Printing-based fabrication of ion sensors using photocurable inks

Abstract

Methods of inkjet printing and / or direct ink writing are provided as are photocurable inks for use in the methods. The methods employ the photocurable inks to form, e.g., ion selective and reference membranes and / or devices such as those used in ion detection.

Description

[0001] PRINTING-BASED FABRICATION OF ION SENSORS USING PHOTOCURABLE INKS CROSS-REFERENCE TO RELATED APPLICATIONS This application claims benefit of United States provisional patent application 63 / 763,984, filed February 27, 2025.

[0002] STATEMENT OF FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0003] This invention was made with government support under grant number R21EB034113 awarded by the National Institutes of Health. The United States government has certain rights in the invention.

[0004] BACKGROUND OF THE INVENTION

[0005] Technical Field

[0006] The invention generally relates to improved methods of inkjet printing and direct ink writing using photocurable ink compositions. In particular, the invention provides photocurable ink compositions and improved methods for their use in inkjet printing and / or direct ink writing, e.g. to form ion selective and reference membranes such as those used in ion detection.

[0007] Description of Related Art

[0008] Ion- selective membranes are water- immiscible membranes that are widely used in electrochemical sensing technologies including ion-selective electrodes and ion-sensitive field-effect transistors. Photocurable poly acrylate and polymethacrylate membranes have previously been investigated as sensing membranes for indicator electrodes in such ion sensors, and photocurable materials have also been reported for reference electrode membranes. For example, see the websites at www.mdpi.com / 2227-9040 / 3 / 2 / 190; doi.org / 10.1002 / elsa.202100145; www.mdpi.com / 1424-8220 / 9 / 9 / 7097; and doi.org / 10.1002 / elan.200704065. However, fabrication of these photocurable membranes has traditionally relied on methods such as manual casting, spin coating, photolithographic patterning, or bulk curing approaches, rather than inkjet printing or direct ink writing offormulated inks.

[0009] Inkjet printing and direct ink writing provide unique advantages over conventional membrane fabrication techniques including manual casting, spin coating, screen printing, and stereolithography (SLA). For example, inkjet printing and direct ink writing are highly material-efficient processes in which minimal ink is wasted, enabling economical use of expensive ionophores, ionic liquids, and specialty chemicals. These printing techniques allow for high spatial resolution and precise control of membrane geometry, with printed features readily reaching dimensions on the order of tens of micrometers, which is particularly advantageous for sensors designed for small- volume biological samples or integrated microfluidic platforms. Furthermore, arbitrary membrane geometries and complex patterns may be digitally designed and directly fabricated, including but not limited to calibration bridges and multiplexed sensor arrays. The printing substrate does not have to be planar and may include curved, flexible, or three-dimensional surfaces. In addition, direct ink writing, as an additive manufacturing technique, enables fabrication of multilayer and three-dimensional membrane architectures that are not readily achievable using traditional methods. Collectively, these capabilities enable new sensor configurations, miniaturized platforms, and multifunctional ion sensing devices that are difficult or impossible to realize using prior fabrication approaches.

[0010] Previously reported inkjet-printed ion sensor membranes have largely relied on polymer solutions dissolved in volatile organic solvents rather than photocurable ink systems (e.g., https: / / doi.org / 10.1021 / acs.analchem.7b03177). Such solvent-based inks require extended drying times following deposition, which slows fabrication and often leads to ink spreading, ink coalescence, and loss of spatial resolution before solvent evaporation is complete. To mitigate nozzle clogging, solvent-based inks are typically highly diluted, necessitating multiple printing cycles to achieve membranes of sufficient thickness and mechanical integrity, further reducing throughput and reproducibility.

[0011] Additive manufacturing and three-dimensional printing technologies have also been applied to fabrication of ion-selective membranes, particularly through stereolithographybased processes (Biosens. Bioelectron. 2025, 117866; Anal. Chem. 2021, 93, 15826).

[0012] However, SLA relies on fundamentally different resin formulations, printing mechanics, and curing geometries than direct ink writing. SLA generally requires large resin volumes in a vat, results in low material utilization efficiency, and is poorly suited for sequentialdeposition of multiple chemically distinct layers required to construct complete electrochemical sensor assemblies. Consequently, SLA-fabricated membranes are often produced as standalone components that must be subsequently assembled with conductive contacts or solid-state transducers fabricated using separate processes.

[0013] In contrast, direct ink writing enables sequential deposition of multiple functional materials using separate cartridges on a single printing platform, allowing streamlined fabrication of complete sensor architectures in a single integrated process. Moreover, rheological requirements for direct ink writing, particularly shear-thinning behavior and yield stress, are fundamentally different from the low-viscosity photopolymer resins employed in SLA.

[0014] It would be of benefit to have available improved compositions and methods of inkjet printing and direct ink writing that reduce or eliminate prior art disadvantages.

[0015] SUMMARY OF THE INVENTION

[0016] Disclosed herein are methods of inkjet printing and direct ink writing using a variety of photocurable ink compositions Membranes printed or written using these methods are utilized, for example, for fabrication of ion-selective electrodes, reference electrodes, calibration bridge, and related electrochemical sensing devices. Novel ink compositions are also provided.

[0017] Other features and advantages of the present invention will be set forth in the description of invention that follows, and in part will be apparent from the description or may be learned by practice of the invention. The invention will be realized and attained by the compositions and methods particularly pointed out in the written description and claims hereof.

[0018] It is an object of this invention to provide a method of making a membrane, comprising: depositing an ink on a substrate in a selected pattern to form a membrane, wherein depositing is performed by inkjet printing and / or direct ink writing, wherein the ink comprises: one or more photocurable, polymerizable monomers in an amount ranging from 5-99 wt%, preferably from 30-95%; one or more polymers in an amount ranging from 0-95 wt%, preferably from 2-70%; one or more photoinitiators in an amount ranging from 0.01 wt% to 10 wt%, preferably from 0.5-5%; one or more multifunctional crosslinkers in an amount ranging from 0.1 wt% to 30 wt%, preferably from about 0.5 wt% to about 5 wt%;i) one or more ionophores and / or one or more ion exchangers or ii) one or more ionic liquids; and, optionally, one or more components selected from the group consisting of: stabilizers, inhibitors, surfactants, pigments, fillers, solvents, viscosity modifiers, rheology modifiers, plasticizers, hydrophobic salts, reactive diluent monomers and / or polar vinyl monomers; and curing the ink to form the membrane. In some aspects, the one or more photocurable polymerizable monomers comprise acrylate monomers and / or methacrylate monomers. In other aspects, the one or more photoinitiators are phosphine oxide photoinitiators. In further aspects, the one or more photoinitiators are activated at light wavelengths ranging from 250 nm to 450 nm, preferably light wavelengths ranging from 320 nm to 420 nm. In yet additional aspects, the one or more multifunctional crosslinkers are diacrylates. In other aspects, the one or more polymers are polyacrylates and / or polymethacrylates .

