A novel coumarin based molecular switch for dual sensing of Zn(II) and Cu(II)

Abstract

An efficient coumarin based molecular switch for the dual sensing of two environmentally as well as biologically important cations Zn²⁺ and Cu²⁺ has been synthesized. The receptor H₂L shows about 6 fold enhancement in fluorescence intensity upon the addition of Zn²⁺ and also exhibits quenching of emission intensity upon the addition of Cu²⁺ without interference of other metal ions present in solution. In the case of other metal ions no significant change in the emission intensity is observed. H₂L presents a tunable system comprising of two INHIBIT logic gates with Zn²⁺ and Cu²⁺ or Zn²⁺ and EDTA as chemical inputs by monitoring the emission mode. An IMPLICATION logic gate is obtained with Cu²⁺ and EDTA as chemical inputs and emission as the output mode.

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Introduction

Zinc(II) and copper(II) play a very crucial role in the human body and serve as the second and third most abundant transition metal ions after iron(III), found in the human body.¹ The highest concentration of zinc in the human body is found in the brain and plays an important role in many biological processes such as regulation of gene expressions, apoptosis, neural signal transmitters and catalytic cofactors.² However metabolic disorders of zinc(II) lead to several neurodegenerative diseases such as Parkinson's and Alzheimer's diseases.³ Again, inadequate level of zinc leads to retardation of growth, decrease in the immunological defense, eye lesion and some skin diseases.⁴ Copper(II) also plays a significant role in biological, environmental and chemical systems.⁵ The disorder in Cu(II) metabolism may lead to severe diseases, such as Alzheimer's and Wilson's diseases, amyotrophic lateral sclerosis, Menkes syndrome and haematological manifestations.^6–11 Thus detection of zinc(II) and copper(II) is of utmost importance from both environmental as well as biological point of views.

Development of artificial chemosensors based on fluorescence technique has emerged out to be a powerful detection tool owing to its simplicity, sensitivity and tunability.¹² In few recent years, several small molecules for the detection of zinc(II),¹³ by the enhancement of fluorescent intensity have been reported while several reported molecules have the capability to detect copper by quenching of fluorescent intensity.¹⁴ Most of the reported chemosensors for zinc suffer the problem of interference from Cd²⁺ because of similarity in the electronic configuration.¹⁵ Till date only a few chemosensors with dual sensing properties for metal ions have been reported.¹⁶ Thus the development of dual chemosensors for the detection of vital elements still remains an active field of research.

Development of various chemical systems to exhibit operations such as AND, OR, NOT and their integrated operations have been carried out.¹⁷ Small molecules with more than one output channel are of utmost interest as they form the basis of molecular logic gates capable of performing several arithmetic operations.¹⁸ Various single molecules have been exploited for the construction of many useful integrated logic gates such as INHIBIT, half subtractor, half adder, full adder, and full subtractor.¹⁹ However very few IMPLICATION gates have been reported so far.²⁰ Recently L. Zhao et al. has reported several molecular logic gates based on salicylidine Schiff base.²¹ In our previous work we have reported an INHIBIT logic gate based on a coumarin Schiff base with Al³⁺ and EDTA as chemical inputs.²²

In this present work we have reported the synthesis and spectral characterizations of a coumarin based organic framework which has the dual sensing property for zinc(II) and copper(II) by subsequent enhancement and quenching of fluorescence intensity. Coumarin framework exhibits various interesting photophysical properties such as Stokes shift and visible excitation and emission wavelengths, and also has high importance as fluorescent dyes.²³ Only a few coumarin based chemosensors are reported so far for the dual sensing of metal ions.²⁴ Till date, there has been no report of coumarin based chemosensor for dual sensing of Zn²⁺ and Cu²⁺. Only a few reported chemosensors are known to selectively recognize these two vital elements zinc and copper. S. Wang et al. has reported a binaphthyl-derived salicylidene Schiff base for dual sensing of Cu(II) and Zn(II).²⁵ Y. Liu et al. has reported a fluorescent ‘off–on–off’ probe for relay recognition of Zn²⁺ and Cu²⁺ derived from N,N-bis(2-pyridylmethyl)amine²⁶ while L. Qua et al. has recently reported a pyridoxal-based dual chemosensor for zinc and copper.²⁷ However, most of these reported chemosensors have lower binding constants for Zn²⁺ and Cu²⁺ and also higher limit of detection, compared to that of our newly developed receptor H₂L. In certain cases, the receptor is synthesized using several steps along with the use of reagents which are difficult to handle. Whereas in the present case, the synthetic route towards H₂L is very facile and economically cheap. The developed chemosensor is highly efficient in the detection of zinc(II) with enhancement of fluorescence intensity by 6 fold while it detects copper by quenching of emission intensity, by 7 folds. H₂L presents a tunable system comprising of two INHIBIT logic gates with Zn²⁺ and Cu²⁺ or Zn²⁺ and EDTA as chemical inputs. An IMPLICATION logic gate is obtained with Cu²⁺ and EDTA as chemical inputs and emission as the output mode. H₂L exhibits very high selectivity only for copper and zinc with no interference from any other metal ions including cadmium. Thus the synthesized chemosensor H₂L is an important addition to the list of few reported simple organic molecules which can detect zinc and copper selectively.

