A facile naphthalene-based fluorescent chemodosimeter for mercury ions in aqueous solution

Abstract

A facile naphthalene-based fluorescence “turn-on” chemodosimeter, 2-((2-(vinyloxy)naphthalen-1-yl)methylene)-malononitrile (MS1), for rapid, selective and sensitive detection of Hg²⁺ by mercury-promoted hydrolysis of the vinylether group has been reported. The probe displayed a fast response time, and a sensitive fluorescence response (100-fold fluorescence enhancement) to the detection of Hg²⁺ in aqueous solution.

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Mercury, which is widely distributed in the environment such as in air, soil, and water due to its use in batteries, dental amalgam, electrical apparatus, and industrial chemicals, is one of the most ubiquitous and poisonous heavy metals.¹ Mercury ions are not biodegradable, and hence can concentrate through the food chain in the tissues of fish and marine mammals. Excess mercury accumulation may induce strong damage to the central nervous system, various cognitive and motor disorders, and Minamata disease.² Due to the toxicity of Hg²⁺, the determination of mercury in biological and environmental samples is crucial both to the monitoring of environmental pollution and to the diagnosis of clinical disorders.

In the past several years, considerable efforts have been made to develop fluorescent chemosensors for Hg²⁺ based on the coordination of Hg²⁺ to heteroatom-based ligands, Hg²⁺ catalyzed desulfurization, and Hg²⁺ promoted hydrolysis of the vinylether group and β-alkynyl ether group.³ However, most of them still have limitations such as interference from other coexisting metal ions, poor water-solubility, and laborious synthesis processes expensive chemicals.⁴ Therefore, for practical applications, it is still desirable to develop simple Hg²⁺ sensors with good water solubility and high selectivity and sensitivity.

Compared with the typically-developed chemosensors,⁵ fluorescent chemodosimeters, based-on highly specific chemical reactions between the dosimeters and the analytes, have received much research attention due to their relatively higher selectivity.⁶ Recently, Peng, Talukdar, Wu, and Ahn's groups have reported fluorescent chemodosimeters based on “deprotection–cyclization strategy” for the detection of fluoride ions,⁶ while the development of chemodosimeters for the specific determination of Hg²⁺ is drawing increasing research efforts. However, among the few available Hg²⁺ chemodosimeters reported,³ most employ the pH-sensitive fluorescein or 7-amino coumarin as the fluorophore and their pH-dependence may pose detection errors to the results. It is therefore strongly desirable to develop simple yet specific fluorescent chemodosimeters for Hg²⁺ that is immune to pH turbulence.

It is known that Hg²⁺ catalyzes hydrolysis of vinylether to form the corresponding hydroxyl group.⁷ We proposed that the Hg²⁺ ion promoted hydrolysis of the vinyl enol ether group in MS1 would generate the hydroxy intermediate, which will readily spontaneous cyclize to form a highly fluorescent chemodosimeter (Scheme 1).

Scheme 1 Hydrolysis of MS1 by mercury ions.

Our research group is actively engaged in the development of novel selective and sensitive fluorescent probes for heavy metal ions.⁸ Herein, we report the synthesis and properties of a deprotection–cyclization reaction based fluorescent chemodosimeter (MS1) that shows high selectivity and sensitivity for Hg²⁺.

As shown in Scheme 2, MS1 can be readily prepared in three convenient steps under facile conditions with high yield starting with commercially available 2-hydroxy-1-naphthaldehyde. The product (MS1) was well characterized by ¹H, ¹³C NMR, and HR-MS (ESI†).

Scheme 2 Synthesis of MS1: (a) 1,2-dibromoethane/K₂CO₃, acetone, reflux, 3 h, 62%; (b) t-BuOK/DMSO, rt, 12 h, 46%; (c) CH₂(CN)₂/piperidine, ethanol, rt, 1 h, 48%.

We firstly assessed the UV-vis spectroscopic properties of MS1 in PBS buffer solution (10 mM, pH = 7.4, containing 1% CH₃CN). MS1 (20 μM) displayed a moderate UV-vis absorption around 538 nm. Upon addition of Hg²⁺ (0–2 equiv.), the absorption band at 538 nm decreased and a new band at 399 nm appeared instantly with an isosbestic point at 439 nm, which is owing to the loss of vinyl enol ether group and the formation of cyclic compound (Fig. 1).

Fig. 1 Absorption spectra of MS1 (20 μM) in PBS buffer solution (10 mM, pH 7.4, containing 1% CH₃CN) in the presence of different concentrations of Hg²⁺ (0–2.0 equiv.).

