A water-soluble highly sensitive and selective fluorescent sensor for Hg²⁺ based on 2-(2-(8-hydroxyquinolin)-yl)benzimidazole via ligand-to-metal charge transfer (LMCT)

Abstract

A water-soluble fluorescent sensor (L) based on 2-(2-(8-hydroxyquinolin)-yl)benzimidazole has been synthesized and characterized. Sensor L displays highly selective and sensitive recognition of Hg²⁺ with a fluorescence “ON–OFF” response in buffered aqueous solution (1% dimethyl sulfoxide (DMSO), Tris–HCl 10 mM, pH = 7.4). X-ray crystal structure of the L–Hg²⁺ complex reveals that the oxygen and nitrogen atoms of 8-hydroxyquinoline and the imine N atom of the benzimidazole unit (N1) bind Hg²⁺ through a 1 : 1 binding stoichiometry. The fluorescence quenching mechanism is qualitatively evaluated by quantum chemical calculations which show that the fluorescence quenching phenomenon is caused by ligand-to-metal charge transfer (LMCT) in the excited state.

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Introduction

Hg²⁺ is a chemical with biological toxicity.¹ The harmful effects of Hg²⁺ on the human body are primarily concentrated in the digestive system, central nervous system, and internal organs due to its high affinity to the thiol group in proteins,² and it has some influence on the eyes, skin, blood and respiratory system.³ It has become one of the most notable environmental pollutants in the world due to its persistence, easy mobility, and high bioaccumulation.⁴ Therefore, accurate detection of trace levels of Hg²⁺ in water near neutral pH is necessary. The design of fluorescent sensors for Hg²⁺ has attracted tremendous attention since they facilitate easy, high sensitivity, rapid detection at low cost by simple fluorescence analysis.⁵ Currently, although many types of Hg²⁺ ion-sensing sensors have been introduced to detect Hg²⁺ and used with some success in biological applications, most of the reported sensors have certain shortcomings such as inefficient selectivity,⁶ poor detectivity,⁷ slow response or poor solubility in aqueous media.⁸ Therefore, more sensors are still needed for the analysis of Hg²⁺ ion in various types of sample.

In order to develop a novel and efficient chemosensor, the detailed sensing mechanism should be thoroughly understood. Among the known sensing mechanisms, ligand-to-metal charge transfer (LMCT) has seldom been reported, although it is a classic mechanism in inorganic coordination compounds. The few reported quenching-type sensors based on the LMCT mechanism have been focused on Cu²⁺ ion.⁹ Usually, Hg²⁺ ion causes fluorescence quenching upon binding with fluorescent sensors owing to spin–orbit coupling, electron transfer and energy transfer.¹⁰ However, the mechanistic details of the quenching response induced by Hg²⁺ are unclear in most cases. To date, reports on Hg²⁺-induced fluorescence quenching based on LMCT are rare. To the best of our knowledge, only one article has mentioned that fluorescence quenching of receptor binding to Hg²⁺ is caused by the LMCT mechanism, but there is no further proof.¹¹ Hence, the detailed photophysical studies of the quenching response will be valuable for future work.

In this work, we synthesized a new water-soluble fluorescent sensor (L) (Scheme 1). Sensor L exhibits highly selective and sensitive recognition of Hg²⁺ in aqueous solution through fluorescence “ON–OFF” behavior. The detailed binding mode of L with Hg²⁺ was disclosed by single-crystal X-ray diffraction analysis of the L–Hg²⁺ complex. Quantum calculation results revealed that the Hg²⁺-induced fluorescence “ON–OFF” response of L can be attributed to the LMCT effect.

Scheme 1 Synthesis of sensor L.

Results and discussion

Synthesis, characterization and fluorescent responses of ligand L

Sensor L was synthesized following the procedures as outlined in Scheme 1. The combination of 8-hydroxyquinoline (8-HQ) and the benzimidazole moiety as the fluorophore is based on their respective excellent photophysical properties.¹² A combination of both moieties can provide more binding sites and improve the fluorescence properties. The diglycol monomethyl ether introduced to quinoline can act as a hydrophilic group to improve water solubility. The structure of L was confirmed by ¹H nuclear magnetic resonance (NMR), ¹³C NMR, high-resolution mass spectrometry (HRMS) (Fig. S1–S3, ESI†) and single-crystal X-ray diffraction analysis.