Results and discussion

Synthesis and spectral characterisation

Synthetic route towards H₂L involves a very facile and economically cheap route using Schiff base condensation of 3-acetyl-4-hydroxycoumarin with p-phenylenediamine in 2 : 1 molar ratio in methanolic medium under refluxing condition for 6 hours (Scheme 1).

Scheme 1 Synthesis of chemosensor H₂L.

The ligand H₂L may exist in equilibrium in the keto and enol form by excited state intramolecular proton transfer process (ESIPT) which is further supported by the small energy gap (ΔE = 8.025 kcal mol⁻¹) between the two tautomeric forms (Scheme 2). The energies calculated by DFT/B3LYP/6-31G(d,p) method indicate that the keto form is more stable than the enol form by 8.025 kcal mol⁻¹. IR spectrum of H₂L taken in KBr disk shows a stretching band at 1698 cm⁻¹ corresponding to lactone C=O, the keto C=O and C=C appears at 1610 cm⁻¹ and 1547 cm⁻¹ respectively (Fig. S1†). ¹H-NMR spectra are recorded in CDCl₃ which shows a band at around δ 15.45 which is due to the hydrogen bonded NH proton (Fig. S4†). This peak vanishes in the H₂L–Zn²⁺ complex indicating co-ordination to the metal centre through N donating site of the enol form (Fig. S5†). The aromatic protons in H₂L appear as expected in the region δ 8.10–7.27. The –N=C(CH₃)– protons appear at δ 2.79 as singlet. All aromatic protons appear at a bit downfield position compared to that of H₂L, which can clearly be explained due to the co-ordination of Zn²⁺ with H₂L. However, the coordination of H₂L with Cu²⁺ could not be studied by NMR spectroscopy owing to the paramagnetic nature of Cu²⁺ ion. Mass spectrum shows m/z peak corresponding to H⁺[H₂L] at 481.2 for H₂L (Fig. S6†). For H₂L–Zn²⁺ complex the strong peak at 703.3 correspond to Na[Zn₂(L)Cl₂]⁺ along with a weak peak at 739.4 corresponding to Na[Zn₂(L)Cl₂(H₂O)₂]⁺ species (Fig. S7†). For H₂L–Cu²⁺ complex the strong peak at 699.2 correspond to Na[Cu₂(L)Cl₂]⁺ along with a weak peak at 717.3 corresponding to Na[Cu₂(L)Cl₂(H₂O)]⁺ (Fig. S8†) species supporting 1 : 2 complex formation for both zinc and copper complexes.

Scheme 2 Keto–enol tautomerism of H₂L and their relative energies calculated by DFT/B3LYP/6-31G(d,p) method.

Cation sensing studies of H₂L

UV-vis study. Receptor H₂L (20 μM) shows a strong absorbance band at 344 nm, and at 238 nm in 1 : 1, v/v CH₃CN : H₂O using HEPES buffered solution at pH = 7.2. Gradual addition of Zn²⁺ (40 μM) shows a decrease in absorbance intensity at 344 nm with the appearance of a new absorption band at 265 nm (Fig. 1). Formation of this new band at 265 nm indicates the co-ordination of the receptor to Zn²⁺. Again, on gradual addition of Cu²⁺ (40 μM) formation of low energy band is observed at 459 nm along with a new absorption band at 270 nm supporting the coordination of Cu²⁺ to the receptor (Fig. 2). UV-vis spectrum of H₂L is also studied in presence of other metals i.e. Na⁺, K⁺, Ca²⁺, Mg²⁺, Mn²⁺, Fe³⁺, Cr³⁺, Co²⁺, Ni²⁺, Al³⁺, Cd²⁺ and Hg²⁺ but no significant changes are observed except for Hg²⁺ and Ni²⁺ (Fig. S9†).

