The maiden report of a fluorescent-colorimetric sensor for expeditious detection of bifluoride ion in aqueous media

Abstract

A fluorescent-colorimetric chemosensor L, for rapid detection of bifluoride ion has been developed based on a simple bis-Schiff base. The chemosensor L is easy to synthesize, eco-friendly and cost effective. It exhibits an unprecedented selectivity and sensitivity towards bifluoride ion through change in absorption as well as fluorescence intensity. The anion recognition occurs through intermolecular H-bond formation exhibiting colour change from yellow to colourless. The structure of the ligand and its sensing behaviour is well established by DFT and electrochemical studies. The sensitivity of the fluorescence based assay (0.62 μM) for bifluoride ion is the lowest ever reported in the literature. The chemosensor L, providing rapid response time with sufficiently low detection limit may be useful as a valuable practical sensor for analysis of bifluoride ion in environmental samples.

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Introduction

Recently significant attention has been paid to the development of chemosensors for anions due to the important role played by them in environmental, security services, health sciences and industrial processes. Consequently anion recognition has been established as a rapidly growing branch of supramolecular and biological chemistry.¹ Nowadays, widely reported examples of chemosensors or reagents for anion detection commonly use one of the following three main approaches: (i) the ‘binding site-signalling subunit’ protocol, (ii) the ‘displacement’ approach and (iii) the ‘chemodosimeter’ paradigm. In the first approach, ‘binding sites’ and the optical ‘signalling subunit’ are covalently bonded in such a way that the interaction of the anion with the binding site induces electronic modulations in the signalling unit, resulting in change in colour or emission intensity.² In the ‘displacement’ protocol, the coordination of a certain anion to the binding site results in the displacement of the signalling subunit, which is usually accompanied with optical changes.³ Finally, the ‘chemodosimeter’ approach has advantage of anion induced chemical reactions, usually irreversible, which results in changes in fluorescence or colour.⁴ However, many synthetic receptors have been reported, in which various binding and sensing mechanisms are employed,⁵ such as hydrogen bonding,^6,7 binding-induced perturbation of the π-conjugation framework of the sensor molecules⁸ or the catalysis effect of anions to trigger a chemical reaction.⁹ For the sensors based on hydrogen bonds, the receptor may contain subunits like urea, thiourea, amide, phenol, pyrrole etc., which can provide one or more H-bond donor sites for selective binding and sensing of anions¹⁰ and the chromophores are generally indoles,¹¹ bisindole,¹² carbazole,¹³ nitrophenyl,¹⁴ quinine,¹⁵ and nitrobenzene/azogroups¹⁶ and other electron-withdrawing moieties¹⁷ covalently attached to an anion receptor. For this kind of sensor, hydrogen-bond-induced π-electron delocalization, or –NH deprotonation are believed to be responsible for signalling the binding event.

Among various anions, the detection and quantification of bifluoride ion is a challenging task. The bifluoride anion HF₂⁻, is of significant structural and theoretical interest. It is a classic example of semi-ionic, three-centered, four-electron bonding and exhibits the strongest known hydrogen bond.¹⁸ HF₂⁻ is used in insecticides,¹⁹ etching borosilicate glasses,²⁰ fluorescence detection of beryllium in occupational hygiene samples,²¹ synthesising borane derivatives,²² understanding supramolecular interaction,²³ preparation of molten salts²⁴ etc. There are lots of reports of different chemosensors available in literature for common anions like carbonate, fluoride, bisulphite, CN⁻ etc.²⁵ But, there is an acute scarcity of bifluoride sensor. So it is of urgent need to design a suitable bifluoride sensor which can provide real time, onsite analysis of bifluoride ion with high selectivity and sensitivity.

