PARA-AMINOBENZOIC ACID SENSITIZED TERBIUM DOPED LAF3 NANOPARTICLES FOR DETECTION OF EXPLOSIVE NITRO COMPOUNDS

Abstract

The patent relates to para amino benzoic acid (pABA) sensitized terbium (Tb.sup.3+) doped spherical LaF.sub.3 nanoparticles used for detection of nitro group containing compounds using the terbium (Tb.sup.3+) doped spherical LaF.sub.3 nanoparticles sensitized by para amino benzoic acid (pABA).

Claims

1. Para amino benzoic acid (pABA) sensitized terbium (Tb.sup.3+) doped spherical LaF.sub.3 nanoparticles comprising pABA in the ratio of 1:1.

2. A process for preparation of para amino benzoic acid (pABA) sensitized terbium (Tb.sup.3+) doped spherical LaF.sub.3 nanoparticles and the said process comprising the steps of: i. mixing of lanthanum nitrate hexahydrate (La(NO.sub.3).sub.3.6H.sub.2O) and terbium nitrate pentahydrate (Tb(NO.sub.3).sub.3.5H.sub.2O) in the ratio ranging between 2 to 10 wt. %; ii. adding citric acid solution to the solution as obtained in step (i) at temperature in the range of 60-70° C.; followed by adding NH.sub.4F to obtain the reaction mixture; iii. refluxing the reaction mixture at temperature in the range of 100 to 110° C. for period in the range of 100 to 120 minutes followed by cooling and drying at temperature in the range of 24-30° C. to obtain nanoparticles; iv. functionalizing the nanoparticles as obtained in step (iii) by dispersing in water followed by adding p-aminobenzoic acid solution at temperature in the range of 60 to 65° C. and refluxing for period in the range of 100 to 120 minutes at temperature in the range of 70 to 75° C. to obtain surface-functionalized nanoparticles.

3. The nanoparticles as claimed in claim 1, wherein the said nanoparticles are useful for detection of nitro group containing compounds by determining the quenching of fluorescence of terbium (Tb.sup.3+) doped spherical LaF.sub.3 nanoparticles by the addition nitro group containing compound.

4. The nanoparticles as claimed in claim 1, wherein the detection level of the nitro compounds in the range of 0.04 to 10 ppm.

5. The method as claimed in claim 3, wherein the nitro group containing compound is selected from aromatic or aliphatic compounds.

6. The method as claimed in claim 3, wherein the nitro group containing compounds are selected from nitrobenzene(NB), o-nitrophenol(2-NP), o-nitrotoluene(2-NT), 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT), 2,4-dinitrophenol(2,4-DNP), picric acid(PA) and 2,4,6-trinitrotoluene (TNT), nitromethane (NM), 1,2,4-butanetriol nitrate (BTTN), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and 1,3,3-Trinitroazetidine(TNAZ).

7. The method as claimed in claim 3, wherein the order of fluorescence quenching is 2,6-dinitrotoluene (2,6-DNT)>o-nitrophenol(2-NP)>2,4-dinitrophenol(2,4-DNP)>nitrobenzene(NB), 2,4,6-trinitrotoluene (TNT)>picric acid(PA)>1,3,5-trinitroperhydro-1,3,5-triazine (RDX)>2,6-dinitrotoluene (2,6-DNT)>>o-nitrotoluene(2-NT)>1,3,3-Trinitroazetidine(TNAZ)>octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)>nitromethane (NM), 1,2,4-butanetriol nitrate (BTTN) and the values of quenching constant (k.sub.Q), obtained for TNT, PA, 2-NP, 2,4-DNT, 2,4-DNP are 12295, 5738, 1683, 3296, 2103M.sup.−1 respectively.

8. The method as claimed in claim 3, wherein the pABA functionalized terbium (Tb.sup.3+) doped spherical LaF.sub.3 nanoparticles shows 100-120 times enhancement in the luminescence intensity in comparison to direct excitation of Tb.sup.3+ ion.

9. The doped, sensitized spherical nanoparticles as claimed in claim 1, wherein said nanoparticles are used for detection of pH acidic or alkaline solution and the said nano particles possess UV-absorption peak at 265 at pH=3.

