Claims:WE CLAIM

1. A compound of formula 1

Formula 1
wherein,
R1 is selected from a group comprising C2-C12 conjugated substituted alkyl, aryl, heteroaryl, substituted aryl, and substituted heteroaryl;
R2 is selected from a group comprising C2-C12 aryl, heteroaryl, substituted aryl, and substituted heteroaryl;
Z is selected from a group comprising O, S, and N; and
n is in the range of 100 to 100000, specifically 10000 to 20000;
wherein,
the conjugated substituted alkyl, substituted aryl, and substituted heteroaryl are substituted by moiety selected from a group comprising, thiophene, alkyl thiophenepyyrole, carbazole, indole, phenyl, benzene, and pyridine.

2. A compound of formula 2

Formula 2
Wherein n is 10000

3. A method of preparation of compound of formula 2, said method comprising steps of

Formula 2
Wherein n is 10000-20000

a) Preparing compound of formula 3, comprising steps of

Formula 3
i) treating diphenyl amine with 1-bromododecane in a solvent in the presence of a catalyst to obtain 1-N-dodecyl diphenylamine; and
ii) treating the 1-N-dodecyl diphenylamine with phosphorousoxychloride in DMF to obtain compound of formula 3;
b) treating the compound of formula 3 with 1,3-diaminothiourea in a solvent in presence of a catalyst and filtering to obtain the compound of formula 2; and
c) optionally purifying to obtain the pure compound of formula 2.

4. A composition for detecting nitrate ion in a test sample, said composition comprising compound of formula 1

Formula 1

wherein,
R1 is selected from a group comprising C2-C12 conjugated substituted alkyl, aryl, heteroaryl, substituted aryl, and substituted heteroaryl;
R2 is selected from a group comprising C2-C12 aryl, heteroaryl, substituted aryl, and substituted heteroaryl; and
Z is selected from a group comprising O, S, and N;
n is ranging from 100 to 100000 specifically 10000 to 20000;
wherein,
the conjugated substituted alkyl, substituted aryl, and substituted heteroaryl are substituted by a moiety selected from a group comprising, thiophene, alkyl thiophenepyyrole, carbazole, indole, phenyl, benzene, and pyridine;
and carbonaceous material and binder in the ratio ranging from about 2:98:0 w/w to about 20:60:20w/w.
5. The composition as claimed in claim 4, wherein the carbonaceous material is selected from a group comprising carbon black, carbon nanotubes, fullerenes, whiskers, wires, fibres, filaments, graphite and mixture thereof.
6. A method of preparation of composition for detecting nitrate ion in a test sample, said composition comprising compound of formula 1

Formula 1

wherein,
R1 is selected from a group comprising C2-C12conjugated substituted alkyl, aryl, heteroaryl, substituted aryl, and substituted heteroaryl;
R2 is selected from a group comprising C2-C12 aryl, heteroaryl, substituted aryl, and substituted heteroaryl; and
Z is selected from a group comprising O, S, and N;
n is ranging from 100 to 100000 specifically 10000 to 20000
wherein,
the conjugated substituted alkyl, substituted aryl, and substituted heteroaryl are substituted by moiety selected from a group comprising, thiophene, alkyl thiophenepyyrole, carbazole, indole, phenyl, benzene, and pyridine.
and carbonaceous material and binder in the ratio ranging from about 2:98:0 w/w to about 20:60:20w/w, said method comprising acts of
a) preparing a solution of the compound of formula 1 in a solvent and agitating the solution,
b) adding the carbonaceous material to the agitated solution to obtain a mixture, and
c) agitating the mixture to obtain the composition.

7. A composition for detecting nitrate ion in a test sample, said composition comprising compound of formula 2

Formula 2
wherein n is 10000 to 20000 and carbon nanotubes and binder in the ratio ranging from about 98:2:0 w/w to about 60:20:20 w/w.

8. A method of preparation of composition for detecting nitrate ion, said composition comprising compound of formula 2

Formula 2

wherein n is 10000 to 20000 and carbon nanotubes and binder is in ratio ranging from about 98:2:0 w/w to about 60:20:20 w/w, said method comprising acts of
a) preparing a solution of the compound of formula 2 in a solvent and agitating the solution,
b) adding the carbon nanotubes to the agitated solution to obtain a mixture, and
c) agitating the mixture to obtain the composition.

9. A sensor for detecting nitrate ions comprising composition of claim 4 or claim 7 coated on electrodes of a circuit.
10. The sensor as claimed in claim 9, wherein the composition of claim 4 or 7 is coated on electrodes of a single layered printed circuit board.
11. The sensor as claimed in claim 9 and claim 10, wherein the electrodes are selected from a group comprising silver, copper, gold, platinum, graphite, Nickel, Tin, and Chrome.
12. A method of fabrication of a sensor of claim 9 for detecting nitrate ions said method comprising steps of
a) preparing composition of claim 4 or claim 7;
b) applying the composition onto the electrodes of the circuit; and
c) drying to obtain the sensor.

