Patent Application
20240310317
The present disclosure relates to ion-selective electrodes (ISEs). and in particular to solid-contact ion-selective electrodes.
The use of ISEs is of interest to many clinical, environmental, and industrial applications. However, upon extended exposure to aqueous samples and under thermal and mechanical stress, the strength of adhesion between these membranes and the underlying substrate often weakens gradually. Eventually, this results in the formation of a water layer at the interface to the underlying electron conductor and in the delamination of the membrane from the electrode body, both major limitations to long-term monitoring.
The miniaturization and improvement of the lifetime of ISEs still pose considerable challenges. For example, pH glass electrodes—most commonly used to measure pH—have a high electrical resistance and are mechanically fragile. Also, pH glass electrodes and conventional ionophore-based ISEs with inner filling solutions are difficult to miniaturize. Efforts to overcome these drawbacks often involve ionophore-doped sensing membranes with solid contacts to underlying electron conductors such as conducting polymers, high-surface-area carbons (such as nanographite or mesoporous carbon), and hydrophobic redox buffers. The life span of these sensors can be improved by use of plasticizer-free polymers as the membrane matrix and the covalent attachment of ionophores or ionic sites to this matrix polymer. Another problem that often limits the lifetimes of such sensors is the formation of a water layer at the interface of the sensing membrane to the underlying electron conductor and the delamination of the sensing membranes from the electrode body. Screw caps and other mechanical devices can be used to prevent such delamination, but this makes miniaturization more difficult and for certain applications can increase the risk of air bubble trapping and damage to the sensing membranes by overtightening.
Because physical adhesion of sensing membranes to the underlying substrate is typically weak, several attempts have been made to covalently attach ISE membranes to underlying substrates. Harrison and co-workers used PVC membranes with hydroxyl groups and attached them to silicon oxide surfaces using SiCl4. T. Satchwill, D. J. Harrison, Journal of Electroanalytical Chemistry 1986, 202, 75-81. Reinhoudt and co-workers functionalized the silicon oxide surface of ion-selective field effect transistor (ISFET) gates with a silylating reagent with terminal methacrylate groups, followed by covalent attachment of photopolymerizable sensing membranes. E. J. R. Sudholter, P. D. van der Wal, M. Skowronska-Ptasinska, A. van den Berg, D. N. Reinhoudt, Sensors and Actuators 1989, 17, 189-194. This method was later modified by incorporation of a hydrogel layer between the gate and the sensing membrane. D. N. Reinhoudt, J. F. J. Engbersen, Z. Brzózka, H. H. van den Viekkert, G. W. N. Honig, H. A. J. Hoiterman, U. H. Verkerk, Analytical Chemistry 1994, 66, 3618-3623; K. Kimura, T. Sunagawa, S. Yajima, S. Miyake, M. Yokoyama, Analytical Chemistry 1998, 70, 4309-4313; and E. J. R. Sudholter, P. D. van der Wal, M. Skowronska-Ptasinska, A. van den Berg, P. Bergveld, D. N. Reinhoudt, Analytica Chimica Acta 1990, 230, 59-65.
However, ISFETs of this type never became widely available, presumably because delamination of the sensing membrane could not be eliminated in a fully satisfactory manner. Bobacka and co-workers covalently attached polyacrylate-based sensing membranes to the conducting polymer pol(3,4-ethylenedioxythiophene) upon functionalization of the latter with methacrylate groups. C. Ocaña, N. Abramova, A. Bratov, T. Lindfors, J. Bobacka, Talanta 2018, 186, 279-285.Such attachment restricts the formation of a water layer between the sensing membrane and the conducting polymer, resulting in the stabilization of the potentiometric response. Unfortunately, it does not prevent the delamination of membranes from the underlying electrode bodies.
To prevent these problems without increasing the complexity of design with a mechanical attachment, photo-induced graft polymerization is used to simultaneously attach sensing membranes covalently both to a high surface area carbon as ion-to-electron transducer and to inert polymeric electrode body materials (i.e., polypropylene and poly(ethylene-co-tetrafluoroethylene)). The sensors provide a high reproducibility (standard deviation of E0 of 0.2 mV), long-term stability (potential drift of 7 μV/h over 260 h), and resistance to sterilization in an autoclave (at 121° C. and 2.0 atm for 30 min). For this work, a covalently attached H+ selective ionophore was used to prepare pH sensors with advantages over conventional pH glass electrodes, but use of other ionophores makes this approach suitable to the fabrication of ISEs for a variety of analytes.