10. The terbium (Tb.sup.3+) doped spherical LaF.sub.3 nanoparticles as claimed in claim 1, wherein the life time value of the surface functionalized nanoparticles in absence of an analyte is 0.1177 ns and in presence of picric acid analyte in the concentration of 18, 61.25 and 125 ppm are 0.1158, 0.0883, and 0.0806 ns respectively.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIG. 1 depicts XRD (X-ray diffraction) and IR spectra of LaF.sub.3:Tb.sup.3+ nanoparticles with and without pABA along with the JCPDS Card No. 82-0690.

[0041] FIG. 2 depicts the morphology of LaF.sub.3:Tb.sup.3+@pABA nanoparticles (a-c) along with (d) its SAED (Selected Area Electron Diffraction) and (e) its EDAX (Energy Dispersive X-ray Analysis) spectra. Rod shape morphology of higher dopant concentration is also shown (f).

[0042] FIG. 3 depicts (a) Absorption, (b) excitation and (c) emission spectra of LaF.sub.3:Tb.sup.3+ with and without pABA along with (d) schematic diagram for the energy transfer in LaF.sub.3:Tb.sup.3+@pABA nanoparticles. Excitation and emission wavelength are fixed at 265 and 546 nm, respectively

[0043] FIG. 4 depicts Effect of pH on the emission spectra LaF.sub.3:Tb.sup.3+@pABA nanoparticles at λ.sub.ex=265 nm. Inset shows the integrated area under the curve for 546 nm peak.

[0044] FIG. 5 depicts Emission spectra of LaF.sub.3:Tb.sup.3+@pABA nanoparticles at λ.sub.ex=265 nm with the change in concentration of (a) PA and (b) TNT. Inset shows their respective Stern-Volmer plots.

[0045] FIG. 6 depicts Emission spectra of LaF.sub.3:Tb.sup.3+@pABA nanoparticles at λ.sub.ex=265 nm with the change in concentration of (a) NP, (b) DNP and (c) DNT. Inset shows their respective Stern-Volmer plots.

[0046] FIG. 7 depicts Bar graph showing the quenching efficiencies (%) for all the analyzed analyte at their fixed concentration of 0.1 mM.

[0047] FIG. 8 depicts overlap of emission spectrum of pABA with UV spectra for (a) NP, 2,4-DNP, PA, DNT and (b) 2,6-DNT, TNT, RDX.

[0048] FIG. 9 depicts Histogram showing the particle size distribution (DLS (Dynamic Light Scattering) measurement) of LaF.sub.3:Tb.sup.3+@pABA nanoparticles

[0049] FIG. 10 depicts Absorption spectra of LaF.sub.3:Tb.sup.3+@pABA nanoparticles at different pH.

[0050] FIG. 11 depicts Emission spectra of LaF.sub.3:Tb.sup.3+@pABA nanoparticles at different concentration of Tb.sup.3+ ions.

[0051] FIG. 12 depicts Emission spectra of LaF.sub.3:Tb.sup.3+@pABA nanoparticles at λ.sub.ex=265 nm with the change in concentration of TNT (ppb level). Inset shows their respective Stern-Volmer plots.

[0052] FIG. 13 depicts Energy level diagram of all the selected nitro compounds.

[0053] FIG. 14 depicts Decay profile for LaF.sub.3:Tb.sup.3+@pABA nanoparticles at varying concentration of (a) PA and (b) TNT.

[0054] FIG. 15 depicts colour change of the particles in presence of explosives under UV light irradiation.

DETAILED DESCRIPTION OF THE INVENTION

[0055] The present invention provide para amino benzoic acid (pABA) sensitized terbium (Tb.sup.3+) doped spherical LaF.sub.3 nanoparticles and a method for the detection of highly explosive nitro compounds and determination of pH of a solution utilizing the para amino benzoic acid (pABA) sensitized terbium (Tb.sup.3+) doped spherical LaF.sub.3 nanoparticles.

[0056] The present invention provides a process for the detection of nitro group containing compound using the terbium (Tb.sup.3+) doped spherical LaF.sub.3 nanoparticles comprising of the steps of: [0057] a) determining the flourescence of terbium (Tb.sup.3+) doped spherical LaF.sub.3 nanoparticles; [0058] b) adding para amino benzoic acid (pABA) to terbium (Tb.sup.3+) doped spherical LaF.sub.3 nanoparticles of step (a) in the ratio ranging between 2 to 10 wt. % and determining the enhanced fluorescence; and [0059] c) adding the sample of step (b) with the nitro group containing compound and determining the quenching of fluorescence of terbium (Tb.sup.3+) doped spherical LaF.sub.3 nanoparticles by nitro group containing compound.

