JP2022537280A - Metal ferrocyanide-doped carbon as a transducer for ion-selective electrodes - Google Patents

Abstract

SUMMARY OF THE DISCLOSURE An ion-selective electrode sensor for measuring the concentration of cations in a liquid is provided. An ion-selective electrode sensor measures the concentration of cations in a liquid with an ion-selective membrane that promotes selective diffusion of cations in a liquid across the ion-selective membrane. and a transducer layer disposed between the ion selective membrane and the electrical contact layer, the transducer layer comprising carbon doped with metal ferrocyanide. The present disclosure also provides methods of manufacturing the above ion-selective electrode sensors. The method comprises depositing an electrical contact layer on an inert substrate, depositing a transducer layer comprising carbon doped with metal ferrocyanide on the electrical contact layer, and depositing an ion selective film on the transducer layer. to form an ion selective electrode.

Description

The present disclosure relates to ion selective electrode sensors for measuring the concentration of cations in liquids. The present disclosure also relates to methods of manufacturing such ion selective electrode sensors.

BACKGROUND OF THE INVENTION A solid-contact ion-selective electrode (SC-ISE) can consist of a conductor directly coated with the desired ionophore-doped membrane, as shown in FIG. 1A. This may provide opportunities for miniaturization and development of low-cost disposable sensors of ions in biological and natural fluids. Practical applications of SC-ISE are limited by challenges in obtaining stable and reliable potential readings.

A major development in this direction is the low interfacial electrostatic charge between the ion selective membrane (ISM) and the electrical contact layer for coated wire electrodes, which can be electrodes with the design shown in FIG. 1A. It is to deal with capacity. Since the coated wire electrode (100) may rely on double layer capacitance, the small surface area between the ISM (104) and conductor (106) of the coated wire electrode (100) Lower interfacial capacitance. The double layer capacitance is based on the charges on the interface and electrical contacts of the ion-selective membrane. The charges in the ion-selective membrane are ionic, whereas the charges in the electrical contacts are electronic. In other words, in this case the double layer capacitance becomes both ionic and electronic. Secondly, small currents of ions at the interface between ISM (104) and electrical contact (106) can lead to relatively large changes in interfacial potential at the backside of membrane (104), which leads to potential instability. can cause

To address this instability, an additional transducer layer (110) located between the ISM (104) and the electrical contacts (106) has been previously considered (FIG. 1B). The transducer layer (110) may include, for example, (i) a conductive polymer that provides electronic and ionic conductivity, or (ii) a large contact surface area that provides double-layer capacitance at the ISM/transducer interface. High surface area nanocarbons may also be included that exhibit an increase. In both cases, the larger the interfacial capacitance, the more stable the potential reading becomes.

Other means developed to address instability include doping redox-active molecules or intercalation compounds into the transducer layer, which has also been successfully demonstrated to improve sensor-to-sensor reproducibility. ing.

Training and standard solutions used for calibration impose additional requirements to obtain accurate measurements, thus complementing the convenience and portability offered by SC-ISE and eliminating the need for calibration will be another attractive prospect.

に従う。 Factors that affect the sensor and require calibration can be evaluated by observing changes in the potential measured between the ISE and the reference electrode. This potential is determined by the Nernst equation

obey.

where E is the measured potential, R, T, F are the gas constant, degrees Kelvin and Faraday's constant, respectively, zI is the charge of the analyte ion _I , _aI is its activity in the sample is. E _I ⁰ includes potential differences at all interfaces other than the ISM/sample interface. For SC-ISEs with conductive polymer transducer layers, there may be a continuum of redox states that causes variations in E _I ⁰ between sensors, requiring calibration before use. there is a possibility. Factors that can affect E _I ^o include changes in crystallinity, time-dependent conformational changes after redox reactions, changes in glass transition temperature due to doping levels, interchain bonding, penetration of counterions into the layer. , and the morphology of the layer as a result of the manufacturing process. Conventionally, the standard deviation of sensors incorporating nanocarbons, such as multi-walled carbon nanotubes, tends to be about 10 mV, and for singly charged ions, the theoretical span is 59.2 mV for measurements within 10 years of concentration. was not suitable.

A calibration-free ISE design revolves around obtaining a defined potential at the ISM/transducer interface with a controlled oxidation state in either the ISM or transducer phases. For sensors using conducting polymers as transducers, electrochemical control of the redox state can be achieved via current pulses by equilibrating the ISE with a standard reference electrode or by controlling the polarization of the transducer layer during electrode fabrication. It can be done by letting

In certain studies performed, solid selective or solid reference electrodes based on colloidal imprinted mesoporous carbon were investigated. In this case the redox-active molecules were mixed with the ion-selective membrane rather than being present in the transducer layer. The presence of the redox couple provided stability in the conditioning solution over the 72 hour test period.