A single crystal, suitable for X-ray diffraction, was obtained by slow evaporation of a dichloromethane solution of L. Single-crystal X-ray diffraction analysis revealed that ligand L crystallizes in an orthorhombic crystal system with a Pbcn space group (Fig. 1). There is one ligand L molecule in the crystallography independent asymmetrical cell. The skeleton formed by fragments of benzimidazole and quinoline, which are connected by ortho carbon atoms, is nearly coplanar. The three O atoms in the flexible chain and the two N atoms in the skeleton are potential chelating sites to certain metal ions.

Fig. 1 Crystal structure of sensor L with the atomic numbering scheme.

In order to examine the metal cation selectivity of L, qualitative experiments were performed by mixing L with various heavy and transition metal ions (Fig. 2). Free L solution (10 μM, 1% DMSO, Tris–HCl 10 mM, pH = 7.4) displays a strong emission at 475 nm (the quantum yield is 0.37). Upon addition of 1.0 equiv. of Hg²⁺, a remarkable fluorescence quenching (90%) was observed and the fluorescence color change from blue to colorless was also detectable by the naked eye (Fig. 2 inset). However, addition of 1.0 equiv. of individual cations such as Cu²⁺, Ag⁺, Pb²⁺, Sr²⁺, Ba²⁺, Cd²⁺, Ni²⁺, Co²⁺, Fe²⁺, Mn²⁺, Fe³⁺, Al³⁺, Cr³⁺, Mg²⁺, K⁺, and Na⁺ promoted no significant fluorescence changes. However, Zn²⁺ could induce a slight emission intensity decrease. This result indicates that L exhibits a high selectivity to Hg²⁺.

Fig. 2 Fluorescence spectra of L (10 μM) upon addition of 1.0 equiv. of metal ions, including Na⁺, K⁺, Mg²⁺, Ca²⁺, Cd²⁺, Cr³⁺, Mn²⁺, Hg²⁺, Ag⁺, Pb²⁺, Fe³⁺, Ni²⁺, Co²⁺ and Cu²⁺, in Tris–HCl buffer (1% DMSO, 10 mM, pH = 7.4) (λ_ex = 348 nm). Inset: fluorescence color change of L (10 μM) upon addition of 1.0 equiv. of Hg²⁺ under a portable UV lamp at 254 nm.

The behavior in the presence of potentially competitive metal ions was investigated by fluorescence spectroscopy. Initially, L (10 μM) was mixed with 1.0 equiv. of various metal ions (Fig. 3, black bars) followed by adding the same amount of HgCl₂ (Fig. 3, red bars). Even in the presence of other metal ions, Hg²⁺ could still coordinate with L and cause strong fluorescence quenching. This result reveals that sensor L recognizes Hg²⁺ in aqueous solution without interference from the other metal ions.

Fig. 3 Fluorescence intensities of L (10 μM) in the presence of various metal ions (1 equiv., black) and additional Hg²⁺ (1 equiv., red) in 10 mM Tris–HCl (1% DMSO, pH = 7.4). Intensity was recorded at 475 nm, excitation at 348 nm.

To check the sensitivity of L toward Hg²⁺, fluorescence titration experiments were performed. Changes in fluorescence spectra of L (10 μM) upon addition of different amounts of Hg²⁺ ion are shown in Fig. 4. The fluorescence intensity of L solution gradually decreased with stepwise increases in the added Hg²⁺ concentration (0 to 1.0 equiv.). Further increasing of the Hg²⁺ concentration did not induce a significant fluorescence emission change, indicating a 1 : 1 complexation of L with Hg²⁺. A Job plot further indicated that L chelated with Hg²⁺ at 1 : 1 stoichiometry (Fig. S4, ESI†). Based on the titration profiles, the association constant (K_a) of L and Hg²⁺ was calculated to be 3.91 × 10⁶ M⁻¹ by nonlinear least-squares fitting (R² = 0.99401) using the 1 : 1 binding equation (Fig. 5).¹³ In addition, a time course study revealed that the Hg²⁺-induced fluorescence quenching can complete within 1 minute (Fig. S5, ESI†), indicating the rapid response of sensor L to Hg²⁺. Moreover, on addition of excess ethylenediamine tetraacetic acid disodium salt (EDTANa₂) to L–Hg²⁺ solution, the fluorescence signal of the resulting solution restored to that of free L (Fig. S6, ESI†), indicating that the Hg²⁺ recognition event is reversible. All these fluorescence study results reveal that sensor L behaves with good sensitivity toward Hg²⁺ and can rapidly and reversibly recognize Hg²⁺ in aqueous solution.

