A sensitive colorimetric and ratiometric fluorescent chemodosimeter for Hg²⁺ and its application for bioimaging

Abstract

A colorimetric and ratiometric fluorescent chemodosimeter for Hg²⁺ has been developed based on mercury ion-promoted hydrolysis of a naphthalimide-derived aryl vinyl ether. The chemodosimeter displays a highly sensitive and selective response with significant changes in both color (from colorless to jade-yellow) and fluorescence (from blue to green) in PBS buffer solution (containing 1% CH₃CN, pH = 7.4). The chemodosimeter exhibits a ratiometric fluorescent response towards Hg²⁺ with a very low detection limit (3.6 nM), which can be used to detect Hg²⁺ ions in drinking water. Furthermore, the chemodosimeter has been successfully applied to the imaging of Hg²⁺ ions in living cells with an emission change from blue to green.

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Introduction

The widespread contamination of highly poisonous mercury species could jeopardize our ecosystem, posing a great threat to human health, because mercury can lead to dysfunction of the brain, kidney, stomach, and central nervous system due to its thiophilic nature for proteins and enzymes.^1–4 As the most prevalent contaminant in biological and environmental samples, Hg²⁺ is widely distributed in air, water, and soil, which continues to be a major environmental and health concern. So, it is urgent to develop versatile tactics to monitor Hg²⁺ ions.

In the past few years, many analytical methods, such as atomic absorption spectroscopy, inductively coupled plasma-mass spectrometry, high performance liquid chromatography, electrochemical sensing, etc., have been applied for the detection of Hg²⁺ concentrations.^5–7 Though these techniques are accurate for Hg²⁺ ions, most of them are rather complicated, costly, and time consuming as well as inappropriate for on-line use or field monitoring, so the wide utilization of these methods is largely limited. Fluorometry, which has the favorable features of operational simplicity, cost-effectiveness, high sensitivity and selectivity, has attracted considerable attention.^8–11 Thus, a number of fluorescent chemosensors for the selective detection of Hg²⁺ ions have been exploited.^12–22 Most of these early fluorescent sensors are based on the coordination of heteroatom-based ligands to Hg²⁺, which usually show a fluorescence quenching response due to the spin–orbit coupling effect of the Hg²⁺ ion and incomplete selectivity over the competing metal ions.^23–25 Since Czarnik and Chae reported the pioneering chemical reaction-based chemodosimeter for Hg²⁺,²⁶ many chemodosimeters based on the mercury-triggered reaction have been reported gradually and have developed rapidly,^27–40 which has provided superior selectivity towards Hg²⁺ with a large spectroscopic change and avoided Hg²⁺-induced fluorescence quenching.^41,42 To the best of our knowledge, most of these chemodosimeters are based on the OFF–ON mechanism only with changes in the fluorescence intensity,^43–45 which can be easy interfered with by the excitation power and detector sensitivity, strongly limiting the quantitative detection of Hg²⁺ ions.^46,47 In contrast, ratiometric fluorescent chemodosimeter, which uses the ratio of two fluorescent bands rather than the absolute intensity of one band, can minimize the background signal and detect the analyte more accurately.^48–51 However, only a handful of ratiometric fluorescent chemodosimeters for Hg²⁺ have been reported.^{28–30,34,36,38,40,48} Moreover, many of these chemodosimeters require a high proportion of organic solvent as the medium for analysis^28,38,40,48 and only a few could be used to image Hg²⁺ ions in living cells.^29,30,34,36 Thus, there remains urgency to develop more accurate and sensitive ratiometric chemodosimeters to detect Hg²⁺ ions in aqueous solution and living cells.

Recently we reported a 4-aminonaphthalimide-based probe for mercury species, which displayed a colorimetric and ratiometric response in a buffer solution via a mercury-promoted cleavage reaction, and detected CH₃HgCl in living cells.⁵² Herein, 4-hydroxynaphthalimide with more desirable photophysical properties was adopted to obtain a much simpler chemodosimeter, (N-butyl-4-vinyl ether-1,8-naphthalimide) (1), which would work though the intramolecular charge transfer (ICT) process, owing to the stronger electron-donating ability of oxygen anions when the vinyl group is removed.⁵³ As expected, in the presence of Hg²⁺, 1 could be hydrolysed readily and 2 released with a free hydroxy group in aqueous solution. This chemical transformation from vinyl ether to hydroxy turned on the ICT process from the oxygen anions to the fluorophore, resulting in ratiometric changes in both color and fluorescence. More importantly, biological application of 1 has been evaluated for the intracellular detection of Hg²⁺ with an emission change from blue to green.