Furthermore, sensor membranes were chemically attached to an inert polymeric sensor platform material. Specifically, polyacrylate-based sensing membranes were covalently attached to surface-modified poly(ethylene terephthalate) and poly(ethylene-co-cyclohexane-1,4-dimethanol terephthalate) by E. L. Anderson, S. A. Chopade, B. Spindler, A. Stein, T. P. Lodge, M. A. Hillmyer, P. Buhlmann. These sensors showed no delamination of membranes from the underlying substrate in both long-term measurements and severe mechanical stress tests, but the surface modification of the substrates required a multistep synthesis and the sensing membranes were not covalently attached to the underlying electron conductor.
This disclosure describes an electrochemical sensor comprising an ion-selective membrane, an electrically non-conducting polymer substrate or electrode body, and an underlying electron conductor, the ion-selective membrane being attached through covalent chemical bonds to the electrically non-conducting polymer substrate or electrode body and, the underlying electron conductor.
This disclosure further describes the sensor wherein the covalent chemical bonds are characterized by photoinitiated surface functionalization and subsequent photoinitiated or thermally initiated graft polymerization.
This disclosure further describes the sensor wherein the covalent chemical bonds between the ion-selective membrane and the inert polymeric electrode body material and the underlying electron conductor are characterized by generation of radicals by a plasma.
This disclosure further describes the sensor wherein the radicals are generated by the plasma treatment comprising pretreatment of the electrically non-conducting polymer substrate or electrode body and the underlying electron conductor with a plasma comprising either argon, helium, oxygen, hydrogen peroxide, hydrogen, chlorine, BCl3, HBr, tetrafluoromethane, fluoroform, CO2, SF6, a fluorocarbon, or a mixture of two thereof.
This disclosure further describes the sensor wherein a subsequent exposure of the electrically non-conducting polymer substrate or electrode body and the underlying electron conductor to oxygen or the ambient atmosphere, created peroxide and hydroperoxide functional groups on surfaces thereof.
This disclosure further describes the sensor wherein the covalent chemical bonds are characterized by a graft-polymerization having been formed by a vapor phase of a monomer having been introduced into the plasma whereby the monomer having formed into a polymer, the polymer being covalently bonded to the electrically non-conducting polymer substrate or electrode body and the underlying electron conductor.
This disclosure further describes the sensor wherein the ion-selective membrane is doped with either (i) an ionophore that is selective for H+ and contains a primary, secondary, or tertiary amine, or a heterocyclic aromatic hydrocarbon being either a pyridine, a quinoline, or a phenanthrene, or (ii) an ionophore with selectivity for a mono- or multivalent ion being either Li+, K+, Na+, Mg2+, Ca2+, Cl−, S042−, carbonate, or phosphate.
This disclosure further describes the sensor wherein an ion exchange capacity in the ion-selective membrane is characterized by having been doped with an ionic site that contains a tetraphenylborate group, a sulfonate group, or a sulfonylimide group.
This disclosure further describes the sensor wherein the ion-selective membrane comprises an alkyl methacrylate homopolymer, an alkyl acrylate homopolymer, or a copolymer of two or more alkyl methacrylates or acrylates, comprising no crosslinker or comprising a crosslinker.
This disclosure further describes the sensor characterized by showing no water layer effect.
This disclosure further describes the sensor wherein the ion-selective membrane comprises a polymeric material, the polymeric material comprising a polycarbonate, polystyrene, polyurethane, polyolefin, silicone, polyamide, polyester, polyether, polyimide, polysulfide, polycarbonate, polyacetal, polymethacrylate or polyacrylate, polyphenylene sulfide, polypropylene, polyethylene, poly(ethylene-co-tetrafluoroethylene), poly(tetrafluoroethylene), poly(vinylchloride), polyvinylidene chloride, polyvinyl acetate, polyacrylonitrile, polyvinyl fluoride, or polyvinylidene fluoride.
This disclosure further describes a reference electrode comprising an ionic liquid doped reference membrane, an inert plastic substrate, and an underlying electron conductor, the ionic liquid doped reference membrane being attached through covalent chemical bonds to both the inert plastic substrate and the underlying electron conductor.