[0060] The present invention provides a process wherein the nitro group containing compound is selected from aromatic or aliphatic compounds.

[0061] The present invention provides a method for the detection of pH of an unknown acidic or alkaline solution by studying the variation of the photoluminescence properties of the para amino benzoic acid (pABA) sensitized LaF.sub.3:Tb.sup.3+ nanomaterials at different pH.

[0062] The present invention provides para amino benzoic acid (pABA) functionalized LaF.sub.3:Tb.sup.3+ nanoparticles (FIG. 3a) with strong broad UV-absorption peak having maximum at 265 nm having absorption intensity highest at pH=3.

[0063] The pABA functionalized nanoparticles show remarkable (>100 times) enhancement in the luminescence intensity as compared to direct excitation of Tb.sup.3+ion as shown in FIG. 3c. The detection level for the selected nitro compound is in the range of 0.04-10 ppm. The surfaced functionalised nanomaterials is able to detect TNT up to 50 ppb.

[0064] The technique of utilizing the Tb.sup.3+ doped NPs sensitized by para amino benzoic acid (pABA) have potential application in the detection of explosives.

[0065] The present invention provides Tb.sup.3+ doped nanoparticles which are highly sensitive as well as selective to the aromatic nitro compounds as compared with the aliphatic nitro compounds.

[0066] The present invention provides detection of nitro group containing compounds selected from the group consisting of nitrobenzene(NB), o-nitrophenol(2-NP), o-nitrotoluene(2-NT), 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT), 2,4-dinitrophenol(2,4-DNP), picric acid(PA) and 2,4,6-trinitrotoluene (TNT), nitromethane (NM), 1,2,4-butanetriol nitrate (BTTN), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and 1,3,3-Trinitroazetidine(TNAZ).

[0067] The present invention provides the order of fluorescence quenching as follows: 2,6-dinitrotoluene (2,6-DNT)>o-nitrophenol(2-NP)>2,4-dinitrophenol(2,4-DNP)>nitrobenzene(NB), 2,4,6-trinitrotoluene (TNT)>picric acid(PA)>1,3,5-trinitroperhydro-1,3,5-triazine (RDX)>2,6-dinitrotoluene (2,6-DNT)>>o-nitrotoluene(2-NT)>1,3,3-Trinitroazetidine(TNAZ)>octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)>nitromethane (NM), 1,2,4-butanetriol nitrate (BTTN). (FIGS. 8 and 13)

[0068] The present invention provides the values of quenching constant (k.sub.Q), obtained using above Stern-Volmer equation for TNT, PA, 2-NP, 2,4-DNT, 2,4-DNP which are as follows 12295, 5738, 1683, 3296, 2103M.sup.−1.

[0069] Life time value of the surface functionalized nanoparticles in absence of any analyte is 0.1177 ns. Further, in presence of 18, 61.25 and 125 ppm picric acid analyte life time values of the surface functionalized nanoparticles are 0.1158, 0.0883, and 0.0806 ns (nanosecond) respectively.

[0070] Life time values of the surface functionalized nanoparticles in presence of 10, 20 and 60 ppm TNT analyte are 0.0790, 0.0889, and 0.0785 ns respectively. (Refer FIG. 14)

EXAMPLES

[0071] Following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention.

Reagents and Materials

[0072] Lanthanum nitrate hexahydrate (La(NO.sub.3).sub.3.6H.sub.2O) (99.99%), terbium nitrate pentahydrate (Tb(NO.sub.3).sub.3.5H.sub.2O) (99.9%), Ammonium fluoride (NH.sub.4F) (99.99%) were purchased from Aldrich. Anhydrous citric acid (99.5%) and dimethyl sulfoxide (99.9%) were purchased from Merck. p-aminobenzoic acid from SDFCL and picric acid (PA) obtained from Aldrich. All the others nitro compounds listed below are obtained from DRDO-HEMRL, Pune. Deionised water was used to make aqueous solutions. All the materials were used as received.

