Technical field

[0001] Various aspects of this disclosure relate to a layer structure for an ion selective electrode sensor. Various aspects of this disclosure also relate to an ion selective electrode sensor and a method of fabricating the layer structure for an ion selective electrode sensor.

Background

[0002] Ion selective electrodes (ISEs) are convenient devices which are used to make ion determination measurements, of which solid contact ISEs (SC-ISEs) offer convenience in their storage and use. However, SC-ISEs with an ion selective membrane (ISM) have poor electrode to electrode potential reproducibility, due to the SC layer being susceptible to solvent dissolution.

[0003] Therefore, there is a need to provide for improved SC-ISEs.

Summary

[0004] Various embodiments concern a layer structure for an ion selective electrode sensor, the layer structure including: an ISM; an SC layer; an electrical conductor disposed on an electrode, for connecting to an electronic circuit; wherein the SC layer is disposed between the ISM and the electrical conductor; wherein the ion selective membrane and the solid contact layer include a same organic salt; and wherein the solid contact layer further comprises a matrix-polymer.

[0005] Various embodiments concern an ion selective electrode sensor including a substrate; the layer structure as defined above disposed on a first portion of the substrate; and a reference electrode disposed on a second portion of the substrate.

[0006] Various embodiments concern a method of fabricating the layer structure for an ion selective electrode sensor as defined above, the method including: disposing the electrical conductor on the electrode; disposing the solid contact layer, including the same organic salt and the matrix-polymer, on the electrical conductor; disposing the ion selective membrane, comprising the same organic salt, from a solution comprising an organic solvent and a membrane precursor on the disposed solid contact layer to form the layer structure. Brief description of the drawings

[0007] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

- FIG. 1 is an illustration showing in A: Segmented potential diagram of a SC-ISE with 3 layers, showing the ISM (1), SC layer (2) and electrical conductor (3) with potential for each and B: diagram showing charge exchange between the 3 layers of an SC-ISE with an organic salt (cation and anion) in both the SC layer and ISM, wherein E signifies the Primary Ion;

- FIG. 2 is a side view of the layered structure according to some embodiments;

- FIG. 3 is a side view of the SC-ISE architecture according to some embodiments;

- FIG. 4 is a schematic drawing of a screen printed electrode;

- FIG. 5 is a graph showing the standard deviation of 3 electrodes immersed in the membrane cocktail of a K ⁺ ISM, with components dissolved in THF at a concentration of 200 mg mL ¹ ;

- FIG. 6 is a graph showing comparative differential scanning calorimetry traces of polyvinylidene difluoride (PVDF) either as as-received powder, cast as a film or loaded with 3 wt% ETH 500, a trace of pure ETH 500 is also included;

- FIG. 7A is a graph showing the potential reproducibility of a set (n = 3) of solid contacts in THF with lwt% ETH 500;

- FIG. 7B is a graph showing the potential reproducibility of a set (n = 3) of solid contacts in dioctyl sebacate (DOS) with lwt% ETH 500;

- FIG. 8A is a graph showing the potential reproducibility of batches (n=3) of bare solid contacts in DOS where the concentration of ETH 500 in DOS is varied, keeping ETH 500 at 0.5 wt% in the solid contact;

- FIG. 8B is a graph showing the potential reproducibility of batches (n=3) of bare solid contacts in DOS where concentration of ETH 500 in the solid contact is varied, keeping ETH 500 at 1 wt% in DOS;

- FIG. 9A is a graph showing SC-ISEs responsive to K ⁺ made with the SC layer according to some embodiments;

- FIG. 9B is a graph showing SC-ISEs responsive to anions made with the SC layer according to some embodiments;

- FIG. 9C is a graph showing SC-ISEs responsive to H ⁺ made with the SC layer according to some embodiments;

- FIG. 10 is a flow chart illustrating the method of fabricating the layer structure for an ion selective electrode sensor; - FIG. 11 shows the SC layer 120a, the electrical conductor 130a and the substrate 210a of a conventional SC-ISE 200a;

- FIG. 12A is a graph showing potential reproducibility of a set (n = 3) of bare solid contacts in THF with lwt% ETH 500;

- FIG. 12B is a graph showing potential reproducibility of a set (n = 3) of bare solid contacts in DOS with lwt% ETH 500; and

- FIG. 12C is a graph showing calibration curves of a set (n = 4) of electrodes with the SC layers introduced in FIG. 11 made into K ⁺ SC-ISEs.

