CN113924481A - Metal ferrocyanide-doped carbon as ion-selective electrode converter - Google Patents
Metal ferrocyanide-doped carbon as ion-selective electrode converter Download PDFInfo
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- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/28—Electrolytic cell components
- G01N27/30—Electrodes, e.g. test electrodes; Half-cells
- G01N27/333—Ion-selective electrodes or membranes
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
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- G01N27/28—Electrolytic cell components
- G01N27/30—Electrodes, e.g. test electrodes; Half-cells
- G01N27/333—Ion-selective electrodes or membranes
- G01N27/3335—Ion-selective electrodes or membranes the membrane containing at least one organic component
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
- G01N27/28—Electrolytic cell components
- G01N27/30—Electrodes, e.g. test electrodes; Half-cells
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Abstract
According to the present disclosure, an ion selective electrode sensor for measuring a concentration of cations in a liquid is provided. The ion-selective electrode sensor comprises an ion-selective membrane that facilitates 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; and a converter layer disposed between the ion-selective membrane and the electrical contact layer, wherein the converter layer comprises carbon doped with metal ferrocyanide. The present disclosure also provides a method of manufacturing the above ion selective electrode sensor. The method includes depositing an electrical contact layer on an inert substrate, depositing a converter layer comprising carbon doped with metal ferrocyanide on the electrical contact layer, and depositing an ion-selective film on the converter layer to form an ion-selective electrode.
Description
Technical Field
The present disclosure relates to ion selective electrode sensors for measuring cation concentration in liquids. The present disclosure also relates to methods of manufacturing such ion selective electrode sensors.
Background
Solid contact ion-selective electrodes (SC-ISE) can be composed of an electrical conductor coated directly with a membrane doped with the desired ionophore as shown in fig. 1A, which can provide opportunities for miniaturization and development of low-cost disposable sensors for ions in biological and natural fluids. Challenges in obtaining stable and reliable potential readings have limited the practical application of SC-ISE.
In the case of a wire-coated electrode, which may be an electrode having the design shown in fig. 1A, the main development in this direction was to address the low interfacial capacitance between the ion-selective membrane (ISM) and the electrical contact layer. Since the wire-coated electrode (100) may rely on electric double layer capacitance, the low surface area between the ISM (104) and the electrical conductor (106) in the wire-coated electrode (100) results in low interfacial capacitance. The electric double layer capacitance is based on the charge at the interface of the ion selective membrane and the electrical contact. The charge within the ion-selective membrane is ionic, while the charge within the electrical contact is electronic. In other words, the electric double layer capacitance is both ionic and electronic in this case. The small current of ions at the interface between the ISM (104) and the electrical contact (106) may then result in a relatively large change in the interface potential at the opposite side of the membrane (104), which may be a source of potential instability.
To address the instability, an additional converter layer (110) located between the ISM (104) and the electrical contacts (106) is conventionally employed (FIG. 1B). The converter layer (110) may comprise, for example, (i) a conductive polymer that provides electronic and ionic conductivity, or (ii) a high surface area nanocarbon that exhibits increased electric double layer capacitance at the ISM/converter interface due to the high surface area of the contact points. The higher capacitance of the interface in both cases enables a much more stable potential reading.
Another approach developed to address the instability involves doping of the redox-active molecule or intercalation compound in the converter layer, which also successfully demonstrates improved reproducibility between sensors.
To complement the convenience and portability provided by SC-ISE, eliminating the need for calibration may be another attractive prospect, as the training and standard solutions used in calibration impose additional requirements on obtaining accurate measurements.
Factors affecting the sensor and resulting in the need for calibration can be assessed by observing the potential change measured between the ISE and the reference electrode, which follows the nernst equation:
where E is the measured potential, R, T and F are the gas constant, temperature in Kelvin and Faraday constant, respectively, zIIs the charge of the analyte ion I, aIIs its activity in the sample. EI 0Including the potential differences of all other interfaces except the ISM/sample interface. In the case of an SC-ISE with a conducting polymer converter layer, there may be successive redox states and this may lead to E between sensorsI 0So that it is likely to have to be calibrated before use. May influence EI 0Including changes in crystallinity, time-dependent conformational changes after redox reactions, changes in glass transition temperature due to doping levels, inter-chain bonds, penetration of counter ions into the layer, and layer morphology resulting from the manufacturing process. The standard deviation of conventional sensors comprising nanocarbons, such as multi-walled carbon nanotubes, is often about 10 mV, and is not suitable for measurements within one concentration decade with a theoretical span of 59.2 mV in the case of monovalent ions.
The calibration-free ISE design is developed centered around the use of controlled oxidation states in the ISM or converter phase to achieve a specific potential at the ISM/converter interface. For sensors with conducting polymers as the transducer, electrochemical control of the redox state can be achieved by current pulsing to allow the ISE to equilibrate with a standard reference electrode, or by polarizing the transducer layer to a controlled degree during electrode fabrication.