13. The method as claimed in claim 12, wherein the composition is applied on to the electrodes by a method selected from dropcasting, dip coating, spin coating, spray coating, pad printing, ink jet printing, screen printing, brush printing, gravure printing, and roll to roll printing.
14. The method as claimed in claim 12, wherein the drying is carried out at a temperature ranging from about 60°C to about 80°C, preferably at about 70°C.
15. The method as claimed in claim 12, wherein the sensor is of size ranging from about 1mm X 1mm to about 5mm X 5 mm.
16. A method of detection of nitrate ions with sensor as claimed in claim 9, said method comprising steps of
a) dipping the sensor in test sample to be tested for nitrate ions; and
b) measuring change in resistance of the sensor to detect nitrate ions.
, Description:TECHNICAL FIELD
The present invention is in relation to detection of nitrate ions. The invention provides a sensor for detecting the nitrate ions in an aqueous test sample; compound and composition for fabricating the sensor. The sensor is resistance based with short response time, easy to handle and economical.
BACKGROUND
The reasons and effects for water contamination are ubiquitously known. Among the various water contaminations, water contamination by the release of chemicals into potable water through various routes like sewage, run off from landfills, pesticides from agricultural fields, open drain are of serious concern. The potable water contamination is to be handled with caution as the chemical pollutants that remain in the water lead to health hazards and environmental crises. Amongst the various chemicals, nitrate ions are of serious concern as they are proven to be hazardous to infants and pregnant women. It is a well-known fact that nitrate gets converted into nitrite in the digestive system. This oxidizes the iron in the haemoglobin of the red blood cells to form methemoglobin. Methemoglobin is devoid of oxygen carrying ability of haemoglobin leading to blue coloured skin and veins. This condition is known as methemoglobinemia often referred to as blue baby syndrome. There are also reports that nitrite ions can cause stomach and gastrointestinal cancer.

The nitrate ions in water and aqueous solutions are detected either by direct determination or indirect determination as it cannot be easily detected by its color, smell or taste. The present methods that are commercially available for detection of nitrate ion in water are ion chromatographic and spectrophotometric method. Griess test is an indirect method of detection where the nitrate ion is reduced to nitrite and the quantity of nitrite is estimated, as the spectroscopic methods have the limitation of the resolution, the lower limit of detection is compromised. These are basically analytical methods and laboratory based techniques that require people with knowledge of the method and instruments. The instruments are bulky and cannot be used for field-based detection. An electrochemical method for nitrate ion detection involves ion selective electrodes with ionophores that selectively bind to nitrate ion, however ion selective electrode needs to remain moist all the time. UV absorbance based nitrate sensors are also available. This method can measure nitrate ion concentration even in turbid conditions, however the response time of these sensors are high. There is a drift in concentration measured associated with the lamp.

US patent 7,160,690 describes about a biosensor for the detection of the nitrate ions. But the method is tedious involving complex instrumentation and procedure.US patent 4,059,599 describes ion selective electrode method involving multiple electrodes, reagents making the system expensive for the nitrate detection. US patents 7,671,336 and 8,444,937 describe method that are applicable only for the detection of nitrate in soil and not for aqueous samples. S. S. M. Hassanet al has described determination of nitrates using iodide electrode, Talanta, vol. 23, no. 10, pp. 738–740, 1976; similarly S.J. Cho et al., describes a systemusing titanium trichloride and an ammonium electrode,Sens. Actuators B, Chem., vol. 85,no. 1–2, pp. 120–125, 2002.Inspite of availability of various methods and sensors for the determination of the nitrate ions, a sensor that can determine the concentration of nitrate ion in aqueous solutions which is fast in response, easy to handle, portable and cost effective is still required to mature to make it commercially viable.

STATEMENT OF INVENTION
Accordingly, the present invention provides a nitrate ion sensor based on change in the resistance of the detector, compound and composition to fabricate the sensor. The invention also provides novel conducting polymers with backbone of planar –NH moiety and is used in combination with carbonaceous substance to enhance the response for detection of nitrate ions as low as 10 -3M.
A compound of formula 1

Formula 1

wherein,
R1 is selected from a group comprising C2-C12 conjugated substituted alkyl, aryl, heteroaryl, substituted aryl, and substituted heteroaryl;
R2 is selected from a group comprising C2-C12 aryl, heteroaryl, substituted aryl, and substituted heteroaryl;
Z is selected from a group comprising O, S, and N; and
n is in the range of 100 to 100000, specifically 10000 to 20000;
wherein,
the conjugated substituted alkyl, substituted aryl, and substituted heteroaryl are substituted by moiety selected from a group comprising, thiophene, alkyl thiophenepyyrole, carbazole, indole, phenyl, benzene, and pyridine.