Detailed description

[0008] The disclosure refers to a layer structure 100 for an ion selective electrode sensor 200. The layer structure 100 includes an ISM 110. The layer structure 100 includes an SC layer 120. The layer structure 100 includes an electrical conductor 130 disposed on an electrode 140. The electrical conductor 130 disposed on an electrode 140 for connecting to an electronic circuit 150. The SC layer 120 is disposed between the ISM 110 and the electrical conductor 130. The ion selective membrane 110 and the SC layer 120 include a same organic salt 160. The SC layer 120 includes a matrix-polymer 170.

[0009] In conventional SC-ISEs, salts are introduced into the SC layer for the creation of a defined electrochemical potential between the SC layer and the ISM. If the compositions of both SC layer and the ISM can be sufficiently controlled, the boundary potential EPB between these two phases can be reproducibly established across a large number of identically made electrodes. This results in SC-ISEs that show a high reproducibility of their standard potential £7° when measured against a common reference electrode. Poor reproducibility between individual electrodes is a known, common phenomenon in the field today. This problem however remains unaddressed, because individual electrodes are normally calibrated before use. Therefore, electrode to electrode deviations are mitigated by this standard procedure, as the calibration curve of a single electrode is investigated immediately before use. However, calibration of individual SC-ISEs does not scale well if measurements need to be made with a large number of electrodes. Additionally, the calibration step can increase potential operator error. Reproducible electrodes can hence enable multiple measurements to be performed with reduced calibration. This can simplify the measurement protocol, which mitigates the possibility or errors and reduces operator training costs.

[0010] A cross section of a potential profile within an SC-ISE is shown in FIG. 1, drawing part A. For reproducible potentials within a batch of electrodes with a standard deviation of 2 to 3 mV to be achieved, a uniform, defined interfacial potential between electrodes should be formed between phases (1) and (2).

[0011] As shown in FIG. 1, drawing B, a salt is incorporated into the SC layer (2) and the ISM (1). This causes the electrochemical potential between the SC and ISM phase to be uniform across individual electrodes. This is reflected in the reproducibility of the potential when a batch of electrodes was used to take readings in sample solution of identical concentration.

[0012] However, during fabrication of the SC-ISE, the ISM is dissolved in an organic solvent, e.g., tetrahydrofuran (THF). During deposition of the ISM in the organic solvent (“ISM cocktail”) on the SC layer, the salt component contained in the SC layer may easily dissolve into the ISM cocktail. This may cause two problems. The first is that the enrichment of the salt in the ISM may negatively affect its properties. In the case of a salt that acts as a cation exchanger, uncontrolled partition of such a salt to the ISM can perturb the ratio of ion exchanger to ionophore in the ISM, leading to deteriorated selectivity coefficients or reduced upper detection limits. Additionally, uncontrolled dissolution of parts of the SC layer may introduce process dependency of SC-ISEs, as the speed and extent of dissolution can vary due to environmental parameters. This results in reproducibility that may not be replicable from batch to batch, or lab to lab, for example, SC-ISEs with poor standard potential reproducibility of ±90 to 110 mV and poor slopes of 40 ± 10 mV dec ¹ may be obtained, despite using similar components. This effect is demonstrated in Example 8, illustrating the susceptibility of the organic salt to an organic solvent.

[0013] The layer structure 100 for an ion selective electrode sensor as described herein mitigates the poor reproducibility between individual electrodes by dissolving a same organic salt 160 in a matrix- polymer 170, acting as a binder, that is not merely inert. In particular, the mitigation of the poor reproducibility between individual electrodes is believed to be the synergistic effect of the organic salt 160 together with the matrix-polymer 170, and additionally the same organic salt 160 being used in the SC layer 120 and the ISM 110.

[0014] In particular, the organic salt 160 (e.g., the lipophilic salt, e.g., tetradodecylamonium tetrakis(4-chlorophenyl)borate (ETH 500)) is fully dissolved in the matrix-polymer 170 (e.g., fluoropolymer, e.g., PVDF). Due to the solvent resistance properties of the matrix-polymer 170, the SC layer 120 is resistant to dissolution by an organic solvent during ISM cocktail deposition. Upon deposition, reproducible potentials are maintained up to a time where the deposited solvent evaporates into air. [0015] Accordingly, when SC layer 120 is used to make a batch of SC-ISEs, the activity of free ions can be determined by matching voltage measured to activity off a calibration curve. The SC-ISE can be used when placed in a solution containing some activity of the target ion. Furthermore, this calibration curve can be highly similar between individual electrodes, reducing the need for calibration. By applying this in conjunction with an ISM composition containing the same organic salt 160, reproducible potentials can be obtained for a variety of different ions.

