. The composite working electrode according to claim 1, wherein, The hydrogen atom is part of the hydroxyl group attached to the carbon ring.

14. The composite working electrode according to claim 1, wherein, The carbon ring includes phenol.

15. The composite working electrode according to claim 1, wherein, The ring structure includes electron-withdrawing groups or electron-donating groups.

16. The composite working electrode according to claim 1, wherein, The carbon ring includes electron-withdrawing groups or electron-donating groups.

17. The composite working electrode according to claim 1, wherein, The active redox substances include hydroxyflavanones.

18. The composite working electrode according to claim 1, wherein, The active redox substance includes 2'-hydroxyflavanone.

19. The composite working electrode according to claim 1, wherein, The electrochemical sensor is configured to measure the pH in a low-buffer-capacity solution.

20. The composite working electrode according to claim 1, wherein, The active redox substance on the working electrode includes monomeric substances, and the potential of the working electrode is oxidatively scanned or maintained at a significantly oxidative potential to induce the oxidation of the monomeric substances, thereby forming electroactive dimers, trimers and / or polymers of the active redox substance.

21. An electrochemical sensor comprising a composite working electrode according to any one of claims 1 to 20.

22. The electrochemical sensor according to claim 21, further comprising a reference electrode and a counter electrode.

23. The electrochemical sensor according to claim 22, wherein, The reference electrode comprises an inactive redox substance that is insensitive to pH.

24. The electrochemical sensor according to claim 22, wherein, The reference electrode includes an active redox reference material that is pH sensitive and sets the pH of the local environment of the low-buffer capacity solution to be close to that of the reference electrode.

25. The electrochemical sensor according to claim 22, further comprising: A potentiostat is configured to scan the volt-ampere signal between the working electrode and the counter electrode to provide information on the oxidation / reduction of the active redox substance on the working electrode.

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