In one particular study that was conducted, solid state selective electrodes or solid state reference electrodes based on colloidal imprinted mesoporous carbon (colloidal imprinted mesoporous carbon) were studied. In this case, the redox-active molecules are incorporated into the ion-selective membrane, rather than being present in the converter layer. The presence of the redox couple brings stability over a test period of 72 hours in the conditioning solution.
In another study, solid-state ion-selective electrodes with redox-active molecules covalently attached to components of the sensor by click chemistry were investigated. In this case, the mode of attachment of redox-active molecules is the focus of research and click chemistry is considered as a means of attaching a range of different molecules to a module.
In the studies of the above mentioned solid selective electrode or solid reference electrode based on colloidally imprinted mesoporous carbon, controlled redox states in the ISM layer have been achieved by mixing redox active compounds of a specific oxidation state directly into the membrane phase when preparing a membrane mixture (membrane cocktail). Although highly stable sensors have been conventionally developed that exhibit low drift, it is necessary to design redox-active molecules that are sufficiently lipophilic to avoid their leaching from the ISM when immersed in solution. For this reason, complex synthetic procedures are routinely designed to obtain redox-active molecules with the desired properties. There is also a conflict between the use of these molecules in ISM. The stable ion-ionophore complexes contribute to a high selectivity in the performance of the sensor, however, this may in turn increase the loss of redox active molecules from the ISM upon contact with the solution. Thus, the stability of the sensor readings may have to compete with the selectivity of the sensor, which forces compromises to be made. Furthermore, conventional ISE manuals generally recommend conditioning for 24 hours, which can be too long to be convenient. Although conventional sensors have been reported to exhibit high stability, they may still experience non-zero potential drift over time, such that calibration, including recalibration, is necessary.
There is therefore a need for a solution that ameliorates and/or solves one or more of the problems mentioned above. The solution described herein should at least provide a sensor and a method of producing such a sensor.
SUMMARY
In a first aspect, there is provided an ion selective electrode sensor for measuring the concentration of cations in a liquid, the ion selective electrode sensor 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; and
a converter layer disposed between the ion-selective membrane and the electrical contact layer, wherein the converter layer comprises carbon doped with metal ferrocyanide.
In another aspect, there is provided a method of manufacturing the ion selective electrode sensor described in the first aspect, the method comprising:
depositing an electrical contact layer on an inert substrate;
depositing a converter layer comprising carbon doped with metal ferrocyanide on the electrical contact layer; and
an ion selective membrane is deposited on the converter layer to form an ion selective electrode.
Brief Description of Drawings
In the drawings, like reference numerals 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:
FIG. 1A shows a schematic view of a wire-coated 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 an electrical contact (106).
FIG. 1B shows a schematic of a modified electrode (112) in which the converter layer (110) is disposed below the selection film layer.
FIG. 2 shows impregnation to 10-3Standard potential term E in mV of ISE with conventional PEDOT: PSS converter in M KCl0(200) Graph of conditioning time (202) in vs. hours. E0By 10-1、10-2、10-3And 10-4The individual calibration curves for each sensor were calculated at calibration of the M KCl solution. All Open Circuit Potential (OCP) measurements were performed against a standard 3M KCl reference electrode. Open diamonds represent data obtained under dry conditions and shaded diamonds represent data obtained under pre-hydrated conditions.
FIG. 3 shows impregnation to 10-3Standard potential term E in mV of ISE with conventional PEDOT: PSS converter in M KCl0 (300) Graph of conditioning time in hours vs (302). No O2Is maintained in the degassed solution for 48 hours, containing O2The group (b) was kept in solution exposed to the laboratory atmosphere. The top curve represents data obtained in the presence of oxygen. The bottom curve represents data obtained in the absence of oxygen.
Fig. 4 shows a side view of a 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 KFeCN doped carbon converter layer (410) sandwiched between a PVC Ion Selective Membrane (ISM) (404) and electrical contacts (406).
FIG. 5 shows a schematic of a conventional Metrohm Dropsens screen printed electrode.
Figure 6 shows a flow chart for performing calibration-free measurements, the effect of conditioning and hydration procedures. A third conditioning and storage state between hours 6 and 12 is recommended. For "no calibration measurements" (no oxygen) between hours 3 to 12 and hours 6 to 12, stable measurements can be made but they require the conditioning solution to remove oxygen. This complexity exceeds the user's ability to perform a separate calibration step and is therefore less preferred for use, although still available. The box labeled "need to calibrate" represents an approach where stable measurements are not possible. The workflow can be generalized into three phases, represented by (600), (602), and (604). At (600), a new sensor is removed from its respective storage atmosphereFor use. At (602), the sensor is immersed at 10-3 M KCl, wherein the duration of impregnation and the dissolved oxygen environment are controlled. Indicates that atmospheric levels of oxygen are present. Is saturated with nitrogen.