Other studies have investigated solid ion-selective electrodes in which redox-active molecules are covalently attached to sensor components by click chemistry. In this case, methods of attachment of redox-active molecules were the focus of research, and click chemistry was viewed as a means of attaching a variety of different molecules to moieties.

In the solid selective electrode or solid reference electrode studies based on colloid-imprinted mesoporous carbon described above, the controlled redox state of the ISM layer was oxidized to a defined oxidation state when preparing the membrane cocktail. It is realized by mixing the reductively active compound directly into the membrane phase. Previously, highly stable sensors have been developed that exhibit low drift, but redox-active molecules must be designed to be sufficiently lipophilic that they do not leach out of the ISM when immersed in solution. . To this end, tedious synthetic protocols are conventionally designed to obtain redox-active molecules with the desired properties. Conflicts also exist in the use of these molecules in ISM. Although stable ion-ionophore complexes contribute to high selectivity in sensor performance, this may in turn increase the loss of redox-active molecules from the ISM when in contact with solutions. As a result, the stability of the sensor reading is necessarily competing with the selectivity of the sensor, and thus a compromise is required. In addition, conventional ISE protocols usually recommend 24 hours of conditioning, which may be too long and inconvenient. Conventional sensors have been reported to exhibit high stability, but are likely to experience non-zero potential drift over time, requiring calibration, including recalibration.

Thus, what is needed is a solution that ameliorates and/or solves one or more of the problems discussed above. The solutions described herein provide at least a sensor and a method of manufacturing such a sensor.

SUMMARY OF THE INVENTION In a first aspect, an ion selective electrode sensor for measuring the concentration of cations in a liquid, comprising:
an ion selective membrane that promotes selective diffusion of cations in a liquid across the ion selective membrane;
an electrical contact layer connected to an external device for measuring the concentration of cations in the liquid;
a transducer layer disposed between the ion selective membrane and the electrical contact layer;
including
wherein the transducer layer comprises carbon doped with metal ferrocyanide;
An ion selective electrode sensor is provided.

In another aspect, a method of making an ion selective electrode sensor as described in the first aspect, comprising:
depositing an electrical contact layer on an inert substrate;
depositing a transducer layer comprising carbon doped with metal ferrocyanide on the electrical contact layer;
depositing an ion selective membrane on the transducer layer to form an ion selective electrode;
A method is provided comprising:

In the drawings, like reference characters generally refer to like parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being generally placed on illustrating the principles of the invention. In the following description, various embodiments of the disclosure are described with reference to the following drawings.

FIG. 2 is a schematic diagram showing a coated wire electrode; The electrodes (100) are 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 electrical contacts (106). Schematic showing a modified electrode (112) with a transducer layer (110) positioned below the selective membrane layer. FIG. 3 shows a plot of the standard potential term E ⁰ (200) in mV versus conditioning time (202) in hours h for an ISE with a conventional PEDOT:PSS transducer immersed in 10 ⁻³ M KCl. E ⁰ was calculated from individual calibration curves for each sensor when calibrated with solutions of 10 ⁻¹ , 10 ⁻² , 10 ⁻³ and 10 ⁻⁴ M KCl solutions. All open circuit potential (OCP) measurements were made against a standard 3M KCl reference electrode. Open diamonds represent data acquired under dry conditions and shaded diamonds represent data acquired under pre-hydration conditions. FIG. 3 shows a plot of the standard potential term E ⁰ in mV (300) versus conditioning time (302) in hours h for an ISE with a conventional PEDOT:PSS transducer immersed in 10 ⁻³ M KCl. The _O2 -free set was kept in the degassed solution for 48 h and the _O2 -containing set was kept in the solution exposed to the laboratory atmosphere. The upper plot represents data acquired in the presence of oxygen. The lower plot represents data acquired in the absence of oxygen. FIG. 10 is a side view of the KFeCN sensor architecture; The electrodes (400) are placed in a liquid sensing environment (402). The electrode (400) is disposed on a support substrate (408) and sandwiched between a PVC ion selective membrane (ISM) (404) and an electrical contact (406), a carbon transducer layer (410) doped with KFeCN. )including. 1 is a schematic diagram showing a conventional screen printed electrode from Metrohm Dropsens; FIG. FIG. 10 is a flow chart for obtaining the effects of calibration-free measurements, conditioning and hydration procedures; FIG. A third conditioning and storage period of 6-12 hours is recommended. The 3-12 hour and 6-12 hour (no oxygen) "calibrate free measurements" allow stable measurements but require the removal of oxygen from the conditioning solution. This complication outweighs the need for the user to perform separate calibration steps, so while still usable, it is not a preferred use. Boxes labeled "Needs Calibration" indicate pathways for which stable measurements cannot be made. The workflow can be generalized into three stages represented by (600), (602) and (604). At (600), a new sensor is removed from each storage atmosphere and used. At (602), the sensor is soaked in 10 ⁻³ M KCl and the soak time and dissolved oxygen environment are controlled. ^* indicates oxygen present at ambient level. ^** indicates saturated with nitrogen. FIG. 7 shows a plot of ^E0 values in mV (700) versus conditioning time in hours h (702). E ⁰ values were calculated by linear fitting the data points obtained from the calibration in 10 ⁻¹ to 10 ⁻⁴ M KCl solutions over 48 hours. 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 solution was degassed by bubbling _N2 . The difference in absolute stable E ⁰ readings between O ₂ -exposed and non-exposed sensors is attributed to mixing errors during small batch production. FIG. 10 shows a plot of ^E0 values in mV (800) versus conditioning time in hours h (802). Specifically, FIG. 8A shows a comparison of a KFeCN sensor that was initially dry conditioned and a sensor that was conditioned without oxygen in solution and subjected to 100% humidity for 24 hours. . The difference in absolute values between the 6-12 hour stable potential values is due to batch-to-batch errors during sensor manufacturing. Hollow diamonds and crosses represent data acquired in dry and pre-hydrated conditions, respectively. FIG. 10 shows a plot of ^E0 values in mV (804) versus conditioning time in hours h (806). Specifically, FIG. 8B shows a comparison of a KFeCN sensor that was initially dry conditioned and a sensor that was conditioned with oxygen in solution and subjected to 100% humidity for 24 hours. Hollow diamonds and crosses represent data obtained under pre-hydrated and dry conditions, respectively. FIG. 10 shows a plot of open circuit potential (OCP) measured in mV (900) versus time h units (902) for a sensor without KFeCN in the transducer layer conditioned with 0.1 M KCl for 48 hours. .