[0019] In some aspects, the ink is formulated to produce an ion- selective membrane and the one or more ionophores and / or one or more ion exchangers are each present at a concentration ranging from 0.1 wt% to 10 wt%. In yet additional aspects, the the ink is formulated to produce a reference membrane and one or more ionic liquids is present at a concentration ranging from 0.1 wt% to 30 wt% and is an imidazolium-based, pyrrolidinium-based, ammonium-based, or phosphonium-based ionic liquid. In yet other aspects, the ink: i) is formulated for inkjet printing and the polyacrylate and / or polymethacrylate are present in a concentration ranging from 0.1 wt% to 10 wt%, or ii) is formulated for direct ink writing and the polyacrylate and / or polymethacrylate are present in a concentration ranging from 5 wt% to 95 wt%.

[0020] In some aspects, the ink is formulated for direct ink writing and further comprises one or more hydrophobic particulate rheology modifiers having an average primary diameter ranging from 2 nm to 500 nm, preferably from 5 nm to 200 nm, and which are present at a loading ranging from 0.1 wt% to 50 wt%, preferably from 1 wt% to 20 wt%. In additional aspects, the one or more hydrophobic particulate rheology modifiers is selected from the group consisting of: surface-modified silica nanoparticles, hydrophobic fumed silica, organosilica nanoparticles, hydrophobic alumina nanoparticles, titania nanoparticles, polymeric microspheres, and hybrid organic-inorganic particles. In further aspects, the plasticizer is present at a concentration ranging from 0 wt% to 70 wt%, preferably from 5 wt% to 50 wt% and is selected from the group consisting of: dioctyl sebacate, bis(2-ethylhexyl) sebacate, o-nitrophenyl octyl ether, and dioctyl phthalate. In yet further aspects,the substrate is planar, curved, flexible, or a three-dimensional surface. In additional aspects, the method comprises multiple steps of depositing to form a multi-layer membrane.

[0021] In other aspects, the method is inkjet printing and the step of curing is performed under reduced oxygen or in an oxygen-free environment. In yet further aspects, the method is direct ink writing and the step of curing is performed under ambient conditions or reduced oxygen environment or in an oxygen-free environment.

[0022] Also provided is a membrane formed using the method of any of the claims as set forth herein.

[0023] Also provided is an ion selective membrane formed using the method set forth herein as claims 1-7 and / or 9-15.

[0024] Also provided is a reference membrane formed using the method set forth herein as claims 1-6 and / or 7-14.

[0025] Also provided is a bridge membrane formed using the methods of any of the claims as set forth herein.

[0026] Also provided is an electrochemical sensing device comprising at least one membrane of claims 16-19 as set forth herein. In some aspects, the electrochemical sensing device comprises an ion-selective electrode and a reference electrode.

[0027] Also provided is an ink for inkjet printing and / or direct ink writing, comprising one or more photocurable, polymerizable monomers in an amount ranging from 5-99 wt%, preferably from 30-95%; one or more polymers in an amount ranging from 0-95 wt%, preferably from 2-70%; one or more photoinitiators in an amount ranging from 0.01 wt% to 10 wt%, preferably from 0.5-5%; one or more multifunctional crosslinkers in an amount ranging from 0.1 wt% to wt%, preferably from about 0.5 wt% to about 5 wt%; and either one or more ionophores and / or one or more ion exchangers or one or more ionic liquids. In some aspects, the ink further comprises one or more components selected from the group consisting of stabilizers, inhibitors, surfactants, pigments, fillers, solvents, viscosity modifiers, rheology modifiers, plasticizers, hydrophobic salts, reactive diluent monomers and / or polar vinyl monomers.

[0028] BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1. A photo of the printed ion-selective electrode described in Example 1. The dark square between the silver layer and the ion-selective membrane is the PEDOT:PSS solidcontact.

[0030] Figure 2. The potentiometric response of the potassium ion-selective electrode described in Example 1. The unit of the concentration is M.

[0031] Figure 3. Potentiometric response of the sodium ion-selective electrode created using the inkjet ink described in Example 2. The unit of the concentration is M.

[0032] Figure 4. A small-size fully inkjet-printed potentiometric sensor comprising a potassium ion-selective electrode and a reference electrode as described in Example 4.

[0033] Figure 5. A photo of the shear-thinning ink and printed potassium ion-selective electrode described in Example 5. Shear-thinning property is tested using a oscillatory rheometer. Figure 6. Potentiometric response of the potassium ion-selective electrode using the ink described in Example 5. The unit of the KC1 concentration is M.

[0034] DETAILED DESCRIPTION

[0035] The present disclosure provides new methods of using a variety of ink compositions for inkjet printing and direct ink writing to create membranes. These methods:

[0036] 1) Use printing technology such as inkjet printing and direct ink writing to deposit an ink. Direct ink writing is a type of 3D printing technology and uses a dispensing tip to deposit inks. Typically, the inkjet printer is based on a piezoelectric print head or a thermal print head. Printing is generally conducted in an oxygen-free environment or a low-oxygen environment to reduce oxygen inhibition of polymerization. For direct ink writing, the printing can be conducted in ambient environment or an oxygen-free environment or a low-oxygen environment.

[0037] 2) Use inks that contain photocurable monomers, photoinitiators, and functional chemicals for sensors. Examples of the photocurable monomers include but are not limited to acrylates, methacrylates, and caprolactams. One exemplary category of photoinitiators is phosphine oxide photoinitiators. The functional chemicals typically include an ionophore and / or an ion exchanger to create an ion- sensing membrane. The functional chemicals typically include a hydrophobic salt such as an ionic liquid to create a reference membrane or a bridge between the sensing membrane and the reference membrane. Such bridges are disclosed, for example, in the publication at pubs.acs.org / doi / 10.1021 / acs.analchem.3c02135) and in pending United States patent application US20240295524A1, the complete contents of which is hereby-1- incorporated by reference in entirety. This bridge can have the same composition as

[0038] the reference membrane or the ion-selective membrane. In some aspects, the ink

[0039] contains crosslinkers (such as diacrylates) to tune the degree of cross-linking and

[0040] thus the properties of the membrane that is formed, e.g. for use in a sensor such as an

[0041] ion sensor. In addition, a plasticizer is an optional ingredient in the inks. If the

[0042] photocured membranes have a glass transition temperature below the temperature in

[0043] which the sensor will be used, then no plasticizer (they are self-plasticized) or only a

[0044] small amount of plasticizer is needed (e.g., < 50 wt% of the total ink). Otherwise, the

[0045] ink contains 10-95 wt% plasticizer. In other designs, the reference membrane ink

[0046] contains photocurable monomers, water, and functional chemicals. Examples of the photocurable monomers include but are not limited to ethylene glycol diacrylate or

[0047] its oligomers / polymers. Functional chemicals are salts containing chloride ions.