Fig. 1 Change in UV-vis spectrum of H₂L (20 μM) upon gradual addition of Zn²⁺ (40 μM) in 1 : 1, v/v CH₃CN : H₂O.

Fig. 2 Change in UV-vis spectrum of H₂L (20 μM) upon gradual addition of Cu²⁺ (40 μM) in 1 : 1, v/v CH₃CN : H₂O.

Fluorescence study. In the absence of metal ions the emission spectrum of the synthesized chemosensor H₂L shows a band with moderate emission intensity and maxima (F₀) at 484 nm (λ_excitation, 344 nm). The fluorescence quantum yield (ϕ = 0.012) is very poor. Gradual addition of Zn²⁺ to the above solution shows fluorescence enhancement by 6 fold (ϕ = 0.058) and the maxima at 484 nm vanished with the formation of new emission maxima at 466 nm (Fig. 3). This blue shift of 18 nm is due to co-ordination of the metal centre to the receptor. Thus the fluorescence enhancement reflects a strong selective OFF–ON fluorescent signaling property of H₂L for Zn²⁺. While on addition of Cu²⁺ to the receptor solution quenching of emission intensity is observed by 7 folds, with the formation of a new emission maxima at 506 nm (Fig. 4). On addition of EDTA to the H₂L–Zn²⁺ solution, fluorescent intensity at 466 nm gradually decreases and the emission band at 484 nm reappears (Fig. S10†). Again, on addition of EDTA to H₂L–Cu²⁺, emission intensity again increases and the emission maxima blue shifted from 506 nm to 484 nm (Fig. S11†).

Fig. 3 Change in emission spectrum of H₂L (20 μM) upon gradual addition of Zn²⁺ (40 μM) in 1 : 1, v/v CH₃CN : H₂O.

Fig. 4 Change in emission spectrum of H₂L (20 μM) upon gradual addition of Cu²⁺ (40 μM) in 1 : 1, v/v CH₃CN : H₂O.

Mole ratio plot obtained from fluorescence titration indicates that the receptor shows an increase in emission intensity till the ratio of Zn²⁺ : H₂L reaches ∼2, after that there is hardly any increase in emission intensity (Fig. S12†). Quenching of fluorescence intensity occurs till the ratio of Cu²⁺ : H₂L reaches 2 (Fig. S13†). Jobs plot of emission intensity shows maxima in the plot corresponding to ∼0.65 mole fraction for H₂L–Zn²⁺ complex (Fig. S14†) and at ∼0.66 mole fraction for H₂L with Cu²⁺, reflecting 1 : 2 complex formation in both the cases (Fig. S15†). From emission spectral change, limit of detection of the chemosensor for Zn²⁺ and Cu²⁺ are determined using the equation LOD = K × SD/S where ‘SD’ is the standard deviation of the blank solution and ‘S’ in the slope of the calibration curve (Fig. S16 and S17†). The limit of detection for Zn²⁺ is 1.94 × 10⁻⁸ M from fluorescent titration while that of Cu²⁺ is found to be 1.87 × 10⁻⁹ M. This result clearly demonstrates that the chemosensor is highly efficient in sensing Zn²⁺ as well as Cu²⁺ even in very minute level. From fluorescent spectral titration the association constant of H₂L with Zn²⁺ and Cu²⁺ are found to be 1.8 × 10⁹ and 1.34 × 10¹⁰ respectively (Fig. S18 and S19†).

Fluorescence emission intensity of H₂L (20 μM) is studied in presence of other metals i.e. Na⁺, K⁺, Ca²⁺, Mg²⁺, Mn²⁺, Fe³⁺, Cr³⁺, Co²⁺, Ni²⁺, Al³⁺, Cd²⁺ and Hg²⁺ (40 μM) in CH₃CN : H₂O (1 : 1, v/v, pH = 7.2) but there is hardly any change in emission intensity of H₂L except in presence of Zn²⁺ and Cu²⁺ (Fig. S20†).