FIG. 7 shows E in mV0Graph of conditioning time (702) in hours for a value (700) vs. By coming from fig. 10-1To 10-4Calculation of E by Linear fitting of 48 hour calibrated data points in M KCl solution0The value is obtained. Open diamonds and crosses represent data obtained from conditioning solutions containing and containing no oxygen for each set of electrodes, respectively. For storage without O2By using N to condition the electrode in a solution2Bubbling and degassing. Exposure to O2And not exposed to O2Stabilization of the sensor of0The absolute difference between readings can be attributed to mixing errors during small volume production.
FIG. 8A shows E in mV0A graph of the conditioning time (802) in hours for the value (800) vs. Specifically, fig. 8A illustrates a comparison of KFeCN sensors conditioned from an initial dry state to sensors subjected to 100% humidity for 24 hours for sensors conditioned without oxygen in solution. The absolute difference between the stable potential values at hours 6 through 12 is attributable to batch-to-batch errors in the sensor fabrication process. Open diamonds and crosses represent data obtained under dry and pre-hydrated conditions, respectively.
FIG. 8B shows E in mV0A graph of the conditioning time (802) in hours for the value (804) vs. Specifically, fig. 8B illustrates a comparison of KFeCN sensors conditioned from an initial dry state to sensors subjected to 100% humidity for 24 hours for oxygen conditioned sensors in solution. Open diamonds and crosses represent data obtained under pre-hydration and dry conditions, respectively.
Fig. 9 shows a graph of Open Circuit Potential (OCP) (900) vs measured in mV versus time in hours (902) for a sensor conditioned in 0.1M KCl for 48 hours without KFeCN in the converter layer.
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. Other embodiments may be utilized and changes may be made without departing from the scope of the present invention. The various embodiments are not necessarily mutually exclusive, as some embodiments may be combined with one or more other embodiments to form new embodiments. The listing or discussion of a prior-published document in this specification is not necessarily to be taken as an admission that the document is part of the state of the art or is common general knowledge.
Features described in the context of one embodiment may apply correspondingly to the same or similar features in other embodiments. Features described in the context of one embodiment may apply correspondingly to other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or substitutions of features as described in the context of one embodiment may be correspondingly applicable to the same or similar features in other embodiments.
The present disclosure provides ion selective electrode sensors and methods of making such ion selective electrode sensors.
The present ion-selective electrode sensors and methods are advantageous because they at least mitigate potential drift, improve reproducibility between sensors with respect to measurements made using the present ion-selective electrode sensors, and reduce the need for pre-use calibration and recalibration. The present ion-selective electrode sensor may be referred to herein as a "solid-contact ion-selective electrode sensor" because it is in solid form and remains in solid form throughout use.
Accordingly, the present disclosure provides an ion selective electrode sensor for measuring the concentration of cations in a liquid. The ion-selective electrode sensor comprises 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; and a converter layer disposed between the ion-selective membrane and the electrical contact layer, wherein the converter layer comprises carbon doped with metal ferrocyanide.
The term "cation" as used herein refers to a positively charged ion. The cation may be a monovalent cation, a divalent cation, or a trivalent cation. The cation may be a metal ion.
For an ion selective electrode sensor, the input signal may be the activity of a particular ion and the output may be a potential.
The present ion-selective electrode sensor is connectable to an external device, which includes a potentiometer for measuring a potential difference between (i) a reference electrode and (ii) an electrical contact. A potentiometer enables a user to read a potential difference measurement, or an ion selective electrode and a reference electrode may also be coupled to the voltmeter for the user to obtain such a reading.
Conversion of a signal input to a potential output based on the activity of a particular ion occurs at the interface between the sample fluid and the ion-selective membrane. In an ion selective membrane, the freely moving charge carriers are ions. Signal conversion also includes the conversion of ion current into electron current (electric current) as ions approach or enter the underlying converter layer. In the case of a conductive polymer as the conversion layer, the conversion occurs due to a redox reaction occurring in the conductive polymer layer. However, when the ion selective membrane is immersed in a liquid, the body and underlying layers of the ion selective membrane may become hydrated, i.e. coated by a layer of water or absorbed molecular water or a volume of water, so as to affect diffusion of ions into the ion selective membrane and ion concentration distribution, thereby causing drift in the potential measurement. The present sensor alleviates this problem.
Advantageously, stable measurements with a standard deviation of less than 4 mV or less can be obtained using the present ion-selective electrode sensor. The present sensor has a transducer layer comprising a carbon electrode doped with metal ferrocyanide. The metal ferrocyanide is a redox pair that improves reproducibility between sensors. This means that the same sensor used to make measurements under the same conditions will not produce different results. This also means that different sensors performing measurements under the same conditions do not produce different results.