This disclosure further describes a reference electrode wherein the covalent chemical bonds are characterized by photoinitiated graft polymerization.
This disclosure further describes a reference electrode wherein the covalent chemical bonds are characterized by radicals formed by a plasma.
This disclosure further describes a reference electrode characterized by pretreatment of the inert plastic substrate and the underlying electron conductor with an argon, helium or oxygen plasma and subsequent exposure to oxygen or ambient atmosphere, peroxide and hydroperoxide functional groups formed on the surfaces thereof.
This disclosure further describes a reference electrode the ionic liquid doped reference membrane comprising a polymer having been formed by the plasma and wherein the plasma having been comprised of a monomer in vapor phase in the plasma and/or other gases that were converted in reactive fragments to form the covalent chemical bonds.
This disclosure further describes a reference electrode wherein the covalent chemical bonds are characterized by radicals formed by thermal polymerization.
This disclosure further describes a reference electrode wherein the ionic liquid doped reference membrane comprises a polymeric material, the polymeric material comprising a polycarbonate, polystyrene, polyurethane, polyolefin, silicone, polyamide, polyester, polyether, polyimide, polysulfide, polycarbonate, polyacetal, polymethacrylate or polyacrylate, polyphenylene sulfide, polypropylene, polyethylene, poly(ethylene-co-tetrafluoroethylene), poly(tetrafluoroethylene), poly(vinylchloride), polyvinylidene chloride, polyvinyl acetate, polyacrylonitrile, polyvinyl fluoride, or polyvinylidene fluoride.
This disclosure further describes a reference electrode characterized by showing no water layer effect.
This disclosure describes the simultaneous covalent attachment of ISE membranes to both inert polymeric electrode bodies and the underlying carbon conductor. While there is precedence for the covalent attachment of sensing membranes to conducting polymers as electron conductor and inert polymeric substrates, the covalent attachment of the sensing membrane to both the inert body polymer and the ion-to-electron transducer has not been shown before. As an example both nanographite (surface area 250 m2/g) and glassy carbon are used as electron conductors and both polypropylene and poly(ethylene-co-tetrafluoroethylene) are used as electrode body materials. The use of other polymeric bodies are within the scope of this disclosure. Notably, these electrode body materials are not only widely used in industry but also have good mechanical, temperature, and chemical resistance suitable for industrial grade sensor bodies. Further electrode body materials suitable for use as electrode body materials include but not limited to polycarbonate, polystyrene, polyurethane, polyolefin, silicone, polyamide, polyester, polyether, polyimide, polysulfide, polycarbonate, polyacetal, polymethacrylate or polyacrylate, polyphenylene sulfide, poly(tetrafluoroethylene), poly(vinylchloride), polyvinylidene chloride, polyvinyl acetate, polyacrylonitrile, polyvinyl fluoride, or polyvinylidene fluoride.
The surface modification and graft polymerization were both achieved by ultraviolet (UV) irradiation. For the first step (Step 1), a THE solution of the photoinitiator 2,2-dimethoxy-2-phenylacetophenone was deposited on the carbon-based conductor surfaces and adjacent inert polymer of the electrode body. Upon UV irradiation, a photo-reduction reaction (hydrogen abstraction) between the photo-initiator and the surface C—H bonds of polypropylene or poly(ethylene-co-tetrafluoroethylene results in surface localized radicals that subsequently combine with benzoyl radicals formed from the photoinitiator, leading to the formation of benzoyl groups on these surfaces. In the case of the carbon-based conductor, direct reaction of the benzoyl radical with sp2 hybridized carbon is likely the dominant reaction. In the second step (Step 2), a solution comprising besides photoinitiator also decyl methacrylate and crosslinker is deposited on the functionalized surfaces. Subsequent UV irradiation cleaves the bonds between the surface-attached benzoyl groups homolytically and initiates graft polymerization of decyl methacrylate, resulting in polymer chains that are chemically bonded onto the substrates. The success of this photo-induced graft polymerization is supported by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, contact angles, weight measurements, and peeling tests, as discussed in the following.