[0048] 3) Uses a light source to cure the ink to form membranes such as an ion-responsive membrane, a reference membrane, or a bridge membrane for self-calibration

[0049] purposes. One exemplary light source is ultraviolet light at 365 or 395 nm. In some

[0050] aspects, the ink is exposed to light during the printing process and / or after the

[0051] printing process. For curing during the printing process, the light source is coupled to

[0052] the printhead.

[0053] DEFINITIONS

[0054] A reference membrane is a specialized, stable, and typically polymer-based film used

[0055] in electrochemical sensors to create a fixed, reproducible electrical potential.

[0056] An ion- selective membrane is a specialized material that shows affinity toward

[0057] specific ions relative to others, commonly used in jon-selective electrodes (ISEs) for Field C electrochemical sensing. These membranes create a measurable potential difference based on

[0058] ion activity, enabling precise, rapid, and cost-effective analysis of ions (e.g., Na+, K+, Cl’ and

[0059] Ca+2in liquid samples.

[0060] An Jon Selective Electrode (ISE) is a membrane-based electrochemical sensor that 1 Field C measures the activity of specific ions (e.g., H+, Na+, F“, etc.) in a solution by converting it into

[0061] an electrical potential. It works via a selective membrane (glass, crystal, or polymer) that

[0062] allows only the target ion to pass, generating a voltage proportional to the ion concentration, typically interpreted using the Nemst equation.INK COMPOSITIONS

[0063] Monomers

[0064] The ink compositions comprise one or more photocurable polymerizable monomers, preferably acrylate and / or methacrylate monomers, together with one or more photoinitiators, multifunctional crosslinkers, optional viscosity modifiers and particulate rheology modifiers, optional plasticizers, and active chemical components for ion recognition or reference potential stabilization. Unless otherwise specified, all weight percentages (wt%) described herein refer to the weight percentage relative to the total weight of the ink composition, including but not limited to polymerizable monomers, crosslinkers, photoinitiators, viscosity modifiers, rheology modifiers, plasticizers, active sensing chemicals, reference electrolyte components, and any optional additives such as stabilizers, inhibitors, surfactants, pigments, colorants, fillers, or solvents. The following description is provided to illustrate representative embodiments of the invention and is not intended to be bound by any specific theory, mechanism of action, physicochemical interaction, or structure-property relationship, whether presently understood or later developed. It will be appreciated that variations, substitutions, and combinations of the disclosed materials and processes may be employed while remaining within the scope of the invention.

[0065] In some aspects, the ink compositions comprise one or more alkyl acrylate monomers having alkyl chain lengths ranging from n = 1 to 25 carbon atoms, such as 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 or 25 carbon atoms. The alkyl groups may be linear, branched, cyclic, bicyclic, or combinations thereof. Mixtures of two or more alkyl acrylates may be employed to tailor viscosity, volatility, solubility of active chemicals, polymer flexibility, hydrophobicity, and electrochemical performance of the cured membrane. When the alkyl chain length is relatively short, such as n = 1-3, the monomers typically exhibit low viscosity and higher vapor pressure, which may result in ink instability, dripping, or spreading after deposition. When the alkyl chain length becomes relatively long, such as n greater than about 12, the monomers become increasingly hydrophobic, which may decrease solubility of certain polar sensing chemicals such as valinomycin, although other less polar ionophores such as calcium ionophore IV could remain highly soluble even in long-chain alkyl acrylates.

[0066] When the alkyl chain length becomes very long, such as n equal to or greater than about 18 for unbranched chains, the monomers may exhibit melting points near or above roomtemperature and become waxy solids, complicating ink preparation unless heating is employed. While elevated temperatures may be used to enhance solubility and maintain fluidity, preferred embodiments utilize alkyl acrylates with chain lengths of approximately n = 4-12 (such as 4, 5, 6, 7, 8, 9, 10, 11 or 12) to provide a balance of low volatility, manageable viscosity, high solubility of sensing chemicals, and convenient room-temperature processing. Cyclic alkyl acrylates such as cyclohexyl acrylate, norbornyl acrylate, or isobornyl acrylate may also be employed to enhance hydrophobicity and mechanical stability.

[0067] In some aspects, the ink compositions comprise alkyl methacrylate monomers having alkyl chain lengths ranging from n = 1 to 25, such as about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 or 25, which may be linear, branched, or cyclic, and may be used alone or in combination with alkyl acrylates. Although methacrylates typically polymerize more slowly than acrylates, their incorporation may improve mechanical robustness of printed membrane structures.

[0068] The monomers are typically present at a concentration ranging from about 5 to about 99 wt%, such as about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 99 wt%.

[0069] Photoinitiators

[0070] The ink compositions comprise one or more photoinitiators capable of generating free radicals upon exposure to ultraviolet and / or visible light. Suitable photoinitiators include, without limitation, Type I cleavage photoinitiators such as benzoin, substituted benzoin ethers including benzoin methyl ether and benzoin ethyl ether, substituted acetophenones such as 2,2-dimethoxy-2-phenylacetophenone, a- hydroxyketones such as 1 -hydroxycyclohexyl phenyl ketone, and phosphine oxide-based photoinitiators including monoacylphosphine oxides and bisacylphosphine oxides such as diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), liquid TPO formulations, and bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (BAPO). Additional photoinitiator systems may include Type II systems such as benzophenone or thioxanthone derivatives used in combination with hydrogen-donor coinitiators including tertiary amines such as ethyl dimethylaminobenzoate or dimethylaminoethyl methacrylate, as well as visible-light-responsive photoinitiators including camphorquinone-based systems, germanium-based initiators, and modified phosphine oxides. The photoinitiators may be activated using light wavelengths ranging from approximately 250 nm to 450 nm, (such as about 250, 300, 350, 400 or 450nm), preferably from about 320 nm toabout 420 nm, and may be present in concentrations ranging from about 0.01 wt% to about 10 wt% (e.g. about 0.01, 0.05, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, or 10 wt% of the total ink composition, with phosphine oxide initiators being preferred in many embodiments due to rapid curing, greater curing depth, and reduced oxygen inhibition.