In order to study the selectivity of H₂L for Zn²⁺ and Cu²⁺, interference experiment is carried out by recording the emission intensity of H₂L (20 μM) in presence of other metal ions like Na⁺, K⁺, Ca²⁺, Mg²⁺, Mn²⁺, Fe³⁺, Cr³⁺, Al³⁺, Co²⁺, Ni²⁺, Cd²⁺ and Hg²⁺ (40 μM) before the addition of Zn²⁺ and Cu²⁺. It is observed that the various competitive metal ions do not cause any significant interference both for Cu²⁺ and Zn²⁺. The addition of Cu²⁺ to H₂L + Zn²⁺ causes sharp quenching of emission intensity as expected from association constant values. Thus H₂L basically shows an OFF–ON–OFF signally pattern in presence of Zn²⁺ and Cu²⁺. To verify whether Cu²⁺ is responsible for the quenching of emission intensity, ascorbic acid is added to mask the effect of Cu²⁺ and the resulting solution shows an enhancement of fluorescence intensity as expected (Fig. 5).

Fig. 5 Emission intensity of H₂L (20 μM) upon addition of Na⁺, K⁺, Ca²⁺, Mg²⁺, Mn²⁺, Fe³⁺, Cr³⁺, Al³⁺, Co²⁺, Ni²⁺, Cu²⁺, Cd²⁺ and Hg²⁺ (40 μM) in CH₃CN : H₂O (1 : 1, v/v, pH = 7.2) (red bars). Zn²⁺ (40 μM) in presence of other metal ions (green bars). Cu²⁺ and Zn²⁺ both 40 μM in presence of other metal ions (navy blue bars).

The effect of pH on the emission intensity of the receptor (H₂L) in absence and presence of Zn²⁺ and Cu²⁺ are studied. In case of H₂L there is hardly any change in fluorescence intensity in the pH range 1–4 (Fig. 6). Below pH 4 high fluorescence intensity is observed due to protonation of imine N and hydroxy O atoms preventing the excited state intramolecular proton transfer (ESIPT) process, which is responsible for the quenching of fluorescence intensity. On addition of 2 equivalents of Zn²⁺ the fluorescence intensity remains almost unchanged in the pH < 4, while there is a sharp increase in fluorescence intensity in the pH range 5–9 compared to H₂L. But, on further increase in pH fluorescence intensity drops drastically due to the formation of Zn(OH)₂ at pH > 9. On addition of Cu²⁺, quenching of fluorescent intensity of H₂L is significant in the pH range 4–9. Thus H₂L forms stable complex with Zn²⁺ and Cu²⁺ in the pH range of 5–9 hence the developed receptor H₂L can detect Zn²⁺ and Cu²⁺ in this pH range. However at low pH values (pH < 4) receptor tends to combine with protons and hence becomes ineffective in detection of Zn²⁺ and Cu²⁺.

Fig. 6 pH dependence of fluorescence intensity of H₂L and its complexes with Zn²⁺ and Cu²⁺.

Application as logic function

The developed chemosensor H₂L can be utilised as a binary logic function with dual stimulating inputs as Zn²⁺ and Cu²⁺ and emission as output. As a result of coordination of H₂L with Zn²⁺(IN1) a new emission band appears at 466 nm. Upon gradual addition of Cu²⁺(IN2), the emission intensity of the band at 466 nm gets quenched. This preference of the receptor H₂L in binding with Cu²⁺ even in presence of Zn²⁺ is well explained from the binding constant values. Addition of Cu²⁺ alone to H₂L also results in quenching of emission intensity. The threshold value of fluorescence intensity is taken to be 190. OUT = 0 when intensity is less than 190; OUT = 1 when intensity is higher than 190. Now OUT = 1 only when Zn²⁺ is present alone. Actually it represents an AND gate with an inverter²⁸ in one of its input. Thus the emission change at 466 nm with Zn²⁺ as well as Cu²⁺ (with an invertor) as inputs can be interpreted as a monomolecular circuit showing an INHIBIT logic function²² (Fig. 7).

Fig. 7 Truth table and the monomolecular circuit based on Zn²⁺ and Cu²⁺.