The metal ferrocyanide may comprise a metal ferricyanide to provide a redox pair. The metal ferrocyanide may be converted to metal ferricyanide and vice versa. The redox couple stabilizes the interface potential between the ion selective membrane and the converter layer even if the ion selective membrane is hydrated. The metal ferrocyanide and/or the metal ferricyanide avoids the use of lipophilic molecules directly mixed into the ion-selective membrane in order to stabilize the interface potential between the ion-selective membrane and the converter layer. The lipophilic molecule may impair selectivity for ions and/or stability of measurement. Furthermore, metal ferrocyanide and/or metal ferricyanide are doped in the carbon, which does not absorb moisture and makes the converter layer resistant to water formation thereon.
The present sensor may be used to measure the concentration of cations in a liquid, wherein the activity may be correlated to the concentration of cations in the liquid. The cation may comprise potassium ion. A metal ferrocyanide that is compatible with the cation may be selected. For example, if the cation comprises potassium ions, the metal ferrocyanide may comprise potassium ferrocyanide. The metal ferricyanide may accordingly comprise potassium ferricyanide.
The ion-selective membrane of the present sensor 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 such that the polyvinyl chloride is in an amount of about 33 weight percent and the plasticizer is about 66 weight percent, where the weight percent is based on the ion selective membrane. Non-limiting examples of plasticizers can be any suitable plasticizer that is compatible with polyvinyl chloride, such as dioctyl sebacate. The plasticizer improves the diffusivity and thus the ionic conductivity of the ion-selective membrane.
The present ion-selective electrode sensor may further comprise an ionophore disposed in the ion-selective membrane, wherein the ionophore facilitates selective diffusion of cations 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 cations in the liquid into and/or out of the ion-selective membrane, wherein the ionophore may comprise, for example, valinomycin. Ionophores, such as valinomycin, can have a loading rate of about 1% by weight in ion selective membranes. The ionophores can selectively bind to cations and help transport them across the membrane to the converter layer. In the case of a converter layer exhibiting both ionic and electronic conductivity, the ionophore releases cations at the interface between the ion-selective membrane and the converter layer.
The present ion-selective electrode sensor may further comprise an inert substrate on which the ion-selective membrane, the electrical contact layer and the converter layer are disposed.
The present disclosure also provides methods of making the ion selective electrode sensors described herein. The method includes depositing an electrical contact layer on an inert substrate, depositing a converter layer comprising carbon doped with metal ferrocyanide on the electrical contact layer, and depositing an ion-selective film on the converter layer to form an ion-selective electrode.
The embodiments and advantages associated with the present ion selective electrode sensor as already described above apply to the present method and vice versa. For the sake of brevity, the elements of the present ion selective electrode sensor should not be repeated, as the various components and configurations and their advantages have been described above. For example, the converter layer may be a carbon layer doped with metal ferrocyanide. The metal ferrocyanide may comprise metal ferricyanide. The metal ferrocyanide/ferricyanide may include potassium ferrocyanide/potassium ferricyanide. In this regard, the method may comprise forming a layer of carbon doped with potassium ferrocyanide.
In the method, depositing the ion-selective membrane can include (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 a converter layer, and (iii) heating the polymer solution to remove cyclohexanone, thereby forming the ion-selective membrane. The use of cyclohexanone is advantageous because it takes significantly longer to dissolve polyvinyl chloride compared to conventional solvents, such as tetrahydrofuran. In other words, while cyclohexanone is the solvent for the polyvinyl chloride used to form the ion selective membrane, this property of cyclohexanone advantageously reduces or hinders dissolution of the deposited carbon converter layer. The deposition of the doped carbon converter layer may involve a polymer binder. When the polymeric binder is contacted with tetrahydrofuran, it may readily dissolve such that the integrity of the doped carbon layer is lost and the doped carbon layer may partially dissolve in the solvent. Thus, the use of cyclohexanone avoids the need for more volatile solvents that may damage the converter layer. As already mentioned above, the polyvinyl chloride used may be about 33% by weight 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% by weight of the ion-selective membrane. In addition, an ionophore may be included with the polymer. The ionophore may be about 1 wt% of the weight of the ion selective membrane. As an example, the ionophore may be valinomycin.
The method may further comprise conditioning the ion-selective electrode sensor for calibration-free measurement of the cation concentration 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 cations prior to use, or (iib) immersing the ion-selective electrode sensor in a solution comprising cations prior to use, wherein the cations may comprise potassium ions.
The storing of the ion-selective electrode may comprise exposing the ion-selective electrode to anhydrous nitrogen or aqueous nitrogen. As already explained above, the carbon converter layer doped with metal ferrocyanide and/or metal ferricyanide provides a redox couple that stabilizes the sensor in the presence of water. Thus, storage of the sensor in moisture-free nitrogen or moisture-containing nitrogen does not significantly impair the stability of the sensor.