[0016] The “ion selective membrane (ISM)”, as used herein, refers to an ISM 110 that forms the outermost layer of an SC-ISE. The ISM 110 controls the sensitivity and selectivity towards a target ion. The ISM 110 may include a polymer matrix, which may be hydrophobic. The polymer matrix may support additional components such as plasticizer, organic salt 160, and/or ionophore. The polymer matrix may include polyvinyl chloride, optionally in a weight percentage of about 1 wt% to about 50 wt% (relative to the total weight of the ISM 110), optionally in a weight percentage of about 10 wt% to about 40 wt%, optionally in a weight percentage of about 20 wt% to about 35 wt%, optionally of about 32.5 wt%. The ISM 110 may include a plasticizer, optionally in a weight percentage of about 50 wt% to about 90 wt%, optionally in a weight percentage of about 60 wt% to about 80 wt%, optionally in a weight percentage of about 65 wt% to about 70 wt%, optionally of about 65 wt%. Non-limiting examples of the plasticizer may be any suitable plasticizer compatible with polyvinyl chloride, such as DOS. The plasticizer may increase diffusivity and hence the ionic conductivity of the ISM 110.

[0017] The ISM 110 may include an ionophore. The ionophore may facilitate selective diffusion of a target ion in the sample into and/or out of the ISM 110, or an ionophore disposed in the ISM 110, wherein the ionophore may facilitate selective diffusion of the target ion in the sample into and/or out of the ISM 110. The ionophore may include, for example, valinomycin. The ionophore, e.g. valinomycin, may have a loading rate of approximately 1 wt% in the ISM 110. An ionophore may selectively bind with the target ion and may help to transport it across the membrane to the SC layer 120

[0018] The “organic salt”, as used herein, refers to a compound that includes a cation and an anion, and wherein at least one, preferably both of the cation or anion includes or substantially consists of an organic component. An organic component may refer to a component including at least one carbon- hydrogen covalent bond. Since the organic salt 160 includes organic components, it may be substantially lipophilic, at least more lipophilic than an inorganic salt, which is typically a hydrophilic salt. Advantageously, when using a same organic salt 160 in both ISM 100 and SC payer 120, this may reduce the membrane charge transfer resistance. [0019] Advantageously, because the organic salt 160 is substantially lipophilic, at least one process step during equilibration of the SC-ISE may be avoided, thereby saving time. In particular, the preparation of the electrodes usually includes an equilibration process in solution before measurements can be taken. A number of processes can happen before the electrode is considered sufficiently equilibrated. Of these processes, the saturation of the electrode (for those with a hydrophilic salt) with water is only one requirement. So the SC-ISE with an organic salt can bypass this one requirement, and this is useful for situations where water saturation is the limiting step. In contrast and in further detail, when hydrophilic salts are mixed in the SC layer, and while the SC-ISE is in contact with an aqueous sample, the electrode experiences an uptake of water through the ISM. The uptake of water has been found to be necessary for stable and reproducible potentials to be attained across multiple devices. This happens as water passes through the ISM and percolates in hydrophilic areas within the SC layer. As this occurs, an equilibrium state can be reached, as the ingress of water within the SC-ISE results in a supersaturated solution of primary ions in the SC layer. The concentration at the solubility limit of primary ion salt within the SC layer forms a controlled boundary potential with the ISM, which also has a defined concentration of primary ion dependent on the concentration and ratio of the ionophore and cation or anion exchanger it contains. For this state to be attained however, a period of immersion of the SC-ISE inside aqueous solution is required before an equilibrium is reached. This has been ascribed to the need for saturation of the SC layer with water. The use of a primary ion salt also means that equilibrium state may change if the SC-ISE experiences prolonged ion exchange with sample solution. This is because prolonged exchange may cause changes to the bulk composition of the primary ion in the SC layer, if it becomes depleted.