Example 1

Synthesis of LaF.SUB.3.:Tb.SUP.3+ Spheres

[0073] Tb.sub.xLa.sub.1-xF.sub.3 (where x=0.02, 0.04, 0.06, 0.08 and 0.1) nanoparticles were synthesized. 3 g of anhydrous citric was dissolved in 20 ml of water in 250 ml RB (Round bottom) flask. 60 ml of DMSO (Dimethyl sulphoxide) and 1 ml conc. NH.sub.4OH were added to the citric acid solution to adjust the pH to 5 and stirred nicely. About 2 mmol (depending upon doping conc.) of lanthanum nitrate hexahydrate (La(NO.sub.3).sub.3.6H.sub.2O) and stoichiometric amount of terbium nitrate pentahydrate (Tb(NO.sub.3).sub.3.5H.sub.2O) were dissolved in 2 ml water. This lanthanide solution was added drop wise to the citric acid solution at 70° C. temperature. A dense white turbidity appeared. White dense turbidity converted to a pale white suspension when 3 ml aq. solution of 7 mmol NH.sub.4F was added slowly. The reaction mixture was refluxed at 110° C. temperature under water circulation for two hours and then cooled to 30° C. The nanoparticles were collected by centrifugation, washed with deionized (DI) water and methanol, and dried at 30° C.

Surface-Functionalization of Nanoparticles

[0074] 0.2 g of the resulting nanoparticles was dispersed in 50 ml water and formed a colloidal solution. 0.2 g of p-aminobenzoic acid was dissolved in 30 ml NaOH solution. This p-aminobenzoic acid solution was added slowly to the nanoparticles colloid at 65° C. and refluxed for two hours at 75° C. The surface-functionalized nanoparticles were separated by centrifugation, washed twice with water and methanol and dried at 30° C.

Example 2

Preparation of Analyte Solution

[0075] 500 mg picric acid was dissolved in 1000 ml water to prepare a stock solution of 500 ppm (2.18 mM) strength. This mother solution was followed by a two-fold serial dilution to prepare the solutions of 250, 125, 62.5, 31, 15.5, 8, 4, 2 ppm concentrations. These analyte solutions were mixed with equal volume of aqueous dispersion of the nanoparticles and analyzed, i.e the analyte concentrations in the experimental solutions were again diluted to half concentration.

Example 3

[0076] 3 mg of 2,4,6-trinitrotoluene was dissolved in 25 ml water to prepare a 120 ppm (0.52 mM) stock solution. This solution was diluted with water to prepare 100 ppm, 80 ppm, 60 ppm, 40 ppm, 20 ppm solutions which on mixing with equal volume dispersion of nanoparticles gave the experimental solutions.

Example 4

[0077] 7 mg of nitrophenol was dissolved in 250 ml of water to prepare a 0.2 mM stock solution (28 ppm) This solution was diluted with water to prepare 14 ppm, 7 ppm, 3.5 ppm, 1.75 ppm solution which on mixing on mixing with equal volume dispersion of nanoparticles gave the experimental solutions.

Example 5

[0078] 9 mg of 2,4-dinitrophenol was dissolved in 250 ml of water to prepared a 0.2 mM stock solution (36 ppm) This solution was diluted with water to prepare 18 ppm, 9 ppm, 4.5 ppm, 2 ppm solution which on mixing with equal volume dispersion of nanoparticles gave the experimental solutions.

Example 6

[0079] 9 mg of 2,4-dinitrotoluene was dissolved in 250 ml of water to prepared a 0.2 mM stock solution (36 ppm) This solution was diluted with water to prepare 18 ppm, 9 ppm, 4.5 ppm, 2 ppm solution which on mixing with equal volume dispersion of nanoparticles gave the experimental solutions.

Example 7

[0080] 6.9 mg of o-nitrotoluene was dissolved in 250 ml water to prepare a 0.2 mM stock solution.

Example 8

[0081] 9 mg of nitrobenzene was dissolved in 250 ml water to prepare a 0.2 mM stock solution.

Example 9

[0082] 9 mg of nitromethane was dissolved in 250 ml water to prepare a 0.2 mM stock solution.

Example 10

[0083] 12 mg of 1,2,4-butanetriol nitrate was dissolved in 250 ml water to prepare a 0.2mM stock solution.

Example: 11

[0084] 9.6 mg of 1,3,3-Trinitroazetidine was dissolved in 250 ml water to prepare a 0.2mM stock solution.

Example 12

[0085] 11 mg of RDX was dissolved in 250 ml water to prepare a 0.2mM stock solution.

Example 13

[0086] 15 mg of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine was dissolved in 250 ml water to prepare a 0.2mM stock solution.