DETAILED DESCRIPTION OF THE INVENTION 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. Also, other embodiments may be utilized and modifications may be made without departing from the scope of the invention. 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 herein should not necessarily be construed as an admission that the document is part of the state of the art or is common knowledge.

Features described in the context of one embodiment may be correspondingly applicable to the same or similar features of other embodiments. Features described in the context of one embodiment, even if not explicitly described in other embodiments, may be correspondingly applicable to those other embodiments. Moreover, additions and/or combinations and/or alternatives described for features in the context of one embodiment may be applicable correspondingly to the same or similar features in other embodiments.

The present disclosure provides ion selective electrode sensors and methods of manufacturing such ion selective electrode sensors.

The ion-selective electrode sensors and methods of the present invention at least mitigate potential drift, improve sensor-to-sensor reproducibility for measurements made using ion-selective electrode sensors of the present invention, and provide calibration prior to use. and in reducing the need for recalibration. Because the ion selective electrode sensor according to the present invention is in solid form and retains the solid form during use, it may be referred to herein as a "solid contact ion selective electrode sensor."

Thus, the present disclosure provides an ion selective electrode sensor for measuring the concentration of cations in liquids. An ion-selective electrode sensor measures the concentration of cations in a liquid with an ion-selective membrane that promotes selective diffusion of cations in the liquid across the ion-selective membrane. and a transducer layer disposed between the ion selective membrane and the electrical contact layer, the transducer layer comprising carbon doped with metal ferrocyanide.

As used herein, the term "cation" refers to positively charged ions. Cations can be monovalent cations, divalent cations or trivalent cations. The cations may be metal ions.

For ion-selective electrode sensors, the input signal can be the activity of a particular ion and the output can be a potential.

An ion selective electrode sensor according to the present invention can be connected to an external device including a potentiometer for measuring the potential difference between (i) a reference electrode and (ii) an electrical contact. The potentiometer allows the user to read the obtained potential difference measurement. Also, the ion-selective electrode and the reference electrode can be coupled to a voltmeter to allow the user to obtain such readings.

Conversion of a signal input to a potential output based on the activity of specific ions occurs at the interface between the sample fluid and the ion-selective membrane. In ion-selective membranes, the freely mobile charge carriers are ions. Signal conversion also includes the conversion of ionic currents into electrical currents as ions approach or enter the underlying transducer layer. In the case of a conducting polymer as the transducer layer, conversion occurs as a result of redox reactions occurring in the conducting polymer layer. However, when the ion-selective membrane is immersed in a liquid, the ion-selective membrane and most of the underlying layer can become hydrated, i.e. the coating or absorption of molecular or bulk water in the water layer. , affecting the diffusion of ions into the ion-selective membrane and the ion concentration profile, which causes potential measurement drift. The sensor according to the present invention alleviates this.

Advantageously, the ion-selective electrode sensor according to the invention can be used to obtain stable measurements with a standard deviation of 4 mV or less. A sensor according to the present invention has a transducer layer comprising carbon electrodes doped with metal ferrocyanide. Metal ferrocyanides are redox couples that enhance reproducibility between sensors. This means that the same sensor used to make measurements under comparable conditions will not produce different results. This also means that different sensors measuring under comparable conditions will not produce different results.