A compound of formula 2

Formula 2
Wherein n is 10000

A method of preparation of compound of formula 2, said method comprising steps of

Formula 2
Wherein n is 10000

a) Preparation of compound of formula 3, comprising steps of

Formula 3
i) treating diphenyl amine with 1-bromododecane in a solvent in the presence of a catalyst to obtain 1-N-dodecyl diphenylamine; and
ii) treating the 1-N-dodecyl diphenylamine with phosphorousoxychloride in DMF to obtain compound of formula 3;
b) treating the compound of formula 3 with 1,3-diaminothiourea in a solvent in presence of a catalyst and filtering to obtain the compound of formula 2; and
c) optionally purifying to obtain the pure compound of formula 2.

A composition for detecting nitrate ion in a test sample, said composition comprising compound of formula 1

Formula 1

wherein,
R1 is selected from a group comprising C2-C12 conjugated substituted alkyl, aryl, heteroaryl, substituted aryl, and substituted heteroaryl;
R2 is selected from a group comprising C2-C12 aryl, heteroaryl, substituted aryl, and substituted heteroaryl; and
Z is selected from a group comprising O, S, and N;
n is ranging from 100 to 100000 specifically 10000 to 20000;
wherein,
the conjugated substituted alkyl, substituted aryl, and substituted heteroaryl are substituted by a moiety selected from a group comprising, thiophene, alkyl thiophenepyyrole, carbazole, indole, phenyl, benzene, and pyridine;
and carbonaceous material and binder in the ratio ranging from about 2:98:0 w/w to about 20:60:20w/w.

A method of preparation of composition for detecting nitrate ion in a test sample, said composition comprising compound of formula 1

Formula 1

wherein,
R1 is selected from a group comprising C2-C12conjugated substituted alkyl, aryl, heteroaryl, substituted aryl, and substituted heteroaryl;
R2 is selected from a group comprising C2-C12 aryl, heteroaryl, substituted aryl, and substituted heteroaryl; and
Z is selected from a group comprising O, S, and N;
n is ranging from 100 to 100000 specifically 10000 to 20000
wherein,
the conjugated substituted alkyl, substituted aryl, and substituted heteroaryl are substituted by moiety selected from a group comprising, thiophene, alkyl thiophenepyyrole, carbazole, indole, phenyl, benzene, and pyridine.
and carbonaceous material and binder in the ratio ranging from about 2:98:0 w/w to about 20:60:20w/w, said method comprising acts of
a) preparing a solution of the compound of formula 1 in a solvent and agitating the solution,
b) adding the carbonaceous material to the agitated solution to obtain a mixture, and
c) agitating the mixture to obtain the composition.

A composition for detecting nitrate ion in a test sample, said composition comprising compound of formula 2

Formula 2
wherein n is 10000 to 20000 and carbon nanotubes and binder in the ratio ranging from about 98:2:0 w/w to about 60:20:20 w/w.

A method of preparation of composition for detecting nitrate ion, said composition comprising compound of formula 2

Formula 2

wherein n is 10000 to 20000 and carbon nanotubes and binder is in ratio ranging from about 98:2:0 w/w to about 60:20:20 w/w, said method comprising acts of
a) preparing a solution of the compound of formula 2 in a solvent and agitating the solution,
b) adding the carbon nanotubes to the agitated solution to obtain a mixture, and
c) agitating the mixture to obtain the composition.

A sensor for detecting nitrate ions comprising composition of present invention coated on electrodes of a circuit.
A method of fabrication of a sensor of present invention for detecting nitrate ions said method comprising steps of
a) preparing composition of present invention;
b) applying the composition onto the electrodes of the circuit; and
c) drying to obtain the sensor.

A method of detection of nitrate ions with sensor of present invention, said method comprising steps of
a) dipping the sensor in test sample to be tested for nitrate ions; and
b) measuring change in resistance of the sensor to detect nitrate ions.

