FIELD OF INVENTION

The present invention relates to a composition used for fabricating self-plasticized sensing membrane to be applied on ion-selective electrode (ISE) and a self-plasticized sensing membrane derived thereof. More specifically, the composition utilizes a dual phase softener to confer the needed plasticizing effect to the fabricated membrane with minimal leaching upon routine usage.

BACKGROUND OF THE INVENTION

Potentiometric ion-selective electrode (ISE) is the most commonly deployed chemical sensors. The application of ISE has evolved to a well-established routine analytical technique in many fields, including clinical and environmental analysis. The essential part of ISE is the ion-selective sensing membrane to recognize target analyte in liquid samples. It is the principle of operation required for measurement of potential difference between working electrode and a reference electrode.

The sensing membrane comprises hydrophobic polymeric matrix, plasticizer, ionophore and lipophilic salt. Poly (vinyl chloride) or PVC has been the most widely used sensing matrix for membrane-based chemical sensors. The main reasons for the wide adoption of PVC, presumably are commercial availability, low cost, ease of preparation and proven long-term results. However, one major drawback with the use of PVC material is its hardness. Plasticizers, In excess of 60 percent of the weight, are required to soften the material and to lower its glass transition temperature in order to make it more suitable as sensing membrane. Plasticizers such as phthalates and ethers are small molecules that gradually leach from the plasticized PVC and the sensing membrane eventually loses its function. Moreover, another one major with the use of PVC is its poor adhesion to electrode surface. PVC is only weakly interacted to solid surface such as silver-silver chloride, conductive polymers such as polypyrrole and other hydrophobic or hydrophilic polymers. For laboratory use that requires minimal only minimal use and allows long time dry storage, the PVC-based membranes do not exhibit short term functional problem. On the other hand prolonged exposure to aqueous media almost certainly results in delamination of the membrane. The sensing membrane peels off from the electrode surface and this causes total functional failure of the chemical sensors.

Acrylic polymers and copolymers have also been frequently employed as sensing membranes. For example, United States patent application no. 5198301 discloses a membrane fabricated from 60 to 98% by weight of resin which is a co-polymer of ethylene and arylic acid, and 2 to 40% by weight of a filler material claiming that the membrane is substantially free of migrating plasticizers or leachable additives. Interestingly acrylates are soft materials with low glass transition temperature such that plasticizers are often not required to make acrylic sensing membranes thus the term self-plasticized sensing membranes. Rais et. al. describes a plasticizer-free sensing membrane for chemical sensor in an International patent with publication no. WO2011040085. specifically, the plasticizer-free sensing membrane is a copolymer membrane of methyl methacrylate and tetrahydrofurfuryl acrylate being photocured under ultraviolet light. Other plasticizer-free sensing membrane can be found in both United States patent publication no. 7201876 and 7226563 which respectively utilizes a uniquely designed methacrylate monomers and ion exchanging molecules.

SUMMARY OF THE INVENTION

The present invention discloses a composition for fabricating self-plasticized sensing membrane. More specifically, the disclosed composition allows formatting of sensing membrane with low glass transition temperature without using plasticizer to solve the problem of plasticizer leached. Another object of the present invention is to offer a sensing membrane free of plasticizer that the sensing membrane is less subjected to leaching problems and delamination of the sensing membrane from the electrode surface. Still another object of the present invention is to provide a sensing membrane which is reliable and with good endurance for routine field use.

At least one of the preceding objects is met, in whole or in part, by the present invention, in which one of the embodiment of the present invention is a composition for fabricating self-plasticized sensing membrane including an organic solvent dissolved homoegeneous mixture comprising 5 to 80% by total weight percentage of poly(vinyl chloride); 5 to 80% by total weight percentage of solid phase polymer or co-polymer selected from the group consisting of methyl methacrylate, tetrahydrofurfuryl acrylate, n-butyl acrylate, hydroxyethyl methacrylate, dodecyl acrylate, tetrafluoroethyl acrylate, glycidyl methacrylate, polyurethane, silicone rubber and any combination derived thereof; 5 to 80% by total weight percentage of liquid phase polymer or copolymer selected from the group consisting of methyl methacrylate, tetrahydrofurfuryl acrylate, n-butyl acrylate, hydroxyethyl methacrylate, dodecyl acrylate, tetrafluoroethyl acrylate, glycidyl methacrylate, poly(n-butyl acrylate) and any combination derived thereof; 1 to 10% by total weight of liphophilic additive; and 1 to 40% by total weight of ionophores.

