Selective and sensitive detection of picric acid in aqueous, sol-gel and solid support media by Ln(III) probes

Sensors and Actuators B 250 (2017) 215–223

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Selective and sensitive detection of picric acid in aqueous, sol-gel and solid support media by Ln(III) probes Jashobanta Sahoo a,b,c , D. Shanthana lakshmi a,b , E. Suresh a,b , P.S. Subramanian a,b,∗ a b c

CSIR-Central Salt and Marine Chemicals Research Institute Gijubhai Badhega Marg Bhavnagar, Gujarat, India Academy of Scientific and Innovative Research (AcSIR),CSIR-CSMCRI, Bhavnagar, Gujarat, 364 002, India Department of Chemistry, CV Raman College of Engineering, Bhubaneswar, Odisha 752 054, India

a r t i c l e

i n f o

Article history: Received 5 January 2017 Received in revised form 24 April 2017 Accepted 26 April 2017 Available online 27 April 2017 Keywords: Lanthanide Explosive Picric acid Sensor Solid films Luminescence

a b s t r a c t The luminescent lanthanide complexes EuL, TbL and its polymeric compounds EuL@PVA & TbL@PVA (where PVA = polyvinylalcohol) were synthesised. The complexes are soluble in water and depict their metal centred luminescence. The luminescent properties are meticulously monitored for sensing various nitro compounds in aqueous, polymeric, solid support and found to be highly selective for picric acid (PA). Remarkably, the minimum detection limit of PA is falling in micro molar (␮M) concentration (i.e., Dlim = 0.5 ␮M) exhibits a strong quenching constant (KSV = 85530 M−1 ) in aqueous medium. Further the detection of PA in micromolar level is demonstrated in sol-gel type polymeric medium and on solid support via contact mode using whatman paper strips. Time dependant quenching of luminescence of PA on paper strips were also investigated by naked eye detection for its user friendly application. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Design and development of new probes for selective detection of nitro compounds such as picric acid (PA) with a focus on low analytical level in aqueous medium is of great interest. Among the various nitro compounds, PA is known for its powerful explosive nature than TNT and others [1]. PA being one of the prime components in the crackers, it is known to contaminate both air and water, caused by leather industries, fire crackers, rocket fuel, pharma, dye industries etc., and thereby leads into serious environmental and health hazardous such as severe irritation, skin allergy, dizziness, nausea, liver, kidney damage, and cancer [2,3]. Thus PA known for its toxic effect, its detection even the trace amount in both aqueous and solid phase, is of high importance for environmental safety, on military operation and homeland security [4–6]. A wide range of sophisticated techniques such as ion mobility spectroscopy (IMS) [7,8], surface enhanced Raman spectroscopy [9], gas chromatography coupled with mass spectroscopy [10], NMR spectroscopy [11] are currently employed for the detection of such nitro explosive compounds. However luminescent based materials (MOF, gel, poly-

∗ Corresponding author at: Inorganic Materials and Catalysis Division, CSIRCentral Salt and Marine Chemicals Research Institute Gijubhai Badhega Marg Bhavnagar, Gujarat, India. E-mail addresses: [email protected], [email protected] (P.S. Subramanian). http://dx.doi.org/10.1016/j.snb.2017.04.170 0925-4005/© 2017 Elsevier B.V. All rights reserved.

mer) [12–41] for detection of PA in solution and solid state have attracted increasing attention by virtue of their high sensitivities, portability, short response times and applicability. Luminescent based Ln(III) complexes or materials for detection of PA had gained much attention, due to its narrow emission bands (<10 nm), long excited lifetimes (ms), large Stoke shifts, and excellent sensitivity. As compared to the above sophisticated techniques and luminescent materials, lanthanide based probes are rarely reported in literature for sensing PA [42–53]. In our recent report [54–56], we have demonstrated a europium based complex for dual sensing of PA and nitrite(NO2 − ) ion in aqueous medium. In such attempts, achieving low analytic detection limit with strong binding constant is still remains challenges. In the course of improving the detection limit and binding constant, we have synthesised a phenanthroline based ligand (L) and its Ln(III) complexes (where Ln(III) = Eu(III) and Tb(III)). In this regard Bhattacharya and co-worker [57] have used p-phenylenevinylene based probes and reported a multimedia tool using water, micelles, organogel, solid supports and demonstrated them for selective detection of NACs (nitro aromatic compounds). Similarly Mukherjee and co-workers [17] reported the electronrich self-assembled discrete molecular sensors and their sensing activities towards different nitroaro compounds both in solution and vapour phase. To the best of our knowledge, the present manuscript demonstrates luminescent Ln(III) based probes for the detection of PA for the first time in aqueous, sol–gel type poly-