As expected, MS1 alone is almost non-fluorescent (λ_ex = 395 nm, Φ = 0.002, Table S1, ESI†) in neutral aqueous solution (10 mM PBS buffer, pH 7.4, containing 1% CH₃CN), while the addition of increasing concentrations of Hg²⁺ gradually enhanced the fluorescent signal and ca. 100-fold increasing was observed when 5.0 equiv. of Hg²⁺ was added (Fig. 2a, Table S1, ESI†), which was attributed to the cleavage of vinyl enol group by mercury ion promoted hydrolysis reaction and the formation of a highly fluorescent cyclic compound (Scheme 1). Moreover, a blue-green fluorescent compound 5 have been isolated from MS1–Hg²⁺ system (ESI†), which was agreed well with the proposed mercury induced deprotection–cyclization mechanism.

Fig. 2 (a) Fluorescence spectra of MS1 (10 μM) in PBS buffer solution (pH 7.4, containing 1% CH₃CN) in the presence of different concentrations of Hg²⁺ (0–50 μM) (λ_ex = 395 nm). Inset: fluorescence intensity changes as a function of Hg²⁺ concentration. (b) Emission spectra of MS1 (10 μM) in PBS buffer solution (pH 7.4, containing 1% CH₃CN) in the presence of various metal ions (λ_ex = 395 nm, 5.0 eq. of Hg²⁺, and 10.0 eq. of Co²⁺, Cr³⁺, Cu²⁺, Fe²⁺, Fe³⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺, Sn⁴⁺, Ag⁺, Ca²⁺, and Zn²⁺, respectively).

Subsequently, the time-dependence of MS1 fluorescence was also evaluated in the presence of different concentration of Hg²⁺. The result shows that the fluorescence of all tested solutions remarkably increased to their maximum value within the 10 minutes. No changes in fluorescence were detected in the absence of Hg²⁺ (Fig. 3).

Fig. 3 Time-dependent fluorescence intensity changes of MS1 (10 μM) upon addition of various concentration of Hg²⁺ (0, 0.2, 1.0, 5.0 equiv. each) in PBS buffer solution (pH 7.4, containing 1% CH₃CN) (λ_ex = 395 nm).

Further, the fluorescence titration of MS1 with various metal ions was conducted to examine the selectivity (Fig. 2b). Much to our delight, the turn-on response of MS1 is highly specific for Hg²⁺ and no obvious change of fluorescent emission was observed when it is treated with Co²⁺, Cr³⁺, Cu²⁺, Fe²⁺, Fe³⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺, Sn⁴⁺, Ag⁺, Ca²⁺, and Zn²⁺. It should be mentioned that MS1 still responds to Hg²⁺ sensitively even in the presence of other relevant competing ions (Fig. 4). Therefore, these results suggest that MS1 displays high selectivity toward Hg²⁺ in neutral aqueous solution.

Fig. 4 Fluorescence responses of MS1 to various metal ions. Black bars represent the addition of 5.0 equiv. of Hg²⁺ and 10.0 equiv. of the other appropriate metal ion to a 10 μM solution of MS1. Red bars represent the addition of 5.0 equiv. of Hg²⁺ to the solutions containing MS1 (10 μM) and the appropriated metals (10.0 equiv.).

Moreover, the Hg²⁺-sensing ability of MS1 at a wide range of pH values was investigated. As depicted in Fig. 5, MS1 alone is inert to pH in the range of 4.0–11.0. But in the presence of Hg²⁺, MS1 have no fluorescence response in the highly basic environment (pH ≥ 9) due to the reaction rate of mercury ion-promoted hydrolysis of vinyl enol ether becomes slow at high pH value.⁶ However, satisfactory Hg²⁺-sensing abilities were exhibited in the range of pH from 4.0 to 8.0, indicating that MS1 could be used in neutral natural systems, or a mildly acidic or basic environment.

Fig. 5 Effect of the pH on the fluorescence emission of MS1 (10 μM) alone and MS1 (10 μM) reacted with Hg²⁺ (3.0 equiv.).

For practical purposes, the detection limit of MS1 for the analysis of Hg²⁺ was also an important parameter. The fluorescence titration curve revealed that the fluorescence intensity of MS1 at 470 nm increased linearly with the amount of Hg²⁺ in the range of 0–5.0 μM (R² = 0.994) (Fig. S1, ESI†). Thus, the detection limit of MS1 for Hg²⁺ was calculated to be 4.31 × 10⁻⁸ M (Hg content = 8.8 ppb), which reveals the high sensitivity for the analysis of the mercury ions.

In conclusion, we have successfully developed a simple naphthalene-based fluorescence probe for Hg²⁺ based on mercury triggered cleavage reaction under mild conditions. The probe has the unique advantage of easy-preparation, good water solubility, and excellent selectivity and sensitivity response towards Hg²⁺ in aqueous solution. We anticipate that the experimental results of this study will inspire the future design of metal-ion sensors in water for a variety of chemical and biological applications.

Acknowledgements

This work was supported by the Zhejiang Provincial Natural Science Foundation of China (LY14B020016 and LQ13B020006) and the Program for Innovative Research Team of Zhejiang Sci-Tech University (13060052-Y).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, synthetic details of MS1, additional spectroscopic data, and copies of NMR spectra. See DOI: 10.1039/c4ra16479b

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