[0001] According to various embodiments, the organic salt 160 may include alkyl chains, such as carbon-containing moieties. These carbon-containing moieties can be linear or branched, substituted or unsubstituted, and are derived from hydrocarbons, typically by substitution of one or more carbon atoms by other atoms, such as oxygen, nitrogen, sulfur, phosphorous, or functional groups that contain oxygen, nitrogen, sulfur, phosphorous. The carbon-containing moieties can comprise any number of carbon atoms, but has preferably a molecular weight of above 500 g/mol or above 800 g/mol.

[0002] In a preferred embodiment, the carbon-containing moieties can be a linear or branched, substituted or unsubstituted alkyl with 5 to 20 carbon atoms; linear or branched, substituted or unsubstituted alkenyl with 5 to 20 carbon atoms; linear or branched, substituted or unsubstituted alkynyl with 5 to 20 carbon atoms; linear or branched, substituted or unsubstituted alkoxy with 5 to 20 carbon atoms; substituted or unsubstituted cycloalkyl with 5 to 20 carbon atoms; substituted or unsubstituted cycloalkenyl with 5 to 20 carbon atoms; substituted or unsubstituted aryl with 5 to 20 carbon atoms; and substituted or unsubstituted heteroaryl with 5 to 20 carbon atoms. Advantageously, when the number of carbon atoms on the alkyl chains is very high, the lipophilicity may be very high, which is more beneficial for the SC-ISE for reasons detailed above.

[0020] The organic moiety can also be a combination of any of the above-defined groups, including but not limited to alkylaryl, arylalkyl, alkyl heteroaryl and the like, to name only a few, all of which may be substituted or unsubstituted. Generally, the organic salt 160 may be any substantially lipophilic salt that shows solubility in both the SC layer 120 and the ISM 110. For example, it may also be a cation or anion exchanger, or similar salts containing both a bulky cation and anion may also be incorporated in the SC layer 120. The organic salt 160 should be sufficiently lipophilic that the presence of ionophore in the ion selective membrane does not cause exchange of the charged salt for primary ion from the aqueous sample when ions from the sample enter the membrane through the sensing process. This would preserve the long term function of the system.

[0021] In some embodiments, the organic salt 160 may have a molecular weight of more than 500 g/mol, preferably of more than 1000 g/mol, and optionally up to a molecular weight of 5000 g/mol. Advantageously, when the molecular weight is very high, the lipophilicity may be very high, since more “organic matter” is provided relative to the charge of the organic salt 160. A high lipophilicity, as mentioned, is more beneficial for the SC-ISE for reasons detailed above. In one example, the organic salt 160 is ETH 500.

[0022] The matrix-polymer 170 is a polymer that is highly dense, crystalline, and that shows resistance to solvent attack. The matrix-polymer 170 may have the function of stabilizing the potential, which is in contrast to the behavior of an inert binder. The advantages of this additional function have been mentioned herein before. The matrix-polymer 170 may include polymers such as polyether ether ketone, polyacrylates that are additionally crosslinked after their deposition, or similar polymers.

[0023] According to various embodiments, the matrix-polymer 170 may be a fluoropolymer. The fluoropolymer may be any polymer that contains the element fluorine. The fluorine may be covalently bonded to a carbon backbone. The fluoropolymer may include interpolymerized monomeric units. Said units may be derived from monomers. The monomers may include the fluorine. The monomers may be selected from tetrafluoroethylene, vinylidene difluoride, or a combination thereof. In one example, the fluoropolymer is PVDF.

[0024] Advantageously, because the fluoropolymer includes fluorine, the fluoropolymer may be substantially resistant to being dissolved in a range of organic solvents, including THF. Because the fluoropolymer does not dissolve easily in organic solvents, and supports the organic salt 160 as a binder or a matrix, this may be beneficial to prevent the organic salt 160 from leaching out of the SC layer 120.

[0025] Because of the matrix-polymer 170 retaining the organic salt within the SC layer 120, it is possible to reduce the amount of the organic salt 160 in the SC layer 120 to less than 5 wt%, or even less than 1 wt% of the total weight of the SC layer 120. Having a lower weight percentage of organic salt 160 being present in the SC layer 120 is advantageous, since this further decreases the probability of the organic salt 160 dissolving into the ISM cocktail.

[0026] While a lower weight percentage of organic salt 160 in the SC layer 120 is advantageous, the upper limit of the weight percentage of organic salt 160 in the SC layer 120 may be determined by the solubility of the organic salt 160 in the matrix-polymer 170, and the ability of the polymer to prevent uncontrolled segregation into the ISM 110 during the fabrication process.