Photo-induced surface modification, shown below for polypropylene. 1st step: attachment of photoinitiator to surface. 2nd step: monomer grafting
ATR-FTTR spectra show for both polypropylene and poly(ethylene-co-tetrafluoroethylene) modified by surface photografting a C═O stretch vibration characteristic for the ester group of poly(decyl methacrylate) (PDMA)[40] (see
The efficiency of graft polymerization was calculated as the ratio of the experimental weight of the grafted membranes and the total amount of polymerizable membrane components used for their preparation (i.e., the decyl methacrylate monomer, the crosslinker, and, if applicable, ionophore). As Table 1 shows, graft polymerization efficiencies of as high as 81% can be achieved with little optimization. The remainder of polymerizable components is lost (in the form of monomers or oligomers) due to evaporation during the photopolymerization or when the membranes are washed with solvent (1 mL THE or methanol) following the polymerization.
Importantly, addition of the H+-selective and polymerizable ionophore 2-(diisopropylamino)ethyl methacrylate and ionic sites (tetrakis(pentafluorophenyl)borate in the form of a K+ salt) into the solution of decyl methacrylate and crosslinker did not interfere with the photoinitiated polymerization (see Table 1) and the covalent attachment onto underlying substrates. Suitable ionophores include without limitation either (i) an ionophore that is selective for H+ and contains a primary, secondary, or tertiary amine, or a heterocyclic aromatic hydrocarbon being either a pyridine, a quinoline, or a phenanthrene, or (ii) an ionophore with selectivity for a mono- or multivalent ion being either Li+, K+, Na+, Mg2+, Ca2+, Cl−, S042−, carbonate, or phosphate. Electroneutrality in the ion-selective membrane may be achieved by doping with an ionic site that contains a tetraphenylborate group, a sulfonate group, or a sulfonylimide group.
To confirm that the 2-step process for photografting indeed results in the covalent attachment of PDMA membranes to the underlying substrates and electrical contacts, peeling tests were performed (see
PDMAmembranes were grafted to polypropylene (
The covalent attachment of the sensing membranes to the polymer and carbon conductor having been confirmed, potentiometric devices were prepared and characterized. For this purpose, gold-coated stainless-steel rods were molded into polypropylene bodies, exposing the gold-coated rod at one end of the body at the bottom of a cavity designed to hold the nanographite and the ion-selective membrane. In
To prevent ionophore leaching into samples from limiting sensor lifetime, 2-(diisopropylamino)ethyl methacrylate was included into the photografting solution. Because of its methacrylate group, this ionophore is incorporated into the PDMA polymer backbone in the course of the photoiniated polymerization. The ionic site tetrakis(pentafluorophenyl)borate, which is required to provide membrane permselectivity for cations, was included into the photografting solution in a 1 to 3 molecular ratio to the ionophore. No attempt was made to covalently attach the ionic sites to the PDMA backbone, as it has been shown that ISEs with both the ionophore and the ionic sites covalently attached exhibit larger electrical resistances and excessive response times and are not suitable for potentiometric analysis.
After overnight conditioning in 10 mM pH 7.1 sodium phosphate buffer, the response of these electrodes to pH was tested in 10.0 mM phosphate buffer solutions by pH adjustment with aliquots of 1 M NaOH and HCl. Three identically prepared electrodes showed a slope of −58.5±1.2 mV/decade and a working range from pH 1.4 to pH 9.7 (see K_(H,Na){circumflex over ( )}Pot
, log
K_(H,K){circumflex over ( )}Pot
, and log
K_(H,Li){circumflex over ( )}Pot
of these membranes were −10.4±0.1, −9.8±0.1, and −10.7±0.1, respectively. These values are close to but slightly different to what was previously reported for the same ionophore covalently attached to PDMA membranes but without covalent attachment of the PDMA membrane to the inert polymer substrate and nanographite as solid contact (see Table 3). The reduction in selectivity is most notable for K+, with log
K_(H,K){circumflex over ( )}Pot
A increasing from −10.7 to −9.8, while log
K_(H,Na){circumflex over ( )}Pot
and log
K_(H,Li){circumflex over ( )}Pot
only increased by 0.6 and 0.4. This difference may be related to use of photoinitiator in Step 1 of the membrane fabrication, in which surface-attached benzoyl groups are formed, or the higher photoinitiator concentration used in Step 2 of this work (1.5 wt %) as compared to our earlier work (0.9 wt %). While there appears to be some room for optimization, this shows that the photoinitiated grafting of PDMA gives well-functioning ISEs.