DESCRIPTION OF FIGURES
The present invention will be readily understood by the following detailed description in conjunction with the accompanying figures, wherein like reference numerals designate like structural elements, and in which:
Figure 1: Design of single layer PCB
Figure 2: Schematic of sensor device
Figure 3: Graphs showing the response with different carbonaceous substances.
Figure 4: Graphs showing the response of poly (DATC) with various concentration of carbonaceous substances.
Figure 5: SEM pictures of the MWCNT in poly (DATC) matrix.
Figure 6: Base resistance distribution of poly (DATC) with and without MWCNT in DI water.
Figure 7: The response of poly (DATC) –MWCNT sensor to 1 M concentration of nitrate ions in DI water.
Figure 8:Response of poly (DATC)-MWCNT to various concentrations of nitrate ions.
Figure 9:Boxplot showing the reproducibility of the poly (DATC)-MWCNT sensors.
Figure 10:Sensor response to various anions.
Figure 11:Reproducibility of the response to various ions.
Figure 12:Response of sensor to cations with common nitrate ion.
Figure 13:Reproducibility of the sensor response to cations.

DETAILED DESCRIPTION OF INVENTION
It is to be understood that this description is not intended to limit the invention to the particular embodiments described in the detailed description, drawings, and claims herein. The disclosure covers all modifications and equivalents that fall within the ambit of the invention. Reference is made to the accompanying figures, which form a part hereof. In the figures, similar symbols typically identify similar components, unless context dictates otherwise.

It may further be noted that as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by person skilled in the art.

The present invention is in relation to the detection of nitrate ions in aqueous medium of a test sample by a resistance based sensor, termed as chemiresistor, wherein a composition comprising novel conjugated polymer of present invention and carbonaceous material optionally with a binding agent like paraffin oil, is applied on to electrodes of PCB as sensor material. The resistance of these materials gets altered in the presence of nitrate thus identifying its presence.Hence the method of detecting nitrate ion with conducting polymer involves detecting change in resistance of the conducting polymer. The carbonaceous substance enhances the signal for its detection.
The present invention is thus in relation to a compound of formula 1

Formula 1

wherein,
R1 is selected from a group comprising C2-C12 conjugated substituted alkyl, aryl, heteroaryl, substituted aryl, and substituted heteroaryl;
R2 is selected from a group comprising C2-C12 aryl, heteroaryl, substituted aryl, and substituted heteroaryl;
Z is selected from a group comprising O, S, and N; and
n is in the range of 100 to 100000, specifically 10000 to 20000;
wherein,
the conjugated substituted alkyl, substituted aryl, and substituted heteroaryl are substituted by moiety selected from a group comprising, thiophene, alkyl thiophenepyyrole, carbazole, indole, phenyl, benzene, and pyridine.

The present invention is also in relation to compound of formula 2

Formula 2
Wherein n is 10000-20000

The present invention is also in relation to a method of preparation of compound of formula 2, said method comprising steps of

Formula 2
Wherein n is 10000-20000

d) Preparation of compound of formula 3, comprising steps of

Formula 3
iii) treating diphenyl amine with 1-bromododecane in a solvent in the presence of a catalyst to obtain 1-N-dodecyl diphenylamine; and
iv) treating the 1-N-dodecyl diphenylamine with phosphorousoxychloride in DMF to obtain compound of formula 3;
e) treating the compound of formula 3 with 1,3-diaminothiourea in a solvent in presence of a catalyst and filtering to obtain the compound of formula 2; and
f) optionally purifying to obtain the pure compound of formula 2.

The present invention is also in relation to a composition for detecting nitrate ion in a test sample, said composition comprising compound of formula 1

Formula 1

wherein,
R1 is selected from a group comprising C2-C12 conjugated substituted alkyl, aryl, heteroaryl, substituted aryl, and substituted heteroaryl;
R2 is selected from a group comprising C2-C12 aryl, heteroaryl, substituted aryl, and substituted heteroaryl; and
Z is selected from a group comprising O, S, and N;
n is ranging from 100 to 100000 specifically 10000 to 20000;
wherein,
the conjugated substituted alkyl, substituted aryl, and substituted heteroaryl are substituted by a moiety selected from a group comprising, thiophene, alkyl thiophenepyyrole, carbazole, indole, phenyl, benzene, and pyridine;
and carbonaceous material and binder in the ratio ranging from about 2:98:0 w/w to about 20:60:20w/w.
In an embodiment of the present invention the carbonaceous material is selected from a group comprising carbon black, carbon nanotubes, fullerenes, whiskers, wires, fibres, filaments, graphite and mixture thereof.
The present invention is also in relation to a method of preparation of composition for detecting nitrate ion in a test sample, said composition comprising compound of formula 1

Formula 1

wherein,
R1 is selected from a group comprising C2-C12conjugated substituted alkyl, aryl, heteroaryl, substituted aryl, and substituted heteroaryl;
R2 is selected from a group comprising C2-C12 aryl, heteroaryl, substituted aryl, and substituted heteroaryl; and
Z is selected from a group comprising O, S, and N;
n is ranging from 100 to 100000 specifically 10000 to 20000
wherein,
the conjugated substituted alkyl, substituted aryl, and substituted heteroaryl are substituted by moiety selected from a group comprising, thiophene, alkyl thiophenepyyrole, carbazole, indole, phenyl, benzene, and pyridine.
and carbonaceous material and binder in the ratio ranging from about 2:98:0 w/w to about 20:60:20w/w, said method comprising acts of
d) preparing a solution of the compound of formula 1 in a solvent and agitating the solution,
e) adding the carbonaceous material to the agitated solution to obtain a mixture, and
f) agitating the mixture to obtain the composition.