As a part of our on-going research work^26,27 in the design and synthesis of chemosensors for anions, cations and neutral molecules, we have synthesized a simple quinoline based bis-Schiff base ligand L, a 1 : 2 condensate of p-phenylenediamine and 4-quinoline carboxaldehyde, for the detection of bifluoride ion in aqueous solution. The chemosensor L demonstrated the presence of bifluoride ion both by change in absorbance and fluorescence intensity accompanied by instantaneous colour change from yellow to colourless.

Experimental section

General information

UV/visible spectra were recorded on a Shimadzu UV 1800 spectrophotometer using a 10 mm path length quartz cuvette. Fluorescence spectra were recorded on a Horiba spectrophotometer. ¹H and ¹³C NMR spectra were recorded on a Bruker Ultrashield 400 MHz spectrometer using CDCl₃ and NMR titrations were carried out by dissolving L and sodium bifluoride in DMSO-d₆ and D₂O respectively at room temperature and the chemical shifts were reported in δ values (ppm) relative to TMS. High resolution mass (HRMS) spectra were recorded on a Waters mass spectrometer using mixed solvent HPLC methanol and triple distilled water. All the chemicals and metal salts were purchased from Merck. All the anions are of sodium salts and re-crystallized from water (Millipore) before use. Solutions of the receptor L (5 × 10⁻⁵ M) and anions (5 × 10⁻⁴ M) were prepared in THF–H₂O (2/1, v/v) and H₂O respectively. Human urine sample collection and determination of bifluoride ion, in those samples were performed in the Dept. of Microbiology, CIMS Hospital, Bilaspur.

Synthesis of the probe L. To a dehydrated methanol solution of p-phenylenediamine (0.108 g, 1 mmol, 50 mL), 4-quinolinecarboxaldehyde (0.314 g, 2 mmol, in 10 mL dehydrated methanol) was added. The mixture was refluxed for 6 h at 45 °C, maintaining dry condition. A light yellow precipitate was obtained. It was filtered off and washed several times with n-hexane and then finally recrystallized from methanol and dried in air. Yield: 0.330 g, 85%. Mp: 212 °C. ¹H NMR (CDCl₃, δ ppm, TMS): 9.20 (s, 2H), 9.08 (d, 2H), 8.92 (d, 2H), 8.23 (d, 2H), 7.97 (d, 2H), 7.82 (m, 2H), 7.70 (m, 2H), 7.47 (s, 4H) (Fig. S1†). ¹³C NMR (CDCl₃, δ ppm, TMS): 158.4, 150.2, 148.2, 139, 132.9, 132, 128, 125.2, 124.1, 122.2, 121 (Fig. S2†). FT-IR (KBr, cm⁻¹): 1613 (–C=N) (Fig. S3†). ESI-MS: m/z [M + H⁺], 387.16 (100%) (Fig. S4†). Anal. calcd (%) for C₂₆H₁₈N₄: C, 80.81; H, 4.69; N, 14.50. Found: C, 80.75; H, 4.74; N, 14.55%.

Synthesis of the control compound L₁. 0.108 g (1 mmol) of solid p-phenylenediamine was dissolved in 50 mL anhydrous methanol. To this solution 10 mL of anhydrous methanol solution of naphthaldehyde (0.312 g, 2 mmol) was added and the reaction mixture was heated under reflux for 6 h under dry condition. On evaporating the solvent at room temperature a yellow solid precipitated out. Yield: 0.326 g, 91%. Mp: 176 °C. ¹H NMR (400 MHz, CDCl₃, δ ppm): 9.28 (2H, s), 8.11 (2H, d), 8.02 (2H, d), 7.94 (2H, d), 7.64–7.60 (6H, m) (Fig. S5†). ¹³C NMR (CDCl₃, δ ppm, TMS): 156.4, 141.9, 136.1, 135.2, 133.7, 131, 129.4, 128, 124.2 (Fig. S6†). FT-IR (KBr, cm⁻¹): 1601 (–C=N) (Fig. S7†). ESI-MS: m/z [M + H⁺], 385.28 (100%) (Fig. S8†). Anal. calcd (%) for C₂₈H₂₀N₂: C, 87.47; H, 5.24; N, 7.29. Found: C, 87.39; H, 5.28; N, 7.22.