[0027] The layer structure 100 may include 3 functional layers, as shown in FIG. 2. According to some embodiments, the 3 layers may be disposed on (e.g. supported) on a substrate 210, as shown in FIG. 3, for forming the ion selective electrode sensor 200. In those embodiments, the ISM 110 and the substrate 210 may be the only parts of the sensor in contact with the sample solution. The electrical contact between the electrical conductor 130 and the electronic circuit 150 may be insulated and the electronic circuit 150 may be an external potentiometer for measurements against a standard reference electrode.

[0028] The SC layer 120 may operate at near zero-current (~10 ¹² A) conditions and may be capacitively coupled to the ISM 110. The ion current through the ISM 110 may be transduced into an electron current that may be measured by standard potentiometers. The SC layer 120 may include polyaniline (PANI), poly-3 -4-ethylenedioxythiophene polystyrene sulfonate (PEDOT:PSS), carbon material, poly(octyl thiophene), polypyrrole, or a combination thereof. In one example, the SC layer 120 includes carbon material.

[0029] As used herein, the term “solid” when used in the phrase “solid contact (SC) layer” takes its normal meaning, and therefore includes references to compositions or substances demonstrating (significant) structural rigidity and resistance to changes of shape or volume (e.g. substances which exhibit no flow). In particular, the term “solid” may refer to substances characterised by their resistance to penetration. The term “solid” is understood not to include a hydrogel.

[0030] The SC layer 120 may have a first main side and a second main side, wherein the second main side is opposite to the first main side. The first main side and the second main side of the SC layer 120 refer to the two largest surfaces of the layer. In particular, a layer typically extends into two directions (perpendicular to each other), while having a thickness in a direction which is perpendicular to the two directions in which the layer extends. The two surfaces that extend into the two directions are referred to herein as the first main side and a second main side. The distance between the two surfaces of the first main side and a second main side may refer to the thickness of the SC layer 120.

[0031] The layer structure 100 according to the disclosure may be arranged in such a configuration that the SC layer 120 is facing (e.g., being in contact with) the ISM 110 with a first main side thereof. The ISM I SC interface as used herein may therefore be positioned on the first main side of the SC layer 120. The SC layer 120 may be facing, with a second main side thereof, the electrical conductor 130.

[0032] The SC layer 120 may have a thickness of about 0.1 pm (micrometer) to about 10 pm, or about 0.5 pm to about 5 pm, or about 0.8 pm to about 2 pm, or about 1 pm.

[0033] The electrical conductor 130 may form an ohmic electrical contact between the SC layer 120 and the wire to the potentiometer. The electrical conductor 130 may include silver.

[0034] According to various embodiments shown in FIG. 3, there is provided an ion selective electrode sensor 200. The ion selective electrode sensor 200 includes a substrate 210. The ion selective electrode sensor 200 includes the layer structure 100 as described herein disposed on a first portion of the substrate 210. The ion selective electrode sensor 200 includes a reference electrode 220 disposed on a second portion of the substrate 210. The ion selective electrode sensor 200 may be free of a counter electrode. Advantageously, the counter electrode is not necessary for such an ion selective electrode sensor 200, as it operates under an effectively zero current (10 ¹² A) regime. Only the working and reference electrodes would be used.

[0035] A material for each of the electrodes may be independently selected from, but not limited to, metals, such as gold, silver, nickel, titanium, platinum. Alternatively, a material for each of the electrodes may be independently selected from polymers including poly(3,4-ethylenedioxythiophene), poly(thiophene)s, polyaniline, polypyrrole. The polymers may also be termed “conducting polymers”. When using conducting polymers as the direct contact to an ion selective membrane electrode material, there are advantages due to their redox capacitance. In particular, the additional redox capacitance provided by the conducting polymer may resist polarization of the interface and hence improves the stability of the electrode response. For metals to perform a similar role approaching the robustness of conducting polymers, depositing a lipophilic monolayer before contact with a membrane may be beneficial, as this may reduce interference by water and gas. [0036] According to various embodiments, there is provided a method of fabricating the layer structure for an ion selective electrode sensor 300 as described herein. The method 300 includes a step 310 of disposing the electrical conductor 130 on the electrode 140. The method 300 includes a step 320 of disposing the SC layer including the organic salt 160 and the matrix-polymer 170 on the electrical conductor 130. The method 300 includes a step 340 of disposing the ISM including the organic salt 160 from a solution comprising a solvent and a membrane precursor on the disposed SC layer 120 to form the layer structure 100.