The present invention is also in relation to a composition for detecting nitrate ion in a test sample, said composition comprising compound of formula 2

Formula 2
wherein n is 10000 to 20000 and carbon nanotubes and binder in the ratio ranging from about 98:2:0 w/w to about 60:20:20 w/w.

The present invention is also in relation to a method of preparation of composition for detecting nitrate ion, said composition comprising compound of formula 2

Formula 2

wherein n is 10000 to 20000 and carbon nanotubes and binder is in ratio ranging from about 98:2:0 w/w to about 60:20:20 w/w, said method comprising acts of
a) preparing a solution of the compound of formula 2 in a solvent and agitating the solution,
b) adding the carbon nanotubes to the agitated solution to obtain a mixture, and
c) agitating the mixture to obtain the composition.

The present invention is also in relation to a sensor for detecting nitrate ions comprising composition of present invention coated on electrodes of a circuit.
In an embodiment of the present invention, the composition of claim 4 or 7 is coated on electrodes of a single layered printed circuit board.
In another embodiment of the present invention, the electrodes are selected from a group comprising silver, copper, gold, platinum, graphite, Nickel, Tin, and Chrome.
The present invention is also in relation to a method of fabrication of a sensor of present invention for detecting nitrate ions said method comprising steps of
a) preparing composition of present invention;
b) applying the composition onto the electrodes of the circuit; and
c) drying to obtain the sensor.

In an embodiment of the present invention, the composition is applied on to the electrodes by a method selected from dropcasting, dip coating, spin coating, spray coating, pad printing, ink jet printing, screen printing, brush printing, gravure printing, and roll to roll printing.
In another embodiment of the present invention, the drying is carried out at a temperature ranging from about 60°C to about 80°C, preferably at about 70°C.
In still another embodiment of the present invention, the sensor is of size ranging from about 1mm X 1mm to about 5mm X 5 mm.
The present invention is also in relation to a method of detection of nitrate ions with sensor, said method comprising steps of
a) dipping the sensor in test sample to be tested for nitrate ions; and
b) measuring change in resistance of the sensor to detect nitrate ions.
Definitions:
Carbonaceous substance includes but not limiting to carbon black, carbon nanotubes, fullerenes, conductive whiskers, wires, fibers, filaments, graphite other carbonaceous materials and mixtures thereof.
Binding agent includes but not limiting to paraffin oil, low molecular weight siloxane, naphthalene oil, wax, and nujol.
Planar NH- moiety includes but not limiting to moieties A-D indicated below.

Test sample includes but not limiting to water, aqueous solutions like beverages, treated industrial effluents, pharmaceutical reagents, water soluble edible food and the like.
Detection of nitrate ions requires suitable moieties that that can attract and collect the nitrate ions and the molecules comprising them that can signal the recognition event. The design of molecules that recognize ions require consideration of the properties of ions such as shape and size of the ions, the medium in which interaction takes place. Cations are spherical and anions have various geometries. Anion recognition requires Lewis acidic sites and hydrogen bonding functionalities. The larger size of anions also requires molecules to possess a much larger binding site than required for cation recognition. Molecules with –NH are capable of hydrogen bonding to anions. The moieties containing –NH indicated above are used in the present invention.
The invention involves development of polymer molecules of formula 1 wherein incorporation of aforementioned moieties into the backbone of a polymer that would favour the binding of anions. The anions can bind in a particular fashion onto the host molecule. The event of binding of planar structure of the polymer, nitrate ion can be signal by the change in the resistance of the polymer. The conducting polymer with anion recognition moiety can signal the event of binding by change of conductivity.

Formula 1
wherein,
R1 is selected from a group comprising C2-C12 conjugated substituted alkyl, aryl, heteroaryl, substituted aryl, and substituted heteroaryl;
R2 is selected from a group comprising C2-C12 conjugated alkyl, aryl, heteroaryl, substituted aryl, and substituted heteroaryl; and
Z is selected from a group comprising O, S, and N;
n ranges from 100 to 100000 more specifically 10000 to 20000
wherein,
the conjugated substituted alkyl, substituted aryl, and substituted heteroaryl are substituted selected from a group comprising thiophene, alkyl thiophene, pyyrole, carbazole, indole, phenyl, benzene, and pyridine.
In an embodiment of the present invention a polymer, poly[N'-(4-(dodecyl(phenyl)amino)benzylidene)-dithiocarbohydrazone] referred as poly(DATC) (formula 2) is prepared.
The mechanism of interaction of poly (DATC) and with nitrate ion is given in scheme1.

n=10000 -20000.
Formula 2: The structure poly (DATC).