Results and discussion

Synthesis of compound 1 is summarized in Scheme 1. N-Butyl-4-hydroxy-1,8-naphthalimide (compound 2) was synthesized according to the literature method.⁵⁴ Then, chemodosimeter 1, N-butyl-4-vinyl ether-1,8-naphthalimide was prepared smoothly from 4-hydroxy-1,8-naphthalimide (2) through nucleophilic substitution and elimination steps in a satisfactory yield. The purity of 1 was fully confirmed by ¹H, ¹³C NMR and ESI-MS analysis.

Scheme 1 Synthesis of chemodosimeter 1.

We firstly assessed the UV-vis spectroscopic properties of 1 in PBS buffer solution (pH = 7.4, containing 1% CH₃CN). 1 (5 μM) displayed a moderate UV-vis absorption around 368 nm. After being treated with Hg²⁺ (5 μM), the absorption band at 368 nm decreased and a new band at 447 nm appeared instantly with an isosbestic point at 400 nm. A marked color change from colorless to light yellow was observed, owing to the disappearance of vinyl ether and the formation of oxygen anions (Fig. 1). Other metal ions including Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺, Cr³⁺, Mn²⁺, Fe³⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ag⁺, Au³⁺, Pd²⁺ and Pt²⁺ (5 μM) incubated in 1 (5 μM) for 30 min have no color change, so Hg²⁺ can be easily detected from other metal ions by the naked eye.

Fig. 1 (a) UV-vis absorption spectra change acquired for a mixture of 5 μM 1 and 5 μM HgCl₂ in PBS buffer solution (pH = 7.4, containing 1% CH₃CN) within 30 min. Inset: color change in 1 (5 μM) upon addition of Hg²⁺ (5 μM); (b) color change of 1 upon addition of various metal ions in PBS buffer solutions (pH = 7.4, containing 1% CH₃CN). [1] = 5 μM, [Mⁿ⁺] = 5 μM.

Then, the fluorescent spectroscopic properties of 1 were studied in PBS buffer solution. 1 (5 μM) exhibits a single emission band at 456 nm and strong fluorescence with a quantum yield (Φ) of ca. 0.285 in PBS buffer solution (pH = 7.4, containing 1% CH₃CN). After being treated with Hg²⁺ (5 μM), a red-shift from 456 nm to 546 nm was also detected in the maximum emission spectrum, with an isosbestic point at 506 nm (Fig. 2). Other mercury compounds, such as Hg(ClO₄)₂ and Hg(OAc)₂, gave similar results (Fig. S2, ESI†).⁵⁵ This marked red shift is due to the ICT process between oxygen anions and the imide moieties in the fluorophore that was switched on via the Hg²⁺-promoted hydrolysis of aryl vinyl ether.

Fig. 2 Fluorescence spectral change of 1 (5 μM) upon treatment with HgCl₂ (5 μM) in PBS buffer solutions (pH = 7.4, containing 1% CH₃CN). λ_Ex = 408 nm. Slit: 10.0 nm/10.0 nm.

To gain insight into the sensing mechanism of 1 towards Hg²⁺, the reaction of 1 with HgCl₂ was conducted under the same conditions as described above. Then, the green fluorescent reaction product was obtained and characterized to be 2 by electrospray ionization mass spectrum (ESI-MS), ¹H NMR and ¹³C NMR (Fig. S13–S15, ESI†), which exhibits weak fluorescence with a quantum yield (Φ) of ca. 0.113 in PBS buffer solution (pH = 7.4, containing 1% CH₃CN). Moreover, titration of 1 with HgCl₂ (from 0 to 1.4 equiv.) showed saturated behavior at 1.0 equiv. of HgCl₂ (Fig. 3), and the results obtained from Job's plots also show the 1 : 1 stoichiometry for the reaction between 1 and Hg²⁺ (Fig. S3, ESI†),^56–58 so the mechanism can be explained by the procedure in Scheme 2.

Fig. 3 The fluorescence intensity ratios change of 1 as a function of equiv. of HgCl₂ in PBS buffer solutions (pH = 7.4, containing 1% CH₃CN). Ex = 408 nm. Slit: 10.0 nm/10.0 nm. Each spectrum was acquired 15 min after HgCl₂ addition.