Fig. 4 Fluorescence emission spectra of L (10 μM) upon addition of different amounts of Hg²⁺ (0 to 1.0 equiv.) in aqueous solution (1% DMSO, Tris–HCl 10 mM, pH = 7.4, λ_ex = 348 nm).

Fig. 5 Non-linear fitting of fluorescence intensity against Hg²⁺ concentration (λ_em = 475 nm).

The binding mode of L with Hg²⁺ and the fluorescence quenching mechanism

To obtain clear insight into the binding mode of L with Hg²⁺, an X-ray diffraction study of the L–Hg²⁺ complex was performed. The single crystal of L–Hg²⁺ was obtained by slow evaporation of the solution of L in methanol in the presence of 1.0 equiv. of HgCl₂. The complex crystallizes in the triclinic system, with space group P1̄ and representing 1 : 1 stoichiometry between L and Hg²⁺ (Fig. 6). There is one ligand L, one Hg²⁺, and two Cl⁻ anions in the crystallographically independent asymmetrical cell. The Hg²⁺ is five-coordinated by two N atoms from the aromatic skeleton, one O atom from the flexible chain, and two Cl⁻ anions (Hg–O1 2.788 Å, Hg–N1 2.315 Å, Hg–N3 2.480 Å, Hg–Cl1 2.384 Å, Hg–Cl2 2.385 Å), which exhibits irregular pyramid geometrical configuration.

Fig. 6 Crystal structure of L–Hg²⁺ with displacement atomic ellipsoids drawn at the 30% probability level. All hydrogen atoms are omitted for clarity.

To clarify the mechanism of the fluorescence quenching by the energy or charge transfer model, we carried out the orbital calculations using the method of PBE0 in time-dependent density functional theory (TD-PBE0). The fluorescence quenching by Hg²⁺ could be rationalized in terms of orbital transition for the lowest energy excitation. According to the TD-PBE0 results (Fig. 7), the HOMO of L–Hg²⁺ is predominantly localized on the benzimidazole and quinoline moieties, whereas, its LUMO is mainly localized on the two N atoms and O atoms coordinated with Hg²⁺. The lowest energy excitation is contributed by HOMO to LUMO+2 energy excitation of configuration coefficient 91%. These excitations correspond to charge transfer from the excited L moiety to the Hg²⁺ center (LMCT) and, thus, provide a main pathway for nonradiative deactivation of the excited state.

Fig. 7 Frontier molecular orbitals of L–Hg²⁺ relevant to the fluorescence quenching, by the TD-PBE0 method.

Determination of the detection limit and effect of pH

To check its practical utility, the fluorescence detection limit of L for Hg²⁺ and the proper pH condition were evaluated. Based on the results of the fluorescence titration experiment, the detection limit was calculated to be 2.54 × 10⁻⁷ M (Fig. 8), which is comparable to those of some previously reported Hg²⁺-selective sensors with similar structure to L.^8d,14

Fig. 8 Normalized response of the fluorescence at various Hg²⁺ concentrations.

Moreover, the acid–base titration experiments showed that the fluorescence intensity of L almost remains constant across the pH range from 5 to 11 (Fig. 9). However, in the presence of the Hg²⁺, there was an obvious fluorescence “ON–OFF” change between pH 5 and 13. Thus, sensor L can detect Hg²⁺ ion over a wide pH range from 5 to 11. This result suggests that buffer solutions are not necessary for the detection of Hg²⁺ ion and sensor L has potential applicability in environmental monitoring work.

