Journal of Science: Advanced Materials and Devices (2019) 237e244 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Novel triazine-based colorimetric and uorescent sensor for highly selective detection of Al3ỵ J Jone Celestina a, L Alphonse a, P Tharmaraj a, *, C.D Sheela b a b PG and Research Department of Chemistry, Thiagarajar College, Madurai 625009, Tamilnadu, India PG and Research Department of Chemistry, The American College, Madurai 625002, Tamilnadu, India a r t i c l e i n f o a b s t r a c t Article history: Received 31 January 2019 Received in revised form 26 April 2019 Accepted May 2019 Available online 16 May 2019 This paper deals with a new fluorescent assay of Al3ỵ ions with (2,20 -(6-((4-nitrophenyl)amino)-1,3,5triazine-2,4-diyl) bis(hydrazine-2-yl-1-ylidene)) bis(indolin-3-one) (NADO) containing triazine moiety developed over other commonly coexisting metal ions The complexation activity of NADO with various metal ions in an ethanolic solution is specifically studied by means of fluorescent spectra The NADO exhibits a significant fluorescence enhancement at 469 nm in presence of Al3ỵ due to the formation of a complex Under an optimized condition the detection limit is found to be 0.09 mM As the concentration of Al3ỵ is increased, the fluorescence intensity gradually increases due to the formation of the complex The 1:1 binding stoichiometry between Al3ỵ and NADO is confirmed by the Job's plot and the HR-LCMS mass spectrum of the metal complex © 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Fluorescent sensor Colorimetric Aluminium ion Triazine Schiff base Introduction Aluminum being the third most abundant element has its widespread use in light alloy, textiles and in treatment of water Al3ỵ remains in blood and tissues for a very long time before it is excreted in the urine World Health Organization holds the view that the average daily human intake of aluminum is about 3e10 mg Human body tolerates an uptake up to mg/kg of the body weight per week Aluminum in excess would lead to environmental contamination and could be toxic to human health since it impedes calcium metabolism, thereby causing Osteomalacia, influences the ingestion of iron in blood and also decreases the liver and kidney functions Excess accumulation of Al3ỵ leads to impairment of the central nervous system, which is associated with the pathology of dementia, anemia, Parkinson's disease, Alzheimer's disease and dialysis [1,2] It is, therefore, desirable to detect and sense Al3ỵ ions and to control their impact in biosphere In previous reports, Zhengqiang Li and co-workers have reported several detection methods for Al3ỵ ions among which fluorescence detection offers several advantages such as high sensitivity, a facile analysis, an intrinsic * Corresponding author E-mail address: rajtc1962@gmail.com (P Tharmaraj) Peer review under responsibility of Vietnam National University, Hanoi selectivity and the capacity for rapid, real-time monitoring compared to other methods [3,4] Schiff base acts as a good optical and fluorescent sensor due to its photophysical properties Triazine Schiff base, formed with active carbonyls and amines, will enhance the property of fluorescence on complexation It has been considered in various fields of chemistry due to its specific properties like co-ordination ability which makes them applicable as a fluorescent sensor in catalysts and in biological fields In view of these facts, we designed the receptor as the target sensor that could strongly bind the metal ions through Schiff base moiety [5,6] In the present study, a functionalized triazine derivative, as a uorescent Al3ỵ sensor, is developed with a lowest detection limit of 0.09 mM Our results exhibit an enhanced fluorescence emission in ethanolic solution upon the addition of Al3ỵ ions [6] Experimental 2.1 Materials All the needed chemicals were purchased from sigma Aldrich India and were used without further purification The metal chloride salts were purchased from Merck chemicals Ethanol was purchased commercially and was further purified by a distillation