[0037] The SC layer 120 may be prepared by dissolving the matrix-polymer 170 and the organic salt 160 in a solvent system before disposing the solid contact layer 120 on the electrical conductor 130. A filler may be added to the solvent system. Advantageously, the filler may be electrically conducting, which may facilitate the ohmic connection to the electrical conductor 130. In particular, the filler may be one or more carbon material, optionally selected from the group consisting of carbon black, carbon nanotube, carbon fibre, graphene, graphene oxide, graphite, carbon mesosphere, and a combination thereof. After deposition on the electrical conductor 130, a relatively low temperature (<100 °C) may be applied for curing of the SC layer 120. Additionally or alternatively, the temperature that is used can be up to the thermal stability of any of the components. Accordingly, the method 300 may include a step 330 of heating the disposed SC layer.

[0038] By “heating” is meant that the temperature of the solution containing a solvent and the membrane precursor is deliberately raised. Heating may thus involve to raise the temperature above room temperature. In various embodiments, the heat is emitted from an oven. The heating may involve heating the disposed SC layer 120 to a temperature of at least 40 °C. The heating may involve heating the disposed SC layer 120 to a temperature of at least 40 °C, or to the boiling point of the solvent.

[0039] “Room temperature”, as used herein, refers to a temperature greater than 4° C, preferably being in the range from 15 °C to 40 °C, or in the range from 15 °C to 30 °C, or in the range from 20 °C to 30 °C, or in the range from 15 °C to 24 °C, or in the range from 16 °C to 21 °C, or around 25 °C. Such temperatures may include, 14 °C, 15 °C, 16 °C, 17 °C, 18 °C, 19 °C, 20 °C, 21 °C, and 25 °C, each of these values including ± 0.5 °C.

[0040] The membrane precursor may have the same composition as described previously in context with the ISM 110.

[0041] The electrode 140 may advantageously have a planar form, such that the electrical conductor 130, the SC layer 120 and the ISM 110 can be easily disposed on the electrode 140 by drop-casting or screen-printing. Hence, each of the ISM 110, the SC layer 120 and the electrical conductor 130 is advantageously amenable to large scale production methods like screen-printing and the SC-ISE may be printed on a planar substrate. Additionally, the electrode 140 may be placed on a heating element for the heating step.

[0042] The solvent system may comprise an organic solvent. The solvent system may be capable to dissolve the matrix-polymer 170, the organic salt 160, and the filler. The solvent of the solvent system may be selected from the group consisting of tetrahydrofuran, propylene carbonate, cyclohexanone, cyclopentanone, or a combination thereof.

[0043] The step 340 of disposing the ISM 110 from a solution may be repeated about 1 to 10 times, or repeated 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 times.

[0044] Embodiments described in the context of the layer structure 100, or the ion selective electrode sensor 200 are analogously valid for the method of fabricating the layer structure for an ion selective electrode sensor 300. Similarly, embodiments described in the context of the method of fabricating the layer structure for an ion selective electrode sensor 300 are analogously valid for the layer structure 100, or the ion selective electrode sensor 200, and vice-versa.

[0045] 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.

[0046] 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.

[0047] As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Examples

[0048] Example 1 - Choice of electrodes:

[0049] Screen printed carbon connected to silver was chosen as the substrate electrode. [0050] Electrodes were purchased from a commercial source with the layout as shown in FIG. 4. Working and pseudo reference electrodes are all situated on the same electrode.

[0051] The electrode has a planar form factor, making it amenable to modification by drop casting or screen printing. The working electrode has a diameter of 4 mm, and only the working electrode was used for measurements.

[0052] Example 2 - Choice of ISM:

[0053] The choice of ISM material was approximately 33 wt% PVC, and 66 wt% per membrane weight composition. More particular, the ISM composition was chosen to be for a K ⁺ membrane: 32.5 wt% PVC, 1 wt% valinomycin, 65.1 wt% DOS, 1 wt% ETH 500, 0.5 wt% potassium tetrakis(4- chlorophenyl)borate, 1 wt% valinomycin. The ISM composition was chosen to be for a H ⁺ membrane: 31.6 wt% PVC, 65.3 wt% DOS, 1.2 wt% potassium tetrakis(4-chlorophenyl)borate (KtCIPB), 3.9 wt% tridodecylamine. The ISM composition was chosen to be for an anion selective membrane: 33 wt% PVC, 65 wt% DOS, 1 wt% ETH 500, 1 wt% tridodecylmethylammonium chloride.