Fig. 9 The fluorescence changes of L and L–Hg²⁺ solution at various pH values (λ_ex = 348 nm).

Conclusions

We have developed a new water-soluble fluorescent sensor L, based on 2-(2-(8-hydroxyquinolin)-yl)benzimidazole, which exhibits highly selective, sensitive and quick recognition of Hg²⁺ with a fluorescence “ON–OFF” response in aqueous solution (1% DMSO, Tris–HCl 10 mM, pH = 7.4). The X-ray crystal structure of the L–Hg²⁺ complex reveals that the oxygen and nitrogen atoms from the 8-hydroxyquinoline moiety, together with the nitrogen atom from the benzimidazole unit (N1) bind Hg²⁺ through a 1 : 1 binding stoichiometry. The density functional theory (DFT) calculations demonstrate that the fluorescence quenching phenomenon is caused by LMCT. The low limit of detection and a wide usable pH range from 5 to 11 make it potentially applicable for determining Hg²⁺ ions in aqueous solution.

Experimental

Instruments and materials

All reagents and other chemicals were obtained from Aladdin Industrial Corporation (Shanghai, China) and used without further purification. NMR spectra were recorded on an Agilent 400-MR NMR spectrometer (Agilent, USA). Chemical shifts (δ) were expressed in parts per million (ppm) and coupling constants (J) in hertz. HRMS was measured on an Agilent 1200 Series Binary SL HPLC/Bruker micrOTOF mass spectrometer (Bruker, Germany). Fluorescence measurements were performed on a Sanco 970-CRT spectrofluorometer (Shanghai, China) and the excitation and emission slit widths were 2 nm. The pH measurements were made with a Model PHS-25B meter (Shanghai, China).

The salts used in stock solutions of metal ions were Ni(NO₃)₂·6H₂O, HgCl₂, Ba(NO₃)₂, Mg(NO₃)₂·6H₂O, AgNO₃, FeSO₄·7H₂O, KNO_3, Al(NO₃)₃·9H₂O, Mn(NO₃)₂, Pb(NO₃)₂, NaNO₃, Sr(NO₃)₂, Cu(NO₃)₂·3H₂O, Co(NO₃)₂·6H₂O, Cd(NO₃)₂, Zn(NO₃)₂·6H₂O, CrCl₃·6H₂O and Fe(NO₃)₃·9H₂O.

General methods

All titration experiments were carried out at room temperature. Fluorescence spectra were measured with a 10 μM solution of sensor L in buffered water solution (1% DMSO, Tris–HCl 10 mM, pH = 7.4) by excitation at 348 nm. The solutions of metal salts were prepared in distilled water. These solutions were used for all spectroscopic studies after appropriate dilution. Double-distilled water was used throughout the experiments. The fluorescence spectra were measured 3 minutes after metal ion addition.

Synthesis

Compound 1. 8-Hydroxyquinaldine (3.825 g, 24 mmol) was dissolved in 50 mL dry acetonitrile and potassium carbonate (4.92 g, 35.6 mmol) was added. After 30 minutes, 2-(2-methoxyethoxy)ethyl-4-methylbenzenesulfonate (5.5 g, 20 mmol) was added and the mixture was refluxed for 10 h under N₂. The obtained solution was cooled to room temperature and then the solvent was removed. The residues were washed by water, extracted with methylene chloride, and the organic layer was dried (Na₂SO₄) and filtered. The solvent was evaporated using rotary evaporators and the crude product was further purified by silica gel chromatography, using ethyl acetate–petroleum ether (1 : 1, v/v) as the eluent to obtain a light brown solid 1 (3.85 g, 74%). ¹H NMR (400 MHz, CDCl₃) δ 8.00 (d, J = 8.4 Hz, 1H), 7.42–7.32 (m, 2H), 7.29 (d, J = 8.4 Hz, 1H), 7.10 (d, J = 6.7 Hz, 1H), 4.44 (t, J = 5.6 Hz, 2H), 4.08 (t, J = 5.6 Hz, 2H), 3.81 (t, 2H), 3.60 (t, 2H), 3.40 (s, 3H), 2.77 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 158.07, 153.87, 139.54, 136.14, 127.63, 125.62, 122.53, 119.73, 109.13, 71.83, 70.73, 69.42, 67.86, 59.07, 25.38.

