WO2020249225A1

WO2020249225A1 - Metal ferrocyanide-doped carbon as transducer for ion selective electrode

Info

Publication number: WO2020249225A1
Application number: PCT/EP2019/065664
Authority: WO
Original assignee: Robert Bosch Gmbh
Priority date: 2019-06-14
Filing date: 2019-06-14
Publication date: 2020-12-17
Also published as: JP2022537280A; KR20220020271A; CN113924481A

Abstract

According to the present disclosure, there is provided for an ion selective electrode sensor for measuring concentration of a cation in a liquid. The ion selective electrode sensor includes an ion selective membrane which facilitates selective diffusion of the cation in the liquid across the ion selective membrane, an electrical contact layer connected to an external device for measuring the concentration of the cation in the liquid, and a transducer layer arranged between the ion selective membrane and the electrical contact layer, wherein the transducer layer comprises carbon doped with a metal ferrocyanide. The present disclosure also provides for a method of fabricating the ion selective electrode sensor described above. The method includes depositing an electrical contact layer on an inert substrate, depositing a transducer layer comprising carbon doped with a metal ferrocyanide on the electrical contact layer, and depositing an ion selective membrane on the transducer layer to form the ion selective electrode.

Description

METAL FERROCYANIDE-DOPED CARBON AS TRANSDUCER FOR ION

SELECTIVE ELECTRODE

FIELD OF THE TECHNOLOGY

[0001] The present disclosure relates to an ion selective electrode sensor for measuring concentration of a cation in a liquid. The present disclosure also relates to a method of fabricating such an ion selective electrode sensor.

BACKGROUND

[0002] Solid-contact ion selective electrodes (SC-ISEs) may consist of an electrical conductor directly coated with the desired ionophore-doped membrane as shown in FIG. 1A, which may provide for opportunities in achieving miniaturization and development of low-cost, disposable sensors of ions in biological and natural fluids. The challenge in obtaining a stable and reliable potential reading has been limiting the practical applications of SC-ISEs.

[0003] A major development in this direction is on addressing the low interfacial capacitance between the ion selective membrane (ISM) and the electrical contact layers in the case of the coated wire electrode, which may be an electrode having the design shown in FIG. 1A. As the coated wire electrode (100) may rely on a double layer capacitance, the low surface area between the ISM (104) and electrical conductor (106) in a coated wire electrode (100) leads to a low interfacial capacitance. The double layer capacitance is based on the charges at the interface of the ion selective membrane membrane and the electrical contact. The charges within the ion selective membrane are ionic while the charges within the electrical contact are electronic. In other words, the double layer capacitance is both ionic and electronic in this case. Small currents of ions at the interface between the ISM (104) and electrical contact (106) may then lead to relatively large changes of the interfacial potential at the reverse side of the membrane (104), which may be a source of potential instability.

[0004] To address the instability, an additional transducer layer (1 10) situated between the ISM (104) and electrical contact (106) has been conventionally worked on (FIG. 1 B). The transducer layer (1 10) may include, for example, (i) a conducting polymer that offers electronic conductivity and ionic conductivity, or (ii) a high surface area nanocarbon that displays increased double layer capacitance, at the ISM / transducer interface due to a high surface area of contact. A greater capacitance of the interface in both cases allows for much more stable potential readings.

[0005] Another means developed to address the instability involves doping of redox active molecules or intercalation compounds in the transducer layer, which also successfully demonstrated improved sensor-to-sensor reproducibility.

[0006] Complementing the convenience and portability that SC-ISEs offer, removing the need for calibration is probably another attractive prospect, as training and standard solutions used in calibration impose additional requirements for obtaining accurate measurements. [0007] Factors that affect a sensor and resulting in need of calibration may be assessed by observing changes in the potential measured between the ISE and a reference electrode, which obeys the Nernst equation:

[0008] where E is the measured potential, R, T and F are the gas constant, temperature in Kelvin and the Faraday constant, respectively, zi is the charge of analyte ion I, ai is its activity in the sample. Ei° encompasses the potential differences of all other interfaces other than the ISM / sample interface. For the case of a SC-ISE with a conducting polymer transducer layer, a continuum of redox states may exist, and this may lead to variations in Ei° between sensors, which likely necessitates calibration before use. Factors that may influence Ei° include variations in crystallinity, time-dependent conformation changes after redox reactions, changes in the glass transition temperature stemming from the doping level, inter-chain bonds, counterion penetration into the layer, and layer morphology as a result of fabrication processes. Standard deviations of sensors that conventionally incorporate nanocarbons like multi-wall carbon nanotubes tend to be about 10 mV, unsuitable for measurements within one decade of concentration with a theoretical span of 59.2 mV, in the case of a monovalent ion.