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meric medium and paper strips. In addition, this luminescent probe hybridized with polyvinyl alcohol (PVA) and paper strip; demonstrate the detection of PA in three different media such as solution, solid and sol-gel state. 2. Experimental section 2.1. Materials and general methods All the chemicals were purchased from Aldrich & Co. Elemental analysis of the ligand and complexes were carried out by using a Vario Micro cube from Elementar. Electronic spectra were recorded on a Shimadzu UV 3600 spectrophotometer using the range 200–800 nm. IR spectra were recorded using KBr pellets on a Perkin-Elmer Spectrum GX FT-IR spectrometer. Massspectrometric analysis was performed by using the positive and negative ESI technique on a Waters Q ToF micro mass spectrometer for all these complexes upon dissolving in CH3 OH. NMR spectra were recorded on a Bruker Avance 200 MHz FT-NMR spectrometer. The chemical shifts for proton resonances are reported in ppm(␦) relative to TMS. The different pHs (ORION VERSA STAR pH meter) were measured by systematic addition of 0.1 M NaOH and 0.1 M HCl and Milli Q water as solvent. Emission spectra were recorded using an Edinburgh instruments model Xe-900 and reported here after applying emission correction. The slit sizes for emission and excitation are adjusted as 3.0/3.0 nm for Eu(III) and 5.0/5.0 for Tb(III). The absolute quantum yield for all these complexes were calculated using standard procedure. 2.2. X-ray crystallography Single-crystal x-ray data collection was performed on a Bruker SMART APEX CCD diffractometer with graphite monochromatized Mo-K␣ radiation (␭ = 0.71073 Å) at 150(2) K for ligand L. The SMART and SAINT software packages were used for data collection and reduction, respectively [58]. In all cases, absorption correction based on multiple scans using the SADABS software was also applied [59]. The structures were solved by direct methods and refined by full-matrix least-squares method on F2 with the programs SHELXS-97 and SHELXL-97 [60,61]. All non-hydrogen atoms were refined anisotropically until convergence was reached. The hydrogen atoms were generated geometrically and treated by a mixture of independent and constrained refinement. The crystallographic data [62] for the ligand, selected bond lengths and angles as well as various H-bonding interactions were given in the ESI.(S17)

4 h. After completion of the reaction, the solution was evaporated and the solvent was removed under vacuum. The resultant light grey powder was isolated. Yield 78%. IR (KBr): ␯ cm−1 = 3420(s), 2974(s), 1628(s), 1046(s). −UV vis(CH3 OH) (␭ = nm (␧, M−1 cm−1 )): 286 (13020), 233 (14295). ESI–MS: m/z (calcd (found)) [EuLCl2 ]+ (m/z = 629.42 (found)/629.09(calcd)) and [(LH)EuCl]+ (m/z = 593.43 (found)/593.11(calcd)). Elemental analysis: Calcd (found) for C24 H32 Cl3 N4 O3 Eu: C 42.21(41.90), H 4.72(5.12), N 8.20 (8.11)%. 2.3.3. TbLCl3 The methanolic solution of ligand L (0.442 g, 0.001 mmol) and TbCl3 ·6H2 O (0.373 mg, 0.001 mmol) were mixed together and allowed for constant stirring at RT for 4 h. After completion of the reaction, the solution was evaporated and the solvent was removed under vacuum. The resultant light grey powder was isolated. Yield. 76%. IR (KBr): ␯ cm−1 = 3415(br), 2969(s), 1625(s), 1046(s). UV Vis(CH3 OH)␭ = nm (␧, M−1 cm−1 ): 287 (10700), 234 (12155). ESI–MS: m/z (calc(found)) [TbLCl2 ]+ (m/z = 635.59(found)/635.10(calcd)) + and [(L-H)TbCl] (m/z = 599.59(found)/599.12(calcd)). Elemental analysis: Calcd(found) for C24 H32 Cl3 N4 O3 Tb: C41.79(41.90), H 4.68(5.12), N 8.12 (8.11)%. 2.4. Procedure for EuL@PVA and TbL@PVA 2.5 g of Polyvinylalcohol (PVA) was mixed gradually in 100 mL of Milli Q water at 60 ◦ C for 8 h. 7 mg of EuL and TbL solution of 1 × 10−3 M concentration was mixed in 10 mL of PVA (2.5 wt%) solution and treated as standard stock solution. The resultant EuL-PVA and TbL-PVA mixture were thoroughly mixed by stirring for 8 h and the resultant homogenous mixture was labelled as EuL@PVA and TbL@PVA. 2.5. Formula for calculating the percentage luminescence quenching efficiency of PA (Fo -F)/Fo x 100% Where, Fo = initial luminescence intensity absence of PA, F = luminescence intensity in presence of PA. 2.6. Detection limit (DL) calculation Detection limit (DL) = [Concentration of complex] × [Eq. of PA at which luminescence changes were observed] 3. Results and discussion