Scheme 2 A plausible hydrolysis mechanism of 1 by HgCl₂ in water.

When 1 equiv. of HgCl₂ was added to 1 in PBS buffer, the fluorescence saturation took about 30 min (Fig. 4b). However, a linear relationship between the concentration of HgCl₂ and the fluorescence intensity ratios at 550 nm and 506 nm (I_550nm/I_506nm) can be easily obtained at 15 min after the addition of HgCl₂ (Fig. S4, ESI†). Therefore, it is not necessary to wait for the full recovery of fluorescence for quantification purposes in drinking water. When the amount of Hg²⁺ exceeds 5 μM in the sample, it can be quantified by diluting the sample, bringing the concentration within 5 μM.

Fig. 4 (a) The time-dependent fluorescence change acquired for a mixture of 5 μM 1 and 5 μM HgCl₂ in PBS buffer solution (pH = 7.4, containing 1% CH₃CN); λ_Ex = 408 nm. Slit: 10.0 nm/10.0 nm. (b) Plot of the fluorescence intensity ratio changes as a function of the reaction time.

Drinking water pretreated with different amounts of HgCl₂ (0–2 μM), and then 1 (20 μL in CH₃CN, [1]_final = 5 μM) was added into the mixture. After 15 min, as shown in Fig. 5, a good linear relationship (R = 0.989) between the low concentration range of HgCl₂ (0–2 μM) and the fluorescence intensity ratios (I_550nm/I_506nm) wasalso found. The detection limit is 3.6 nM, lower than many reported results^34,36,45,52 and satisfies the U.S. Environmental Protection Agency (EPA) limit (10 nM) of Hg²⁺ detection in drinking water.⁵⁹

Fig. 5 The linear relationship between Hg²⁺ concentration within the range 0–2 μM and the fluorescence intensity ratios at 550 nm and 506 nm (I_550nm/I_506nm) in drinking water (containing 1% CH₃CN). λ_Ex = 408 nm. Slit: 10.0 nm/10.0 nm.

The fluorescence spectrum of 1 was measured in the presence of other metal ions including Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺, Cr³⁺, Mn²⁺, Fe³⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ag⁺, Au³⁺, Pd²⁺ and Pt²⁺ under identical conditions. After incubating 1 (5 μM) with metal ions (5 μM) individually for 30 min, all of these ions showed relatively little or no response to the emission of 1 (Fig. 6a), owing to little hydrolysis of the vinyl ether with these metals. To further explore the selectivity of Hg²⁺ in the presence of other metal ions, competition experiments were carried out. Fortunately, chemodosimeter 1 showed a full fluorescence response to mercury ions in the presence of 5 equiv. of all other metal ions, and even the good chelating agent, EDTA (Fig. 6b). The competition experiment showed that other metal species and EDTA have a negligible influence on the sensing of Hg²⁺. Moreover, the fluorescence spectrum of 1 was also measured in the presence of some anions, such as F⁻, Cl⁻, NO₃⁻, ClO₄⁻, AcO⁻, CO₃²⁻ and SO₄²⁻. Both 1 and the 1–Hg²⁺ system showed little response to these anions tested (Fig. S5 and S6, ESI†). These results confirm that 1 shows a good sensitivity and selectivity towards Hg²⁺ over other competitive ions.

Fig. 6 (a) Fluorescence spectra of 1 in the absence and presence of different metal ions Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺, Cr³⁺, Mn²⁺, Fe³⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ag⁺, Au³⁺, Pd²⁺, Pt²⁺ and Hg²⁺ (as their Cl⁻ or ClO₄⁻ salts) in PBS buffer solutions (pH = 7.4, containing 1% CH₃CN); (b) fluorescence responses of 1 (5 μM) upon the addition of various metal ions in PBS buffer solutions. Bars represent the fluorescence intensity ratio in the presence (R) and absence (R₀) of various metal ions. Black bars represent the addition of 25 μM different metal ions (or EDTA) to the solution of 1 (5 μM). Red bars represent the subsequent addition of 5 μM Hg²⁺ to the solution. λ_Ex = 408 nm. Slit: 10.0 nm/10.0 nm; R = I_550nm/I_506nm. Each spectrum was acquired 30 min after metal ion addition.