[0009] Calibration-free ISE design has revolved around the use of a controlled oxidation state, either in the ISM or transducer phase to obtain a defined potential at the ISM / transducer interface. For sensors with a conducting polymer as a transducer, electrochemical control of the redox state may be through current pulses, allowing ISEs to equilibrate with a standard reference electrode, or by polarizing the transducer layer to a controlled degree during electrode fabrication.

[0010] In a particular study conducted, solid state selective electrodes or solid state reference electrodes based on colloid imprinted mesoporous carbon were investigated. In this case, the redox active molecules were mixed in with the ion selective membrane, instead of being present in the transducer layer. The presence of the redox couple led to stability across a tested time of 72 hours in a conditioning solution.

[0011] In another study, solid state ion selective electrodes that have redox active molecules covalently attached to a component of the sensor by click chemistry were looked into. In this case, the mode of attachment of the redox active molecules is the focus of the study and click chemistry was considered as the means to attach a range of different molecules to the component.

[0012] In the study of solid state selective electrodes or solid state reference electrodes based on colloid imprinted mesoporous carbon mentioned above, a controlled redox state in the ISM layer has been achieved by directly mixing redox-active compounds of a defined oxidation state into the membrane phase when preparing the membrane cocktail. While highly stable sensors that exhibit low drift have been conventionally developed, redox-active molecules need to be designed to be sufficiently lipophilic to avoid their leaching from the ISM when immersed in solution. To this end, complicated synthesis protocols are conventionally designed to obtain redox active molecules with the desired properties. A contradiction also exists between use of these molecules in the ISM. Stable ion-ionophore complexes contribute to high selectivities in sensor performance, however, this may conversely increase loss of redox-active molecules from the ISM when in contact with a solution. As a result, the stability of the sensor reading may have to compete with the selectivity of the sensor, which forces a compromise. Further, conventional ISE manuals typically recommend 24 hours of conditioning, which may be too long a time to be convenient. Even though conventional sensors have been reported to exhibit high stability, they remain likely subject to non-zero potential drifts over time, which necessitates calibration, including recalibration.

[0013] There is thus a need for a solution that ameliorates and/or resolves one or more of the issues mentioned above. The solution described herein should at least provide for a sensor and a method of producing such sensor.

SUMMARY

[0014] In a first aspect, there is a provided for an ion selective electrode sensor for measuring concentration of a cation in a liquid, the ion selective electrode sensor comprising:

an ion selective membrane which facilitates selective diffusion of the cation in the liquid across the ion selective membrane;

an electrical contact layer connected to an external device for measuring the concentration of the cation in the liquid; and

a transducer layer arranged between the ion selective membrane and the electrical contact layer, wherein the transducer layer comprises carbon doped with a metal ferrocyanide.

[0015] In another aspect, there is provided for a method of fabricating the ion selective electrode sensor described for the first aspect, the method comprising:

depositing an electrical contact layer on an inert substrate;

depositing a transducer layer comprising carbon doped with a metal ferrocyanide on the electrical contact layer; and

depositing an ion selective membrane on the transducer layer to form the ion selective electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In the drawings, like reference characters generally refer to like parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present disclosure are described with reference to the following drawings, in which:

[0017] FIG. 1A shows a schematic diagram of a coated wire electrode. The electrode (100) is placed in a liquid sensing environment (102). The electrode (100) is disposed on a support substrate (108) and includes a PVC ion selective membrane (ISM) (104) and the electrical contact (106). [0018] FIG. 1 B shows a schematic diagram of an electrode (1 12) that has been changed, wherein a transducer layer (1 10) is disposed under selective membrane layer.

[0019] FIG. 2 shows a plot of standard potential term E° in mV (200) against conditioning time in hours (202) for ISEs with conventional PEDOT SS transducer immersed into 10 ³ M KCI. E° was calculated from the individual calibration curves of each sensor when calibrated with solutions of 10 \ 10 ², 10 ³ and 10⁴ M KCI solutions. All open circuit potential (OCP) measurements were taken against a standard 3 M KCI reference electrode. The hollow diamonds represent data obtained under dry conditions and shaded diamonds represent data obtained under prehydrated conditions.

[0020] FIG. 3 shows a plot of standard potential term E° in mV (300) against conditioning time in hours (302) for ISEs with conventional PEDOT :PSS transducer immersed into 10 ³ M KCI. The set with no O2 was kept in deaerated solution across 48 hours and the set with O2 was kept in solution exposed to lab atmosphere. The top plot represents for data obtained in the presence of oxygen. The bottom plot represents for data obtained in absence of oxygen.

[0021] FIG. 4 shows a side view of KFeCN sensor architecture. The electrode (400) is placed in a liquid sensing environment (402). The electrode (400) is disposed on a support substrate (408), and includes a carbon transducer layer (410) doped with KFeCN sandwiched between a PVC ion selective membrane (ISM) (404) and the electrical contact (406).

[0022] FIG. 5 shows a schematic of a conventional Metrohm Dropsens screen printed electrode.