2.3. Synthesis and characterization 2.3.1. Ligand L (2,2’-((1E,1’E)-((1,10-phenanthroline-2,9diyl)bis(methanylylidene))bis(azanylylidene))bis(3-methylbutan-1-ol) 2,9-dialdehyde-1,10-phenanthroline (0.272 g, 0.001 mmol) was dissolved in 50 mL of CH3 OH. To this methanolic solution (R)2-Amino-3-methyl-1-butanol (0.243 g, 0.002 mmol) was added drop-wise. Upon completion of this addition, the solution was allowed for continuous stirring upto 24 h at 50 ◦ C. The resultant solution was evaporated through rotavapor and dried. Yield.75%. IR (KBr): ␯ cm−1 = 3364(s), 2964(s), 1635(s), 1030(s). MS (ESI): m/z (calcd (found)) m/z = 407.22(407.22), 429.20(429.20) for [L + H]+ and [L + Na]+ Elemental analysis: Calcd (found) C25 H36 N4 O4 C 65.77(65.68), H 7.95(8.03), N 12.27(12.19). 2.3.2. EuLCl3 The methanolic solution of ligand L (0.442 g, 0.001 mmol) and EuCl3 ·6H2 O (0.366 mg, 0.001 mmol) were mixed together and allowed for constant stirring at room temperature for

Phenanthroline based ligand L was obtained following simple Schiff base condensation by treating 2,9-dialdehyde-1,10phenanthroline and (R)-2-Amino-3-methyl-1-butanol. Elemental data, LC–MS, 1 H NMR, and single crystal molecular structure [56], confirm the formation of L (Scheme 1). The 1 H NMR of L depicting characteristic peak at 8.03␦ correspond to CH N azomethine proton, confirms the formation of Schiff base (S2). MS spectra (S3) of L depicting two distinct peaks at m/z = 407.22 and 429.20 in the positive ion mode, are assigned to the [L + H]+ and [L + Na]+ ions respectively. Treating L with respective LnCl3 salt in 1:1 stoichiometric ratio, the corresponding Ln-complexes EuLCl3 and TbLCl3 were synthesized. Both the complexes were characterized by LC–MS, elemental analysis, IR, UV–vis and fluorescence spectra. MS spectra of EuL (S4) depict m/z peaks attributable to two distinct monocation species i.e., [EuLCl2 ]+ (m/z = 629.42 (found)/629.10(calc)) and [(L-H)EuCl]+ (m/z = 593.43 (found)/593.11(calcd)). The TbL (S5) also gave similar ionization pattern of monocation correspond to [TbLCl2 ]+ (m/z = 635.59(found)/635.10(calc)) and [(L-H)TbCl]+

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Scheme 1. Synthesis of ligand L and its complexes (i) EuL; (ii) TbL and (iii). crystal structure of ligand L.

