FIELD OF THE INVENTION The present invention relates to a nitrate sensor. More specifically it relates to nitrate sensor with polymeric membrane and photocurable component.

BACKGROUND ART

Since the introduction of ion-selective electrode (ISE) in 1900's, much attention has been given to improve the sensing membrane components which based on potentiometric detection. The sensing membrane comprises polymer or co-polymer backbone, ionophore to transport the target ion from an aqueous medium into a hydrophobic phase, and lipophilic salt to create ionic site. In recent years, the possibility to improve the potentiometric detection was demonstrated by doping membrane with ion-exchanger such as quaternary ammonium salts. This ion- exchanger has a selectivity-modifying influence since its concentration in the membrane determines the amount of exchangeable ions of opposite charge. Traditionally, ISE sensor membrane is based on hydrophobic plasticized polymeric membranes or films that are doped with ionophore and plasticizer as matrix softening agent. Unfortunately low molecular weight ionophore and plasticizer are prone to leaching - the sensor components are lost into the analyte and this causes degradation of the sensor response. SUMMARY OF THE INVENTION

Accordingly, the present invention provides a nitrate sensor, wherein the sensor having covalently bonded lipophilic ammonium salt and methyl-methacrylate and tetrahydrofurfuryl acrylate photo-polymerized co-polymer.

Further, there is also provided a process for producing a low impedance nitrate ion selective electrode (ISE) sensor with tridodecyl-11-acrylundecyl ammonium bromide salt having 2 parts of methyl methacrylate and 8 parts of tetrahydrofurfuryl acrylate, the process includes the steps of (a) preparing an internal hydrophiiic layer composition using a monomer of 2-hydroxylethyl methacrylate (HEMA) and 2,2-dimethoxyl-2- phenylacetopenone (DMPP) photoinitiator to form a mixture, (b) applying the mixture from step (a) onto a microelectrode, (c) photocuring the mixture from step (b) under ultraviolet (UV) exposure with an inert gas such as nitrogen gas, (d) hydrating glassy poly HEMA membrane with potassium nitrate solution to produce an internal layer, (e) mixing methyl-methacrylate monomer with tetrahydrofurfuryl acrylate monomer to form a mixture, (f) adding 2-hexenedioldiacrylate (HDDA) crosslinker to the mixture as obtained in step (e), (g) mixing a suitable amount of DMPP and tridodecyl-11-undecyl ammonium bromide to form a polymeric membrane cocktail, (h) depositing the cocktail on top of the internal layer as obtained in step (d) and (i) photocuring the cocktail from step (h) under UV exposure with an inert gas such as nitrogen gas.

Also provided is a process for producing a nitrate ion selective electrode (ISE) sensor with tetraoctyi ammonium nitrate (TOAN) having 2 parts of methyl methacrylate and 8 parts of tetrahydrofurfuryl acrylate, the process includes the steps of (a) preparing an internal hydrophiiic layer composition using a monomer of 2-hydroxylethyl methacrylate (HEMA) and 2,2-dimethoxyl-2-phenylacetopenone (DMPP) photoinitiator to form a mixture, (b) applying the mixture from step (a) onto a microelectrode, (c) photocuring the mixture from step (b) under ultraviolet (UV) exposure with an inert gas such as nitrogen gas, (d) hydrating glassy poly HEMA membrane with potassium nitrate solution to produce an internal layer, (e) mixing methyl-methacrylate monomer with tetrahydrofurfuryl acrylate monomer to form a mixture, (f) adding 2- hexenedioldiacrylate (HDDA) crosslinker to the mixture as obtained in step (e), (g) mixing a suitable amount of DMPP and tetraoctyl ammonium nitrate to form a polymeric membrane cocktail, (h) depositing the cocktail on top of the internal layer as obtained in step (d) and (i) photocuring the cocktail from step (h) under UV exposure with an inert gas such as nitrogen gas.