[0054] For the H ⁺ membrane composition that did not contain the ETH 500, the cation exchanger KtCIPB contains a common anion tetrakis(4-chlorophenyl)borate, which results similarly in defined interfacial potentials.

[0055] As a model system, the membrane cocktail of the K ⁺ ISM was used to show reproducibility of the potential between electrodes during the casting process. ISM components were dissolved in the solvent, THF, at 200 mg of membrane components per mL of solvent. When a batch of 3 bare SC layers was inserted, the potentials measured against an Ag/AgCl wire showed a standard deviation of 2.6 mV retaining a high reproducibility, as seen in FIG. 5.

[0056] For each membrane cocktail, the concentration of membrane components in THF was kept at 200 mg mF ¹ , and 2 depositions of 20 pL each were drop cast on bare SC layers.

[0057] Example 3 - Choice of SC layer:

[0058] The SC layer consists of several components. An electrically conducting filler for ohmic connection to the electrical contact, a binder to resist solvent ingress and to retain a controlled composition of lipophilic salt, and a controlled concentration of lipophilic salt to form defined boundary potentials with the ISM. [0059] During preparation of the SC layer, the binder was first prepared by mixing 50 mg of PVDF with 0.75 mL each of THF and propylene carbonate. These were mixed overnight at a temperature of 70 °C in a glove box.

[0060] 250 mg graphite, 5 mg carbon black and 1.5 mg ETH 500 were then added to the mixture and allowed to stir for 20 min at 70 °C. This was followed by a further 10 min stirring step at room temperature, and 10 min of allowing the mixture to stand undisturbed.

[0061 ] 4 pL of the supernatant was then drawn and deposited onto the 4 mm diameter silver screen printed electrode. The electrodes were then put into an oven at 50 °C for 30 min, before raising the temperature to 70 °C for 60 min.

[0062] Example 4: Materials choice

[0063] The carbon black and graphite were chosen as cheap, abundant filler. The particulate nature of the filler also allows for inks suitable for screen printing to be made.

[0064] ETH 500 was chosen since it can be dissolved into the binder. This is shown in differential scanning calorimetry results in FIG. 6, where the melting peak of ETH 500 disappears when it is loaded into PVDF at 3 wt%, the same proportion as in the SC layer.

[0065] PVDF was chosen for the reason above, as it is able to retain its integrity when in contact with both DOS and THF. This leads to high reproducibility of the potential when batches of electrodes with the proposed SC were inserted in solvents containing 1 wt% ETH 500, as seen in FIG. 7A and FIG. 7B. In comparison with FIG. 12A and FIG. 12B, SC layers with a PVDF binder can retain potential reproducibility in both DOS and THF, compared to only in DOS for SC layers with a PVC binder.

[0066] Example 5: Choice of electrical contact:

[0067] Silver is a proven electrical conductor ink suited to screen printing, and is stable at the electrical potentials and temperatures experienced during measurements.

[0068] Example 6: Minimum quantity of lipophilic salt required for potential reproducibility

[0069] FIG. 8 shows the minimum concentration of organic salt in the A: ISM and B: SC layer required for good reproducibility of the potential in a batch of electrodes. For each potential trace, a batch of 3 electrodes were inserted into DOS and their potentials measured against an Ag/AgCl wire. When ETH 500 in DOS was varied, the SC contained 0.5 wt% of ETH 500, and when ETH 500 in the SC was varied, the DOS contained 1 wt% ETH 500. [0070] Observations

[0071] Highly reproducible potentials below a standard deviation of 3 mV were observed at > 0.2 wt% ETH 500 in the DOS, and > 0.5 wt% in the SC layer.

[0072] A batch of bare SC layer with only carbon and binder, without ETH 500 showed poor reproducibility of a standard deviation > 12 mV.

[0073] A batch of bare Ag electrodes measured in DOS resulted in a blocked interface, with little to no charge transfer. The high impedance of the system made it susceptible to ambient electrical noise, resulting in the oscillating signal seen in the batch of electrodes.