(m/z = 599.59 (found)/599.12(calcd)). The bis-imine ligand was soluble in almost all organic solvents such as CH3 OH, DMF and DMSO, but insoluble in water. However the respective complexes EuL and TbL are soluble in organic solvents such as CH3 OH, DMF and DMSO including water. The electronic spectra of EuL and TbL showing two distinct bands centered at 286 and 233 nm, are attributed to the ligand centered n-* and -* transitions (S7). The excitation spectra of both complexes monitored under the characteristic emission of 614 nm for Eu(III) and 545 nm for Tb(III) ion exhibited a broad band with a maximum intensity at 300 nm. The emission spectra shown in Fig. 1 are obtained upon excitation at ␭ex = 300 nm. EuL illustrating “luminous red” reveals hypersensitive emission peak at 614 nm attributable to 5 D0 →7 F2 transition. Similarly TbL with its characteristic “green emission” correspond to 5 D4 →7 F5 transition appeared at 545 nm was found hypersensitive along with other bands originated from 5 D4 excited state to 7 FJ (J = 1, 2, 3, 4, 5, 6). Considering the possibility of higher coordination number for Ln(III), the ligand L fulfills hexadentate coordination sites and the remaining sites at Eu(III) and Tb(III) metal centers are established following the excitation state lifetime measurements. Thus aiming to establish the hydration number, for Eu(III) and Tb(III) the lifetime(␶) were measured [63,64] in H2 O and D2 O (Table 1). Adapt-

Fig. 1. Normalized emission intensity of excitation and emission spectra of EuL and TbL in HEPES buffer pH 8.0 (␭ex = 300 nm).

ing the Eq. (1), the inner sphere O H oscillator (q) for EuL and TbL were calculated as 2.62 and 3.24. This indicates that Eu(III) and

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Table 1 Lifetime in H2 O and D2 O for EuL − TbL in HEPES buffer pH,8.0. Complex

H2 O (ms)

D2 O(ms)

qcorr a

CN

 HEPES (ms)

˚obs

Eu-L Tb-L

0.32 0.18

1.60 0.35

2.62 3.24

9 9

0.55 0.41

15.6% 4%

a qcorr value were determined adapting by A’ = 1.2ms(Eu) and 5ms(Tb) &kcorr = −0.25 ms−1 (Eu), −0.06 ms−1 (Tb) were adapted.

Tb(III) in EuL and TbL were existing with three water molecules respectively. qcorr = A’kcorr (wherekcorr = (kH2O -K D2O ))

(1)

As pH is known to modulate emission intensity, a detailed investigation by varying pH from acidic to alkaline indicates that complexes are stable in the range pH = 4-9 (S8). Considering the bright luminescent intensity of both complexes (EuL and TbL), we have calculated the absolute quantum yield [65]. The absolute quantum yield (˚obs ) for EuL and TbL were determined from the ratio of the observed ( obs ) and radiative ( rad ) lifetime using Eqs. (2), (3) and are presented in Table 1. Based on the quantum yield (˚Ln ) of EuL (˚ = 15.6%) and TbL (˚ = 4%), this is obvious to understand that Eu(III) is sensitized by L effectively than Tb(III) in their respective complexes. ˚Ln =  obs / rad

(2)

1/ rad = 14.65s−1 .3 . (I tot /I(5 D0 -7 F1 ))

(3)