Scheme 1: Mechanism of binding of nitrate ion to poly (DATC).
The carbonaceous substance in the composition enhances the response of the conjugated polymer, making it a low resistance sensor. The resistance of polymer is few giga ohms. The change in the resistance on introducing nitrate ions would not be observable because of the noise. Adding conductive carbonaceous particles such as carbon black, carbon nano tubes, and graphite powder decreases the resistance of the polymer composite, they have also shown to increase the response of the polymer resistive based sensors at their percolation threshold as shown in figure 6 and also preserve the integrity of the film.
EXPERIMENTATION AND RESULTS
Dimethyl propylene urea (DMPU) is obtained from Spectrochem Pvt. Ltd. Potassium salts of nitrate, nitrite, fluoride, chloride, dihydrogen phosphate, carbonate, sodium nitrate and calcium nitrate used are of reagent grade and were used as received. All solutions are prepared using de ionized water with a resistivity of 18 MO cm.
Carbon nanotube, graphite powder and carbon black were obtained commercially.
A. Synthesis of poly(DATC)
Example 1
Synthesis of compound 1-N-dodecyl diphenylamine
Sodium hydride (0.72g) is added to the solution of diphenylamine (5 g)dissolved in 50 mL DMF) and the resulting mixture is stirred for approximately 30 min. 1-Bromo dodecane(7.35 g) is then slowly added to the reaction mixture, and the mixture is stirred for 5 h at room temperature. After completion of the reaction, the resulting mixture is extracted with ethyl acetate/brine and then dried with Na2SO4. The solvent is removed by evaporation. The resulting crude product is purified by column chromatography using hexane and ethyl acetate. Yield 82 %.
Synthesis of N-dodecyl-diphenylamine-4,4'-dicarbaldehyde 1
Freshly distilled phosphorous oxychloride (12.6 ml) is added drop-wise to 5.5 ml of anhydrous DMF at 0 °C over a period of 30 min. Later, 1-N-dodecyl diphenylamine(5 g, in 20 ml of 1,2-dichloroethane) is added to the above solution and stirred at 90 °C for 48 h. This solution is cooled to room temperature, poured into ice water, and neutralized to pH 6–7 by the drop-wise addition of saturated sodium hydroxide solution. The dialdehyde 1 is extracted with ethyl acetate. The organic layer was dried with anhydrous Na2SO4, and the solvent is subsequently removed under reduced pressure. The crude product is purified by column chromatography. A light-orange solid is obtained. Yield: 71 %.
Synthesis of polymer Poly (DATC)
To a stirred solution of compound 1 (0.5 g) and 1, 3-diaminothiourea (2)(0.13 g) in ethanol, catalytic amount of concentrated sulphuric acid is added under nitrogen atmosphere. The reaction mixture is refluxed for 12 h. The reaction mixture was cooled to room temperature and the solid is filtered off. The crude product is washed with water and hot ethanol. The solid yellow powder is dried under vacuum at 40° C for overnight. Yield- 60 %, melting point> 125 °C.
The reaction is schematically represented in scheme 2

Scheme-2

Example 2
9-butyl carbazole (3) istreated with vilsmeir reagent in dichloroethane and heated to reflux to get 9-butyl-9H-carbazole-3,6-dicarbaldehyde(4). 4 is polymerized with 2 in ethanol under acid catalyzed conditions to get the desired product (5).

Example 3
3,5 dimethyl pyridine (6) is oxidized with KMnO4 to get pyridine-3,5-dicarboxylic acid(7), which is later reduced to get pyridine-3,5-dicarbaldehyde (8). Pyridine-3,5-dicarbaldehyde (8 ) is polymerized with 2 in ethanol under acid catalyzed conditions to get the desired product (9).

