Field of Invention The present invention relates generally to a sensing membrane, more particularly to a plasticizer-free sensing . membrane for chemical sensors.

Background of the Invention

Ion-selective electrodes (ISEs) are the chemical sensors of longest history and probably the most frequent routine application. They have found widespread use in clinical laboratories and are ^' being explored for numerous other applications. ISEs produce a measurable electrical change upon contact with a fluid sample containing target ions. Potentiometric detection based on ion-selective electrodes offers several advantages, such as speed and ease of preparation and procedures, simple instrumentation, relatively fast response, wide dynamic range, reasonable selectivity and low cost. Besides, they are also suitable for on-site analysis and, nowadays, are found to be applicable in the analysis of some biologically relevant ions, process control and environmental analysis. The most common ion selective electrode construction requires an ion-selective membrane of polymeric type containing the ionophore. Some typical examples are poly (vinyl chloride) or PVC. This is primarily due to its high tensile strength, chemical inertness and plasticizer compatibility. However, there have been problems with attempts to fabricate a solid- state sensor with this polymer, because PVC exhibits poor adhesion toward ion-selective electrode, mostly performed by solvent casting procedures and requires plasticizer for the purpose of softening the polymer. It could not be used without plasticizer.

There are problems with the use of plasticized PVC in ion- selective sensors and one of which is plasticizer leaching. Leaching of plasticizers in biomedical devices has prompted the issuance of a health warning on biomaterials . Therefore various efforts made to reduce or eliminate plasticizer content in such devices. However, plasticizer is one of the principle factors directly affecting sensor lifetime and reduction in plasticizer may results in an increased membrane resistance and may decrease the solubility of the active sensing components within the membrane.

The photopolymerized acrylate membranes could be choice a alternative of sensing membranes. Because the photopolymerize acrylate membranes has many advantages, e.g. self plasticising, without the need of any external plasticiser and photocurable . The polymer also exhibits excellent adhesion on solid surfaces and this makes it very suitable for application in solid-state ion sensors.

Photopolymerization offers multiple advantages which include spatial and temporal control, solventless polymerizations, as well as compatibility with multiple solvents. Due to the multitude of advantages it offers, photopolymerization of acrylates has been used for a variety of applications including sensor material, biomaterial, dental material, contact lens and adhesive. Photoinitiated free radical polymerization of multifunctional monomers produces highly crosslinked networks with high thermal stability, mechanical strength and resistance to solvent absorption.

The copolymer membranes of the invention are fabricated from methyl methacrylate and tetrahydrofurfuryl acrylate monomers. The disclosed copolymer exhibits excellent properties as a sensing membrane that includes clear and colorless appearance, good flexibility, not sticky, and good adhesion onto surface. The polarity of this copolymer can be varied depend ratio between methyl methacrylate and tetrahydrofurfuryl acrylate, and thus can be used to variety sensors fabricated. Other objects of this invention will become apparent on the reading of this entire disclosure.

Summary of the Invention

In the present invention, a copolymer membrane for use on an ion detective sensor as a sensing membrane, the membrane comprises a plasticizer-free copolymer including first monomer units of methyl methacrylate and second monomer units of tetrahydrofurfuryl acrylate, wherein the copolymer is mixed with sensing components and photocured under ultraviolet (UV) light.

In one aspect of the invention, the sensing components comprise liphophilic salt, photoinitiator, ionophore and cross linker diacrylate.