It is well known that formation of a thin water layer between the polymeric membrane and the solid contact of an ISE causes not only drifting potentials and a poor emf repeatability but also ultimately delamination of the membrane. To assess whether such a water layer is formed in the case of photografted PDMA membranes, we used the well-established water layer test. M. Fibbioli, W. E. Morf, M. Badertscher, N. F. De Rooij, E. Pretsch, Electroanalysis 2000, 12, 1286-1292; B. Hambly, M. Guzinski, B. Pendley, E. Lindner, Electroanalysis 2020, 32, 781-791. As shown by
To assess the improvement of long-term durability provided by the covalent attachment of the H+ selective membranes, three tests were performed. First, three electrodes were stored in pH-buffer solutions for 6 months. Then, their response to pH and selectivity were measured, and a water layer test was once again performed. The three membranes maintained theoretically expected Nernstian slopes (−58.4 mV±0.8 mV/decade) with a linear response range from pH 1.4 to 10.0 and no changes in selectivity coefficients when stored for 6 months in 10 mM sodium buffer solution (pH 7.1; see Table 3). Moreover, the water layer test demonstrated that no water layer had formed over this time period (see
As a second test to assess the covalent attachment of the sensing membrane, the electrodes were exposed to high pressure (1500 Torr) and heat (121° C.) in an autoclave. Substantially higher temperatures would damage the polypropylene-based electrode bodies, as commercial isotactic propylene melts in the range of 160 to 166° C., while crosslinked methacrylates do not exhibit melting and only decompose at much higher temperatures.
In separate experiments, electrodes were also exposed to 10 wt. % ethanol solutions for 1 day. The response slopes and selectivities remained unchanged after both the autoclave and the ethanol treatment (see Table 3), and a water layer was not observed either (see
The third durability test was a long-term emf drift test. For this purpose, we used a capillary-based reference electrode (prepared as reported in E. L. Anderson, B. K. Troudt, P. Buhlmann, ACS Sensors 2021, 6, 2211-2217) to minimize the drift contributed by the reference electrode. As
In summary, the simultaneous attachment of ISE membranes to both polymeric electrode bodies and the underlying electron conductor provides mechanically robust sensors that exhibit excellent long-term stabilities. The covalent photografting inhibits formation of water layers at the interface of the electrode body and sensing membrane, preventing membrane delamination, and it counteracts the formation of a water layer at the interface to the carbon solid contact, preventing emf signal drift. It is within the present disclosure that the approach described herein can be adapted to detect other ions by using different ionophores, provided that the ionophore is compatible with the photopolymerization. The approach appears particularly suitable for wearable and implantable sensors that require long term stability with minimal or no recalibration.
Plasma irradiation methods were also applied to graft sensor membranes to both electrode bodies and solid contacts made of a conductive carbon material. The plasma provided 5 times faster efficiency than ultraviolet (UV) treatment. Argon, helium, and oxygen plasmas produces radicals on the polymer surface. The new generated radicals initiated the polymerization of monomers, resulting in graft polymer chains chemically bonded onto the substrates. Other suitable plasmas include without limitation, hydrogen peroxide, hydrogen, chlorine, BCl3, HBr, tetrafluoromethane, fluoroform, CO2, SF6, a fluorocarbon, or mixtures thereof.
Using one method, the components of the sensing membrane were deposited onto polypropylene and poly(ethylene-co-tetrafluoroethylene) platform substrates, and those were then exposed to an argon or helium plasma. The plasma generated radicals in the deposited sensing material and on the substrate surface, which led to graft-polymerization of the sensing membrane to the substrate.
Using another method, the polymeric substrates were pretreated with an argon or helium or oxygen plasma and subsequently exposed to oxygen or the ambient atmosphere, creating peroxide and hydroperoxide functional groups on the surface. The monomeric components of the sensing membranes were then deposited onto these substrates and radical polymerization was induced by UV-irradiation or thermal treatment, graft-polymerizing the sensing membranes onto the substrates. The plasma parameters (power, pressure, and flow rate) were used to control the polymerization efficiency and thickness of the resulting sensor membranes. High power values may damage the ion-selective membrane and polymer electrode body.
The simultaneous attachment of ISE membranes to both polymeric electrode bodies and the underlying electron conductor provides mechanically robust sensors that exhibit excellent long-term stabilities. The covalent photografting or plasma grafting inhibits formation of water layers at the interface of the electrode body and sensing membrane, preventing membrane delamination, and counteracts the formation of a water layer at the interface to the carbon solid contact, preventing emf signal drift. It is evident that this approach can be adapted to detect other ions by using different ionophores, provided that the ionophore is compatible with the photopolymerization. The approach appears particularly suitable for wearable and implantable sensors that require long term stability with minimal or no recalibration.