Metal ferrocyanide can include metal ferricyanide to provide a redox couple. A metal ferrocyanide can be converted to a 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 is hydrated. With metal ferrocyanide and/or metal ferricyanide, it is possible to use lipophilic molecules mixed directly into the ion selective membrane to stabilize the interfacial potential between the ion selective membrane and the transducer layer. Avoided. Lipophilic molecules may impair the selectivity for ions and/or the stability of the measurement. In addition, the metal ferrocyanide and/or metal ferricyanide is doped with carbon, which is not hygroscopic, so the transducer layer is resistant to water formation on top.

A sensor according to the invention can be used to measure the concentration of cations in a liquid, and the activity can be correlated with the concentration of cations in the liquid. Cations may include potassium ions. Metal ferrocyanides can be selected to be compatible with cations. For example, if the cations include potassium ions, the metal ferrocyanide can include potassium ferrocyanide. Accordingly, the metal ferricyanide may include potassium ferricyanide.

The ion selective membrane of the sensor according to the invention may comprise polyvinyl chloride or polyvinyl chloride in an amount of 33% by weight of the ion selective membrane. The ion-selective membrane may also include a plasticizer in an amount of about 33 wt% polyvinyl chloride and about 66 wt% plasticizer, where wt% is the ion selective Based on the sex membrane. A non-limiting example of a plasticizer may be any suitable plasticizer that is compatible with polyvinyl chloride, such as dioctyl sebacate. The plasticizer improves diffusivity, which improves the ionic conductivity of the ion-selective membrane.

The ion-selective electrode sensor of the present invention is an ionophore disposed in an ion-selective membrane that facilitates selective diffusion of cations in a liquid to and/or out of the ion-selective membrane. or an ionophore located in an ion-selective membrane that promotes selective diffusion of cations in a liquid to and/or out of the ion-selective membrane and may include valinomycin It may further contain an ionophore. Ionophores, such as valinomycin, can have loading rates of approximately 1% by weight on ion-selective membranes. Ionophores can selectively bind cations and help transport the cations across the membrane to the transducer layer. For transducer layers that exhibit both ionic and electronic conductivity, ionophores emit positive ions at the interface between the ion-selective membrane and the transducer layer.

An ion-selective electrode sensor according to the present invention may further comprise an inert substrate on which the ion-selective membrane, the electrical contact layer and the transducer layer are arranged.

The present disclosure also provides methods of manufacturing the ion selective electrode sensors described herein. The method comprises depositing an electrical contact layer on an inert substrate, depositing a transducer layer comprising carbon doped with metal ferrocyanide on the electrical contact layer, and depositing an ion selective film on the transducer layer. to form an ion selective electrode.

Embodiments and advantages relating to the ion-selective electrode sensor according to the invention already mentioned above are applicable to the present method and vice versa. Since the various components and configurations and their advantages have already been described above, the elements of the ion selective electrode sensor according to the invention will not be repeated for the sake of brevity. For example, the transducer layer may be a carbon layer doped with metal ferrocyanide. Metal ferrocyanides can include metal ferricyanides. The metal ferrocyanide/ferricyanide can include potassium ferrocyanide/ferricyanide. In this regard, the invention may include forming a layer of carbon doped with potassium ferrocyanide.

In the method, depositing the ion-selective membrane comprises (ia) dissolving the polymer in cyclohexanone to form a polymer solution, or (ib) dissolving the polymer in cyclohexanone to form a polymer solution. (ii) forming a polymer solution on the transducer layer; (iii) heating the polymer solution to remove cyclohexanone, thereby forming an ion selective membrane; and forming a. The use of cyclohexanone is advantageous in that it takes much longer to dissolve polyvinyl chloride compared to conventional solvents such as tetrahydrofuran. In other words, although cyclohexanone is a solvent for polyvinyl chloride to form ion-selective membranes, this property of cyclohexanone advantageously reduces or hinders dissolution of the deposited carbon transducer layer. Deposition of the doped carbon transducer layer can be accompanied by a polymeric binder. When the polymer binder comes into contact with tetrahydrofuran, it dissolves easily, and the integrity of the doped carbon layer is lost, and the doped carbon layer can partially dissolve in the solvent. Therefore, the use of cyclohexanone eliminates the need for more volatile solvents that can damage the transducer layer. As already mentioned above, the polyvinyl chloride used can be about 33% by weight of the ion selective membrane. A plasticizer may be included in the polymer. For example, a plasticizer may be included in polyvinyl chloride. The plasticizer can be about 66% by weight of the ion selective membrane. Additionally, ionophores can be included in the polymer. The ionophore can be about 1% by weight based on the weight of the ion selective membrane. The ionophore may be, for example, valinomycin.