UV-Vis titrations

L (3.86 mg, 0.01 mmol) was dissolved in the solvent mixture THF–H₂O (2/1, v/v) (10 mL) and 30 μL of it was diluted to 3 mL with the solvent mixture to make a final concentration of 10 μM. NaHF₂ (6.2 mg, 0.1 mmol) was dissolved in 10 mL of triple distilled water and 1.5–90 μL of the bifluoride ion solution (10 mM) were transferred to the solution of L (10 μM) prepared above. After mixing them for a few seconds, UV-Vis spectra were obtained at room temperature.

Fluorescence titrations

L (3.86 mg, 0.01 mmol) was dissolved in 10 mL of mixed solvent THF–H₂O (2/1, v/v) to make a solution of 1 × 10⁻³ M and 30 μL of this solution were diluted with 2.97 mL of solvent mixture to make the final concentration of 10 μM. NaHF₂ (6.2 mg, 0.1 mmol) was dissolved in triple distilled water (10 mL) and 1.5–60 μL of this bifluoride solution (10 mM) were transferred to each receptor solution (10 μM) to give 0.5–20 equiv. After mixing them for a few seconds, fluorescence spectra were obtained at room temperature.

Competition with other anions

L (3.86 mg, 0.01 mmol) was dissolved in afore-mentioned solvent mixture (10 mL) and 30 μL of it was diluted to 3 mL with the solvent mixture to make a final concentration of 10 μM. Sodium salts of F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, NO₃⁻, OAc⁻, HCO₃⁻, HSO₃⁻, CO₃²⁻, SO₄²⁻, SO₃²⁻, N₃⁻, CN⁻, S₂⁻ and HF₂⁻ (0.1 mmol) were dissolved in 10 mL of triple distilled water and 6 μL of each solution (10 mM) were added to 3 mL of the solution of receptor L (10 μM) to give 2 equiv. of anion conc. Then, 6 μL of HF₂⁻ solution (10 mM) were added to the mixed solution of each anion and L to make 2 equiv. After mixing them for a few seconds, absorbance spectra were obtained at room temperature.

Job plot measurements

L (3.86 mg, 0.01 mmol) was dissolved in THF (10 mL). 100, 90, 80, 70, 60, 50, 40, 30, 20, 10 and 0 μL of the L solution were taken and transferred to vials. Each vial was diluted with 2.9 mL of a mixed solvent. NaHF₂ (0.01 mmol) was dissolved in triple distilled water (10 mL). 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 μL of the bifluoride ion solution were added to each diluted L solution. Each vial had a total volume of 3 mL. After shaking them for one minute, UV-Vis spectra were obtained at room temperature.

Cyclic voltammetry

Electrochemical experiments were carried out at room temperature using BAS electrochemical workstation in a conventional three electrode system with Ag/AgCl (3 M KCl) as the reference electrode, a platinum wire as the counter electrode and a glassy carbon disc (GC) as working electrode. Tetrabutylammonium perchlorate (0.1 M) solution was used as supporting electrolyte. Nitrogen gas was purged through the electrolytic solution for at least 5 min to remove any dissolved oxygen before every experiment. Nitrogen atmosphere was maintained over the electrolytic solutions during each experiment. Prior to every experiment the GC electrode was cleaned.

pH effect test

A series of buffers with pH values ranging from 2 to 11 was prepared using 100 mM HEPES buffer. After the solution with a desired pH was achieved, receptor L (3.86 mg, 0.01 mmol) was dissolved THF–H₂O (2/1, v/v) (10 mL), and then 30 μL of this solution (1 mM) was diluted to 3 mL with above-mentioned buffers to make the final concentration of 10 μM NaHF₂ (6.2 mg, 0.1 mmol) was dissolved in HEPES buffer (10 mL, pH 7.00). 30 μL of the bifluoride ion solution (10 mM) were transferred to each receptor solution (10 μM) prepared above. After mixing them for a few seconds, fluorescence spectra were obtained at room temperature.