When EDTA (40 μM) is added to the H₂L–Zn²⁺ complex the solution shows a decrease of emission intensity and the band at 466 nm is disappeared, suggesting that the receptor H₂L has again returned to its free form. However in the absence of Zn²⁺, EDTA does not have any effect on the emission intensity of the receptor H₂L (Fig. 8). Thus with two chemical inputs as Zn²⁺ and EDTA, H₂L functions as an AND gate with an inverter in the EDTA input by monitoring the emission output. This function can be interpreted as a monomolecular circuit showing an INHIBIT logic function. On the other hand when EDTA is added to the H₂L–Cu²⁺ solution, an obvious enhancement of fluorescence intensity is observed due to complex formation of EDTA with Cu²⁺ making the receptor free. Thus the emission intensity value at 484 nm is low only when Cu²⁺ is present. When Cu²⁺ is present along with EDTA the emission intensity at 484 nm is high. Thus it actually represents an OR with an inverter in one of its input which is also called an IMPLICATION logic gate.²⁹

Fig. 8 Truth table and the monomolecular circuit based on Zn²⁺ with EDTA and EDTA with Cu²⁺.

Electronic structure and sensing mechanism

To interpret the electronic structure of H₂L geometry optimization has been performed by DFT/B3LYP method for keto and enol forms in singlet ground state (S₀) (Scheme 2). The energy calculation in S₀ state reveals that the keto form is more stable by 8.025 kcal mol⁻¹ than the corresponding enol form which is consistent with the X-ray structure of this type of molecules.³⁰ The geometry of H₂L–Zn²⁺ and H₂L–Cu²⁺ have been optimized and the energy minimized structures are shown in Fig. S21 and S22† respectively. In the complexes the chemosensor H₂L binds to Zn²⁺ and Cu²⁺ through phenolic-O atom and imine-N. Contour plots of some selected molecular orbitals of H₂L and its complexes with Zn²⁺ and Cu²⁺ are shown in Fig. S23–25.†

To interpret the changes in electronic spectra TDDFT calculation by DFT/B3LYP/CPCM method has been carried out in acetonitrile. The intense band at 338 nm for chemosensor H₂L corresponds to HOMO → LUMO transition (Table S1†). In H₂L–Zn²⁺ the intense HOMO → LUMO transition is observed at 340 nm having ILCT character. For H₂L–Cu²⁺ the very weak transition at 482 nm corresponds to ligand to metal charge transfer transition (LMCT) along with a strong transition at 359 nm corresponding to HOMO−1(β) → LUMO+2(β) transition has been observed.

The chemosensor H₂L shows a weak emission band centered around 484 nm. Upon gradual addition of Zn²⁺, there is an enhancement of fluorescence intensity and a new emission band appears at 466 nm. To interprete whether the excited state intramolecular proton transfer (ESIPT)³¹ is responsible for the quenching of fluorescence intensity for H₂L, theoretical calculations are carried out. The possible intramolecular proton transfer process in ground (S₀) state has been considered (Scheme 2) and there is only 8.025 kcal mol⁻¹ of energy difference between the two forms. So, the hydrogen transfer takes place easily resulting in quenching of fluorescence for H₂L. On coordination with Zn²⁺ this ESIPT process is inhibited resulting in fluorescence intensity enhancement. However, the significant quenching of fluorescence intensity of H₂L upon addition of Cu²⁺ is expected due to the paramagnetic nature of Cu²⁺ ion.³²

Experimental

Material and methods

4-Hydroxycoumarin and 1,4-diaminobenzene were purchased from Aldrich. All other organic chemicals and inorganic salts were available from commercial suppliers and used without further purification.

Elemental analysis was carried out in a 2400 Series-II CHN analyzer, Perkin Elmer, USA. HRMS mass spectra were recorded on Waters (Xevo G2 Q-TOF) mass spectrometer. Infrared spectra were taken on a RX-1 Perkin Elmer spectrophotometer with samples prepared as KBr pellets. Electronic spectral studies were performed on a Perkin Elmer Lambda 25 spectrophotometer. Luminescence property was measured using Perkin Elmer LS 55 fluorescence spectrophotometer at room temperature (298 K). NMR spectra were recorded using a Bruker (AC) 300 MHz FTNMR spectrometer in CDCl₃.

The luminescence quantum yield was determined using carbazole as reference with a known ϕ_R of 0.42 in MeCN. The complex and the reference dye were excited at the same wavelength, maintaining nearly equal absorbance (∼0.1), and the emission spectra were recorded. The area of the emission spectrum was integrated using the software available in the instrument and the quantum yield is calculated according to the following equation:

ϕ_S/ϕ_R = [A_S/A_R] × [(Abs)_R/(Abs)_S] × [η_S²/η_R²].