The method may further include conditioning the ion-selective electrode sensor for calibration-free measurement of the concentration of cations in the liquid. Conditioning includes (i) storing the ion-selective electrode in an inert environment; (ii) soaking the ion-selective electrode sensor in a solution containing cations prior to use; or (iib) prior to use. immersing the ion-selective electrode sensor in a solution containing cations, wherein the cations may include potassium ions.

Storing the ion selective electrode can include placing the ion selective electrode in anhydrous or hydrous nitrogen. As already mentioned above, a carbon transducer layer doped with metal ferrocyanide and/or metal ferricyanide provides a redox couple that stabilizes the sensor in the presence of water. Therefore, storage of the sensor in dry or moist nitrogen does not significantly compromise sensor stability.

Soaking the ion-selective electrode sensor includes immersing the ion-selective electrode in solution in the absence of oxygen for 6-12 hours, or immersing the ion-selective electrode in solution in the presence of oxygen for 6-12 hours. can include soaking.

Since the method introduces a transducer layer comprising metal ferrocyanide and/or carbon doped with metal ferricyanide, conditioning of the ion-selective electrode sensor prior to use can be done in the presence or absence of oxygen. It can be carried out. As already mentioned above, metal ferrocyanides and/or ferricyanides provide a redox couple that mitigates potential drift. In other words, the transducer layer of the present invention exhibits improved resistance to the effects of oxygen in the sample solution during the specified 6-12 hours and improved potential measurement stability. This makes the conditioning of ion-selective sensors according to the invention more versatile and not limited by the need or absence of oxygen.

The solution may contain cations, the concentration of which is being measured. The cation can be potassium. The solution may contain potassium chloride. Potassium chloride can range in concentration from 10 ⁻³ to 10 ⁻⁵ M to condition ion selective electrode sensors according to the present invention.

In this disclosure, the word "substantially" does not exclude "completely", for example a composition "substantially free" of Y may be completely free of Y. If desired, the word "substantially" may be omitted from the specification of the invention.

In the context of various embodiments, the articles "a," "an," and "the" used in connection with features or elements include references to one or more features or elements.

In the context of various embodiments, the term "about" or "approximately" as applied to numerical values encompasses exact values and reasonable variances.

As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitation not specifically disclosed herein. . Thus, for example, the terms "comprising," "including," "containing," etc. shall be interpreted expansively and non-limitingly. Thus, the term “comprises” or “comprises” or variations such as “comprising” are meant to include the recited integer or group of integers, but any other integer or It is understood that it is not meant to exclude groups of integers. Moreover, the terms and expressions used herein are used as terms of description rather than of limitation, and use of such terms and expressions does not imply any equivalents of the features illustrated and described, or portions thereof. is not intended to exclude, but it is recognized that various modifications are possible within the scope of the invention as claimed. Accordingly, while the present invention has been specifically disclosed by way of exemplary embodiments and optional features, modifications and variations of the invention embodied herein disclosed herein will occur to those skilled in the art. and that such modifications and variations are considered to be within the scope of the present invention.

EXAMPLES The present disclosure provides ion-selective electrode sensors for measuring the concentration of cations in liquids and methods of making ion-selective electrode sensors. In order that the present disclosure may be readily understood and implemented in practice, specific embodiments are illustrated by the following non-limiting examples.

Example 1A Potential Drift of Conventional Sensors In solid contact ion sensors, potential drift can occur where the potential recorded in the same solution changes at different times. When a dry electrode is immersed in a solution, water is taken up by the membrane and the hydration state of the membrane is changed. This changes the activity coefficient of the primary ion (in this case K ⁺ ) in the membrane. Since the potentiometric signal is the sum of all interfacial potentials between the metal contacts of the ISE and the reference electrode, changes in the membrane's ion concentration profile as a function of hydration will change the membrane's interfacial potential. This is observed as an overall shift in the absolute value of the recorded potential. An example comparing readings for the standard potential term E ⁰ 1-12 hours after the sensor was first immersed in a 10 ⁻³ M KCl conditioning solution is shown in FIG. The pre-hydrated sensors were exposed to a 100% humidity _N2 atmosphere for 24 hours, whereas the dried sensors were immersed directly in the liquid for a test time of 0 hours. The difference in the two sets of data explains the difference in hydration state between the two sets of ion-selective membranes.

Water in PVC membranes has been found to have two diffusion coefficients. A first, faster factor is found to dominate the initial hydration of the membrane. This can be seen from the much higher drift velocity for the dry film in FIG. 2 during the first 12 hours of hydration. A second slower hydration process can then be seen from the data points after 24 hours of hydration has passed and can be observed as a slower process of drift.