Similarly, another aspect of the disclosed invention involves a self-plasticized sensing membrane fabricated from an organic solvent dissolved homoegeneous mixture comprising 5 to 80% by total weight percentage of poly(vinyl chloride); 5 to 80% by total weight percentage of solid phase polymer or co-polymer selected from the group consisting of methyl methacrylate, tetrahydrofurfuryl acrylate, n-butyl acrylate, hydroxyethyl methacrylate, dodecyl acrylate, tetrafluoroethyl acrylate, glycidyl methacrylate, polyurethane, silicone rubber and any combination derived thereof; 5 to 80%) by total weight percentage of liquid phase polymer or co-polymer selected from the group consisting of methyl methacrylate, tetrahydrofurfuryl acrylate, n-butyl acrylate, hydroxyethyl methacrylate, dodecyl acrylate, tetrafluoroethyl acrylate, glycidyl methacrylate and any combination derived thereof; 1 to 10% by total weight of liphophilic additive; and 1 to 40% by total weight of ionophores.

In another aspect, the organic solvent in both composition and the membrane is polar solvent. Preferably, it is tetrahydrofuran, diethyl ether, methylene chloride, methoxy ethanol, diethyl carbonate, dimethyl sulfoxide, toluene, acetone, methyl ethyl ketone or any mixture thereof. In another aspect, the lipohilic additive is lipophilic tetraalkyl ammonium halides, lipophilic tetraphenyl borates or any combination thereof, while the ionophore is valinomycin, nonactin, tridodecylamine, tetraoctyl ammonium nitrate, Bisthiourea, bis-crown ether, calcimycin, ionomycin, or monactin. Further, the lipophilic additive and ionophores are equivalent in molar ratio within the composition and membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a diagram illustrating basic membrane matrix without doping by the ionophores; Figure 2 is a diagram illustrating basic membrane matrix doped with the ionophores and lipophilic salts;

Figure 3 is a flow chart showing one of the process flow to prepare the disclosed sensing membrane;

Figure 4 is a flow chart showing fabrication of a chemical sensor using the disclosed composition; and

Figure 5 is a graph illustrating the tested result of the fabricated chemical sensor as described in example 1 and 2. DETAILED DESCRIPTION OF THE INVENTION

One skilled in the art will readily appreciate that the present invention is well adapted to carry out the objects and obtain the ends and advantages mentioned, as well as those inherent therein. The embodiment describes herein is not intended as limitations on the scope of the invention.

According to one preferred embodiment, the present invention discloses a composition for fabricating self-plasticized sensing membrane which is used for preparing potentiometric ion-selective electrode (ISE), particularly as a means to recognize a targeted analyte available in a liquid sample. Preferably, the disclosed composition includes an organic solvent dissolved homoegeneous mixture comprising 5 to 80% by total weight percentage of poly(vinyl chloride); 5 to 80% by total weight percentage of solid phase polymer or co-polymer selected from the group consisting of methyl methacrylate, tetrahydrofurfuryl acrylate, n-butyl acrylate, hydroxyethyl methacrylate, dodecyl acrylate, tetrafluoroethyl acrylate, glycidyl methacrylate, polyurethane, silicone rubber and any combination derived thereof; 5 to 80% by total weight percentage of liquid phase polymer or co-polymer selected from the group consisting of methyl methacrylate, tetrahydrofurfuryl acrylate, n-butyl acrylate, hydroxyethyl methacrylate, dodecyl acrylate, tetrafluoroethyl acrylate, glycidyl methacrylate, poly(n- butyl acrylate) and any combination derived thereof; 1 to 10% by total weight of liphophilic additive; and 1 to 40% by total weight of ionophores.