Reagents and Materials. Magnesium sulfate, sodium chloride, lithium chloride, lithium hydroxide, 2,2-dimethoxy-2-phenylacetophenone, 1,6-hexanediol dimethacrylate, and 2-(diisopropylamino)ethyl methacrylate were purchased from Sigma Aldrich (St. Louis, MO, USA). Decyl methacrylate (97%) was purchased from Pfaltz & Bauer (Waterbury, CT, USA). Potassium chloride, potassium hydroxide, basic alumina, sodium hydroxide, and tetrahydrofuran were purchased from Fisher Chemical (Waltham, MA, USA). Nanographite powder (GS-4827, BET surface area of 250 m2/g, particle size distribution from 0.10 μm to 10 μm) was purchased from Graphitestore (Northbrook, IL, USA). Potassium tetrakis(pentafluorophenyl)borate (KTPFB) was purchased from Alfa Aesar (Tewksbury, MA, USA). Polypropylene sheets and poly(ethylene-co-tetrafluoroethylene) (also known under the brand name Tefzel) films were purchased from McMaster-Carr (Chicago, IL, USA). A glassy carbon plate (SPI-glas 11 grade, 50 mm×50 mm) was purchased from SPI Supplies (West Chester, PA, USA). For delamination tests, the glassy carbon plate was cut into 1×1 cm2 pieces using a rotary cutting wheel tool (Dremel, Mt. Prospect WI, USA). Decyl methacrylate, 1,6-hexanediol dimethacrylate, 2-(dimethylamino)ethyl methacrylate, 2-(diisopropylamino)ethyl methacrylate, and anhydrous inhibitor-free tetrahydrofuran (THF) was each passed through a pipet column filled with basic alumina separately before use. All aqueous solutions were prepared with deionized and charcoal-treated water (18.2 MΩ/cm specific resistance) using a Milli-Q Plus reagent-grade water system (Millipore, Bedford, MA, USA).
Contact Angle Measurements. Contact angles before and after surface modification of polypropylene substrates and poly(ethylene-co-tetrafluoroethylene) film were measured with a contact-angle goniometer (Erma, Tokyo, Japan) using the sessile drop method (R. Good, Journal of Adhesion Science and Technology 1992, 6, 1269-1302) with a drop increasing from 5 to 10, 15, and 20 μL of purified H2O to measure advancing angles, followed by three times removal of 5 μL aliquots from the droplet for the measurement of receding angles. Advancing contact angles for 10, 15, and 20 μL drops were within error identical, while the advancing angle for the 5 μL drop was slightly larger and considered to be affected by a systematic error caused by the small volume of the drop. Receding angles were within error identical for all drop volumes. Averages were calculated from the three advancing angles for 10, 15, and 20 μL drops and the 15, 10, and 5 μL drop receding angles and three different sample surfaces.
DSC: A TA Instrument Q1000 DSC (New Castle, DE) differential scanning calorimeter with a liquid nitrogen cooling system was used for all DSC measurements. The capped sample pans were thermally equilibrated at 100° C. for 1 min. Then samples were scanned to −90° C. and back to 100° C., with a 10° C./min heating and cooling rate. Glass transition temperatures were determined from the midpoints of transition zones.
Other Instrumentation. ATR-FTIR spectroscopy was performed with a UV-1800 spectrometer from Shimadzu Corporation (Canby OR, USA). Electrodes were autoclaved with a 2340M manual sterilizer from Tuttnauer (Hauppauge, NY). A 3UV lamp was purchased from Analytik Jena (Jena, Germany).
Attachment of Surface Initiator to Solid Surface. A total of 30 μL of 20 wt % THE or methanol solution of 2,2-dimethoxy-2-phenylacetophenone was drop cast onto the gold electrodes coated with nanographite and the surrounding polypropylene (in four portions of 10, 10, 5, and 5 μL to avoid flushing aside the nanographite). The electrodes were placed into a well-sealed box covered by a UV-transparent quartz glass plate, and the box was flushed with argon for 10 min. The photoinitiator was then grafted to the polypropylene and nanographite solid contact by UV irradiation (peak output 365 nm, with significant output in the 350 to 390 nm range) over 20 min. The electrodes were dried at room temperature in air for 1 h.