[0023] FIG. 6 shows a flow chart to take a calibration-free measurement, impact of conditioning and hydration procedures. The third conditioning and storage state, which is between the 6^th to 12^th hour, is the recommended. For the“Calibration-free measurement” between 3^rd to 12^th hour and 6^th to 12^th hour (without oxygen), stable measurements can be taken but they require the conditioning solution to be depleted of oxygen. This complication outweighs the user performing a separate calibration step, and hence is less preferably used, albeit still useable. The box indicating“calibration required” signifies the route in which stable measurements are not possible. The workflow may be generalized into three stages represented by (600), (602) and (604). In (600), a fresh sensor from its respective storage atmosphere is taken for use. In (602), the sensor is immersed in 10 ³ M KCI, wherein the immersion duration and the dissolved oxygen environment is controlled. * denotes oxygen present at atmospheric levels. ** denotes saturated with nitrogen.

[0024] FIG. 7 shows a plot of E° values in mV (700) against conditioning time in hours (702). The E° values were calculated by linear fitting of the data points from calibration in 10 ¹ to 10⁴ M solutions of KCI over the course of 48 hours. The hollow diamonds and crosses represent data obtained from conditioning solutions with and without oxygen, respectively, for each set of electrodes. For electrodes stored in conditioning solution without O2, the solutions were deaerated by bubbling with N2. The difference in absolute value between the stable E° readings for sensors exposed and not exposed to O2 is attributed to mixing errors during small batch production.

[0025] FIG. 8A shows a plot of E° values in mV (800) against conditioning time in hours (802). Specifically, FIG. 8A illustrates for comparison of KFeCN sensors conditioned from an initially dry state, compared to sensors subjected to 24 hours of 100% humidity for sensors conditioned without oxygen in solution. Differences in absolute values between the stable potential values of 6^th to 12^th hour are attributed to batch-to-batch errors during sensor fabrication. The hollow diamonds and crosses represent data obtained under dry and prehydrated conditions, respectively.

[0026] FIG. 8B shows a plot of E° values in mV (804) against conditioning time in hours (806). Specifically, FIG. 8B illustrates for comparison of KFeCN sensors conditioned from an initially dry state, compared to sensors subjected to 24 hours of 100% humidity for sensors conditioned with oxygen in solution. The hollow diamonds and crosses represent data obtained under prehydrated and dry conditions, respectively.

[0027] FIG. 9 shows a plot of the open circuit potential (OCP) measured in mV (900) against time in hours (902) for sensors without KFeCN present in the transducer layer, conditioned in 0.1 M KCI for 48 hours.

DETAILED DESCRIPTION

[0028] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized, and changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. The listing or discussion of a previously published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

[0029] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0030] The present disclosure provides for an ion selective electrode sensor and a method of fabricating such ion selective electrode sensor.

[0031] The present ion selective electrode sensor and method are advantageous as it at least mitigates potential drifts, improves sensor-to-sensor reproducibility in terms of measurements taken using the present ion selective electrode sensor, and reduces the need for calibration and re-calibration before use. The present ion selective electrode sensor may be referred to herein as a“solid-contact ion selective electrode sensor”, as it is in a solid form and retains the solid form throughout use.

[0032] Accordingly, the present disclosure provides for an ion selective electrode sensor for measuring concentration of a cation in a liquid. The ion selective electrode sensor comprises an ion selective membrane which facilitates selective diffusion of the cation in the liquid across the ion selective membrane, an electrical contact layer connected to an external device for measuring the concentration of the cation in the liquid, and a transducer layer arranged between the ion selective membrane and the electrical contact layer, wherein the transducer layer comprises carbon doped with a metal ferrocyanide.

[0033] The term“cation” used herein refers to a positively charged ion. The cation may be a monovalent cation, bivalent cation, or trivalent cation. The cation ion may be a metal ion.

[0034] For an ion selective electrode sensor, the input signal may be the activity of a specific ion and the output may be an electrical potential.

[0035] The present ion selective electrode sensor may be connected to an external device comprising a potentiometer for measuring a difference in potential between (i) a reference electrode, and (ii) the electrical contact. The potentiometer allows a user to read the potential difference measurements taken, or the ion selective electrode and the reference electrode may also be coupled to a voltmeter for a user to obtain such readings.

[0036] The transduction of a signal input based on activity of a specific ion into an electrical potential output occurs at the interface between a sample fluid and the ion selective membrane. In the ion selective membrane, the charge carriers that are free to move are ions. The signal transduction also includes a transduction of an ionic current into an electric current when the ions approach or enter the underlying transducer layer. In the case of a conducting polymer as a transducer layer, the transduction occurs as a result of a redox reaction occuring in the conducting polymer layer. However, when an ion selective membrane is immersed into a liquid, the bulk of the ion selective membrane and underlying layers may become hydrated, i.e. coated with a layer of water or absorbing molecular or bulk water which affects diffusion of ions into the ion selective membrane and the ion concentration profile, thereby causing a drift in potential measurement. The present sensor mitigates this.