Colorimetric test kit

Receptor L (3.86 mg, 0.01 mmol) was dissolved in THF (10 mL) to get 1 mM solution. Receptor L-test kits were prepared by immersing filter papers into receptor L solution (1 mM), and then dried in air to get rid of the solvent. Sodium salts (SO₄²⁻, SO₃²⁻, S₂⁻, HSO₃⁻, F⁻, OAc⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, HCO₃⁻, HF₂⁻, N₃⁻, NO₃⁻, CN⁻ and CO₃²⁻; 0.001 mmol) was dissolved in distilled water (10 mL), respectively. The test kits prepared above were added into different anion solutions (10 mL), and then dried at room temperature.

Computational details

The GAUSSIAN-09 Revision C.01 program package was used for all calculations.²⁸ The gas phase geometries of the compound was fully optimized without any symmetry restrictions in singlet ground state with the gradient-corrected DFT level coupled with the hybrid exchange–correlation functional that uses Coulomb-attenuating method B3LYP.²⁹ Basis set 6-31++G was found to be suitable for the whole molecule. The electronic spectrum of the receptor L was calculated with the TD-DFT method and the solvent effect (in methanol) was simulated using the polarizing continuum model with the integral equation formalism (C-PCM).^30,31

Results and discussion

Synthesis, structure and DFT study on the receptor L

Receptor L and the control compound L₁ were obtained by 1 : 2 condensation reactions of p-phenylenediamine with 4-quinoline carboxaldehyde/1-naphthaldehyde in methanol with 85% yield (Scheme 1) and characterized by ¹H NMR, ¹³C NMR, IR, ESI-mass spectrometry and elemental analysis.

Scheme 1 Synthetic procedure of the receptor L and control compound L₁.

In order to get the structural information of L and to co-relate its spectral property, DFT calculations were performed on the molecule L. The geometry optimizations staring from gauss view structure of L lead to a global minimum as stationary level. The optimized structure of the L is shown in Fig. 1. The diagnostic experimental and calculated (non-scaled) IR frequencies of L is shown in Fig. S9.† The calculated vibrational stretching frequencies of L, is in good agreement with the experimentally observed data. The simulated absorption spectra of L in presence of the solvent employing the TD-DFT are in good agreements with the experimental data (Fig. S10†). A schematic representation of the energy of the MOs and contours of selected orbitals of the receptor L is presented in Fig. S11.† The HOMO to LUMO energy difference for L is 1.039 eV.

Fig. 1 Geometry optimized structure of ball and stick diagram of L.

Absorption studies of L towards different anions

To investigate the selectivity and specificity of the fluorescent-colorimetric sensing agent L for bifluoride ion, absorption and fluorescence studies of L were performed with other representative analytes, including common metal cations and anions under identical conditions.

The colorimetric selective sensing abilities of chemosensor L were primarily investigated by UV-Vis absorption spectrometry in a THF–H₂O solution (2 : 1, v/v) with various anions (SO₄²⁻, SO₃²⁻, S₂⁻, HSO₃⁻, F⁻, OAc⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, HCO₃⁻, N₃⁻, NO₃⁻, CN⁻, CO₃²⁻ and HF₂⁻) (Fig. 2) and metal cations (Al³⁺, Mg²⁺, Mn²⁺, Fe³⁺, Ba²⁺, Co²⁺, K⁺, Ni²⁺, Ca²⁺, Cu²⁺, Cr³⁺, Zn²⁺, Cd²⁺ and Na⁺) (Fig. S12†). Remarkably, the results showed an excellent selectivity and specificity towards bifluoride ion over all other tested analytes. The probe L without bifluoride ion exhibited three absorption bands at 241, 335 and 381 nm respectively. Among them two bands at 245 nm and 335 nm were assigned to the phenyl π–π* electron transition, while the third band at 381 nm was accredited to the n–π* electron transition that originates from the promotion of non-bonding electrons on terminal N atoms of quinoline moiety to an anti-bonding orbital of L. But, addition of bifluoride ion to a solution of L led to an abrupt decrease in the absorption intensity at 381 nm and 335 nm. This change in absorption spectra is also accompanied by easily discernible colour change from yellow to colourless (Fig. 3).