Where  is the refractive index of the medium, Itot is the integrated emission intensity of 5 D0 → 7 FJ = 0-6 transitions and IMD corresponds to I(5D0→7F1) of the integrated intensity of band related to J = 1. 3.1. Sensing in aqueous medium The luminescent properties of EuL and TbL were applied to detect various nitro compounds. The nitro compounds such as PA (trinitro phenol), 2,6-DNT (2,6-dinitro toluene), 2,4-DNT (2,4-dinitro toluene), 1,4-DNB (1,4-dinitro benzene), 1,3-DNB (1,3-dinitro benzene), 2-NT (2-nitrotoluene), 3-NT (3nitrotoluene), 4-NT (4-nitrotoluene), NB (Nitrobenzene), 2-NPr (2-nitropropane), NMe (nitromethane), NEt (nitroethane), DNDMB (dinitrodimethylbutane), 2,4-DNP (2,4-dinitrophenol) and 4-NP (4nitro phenol) were scanned in an aqueous medium (pH = 8.0) and the respective emission responses are depicted in Fig. 2a & b. Among the nitro compounds (100 eq) screened, PA showed 89% quenching of luminescence [66] on EuL (Fig. 2a), and 90% quenching on TbL (Fig. 2b). Competitive experiments (Fig. 3a & b) were performed for PA in presence of other nitrocompounds. In this regard, the solution mixture of EuL and series of different nitrocompounds were treated sequentially treated with constant amount of PA and the respective luminescence responses are plotted in Fig. 3a. Similar interference study with TbL and PA mixture, performed against various nitrocompounds is depicted in Fig. 3b. The luminescence intensity of PA mixture of EuL and TbL thus, remains unchanged, even in the presence of other nitrocompounds and illustrates their superior selectivity towards PA. A systematic titration on EuL against a function of concentration of PA by varying 0.05eq to 20eq was performed. The respective luminescence changes are depicted in Fig. 4a. A similar titration for TbL varying PA from 0.025 eq to 9 eq is depicted in Fig. 4b. Based on the responses derived from above experiments, the luminescent quenching efficiency was determined quantitatively adapting Stern-Volmer (SV) equation-4. F0 = 1 + KSV [M] F

(4)

Fig. 2. Luminescence responses of a) EuL (2 × 10−5 M); b) TbL (2 × 10−5 M) against the addition of various anions(100eq) in HEPES buffer at pH = 8.0 (␭ex = 300 nm).

Where F0 and F are the luminescence intensity in absence and presence of PA, KSV is quenching constant. Applying Eq. (4) with increasing concentration of PA, a linear fit drawn between F0 /F and [PA] for EuL, derived KSV = 62960 M−1 (R2 = 0.9920); for TbL the Ksv is derived as 85530 M−1 (R2 = 0.9939). The dynamic detection range [67,68] for EuL is determined as 1 ␮M–400 ␮M and for TbL is 0.5 uM–180 uM. Binding stoichiometry of both these complexes with PA in aqueous phase has also been derived by using the Benesi-Hildebrand Eq. – (5) [69]. 1 1 1 = + F0 − F (Fmax − F0 ) Ka (Fmax − F0 ) [A− ]n

(5)

Where F0 and F are the luminescence intensity of before and after addition of PA. Ka is the association constant; A represents the concentration of the quencher PA. The linear least square fit (S9, S10) drawn between (1/F0 -F) vs 1/[PA] applying Eq. (5) and the Jobs plot (S11, S12) together confirms the formation of 1:1 species. MS analysis of this 1:1 mixture (S13, S14) of both EuL and TbL with PA, reveal peaks at m/z = 804.64 and 882.81 respectively along with isotopic pattern, attributed to [(L-H+ )EuL(PA)H2 O]+ and [TbLCl(PA).3(H2 O)]+ species. 3.2. Sensing in polymeric solution Both the luminescent probes although are found stable, on standing in aqueous medium they begin to precipitate after more

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Fig. 3. Emission changes a) Interference study of EuL. b) Interference study of TbL with different analyte in HEPES buffer at pH = 8.0 (␭ex = 300 nm).

than 5 h. The precipitate collected was analyzed through SEM and the respective images illustrate a 2D sheet layered like aggregation pattern (Fig. 5). To overcome this aggregation problem, the luminescent probes were doped with aqueous solution of PVA. Accordingly EuL or TbL (7 mg) of 1 × 10−3 M solution was mixed with 10 mL of PVA (2.5 wt%) solution. The resultant sol-gel type PVA (2.5 wt%) stock solutions EuL@PVA and TbL@PVA were found transparent, luminescent and stable for few months. Hence following this luminescence and stability, we have investigated the compounds for detection of various explosive nitrocompounds (100eq). Interest-

Fig. 4. Change in luminescence intensity of a) EuL (2 × 10−5 M), b) TbL (2 × 10−5 M) upon addition of PA in HEPES buffer at pH = 8.0 and the inset shows the corresponding Stern-Volmer plot (␭ex = 300 nm).

ingly among the various nitro compounds screened, PA showed significant luminescent quenching in both polymeric solutions similar to that observed in their aqueous state. The PA quenches 85% of EuL@PVA (Fig. 6a) and 88% of TbL@PVA (Fig. 6b) luminescent intensity and suggests that PA is again the superior quencher than other nitrocompounds in the PVA based sol-gel. A further insight regarding the selectivity of PA with EuL@PVA and TbL@PVA in presence of other nitro analytes, was obtained

Fig. 5. SEM images of complex EuL after precipitation.