[0037] Advantageously, stable measurements having a standard deviation of less than 4 mV or less are obtainable using the present ion selective electrode sensor. The present sensor has a transducer layer comprising a carbon electrode doped with a metal ferrocyanide. The metal ferrocyanide is a redox couple that increases sensor-to-sensor reproducibility. This means that the same sensor used for taking a measurement under identical conditions does not produce different results. This also means that different sensors taking a measurement under identical conditions do not produce different results.

[0038] The metal ferrocyanide may include a metal ferricyanide for providing the redox couple. The metal ferrocyanide may be convertible to the metal ferricyanide, and vice versa. The redox couple stabilizes the interfacial potential between the ion selective membrane and the transducer layer even when the ion selective membrane gets hydrated. The metal ferrocyanide and/or metal ferricyanide circumvents the use of lipophilic molecules that are mixed directly into the ion selective membrane for stabilizing the interfacial potential between the ion selective membrane and the transducer layer. The lipophilic molecule may compromise selectivity toward the ion and/or stability of measurements. Moreover, the metal ferrocyanide and/or metal ferricyanide are doped in carbon, which is not hydrogroscopic and renders the transducer layer resistant to formation of water thereon.

[0039] The present sensor can be used to measure concentration of a cation in a liquid, wherein activity may be correlated to concentration of the cation in the liquid. The cation may comprise a potassium ion. The metal ferrocyanide may be selected to be compatible with the cation. For example, if the cation comprises a potassium ion, the metal ferrocyanide may comprise potassium ferrocyanide. The metal ferricyanide may accordingly comprise potassium ferricyanide.

[0040] The ion selective membrane of the present sensor may comprise polyvinyl chloride or polyvinyl chloride in an amount of 33 wt% of the ion selective membrane. The ion selective membrane may also include a plasticizer, such that the polyvinyl chloride is in an amount of about 33 wt% and the plasticizer is about 66 wt%, wherein the wt% is based on the ion selective membrane. Non-limiting examples of the plasticizer may be any suitable plasticizers compatible with polyvinyl chloride, such as dioctyl sebacate. The plasticizer increases diffusivity and hence the ionic conductivity of the ion selective membrane.

[0041] The present ion selective electrode sensor may further include an ionophore disposed in the ion selective membrane, wherein the ionophore facilitates selective diffusion of the cation in the liquid into and/or out of the ion selective membrane, or an ionophore disposed in the ion selective membrane, wherein the ionophore facilitates selective diffusion of the cation in the liquid into and/or out of the ion selective membrane, wherein the ionophore may comprise, for example, valinomycin. The ionophore, e.g. valinomycin, may have a loading rate of approximately 1 wt% in the ion selective membrane. An ionophore may selectively bind with the cation and helps to transport them across the membrane to the transducer layer. In the case of transducer layers that exhibit both ionic and electronic conductivity, at the interface between the ion selective membrane and the transducer layer, the ionophore releases the cation.

[0042] The present ion selective electrode sensor may further comprise an inert substrate which the ion selective membrane, the electrical contact layer, and the transducer layer are disposed thereon.

[0043] The present disclosure also provides for a method of fabricating the ion selective electrode sensor described herein. The method includes depositing an electrical contact layer on an inert substrate, depositing a transducer layer comprising carbon doped with a metal ferrocyanide on the electrical contact layer, and depositing an ion selective membrane on the transducer layer to form the ion selective electrode.

[0044] Embodiments and advantages associated with the present ion selective electrode sensor as already described above are applicable to the present method, and vice versa. Elements of the present ion selective electrode sensor shall not be iterated for the sake of brevity, as the various components and configurations, and their advantages, have already been described above. For example, the transducer layer may be a carbon layer doped with metal ferrocyanide. The metal ferrocyanide may include metal ferricyanide. The metal ferrocyanide/ferricyanide may include a potassium ferrocyanide/ferricyanide. In this regard, the present may comprise forming a layer of carbon doped with potassium ferrocyanide.

In the present method, depositing the ion selective membrane may comprise (ia) dissolving a polymer in cyclohexanone to form a polymer solution, or (ib) dissolving a polymer in cyclohexanone to form a polymer solution, wherein the polymer comprises polyvinyl chloride, (ii) forming the polymer solution on the transducer layer, and (iii) heating the polymer solution to remove the cyclohexanone, thereby forming the ion selective membrane. The use of cyclohexanone is advantageous as it takes significantly longer to dissolve polyvinyl chloride, compared to conventional solvents such as tetrahydrofuran. Said differently, while cyclohexanone is a solvent for polyvinyl chloride for forming the ion selective membrane, this property of cyclohexanone advantageously reduces or hinders dissolution of the deposited carbon transducer layer. The deposition of the doped carbon transducer layer may involve a polymer binder. When the polymer binder comes into contact with tetrahydrofuran, it may dissolve easily, rendering lost of integrity of the doped carbon layer and the doped carbon layer may partially dissolve in the solvent. Hence, use of cyclohexanone avoids the need for even more volatile solvents that may damage the transducer layer. As already mentioned above, the polyvinyl chloride used may be about 33 wt% of the ion selective membrane. A plasticizer may be included with the polymer. For example, a plasticizer may be included with the polyvinyl chloride. The plasticizer may be about 66 wt% of the ion selective membrane. Further, an ionophore may be included with the polymer. The ionophore may be about 1 wt% per weight of the ion selective membrane. The ionophore may be valinomycin as an example.