Characterization

1. XRD Study

[0087] All the samples analyzed (before and after surface functionalization) had clear resemblance with the tysonite structure of LaF.sub.3 (JCPDS card 82-0690) as shown in FIG. 1(a-c). The average crystallite size of the nanoparticles were calculated using Scherrer equation (d=kλ/β cos θ), where λ is the X-ray wavelength (0.154 nm), β is the full width at half-maximum (fwhm) of a diffraction peak, θ is the diffraction angle, k is a constant (0.89). Average crystallite size of LaF.sub.3:Tb.sup.3+ and LaF.sub.3:Tb.sup.3+@pABA nanoparticles are in the range of (5.52-6.29) and (4.50-5.53) nm respectively, at different concentration of the dopant, Tb.sup.3+ as summarized in Table 1.

[0088] There is a broadening of peaks after the surface functionalization as can be observed from the gradual increase in the FWHM value of the (111) plane from (2.49-2.71) to (2.70-3.12) nm on moving from LaF.sub.3:Tb.sup.3+ to LaF.sub.3:Tb.sup.3+@PABA nanoparticles (Table 1).

[0089] The reason for the shrinking phenomenon of the unit cell volume is due to the smaller ionic radius of Tb.sup.3+ (0.923 Å) as compared to that of La.sup.3+ ion (1.032 Å). All the above analysis indicates that Tb.sup.3+ ions have been successfully doped into the lattice of LaF.sub.3 nanomaterials.

TABLE-US-00001 TABLE 1 Crystallite size, Lattice parameter, Unit cell volume and FWHM (Full Width Half Maximum) of pABA functionalised and non- functionalised LaF.sub.3:Tb.sup.3+ NPs. Unit Crystal- Lattice cell S. lite size parameter volume FWHM No. Sample (nm) a = b c (Å.sup.3) (111) 1 LaF.sub.3:Tb.sup.3+ (2%) 5.52 7.19 7.43 332.90 2.70 2 LaF.sub.3:Tb.sup.3+ (4%) 5.90 7.17 7.44 331.99 2.50 3 LaF.sub.3:Tb.sup.3+ (6%) 5.74 7.17 7.50 334.10 2.61 4 LaF.sub.3:Tb.sup.3+ (8%) 5.59 7.17 7.42 330.95 2.71 5 LaF.sub.3:Tb.sup.3+ (10%) 6.29 7.16 7.31 325.23 2.49 6 LaF.sub.3:Tb.sup.3+ 4.50 7.19 7.43 340.67 3.12 (2%)@PABA 7 LaF.sub.3:Tb.sup.3+ 5.28 7.17 7.44 336.10 2.70 (4%)@PABA 8 LaF.sub.3:Tb.sup.3+ 5.08 7.17 7.50 341.27 2.93 (6%)@PABA 9 LaF.sub.3:Tb.sup.3+ 4.57 7.17 7.42 338.62 3.02 (8%)@PABA 10 LaF.sub.3:Tb.sup.3+ 5.53 7.16 7.31 332.71 3.14 (10%)@PABA

2. IR Study

[0090] The broad peak (FIG. 1d) in the range of 3000-3500 cm.sup.−1 can be assigned to the stretching vibration of O—H group of carboxylic acid. Peaks at 1583 cm.sup.−1 and 1410 cm.sup.−1 are assigned to asymmetric and symmetric stretching vibrations of carboxylic group. Strong peak at 1016 cm.sup.−1 is due to C—O stretching of citric acid. The spectra of the functionalized nanoparticle (FIG. 1e) show two new peaks at 1497 cm.sup.−1 and 747 cm.sup.−1 which is due to C—C stretching vibration and out of plane C—H bending vibrations of benzene ring, respectively. The peaks at 1558 cm.sup.−1 and 1400 cm.sup.−1 are due to the carboxylic acid group of pABA. Broad peak above 3000 cm.sup.−1 can be assigned to amine group of pABA. The above analysis confirms that pABA has been successfully attached on the surface of the nanoparticles.

3. Morphology and Elemental Composition Analysis

[0091] FIG. 2(a-c) shows the HRTEM (High Resolution Transmission Electron Microscope). images of as prepared surface functionalized LaF.sub.3:Tb.sup.3+@pABA NPs which shows the morphology of spherical shape. The average size of the spherical nanoparticles is 5.2 nm which is in good agreement with the value calculated from XRD. The lattice fringes are clearly observed and the experimentally observed d-spacing is found to be 0.32 nm which coincides with the (111) plane of tysonite structure (JCPDS card 82-0690) of LaF.sub.3. Crystalline nature of the nanoparticles is well understood from the SAED pattern (FIG. 2d) and the major diffraction has been assigned to their corresponding planes. FIG. 2e shows the EDAX spectra of as-prepared LaF.sub.3:Tb.sup.3+@pABA NPs. All the typical peaks corresponding to La, F, Tb are observed in the spectra. At higher dopant concentration (10 at % of Tb.sup.3+), the spherical shape morphology of the nanoparticles get converted to a rod shape morphology with an average length ˜10 nm and width of ˜2 nm as shown in FIG. 2f.