For preparing the disclosed composition, one of the possible ways is illustrated in figure 3. Preferably, the solid-phase softening polymer is prepared by forming a co- polymer of methyl methacrylate, tetrahydrofurfuryl acrylate, n-butyl acrylate, hydroxyethyl methacrylate, dodecyl acrylate, tetrafluoroethyl acrylate, or glycidyl methacrylate polyurethane or silicone rubber through mixing and reacting monomers of the selected polymers under influence of suitable radical initiator. More preferably, the solid phase polymer or co-polymer are of 25% to 50% by weight of the total composition. The solid phase co-polymer is composed of, but not limited to, methyl methacrylate and n-butyl acrylate in a ratio of 1 -4: 6-9 by part. Another solid phase co-polymer used in the present invention maybe synthesized by reacting 4 to 6 parts of methyl methacrylate and 4 to 6 parts tetrahydrifurfuryl acrylate. Possibly, polyurethane-based or silicone rubber-based polymers can be used as well apart from the co-polymer mentioned above. On the other hand, liquid phase polymer or copolymer selected from the group consisting of methyl methacrylate, tetrahydrofurfuryl acrylate, n-butyl acrylate, hydroxyethyl methacrylate, dodecyl acrylate, tetrafluoroethyl acrylate, glycidyl methacrylate, poly(n-butyl acrylate) and any combination derived thereof as mentioned above more preferably has a concentration around 25% to 50% by weight of total composition. For example, the liquid phase copolymer can be synthesized from 1 to 3 parts of methyl methacrylate and 10 to 12 parts of n-butyl acrylate. Or, in another embodiment, the liquid phase co-polymer is composed of 1 to 2 parts of methyl methacrylate and 6 to 9 parts of tetrahydrofurfuryl acrylate. In the embodiment of employing sole liquid polymer, liquid poly(n-butyl acrylate) is used. Both the solid and liquid phase co-polymers serve as substitution for common plasticizer to offer the fabricated membrane with much lower glass transition temperature to attain a solid state once adhered onto the conductor. Preparation of the co-polymers is preferably conducted under an inert condition. For example, the polymerization may be carried out under an inert gas like nitrogen with mild heating or reflux. The formed co-polymers may be subjected to repeated washing step for removing non-reacted residues. Most preferably, the solid phase polymer or co- polymer in the disclosed composition is different from the types of liquid phase polymer or co-polymer.

Pursuant to the preferred embodiment, the basic sensing membrane prior to doping with ionophores and lipohilic salts is illustrated in figure 2. The liquid phase co- polymer is preferably intercalating in between monomers of polymerized poly(vinyl chloride), while the solid phase co-polymer is disposed in between polymerized poly(vinyl chloride). Such arrangement in the molecular structure has shown to be effective in lowering glass transition temperature of the fabricated sensing membrane especially limiting free movement in between the macromolecules of polymerized poly(vinyl chloride). Preferably, the poly(vinyl chloride) monomers and the prepared solid phase and liquid phase polymer or co-polymers are homogeneously mixed in an organic solvent, more preferably a polar organic solvent such as tetrahydrofuran, diethyl ether, methylene chloride, methoxy ethanol, diethyl carbonate, dimethyl sulfoxide, toluene, acetone, methyl ethyl ketone, various short chain alcohols or any mixtures derived thereof. Yet, tetrahydrofuran is used in the most preferred embodiment. Further, the poly(vinyl chloride) used in the present invention is preferably has relatively high molecular weight.

Referring to figure 2, matrix of the fabricated sensing membrane doped with the ionophores and the lipohilic additive is shown. The ionophores and lipophilic are enclosed within the void spaces of the polymerized poly(vinyl chloride) and the softening co-polymers. Preferably, the doping is conducted by mixing the ionophores and lipophilic additive into the mixture of co-polymers and poly( vinyl chloride) in the organic solvent. The ionophore used in present invention is valinomycin, nonactin, tridodecylamine, tetraoctyl ammonium nitrate, bisthiourea, bis-crown ether, calcimycin, ionomycin, or monactin, while the lipophilic additives is lipophilic tetraalkyl ammonium halides, lipophilic tetraphenyl borates or any combination thereof. More preferably, the lipophilic additive and ionophores are equivalent in molar ratio within the composition to facilitates precise ion selection.

As in the setting forth, another embodiment of the present invention is a self- plasticized sensing membrane fabricated from an organic solvent dissolved homoegeneous mixture comprising 5 to 80% by total weight percentage of poly(vinyl chloride); 5 to 80% by total weight percentage of solid phase polymer or co-polymer selected from the group consisting of methyl methacrylate, tetrahydrofurfuryl acrylate, n-butyl acrylate, hydroxyethyl methacrylate, dodecyl acrylate, tetrafluoroefhyl acrylate, glycidyl methacrylate, polyurethane, silicone rubber and any combination derived thereof; 5 to 80%) by total weight percentage of liquid phase polymer or co-polymer selected from the group consisting of methyl methacrylate, tetrahydrofurfuryl acrylate, n-butyl acrylate, hydroxyethyl methacrylate, dodecyl acrylate, tetrafluoroethyl acrylate, glycidyl methacrylate, poly(n-butyl acrylate) and any combination derived thereof; 1 to 10% by total weight of liphophilic additive; and 1 to 40%) by total weight of ionophores. Accordingly, the organic solvent in homogeneous mixture is polar organic solvent such as tetrahydrofuran, diethyl ether, methylene chloride, methoxy ethanol, diethyl carbonate, dimethyl sulfoxide, toluene, acetone, methyl ethyl ketone, various short chain alcohols or any mixtures derived thereof. Yet, tetrahydrofuran is used in the most preferred embodiment to disperse the various molecules in a homogeneous matrix .