A plasticizer-free ion detective sensor, the sensor comprising sensing element for detecting chemical species, including a copolymer membrane enclosing the sensing element, the membrane providing high polarity and low impedance, the improvement comprising forming the membrane from first monomer units of methyl methacrylate and second monomer units of tetrahydrofurfuryl acrylate, wherein the copolymer is mixed with sensing components and photocured under ultraviolet (UV) light. Preferably the ion detective sensor is a potentiometric chemical sensor based ion selective electrode (ISE) or an ion sensitive field effect transistor (ISFET) . Brief Description of the Drawings

Other objects, features, and advantages of the invention will be apparent from the following description when read with reference to the accompanying drawings. In the drawings, wherein like reference numerals denote corresponding parts throughout the several views:

Figure 1 shows the structures of methyl methacrylate and tetrahydrofurfuryl acrylate;

Figure 2 depicts the structures of photocured MT copolymer membrane of the present invention;

Figure 3 illustrates the result of the response of magnesium ion selective electrode based on copolymer MT 28;

Figure 4 illustrates the result of the response of potassium ion use generic cation selective electrode based on copolymer MT 28; and Figure 5 illustrates the result of the response of CI ion using generic anion selective electrode based on copolymer MT 28. Detailed Description of the Preferred Embodiments

In the following detailed description, numerous specific details are set forth in order to provide a ^% thorough understanding of the invention. However, it will be understood by those of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known methods, procedures and/or components have not been described in detail so as not to obscure the invention. Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

The present invention is directed to a plasticizer free ion detective sensor wherein the sensor provides copolymer membranes which are fabricated from methyl methacrylate (11) and tetrahydrofurfuryl acrylate (12) monomers as shown in Figure 1. It has been discovered that the use of such membranes provides many advantages including of good characteristics as a sensing membrane, provides fast and stable sensor response and thus an excellent choice for chemical sensor, ease of handling, economic benefits of low costs of the monomers and low impedance due to polar tetrahydrofurfuryl substituent and compatibility with sensor components . Different volume ratios of methyl methacrylate (11) and tetrahydrofurfuryl acrylate (12) monomers under UV photocuring condition afford a self-plasticising sensing membrane called MT membrane (13) as shown in Figure 2. In typical sensor application, particularly preferred is a specific volume ratio of the monomers such as 2 part of methyl methacrylate (11) and 8 part of tetrahydrofurfuryl acrylate (12), along with the rest of the sensing membrane components such as liphophilic salt, photoinitiator, ionophore and cross linker diacrylate are mixed together in the absence of plasticizer, and without the assistance of solvent, and photocured under UV light.

In another aspect of the invention, the ratio between the methyl methacrylate (11) and tetrahydrofurfuryl acrylate (12) can be varied to obtain different polarity of copolymer membrane to be used to variety sensors fabricated. Another example of volume ratio of the monomers is 3 part of methyl methacrylate (11) and 7 part of tetrahydrofurfuryl acrylate (12) . For the purpose of example but not to be limiting, the inventive copolymer membranes- of the present invention may also be used in an ion sensitive field effect transistor (ISFET) . Therefore as will be appreciated by those skilled in the art, variations in the volume ratio of such monomers will be able to achieve the desired physical characteristics of the membranes . The present invention is further described in the following examples ;

Example 1 - Magnesium Sensor based on Methyl Methacrylate- Tetrahydrofurfuryl Acrylate (MT 28) Membrane

(i) Inner Layer Preparation

1 g of 2-hydroxylethyl methacrylate (HEMA) monomer is mixed with 0.016 g of photoinitiator ^' 2 , 2-dimethoxyl-2- phenylacetopenone (DMPP) and dropped onto Ag/AgGl electrode surface . The mixture was photocured under UV radiation in UV- exposure unit under constant flow of nitrogen gas for approximately 3 minutes. The polymer film formed was then hydrated with 0.1 M of Magnesium Chloride solution for 10 minutes to form the inner layer of the sensor.

(ii) Magnesium Sensor Preparation.

200 pL of methyl methacrylate (MMA) monomer mixed with 800 pL tetrahydrofurfuryl acrylate (THFA) monomer and 1.05 pL 2- hexenedioldiacrylate (HDDA) as crosslinker . Then 100 L from the mixture of monomers mixed with 1 mg DMPP was added with 2mg sodium tetrakis [bis-3 , 5 ( trifluoromethyl ) phenyl ] borate (NaTFPB) and 0.77 mg Magnesium Ionophore I (from Fluka) . The homogenous cocktail was deposited on top of the freshly prepared and hydrated inner layer and photocured under UV radiation in UV-exposure apparatus under constant flow of nitrogen gas for approximately 3 minutes to form copolymer MT 28 as a sensing layer. Further, the magnesium ion selective electrode was tested versus a commercial Ag/AgCl double junction reference electrode. The results were shown in the following Table 1 and Figure 3.