[0092] The particles are found to be agglomerated in nature. FIG. 9 shows the histogram of particle size distribution from the DLS measurement. The individual particles are found to be agglomerated in the size range of ˜120-170 nm with ˜150 nm constituting half of the total populations.

4. Absorption Study

[0093] The absorption spectra of pABA functionalized and non-functionalized LaF.sub.3:Tb.sup.3+ nanoparticles is shown in FIG. 3a. In case of functionalized nanoparticles a new strong broad absorption peak having maximum at 265 nm appears which is not present for as prepared non-functionalized LaF.sub.3:Tb.sup.3+ nanoparticles. The functionalized nanoparticles show lowest absorption intensity at pH=1 (FIG. 10). Absorption intensity is highest at pH=3 which is almost double as compared to the absorption intensity at pH=7. The above analysis further confirms that the pABA sensitized LaF.sub.3:Tb.sup.3+ shows highest absorption activity at pH=3 and subsequently all the photoluminescence analysis has been performed at this optimum pH. Very low UV absorption in the alkaline medium where free —NH.sub.2 group is present is due to very little dispersibility of the nanoparticles at pH=13.

5. Photoluminescence Studies

[0094] All the photoluminescence studies were performed in aqueous medium. Taking 50 mg/250 ml aqueous (pH=3) dispersions of pABA functionalized and non-functionalized LaF.sub.3:Tb.sup.3+ samples to examine the energy transfer or sensitization of Tb.sup.3+ by pABA (FIG. 3b). When the functionalised nanoparticles are excited at pABA absorption maximum at 265 nm, it gives ˜100 times stronger luminescence as compared to direct excitation (360 nm) of Tb.sup.3+ ion as shown in FIG. 3c. The energy transfer process can be explained through the schematic diagram in FIG. 3d.

[0095] Radiative transitions of Tb.sup.3+ ions gives typical three characteristic strong emission bands centered at 490 nm, 546 nm and 586 nm, among which the peak at 546 nm is the strongest one. Sensitized luminescence intensity increases with increase in Tb.sup.3+ ion concentration in the LaF.sub.3 nanoparticles (FIG. 11). But at 10% Tb.sup.3+ concentration luminescence reduces due to concentration quenching. In the present study, the optimum concentration of the Tb.sup.3+ ion was found to be 8%.

Example: 14

Variation on the Photoluminescence Properties with Change of pH

[0096] A 1000 ml of 0.2M HCl solution was prepared by diluting 17 ml conc. HCl solution to 1000 ml. This solution was followed by ten fold serial dilution to prepare HCl solutions of strength 0.02, 0.002, 0.0002, 0.00002 and 0.000002(M).

[0097] Equal volume of 50 mg in 250 ml water dispersion of LaF.sub.3:Tb.sup.3+(8%) @ pABA nanoparticles were mixed with the acid solution to prepare the experimental solutions of pH 1, 2, 3, 4, 5, 6. 0.4 g of NaOH was dissolved in 50 ml water to prepare a 0.2 (M) solution. And following the above procedure experimental solutions of pH 13, 12, 11, 10, 9 and 8 were prepared. All the samples were excited at 265 nm and variation in luminescence intensity of these samples were recorded in the wavelength range of 450-700 nm (FIG. 4).

[0098] Luminescence intensities in the pH range of 4-7 are almost same as shown in FIG. 4a. The integrated area of 546 nm peak shows that maximum at pH=3 (FIG. 4a). After pH=7, a slight enhancement in the luminescence is observed till pH=9. In the strong alkaline medium (pH=10-13), luminescence reduces below the neutral medium luminescence (FIG. 4b). Very low luminescence intensity in strongly acidic medium (pH=1) and strongly alkaline medium (pH=13) is experimentally supported by very low UV absorption intensities at that pH values (FIG. 10). Surface area to volume ratio (4 πr.sup.2/4 πr.sup.3/3) of these very small nanoparticles (˜5 nm) is around 10.sup.7 cm.sup.−1. A plot of integrated area of the most intense peaks at 546 nm vs pH shows a straight line having negative slope and taking an intercept on y-axis (Inset of FIG. 4b). This linear relationship can be used to detect pH of unknown alkaline medium.