[0045] The present method may further comprise conditioning the ion selective electrode sensor for calibration-free measurements of concentration of the cation in the liquid. The conditioning may comprise (i) storing the ion selective electrode in an inert environment, and (iia) immersing the ion selective electrode sensor in a solution comprising the cation before use, or (iib) immersing the ion selective electrode sensor in a solution comprising the cation before use, wherein the cation may comprise a potassium ion.

[0046] Storing of the ion selective electrode may include placing the ion selective electrode in anhydrous nitrogen or hydrous nitrogen. As already explained above, the carbon transducer layer doped with metal ferrocyanide and/or metal ferricyanide provide for a redox couple that stabilizes the sensor in the presence of water. Hence, storage of the sensor in nitrogen free of moisture or nitrogen with moisture does not significantly compromise stability of the sensor.

[0047] Immersing the ion selective electrode sensor may include soaking the ion selective electrode in the solution in the absence of oxygen for 6 hours to 12 hours, or soaking the ion selective electrode in the solution in the presence of oxygen for 6 hours to 12 hours.

[0048] As the present method introduces a transducer layer comprising carbon doped with the metal ferrocyanide, and/or metal ferricyanide, conditioning of the ion selective electrode sensor before use can take place in the presence or absence of oxygen. As already mentioned above, the metal ferrocyanide and/or ferricyanide provides a redox couple that mitigates potential drifts. In other words, the present transducer layer demonstrates improved resistance to the impact of oxygen in a sample solution during the identified 6^th to 12^th hour period with improved potential measurement stability. This renders conditioning of the present ion selective sensor more versatile and not restricted by the need or absence of oxygen.

[0049] The solution may comprise the cation in which its concentration is to be measured. The cation may be potassium. The solution may comprise potassium chloride. The potassium chloride may be in a concentration ranging from 10 ³ to10 ⁵ M for conditioning the present ion selective electrode sensor.

[0050] In the present disclosure, the word“substantially” does not exclude“completely” e.g. a composition which is“substantially free” from Y may be completely free from Y. Where necessary, the word“substantially” may be omitted from the definition of the invention.

[0051] In the context of various embodiments, the articles“a”,“an” and“the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0052] In the context of various embodiments, the term“about” or“approximately” as applied to a numeric value encompasses the exact value and a reasonable variance.

[0053] As used herein, the term“and/or” includes any and all combinations of one or more of the associated listed items.

[0054] The invention illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms “comprising”, “including,” containing”, etc. shall be read expansively and without limitation. The word "comprise" or variations such as "comprises" or "comprising" will accordingly be understood to imply the inclusion of a stated integer or groups of integers but not the exclusion of any other integer or group of integers. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by exemplary embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

EXAMPLES

[0055] The present disclosure provides for an ion selective electrode sensor for measuring concentration of a cation in a liquid, and a method of fabricating the ion selective electrode sensor. In order that the present disclosure may be readily understood and put into practical effect, particular embodiments will now be described by way of the following non-limiting examples.

[0056] Example 1A: Potential Drifts in Conventional Sensors [0057] Solid contact ion sensors may suffer from potential drifts, which is the changing of the potential recorded for the same solution at different points of time. As a dry electrode is immersed in solution, water uptake occurs in the membrane, leading to a change in the hydration state of the membrane. This in turn changes the activity coefficients of the primary ion (K⁺ in this case) in the membrane. As the potentiometric signal is the sum of all the interfacial potentials between the metal contacts of the ISE to the reference electrode, the change of the ion concentration profile in the membrane as a function of hydration causes changes to the interfacial potentials at the membrane. This is in turn observed as an overall shift in the absolute value of the potential recorded. An example is shown in FIG. 2, when comparing the readings of the standard potential term, E°, from 1 to 12 hours after first immersing the sensors in 10 ³ M KCI conditioning solution. Prehydrated sensors were exposed to an N2 atmosphere of 100% humidity for 24 hours, compared to the dry sensors which were directly immersed in liquid at a test time of 0 hour. The difference in the 2 sets of data accounts for the difference in hydration state between the 2 sets of the ion selective membranes.