Fig. 2 Changes in the absorption spectra of L (50 μM) in the presence of 10 equiv. of different anions.

Fig. 3 Visible colour change of probe L on addition of bifluoride and other anions.

However absorption studies carried out with other anions and metal ions (except Al³⁺) did not cause such spectral and visible colour changes, indicating their non-interactive nature with L. Only Al³⁺ has some influence on the absorption behaviour of L showing the same type of colour change as was shown by bifluoride ion.^27a This indicates that under signalling conditions, the possible interference by common metal ions or anions is not of practical problem in bifluoride ion sensing by the probe L. In the titration experiment, upon increasing the concentration of bifluoride ion, the absorption intensity of L at 381 nm significantly decreased with a red shift of 14 nm accompanied by four well-defined isosbestic points at 235, 260, 290 and 407 nm respectively (Fig. 4). The π conjugate system of the probe L undergoes intramolecular charge transfer (ICT) from the donor to the acceptor upon excitation by light, and so the association of bifluoride with L through hydrogen bonding interactions will affect the efficiency of intramolecular charge transfer and reduce the electron-donating ability of quinolinyl-nitrogen atoms leading to the decrease in intensity at 381 nm.

Fig. 4 Change in the absorption spectra of L after the addition of HF₂⁻ up to 10 equiv.

Fluorescence spectroscopy

The fluorometric detection of bifluoride ion by L was also very much distinct. The fluorescence behaviours of receptor L towards various anions (SO₄²⁻, SO₃²⁻, S²⁻, HSO₃⁻, F⁻, OAc⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, HCO₃⁻, HF₂⁻, N₃⁻, NO₃⁻, CN⁻ and CO₃²⁻) were investigated in THF–H₂O (2/1, v/v) (Fig. 5). The low fluorescence intensity of L (λ_em = 450 nm) can be ascribed to the photo-induced electron transfer (PET) process caused by the electron transmission from the two terminal quinolinyl nitrogen atoms to the large π-conjugation system including two [double bond splayed left]C=N– groups and four aromatic rings. On addition of bifluoride ion, the two terminal quinolinyl nitrogen atoms of the L formed H-bonds with the hydrogen atom of the bifluoride ion.

Fig. 5 Fluorescence spectra of L (50 μM) before and after addition of 10 equiv. of various anions in THF–H₂O (2 : 1, v/v).

This hydrogen bonding impeded the PET process resulting in a significant fluorescence enhancement of L–HF₂⁻ adduct accompanied by a prominent blue shift of about 15 nm. As shown in Fig. 5, L exhibited an extremely weak fluorescence on excitation at 330 nm (Q = 0.0169). But on addition of ten equivalent of bifluoride ion, the fluorescence intensity was dramatically increased with high quantum yield (Q = 0.53). As sensitivity is a very important factor in designing a fluorescent chemosensor, the probe L was titrated with bifluoride ion to check its sensitivity towards bifluoride ion (Fig. 6). Upon successive addition of bifluoride ion, the fluorescence intensity gets increased regularly. Fig. 7, which elaborates the titration data in a narrow range shows that the emission intensities of the probe L as a function of added bifluoride sharply increased up to two equivalent of bifluoride ion added, and then the curve became a plateau with further addition of bifluoride ion. From the fluorescence titration profiles, the association constant for L–HF₂⁻ in THF–H₂O mixed solvent was determined as 2.03 × 10⁵ M⁻¹ by a Hill plot (Fig. S13†). By using the above-mentioned titration results, the detection limit for the L–HF₂⁻ adduct was determined to be 0.62 μM on the basis of 3σ/K (Fig. S14†). Importantly, the low detection limit of L for HF₂⁻ suggests that L could be an effective sensor for the detection of bifluoride ion in drinking water. These findings inferred that the chemo-sensor L would be potentially useful for detection of bifluoride ion in different environmental samples.