Detection of Nitro Explosives

[0099] All the experiment for the detection of the nitro compounds (Example 15-26) were performed at the pH=3 and Tb.sup.3+=8% ions concentration. Energy of LUMO of most of the aliphatic nitro explosives lies above that of pABA and hence no electron transfer as well as luminescence quenching is observed by the aliphatic nitro explosives (FIG. 13).

Example 15

[0100] To the aqueous dispersion of the pABA sensitized LaF.sub.3:Tb.sup.3+nanoparticles, aqueous solution of nitrobenzene (NB), was added and its photoluminescence property was studied.

Example 16

[0101] To the aqueous dispersion of the pABA sensitized LaF.sub.3:Tb.sup.3+nanoparticles, aqueous solution of o-nitrophenol (2-NP), was added and its photoluminescence property was studied. Photoluminescence spectra (FIG. 6) of LaF.sub.3:Tb.sup.3+@pABA in NP show that this nitro compound can quench the luminescence at very low concentrations of the analyte (2 ppm or 12.5 μM). Calculated values of quenching constant (k.sub.Q), obtained for 2-NP is 1683M.sup.−1 (FIG. 7).

Example 17

[0102] To the aqueous dispersion of the pABA sensitized LaF.sub.3:Tb.sup.3+nanoparticles, aqueous solution of o-nitrotoluene(2-NT) was added and its photoluminescence property was studied.

Example 18

[0103] To the aqueous dispersion of the pABA sensitized LaF.sub.3:Tb.sup.3+nanoparticles, aqueous solution of 2,4-dinitrotoluene (2,4-DNT) was added and its photoluminescence property was studied. Photoluminescence spectra (FIG. 6) of LaF.sub.3:Tb.sup.3+@pABA in 2,4-DNT show that this nitro compounds can quench the luminescence at very low concentrations of the analyte (2 ppm or 12.5 μM). Calculated values of quenching constant (k.sub.Q), obtained for 2,4-DNT is 3296M.sup.−1 (FIG. 7).

Example 19

[0104] To the aqueous dispersion of the pABA sensitized LaF.sub.3:Tb.sup.3+nanoparticles, aqueous solution of 2,6-dinitrotoluene (2,6-DNT) was added and its photoluminescence property was studied.

Example 20

[0105] To the aqueous dispersion of the pABA sensitized LaF.sub.3:Tb.sup.3+nanoparticles, aqueous solution of 2,4-dinitrophenol(2,4-DNP) was added and its photoluminescence property was studied. Photoluminescence spectra (FIG. 6) of LaF.sub.3:Tb.sup.3+@pABA in 2,4-DNP show that this nitro compounds can quench the luminescence at very low concentrations of the analyte (2 ppm or 12.5 μM). Calculated values of quenching constant (k.sub.Q), obtained for 2,4-DNP is 2103M.sup.−1 (FIG. 7).

Example 21

[0106] To the aqueous dispersion of the pABA sensitized LaF.sub.3:Tb.sup.3+nanoparticles, aqueous solution of picric acid (PA) was added and its photoluminescence property was studied. Luminescence quenching is observed at very low concentrations of picric acid (2.25 ppm or 10 μM) (FIG. 5). Calculated values of quenching constant (k.sub.Q), obtained for PA is 5738 M.sup.−1 (FIG. 7).

Example 22

[0107] To the aqueous dispersion of the pABA sensitized LaF.sub.3:Tb.sup.3+nanoparticles, aqueous solution of 2,4,6-trinitrotoluene (TNT) was added and its photoluminescence property was studied. Luminescence quenching is observed at very low concentrations of TNT (10 ppm or 50 μM) (FIG. 5). Calculated values of quenching constant (k.sub.Q), obtained for TNT is 12295 M.sup.−1 (FIG. 7).

Example 23

[0108] To the aqueous dispersion of the pABA sensitized LaF.sub.3:Tb.sup.3+nanoparticles, aqueous solution of nitromethane (NM) was added and its photoluminescence property was studied.

Example 24

[0109] To the aqueous dispersion of the pABA sensitized LaF.sub.3:Tb.sup.3+nanoparticles, aqueous solution of 1,2,4-butanetriol nitrate (BTTN) was added and its photoluminescence property was studied.