[0071] Crosslinkers

[0072] The ink compositions preferably comprise one or more multifunctional crosslinkers capable of forming three-dimensional polymer networks upon photopolymerization, including without limitation aliphatic diacrylates and dimethacrylates such as 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, ethylene glycol diacrylate, polyethylene glycol diacrylate, neopentyl glycol diacrylate, triethylene glycol diacrylate, cycloaliphatic and aromatic diacrylates such as bisphenol A diacrylate and cyclohexane dimethanol diacrylate, and multifunctional acrylates such as trimethylolpropane triacrylate, pentaerythritol triacrylate, and pentaerythritol tetraacrylate. Additional crosslinkers may include urethane acrylates, polyester acrylates, epoxy acrylates, and oligomeric multifunctional acrylates containing two or more polymerizable groups. Crosslinker content may range from about 0.1 wt% to about 30 wt% (such as from about 0.1, 0.5, 1, 5, 10, 15, 20, 25 or 30 wt%), preferably from about 0.5 wt% to about 5 wt% (such as about 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.0 or 5.0 wt%), depending on the desired membrane stiffness, permeability, swelling resistance, and electrochemical performance, with 1,6-hexanediol diacrylate being particularly preferred in many embodiments.

[0073] Polymers

[0074] The ink compositions may further comprise polyacrylates formed from alkyl acrylate monomers with chain lengths of n = 1-25 (such as about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 or 25) and degrees of polymerization ranging from about 3 to about 1,000,000, which may be incorporated to increase viscosity, improve jetting or extrusion behavior, suppress satellite droplet formation, or provide yield stress for direct ink writing.

[0075] For inkjet printing, polyacrylate concentrations may range from about 0.1 wt% to about 10 wt%, (such as about 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 wt%) while for direct ink writing much higher loadings ranging from about 5 wt% to about 95 wt% (such as about 5, 10, 15, 20, 3, 30, 35, 40, 45, 50, 55, 60, 65, 70, 76, 80, 85, 90 or 95 wt% may be employed toachieve high viscosity and shape retention. Similarly, the ink compositions may comprise polymethacrylates formed from alkyl methacrylate monomers with chain lengths of n = 1-25 (such as about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 or 25) and degrees of polymerization ranging from about 3 to about 1,000,000. Some polymer compositions may contain organic solvents such as toluene.

[0076] Rheology modifiers

[0077] For direct ink writing, the ink compositions may further comprise hydrophobic particulate rheology modifiers including surface-modified silica nanoparticles, hydrophobic fumed silica, organosilica nanoparticles, hydrophobic alumina or titania nanoparticles, polymeric microspheres, or hybrid organic-inorganic particles. The particles may have average primary diameters ranging from about 2 nm to about 500 nm such as about 2, 10, 50, 100, 150, 200, 250, 300, 350, 400, 450 or 500 nm) and preferably from about 5 nm to about 200 nm (such as about 5, 10, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 200 nm) and may form reversible thixotropic networks within the ink. Hydrophobic surface functionalizers may include alkylsilanes such as octyltrimethoxysilane, octyltriethoxysilane, dodecyltrimethoxysilane, hexadecyltrimethoxysilane,

[0078] organopoly siloxanes, fluorinated silanes such as perfluoroalkylsilanes, and mixtures thereof. These coatings can be applied onto silica nanoparticles and other nanoparticles. Particle loading may range from about 0.1 wt% to about 50 wt% (such as about 0.1, 0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45 or 50 wt%), preferably from about 1 wt% to about 20 wt% (such as about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 wt%) to increase viscosity, impart shear-thinning behavior and structural stability.

[0079] Plasticizers

[0080] The ink compositions may optionally include plasticizers commonly employed in ion-selective membranes, including but not limited to dioctyl sebacate, bis(2-ethylhexyl) sebacate, o-nitrophenyl octyl ether, dioctyl phthalate, and related hydrophobic esters, which may enhance ion mobility, reduce membrane resistance, and improve mechanical flexibility. Plasticizer content may range from about 1 wt% to about 70 wt%, (such as about 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65 or 70 wt%), preferably from about 5 wt% to about 50 wt% (such as about 5, 10, 15, 20, 25, 30, 35, 40, 45 or 50 wt%), In some embodiments, separate plasticizers are unnecessary where the cured polymer itself exhibits a glass transition temperature below room temperature, forming self-plasticized membranes, such as thosederived from 2-ethylhexyl acrylate or other low-Tg alkyl acrylates.

[0081] Additional monomers

[0082] The photocurable membrane ink compositions may further comprise one or more reactive diluent monomers and / or polar vinyl monomers incorporated to adjust viscosity, reduce oxygen sensitivity, and modify mechanical properties of the cured membrane. Such reactive diluents may include, without limitation, vinyl lactams including N-vinylcaprolactam, N-vinylpyrrolidone, and substituted vinyl lactams; vinyl amides and vinyl imides; hydroxyfunctional acrylates and methacrylates such as hydroxyethyl acrylate and hydroxyethyl methacrylate; alkoxy-functional acrylates; and other low-viscosity polymerizable monomers exhibiting polar functional groups capable of hydrogen bonding or dipole interactions. The reactive diluent monomers may be present individually or in combinations and may be incorporated in amounts ranging from about 1 wt% to about 70 wt%, (such as about 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65 or 70 wt%) and more preferably from about 5 wt% to about 40 wt%, (such as about 5, 10, 15, 20, 25, 30, 35 or 40 wt%), of the total ink composition. Upon photopolymerization, the reactive diluents become chemically integrated into the polymer network and contribute to the structural integrity and flexibility of the cured membrane. It should be noted that these reactive diluent monomers and / or polar vinyl monomers are optional. They are also possible to compromise selectivity of the ion-selective membranes.

[0083] For inkjet printing, the ink viscosity typically ranges from about 1 to about 20 mPa-s, (such as about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 mPa-s) and more preferably from about 2 to about 12 mPa-s, (such as about 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 mPa-s, as measured at the jetting temperature.