As a result of drift, the potential difference signal of each sensor is not stable in the order of time, and the speed of drift varies with time, making it difficult to predict the extent of drift.

The equilibration process of a solid-contact ISE is complicated by the equilibration process that occurs at the ISM/transducer layer interface. It has previously been speculated that PEDOT:PSS promotes further downward drift of the newly prepared electrode and the extent to which this competes with the equilibration occurring in the ISM layer. Nonetheless, what is consistent is a continued downward drift during the 48 hour experiment. Therefore, the absolute potential over time is always time dependent. This indicates the need for calibration between conventional sensor measurements.

Example 1B: Requirements for Calibration of Conventional Sensors In conventional ISEs, potential instabilities due to drift and poor sensor-to-sensor reproducibility require calibration to perform effective measurements. To overcome this, automation or training must be used, which increases resource usage and can introduce measurement errors.

Example 1C: Oxygen Sensitivity for Storage of Conventional Sensors There are two ways oxygen in the storage environment affects the ^E0 value. In Figure 3, a clear divergence of the ^E0 values is seen after 9 hours. Here, the sensor values stored in the degassed solution show a greater drop in ^E0 values compared to those stored in an environment with _O2 present. This effect has been previously reported in conventional films of PEDOT:PSS.

Oxygen also significantly reduces the reproducibility of ^E0 values between sensors. Sensors in _O2 showed a standard deviation of about 20 mV in ^E0 values. According to the Nernst equation, conventional potentiometric sensors show a 59.2 mV per decade difference in analyte concentration for monovalent analyte ions, with a standard deviation of 20 mV yielding the same This will result in poor correlation of analyte activity to voltage readings.

Example 2: Structure of a sensor according to the present invention A K ⁺ sensor according to the present invention is based on a carbon transducer doped with potassium ferrocyanide (KFeCN) and the sensor is immersed in an aqueous solution to detect free K ⁺ ions. can do. The sensor design according to the present invention enables similarly high performance when used with other metal cation sensitive membranes and having ferrocyanide in the transducer layer. A sample-dependent potential is then developed across the sample/ISM interface. The free metal cation activity can be determined by matching the measured voltage to the activity outside the calibration curve.

Compared to conventional sensors, the sensors according to the invention exhibit stable potentials and high reproducibility between sensors. Sensors according to the present invention are also cost-effective and easy to manufacture, as they do not require the manufacture of materials and can be manufactured with commercially available reagents. A sensor according to the present invention can be conveniently prepared by exposure to an aqueous solution of 10 ⁻³ M potassium chloride (KCl) for 6-12 hours. Stable measurements ( ^E0 standard deviation less than about 4 mV) can be obtained by immediately removing the sensor from the KCl solution and using it for measurements. The overall workflow is shown in Figure 6.

Overall, a sensor according to the present invention may comprise three layers supported on an inert substrate (408), as shown in FIG. Each layer is suitable for large scale production methods such as screen printing and the sensor can be printed on a planar substrate (408). The ISM (404) and substrate (408) may be the only parts of the sensor that come into contact with the analyte solution (402). Electrical contacts (406) are insulated and can be connected to an external potentiometer for measurement against a standard reference electrode.

ISM (404) provides sensitivity and selectivity for K ⁺ . A non-limiting example of an ISM formulation may be a membrane consisting of about 33% polyvinyl chloride and 66% plasticizer by weight. Here, weight percentages are based on ISM. On the other hand, the transducer layer (410) provides:

により表すことができる。 The transducer layer (410) of the sensor of the present invention greatly improves sensor-to-sensor reproducibility due to the presence of redox couples in the carbon. This stabilizes the interfacial potential at the ISM/transducer layer interface. The presence of redox couples in transducer (410) is in contrast to conventional section designs as described in the Background above. In the design of conventional sections, the design and synthesis of molecules that are sufficiently lipophilic so that the positions of the redox couples within the ISM (404) are not lost to the sensing environment (402) during use of the sensor. Needed. The aforementioned trade-off between sensor selectivity and reading stability can also be avoided by the sensor according to the invention. Carbon transducers (410) are less hygroscopic compared to PEDOT:PSS, making them more resistant to the formation of water layers that cause instability during measurements. Based on the Nernst equation, the potential at the ISM/transducer layer interface depends on the redox state of the potassium ferrocyanide/potassium ferricyanide redox couple, and its stability depends on this overall It can depend on external factors to change the redox state. Such factors can include oxygen, which can oxidize wet ferrocyanide to ferricyanide upon prolonged exposure, or the purity of ferrocyanide or ferricyanide during the manufacturing process. The redox state is given by the formula:

can be represented by

Since the overall redox state of the transducer (410) remains constant, the sensor can be used to make calibration-free measurements. The transducer (410) also exchanges ions with the ISM (404) at near zero current (approximately 10 ⁻¹² A) and this ion current is converted to an electronic current that can be measured by a potentiometer.