Impregnating the ion-selective electrode sensor may include soaking the ion-selective electrode in a solution in the absence of oxygen for 6 hours to 12 hours, or soaking the ion-selective electrode in a solution in the presence of oxygen for 6 hours to 12 hours.
Because the present method incorporates a converter layer comprising carbon doped with metal ferrocyanide and/or metal ferricyanide, conditioning of the ion-selective electrode sensor prior to use can be performed in the presence or absence of oxygen. As already mentioned above, the metal ferrocyanide and/or ferricyanide provide a redox pair to mitigate potential drift. In other words, the present converter layer exhibits improved resistance to the influence of oxygen in the sample solution during the identified 6 th to 12 th hours, with consequent improvement in potentiometric stability. This makes the conditioning of the present ion selective sensor more versatile and not limited to aerobic or anaerobic conditions.
The solution may contain cations, the concentration of which is to be measured. The cation may be potassium. The solution may comprise potassium chloride. The potassium chloride may be 10-3To 10-5The concentration of M to condition the present ion selective electrode sensor.
In the present disclosure, the word "substantially" does not exclude "completely", e.g., a composition that is "substantially free" of Y may be completely free of Y. The word "substantially" may be omitted from the definition of the invention if necessary.
In the context of various embodiments, the articles "a," "an," "the," and "said" as used with respect to a feature or element include reference to one or more features or elements.
In the context of various embodiments, the term "about" or "approximately" as applied to a numerical value includes both precise 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 limitations, not specifically disclosed herein. Thus, for example, the terms "comprising," "including," "containing," and the like, are to be construed broadly and not restrictively. The word "comprise" or variations such as "comprises" or "comprising", will thus be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Additionally, the terms and phrases used herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and phrases to exclude 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 specifically disclosed herein 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
The present disclosure provides ion-selective electrode sensors for measuring cation concentration in liquids and methods of making ion-selective electrode sensors. In order that the present disclosure may be more readily understood and put into practical effect, specific embodiments will now be described by way of the following non-limiting examples.
Example 1A potential Drift in conventional Sensors
Solid contact ion sensors may suffer from potential drift, i.e., changes in the recorded potential of the same solution at different points in time. When the dry electrode is immersed in the solution, water absorption occurs in the membrane, so that the hydration state of the membrane changes. This in turn changes the primary ion (in this case K)+) Activity coefficient in the membrane. Since the potential signal is the sum of all the interface potentials between the metal contact point of the ISE and the reference electrode, changes in the ion concentration distribution in the membrane with hydration cause changes in the interface potential at the membrane. This is again observed as an overall shift in the absolute value of the recorded potential. One example is shown in FIG. 2, which compares the initial immersion of the sensor at 10-3Standard potential term E1 to 12 hours after M KCl conditioning solution0Is read. Pre-hydrated sensor was exposed to N at 100% humidity, as compared to a dry sensor immersed directly in liquid at 0 hour test time2Atmosphere for 24 hours. The differences in the 2 sets of data explain the differences in hydration state between the 2 sets of ion selective membranes.
It has been found that water has 2 diffusion coefficients in PVC films. It has been found that the first faster factor dominates at initial hydration of the membrane. This can be seen in fig. 2 from the much higher drift rate during the initial 12 hours of hydration in the case of the dry film. The second slower hydration process can then be seen from the data points after 24 hours of hydration and can be observed as a slower drift process.
The potential signals of the individual sensors are unstable on the hourly level due to drift, and the extent of drift is difficult to predict due to different drift rates over time.
The balancing process that occurs at the ISM/converter layer interface also complicates the balancing process of solid contact ISE. PSS contributes to further downward drift of the freshly prepared electrode and the extent to which this competes with the equilibrium occurring at the ISM layer can only be presumed so far. Nevertheless, it is consistent that during the 48 hours of the experiment there is a constant downward drift, so the absolute potential over the hours is always time dependent. This confirms that calibration is therefore required between measurements of conventional sensors.
Example 1B calibration of conventional sensor
Conventional ISE requires calibration to make an effective measurement due to potential instability from drift and poor reproducibility between sensors. This must be addressed using automation or training, so as to increase the use of resources and the likelihood of measurement errors.
Example 1C stored oxygen sensitivity of conventional sensor
Oxygen in the storage environment affects E in two ways0The value is obtained. In FIG. 3E is visible after 9 hours0Clear divergence of values (divergence) in which the value of the sensor stored in the degassed solution is compared with the value stored in the presence of O2Exhibit a larger E than those in the environment of0The value decreases. This effect has been described earlier for conventional PEDOT: PSS films.