Fig. 6 Fluorescence spectra of L (50 μM, λ_ex = 330 nm) after addition of increasing amounts of bifluoride ion (up to 30 equiv.) in THF–H₂O (2 : 1, v/v) at room temperature.

Fig. 7 Change in fluorescence intensity of probe L with equiv. of bifluoride ion added.

The selectivity behaviour is obviously one of the most important characteristics of a chemosensor, that is, the relative sensor response for bifluoride ion over other analytes present in solution. In order to evaluate the selectivity of the probe L towards bifluoride, fluorescence studies on L were performed with different anions and metal cations under the similar conditions: the concentration of L was kept at 5.0 × 10⁻⁵ mol dm⁻³ and ten equiv. of analytes were added. As shown in Fig. S15 and S16,† no change in fluorescence intensity is observed in the emission spectra of L after addition of other analytes (except Al³⁺). In presence of Al³⁺, fluorescence intensity of L gets enhanced to some extent but this enhancement is smaller than that induced by bifluoride ion. To further explore the selectivity of L for bifluoride ion, we measured the fluorescence intensity of L in the presence of bifluoride ion mixed with various anions in water (Fig. 8). The emission intensity of probe L and its bifluoride adduct remain unperturbed in the presence of 10 equiv. of other competing ions, indicating excellent selectivity for bifluoride ion over these competing anions.

Fig. 8 Competitive experiment in presence of other anions ((1) L; (2) L + HF₂⁻; (3) L + HF₂⁻ + SO₃²⁻; (4) L + HF₂⁻ + HSO₃⁻; (5) L + HF₂⁻ + F⁻; (6) L + HF₂⁻ + OAc⁻; (7) L + HF₂⁻ + Cl⁻; (8) L + HF₂⁻ + Br⁻; (9) L + HF₂⁻ + I⁻; (10) L + HF₂⁻ + H₂PO₄⁻; (11) L + HF₂⁻ + HCO₃⁻; (12) L + HF₂⁻ + N₃⁻; (13) L + HF₂⁻ + NO₃⁻; (14) L + HF₂⁻ + SO₄²⁻, (15) L + HF₂⁻ + CO₃²⁻ and (16) L + HF₂⁻ + CN⁻).

Sensing mechanism

The anion recognition pattern is mainly based on H-bonding induced mechanism. As bifluoride ion has the strongest aptitude of forming H-bonds among all common anions, it makes firm association with the terminal quinolinyl N atoms through H-bond formations (Scheme 2). To get the complete understanding about the sensing mechanism, another receptor L₁, lacking two quinolone nitrogen atoms was subjected to interact with bifluoride ion, but no perceptible colour change and subsequently no significant change in absorbance spectra was found in comparison to probe L itself (Fig. S17†), suggesting the requirement and involvement of two terminal quinolinyl nitrogen atoms in the probe for efficient fluorescence enhancement.

Scheme 2 Sensing process based on the hydrogen bond recognition mechanism.