The electrical contacts (406) act as conductors making electrical contact between the transducer (410) and the wires to the potentiometer.

Example 3: Material Selection for Device Fabrication Screen-printed carbon sensors were employed for electrode selection. Carbon doped with a potassium ferrocyanide redox couple was also employed. Electrodes were purchased from Metrohm with the layout shown in FIG.

The working, counter and pseudo-reference electrodes were all placed on the same electrode. Because the electrodes are planar, they can be modified by processes such as drop casting or screen printing. The diameter of the working electrode was 4 mm and only the working electrode was used for the measurements.

The selection of ion-selective membranes is similar to those used in previously studied ion-selective electrodes, with approximately 33 wt% PVC and 66 wt% such as dioctyl sebacate per weight composition of the membrane. % plasticizer was used. The ionophore valinomycin was used, which has high selectivity for K ⁺ . The membrane was dissolved in a cyclohexanone solvent containing about 6% by weight membrane component and 10 μl of the membrane solution was drop-cast or screen-printed onto each electrode. Cyclohexanone was chosen because it takes a very long time to dissolve PVC compared to more commonly used solvents such as tetrahydrofuran. This property of cyclohexanone greatly reduces the dissolution of screen-printed carbon electrodes, since the inks used in the screen-printing process are composed of polymeric binders. Otherwise, using a solvent that is too volatile can cause the carbon layer to lose integrity and partially dissolve. Each electrode was then placed on a hot plate at 70° C. for about 5 minutes.

The transducer layer was chosen for its screen-printable properties, which serve as proof that high performance can be achieved by mass production techniques. The types of redox couples available on carbon working electrodes consist of simple, readily available redox-active compounds without the need to develop new manufacturing methods. Carbon, unlike conventional PEDOT:PSS, is not hygroscopic, resulting in less formation of a water layer between the ISM and transducer layers.

Silver was chosen because it can be in the form of an ink suitable for screen printing and is a conductor that has been proven stable at the potentials and temperatures required for various measurements.

Example 4A: Fabrication of Sensor The conditions under which calibration-free measurements are possible are shown below. This can be seen in FIG. A calibration-free measurement requires that the sensors are ¹ ) free of significant drift E0 values for each sensor over a period of time (in hours h) and ² ) highly reproducible between sensors (i.e., the standard deviation of E0 is less than 4 mV).

Conditioning of potentiometric ion-selective electrodes is a method for hydrating the membrane prior to use. The electrodes may be subjected to _O2 exposure or _O2 removal during conditioning. Initial conditions of the electrode for 24 hours in either dry _N2 or 100% humid _N2 can be used to determine the window in which calibration-free measurements can be performed.

Considering the balance between performance and conditioning convenience of the sensor according to the present invention, dry _N2 is present during storage and the sensor is immersed in the presence of oxygen for 6-12 hours, as shown in FIG. can be a condition. It was chosen because it requires almost no care or precautions required when handling conditioning solutions.

Example 4B: Extended Conditioning Associated with No Calibration Condition FIG. 7 shows initial conditioning for 1-48 hours of conditioning in 10 ⁻³ M KCl solution with and without O ₂ in solution. The ^E0 value of the process is shown. For each individual study, a newly prepared set of electrodes was used each time, and three sets of electrodes were each prepared by the same process for each electrode. ^E0 values for each set of sensors were studied over a period of 48 hours from initial contact with conditioning solution. All test solutions were prepared using KCl in deionized water and exposed to atmosphere. The observation results are as follows.

The most reproducible and stable ^E0 values are observed at 6, 9 and 12 hours (represented by the shaded areas) regardless of the presence of oxygen in the conditioning solution. Sensors exposed to oxygen were more adversely affected, with reduced sensor-to-sensor reproducibility after 24 hours. Mean values of E ⁰ showed little change from 24 to 36 hours, but values at 48 hours showed unpredictable changes. A high stability of the sensor according to the present invention is achieved, as seen by the small difference in the E ⁰ readings from 6 to 12 hours and the small variance, making calibration-free measurements during this time. be able to. The sensor according to the invention showed the most stable and least fluctuating potential values from 6 to 12 hours, regardless of whether it was conditioned in a solution with _O2 present, which is consistent with the present invention. This means that the sensor stability window tends to be long, anywhere from 12 to 24 hours.

Example 4C: Effect of pre-hydration of ISM Figure 8 shows the change in ^E0 of the sensor from a dry state or from being first exposed to 100% humidity in a _N2 atmosphere for 24 hours prior to testing. showing. The behavior of sensors conditioned with O ₂ in conditioning solutions was compared to solutions degassed with N ₂ .