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Fig. 6. Emission spectra of a) EuL@PVA b) TbL@PVA of 2 × 10−5 M against the addition of various anions (100eq) in PVA solution(␭ex = 300 nm).

Fig. 7. Emission interference of a) EuL@PVA and b) TbL@PVA with different analyte (100 eq) in PVA solution (␭ex = 300 nm).

by performing interference study. Fig. 7a and b illustrates the luminescence intensity of EuL@PVA-PA and TbL@PVA-PA with mixture of PA and other nitrocompounds. The luminescent intensity of EuL@PVA-PA and TbL@PVA-PA thus remaining unaffected declares that the other nitrocompounds does not affect the luminescence intensity in presence of PA. Thus PA shows a high selectivity compared to other nitro analytes. The respective titrations using EuL@PVA and TbL@PVA against PA (Fig. 8) demonstrates that the emission intensity is gradually decreased, upon increasing the concentration of PA. The respective SternVolmer quenching constant (KSV ) with detection limit were derived as 13930 M−1 and 2 ␮M for EuL@PVA, while TbL@PVA shows the KSV = 49620 M−1 and the dynamic detection limit (Dlim ) = 1 ␮M. Adapting the Benesi-Hildebrand equation the stichometric ratio of EuL@PVA and TbL@PVA with PA was estimated as 1:1 (S15, S16).

with PA. Accordingly the absorption maxima of PA and emission maxima of L, overlapping in their respective spectra (S19) indicate a higher probability of energy transfer from L to PA.

3.3. Mechanism The linear fit in the inset of Figs. 4 & 8 of SV plot with increasing concentration of quencher in aqueous or polymeric medium, represents either static or dynamic quenching. The lifetime measurement of EuL and TbL in absence and presence of multiple addition of PA (S18) suggests that the lifetime remains almost unchanged. This leads to the formation of non-emissive ground state between the complex and quencher (PA) and this observation confirms that the quenching process follows only static mechanism. Apart from the static mechanism, both these complexes were investigated for their energy transfer mechanism when treated

3.4. Sensing on solid support So far the study has been demonstrated the PA detection in solution state. However in the firecracker and detonator fields, the health hazardous contamination of PA is quite high. Hence in addition to solution state detection, PA needs to be detected by simple methods in its contact mode through solid support. The present EuL being highly luminescent, sensitive and selective towards PA, we inspired to prepare an economically cheap, simple, easy handling solid based material. Accordingly whatman paper has been chosen for this purpose. Whatman filter paper has been dip-coated in EuL solution of 1 × 10−3 M and dried. These luminescent whatman strips were studied for their response against PA both in solution and solid state respectively. The luminescent strips were dipped into aqueous solution of different concentration of PA ranging from 0.1eq to 20eq as shown in Fig. 9. These paper strips were monitored under UV lamp (␭ex = 254 nm) and respective luminescent changes were recorded in different time intervals upto 5 min. The naked eye visualization clearly indicates that at lower concentration (0.1eq/1 ␮M) the luminescent strip becomes non-luminescent after 5 min. The minimum amount of PA, detectable by naked eye was upto ppm level. Similarly, PA crystals were placed over these paper-strips for five minutes to test the contact mode response of EuL (Fig. 10). Upon illumination with UV lamp, black spots were

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Fig. 9. Photographs (under 254 nm UV light) of EuL on test strips The luminescence quenching of EuL on test trip for visual detection of PA at different concentration through timing.