[0084] Ion-sensing chemicals and reference chemicals

[0085] In some aspects, the ink is formulated to produce an ion selective membrane. In ion selective membranes, generally one or more ionophores is present at a concentration ranging from 0 wt% to 10 wt%, e.g. from about 0, 1, 2, 3, 4, 56, 7, 8, 9 or 10 wt%. Preferably, the one or more ionophores is present from 0.5 wt% to 3 wt%, e.g. from about 0.5, 1.0, 1.5, 2.0, 2.5 or 3.0 wt%. The ionophore is capable of selectively binding, extracting, complexing, or otherwise interacting with a target ion. Suitable ionophores include, but are not limited to, natural ionophores, synthetic neutral carriers, macrocyclic compounds, crown ethers, cryptands, calixarenes, tripodal ligands, organophosphates, hydrogen-bonding receptors,steroid-based carriers, and derivatives, analogs, substituted variants, functionalized forms, and combinations thereof. Non-limiting examples include proton ionophores such as hydrogen ionophore I, hydrogen ionophore II, hydrogen ionophore III, tridodecylamine, trioctylamine, substituted benzimidazoles, and alkylated imidazoles; potassium ionophores such as valinomycin, nonactin, monactin, dinactin, trinactin, tetranactin, potassium ionophore I, potassium ionophore II, potassium ionophore III, potassium ionophore IV, dibenzo- 18-crown-6 and derivatives thereof, and calix[4]arene-based potassium carriers; sodium ionophores such as sodium ionophore I, sodium ionophore II, sodium ionophore III, sodium ionophore VI, sodium ionophore X, ETH 157, ETH 227, sodium-selective crown ethers, and calixarene-based sodium carriers; calcium ionophores such as calcium ionophore I, calcium ionophore II, calcium ionophore IV, calcium ionophore V, ETH 1001, ETH 129, ETH 5234, A23187 (calcimycin), ionomycin, and bis(dialkylphosphate) calcium carriers; magnesium ionophores such as magnesium ionophore I, magnesium ionophore II, magnesium ionophore III, magnesium ionophore IV, ETH 5214, ETH 5506, phosphorylated crown ethers, and bisphosphate magnesium carriers; lithium ionophores such as lithium ionophore I, lithium ionophore II, lithium-selective crown ethers, and cryptand-based lithium carriers; ammonium ionophores such as ammonium ionophore I, ammonium ionophore II, nonactin derivatives, and macrocyclic lactone carriers; chloride ionophores such as mercuracarborand, tridodecylmethylammonium chloride (TDMAC), Aliquat 336, steroid-based chloride carriers, and calixarene anion receptors; nitrate ionophores such as nitrate ionophore I, nitrate ionophore II, nitrate ionophore VI, quaternary ammonium nitrate exchangers, and urea-based anion carriers; nitrite ionophores such as nitrite ionophore I, nitrite ionophore II, nitrite ionophore VI, organotin carriers, and hydrogen-bonding anion receptors; bicarbonate and carbonate carriers including quaternary ammonium exchangers and steroidal anion transporters; phosphate-binding ionophores including metal-complex phosphate carriers and guanidinium-based receptors; and metal-selective ionophores including copper ionophore I and II, lead ionophore I and IV, cadmium ionophore I, zinc ionophore I and II, mercury ionophore I, and silver ionophore I, and any derivatives, structural analogs, substituted variants, functionalized forms, mixtures, or combinations thereof.

[0086] In addition, in inks formulated to produce an ion selective membrane one or more ion exchangers is generally present. The concentration of the one or more ion exchangers typically ranges from 0-10 wt%, such as about 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 wt%. Preferably, the rangeis from about from 0.5 wt% to 3 wt% such as about 0.5, 1.0, 1.5, 2.0, 2.5 or 3.0 wt% . The one or more ion exchangers is, for example, a tetraphenylborate derivative or a quaternary ammonium salt, and combinations thereof. Examples of tetraphenylborate derivatives include but are not limited to: sodium tetraphenylborate (NaTPB), potassium tetrakis(4-chlorophenyl)borate (KTC1PB or KTCB), sodium tetrakis(4-fluorophenyl)borate, potassium or sodium tetrakis[3,5-bis(trifhioromethyl)phenyl]borate (KTFPB or NaTFPB), potassium tetrakis[(4-allyloxy)phenyl]borate (KTAPB), tetrakis(pentafluorophenyl)borate (TinPFPB), and the like. These cation exchangers are usually used in cation ion-selective electrodes. Examples of quaternary ammonium salts include but are not limited to: tridodecylmethylammonium chloride, tetradodecylammonium chloride, trioctylmethylammonium chloride, trioctylmethylammonium bromide, methylated melamine grafted poly vinyl benzyl chloride (mm-qPVBz / Cl-), tetrahexylammonium chloride (THAC1), tetraoctylammonium chloride (TOAC1), and the like. These anion exchangers are usually used in anion ion-selective electrodes.

[0087] In some aspects, the ink is formulated to produce a reference membrane or a selfcalibration membrane. In these membranes, generally one or more hydrophobic salts is present at a concentration ranging from 0.1 wt% to 20 wt%, e.g. from about 0.1, 1, 2, 3, 4, 5 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 wt%. Preferably, the one or more hydrophobic salts is present from 1 wt% to 5 wt%, e.g. from about 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0 wt%. Examples of hydrophobic salts include, but are not limited to, tetrakis(4-chlorophenyl)borate tetradodecylammonium salt (ETH 500), tetrakis(3,5-bis(trifluoromethyl)phenyl)borate salts, tetrakis(pentafluorophenyl)borate salts, tetraphenylborate salts, and substituted arylborate salts paired with long-chain ammonium, phosphonium, pyrrolidinium, imidazolium, or sulfonium cations, including tridodecylmethylammonium, tetradodecylammonium, trioctylmethylammonium, and trihexyl(tetradecyl)phosphonium counterions. Additional suitable hydrophobic salts include bis(trifhioromethanesulfonyl)imide (TFSI ) salts, hexafluorophosphate (PFe") salts, tetrafluoroborate (BE1) salts, perchlorate salts, long-chain alkyl sulfates, alkyl sulfonates, fluorinated borates, and combinations or derivatives thereof. Another group of hydrophobic salts are ionic liquids, which include but are not limited to, imidazolium-based ionic liquids such as l-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PFe]), l-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), l-ethyl-3-methylimidazoliumbi s ( trifl noromethanes nlfo nyl )i m ide ([EMIM] [TFSI]), l-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, and l-methyl-3-octyl-lH-imidazol-3-ium bis((trifluoromethyl)sulfonyl)amide; pyrrolidinium-based ionic liquids such as N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([BMPyrr][TFSI]) and N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide; ammonium-based ionic liquids such as tetraethylammonium bis(trifluoromethanesulfonyl)imide and trioctylmethylammonium bis(trifluoromethanesulfonyl)imide; phosphonium-based ionic liquids such as trihexyl(tetradecyl)phosphonium chloride ([PeeeuHCl]), trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)imide ([PeeeuHTFSI]), and trihexyl(tetradecyl)phosphonium dicyanamide; and sulfonium-based ionic liquids, including long-alkyl-substituted hydrophobic derivatives thereof. These hydrophobic salts may function as hydrophobic electrolytes, ion reservoirs, junction phases, or stabilizing components in liquid junction reference electrodes, polymer-membrane reference electrodes, solid-contact reference electrodes, quasi-reference electrodes, and microfluidic reference electrodes. The reference membrane can be extended to the ion-selective electrodes to serve as a self-calibration membrane.