[0074] Discussion/Conclusion

[0075] From the potential reproducibilities, it can be inferred that ETH 500 can be exchanged between the proposed SC layer and the contacting DOS phase. When sufficient ETH 500 is present in both DOS and the SC, the cation and anion become the potential determining species across the SC | DOS interface, resulting in reproducible potentials due to the controlled composition of ETH 500 in each phase. Comparing this to the result in FIG. 5, the potential reproducibility is retained even when the surrounding matrix (THF) also contains other ions such as the K ⁺ cation and the tetrakis[3,5-bis(tri- fluoromethyl)phenyl]borate anion.

[0076] Due to the ability of ETH 500 to transfer between the SC and DOS, it is also expected that other charged species like anion or cation exchangers may be able to exhibit similar transport. Hence, further optimization of the system may necessitate finding an equilibrium concentration of all charged species in the SC and ISM to achieve stable and reproducible potentials between electrodes.

[0077] Example 7: Reproducibility of calibration curves between batches of SC-ISEs

[0078] FIG. 9 shows SC-ISEs responsive to K ⁺ , anions and H ⁺ made from the proposed SC layer containing 0.5 wt% ETH 500. As predicted from the evidence shown during testing of the bare SC layer, high reproducibility of the standard potential Eo were achieved across sets of 3 or 4 electrodes. The slopes recorded in the range from 10 ¹ to 10 ⁴ M of ions for K ⁺ and Cl show good linearity, and the linear range extends to 10 ¹ M, compared to the reduced high detection limit from the SC-ISEs proposed in the prior art.

[0079] Example 8: illustrating the solvent susceptibility of a conventional SC layer [0080] FIG. 11 describes a conventional example of a conventional SC layer 120a, showing a conventional ion selective electrode sensor 200a, including conventional SC layer 120a, conventional electrical conductor 130a disposed on a conventional substrate 210a. Conventionally, ISMs are formed on SC-ISEs by deposition of an ISM cocktail on top of an existing SC layer. This ISM cocktail comprises solid components dissolved in an organic solvent, commonly tetrahydrofuran (THF). After deposition, the device is left for the solvent to evaporate. During this time, solvent contact of the SC layer may cause dissolution of part of, or all of the SC layer.

[0081] This phenomenon is shown in the case of an SC layer 120a consisting of the lipophilic salt ETH 500, polyvinyl chloride (PVC), graphite and carbon black.

[0082] Three identical electrodes were inserted in either THF (FIG. 12A) or DOS (FIG. 12B) each containing lwt% ETH 500. Over a time of 600 s from immersion, the standard deviation of the potential measured against an Ag/AgCl wire was recorded.

[0083 ] A clear difference in potential reproducibility between the two sets of electrodes can be seen. The electrodes in DOS show a far greater reproducibility over time compared to those in THF. The reproducibility of the electrodes in THF also varies widely across the time measured. This is due to the interaction between PVC and both the organic liquids. While DOS is a plasticizer and does not change the bulk of PVC in a short contact time, PVC is highly susceptible to solvent attack by THF.

[0084] While a steady composition of ETH 500 in the electrodes and in the surrounding matrix is sufficient to define the electrode | matrix boundary potential, this cannot be observed if the electrode materials are susceptible to solvent attack. If this occurs, the electrode may disintegrate, the boundary potential becomes poorly defined, and the potential between electrodes becomes poorly reproducible.

[0085] For SC-ISEs with an SC Layer susceptible to solvent dissolution, the lack of a defined interfacial potential due to the dissolution of the SC layer by the ISM cocktail leads to poor electrode to electrode potential reproducibility.

[0086] A set of SC layers introduced in FIG. 11 (n = 4) showed poor potential reproducibility when these were used to make a set of K ⁺ selective SC-ISEs. Calibration curves showing the electrodes’ responses in solutions of 10 ¹ to 10 ⁴ M KC1 are shown in FIG. 12C. According to the Nemst equation, 1 mV of uncertainty corresponds to 4% error in ion activity determination for a monovalent ion. A variation of ± 8.8 mV hence introduces error greater than what is acceptable in many sensing use cases, and switching from one electrode to another would necessitate calibration. [0087] Furthermore, the slopes show significant deviation from the expected Nemstian response of 59.2 mV dec ¹ (> 5% deviation), which may indicate that charge transfer within the SC-ISE is hindered by a kinetic process, and does not fully adhere to the thermodynamic value.

[0088] While the disclosure has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.