The host–guest interaction based on hydrogen bond mechanism can be proved further by experimenting the sensing behaviour of L in both protic and non-protic solvents. In protic solvents (methanol, ethanol) the sensing ability of probe L is retarded to some extent due to formation H bonds between bifluoride ion and solvent molecules establishing the fact of H-bond mechanism. Furthermore the Job's plot measurement confirms the stoichiometry involved in L–HF₂⁻ adduct. It shows 1 : 2 stoichiometric ratios between receptor L and bifluoride ion (Fig. S18†). Both the stoichiometry and binding site involved in sensing mechanism was further vindicated by NMR spectra (Fig. 9). The ¹H-NMR spectra of the probe L in presence of bifluoride ion has been compared with that of the sensor L. In ¹H-NMR spectra of L, the four phenyl protons of the central benzene ring resonate at 7.5 ppm as a sharp singlet. But, in L–HF₂⁻ adduct these protons appear at 7.35 ppm. The imine protons (–CH=N–) of the probe L appear as singlet at 9.20 ppm but these protons are slightly up-fielded in L–HF₂⁻ adduct, appearing at 9.14 ppm. Other quinolinyl protons are also shifted toward upfield. The shift of the ¹H NMR signals of the probe L on addition of 2 equivalent of HF₂⁻ has been shown in Fig. 9. No significant shift of the ¹H NMR signals is observed on increasing the amount of HF₂⁻ concentration more than two equivalents. This may be due to the formation of strong hydrogen bond between bifluoride H atom and quinolinyl N atom.

Fig. 9 ¹H NMR spectra of L and L–HF₂⁻ adduct.

DFT study on sensing mechanism

The sensing mechanism based on hydrogen bonding recognition was also well documented by DFT studies. To calculate the hydrogen bonding energy we have used the same basis sets and the same functional as is used to optimize the structures of the ligand and bifluoride. Bifluoride can form hydrogen bonding with the ligand in two positions, viz. nitrogen in the chain and the terminal position of the quinolinyl ring. The hydrogen bonding is stronger in the terminal one, than inside the chain. The hydrogen bond distance is about 2.32 Å. The optimized structure of L–2(HF₂⁻) has been shown in Fig. 10. A schematic representation of the energy of the MOs and contours of selected orbitals of the receptor L–2(HF₂⁻) adduct is presented in Fig. S19.† The calculated HOMO to LUMO energy difference for L–2(HF₂⁻) is 0.42 eV.

Fig. 10 Geometry optimized structure of ball and stick diagram of L–2(HF₂⁻) adduct.

Cyclic voltammetry

The interaction of bifluoride ion with the chemosensor L has also been investigated by the electrochemical studies. Cyclic voltammogram of L in DMSO–H₂O solvent mixture (1 : 1, v/v) at glassy carbon electrode and Ag–AgCl reference electrode yielded an irreversible oxidation peak at 1.40 V at a scan rate 0.10 V s⁻¹. On addition of aqueous solution of bifluoride ion into the electrolytic medium the oxidation peak gets vanished (Fig. 11).

Fig. 11 Cyclic voltammetric response in DMSO–H₂O solvent mixture (1 : 1, v/v) in presence of 0 mM HF₂⁻ (green line) and 0.50 mM HF₂⁻ (red line) at scan rate 0.1 V s⁻¹.

pH and time effect

The stability of the chemosensor L towards acid and base was checked by ESI-MS of L at different pH values and it has been found that it is stable from pH 4–11 (Fig. S20†). For realistic application, we measured the fluorescence intensity of L at various pH values in presence and absence of bifluoride ion (Fig. 12). The emission peak of L was observed at 452 nm and no appreciable change was observed under neutral and alkaline conditions. However, under acidic conditions (pH = 2 to 4), the emission peak of L was enhanced to some extent with slight blue shift, probably as a consequence of protonation at the quinolinyl nitrogen atoms. Interestingly, L showed good fluorescence sensing ability to bifluoride ion over a wide range of pH values. As shown in Fig. 12, L displayed no appreciable fluorescence response to bifluoride ion at a pH below 4, which may be due to the competition with H⁺ ion. However, L exhibited satisfactory bifluoride sensing abilities when the pH is in the range of 5–11. The intense and almost stable fluorescence of L–HF₂⁻ adduct in wide pH range warrants its application under physiological conditions, without any change in detection results.