[0088] Notably, this invention is not restricted to specific structures of ionophores, ion exchangers, or hydrophobic salts. Ionophores represent hydrophobic chemicals that can be included in the ion- selective membrane to impart selectivity. Any chemicals that bind to the analyte ions can be considered as ionophores. Ion exchangers are salts that comprise at least one hydrophobic ion. Hydrophobic salts are salts that can be dissolved in the ion-selective or reference membranes. They are more hydrophobic than simple water-soluble inorganic salts such as sodium chloride.

[0089] METHODS

[0090] The inks disclosed herein are used in inkjet printing and direct ink writing. Overall, the methods comprise depositing at least one ink on a substrate and then curing the ink while on the substrate to form a membrane. In some aspects, multiple membrane layers are formed, one over (on top of) the other with curing or each layer in between depositions to build a cured multilayer structure. In some aspects, a 3-dimensional structure is formed by the deposition.

[0091] Inkjet printing is a non-contact digital printing technology that creates patterns by depositing (e.g. jetting / spraying / propelling) precise, microscopic droplets (e.g., picoliter andnanoliter sizes) of liquid ink onto various substrates, including paper, plastic, metal, textiles, and the like. For inkjet printing, a print head moves across the material, ejecting ink through tiny nozzles. There are two main types: thermal inkjet, which uses heat to create a vapor bubble that forces ink out; and piezoelectric inkjet, which uses electrical charges to change the shape of a material, propelling the ink. For inkjet printing, curing (generally photocuring) of the acrylate or methacrylate-based inks is typically done in a low or no oxygen environment.

[0092] Direct ink writing (DIW) is an extrusion-based additive manufacturing technique that uses a nozzle to deposit viscoelastic inks (such as polymers, ceramics, metals, or hydrogels) layer-by-layer to create three-dimensional (3D) structures. Known as "robocasting," this method is performed at room temperature, requiring inks to possess shear-thinning properties. According to this method, a syringe or nozzle applies pressure to extrude viscous material, which solidifies upon deposition through drying, curing, or cooling. DIW is capable of printing with high-viscosity materials, including acrylate or methacrylate-based inks, biomedical gels, conductive inks, cement, and silicone. DIW is primarily used for micro-scale, high-resolution, complex, or customized, 3D printed structures, particularly in electronics (embedded circuits), sensors, and tissue engineering. It allows for 100% solid, high-density parts compared to other, more porous methods. The technique is often used in low-volume prototyping and research where material customization is critical. For direct ink writing, curing (generally photocuring) may be done in a low or no oxygen environment but may also be done under ambient conditions.

[0093] ION SENSING DEVISES

[0094] In some aspects, the structure that is formed by the methods described herein is an ion sensing device. In this aspect, membranes are printed or formed on any planar or non-planar substrate. Examples of planar substrates include but are not limited to a plastic or paper sheet. Examples of the non-planar substrates include but are not limited to tubes and 3D objects. The substrate itself may be electrically conductive (e.g., conductive tape or a substrate with laser-engraved graphene). Otherwise, the substrate is first coated with an electrical contact layer. This electrical contact layer comprises or consists of metal and / or carbon materials. This coating process is preferably conducted by the same printing technology as that for the photocurable membranes, but it can also be conducted by a different fabrication protocol such as screen printing. An optional solid contact layer may be further coated onto the electrical contact layer before the ion-selective membrane and / orreference membrane is printed. Lastly, a dielectric layer may be created for insulation.

[0095] The ink may be dispensed using piezoelectric or thermal drop-on-demand jetting mechanism. Photocuring may be performed immediately following droplet deposition using light sources integrated with the printhead, or alternatively after printing using a separate irradiation source. In certain embodiments, (e.g. for inkjet printing) curing is carried out under reduced-oxygen or oxygen-free environments to enhance radical polymerization efficiency and minimize oxygen inhibition effects. Such environments may be achieved using glove boxes, enclosed printing chambers, or purging with inert gases such as nitrogen or argon. Due to the extremely small volume and high surface-area-to-volume ratio of jetted droplets, oxygen inhibition can be more pronounced and thus reduced-oxygen conditions are preferred to ensure rapid and complete curing of the printed membrane features. For direct ink writing, the ink viscosity is substantially higher and typically ranges from about 102to about 107mPa- s at the shear rates experienced during extrusion through the nozzle, with shear-thinning behavior being particularly advantageous. The ink may be extruded through a nozzle to form continuous filaments or patterned structures, with photocuring performed simultaneously with extrusion or sequentially following deposition to enable multilayer constructs, free-standing features, or three-dimensional membrane architectures. Curing may be conducted under ambient atmospheric conditions or within controlled environments. In embodiments employing photoinitiators with reduced sensitivity to oxygen inhibition, sufficient photocuring may occur even in the presence of oxygen, enabling printing and curing to proceed under open-air conditions.

[0096] The printed photocurable membranes may be deposited onto one or more electrically conductive layers including, without limitation, carbon-based conductors such as graphite, carbon ink, graphene, or carbon nanotube networks; noble metals such as gold, platinum, and silver; conductive metal oxides such as indium tin oxide or fluorine-doped tin oxide; conductive polymers; and other suitable conductive materials known in the art. The sensor architecture may optionally include one or more solid-contact interlayers positioned between the conductive layer and the photocurable membrane, such interlayers comprising conducting polymers, carbon nanotubes, graphene and related two-dimensional materials, metal or metal oxide nanoparticles, porous carbon structures, nanocomposites, or other high- surface-area electronic transduction materials configured to improve charge transfer efficiency, reduce potential drift, enhance long-term stability, and facilitate signal transduction.In certain embodiments, one or more dielectric layers may be incorporated to define exposed sensing or reference areas and electrically isolate adjacent sensor elements. The dielectric layer may itself be deposited by inkjet printing or direct ink writing and subsequently photocured in a manner similar to the membrane layers described herein. The ink formulation for the dielectric layer may be based on similar photocurable resin systems as disclosed in this invention but may omit active chemicals such as ionophores, ion exchangers, ionic liquids, or reference salts used for ion recognition or potential stabilization. In preferred embodiments, the dielectric layer is formulated to yield a more rigid, less permeable polymer network to suppress penetration of water, ions, and chemical species into underlying conductive features. Increased rigidity and chemical resistance may be achieved through higher crosslink density, incorporation of multifunctional acrylates, selection of higher glass-transition monomers, elimination of plasticizers, and / or inclusion of reactive diluent monomers and polar vinyl monomers capable of forming tightly crosslinked polymer matrices upon photopolymerization.