Table 1

Example 2-Generic Cation Sensor based on Methyl Methacrylate- Tetrahydrofurfuryl Acrylate Membrane (MT 28)

(i) Inner layers Preparation

1 g of 2-hydroxylethyl methacrylate (Hema) monomer is mixed with 0.016 g of photoinitiator 2 , 2-dimethoxyl-2- phenylacetopenone (DMPP) and dropped onto Ag/AgCl electrode surface . The mixture was photocured under UV radiation in UV- exposure unit under constant flow of nitrogen gas for approximately 3 minutes. The polymer film formed was then hydrated with 0.1 M of Potassium Chloride solution for 10 minutes to form the inner layer of the sensor.

(ii) Generic Cation Sensor Preparation

200 μΐ, of methyl methacrylate ( MA) monomer mixed with 800 L tetrahydrofurfuryl acrylate (THFA) monomer and 1.05 pL 2- hexenedioldiacrylate (HDDA) as crosslinker . Then 100 L from the mixture of monomers mixed with 1 mg DMPP was added with 2mg sodium tetrakis [bis-3, 5 ( trifluoromethyl) phenyl] borate (NaTFPB) (from Fluka) . The homogenous cocktail was deposited on top of the freshly prepared and hydrated inner layer and photocured under UV radiation in UV-exposure apparatus under constant flow of nitrogen gas for approximately 3 minutes to form copolymer MT 28 as a sensing layer. Further, the generic cation ion selective electrode was tested versus a commercial Ag/AgCl double junction reference electrode. The results were shown in the following Table 2 and Figure 4.

Table 2

Example 3- Generic Anion Sensor based on Methyl Methacrylate- Tetrahydrofurfuryl Acrylate Membrane (i) Inner layers Preparation

lg of 2-hydroxylethyl methacrylate (HEMA) monomer is mixed with 0.016g of photoinitiator 2 , 2-dimethoxyl-2- phenylacetopenone (D PP) and dropped onto Ag/AgCl electrode surface . The mixture was photocured under UV radiation in UV- exposure unit under constant flow of nitrogen gas for approximately 3 minutes. The polymer film formed was then hydrated with 0.1 M of Potassium Chloride solution for approximately 10 minutes to form the inner layer of the sensor.

(ii) Generic Anion Sensor Preparation

200 μΐ, of methyl methacrylate (MMA) monomer mixed with 800 ]iL tetrahydrofurfuryl acrylate (THFA) monomer and 1.05 μΐ, 2- hexenedioldiacrylate (HDDA) as crosslinker . Then 100 μΐ, from the mixture of monomers mixed with 1 mg DMPP was added with 2mg tetradodecyl ammonium chloride (TDDAC1) (from Fluka) . The homogenous cocktail was deposited on top of the freshly prepared and hydrated inner layer and photocured under UV radiation in UV-exposure apparatus under constant flow of nitrogen gas for approximately 3 minutes to form copolymer MT 28 as a sensing layer. Further, the generic anion ion selective electrode was tested versus a commercial Ag/AgCl double junction reference electrode. The results were shown in the following Table 3 and Figure 5. Table 3

As can be seen from these results, the copolymers exhibit excellent properties as a sensing membrane which includes good flexibility. The capability to vary the composition of the copolymer to achieve certain specific properties is a key feature of this invention.

As will be readily apparent to those skilled in the art, the present invention may easily be produced in other specific forms without departing from its essential characteristics. The present embodiments is, therefore, to be considered as merely illustrative and not restrictive, the scope of the invention being indicated by the claims rather than the foregoing description, and all changes which come within therefore intended to be embraced therein.