It can be seen that the sensor exposed to high humidity had a lower standard deviation of ^E0 . Also observed is the measurement stability of the sensor conditioned with dry nitrogen. Sensors tested from dry showed stable readings with low deviations at 6, 9 and 12 hours of calibration. Compared to sensors conditioned with O ₂ in solution, sensors conditioned without O ₂ showed significantly better potential stability, especially even in pre-hydrated sensors. Sensors that had been _prehydrated but stored without ^O achieved stable E0 values by 3 hours of conditioning. Pre-hydration of the ISM was effective in reducing the time required for each sensor to achieve a stable potential value, which was reduced from 6 hours to 3 hours. The data points for the first hour look promising, but were not taken into account due to the presence of slopes in the calibration curves for each of the three sensors measured in the first hour, which constitutes noise in the data.

Example 4D Response of a Sensor Containing Only Carbon in the Transducer Layer FIG. 9 shows the conditioning behavior of a sensor with a transducer made entirely of screen-printed carbon, i.e. without KFeCN doping. there is Observations were made as follows.

A fast drift was observed for the first 12 hours, followed by an almost linear drift for the remainder of the process. Furthermore, no stable potential was reached during the conditioning period.

The sensor without the redox couple was unstable and did not reach a stable potential in the 48 hour test period, whereas the sensor according to the invention with the redox couple showed 3-6 hours. The effect of the redox couple was indicated by reaching a stable potential at .

Claims (13)

In an ion-selective electrode sensor for measuring the concentration of cations in liquids,
an ion-selective membrane that facilitates selective diffusion of said cations in said liquid across said ion-selective membrane;
an electrical contact layer connected to an external device for measuring the concentration of the cation in the liquid;
a transducer layer disposed between the ion selective membrane and the electrical contact layer;
wherein the transducer layer comprises carbon doped with metal ferrocyanide;
Ion-selective electrode sensor. 2. The ion selective electrode sensor of claim 1, wherein said cations comprise potassium ions and/or said metal ferrocyanide comprises potassium ferrocyanide. The ion-selective membrane is
polyvinyl chloride, or
3. The ion selective electrode sensor of claim 1 or 2, comprising polyvinyl chloride in an amount of 33% by weight of the ion selective membrane. The ion-selective electrode sensor is
an ionophore disposed in the ion-selective membrane, the ionophore facilitating selective diffusion of cations in the liquid to and/or from the ion-selective membrane; or
an ionophore disposed in said ion selective membrane, said ionophore facilitating selective diffusion of cations in said liquid to and/or from said ion selective membrane, said ionophore comprising valinomycin; 4. The ion selective electrode sensor of any one of claims 1-3, further comprising. 5. The ion selective electrode sensor of any one of claims 1-4, wherein the electrical contact layer comprises silver. Ion selection according to any one of claims 1 to 5, wherein the ion selective electrode sensor further comprises an inert substrate on which the ion selective membrane, the electrical contact layer and the transducer layer are arranged. sex electrode sensor. 7. The ion-selective electrode sensor of any one of claims 1-6, wherein the external device comprises a potentiometer for measuring the potential difference between (i) a reference electrode and (ii) an electrical contact layer. A method of manufacturing an ion selective electrode sensor according to any one of claims 1 to 7, comprising:
depositing an electrical contact layer on an inert substrate;
depositing a transducer layer comprising carbon doped with metal ferrocyanide on the electrical contact layer;
depositing an ion selective film over the transducer layer to form the ion selective electrode;
method including. 9. The method of claim 8, wherein depositing the transducer layer comprises forming a layer of potassium ferrocyanide-doped carbon. Depositing the ion selective membrane comprises:
(ia) dissolving the polymer in cyclohexanone to form a polymer solution, or
(ib) dissolving a polymer in cyclohexanone to form a polymer solution, said polymer comprising polyvinyl chloride;
(ii) forming the polymer solution on the transducer layer;
(iii) heating the polymer solution to remove the cyclohexanone, thereby forming the ion selective membrane;
10. A method according to claim 8 or 9, comprising The method further comprises conditioning the ion-selective electrode sensor for calibration-free measurement of the concentration of the cation in the liquid, the conditioning comprising:
(i) storing the ion selective electrode in an inert environment;
(iia) immersing the ion-selective electrode sensor in a solution containing the cations prior to use, or
(iib) immersing the ion selective electrode sensor in a solution containing the cations prior to use, the cations comprising potassium ions;
11. The method of any one of claims 8-10, comprising 12. The method of claim 11, wherein storing the ion selective electrode comprises placing the ion selective electrode in anhydrous or hydrous nitrogen. Immersing the ion-selective electrode sensor comprises:
immersing the ion-selective electrode in the solution in the absence of oxygen for 6-12 hours, or
A method according to claim 11 or 12, comprising soaking the ion selective electrode in the solution in the presence of oxygen for 6-12 hours.

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