The oxygen also obviously reduces E0Inter-sensor reproducibility of values, at O2The sensor in (1) shows an E of about 20 mV0Value standard deviation. According to the nernst equation, conventional potentiometric sensors exhibit a 59.2 mV difference for each decade of analyte concentration of monovalent analyte ions, with a 20 mV standard deviation resulting in a poor correlation of analyte activity between sensors at the same voltage reading.
Example 2 BenzhuanStructure of sensor
This K+The sensor is based on a carbon converter doped with potassium ferrocyanide (KFeCN), wherein the sensor can be immersed in an aqueous solution to detect free K+Ions. The present sensor design enables similarly high performance when used with other metal cation sensing membranes and having ferrocyanide in the converter layer. A sample-dependent potential can then be developed across the sample/ISM interface and the activity of the free metal cations can be determined by matching the measured voltage to the activity of the calibration curve.
The present sensor exhibits a stable potential and a high degree of inter-sensor reproducibility compared to conventional sensors. The present sensor is also cost effective and can be conveniently produced because it does not require the production of any material and can be manufactured with commercially available reagents. The sensor can be manufactured by exposing it to 10-3Aqueous M-potassium chloride (KCl) solution is prepared in a convenient manner over a period of 6 to 12 hours. When the sensor is taken out directly from the KCl solution and used for measurement, a stable measurement result (E of about 4 mV or less) can be obtained0Standard deviation). The complete workflow is shown in fig. 6.
Overall, the present sensor may include three layers supported on an inert substrate (408) as shown in fig. 4. The layers are suitable for mass production methods, such as screen printing, and the sensors can be printed on a planar substrate (408). The ISM (404) and the substrate (408) may be the only sensor components in contact with the analyte solution (402). The electrical contacts (406) may be insulated and connected to an external potentiometer for measurement against a standard reference electrode.
ISM (404) provides pairs K+Sensitivity and selectivity. A non-limiting example of an ISM recipe can be a film consisting of about 33 wt% polyvinyl chloride and 66 wt% plasticizer, where wt% is based on ISM. Meanwhile, the converter layer (410) provides the following.
The transducer layer (410) of the present sensor has significantly improved inter-sensor reproducibility due to the presence of a redox couple in the carbon to stabilize the interface potential at the ISM/transducer layer interface. The presence of a redox couple in the transducer (410) is different from conventional art designs, such as those described in the background above, where the presence of the redox couple in the ISM (404) results in a need to design and synthesize molecules that are sufficiently lipophilic so as not to be lost to the sensing environment (402) during use of the sensor. The tension between the selectivity of the sensor and the stability of the reading, described earlier, is also avoided by the present sensor. The carbon converter (410) is not hygroscopic compared to PEDOT: PSS, so that it is more resistant to water layer formation leading to instability during measurement. Based on the nernst equation, the potential at the ISM/converter layer interface depends on the redox state of the potassium ferrocyanide/potassium ferricyanide redox couple, and its stability may depend on external factors that change this overall redox state in the layer. Such factors may include oxygen, which can oxidize wet ferrocyanide to ferricyanide upon prolonged exposure, or the purity of ferrocyanide or ferricyanide during manufacture. The redox state can be represented by the following equation:
the overall redox state of the transducer (410) remains constant so that calibration-free measurements can be made with the sensor. The converter (410) is also at near zero current (about 10)-12A) Ions are exchanged with the ISM (404) under conditions and this ion current is converted into an electron current, which can then be measured by a potentiometer.
The electrical contact (406) acts as a conductor that forms an electrical contact between the transducer (410) and the wire connected to the potentiometer.
Example 3 Material selection for device fabrication
For the selection of electrodes, screen printed carbon sensors were used. Carbon doped with a potassium ferrocyanide redox couple is also used. The electrodes were purchased from Metrohm and had the layout shown in figure 5.
The working electrode, counter electrode and pseudo-reference electrode are all disposed on the same electrode. The electrode has a form factor such that it is suitable for modification by methods such as drop casting or screen printing. The working electrode had a diameter of 4 mm and only the working electrode was used for the measurement.
The ion-selective membrane was selected similarly to that used in conventionally studied ion-selective electrodes, having about 33 wt% PVC and 66 wt% plasticizer, such as dioctyl sebacate, relative to the membrane weight composition. As ionophore, valinomycin was used, since it is responsible for K+Has high selectivity. The membrane was dissolved in cyclohexanone solvent in the form of approximately 6% by weight of membrane components, and 10. mu.l of membrane solution was drop-coated or screen-printed on the respective electrodes. Cyclohexanone is chosen because it takes significantly longer to dissolve PVC than the more common solvents such as tetrahydrofuran. This property of cyclohexanone greatly reduces the dissolution of screen printed carbon electrodes, since the inks used in screen printing processes consist of a polymer binder. Otherwise, when too volatile solvents are used, the carbon layer may lose integrity and become partially dissolved. Each electrode was then placed on a hot plate at 70 ℃ for approximately 5 minutes.