4. Conclusions

Fig. 8. Change in luminescence intensity of a) EuL@PVA; b) TbL@PVA of 2 × 10−5 M upon addition of PA; the inset shows the corresponding SV plot (␭ex = 300 nm).

observed in the contact area. This illustrates a simple and efficient method for the detection of picric acid in both solid and solution states. It is also important to compare few reported lanthanide compounds with the present complexes (S20) using its quenching constant (Ksv), detection limit (Dlim ) and medium. The complexes EuL and TbL show a high selectivity and sensitivity in an ecofriendly water medium and gains importance. The latter complex TbL show the highest quenching constant with lowest detection limit compared to reported values.

In summary, lanthanide luminescent probes EuL, TbL and its polymeric matrices EuL@PVA and TbL@PVA were prepared. All of them were successfully demonstrated for the detection of picric acid in presence of other interfering nitro compounds in three different medium such as aqueous, sol-gel and solid. This present Ln(III) complexes are found to perform the lowest detection level upto 0.5 ␮M with highest binding constant (KSV = 85530 M−1 ). Complex EuL with its highest sensitivity and selectivity serves as an efficient probe for PA. The respective paper strips serve as a simple, portable, sensitive, fast and low cost tool for the detection of PA in solid state. However, these luminescent complexes although show their efficient sensitivity and selectivity, the precipitation problem of EuL and TbL on standing long time in aqueous medium is considered as disadvantage for its long time use. Hence the complexes should be used immediately after preparing the solution and certainly not to be used after 4 h. To over this problem precipitation problem, the respective sol-gel compounds EuL@PVA, TbL@PVA can be used is considered as advantage. Our group is currently involved in working with the same area of detection of nitro compounds and their real time application.

Fig. 10. Contact mode detection of EuL on test strips (A) without UV light passed on test strip (B) Under UV light test strip (under 254 nm UV light) (C) PA crystal placed on test strip, (D) after removal of PA crystals.

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Acknowledgements CSMCRI communication number CSIR-CSMCRI 208/2015. Author JS acknowledge CSIR New Delhi for CSIR-SRF. Members of Analytical Division and Centralized Instrument Facility (ADCIF) of CSMCRI are acknowledged for their instruments supports. Prof B. Jha is acknowledged for allowing Fluorescence Microscopy facility.

[24]

[25]

[26]

Appendix A. Supplementary data

[27]

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2017.04.170.

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Biographies Jashobanta Sahoo received his M.Sc. from Indian Institute of Technology Guwahati in 2011. He is currently pursuing his doctoral research at CSIR-Central Salt and Marine Chemical Research Institute under the supervision of Dr. P. S. Subramanian. His research interests are design and development of luminescence based lanthanides complexes and materials for applications as sensors. Dr. D. Shanthana lakshmi received her Ph.D. from Anna University, Chennai in the area of “Liquid Membrane Technology” followed by Marie-Curie International Incoming Researcher at ITM-CNR, Italy. She is currently “Scientist Fellow” at CSIRCentral Salt and Marine Chemical Research Institute, Bhavnagar. Her research interests are material fabrication, membrane technology and water purification applications. Dr. Eringathodi Suresh obtained his PhD in Chemistry from the Bhavnagar University, Gujarat, for work on the synthesis and X-ray crystallographic studies on coordination polymers at CSIR-CSMCRI, Bhavnagar, India. Subsequently, he gained postdoctoral research experience at the Institute of Molecular Science, Okazaki, Japan, with a JSPS postdoctoral fellowship. He is also the recipient of a CSIR-DAAD fellowship (2011) for senior scientists. For the last two decades, he has been working in various capacities at CSIR-CSMCRI and continuing as a principal scientist. Dr. Suresh has been working in the area of coordination polymers (CPs). Dr. P.S. Subramanian obtained his PhD in Chemistry from the Bhavnagar University, Gujarat, affiliated to CSMCRI Bhavnagar. Subsequently, he gained postdoctoral research experience at the University of Pavia, Pavia, Italy, with a BOYSCAST Fellowship. He is also the recipient of a CSIR-DAAD fellowship (2010) for senior scientists and worked as DAAD Fellow in University of Aachen Germany. For the last two decades, he has been working in various capacities at CSIR-CSMCRI and continuing as a principal scientist. Dr. Subramanian has immensely contributed in the area of sensing and molecular recognition.