The present invention consists of several novel features and a combination of parts hereinafter fully described and illustrated in the accompanying description and drawings, it being understood that various changes in the details may be made without departing from the scope of the invention or sacrificing any of the advantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, wherein:

FIG. 1 shows polymerizable lipophilic ammonium salts;

FIG. 2 shows monomers for low impedance membrane i.e. methyl-methacrylate (1 ) and tetrahydrofurfuryl acrylate (2);

FIG. 3 shows photo-polymerization of methyl-methacrylate (1) and tetrahydrofurfuryl acrylate (2) to give low-impedance methyl methacrylate and tetrahydrofurfuryl acrylate (MT) membrane;

FIG. 4 shows the response of nitrate ion selective electrode based on MT 19, 28, 37, 46 and 55 co-polymer acryl ammonium bromide ion exchanger;

FIG. 5 shows the response of nitrate ion selective electrode based on MT 28 copolymer and tetraoctyl ammonium nitrate (TOAN) ion exchanger.

S

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a nitrate sensor. Hereinafter, this specification will describe the present invention according to the preferred embodiments of the present invention. However, it is to be understood that limiting the description to the preferred embodiments of the invention is merely to facilitate discussion of the present invention and it is envisioned that those skilled in the art may devise various modifications and equivalents without departing from the scope of the appended claims. The present invention provides a use of acrylated lipophilic ion exchanger gives selectivity towards nitrate ion, and the lipophilic salt is polymerized into the polymer backbone. FIG. 1 shows the polymerizable lipophilic ammonium salts.

Moreover, the polymer backbone is a novel low impedance co-polymer of methyl- methacrylate (M) and tetrahydrofurfuryl acrylate (T). FIG. 2 shows the monomers for low impedance membrane; methyl-methacrylate (1) and tetrahydrofurfuryl acrylate (2).

The present invention provides a low impedance nitrate sensor based ion acrylated lipophilic ammonium nitrate (I) and bromide (II). The ion exchangers give good selectivity to nitrate when immobilized to co-polymer of methyl methacrylate and tetrahydrofurfuryl acrylate (MT copolymer). Different ratios of the co-polymer have been experimented as described in detail in the examples. A wide range of MT ratios of 1 :9, 2:8, 3:7, 4:6, 5:5, 6:4 and 7:3 have been demonstrated to give nitrate sensors with good response slope and correlation coefficient. FIG. 3 shows the photo-polymerization of m ethyl -meth aery I ate (1) and tetrah drofurfuryl acrylate (2) to give low-impedance MT membrane. The co- polymerization reaction takes place efficiently under UV condition within 3 minutes to give firm and sticky-free membrane.

The freshly prepared acrylated ammonium bromide has been used as ion exchanger for nitrate ISE sensor based on photocurable methyl methacrylate-tetrahydrofurfuryl acrylate (MT) with MT monomer ratios of 1 :9, 2:8, 3:7, 4:6, and 5:5. The results of the nitrate sensor characteristics are described in the following examples.

Examples Example 1 - Nitrate ISE Sensor with Tridodecyl-11-acrylundecyl Ammonium Bromide and Low Impedance MT 28 Membrane a) Preparation of Internal Layer 1 g monomer of 2-hydroxylethyl meth acrylate (HEMA) and 0.016 g of photoinitiator 2,2-dimethoxyl-2-phenylacetopenone (DMPP) was prepared to form internal hydrophilic layer composition. The mixture was applied onto Ag/AgCl microelectrode and photocured under UV exposure with continuous flow of nitrogen gas for 3 minutes. The glassy poly HEMA membrane was then hydrated with 0.1 M potassium nitrate solution for 10 minutes to form sensor internal layer. b) Preparation of Nitrate ISE Sensor with Tridodecyl-11-acrylundecyI Ammonium Bromide and Low Impedance WIT 28 Membrane