[0097] The printed ion-selective membranes and reference membranes may be employed together or independently and may be integrated with conventional electrode structures, printed electronic circuits, flexible or stretchable substrates, microfabricated platforms, microfluidic devices, wearable systems, implantable sensors, or hybrid sensor assemblies to form complete electrochemical sensing systems. The photocurable membrane technology described herein is not limited to any particular sensor geometry, electrode arrangement, substrate material, or device architecture, and is likewise not restricted to specific electrochemical transduction mechanisms, which may include, without limitation, potentiometric sensors, ion-selective field-effect transistors (ISFETs), capacitive sensors, amperometric sensors, conductometric sensors, optical-electrochemical hybrid sensors, and combinations thereof.

[0098] It is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

[0099] Where a range of values is provided, it is understood that each intervening value, tothe tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.

[0100] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Representative illustrative methods and materials are herein described; methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention.

[0101] All publications and patents cited in this specification are herein incorporated by reference as if each individual publication or patent were specifically and individually indicated to be incorporated by reference and are incorporated herein by reference to disclose and describe the methods and / or materials in connection with which the publications are cited. The citation of any publication is for its disclosure prior to the filing date and should not be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual dates of public availability and may need to be independently confirmed.

[0102] It is noted that, as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as support for the recitation in the claims of such exclusive terminology as "solely," "only" and the like in connection with the recitation of claim elements, or use of a "negative" limitations, such as "wherein [a particular feature or element] is absent", or "except for [a particular feature or element]", or "wherein [a particular feature or element] is not present (included, etc.)...".

[0103] As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of theother several embodiments without departing from the scope or spirit of the present invention. Any recited method can be carried out in the order of events recited or in any other order which is logically possible.

[0104] The invention is further described by the following non-limiting examples which further illustrate the invention, and are not intended, nor should they be interpreted to, limit the scope of the invention.

[0105] EXAMPLES

[0106] Example 1.

[0107] An exemplary formulation of an ink for inkjet printing of a potassium ion-selective membrane is as follows: The ink contains 87.2% 2-ethylhexyl acrylate (monomer), 5% Poly-2-ethylhexyl acrylate (polymer), 4% phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (photoinitiator), 1% 1,6-hexanediol diacrylate (crosslinker), 2.1% valinomycin (ionophore), and 0.7% potassium tetrakis(4-chlorophenyl)borate (ion-exchanger). This formulation creates self-plasticized membranes for potassium ions and can function without using a plasticizer. Using Dimatix DMP-2850 printer equipped with a Samba cartridge, we printed and cured a silver layer as the electrical contact layer and a PEDOT:PSS layer (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) as the solid contact layer. Then the potassium ion-selective ink was printed and photocured on the same printing platform. A PEL UV CURE SYSTEM is attached to the printer head for UV curing during the jetting process. A photo of the printed ion-selective electrode is shown in Figure 1. The solid contact layer is optional. The electrical contact layer can be other materials such as gold and carbon. The potentiometric response of the potassium ion- selective electrode created using this inkjet ink is shown in Figure 2.

[0108] Example 2.

[0109] An exemplary formulation of an ink for inkjet printing of the sodium ion-selective membrane is as follows: The ink contains 87.2% 2-ethylhexyl acrylate (monomer), 5% Poly-2-ethylhexyl acrylate (polymer), 4% phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (photoinitiator), 1% 1,6-hexanediol diacrylate (crosslinker), 2.1% 4- / <y / -biitylcalix[4]arene-tetraacetic acid tetraethyl ester (sodium ionophore X), and 0.7% sodium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (ion-exchanger). This formulation creates self-plasticized membranes for potassium ions and can function without using a plasticizer.Similarly, a calcium ion- selective electrode can be created using the following formulation. The ink contains 87.2% 2-ethylhexyl acrylate (monomer), 5% Poly-2-ethylhexyl acrylate (polymer), 4% phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (photoinitiator), 1% 1,6-hexanediol diacrylate (crosslinker), 2.1% N,N,N',N'-tetracyclohexyl-3-oxapentanediamide (Calcium ionophore II), and 0.7% sodium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (ion-exchanger).

[0110] The potentiometric response of the sodium ion-selective electrode created using this inkjet ink is shown in Figure 3.

[0111] Example 3.

[0112] An exemplary formulation of an ink for inkjet printing of a reference membrane is as follows: The ink contains 87.2% 2-ethylhexyl acrylate (monomer), 5% Poly-2-ethylhexyl acrylate (polymer), 4% phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (photoinitiator), 1% 1,6-hexanediol diacrylate (crosslinker), and 1% l-methyl-3-octyl-lH-imidazol-3-ium bis((trifluoromethyl)sulfonyl)amide (ionic liquid), based on total ink formulation.

[0113] Example 4.

[0114] An exemplary small-size fully inkjet-printed potentiometric sensor comprising a potassium ion-selective electrode using a membrane made using the ink formulation described in example 1 and a reference electrode using a membrane made using the ink formulation described in example 3 is shown in Figure 4. The electrical contact layer is silver and a dedicated solid contact layer was not used in this example.

[0115] The indicator electrode comprises an electrical contact layer (silver in this example) and the photocured polyacrylate membrane with an ionophore and an ion exchanger. The reference electrode comprises an electrical contact layer (silver in this example) and the photocured polyacrylate membrane with an ionic liquid. The dielectric layer is printed from a commercial SunTronic® UV curing jettable insulator for low-K dielectric applications. Example 5.

[0116] An exemplary formulation of an ink for direct ink writing of potassium ion-selective membrane is as follows: The ink contains 40% 2-ethylhexyl acrylate (monomer), 53% Poly-2-ethylhexyl acrylate (Polymer), 2.2% phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (photoinitiator), 2% 1,6-hexanediol diacrylate (crosslinker), 2.1% valinomycin, 0.7% potassium tetrakis(4-chlorophenyl)borate and 8% amorphous fumed silica particles (surface treatment with dimethyldichlorosilane, ~325 mesh powder) based on total ink formulation.Formulation of the ink for direct writing of the reference membrane is as follows. The ink contains 40% 2-ethylhexyl acrylate (monomer), 53% Poly-2-ethylhexyl acrylate (Polymer), 2.2% phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (photoinitiator), 2% 1,6-hexanediol diacrylate (crosslinker), 1% l-methyl-3-octyl-lH-imidazol-3-ium bis((trifluoromethyl)sulfonyl)amide (ionic liquid) and 8% amorphous fumed silica particles (surface treatment with dimethyldichlorosilane, ~325 mesh powder) based on total ink formulation.