The converter layer is selected for its properties suitable for screen printing, which is 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 simple, readily available redox active compounds, which does not require the development of new production methods. Unlike conventional PEDOT: PSS, carbon does not absorb moisture and therefore forms a less readily water layer between the ISM and the converter layer.
Silver is chosen because it is a well-established electrical conductor that can be in the form of an ink suitable for screen printing and is stable at the potentials and temperatures required for various measurements.
Example 4A sensor fabrication
The conditions under which the calibration-free measurement can be performed are shown below. This can be seen in fig. 6. A calibration-free measurement is defined as a measurement over a period of time in which sensor 1) does not exhibit E for each sensor over a period of time (in hours)0Significant drift in value and 2) exhibit high inter-sensor reproducibility (i.e., E of less than 4 mV0Standard deviation).
Conditioning of the potentiometric ion-selective electrode is the practice of hydrating the membrane prior to its use. The electrode may be exposed to O during conditioning2Or removing O2. In the dry of N2Or 100% humidity N2The electrode initial state for the middle 24 hours can be used to determine the window over which calibration-free measurements can be made.
A balance between the performance of the present sensor and the ease of conditioning may be considered, which may be the presence of dry N during storage as shown in FIG. 62And the sensor is immersed in the presence of oxygen for a period of 6 to 12 hours. This condition is chosen because little control or precaution is required to manipulate the conditioning solution.
Example 4B Extended Conditioning relative to No calibration State
FIG. 7 shows the presence or absence of O in solution 210 of-3E for conditioning the first conditioning process from 1 st to 48 th hours in M KCl solution0The value is obtained. For each individual study, a newly manufactured set of electrodes was used each time, wherein sets of 3 electrodes each were manufactured in the same way for each electrode. E of each group of sensors was studied over a period of 48 hours from their initial contact with the conditioning solution0The value is obtained. All test solutions were prepared with KCl in deionized water and exposed to atmospheric air. The observation is as follows.
The highest reproducible stability E was observed at hours 6, 9 and 120Values (indicated by shaded areas) regardless of the presence of oxygen in the conditioning solution. Oxygen-exposed sensors are more adversely affected and the reproducibility between sensors decreases after 24 hours. Although E0The average values showed small changes from hour 24 to 36, but values at hour 48 showed unpredictable changes. From 6 th to 12 th hour E0The small difference and low variance between readings can be seen to achieve high stability of the present sensor, enabling calibration-free measurements during this period. The present sensors exhibit the most stable and least changing potential values from hour 6 to hour 12, regardless of whether they are in the presence of O2By conditioning in solution, it is meant that their stability window is as good asFor any time longer, i.e. from 12 to 24 hours.
Example 4C Effect of Pre-hydration of ISM
FIG. 8 shows the test in a dry state or first at N before testing2E in a sensor subjected to 100% humidity for 24 hours in the atmosphere0And (4) changing. Will contain O in the conditioning solution2Behavior of the conditioned sensor with N2The degassed solutions were compared.
It can be seen that E of the sensor exposed to high humidity0The standard deviation is low. The sensor conditioned in dry nitrogen also had measurement stability. Those sensors tested from the dry state showed stable readings with low bias during the 6 th, 9 th and 12 th hour calibrations. With O in solution2Compared with the conditioned sensor in the case of (1), no O2The conditioned sensors showed significantly better potential stability, even especially those that were pre-hydrated. Is pre-hydrated but has no O2The stored sensor achieved E stabilized by 3 hours of conditioning0The value is obtained. Prehydration of the ISM effectively reduced the time required for each sensor to achieve a stable potential value, from 6 hours to 3 hours. The data point at hour 1 looked good, but due to the presence of a slope (which causes noise in the data), the calibration curves for each of the 3 sensors measured during the first hour 1 were not considered.
Example 4D response of a sensor with carbon only in the conversion layer
Fig. 9 shows the conditioning behavior of a sensor with a transducer made only of screen printed carbon (i.e. without any doping of KFeCN). The following was observed.
A faster drift during the first 12 hours was observed followed by a drift that was nearly linear for the remainder of the process. Furthermore, a stable potential was not reached during conditioning.
The effect of the redox couple was shown by the instability of the sensor without the redox couple and did not reach a stable potential within 48 hours of the test time, compared to the present sensor with the redox couple reaching a stable potential at hours 3 to 6.
Claims (13)
1. An ion-selective electrode sensor for measuring the concentration of cations in a liquid, the ion-selective electrode sensor 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; and
a converter layer disposed between the ion-selective membrane and the electrical contact layer, wherein the converter layer comprises carbon doped with metal ferrocyanide.