The effect of pH on the fluorescence of 1 and the 1–Hg²⁺ system have been studied. As seen from Fig. 7, 1 is pH insensitive, the emission intensity of 1 remaining unaffected from pH 1.60 to pH 12.12 after incubation for 30 min. Therefore, it reacted with Hg²⁺ within the biologically relevant pH range (1.60–12.12). Based on these findings, it is possible that 1 could be applied to image Hg²⁺ ions in living cells without interference from pH effects.

Fig. 7 The fluorescence intensity of 5 μM 1, 1–Hg²⁺ or 2 as a function of pH in aqueous solutions (containing 1% CH₃CN). Each spectrum was acquired 30 min after 1, 1–Hg²⁺ or 2 addition.

To further demonstrate the ability of 1 to image Hg²⁺ ions in living systems, we carried out experiments in live, human hepatocarcinoma SMMC-7721 cells. The cells incubated with 10 μM 1 in RPMI-1640 for 0.5 h at 37 °C exhibited blue fluorescence (Fig. 8b). However, when the cells were treated with 10 μM 1 and 20 μM HgCl₂ they displayed green fluorescence (Fig. 8d). Obvious changes indicated that 1 can penetrate the cell membrane and enable ratiometric imaging of Hg²⁺ ions in the living cells. Brightfield imaging (Fig. 8a and 8c) confirmed cell viability during the imaging experiment.

Fig. 8 Images of SMMC-7721 cells (Olympus BX53 fluorescence microscope, 40× objective lens): (a) brightfield image of SMMC-7721 cells incubated with 10.0 μM 1 for 0.5 h; (b) fluorescence image of (a); (c) brightfield image of SMMC-7721 cells incubated with 10.0 μM 1 for 0.5 h, and then further incubation with 20 μM HgCl₂ for 1.0 h; (d) fluorescence image of (c).

Conclusion

In conclusion, we have developed a new and simple reactive fluorescent chemodosimeter for Hg²⁺ based on the mercury ion-promoted hydrolysis of aryl vinyl ether under mild conditions. The chemodosimeter showed specific response to Hg²⁺ and ratiometric fluorescence in PBS buffer solution (containing 1% CH₃CN, pH = 7.4), with a marked color change and a low detection limit (3.6 nM). Besides, the chemodosimeter can enter live SMMC-7721 cells and indicate intracellular Hg²⁺ ions. Therefore, the chemodosimeter has potential applications for the study of the toxicity of Hg²⁺ and the detection of Hg²⁺ in living cells.

Experimental section

Materials and general methods

All reagents and solvents were obtained commercially and used without further purification unless otherwise noted. ¹H NMR and ¹³C NMR spectra were recorded using a Bruker DRX400 spectrometer and referenced to the solvent signals. Mass spectra (ESI) were performed using an LQC system (Finngan MAT, USA). The melting points were measured using an X-6 melting point apparatus without calibration (Beijing Fuka Keyi Science and Technology Co., LTD). All UV-visible spectra and fluorescence spectra were recorded using a Varian Cary 100 spectrophotometer and a Hitachi F-4500 luminescence spectrometer, respectively. Fluorescence quantum yields were determined by an absolute method using an integrating sphere on FLS920 of Edinburgh Instruments. All pH measurements were made with a pH-10C digital pH meter. All spectra were recorded at 25 °C. Fluorescence microscopy experiments were performed using an Olympus BX53 fluorescence microscope.

Solutions of HgCl₂, Hg(OAc)₂, Hg(ClO₄)₂, LiClO₄, NaCl, KCl, CaCl₂, MgCl₂, BaCl₂, CrCl₃, MnCl₂, FeCl₃, FeCl₂, CoCl₂, NiCl₂, CuCl₂, ZnCl₂, CdCl₂, and AgClO₄ were prepared in acetonitrile with a concentration of 10 mM, respectively. All the anion solutions were prepared from NaF, NaCl, NaNO₃, NaClO₄, NaAcO, Na₂SO₄, and Na₂CO₃ in distilled water, with a concentration of 10 mM, respectively. PdCl₂ was prepared in 1 : 3 brine–MeOH solution with a concentration of 10 mM. PtCl₂ and AuCl₃ were prepared in DMSO with a concentration of 10 mM.