[0058] Water in PVC membranes has been found to have 2 diffusion coefficients. The first, faster coefficient has been found to dominate at the initial hydration of the membrane. This can be seen from the much higher rate of drift in the case of the dry membrane in FIG. 2 during the initial 12 hours of hydration. The second, slower hydration process can then be seen from the data points after 24 hours of hydration have passed and can be observed as a slower process of drift.

[0059] As a result of drift, the potentiometric signal of each sensor is not stable on the order of hours, and owing to the different rates of drift across time, the degree of drift is difficult to predict.

[0060] The equilibration process of solid contact ISEs is also complicated by the equilibration process taking place at the ISM / transducer layer interface. PEDOT:PSS contributes to a further downward drift of freshly prepared electrodes and the degree this competes with the equilibration happening at the ISM layer has thus far only been speculated upon. Nevertheless, what is consistent is the continuous downward drift over the course of 48 hours of the experiment, and hence the absolute potential, over hours, is always dependent on time. This demonstrates that calibration is thus required between the measurements for conventional sensors.

[0061] Example 1 B: Requirement For Calibration of Conventional Sensor

[0062] As a result of potential instabilities from drift, as well as poor sensor to sensor reproducibility, conventional ISEs require calibration for effective measurements to be performed. This necessitates the use of automation or training to overcome, which leads to increased use of resources, as well as potential for measurement error.

[0063] Example 1 C: Oxygen Sensitivity For Storage of Conventional Sensor

[0064] Oxygen in storage environment impacts the E° value in 2 ways. A clear divergence in E° values can be seen after 9 hours in FIG. 3, where the values for the sensors stored in deaerated solution showed a greater drop in E° values compared to those stored in environments where O2 is present. This effect has been documented earlier for conventional films of PEDOT SS. [0065] Oxygen also significantly decreased sensor to sensor reproducibility of the E° value, with sensors in O2 exhibiting standard deviations of about 20 mV in E° values. According to the Nernst equation, conventional potentiometric sensors displayed a difference of 59.2 mV per decade of analyte concentration for monovalent analyte ions, and a 20 mV standard deviation results in poor correlation of analyte activities for the same voltage reading across sensors.

[0066] Example 2: Structure of Present Sensor

[0067] The present K⁺ sensor is based on a potassium ferrocyanide (KFeCN) doped carbon transducer, wherein the sensor may be immersed in an aqueous solutions for detecting free K⁺ ions. Design of the present sensor allows for similarly high performance when used with other metal cation sensing membranes, and having the ferrocyanides in the transducer layer. A sample- dependent potential may then develop across the sample/ISM interface and activity of the free metal cation can be determined by matching voltage measured to activity off a calibration curve.

[0068] Compared to conventional sensors, the present sensor exhibits a stable potential and a high degree of sensor-to-sensor reproducibility. The present sensor is also cost effective and conveniently producible since it does not require production of any materials and can be fabricated with commercially available reagents. The present can be prepared in a convenient manner by exposing it to an aqueous solution of 10 ³ M potassium chloride (KCI) for a period between 6 to 12 hours. A stable measurement (E° standard deviation of about 4 mV or less) can be obtained when the sensor is immediately removed from the KCI solution and used to take a measurement. The full workflow is shown FIG. 6.

[0069] Holistically, the present sensor may include three layers supported on an inert substrate (408) as shown in FIG. 4. Each layer is amenable to large scale production methods like screen printing and the sensor may be printed on a planar substrate (408). The ISM (404) and substrate (408) can be the only parts of the sensor in contact with the analyte solution (402). The electrical contact (406) can be insulated and connected to an external potentiometer for measurements against a standard reference electrode.

[0070] The ISM (404) provides for sensitivity and selectivity towards K⁺. Non-limiting examples of the ISM formulation may be a membrane consisting of about 33 wt% polyvinyl chloride and 66 wt% plasticizer, wherein the wt% is based on the ISM. Meanwhile, the transducer layer (410) provides the following.

[0071] The transducer layer (410) of the present sensor has significantly improved sensor-to- sensor reproducibility due to presence of the redox couple in carbon, which stabilizes the interfacial potential at the ISM/transducer layer interface. The presence of the redox couple in the transducer (410) is in contrast to designs in conventional section, such as those described in the background above, where the location of the couple in the ISM (404) necessitates design and synthesis of molecules that are sufficiently lipophilic so as not to be lost to the sensing environment (402) during use of the sensor. The earlier described tension between selectivity of the sensor and stability of the reading can also be avoided by the present sensor. The carbon transducer (410) is not hygroscopic compared to PEDOT SS, which makes it more resistant to the formation of a water layer that causes instability during measurements. Based on the Nernst equation, the potential at the ISM/transducer layer interface depends on the redox state of the potassium ferro/ferricyanide redox couple, and its stability may depend on external factors that alter this overall redox state in the layer. Such factors may include oxygen which can oxidize wet ferrocyanide to ferricyanide under prolonged exposure, or the degree of purity of the ferrocyanide or ferricyanide during the manufacturing process. The redox state may be represented by the following equations:

K⁺ + Fe(CN)i^~ ® KFe(CN)l^~

K⁺ + Fe(CN)l^~ ® KFe(CN)l^~

[0072] The overall redox state of the transducer (410) remains constant so that calibration-free measurements may be taken with the sensor. The transducer (410) also exchanges ions at near zero-current (about 10 ¹² A) conditions with the ISM (404), and this ion current is transduced into an electron current that can be then measured by potentiometers.