method Melting point of the synthesized ligand and its metal https://doi.org/10.1016/j.jsamd.2019.05.001 2468-2179/© 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/) 238 J.J Celestina et al / Journal of Science: Advanced Materials and Devices (2019) 237e244 complex were analyzed by an electrical heating method at using capillary tubes 2.2 Characterization The 1H NMR and 13C NMR was recorded using a 400 MHz Bruker NMR instrument A FT-IR spectrometer in the range 4000e400 cmÀ1 was used to record the spectra using a KBr pellet The LCMS and HR-LCMS mass spectra were recorded in the Bruker mass spectrometer using acetonitrile as the solvent Absorption and emission properties of NADO with different metal ions were recorded in UV-Visible Jasco (V-530) and FP-6200 Spectrophotometer instruments 2.3 Synthesis of NADO The triazine Schiff base ligand was prepared using ethanol 4nitroaniline (0.50 g, mmol) in acetone, was stirred with cyanuric chloride under ice cold condition for about h and the yellow solid obtained was filtered and dried The precipitate obtained was allowed to stir for an hour with hydrazine hydrate to get (a) 4, 6-dihydrazinyl-N-(4-nitrophenyl)-1,3,5-triazine-2 amine To the dried precipitate (0.20 g, mmol), Isatin (b) (0.24 g, mmol) in ethanol was gradually added at 80 C and refluxed for 24 h As a completion of the reaction, a dark yellow precipitate NADO (c) starts separating out It is shown in Fig.S1 The precipitate obtained was collected by filtration and washed several times with ethanol and dried in vacuum It was purified by column chromatography using neutral alumina in a mixture of (Ethyl acetate/Hexane) 2.4 Synthesis of Al3ỵ-NADO complex The complex was prepared using an ethanolic solution of AlCl3 (0.15 g, mmol) and an ethanolic solution of NADO (0.32 g, mmol) in 1:1 (ligand:metal) molar ratio by refluxing for two hours as shown in Fig.S2 The obtained precipitate was washed with absolute ethanol and dried 2.5 Job's plot measurements NADO (1 mg) and 0.1 mg of AlCl3 are dissolved in ethanol The solution mixture is taken and poured into the glass vials Different Table Physical and analytical data of NADO and [Al(NADO)Cl]Cl2 complex Compound C25H17N11O4 (NADO) [Al(NADO)Cl]Cl2 M.W g/molÀ1 535.47 667.10 Colour Yellow Dark Yellow M.p ( C) Calculated (Found) (%) C H O N M 56.08 (56.05) 50.14 (50.16) 3.20 (3.18) 3.03 (3.04) 11.95 (11.88) 25.73 (25.70) 28.77 (28.79) 22.01 (21.98) e (4.51) 4.53 195e198 C 254e256 C Fig (a) UVevis spectrum of ligand (NADO), (b) UVevis spectra of NADO (20 mM) after addition of metal cations and anions (c) UVevis spectra of NADO after adding (0.10e0.90) mM of Al3ỵ ion J.J Celestina et al / Journal of Science: Advanced Materials and Devices (2019) 237e244 239 concentrations (0.1e1.0 mM) of AlCl3 are added to the vials containing the NADO and shaked for getting clear solutions After shaking the solution is transferred into the UV-visible cuvette and the UVevis absorption spectrum is taken using UV-visible spectrophotometer between the ranges 200 nme800 nm at room temperature 2.6 Elemental analysis and conductivity measurements The synthesized NADO and [Al(NADO)Cl]Cl2 complex was obtained in good yield with high purity The data of the complex confirms the formation of 1:1 (M:L) coordination Al3ỵ with NADO as shown in Table The percentage of C,H,N was measured in a Vario EL- III CHNS Analyzer Melting points of the synthesized NADO and its [Al(NADO)Cl]Cl2 complexes were analyzed in the buchi melting point analyzer The electrolytic conductance of the synthesized complex was studied with the help of the conductometer with ethanol as the solvent Initially the cell constant value is Fig FT-IR Spectrum of NADO Results and discussion 3.1 Absorption studies of NADO The sensing property of NADO with different metal cations was investigated by preparing stock solutions of NADO (1 Â 10À3 mM) and metal cations in anhydrous ethanol [7] and its coordination properties with various metal cations were measured by the UVeVis Spectrometer The free NADO showed maximum absorption bands at 268 nm (37,313 cmÀ1) and 