Fig. 12 Change of fluorescence intensity of L and L + HF₂⁻ adduct in different pH.

The time evolution of the receptor L in the presence of 10 equiv. of bifluoride ion in afore-mentioned solvent mixture was investigated (Fig. S21†). In THF–H₂O solvent mixture, the recognition interaction gets almost completed just after the addition of 10 equiv. of bifluoride ion and the fluorescence intensity remains almost the same up to 120 min. In protic solvents (methanol, ethanol), the sensing of bifluoride ion by the probe L is delayed in comparison to the solvent mixture used and the sensing capability is best observed in THF. This observation ensures the receptor L to be a sensitive sensor for bifluoride ion, which can be applied in environmental analysis.

Application of chemosensor L

For the practical application of the receptor L, test kits were prepared by immersing filter papers in a THF solution of L (1 mM) and then drying in air. These test kits were utilized to sense HF₂⁻ among different anions. As shown in Fig. 13, when the test kits coated with L were added to different anion solutions, the obvious colour change from yellow to colourless was observed only with HF₂⁻ in THF–H₂O solution (2 : 1, v/v). Therefore, the test kits coated with the receptor L solution would be convenient for detecting HF₂⁻. These results showed that receptor L could be a valuable practical sensor for environmental analyses of HF₂⁻.

Fig. 13 Photographs of the test kits with L (1 mM) for detecting bifluoride ion among other anions.

Determination of bifluoride ion in real samples

In order to evaluate the practical feasibility of the sensor for determination of bifluoride ion, water samples collected from tap, rain water and human urine samples were employed (Table 1). The spiked bifluoride ion concentrations were of 20 and 30 μM. The determined concentrations are: tap water (19.7 and 31.3 μM); rain water (21.7 and 31.8 μM) and urine sample (19.2 and 28.9 μM) respectively. The corresponding recoveries are: tap water (98.5% and 104%); rain water (108% and 106%) and urine sample (96% and 96.3%) respectively. Appreciable recoveries achieved in the determination of bifluoride ion in various water samples and simulated urine samples revealed good practical feasibility of the sensor in quantitative estimation of bifluoride ion of different environmental and biological samples.

Table 1 Determination of HF₂⁻ recovery in different water samples

Samples	Added (μM)	Found (μM)	Recovery (%)	RSD^a (%)
a Relative standard deviation of 3 individual measurements.
Tap water	20	19.7	98.5	1.7
	30	31.3	104	2.3
Rain water	20	21.7	108	2.1
	30	31.8	106	3
Urine sample	20	19.2	96	1.8
	30	28.9	96.3	1.6

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Conclusion

We have developed, for the first time, a simple quinoline-based fluorescent-colorimetric chemosensor L, which exhibits high selectivity and sensitivity for detection of bifluoride ion in aqueous solution. The receptor bound to bifluoride ions in a 1 : 2 stoichiometric manner, induces a fast colour change from yellow to colourless for bifluoride ion over other anions with significant change in both absorbance and fluorescence spectra. These results could be explained by the hydrogen bonding ability of the bifluoride ion. There are several advantages associated with L for bifluoride detection (i) a simple synthesis of receptor L; (ii) high selectivity over other competing anions such as, HCO₃⁻, H₂PO₄⁻ and HSO₃⁻ in aqueous solution; (iii) practical application of L by using test kits and (iv) real sample analysis. Therefore, receptor L may constitute a simple and inexpensive chemosensor which demonstrates a highly viable and useful application for the detection of bifluoride ion in aqueous environment.

Acknowledgements

G. K. P. would like to thank the Department of Science and Technology and Department of Biotechnology, Government of India, New Delhi for financial support. The authors are indebted to Dr R. Murthy, Medical Superintendent, CIMS Hospital Bilaspur for allowing to perform few experiments in CIMS Hospital, Bilaspur.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21868g

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