Preparation of chemodosimeter

N-Butyl-4-(2-bromoethoxy)vinyl ether-1,8-naphthalimide (3). To a solution of 2 (403.5 mg, 1.5 mmol) in 1 mL dimethylformamide (DMF) and 10 mL acetone was added 1,2-dibromoethane (1.3 mL, 15.0 mmol) and K₂CO₃ (310.5 mg, 3.0 mmol). Then the reaction mixture was refluxed for 4 h. The reaction mixture was cooled to room temperature and the solvent was evaporated in vacuo, then poured into brine (10 mL), and extracted with CH₂Cl₂ (50 mL). The combined organic layers were dried over Na₂SO₄, filtered, concentrated and chromatographed to provide compound 3 as a yellow powder. Yield: 524.5 mg, (93%). M.p. 148.5–150.0 °C. ¹H NMR (CDCl₃, 400 MHz, ppm) δ 8.57 (d, J = 1.9 Hz, 1H), 8.55 (d, J = 3.1 Hz, 1H), 8.49 (d, J = 8.2 Hz, 1H), 7.74–7.66 (m, 1H), 6.98 (d, J = 8.3 Hz, 1H), 4.58 (t, J = 5.9 Hz, 2H), 4.37–4.04 (m, 2H), 3.84 (t, J = 5.9 Hz, 2H), 1.71 (dt, J = 21.1, 7.6 Hz, 2H), 1.56–1.34 (m, 2H), 0.98 (t, J = 7.4 Hz, 3H). ¹³C NMR (CDCl₃, 100 MHz, ppm): δ 164.3, 163.6, 158.8, 132.9, 131.5, 129.2, 128.4, 126.0, 123.2, 122.3, 115.7, 105.8, 68.3, 40.0, 30.1, 28.4, 20.3, 13.8.

N-Butyl-4-vinyl ether-1,8-naphthalimide (1). A mixture of compound 3 (225.6 mg, 0.6 mmol) and potassium tert-butoxide (BuOK) (112.0 mg, 10.0 mmol) in dimethyl sulfoxide (DMSO) (10 mL) was stirred at room temperature for 30 min until the reaction was completed. Then the reaction mixture was quenched with water and extracted three times with CH₂Cl₂. The combined organic layers were dried, filtered, concentrated and chromatographed to provide 1. Yield: 140.0 mg, (80%). M.p. 81.6–82.9 °C. ESI-MS: m/z = 296.2 [M + H]⁺. ¹H NMR (CDCl₃, 400 MHz, ppm) δ 8.56 (d, J = 7.3 Hz, 1H), δ 8.52 (dd, J = 12.2, 8.4 Hz, 2H), 7.73–7.67 (m, 1H), 7.13 (d, J = 8.2 Hz, 1H), 6.87 (dd, J = 13.6, 5.9 Hz, 1H), 5.16 (dd, J = 13.6, 1.9 Hz, 1H), 4.83 (dd, J = 5.9, 1.9 Hz, 1H), 4.19–4.12 (m, 2H), 1.78–1.64 (m, 2H), 1.44 (dp, J = 14.4, 7.2 Hz, 2H), 0.98 (t, J = 7.4 Hz, 3H). ¹³C NMR (CDCl₃, 100 MHz, ppm): δ 164.2, 163.6, 157.7, 146.0, 132.6, 131.7, 129.4, 128.3, 126.4, 123.3, 122.5, 117.0, 109.2, 99.7, 40.1, 30.2, 20.4, 13.8.

Cell incubation and imaging

The human cell line SMMC-7721 was maintained in RPMI-1640 medium supplemented with 10% heat-inactivated fetal calf serum, 100 units per mL penicillin, and 100 μg mL⁻¹ streptomycin at 37 °C in a humidified atmosphere containing 5% CO₂ for 24 h. Cells (5 × 10⁸ L⁻¹) were plated on 18 mm glass coverslips and allowed to adhere for 24 h. Then the cells were treated with the relative compound for fluorescence microscopic imaging on an Olympus BX53 fluorescence microscope.

After SMMC-7721 cells were grown on a 12′′ orifice plate at 37 °C and in 5% CO₂ atmosphere for 24 h, then treated with 1 (10 μM) and incubated for 0.5 h. Subsequently, the cells were treated with 20 μM HgCl₂. Cells were incubated for 1.0 h and rinsed with PBS three times to remove free compound and ions before analysis. SMMC-7721 cells only incubated with 10 μM 1 for 1.5 h acted as a control. All fluorescence microscopic images were collected using an Olympus BX5 fluorescence microscope.

Acknowledgements

This study was supported by the NSFC (Grant 20931003, 91122007) and the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20110211130002).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, characterization of the compounds, and additional spectroscopic data, NMR and MS spectra. See DOI: 10.1039/c3nj00766a

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