326 nm (30,675 cmÀ1) which are due to the p / p* transition [8] as shown in Fig 1(a) When the metal ions were added to the NADO, there was no specific change observed with the metal cations and anions like (Zn2ỵ, Ni2ỵ, Fe2ỵ, Fe3ỵ, Cu2ỵ, Al3ỵ, Co2ỵ, Mg2ỵ, Ca2ỵ, Bi2ỵ, 3 Hg2ỵ, As2ỵ, Cr3ỵ, CO2 and Cr2O2 , SO4 , PO4 , NO , NO ) except for Al3ỵ which gave a new band with increasing intensity Table UV-Vis spectral data of NADO and [Al(NADO)Cl]Cl2 complex Compounds NADO [Al(NADO)Cl]Cl2 Frequency cmÀ1 30,675 37,313 19,417 15,220 Transition p / p* 3 A2g(F)/ T1g(P) T1g(F)/4A2g(F) Geometry Electrolytic conductance S/m e e distorted octahedral 173 Fig FT-IR Spectrum of the complex [Al(NADO)Cl]Cl2 at 515 nm (19,417 cmÀ1) corresponding to the 3A2g(F) / 3T1g(P) transition and at 657 nm (15,220 cmÀ1) corresponding to the T1g(F) / 4A2g(F) region as clearly shown in Fig 1(b) [9e11] The Fig (a) Emission spectra of NADO with different metal ions and (b) Emission intensity at 469 nm as a function of Al3ỵ at different concentrations (0.10e0.80 mM) 240 J.J Celestina et al / Journal of Science: Advanced Materials and Devices (2019) 237e244 Table Infrared spectral data of the compound NADO and the [Al(NADO)Cl]Cl2 complex Compound n C¼N cmÀ1 n C¼N cmÀ1 triazine ring n N-H cmÀ1 n Al-N cmÀ1 n Al-O cmÀ1 n C-N cmÀ1 Ligand (NADO) [Al(NADO)Cl]Cl2 1626 1590 1401 1398 3139 3129 e 568 e 749 1324 1327 latter transition might be due to the formation of a distorted octahedral complex of Al3ỵ with NADO [1] as presented in Table Though Zn2ỵ gave a small shift in the absorbance it is considered negligible and it is not because of the complex formation [12] The interference experiments clearly show that Zn2ỵ has no interference in selectively detecting Al3ỵ In Fig 1(c) with increasing equivalents of Al3ỵ added to the solution of NADO, the concentration of Al3ỵ increases with the consecutive decrease in the concentration of NADO which conrms the presence of the Al3ỵ complex in equilibrium with the free NADO A well-defined isosbestic point at 401 nm clearly confirms the complex formation of Al3ỵ with the NADO resulting in a red shift [13,18,34] 3.2 Fluorescent studies of NADO Fig Jobs Plot for determining the stoichiometry of NADO and Al3ỵ in ethanol The selective ability of NADO towards Al3ỵ in uorescence in the presence of various metal ions was investigated in ethanol, as shown in Fig 2(a) Free NADO showed weak fluorescence emission bands due to fluorescent quenching by lone pair of electrons from oxygen and nitrogen atoms, which results in the photo-induced electron transfer (PET) [14] When Al3ỵ ions were added, the fluorescence intensity of the ligand at 469 nm was enhanced without any spectral changes which indicates a high selectivity of NADO for Al3ỵ ions by means of chelation [11,15] Upon addition of (Zn2ỵ, Ni2ỵ, Fe2ỵ, Fe3ỵ, Cu2ỵ, Al3ỵ, Co2ỵ, Mg2ỵ, Ca2ỵ, Bi2ỵ, Hg2ỵ, 2 As2ỵ, Cr3ỵ, CO2 and Cr2O2À , SO4 , PO4 , NO , NO ) ions into NADO, no obvious fluorescence response could be observed except for Zn2ỵ and Fe2ỵ which ions slightly increased the emission intensity with little interference in detecting Al3ỵ The Fluorescence response of the NADO to various concentrations of Al3ỵ (0.10e0.80 mM) showed a gradual increase in intensity [12,34] as shown in Fig 2(b) At 0.80 mM the maximum precipitation occurs Fig Proposed sensing mechanism of Al3ỵ ion by NADO J.J Celestina et al / Journal of Science: Advanced Materials and Devices (2019) 237e244 241 Fig HOMO and LUMO energy levels of NADO and [Al(NADO)Cl]Cl2 The interference studies clearly show the selective sensing ability of Al3ỵ ion compared with that of other cations with the highest fluorescence intensity at 469 nm It underlines the excellent specicity to Al3ỵ 3.3 FT-IR spectral studies In Fig 3, the NADO has a broad absorption band in the region of 3139 cmÀ1 due to the e NH stretching vibration [12] This band is slightly shifted by the formation of the Al3ỵ complex and a new band appears at 3129 cmÀ1 [16e18] which indicates the formation of a metal complex with nitrogen present in the NH group that is present in NADO [19,20] The sharp intense band in the region of 1324 cmÀ1 in the ligand is attributed to the carbonyl C-N stretching vibration and it was shifted to higher frequency 1327 cm1 for the Al3ỵ complex The band at 1401 cmÀ1 is characteristic of triazine moiety which is shifted to 1398 cmÀ1 The sharp peak at 749 cmÀ1 strongly supports the bond formation nAl-O of metal through carbonyl oxygen of the Schiff base ligand to the Al3ỵ ion [20,21] The metal complex formation was further confirmed by the appearance of sharp peaks at 568 cmÀ1 nAl-N in the spectrum of the metal complex in Fig which is assignable to stretching vibrations [22] The coordination through azomethine nitrogen was supported by the shifting of nC ¼ N towards higher frequencies 242 J.J Celestina et al / Journal of Science: Advanced Materials and Devices (2019) 237e244 is attributed to the [NADO] [Al (NADO)Cl]Cl2 complex (calculated: 667.01) [36e39] and is shown in Fig S7 The mass data, therefore, confirm the binding of Al3ỵ to ligand with 1:1 stoichiometry [16] The conductivity measurement value of 173 S/m corresponds to the proposed structure with two chlorine ions present outside the coordination sphere in the metal complex (Fig 6) 3.5 Theoretical studies Fig Effect of pH on [Al(NADO)Cl]Cl2 complex with respect to that observed at 1626 cmÀ1 in the free ligand while the new shifted peak appears at 1590 cmÀ1 in the metal complex given in Table [11,16,23] The absence of the carbonyl amide band at 1660 cmÀ1 in NADO is a strong evidence for amidoeimido tautomerism which takes place before complexation It is also correlated by the decrease in the NH bending vibrations of the Al3ỵ metal complex with NADO and by the presence of a new peak at 1589 cm1 in the spectrum of the Al3ỵ NADO complex [24] In the metal complex, the bands shifts to lower frequency indicate that the tautomerism is inhibited in the Al3ỵ complex and that the imine nitrogen and carbonyl oxygen are involved in the coordination to the metal [25] 3.4 NMR and mass spectral studies To understand the binding mode of NADO with Al3ỵ, a 1H NMR spectrum analysis is done in DMSO-d6 The ne spectra of the NADO and Al3ỵ metal complex are shown in Fig.S3 and Fig.S4 The H NMR spectrum of NADO shows the characteristic imine peak at 8.11 ppm and the aromatic protons are centered from 6.96 ppm to 7.87 ppm The 1H NMR spectrum of the Al3ỵ complex shows the merging of two singlet peaks at 6.98 ppm, a doublet at 7.6 ppm and they are downshifted [26] A peak at 8.2 ppm undergoes a downshift as well Complexation of NADO with Al3ỵ was conrmed by the shifted proton peaks towards lower magnetic fields due to the reduction of the electron density upon coordination to the metal ion [27e31] 13C NMR spectrum of NADO as given in Fig.S5 also confirms the formation of NADO The Mass spectrum of the NADO and Al3ỵ are examined by LCMS and HRLCMS mass spectrum analysis using acetonitrile as the solvent [32,33] Fig S6 clearly confirms the formation of the NADO with the observed m/z peak at 536.53 (M ỵ H)ỵ [34,35] The observed m/z peak for NADO in the presence of Al3ỵ at 666.06 (M-H)- peak In order to support the results obtained from the 1H- NMR and HR-LCMS mass spectral analysis, an optimization of the structures of both NADO and [Al (NADO)Cl]Cl2 were performed at the hybrid basic sets B3LYP/6-3 g(d) level in the Gaussian program The obtained result is in accordance with the results of the NMR and Mass spectral data [40] The optimized results exposed an octahedral structure It is found that [Al (NADO)Cl]Cl2 is better stabilized than NADO The energy gap of HOMO and LUMO of NADO and the Al3ỵ complex is (DE ẳ 3.5919 eV and DE ẳ 2.4038 eV) The value of the Al3ỵ complex is found to be comparatively lower than that of NADO, indicating a bathochromic shift towards longer wavelength in the absorption spectrum In the optimized structure of the Al3ỵ NADO complex HOMO as given in Fig 7, electrons are mostly occupied in the ligand rings whereas, on the other hand, LUMO electrons are occupied both by the ligand rings and the Al center In view of these results, ICT (intramolecular charge transfer) is taking place efficiently in the mechanism of forming the Al3ỵ complex [41] (Fig 8) 3.6 Stoichiometry and binding mechanism of NADO with Al3ỵ The binding mechanism of NADO e Al3ỵ was studied by uorescence spectral changes, by mass spectrometry [42] and by 1H NMR and FT-IR spectral studies [38] Free NADO, which has poor fluorescence, is enhanced by the isomerization of C¼N (azomethine carbon and hydrogen) double bond in the excited state [36] by Al3ỵ metal on stable chelation Increase in fluorescence takes place and the Photo Induced Electron Transfer (PET) is inhibited (when the metal gets binded to NADO, the PET is stopped) In Fig 2, NADO, as such being low in fluorescence because of intramolecular photoinduced electron transfer, on rapid addition of Al3ỵ shows enhanced fluorescence Thus the enhancement in fluorescence ultimately leads to the complex formation [11] The detection limit (DL) of NADO to Al3ỵ was found to be lowest with the value of 0.09 mM compared with the recent literatures as given in Table The binding stoichiometry of ligand NADO to Al3ỵ was calculated by the Job's method on the basis of fluorescence emission spectrum In Fig the fluorescence intensity at 469 nm exhibited a maximum mole fraction at 0.50 demonstrating a possible 1:1 binding stoichiometry between Al3ỵ and NADO 3.7 Effect of pH on uorescence in presence of Al3ỵ with NADO To observe the binding interaction of NADO with Al3ỵ at different pH's, both acidic and basic pH solutions were used The interaction between NADO and the Al3ỵ ion was investigated at a Table Detailed comparison of lod of various Al3ỵ sensors by uorescence responses Probe Selectivity Method Solvent LR Lod (mM) Ref Naphthol- Quinoline Fluorescent chemosensor Fluorescent chemosensor Amphiphilic Carbon dots Fluorescent chemosensor Triazine schiff base Al3ỵ Al3ỵ Al3ỵ Al3ỵ Al3ỵ Fluorescence Fluorescence Fluorescence Fluorescence Fluorescence DMF DMSO Ethanol CH3OH/Water Ethanol 0-16 equivalence 0.1e0.5 mM 8e20 mM 0.5e10 mM 0.1e0.8 mM 1.0 mM 0.1 mM 1.1 mM 0.43 mM 0.09 mM [15] [12] [30] [36] Present work J.J Celestina et al / Journal of Science: Advanced Materials and Devices (2019) 237e244 pH range from to 14 [26] The pH of the solution was adjusted using either NaOH or HCl solutions [32] This experiment was carried out at a xed concentration of the NADO and Al3ỵ as 20 mM and 200 mM respectively in ethanol As shown in Fig initially the fluorescence is low and increases with increase in pH [42], it attains a maximum intensity of 469 nm at pH and again starts to decrease in highly basic pH This is due to the precipitation of Aluminium hydroxide [19] Thus, the [Al (NADO)Cl]Cl2 has a higher intensity value at pH and the intensity starts to decrease at higher basic pH Conclusion In this work, a novel triazine based Schiff base, NADO as a fluorescent sensor is developed and its ability of sensing for a wide range of metal ions is studied NADO showed a signicant enhancement in uorescence intensity after adding Al3ỵ ions It can operate as an efficient sensor in the detection of Al3ỵ The NADO displayed a better selectivity for Al3ỵ than that of other competitive metal ions The detection limit is low with the value of 0.09 mM The coordination of NADO with Al3ỵ was conrmed by mass spectrum and Job's plot analysis and the coordination was found to be the 1:1 stoichiometry On the basis of the results, NADO could be applied for the selective detection and recognition of Al3ỵ present in the environment It can be utilized for various applications Acknowledgements The authors thank the Management of Thiagarajar College Madurai for providing the analytical facilities Appendix A Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsamd.2019.05.001 References [1] X.-L Yue, Z.-Q Wang, C.-R Li, Z.-Y Yang, Naphthalene-derived Al3ỵ-selective uorescent chemosensor based on PET and ESIPT in aqueous solution, Tetrahedron Lett 58 (2017) 4532e4536 [2] A Ayman, A Abdel, G.M Rania, M Fatima, Elantabli, M Samir, E.I Medani, A novel fluorimetric 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