Compound 2. To a solution of SeO₂ (1.65 g, 15 mmol) in 1,4-dioxane (20 mL), compound 1 (2.61 g, 10 mmol) in 1,4-dioxane (20 mL) was added dropwise under N₂, and then the mixture was stirred for 10 h at 75 °C, cooled, filtered and further purified by silica gel chromatography, using ethyl acetate–petroleum ether (1 : 1, v/v) as the eluent to obtain 2 as yellow liquid (2.2 g, 80%). ¹H NMR (400 MHz, CDCl₃) δ 10.28 (s, 1H), 8.28 (d, J = 5.2 Hz, 1H), 8.06 (d, J = 5.2 Hz, 1H), 7.61 (t, J = 5.2 Hz, 1H), 7.48 (d, 1H), 7.21 (d, J = 5.2 Hz, 1H), 4.49 (t, 2H), 4.13 (t, 2H), 3.84 (t, 2H), 3.61 (t, 2H), 3.40 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 193.93, 155.48, 151.35, 140.09, 137.30, 131.54, 129.94, 119.81, 117.76, 110.13, 71.92, 71.00, 69.40, 68.73, 59.06.

Compound L. o-Phenylenediamine (220 mg, 2 mmol), NaHSO₃ (624 mg, 6 mmol) and compound 2 (550 mg, 2 mmol) were added into dry ethanol (50 mL). The mixture was refluxed for 4 h and was then cooled and filtered. The solvent was removed and the crude product was further purified by silica gel chromatography with ethyl acetate–methylene chloride (1 : 10, v/v) as the eluent to give 175 mg (24%) of L as a fulvous solid. ¹H NMR (400 MHz, CDCl₃) δ 12.87 (s, 1H), 8.62 (d, J = 8.6 Hz, 1H), 8.27 (d, J = 8.6 Hz, 1H), 7.83 (d, J = 7.9 Hz, 1H), 7.44 (d, J = 4.4 Hz, 2H), 7.27–7.05 (m, 4H), 4.31 (t, J = 4.5 Hz, 2H), 3.83 (t, J = 4.5 Hz, 2H), 3.58 (m, 2H), 3.47 (m, 2H), 3.34 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 154.56, 151.22, 147.77, 144.48, 139.54, 136.99, 134.73, 129.66, 127.04, 123.52, 121.91, 119.94, 119.60, 111.84, 109.78, 71.80, 70.49, 69.21, 68.34, 58.77. HRMS (ESI†) m/z, calculated for C₂₁H₂₁N₃O₃ [M + H]⁺: 364.1583; found: 364.1656.

X-ray crystal structure determination

The X-ray diffraction data were collected under a nitrogen stream at 293 K on a Bruker D8 SMART APEX CCD single-crystal diffractometer with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. Data collection, indexing, and initial cell refinements were processed using the SMART software while frame integration and final cell refinements were carried out using the SAINT software. The final cell parameters were determined from the least-squares refinement of total reflections. The structures were determined through direct methods (SHELXL97) and difference Fourier maps (SHELXL97). The final results of crystal data and structure refinement are summarized in Table S1 (ESI†).

DFT calculations

DFT calculations¹⁵ with hybrid-type Perdew–Burke–Ernzerhof exchange correlation functional (PBE0).¹⁶ In calculations, a “double-ξ” quality basis set LANL2DZ¹⁷ associated with the pseudopotential was employed on the mercury atom and the 6-31G*¹⁸ basis set was used on C, H, N, Cl and O atoms. All calculations were performed with the Gaussian 09 package.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (no. 21304009, 21176029), the National Key Technologies R&D Program of China during the 12th Five-Year Plan Period (no. 2012BAD29B06), Food Safety Key Laboratory of Liaoning Province (no. LNSAKF2011025), the Program for Liaoning Excellent Talents in University (LJQ2012096) for financial support.

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Footnote

† Electronic supplementary information (ESI) available: NMR and HRMS spectra of sensor L, crystal data and supplementary spectra. CCDC 971509 and 971510. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra00060a

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