Here, ϕ_S and ϕ_R are the luminescence quantum yields of the sample and reference, respectively. A_S and A_R are the area under the emission spectra of the sample and the reference respectively, (Abs)_S and (Abs)_R are the respective optical densities of the sample and the reference solution at the wavelength of excitation, and η_S and η_R are the values of refractive index for the respective solvent used for the sample and reference.

Synthesis of the receptor (H₂L)

3-Acetyl-4-hydroxy-2H-chromen-2-one (L)³³ (0.204 g, 1.0 mmol) and 1,4-diaminobenzene (0.054 g, 0.5 mmol) were refluxed for 6 hours in methanolic medium. Solvent was evaporated under reduced pressure and then dissolved in dichloromethane which is then further subjected to silica gel (60–120 mesh) column chromatographic separation. The desired light yellow solid product was obtained by elution with 35% ethyl acetate–pet-ether (v/v) mixture. Yield was, 0.399 g, 83%.

Anal. calc. for C₂₈H₂₀N₂O₆ (H₂L): calc. (%) C 69.99, H 4.20, N 5.83. Found (%), C 69.03, H 4.01, N 5.21. IR data (KBr, cm⁻¹): 1698 ν(lactone C=O); 1610 ν(keto C=O), 1547 ν(C=C). ¹H NMR data (CDCl₃, 300 MHz): δ 15.45 (2H, s), 8.10 (2H, d, J = 7.4 Hz), 7.60 (2H, t, J = 7.2 Hz), 7.38 (4H, s), 7.27–7.32 (4H, m), 2.79 (6H, s).

General method for UV-vis and fluorescence titration

Stock solution of the receptor H₂L (10 μM) in [(CH₃CN/H₂O), 1 : 1, v/v] (at 25 °C) using HEPES buffered solution at pH = 7.2 was prepared. The solution of the guest cations using their chloride salts in the order of 100 μM were prepared in deionised water. Solutions of various concentrations containing host and increasing concentrations of cations were prepared separately. The spectra of these solutions were recorded by means of UV-vis methods. EDTA solution of 100 μM was added to the same solution where Zn²⁺ and Cu²⁺ were added gradually to H₂L and emission spectra recorded. The spectra of all these solutions were also recorded by means of fluorescence methods.

Job's plot by fluorescence method

A series of solutions containing H₂L (10 μM), ZnCl₂ and CuCl₂ (10 μM) were prepared in such a manner that the sum of the total metal ion and H₂L volume remained constant (4 ml). CH₃CN : H₂O (1 : 1, v/v) was used as solvent at pH 7.2 using HEPES buffer. Job's plots were drawn by plotting ΔF versus mole fraction of Zn²⁺ and Cu²⁺.

Computational method

All calculations were carried out at the B3LYP³⁴ level using Gaussian 09 software package.³⁵ The 6-31G(d,p) basis set was assigned for the elements except for zinc and copper. The LANL2DZ basis set, with an effective core potential for Zn and Cu, was used.³⁶ Vertical electronic excitations based on B3LYP optimized geometries were computed using the time-dependent density functional theory (TDDFT) formalism³⁷ in acetonitrile using conductor-like polarizable continuum model (CPCM).³⁸

Conclusions

Thus we have successfully developed a new coumarin based chemosensor for the selective dual sensing of Cu²⁺ and Zn²⁺ over other metal ions. The receptor H₂L shows about 6 fold increase in fluorescent intensity upon addition of Zn²⁺ and also exhibits quenching of emission intensity upon addition of Cu²⁺ without the interference of other metal ions present in solution. It exhibits two sets of integrated logic gates: (a) one INHIBIT logic gate with Zn²⁺ and Cu²⁺ or Zn²⁺ and EDTA as chemical inputs (b) one IMPLICATION logic gate with Cu²⁺ and EDTA as chemical inputs. We belief that in near future, our designed chemosensor H₂L will lead to several important openings in the synthesis of other important chemosensors with additional application in biological systems.

Acknowledgements

Financial supports received from the Department of Science and Technology, New Delhi, India (no. SB/EMEQ–242/2013) is gratefully acknowledged. D. Sarkar and A. K. Pramanik are thankful to CSIR, New Delhi, India, for their fellowships.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Association constant determination, detection limit determination, ¹H NMR, HRMS, UV-vis titration spectra of HL with different metal ions etc. See DOI: 10.1039/c4ra12920b

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