2. The ion-selective electrode sensor of claim 1, wherein the cation comprises potassium ions, 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 promotes selective diffusion of cations in the liquid into and/or out of the ion selective membrane; or
An ionophore disposed in an ion selective membrane, wherein the ionophore facilitates selective diffusion of cations in a 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, wherein the ion selective membrane, the electrical contact layer, and the converter 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 the potential difference between (i) the reference electrode and (ii) the electrical contact point.
8. A method of manufacturing 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;
depositing a converter layer comprising carbon doped with metal ferrocyanide on the electrical contact layer; and
an ion selective membrane is deposited on the converter layer to form an ion selective electrode.
9. The method of claim 8, wherein depositing the converter 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 a converter layer; and
(iii) heating the polymer solution to remove cyclohexanone, thereby forming an ion selective membrane.
11. The method of any one of claims 8 to 10, further comprising conditioning the ion-selective electrode sensor for calibration-free measurement of cation concentration 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 cations prior to use; or
(iib) immersing the ion-selective electrode sensor in a solution comprising cations prior to use, wherein the cations comprise potassium ions.
12. The method of claim 11, wherein storing the ion-selective electrode comprises exposing the ion-selective electrode to anhydrous nitrogen or aqueous nitrogen.
13. The method of claim 11 or 12, wherein impregnating the ion-selective electrode sensor comprises:
soaking the ion-selective electrode in a solution without oxygen for 6 to 12 hours; or
The ion-selective electrode is soaked in the solution in the presence of oxygen for 6 to 12 hours.
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| JPS52142584A (en) * | 1976-05-19 | 1977-11-28 | Eastman Kodak Co | Ion selective electrode |
| CA1093641A (en) * | 1976-05-19 | 1981-01-13 | Charles J. Battaglia | Ion-selective electrode |
| JPS60243555A (en) * | 1984-05-18 | 1985-12-03 | Fuji Photo Film Co Ltd | Ion-selecting electrode and manufacture thereof |
| EP0231476A1 (en) * | 1985-12-23 | 1987-08-12 | Siddiqi, Iqbal W., Dr. | Selectively ion-permeable electrodes for analyzing selected ions in aqueous solution |
| US5286365A (en) * | 1992-01-15 | 1994-02-15 | Beckman Instruments, Inc. | Graphite-based solid state polymeric membrane ion-selective electrodes |
| CN101915736A (en) * | 2005-10-28 | 2010-12-15 | 松下电器产业株式会社 | Measuring instrument, measuring device and measuring method |
| US20180246055A1 (en) * | 2015-03-16 | 2018-08-30 | Crc Care Pty Ltd | Method of recalibrating a device for assessing concentration of at least one analyte ion in a liquid |
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|---|---|---|---|---|
| EP0230572B1 (en) * | 1985-12-23 | 1990-07-04 | F. Hoffmann-La Roche Ag | A method of manufacturing ion-selective electrodes for analyzing selected ions in solution |
| AT409798B (en) * | 1998-11-19 | 2002-11-25 | Hoffmann La Roche | ELECTRODE SYSTEM |
| WO2018052937A1 (en) * | 2016-09-13 | 2018-03-22 | Erno Lindner | Superhydrophobic solid contact ion-selective electrodes |
-
2019
- 2019-06-14 WO PCT/EP2019/065664 patent/WO2020249225A1/en active Application Filing
- 2019-06-14 CN CN201980097423.0A patent/CN113924481A/en active Pending
- 2019-06-14 KR KR1020217040596A patent/KR20220020271A/en not_active Withdrawn
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Patent Citations (8)
| Publication number | Priority date | Publication date | Assignee | Title |
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| JPS52142584A (en) * | 1976-05-19 | 1977-11-28 | Eastman Kodak Co | Ion selective electrode |
| CA1093641A (en) * | 1976-05-19 | 1981-01-13 | Charles J. Battaglia | Ion-selective electrode |
| GB1584788A (en) * | 1976-05-19 | 1981-02-18 | Eastman Kodak Co | Ion-selective electrode |
| JPS60243555A (en) * | 1984-05-18 | 1985-12-03 | Fuji Photo Film Co Ltd | Ion-selecting electrode and manufacture thereof |
| EP0231476A1 (en) * | 1985-12-23 | 1987-08-12 | Siddiqi, Iqbal W., Dr. | Selectively ion-permeable electrodes for analyzing selected ions in aqueous solution |
| US5286365A (en) * | 1992-01-15 | 1994-02-15 | Beckman Instruments, Inc. | Graphite-based solid state polymeric membrane ion-selective electrodes |
| CN101915736A (en) * | 2005-10-28 | 2010-12-15 | 松下电器产业株式会社 | Measuring instrument, measuring device and measuring method |
| US20180246055A1 (en) * | 2015-03-16 | 2018-08-30 | Crc Care Pty Ltd | Method of recalibrating a device for assessing concentration of at least one analyte ion in a liquid |
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| KR20220020271A (en) | 2022-02-18 |
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