[0117] Example 6. An exemplary formulation of a shear thinning ink for direct ink writing of potassium ion-selective membrane was developed. The dielectric layer is the commercial SunTronic® UV curing jettable insulator ink mixed with amorphous fumed silica particles (surface treatment with dimethyldichlorosilane, ~325 mesh powder, 30% of the total ink weight). A Physik gantry-based printing station with a Nordson EFD precision fluid dispenser and a UV light source is used to prepare the electrode. The electrical contact layer is silver in this example, but it can be other conductive materials like carbon-based materials.

[0118] Figure 5 shows the shear thinning ink for direct ink writing of potassium ion-selective membrane described in this example, the shear rate and a photo of the printed ion-selective electrode. Figure 6 shows the potentiometric response of potassium ion-selective electrodes created using this ink.

[0119] While the invention has been described in terms of its several exemplary embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims. Accordingly, the present invention should not be limited to the embodiments as described above but should further include all modifications and equivalents thereof within the spirit and scope of the description provided herein.

Claims

CLAIMSWe claim:

1. A method of making a membrane, comprisingdepositing an ink on a substrate in a selected pattern to form a membrane, wherein depositing is performed by inkjet printing and / or direct ink writing, wherein the ink comprises:one or more photocurable, polymerizable monomers in an amount ranging from 5-99 wt%, preferably from 30-95%;one or more polymers in an amount ranging from 0-95 wt%, preferably from 2-70%; one or more photoinitiators in an amount ranging from 0.01 wt% to 10 wt%, preferably from 0.5-5%;one or more multifunctional crosslinkers in an amount ranging from 0.1 wt% to 30 wt%, preferably from about 0.5 wt% to about 5 wt%;i) one or more ionophores and / or one or more ion exchangers or ii) one or more hydrophobic salts;and, optionally, one or more components selected from the group consisting of: stabilizers, inhibitors, surfactants, pigments, fillers, solvents, viscosity modifiers, rheology modifiers, plasticizers, hydrophobic salts, reactive diluent monomers and / or polar vinyl monomers;andcuring the ink to form the membrane.

2. The method of claim 1, wherein the one or more photocurable polymerizable monomers comprise acrylate monomers and / or methacrylate monomers.

3. The method of claims 1-2, wherein the one or more photoinitiators are phosphine oxide photoinitiators.

4. The method of any of claims 1-3, wherein the one or more photoinitiators are activated at light wavelengths ranging from 250 nm to 450 nm, preferably light wavelengths ranging from 320 nm to 420 nm.

5. The method of any of claims 1-4, wherein the one or more multifunctional crosslinkers are diacrylates.

6. The method of any of the preceding claims, wherein the one or more polymers are poly acrylates and / or polymethacrylates.

7. The method of any of the preceding claims, wherein the the ink is formulated to produce an ion-selective membrane and the one or more ionophores and / or one or more ion exchangers are each present at a concentration ranging from 0.1 wt% to 10 wt%.

8. The method of claims 2-6, wherein the the ink is formulated to produce a reference membrane and one or more hydrophobic salts is present at a concentration ranging from 0.1 wt% to 30 wt% and is an imidazolium-based, pyrrolidinium-based, ammonium-based, or phosphonium-based ionic liquid.

9. The method of claims 2-8, wherein the ink:i) is formulated for inkjet printing and the polyacrylate and / or polymethacrylate are present in a concentration ranging from 0.1 wt% to 30 wt%, orii) is formulated for direct ink writing and the polyacrylate and / or polymethacrylate are present in a concentration ranging from 5 wt% to 95 wt%.

10. The method of any of the preceding claims, wherein the ink is formulated for direct ink writing and further comprises one or more hydrophobic particulate rheology modifiers having an average primary diameter ranging from 2 nm to 500 nm, preferably from 5 nm to 200 nm, and which are present at a loading ranging from 0.1 wt% to 50 wt%, preferably from 1 wt% to 20 wt%.

11. The method of claim 10, wherein the one or more hydrophobic particulate rheology modifiers is selected from the group consisting of: surface-modified silica nanoparticles, hydrophobic fumed silica, organosilica nanoparticles, hydrophobic alumina nanoparticles,titania nanoparticles, polymeric microspheres, and hybrid organic-inorganic particles.

12. The method of any of the preceding claims, wherein the plasticizer is present at a concentration ranging from 0 wt% to 70 wt%, preferably from 5 wt% to 50 wt% and is selected from the group consisting of: dioctyl sebacate, bis(2-ethylhexyl) sebacate, o-nitrophenyl octyl ether, and dioctyl phthalate.

13. The method of any of the preceding claims, wherein the substrate is planar, curved, flexible, or a three-dimensional surface14. The method of any of the preceding claims, wherein the method comprises multiple steps of depositing to form a multi-layer membrane.

15. The method of any of claims 1-9 and 12-13, wherein the method is inkjet printing and the step of curing is performed under reduced oxygen or in an oxygen-free environment.

16. The method of any of the preceding claims, wherein the method is direct ink writing and the step of curing is performed under ambient conditions or a reduced oxygen environment or in an oxygen-free environment.

17. A membrane formed using the method of any of the preceding claims.

18. An ion selective membrane formed using the method of claims 1-7 and / or 9-16.

19. A reference membrane formed using the method of claims 1-6 and / or 8-16.

20. A bridge membrane formed using the methods of any of the preceding claims.

21. An electrochemical sensing device comprising at least one membrane of claims 17-20.

22. The electrochemical sensing device of claim 21 which comprises an ion-selective electrode and a reference electrode.

23. An ink for inkjet printing and / or direct ink writing, comprisingone or more photocurable, polymerizable monomers in an amount ranging from 5-99 wt%, preferably from 30-95%;one or more polymers in an amount ranging from 0-95 wt%, preferably from 2- 70%;one or more photoinitiators in an amount ranging from 0.01 wt% to 10 wt%, preferably from 0.5-5%;one or more multifunctional crosslinkers in an amount ranging from 0.1 wt% to 10 wt%, preferably from about 0.5 wt% to about 5 wt%; and eitheri) one or more ionophores and / or one or more ion exchangers or ii) one or more hydrophobic salts.

24. The ink of claim 23, further comprising, one or more components selected from the group consisting of stabilizers, inhibitors, surfactants, pigments, fillers, solvents, viscosity modifiers, rheology modifiers, plasticizers, hydrophobic salts, reactive diluent monomers and / or polar vinyl monomers.

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