[0073] The electrical contact (406) acts as a conductor that forms an electrical contact between the transducer (410) and the wire to the potentiometer.

[0074] Example 3: Materials Selection for Device Fabrication

[0075] For the choice of electrode, screen printed carbon sensors were employed. Carbon doped with potassium ferrocyanide redox couple was also adopted. Electrodes were purchased from Metrohm with the layout shown in FIG. 5.

[0076] Working, counter and pseudo reference electrodes were all disposed on the same electrode. The electrode had a planar form factor, making it amenable to modification by processes like drop casting or screen printing. The working electrode had a diameter of 4 mm, and only the working electrode was used for measurements

[0077] The choice of ion selective membrane was similar to that used in conventionally researched ion selective electrodes, with approximately 33 wt% PVC, and 66 wt% plasticizer, such as dioctyl sebacate, per membrane weight composition. The ionophore, valinomycin was used as it has high selectivities towards K⁺. The membranes were dissolved in a cyclohexanone solvent with about 6 wt% of membrane components, and 10 m I of membrane solution was either drop cast or screen printed on the individual electrodes. Cyclohexanone was chosen as it takes significantly longer to dissolve PVC, compared to a more commonly used solvent such as tetrahydrofuran. This property of cyclohexanone greatly reduces the dissolution of the screen printed carbon electrode, as the ink used in the screen printing process consists of a polymer binder. Otherwise, the carbon layer may lose integrity and become partially dissolved when a solvent that is too volatile is used. Each electrode was then put on a hot plate at 70°C for about 5 mins.

[0078] The transducer layer was chosen for its screen printable property, which serves as a proof that high performance can be achieved with mass production techniques. The type of redox couple available on the carbon working electrode consists of a simple, easy to obtain redox-active compound, which does not require new methods of production to be developed. Carbon, unlike conventional PEDOT SS, is not hygroscopic, and as a result is less susceptible to the formation of a water layer between the ISM and transducer layer.

[0079] Silver was selected as it is a proven electrical conductor that can be in the form of an ink suitable for screen printing, and is stable at the electrical potentials and temperatures required for various measurements.

[0080] Example 4A: Sensor Fabrication

[0081] In the following, the conditions for which calibration-free measurements can be taken are shown. This can be seen in FIG. 6. Calibration-free measurements are defined within a time period in which sensors show 1 ) no significant drift E° values for each sensor for a time period (in hours) and 2) high sensor-to-sensor reproducibility (i.e. standard deviation of E° less than 4 mV).

[0082] Conditioning of potentiometric ion selective electrodes is a practice for hydrating the membrane before its use. Electrodes may be subjected to exposure or depletion of O2 during conditioning. The initial state of the electrodes, either in dry N2, or 100% humidity N2, for 24 hours may be used to determine the window in which calibration-free measurements could be taken.

[0083] A balance between the present sensor’s performance and convenience for conditioning can be considered, and this may be a condition where dry N2 is present during storage and the sensor is immersed for 6 to 12 hours in the presence of oxygen as shown in FIG. 6. This was chosen because few controls or precautions are needed when handling the conditioning solution.

[0084] Example 4B: Extended Conditioning in Relation to Calibration-Free State

[0085] FIG. 7 shows the E° value for the first conditioning process for the 1^st to 48^th hour of conditioning in 10 ³ M KCI solution with or without O2 present in the solution. For each separate investigation, a newly made set of electrodes was used each time, wherein sets of 3 electrodes each were made with an identical process for each electrode. E° values of each set of sensors were investigated over the course of 48 hours from their initial contact with conditioning solution. All test solutions were prepared with KCI in deionized water and exposed to atmospheric air. Observations were as follows.

[0086] Most highly repeatable, stable E° values are observed at the 6^th hour, 9^th hour and 12^th hour (represented by the shaded region), regardless of the presence of oxygen in the conditioning solution. The sensors exposed to oxygen were more adversely affected with reduced sensor-to- sensor reproducibility after 24 hours. While the average values of E° showed a small change from 24^th to 36^th hour, the values at 48^th hour showed an unpredictable change. High stability of the present sensor was achieved as it can be seen from the small difference and low variances between E° readings from the 6^th to 12^th hour, allowing for calibration-free measurements to be made within this time period. The present sensors showed the most stable and least varying potential values at the 6^th to 12^th hour regardless of whether they are conditioned in solutions with O2 present, meaning that their window of stability tends to last longer, i.e. any time from 12 to 24 hours. [0087] Example 4C: Influence of Prehvdration of the ISM

[0088] FIG. 8 shows the E° changes in sensors either from a dry state, or first subjected to 100% humidity for in a N2 atmosphere for 24 hours before testing. The behaviour of sensors that were conditioned with O2 in the conditioning solution was compared with solutions deaerated with N2.