B) Sensor design and fabrication.
The design of a single layer sensor element is as shown in Figure 1. Silver-coated copper electrodes printed on single layer printed circuit board (PCB) are procured locally. Three sensor elements are printed over 2.7 cm x 2.5 cm substrates of single layer PCB of thickness 0.15 cm using removable masks. Individual sensor elements have a space of 0.1 cm width between two electrodes.

a) Sensor fabrication steps and testing
(i) Preparation of the composition of polymer and carbonaceous material
94 wt % solution of Poly (DATC) in DMPU is prepared and sonicated for ~ 4 min.
6 wt % multiwalled carbon nanotubes (MWCNT) is added to this solution and sonicated for ~ 30 min to make 8 wt % of poly (DATC)–MWCNT stock solution. The single layer PCB substrate is masked with Kapton tape to isolate each sensor element and to obtain the same size of the sensor element.8 µl of said solution is drop casted over the electrodes. The films are dried in oven for ~ 4 h at 70 °C. The size of the sensing element is 3mm × 3 mm. The final sensor device is shown in the Figure 2.
ii) Testing using sensor
The sensors are connected to the differential multiplexer with 6-pin edge connector.The change in resistance is measured using Keithley Instruments Inc, model 2700 multimeter/data acquisition (DMM) with model 7700 differential multiplexer with 20 channels.
The percentage change of sensor response is given by equation (1).
[(Ri-Ro)/Ro] × 100 or dR/Ro ×.100 ……… (1)
Where dR= (Ri-Ro), Ro is the value of the sensor signal in water in the absence of anions, and Riis the sensor signal observed in water during presence of each ion i.e., Ri is the sensor response.
The sensor response is measured at 25° C and pH 7. To test for anions, the inorganic salts used are potassium salts of nitrate, chloride, dihydrogen phosphate, fluoride, carbonate nitrite. And for cations potassium nitrate, sodium nitrate and calcium nitrate are used. For testing ammonia, ammonia solution was used. To maintain pH 7 in DI water, buffer tablets pH 7 is used except for Ca (NO3)2 solution, buffer tablet in DI water is not used as calcium nitrate is not soluble. Calcium nitrate is soluble in DI water.

B. Sensor characterization
(i) Sensor with poly(DATC) and different carbonaceous materials
The resistance of the sensoris measured in DI water (pH 7). This is the base line resistance. When nitrate ion are introduced the resistance of the film decreased. When the nitrate ions are withdrawn the resistance recovered as demonstrated in figure 3 (i)- for carbon black, figure 3 (ii) for MWCNT and figure 3 (iii) for graphite powder.
For CNT (Figure 3 (ii)) the initial resistance is 250 k ?in water and it decreased to ~100 k? after introducing nitrate ions.
For graphite powder also(figure 3(iii))a similar response is observed.For graphite powder the current is measured by applying 1.2 V. The current is measured in DI water and then measured in 1 M nitrate ions. The current increased in presence of nitrate ions.
(ii) Testing with Different compositions of the poly (DATC), various carbonaceous materials (graphite powder, MWCNT and carbon black) and binder.
Different compositions of the poly (DATC), Graphite powder and binder are used in the sensor to detect and test the nitrate ion concentration and the results are tabulated in Table 1, Figure 3 (i-iii). Accordingly it is found that the change is resistance is maximum wherein the composition is 0.5%w/w of poly (DATC), 89.5% w/w of graphite powder and 10%w/w binder.
Table 1- change in resistance of different compositions of poly (DATC), various carbonaceous materials (graphite powder, MWCNTand carbon black) and binder
Composition Current (?A) Resistance (M O) Change in resistance (%)
Blank 1 M KNO3 Blank 1M KNO3
(a) (0.5-89.5) wt. % poly (DATC)– Graphite powder (10 wt % binder) -0.68 -59 1.74 0.0204 98
(b) (1 -89)wt % poly (DATC)-graphite powder (10 wt % binder) -37 -130 0.032 0.00955 70
(c) 2 -78 wt. %poly (DATC)-graphite powder (20 wt % binder) -14.9 -100 0.08 0.011 85
(d) (100 -0) wt % poly (DATC)-MWCNT (0 wt % binder) 0.56 37.1 2.14 0.03 98
(e) (94 -6 ) wt % poly (DATC)-MWCNT (0 wt % binder) 0.23 0.06 73
(f) (73 – 27) wt. % poly (DATC) –carbon black(0 wt % binder) 460 (Ohm) 561 (Ohm) 22

Further fabrication and testing of the sensor are carried out with the composition comprising 94 % w/w of poly (DATC) and 6% w/w of MWCNT and 0% w/w binder.

C. Sensor characterization

(iii)Resistance of poly(DATC) and poly (DATC)-MWCNT
The two-probe resistance of poly (DATC) is measured in DI water. The resistance is ~ 500 kO. In order to obtain a low resistance sensor, MWCNT is loaded in the polymer figure 5 shows the electron microscope pictures of the MWCNT in poly (DATC) matrix.The resistance of the polymer decreases to ~200 kO. The resistance of poly (DATC) and poly (DATC)-MWCNT is shown in Figure 6.