Relying on the type of target ion to be analyzed by the chemical sensor using the sensing membrane, different types of ionophores capable of selecting specific ion type can be incorporated into the sensing membrane. The representative example of various ionophores can be used in the present invention is valinomycin, nonactin, tridodecylamine, tetraoctyl ammonium nitrate, bisthiourea, bis-crown ether, calcimycin, ionomycin, or monactin. The sensing membrane requires a lipohilic additive, preferably a lipophilic salt, for the ion selection. The lipophilic additive can be, but not limited to, lipophilic tetraalkyl ammonium halides, lipophilic tetraphenyl borates or any combination thereof. More preferably, the lipophilic additive and ionophores are equivalent in molar ratio within the sensing membrane to facilitate precise reading for the produced chemical sensor. The following example is intended to further illustrate the invention, without any intent for the invention to be limited to the specific embodiments described therein.

Example 1

Preparation of Solid Polymer (MB28)

Methyl methacrylate (MMA, 2 mL) and 8 mL n-butyl acrylate (nBA) were added into a 50 mL three-neck round bottom flask. 15 mL of Toluene and 1 mg benzoyl peroxide radical initiator were added into the mixture of monomers. The reaction mixture was gently refluxed by maintaining the temperature at 95°C while stirring under nitrogen blanket for 7 hours. After 7 hours of reflux the heating was discontinued and the mixture gradually cooled to room temperature. The viscous polymeric material was transferred into a 50 mL beaker, and the solid material became cloudy. The cloudy polymeric material was washed three times with 5 mL portions of petroleum ether (b.p. 80°C-100°C) until it became clear. The bulk polymeric membrane was air dried at room temperature overnight.

Example 2

Preparation of Liquid Polymer (MT28)

Methyl methacrylate (MMA, 2 mL) and 8 mL tetrahydrofurfuryl acrylate (T) were added into a 50 mL three-neck round bottom flask. 15 mL of Toluene and 1 mg benzoyl peroxide radical initiator were added into the mixture of monomers. The reaction mixture was gently refluxed by maintaining the temperature at 95°C while stirring under nitrogen blanket for 7 hours. After 7 hours of reflux the heating was discontinued and the mixture gradually cooled to room temperature. The viscous liquid polymer was transferred into a 50 mL beaker and washed three times with 5 mL portions of petroleum ether (b.p. 80°C-100°C) until it became clear. The liquid polymer air dried for 12 hours to remove the solvents.

Example 3

Preparation of Ammonium Sensor with PVC-MB28 and MT28 Self-Plasticized Membrane

Ammonium Sensor composition was prepared by mixing 40 mg poly(vinyl) chloride (PVC), 1 mg potassium tetrakis[bis-3,5(p-chlOrophenyl] borate (KTpClPB), 3 mg nonactine (Ammonium Ionophore I), 30 mg methyl methacrylate-n-butyl acrylate solid copolymer (MB 28) and 30mg methyl methacrylate- tetrahydrofurfuryl acrylate liquid polymer (MT28). The mixture was dissolved with 1 mL tetrahydofuran (THF) solvent. Screen printed electrodes (SPE) with 4 mm diameter were cleaned ultrasonically with deionised water for 1 min . Pyrrole (0.5M) doped with 1M potassium chloride was electrochemically polymerisation using Autolab PGSTAT MODEL 128N for 90 seconds and 2 mA cm ^'2 current density in a three-electrode cell; platinum stick counter electrode and Ag/AgCl double junction reference electrode. The homogenous ammonium cocktail containing 30 weight percent MB28 solid polymer and 30 weight percent MT28 liquid polymer was drop coated on the freshly prepared polypyrrole layer and dried under continous flow of nitrogen gas for 2 hours or air dried at ambient temperature for 12 hours. This ammonium sensor was tested using commercial Ag/AgCl double junction reference electrode with 0.1M LiOAc as outer solution. Before tested, ammonium sensor was soaked into 0.1 M ammonium chloride until overnight to conditioning. The results were shown in Table 1 and plotted in Figure 4. The plot of emf response versus activity of ammonium ion shows good Nernstian response, and good linearity.

Table 1: Response of ammonium ISE with self-plasticized membrane; PVC-

MB28-MT28 with 40:30:30 weight ratio

It is to be understood that the present invention may be embodied in other specific forms and is not limited to the sole embodiment described above. However modification and equivalents of the disclosed concepts such as those which readily occur to one skilled in the art are intended to be included within the scope of the claims which are appended thereto.