[0089] It can be seen that the standard deviations of E° were low for the sensors that were exposed to high humidity. There is also measurement stability for the sensors conditioned in dry nitrogen. Those that were tested from a dry state showed stable readings with low deviation during the 6^th, 9^th and 12^th hour calibrations. Compared to the sensors that were conditioned with O2 in solution, the sensors conditioned without O2 showed significantly better potential stability, even especially for those which had been prehydrated. The sensor that had been prehydrated but stored without O2 achieved a E° value that was stable by the 3^rd hour of conditioning. Prehydration of the ISM is effective in shortening the time required for each sensor to achieve a stable potential value, reducing it from 6 hours to 3 hours. The data point at the 1^st hour looks promising but the calibration curves for each of the 3 sensors measured during the initial 1^st hour were not considered due to the presence of the slope, which constitutes noise to the data.

[0090] Example 4D: Response of Sensors Having Only Carbon in the Transducing Laver

[0091] FIG. 9 shows the conditioning behaviour of a sensor having a tranducer that is made only from screen printed carbon, i.e. without doping of any KFeCN. The following observations were made.

[0092] Faster drift during the first 12 hours was observed, followed by a drift that was approximately linear for the rest of the process. In addition, a stable potential was not reached during conditioning time.

[0093] The impact of redox couple was shown by the instability of sensors without the couple, and no stable potential was reached in 48 hours of testing time as compared to a stable potential reached at the 3^rd to 6^th hour for present sensors with the redox couple.

Claims

1. An ion selective electrode sensor for measuring concentration of a cation in a liquid, the ion selective electrode sensor comprising:

2. The ion selective electrode sensor of claim 1 , wherein the cation comprises a potassium ion, and/or the metal ferrocyanide comprises potassium ferrocyanide.

3. The ion selective electrode sensor of claim 1 or 2, wherein the ion selective membrane comprises:

polyvinyl chloride; or

polyvinyl chloride in an amount of 33 wt% of the ion selective membrane.

4. The ion selective electrode sensor of any one of claims 1 to 3, further comprising:

an ionophore disposed in the ion selective membrane, wherein the ionophore facilitates selective diffusion of the cation in the liquid into and/or out of the ion selective membrane; or an ionophore disposed in the ion selective membrane, wherein the ionophore facilitates selective diffusion of the cation in the liquid into and/or out of the ion selective membrane, wherein the ionophore comprises valinomycin.

5. The ion selective electrode sensor of any one of claims 1 to 4, wherein the electrical contact layer comprises silver.

6. The ion selective electrode sensor of any one of claims 1 to 5, further comprising an inert substrate which the ion selective membrane, the electrical contact layer, and the transducer layer are disposed thereon.

7. The ion selective electrode sensor of any one of claims 1 to 6, wherein the external device comprises a potentiometer for measuring a difference in potential between (i) a reference electrode and (ii) the electrical contact.

8. A method of fabricating the ion selective electrode sensor of any one of claims 1 to 7, the method comprising:

depositing an electrical contact layer on an inert substrate;

9. The method of claim 8, wherein depositing the transducer layer comprises forming a layer of carbon doped with potassium ferrocyanide.

10. The method of claim 8 or 9, wherein depositing the ion selective membrane comprises:

(ia) dissolving a polymer in cyclohexanone to form a polymer solution; or

(ib) dissolving a polymer in cyclohexanone to form a polymer solution, wherein the polymer comprises polyvinyl chloride;

(ii) forming the polymer solution on the transducer layer; and

(iii) heating the polymer solution to remove the cyclohexanone, thereby forming the ion selective membrane.

1 1. The method of any one of claims 8 to 10, further comprising conditioning the ion selective electrode sensor for calibration-free measurements of concentration of the cation in the liquid, wherein the conditioning comprises:

(i) storing the ion selective electrode in an inert environment; and

(iia) immersing the ion selective electrode sensor in a solution comprising the cation before use; or

(iib) immersing the ion selective electrode sensor in a solution comprising the cation before use, wherein the cation comprises a potassium ion.

12. The method of claim 1 1 , wherein storing the ion selective electrode comprises placing the ion selective electrode in anhydrous nitrogen or hydrous nitrogen.

13. The method of claim 11 or 12, wherein immersing the ion selective electrode sensor comprises:

soaking the ion selective electrode in the solution in the absence of oxygen for 6 hours to 12 hours; or

soaking the ion selective electrode in the solution in the presence of oxygen for 6 hours to 12 hours.