Sensor response measurement
The response of poly (DATC) –MWCNT sensor to 1 M concentration of nitrate ions in DI water is shown in Figure 7. The resistance of poly (DATC) sensor is measured in DI water (pH 7) for 15 min. This is the base line resistance. The DI water is withdrawn and ion solution is introduced. The resistance of the sensor decreases on introducing the ion containing solution. This is the response of the sensor. The change in resistance of the sensor is monitored. On removal of the water containing nitrate ions and replacing it with fresh DI water, the resistance of the sensor comes back slowly to its original position.
Response to low concentrations of nitrate ions
The response of the sensor for various concentrations of nitrate ions is shown in Figure 8.
It is observed that the response of the sensor decreases with the decrease in concentration of nitrate ions. The sensor response upto 10-3 M concentration is observable and the change in resistance is ~ 1 %. The sensor doesnot show response for 10-4 M concentration of nitrate ions. The sensor response is observed to be reproducible if the loading of the MWCNT in the composite and thickness of the sensing film is same.

As poly (DATC)-MWCNT sensor is a resistance-based sensor, care is taken to have the resistances of all sensors as same. However the agglomeration of MWCNT and the film thickness will cause change in percolation path and hence change in resistance. Therefore sensors with certain resistance range are chosen. Since it is important to monitor the change in resistance without and with nitrate ions, the resistance of the poly (DATC) - MWCNT sensor without ions is monitored in DI water. The response of the sensor is repeatable for those with the resistance of in DI water in the range ~ 150-250 kO. The change in the resistance and the response time to reach the maximum change in resistance is given in Table 2.
Table 2: The maximum change in resistance and the response time to reach the maximum change in resistance.
Concentration (M) Nitrate ion concentration (ppm) dRp/Ro (%) Response time (s)
1 62000 70.8 28
0.1 6200 46.6 72
0.01 620 5.16 43
0.001 62 1.18 43

The reproducibility of sensors is shown in Figure 9. The response of the sensor for 1M concentration of nitrate ion is observed to be ~ 64 % , 0.1 M is ~ 42 %, 0.01 M is ~ 3 % and for 0.001 M is ~ 1 %for various concentration of nitrate ions.
(iv) Cross sensitivity of the poly (DATC)-MWCNT sensor to various ions.
(a) Response for various anions
The poly (DATC) -MWCNT sensor is tested for various anions other than nitrate ions like fluoride, chloride, nitrite, carbonate and phosphate ions for studying interference effect.Table 3 and Figure 10 shows a plot of change in resistance of the sensors for various anions. Here the response to various ions is observed by the change in the ?R/Ro. It can be observed that the maximum response is for fluoride ions and least for observed for dihydrogen phosphate ions. It is generally observed that the sensors for nitrate use indirect determination. That is the nitrate is reduce to nitrite and then detection of nitrite is used for detection of nitrate ion. With this method of detection, both nitrate and nitrite ion can be detected. The response for nitrate is ~ 70 % and for nitrite is ~60 %. The boxplot in Figure 11 shows the reproducibility of poly (DATC)-MWCNT sensor for the anions. The invention is able to clearly differentiate between nitrate and any other anions present.Thus the sensor is capable of differentiating from other anions present in a test sample.

Table 3: The ionic strength, maximum change in resistance and the response time of the sensor.

Anion Ionic strength dRp/Ro (%) Response time (s)
Nitrate 0.5 69.4 27
Nirtite 0.5 58.9 69
Ammonia 0.5 46.5 100
Fluoride 0.5 84.6 80
Chloride 0.5 55.62 90
Carbonate 2 19.67 142
Dihydrogen phosphate 0.5 20.24 248

(ii) Response of various cations
The effect of cations is also tested on the sensor. The test is carried out for potassium, sodium and calcium ions with 1M nitrate ions being common with1M concentration of other ions. It can be observed from Table 4, Figure 12 that the response of the poly (DATC) –MWCNT sensor for the three types of cations is almost same. From figure 13it can be observed that the response for cationsis same as that for nitrate ions. This shows that the cation do not affect the sensor response to nitrate. In other words the response observed is because of the nitrate ions. Figure 13 shows the sensor response is reproducible.
Table 4: The concentration of cation and ionic strength in the 1 M nitrate ion solution
Cation Concentration (ppm) Ionic strength
Potassium 39090 0.5
Sodium 22980 0.5
Calcium 40070 2

Thus the present invention provides simple method using simple architecture.The method of detection is a direct method for detection of nitrate ions and distinguish between other ions. The response time of the sensor to nitrate ion has decreased. The sensor is commercially viable as it involves low cost reagents with simple fabrication process and is one time useable devoid of any preconditioning of the sensor.