Grafting of Crosslinked PDMA onto the Functionalized Inert Polymer and Solid Contact Carbon. This procedure was analogous to the attachment of surface initiator to the inert polymers or solid contact carbon, except that besides the 2,2-dimethoxy-2-phenylacetophenone also 1.5 wt % crosslinker and 15 or 50 wt % decyl methacrylate was used (for concentrations, see Table 1).
Preparation of pH Sensing Membrane Precursor Solutions. Solutions for the preparation of PDMA membranes with covalently attached ionophore were prepared by mixing 1.5 wt % 2,2-dimethoxy-2-phenyl-acetophenone (photoinitiator), 93 wt % decyl methacrylate, 1.5 wt % 1,6-hexanediol dimethacrylate (crosslinker), 4 wt % potassium tetrakis(pentafluorophenyl)borate (ionic sites), and covalently attachable ionophore 2-(diisopropylamino)ethyl methacrylate (300 mol % with respect to the ionic sites, which corresponds to 18.2 mg per 694.7 mg of the other membrane components). No additional solvent was used.
Grafting of the pH Sensor Membrane onto the Functionalized Inert Polymer and Solid Contact Carbon. This procedure was analogous to the attachment of surface initiator to the inert polymer and solid contact carbon, except that the pH sensing membrane precursor solutions contained besides the 2,2-dimethoxy-2-phenylacetophenone solutions also ionophore, monomer and crosslinker (see preceding paragraph). A total of 30 μL (in 10, 10, 5 and 5 μL portions) of the membrane precursor solution was drop-cast onto the polypropylene area and nanographite previously activated with surface initiator.
Grafting Degree/Efficiency, and Membrane Thickness. The grafting degree, membrane thickness, and grafting efficiency were calculated as follows:
Potentiometry. Potentiometric measurements were carried out in stirred solutions with a EMF 16 high-impedance voltmeter controlled by EMF Suite 1.03 software (Lawson Labs, Malvern, PA, USA) against a double-junction reference electrode with AgCl-saturated 3.00 M KCl reference electrolyte and 1.0 M KCl bridge electrolyte (DX200, Mettler Toledo, Switzerland). A pH glass electrode (InLab 201, Mettler Toledo, Columbus, OH, USA) was used to separately determine the pH of aqueous solutions. To measure pH responses of ISEs, the pH of 10 mM pH 7.1 sodium phosphate buffer solutions was changed by adding aliquots of 1 M NaOH or 1 M HCl. Selectivity coefficients against Na+, K+, and Li+ were measured using the fixed interference method (FIM) (E. Bakker, E. Pretsch, P. Buhlmann, Analytical Chemistry 2000, 72, 1127-1133.) Long-term drifts test were measured relative to a capillary-based electrode (E. L. Anderson, B. K. Troudt, P. Buhlmann, ACS Sensors 2021, 6, 2211-2217) in 1.0 mM sodium phosphate buffer solution or to a Ag—Cl-coated Ag wire in 1.0 mM NaCl solution in a temperature-controlled Faraday cage at 32° C.
Electrode Bodies. A cylindrical polypropylene-based electrode body was prepared by molding a stainless steel rod coated with gold into polypropylene, The metal rod pops out a bit from the bottom of a cavity at the end of the electrode body (see
a Photografting solutions contained photoinitiator, decyl methacrylate, and crosslinker.
b Photografting solutions contained photoinitiator, decyl methacrylate, crosslinker, ionophore, and ionic sites.
Table 2. Shows contact angles of H2O on unmodified polypropylene sheets, unmodified poly(ethylene-co-tetrafluoroethylene) films. as well as PDMA membranes attached onto polypropylene and poly(ethylene-co-tetrafluoroethylene).
Table 3 shows comparison of the initial potentiometric responses to the potentiometric response after 6 months in 10 mM sodium phosphate buffer solution (pH 7.1), as well as after exposure for heat and pressure using an autoclave (at 121° C. and 2.0 atm for 30 min) or to 10 wt % ethanol for one day.
aThe working range is determined here by the intersection of the Nernstian (linear) region and constant (plateau) emf region at the lower or upper detection limits.
| Number | Date | Country | |
|---|---|---|---|
| 63451741 | Mar 2023 | US |