Example 25

[0110] To the aqueous dispersion of the pABA sensitized LaF.sub.3:Tb.sup.3+nanoparticles, aqueous solution of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) was added and its photoluminescence property was studied.

Example 26

[0111] To the aqueous dispersion of the pABA sensitized LaF.sub.3:Tb.sup.3+nanoparticles, aqueous solution of 1,3,3-Trinitroazetidine(TNAZ) was added and its photoluminescence property was studied. FIG. 5 shows the photoluminescence spectra of LaF.sub.3:Tb.sup.3+@pABA in picric acid and T.N.T solution. Luminescence quenching is observed at very low concentrations of TNT (10 ppm or 50 μM) and picric acid (2.25 ppm or 10 μM). Photoluminescence spectra (FIG. 6) of LaF.sub.3:Tb.sup.3+@pABA in NP, 2,4-DNP and 2,4-DNT show that these nitro compounds can also quench the luminescence at very low concentrations of the analyte (2 ppm or 12.5 μM).

[0112] FIG. 12 shows very sensitive detection of TNT at ppb level. At such low concentration the luminescence intensity change at .sup.5D.sub.4-.sup.7F.sub.4 is more clearly seen than for .sup.5D.sub.4-.sup.7F.sub.5 transition. The luminescence at 490 nm was used for Stern-Volmer plot. The surfaced functionalised nanomaterials were able to detect TNT upto 50 ppb.

[0113] Calculated values of quenching constant (k.sub.Q), obtained using above Stern-Volmer equation for TNT, PA, 2-NP, 2,4-DNT, 2,4-DNP are 12295, 5738, 1683, 3296, 2103M.sup.−1 respectively. Among the above analysed nitrocompounds, TNT has remarkably high quenching constant. A comparison of quenching efficiency of the all the selected nitrocompounds at a particular concentration (0.1 mM) is shown in the bar-graph of FIG. 7. It shows negligible quenching by aliphatic nitro explosives except for RDX (explained below). Thus the nanoparticles are highly sensitive as well as selective to the aromatic nitro compounds as compared with the aliphatic nitro compounds.

[0114] The order of quenching for the above analysed nitrocompounds was; 2,6-dinitrotoluene (2,6-DNT)>o-nitrophenol (2-NP)>2,4-dinitrophenol(2,4-DNP)>nitrobenzene(NB), 2,4,6-trinitrotoluene (TNT)>picric acid(PA)>1,3,5-trinitroperhydro-1,3,5-triazine (RDX)>2,6-dinitrotoluene (2,6-DNT)>>o-nitrotoluene (2-NT)>1,3,3-Trinitroazetidine(TNAZ)>octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)>nitromethane (NM), 1,2,4-butanetriol nitrate (BTTN). (FIGS. 8 and 13) UV-absorption study as shown in the FIG. 8 reveals that emission spectra of pABA overlaps effectively with the UV absorption spectra of 2,4-DNT, 2-NP, 2,4-DNP and picric acid. This can explain high quenching performance of 2,4-DNT, 2-NP and 2,4-NP. There is a very less overlap for 2,6-DNT, TNT and RDX. So the quenching mechanism for 2,6-DNT and TNT is mainly electron transfer mechanism. This observation can also explain very low quenching performance by 2,6-DNT and RDX. Hence, pABA sensitised LaF.sub.3:Tb.sup.3+ NPs can be potential materials for using as a sensor for the detection of highly explosive nitrocompounds.

Example 27

Lifetime Measurements

[0115] Life time of the nanoparticles were studied in presence and absence of the analytes (PA and TNT) and were recorded by fixing the excitation and emission wavelengths at 265 nm and 546 nm respectively as shown in FIG. 14. Life time of the surface functionalized nanoparticles in absence of any analyte is 0.1177 ns. In presence of 18, 61.25 and 125 ppm picric acid life time values are 0.1158, 0.0883, and 0.0806 ns respectively. Life time values for 10, 20 and 60 ppm TNT are 0.0790, 0.0889, and 0.0785 ns respectively. (FIG. 15)

ADVANTAGES OF INVENTION

[0116] 1. High detection level.

[0117] 2. Method can detect nitro containing explosives.

[0118] 3. It can be used for determining the pH of a solution.

[0119] 4. Process of synthesis of Tb.sup.3+ doped LaF.sub.3 is simple and quick.