200 uL of methyl methacrylate (M) monomer was mixed with 800 uL tetrahydrofurfuryl acrylate (T) monomer and t .05 uL of 2-hexenedio)diacrylate (HDDA) crosslinker was added to this mixture. 00 uL of this mixture was mixed with 2 mg DMPP and 3 mg tridodecyl-11 -undecyl ammonium bromide (II) to form a polymeric membrane cocktail. Then, the homogenous cocktail was deposited on top of internal layer and photocured under UV exposure with continuous flow of nitrogen gas for 90 seconds to form co- polymer MT 19, 28, 37, 46 and 55 with acryl ammonium bromide II ion exchanger as a sensing layer. This nitrate sensor was tested using commercial Ag/AgCI double junction reference electrode. The results were shown in Table 1 and FIG. 4.

Table 1

Sensitivity of nitrate sensor

Log [N03-]

MT 19 MT 28 MT 37 MT 46 MT 55

-4 222.2 231.9 231.0 249.9 248.1

-3 166.5 175.5 174.7 196.3 195.4

-2 105.0 110.3 109.9 131.9 130.2

-1 42.7 47.1 47.8 68.7 66.0

Slope (mv/dec) -60.0 -62.0 -61.4 -60.8 -61.1

Intercept, C -16.0 -13.7 -12.7 9.7 7.1

Regression (R ² ) 0.9993 0.9991 0.9992 0.9984 0.9977 Selectivity coefficient, K ^pot NO3-J

Interfering ions

MT 19 MT 28 MT 37 MT 46 MT 55

Chloride (CI ^" ) 3.2 X 10 ^"2 3.2 X 10 ^"2 3.2 X 10 ^"2 3.2 X 10 ^"2 3.2 X 10 ^"2

Sulfate (S04 ^a" ) 6.3 X 10 ^"4 1.0 X 10 ^"3 1.0 X 10 ^"3 1.0 X 10 ^"3 3.9 X 10 ^"3

Dibasic phosphate,

7.9 X 10 ^"2 1.0 X 10 ^"1 1.0 X 10 ^"1 3.9 X 10 ^"1 1 .0 X 10 ^"1 (HP0 ₄ ² -)

Example 2 - Nitrate ISE Sensor with Tetraoctyl Ammonium Nitrate and Low Impedance MT 28 Membrane a) Preparation of Internal Layer

1 g monomer of 2-hydroxylethyl methacrylate (HEMA) and 0.016 g of photoinitiator 2,2-dimethoxyl-2-phenylacetopenone (DMPP) was prepared to form internal hydrophiiic layer composition. The mixture was applied onto Ag/AgCI microelectrode and photocured under UV exposure with continuous flow of nitrogen gas for 3 minutes. The glassy poly HEMA membrane was then hydrated with 0.1 M potassium nitrate solution for 10 minutes to form sensor internal layer. b) Preparation of Nitrate Ion-selective Membrane with Tetraoctyl Ammonium Nitrate (TOAN) as ion exchanger

190 uL of methyl-methacrylate (MMA) monomer was mixed with 760 μΐ_ tetrahydrofurfuryl acrylate (THF) monomer and 1.05 uL 2-hexenedioldiacrylate (HDDA) crosslinker. 100 uL of this mixture was mixed with 2 mg DMPP and 3.9 mg Tetraoctyl Ammonium Nitrate (TOAN) (from Fluka). The homogenous cocktail was deposited on top of internal layer and photocured under UV exposure with continuous flow of nitrogen gas for 90 seconds to form copolymer MT 28 with TOAN ion exchanger as a sensing layer. Further, the nitrate ion selective electrode was tested with commercial Ag/AgCI double junction reference electrode. The results were shown in Table 2 and FIG. 5.

Table 2

Selectivity coefficient, K ^pot _N0 3-j

Interfering ion Log K" ⁰¹ ,., Chloride (CI ^' ) 3.2 X 10 ^'2