A novel water soluble chemosensor based on carboxyl functionalized NDI derivatives for selective detection and facile removal of mercury(II)

Abstract

Mercury(II) (Hg²⁺) has acute toxicity. It is still a challenge to design and synthesize chemosensors for selective detection and removal of Hg²⁺ in water solutions. By rationally combining the carboxyl group and naphthalene diimide moieties, we obtained a novel water-soluble Hg²⁺ chemosensor (M2). Interestingly, the sensor M2 showed a dramatic fluorescent “turn-on” response for Hg²⁺ in water. Moreover, the sensor M2 displayed a high specificity for Hg²⁺, other cations (including Fe³⁺, Ag⁺, Ca²⁺, Cu²⁺, Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Zn²⁺, Cr³⁺, and Mg²⁺) had no influence on the Hg²⁺ detection process. Moreover, the sensor M2 showed high sensitivity for Hg²⁺, with detection limits of 1.18 × 10⁻⁶ M. Even more meaningfully, the sensor M2 can remove Hg²⁺ from water solutions effectively via the formation of a M2–Hg²⁺ coordination polymer, which can increase the possibility of M2 being used for practical applications.

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Introduction

As we all know, ions play a significant role in life and technological processes.^1–4 Moreover, heavy metal ions pose a big threat to human health and the environment due to their high toxicity and bioaccumulation.^5,6 Among the heavy metals, mercury is one of the most toxic.^7–11 However, large quantities of mercury salts are widely used in industrial chemicals, electricals, apparatus, dental amalgams, and batteries.^12–15 Hence, the rational design and synthesis of efficient sensors to selectively detect and remove Hg²⁺ ions has attracted much attention.^16–18 Moreover, due to most of the biological or environmental procedures being carried out in water systems, it is very important to develop a Hg²⁺ chemosensor, which can detect and remove Hg²⁺ in water.

Thus far, some methods based on organic fluorophores¹⁹ or chromophores,²⁰ semiconductor nanocrystalline materials,²¹ cyclic voltammetry,²² polymeric materials,²³ and proteins²⁴ have been established for the detection of Hg²⁺. However, most of these materials are difficult to be applied in the detection of mercury ions in water solutions due to their poor water solubility. Moreover, these chemosensors cannot remove Hg²⁺ in water simultaneously. Therefore, the search for effective sensing systems in a water environment is still a great challenge. At the same time, various absorbents, such as activated carbon,²⁵ resins,²⁶ silica,²⁷ zeolites²⁸ and metal sulfides²⁹ have been applied to remove Hg²⁺ from wastewater; however, these absorbents cannot detect Hg²⁺ in water. The reports on simultaneous detection and removal of Hg²⁺ in water are still very scarce.^30–33 Therefore, the development of water soluble Hg²⁺ chemosensors, which can synchronously detect and remove Hg²⁺ in water is an important task.

In view of this and based on our research in ion recognition,^34,35 we report a water-soluble Hg²⁺ sensor M2 based on naphthalene diimide derivatives. In order to achieve fluorescent sensing and efficient removal of Hg²⁺ in water, we rationally introduced the carboxyl group into the sensor molecule as a hydrophilic group and Hg²⁺ binding site. Moreover, we introduced a naphthalene diimide moiety as the fluorescent signal group and π–π stacking site. As a result, the sensor M2 showed good solubility in water and could fluorescently “turn on” when it detected Hg²⁺ with high selectivity and sensitively in water solution. More interestingly, the sensor M2 could efficiently remove Hg²⁺ in water simultaneously.

The sensor M2 was synthesized by a simple dehydration condensation procedure (Scheme 1) and was characterized by ¹H NMR spectroscopy and ESI-MS (Fig. S1 and S2†). The recognition properties of the chemosensor M2 towards various metal cations, such as Fe³⁺, Hg²⁺, Ca²⁺, Cu²⁺, Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Zn²⁺, Ag⁺, Cr³⁺, and Mg²⁺, were primarily investigated by fluorescence emission spectra in water solution (buffered with HEPES, pH = 7.2). The compound M2 alone displayed no fluorescence when it was excited at 365 nm (Fig. 1). However, when 10 equiv. of Hg²⁺ was added to the water solution of M2 (2.0 × 10⁻⁴ M), the solution emitted bright blue fluorescence emission. Simultaneously, in the fluorescence spectrum, an emission peak at 425 nm could be observed (Fig. S3†). To validate the selectivity of sensor M2 for Hg²⁺, the same tests were carried out for other cations (including Fe³⁺, Ca²⁺, Cu²⁺, Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Zn²⁺, Cr³⁺, Ag⁺, and Mg²⁺); however, these cations could not induce any significant fluorescent changes in the fluorescence spectrum (Fig. 1). These results indicate that the sensor M2 can selectively fluorescently “turn-on” when sensing Hg²⁺ in water solution.

Scheme 1 Synthesis of compound M2.

Fig. 1 (a) Photograph of M2 upon adding 10 equiv. of various cations in, which was taken under an UV-lamp (365 nm) at room temperature. (b) Fluorescence spectra responses in water solution (buffered with HEPES, pH = 7.2) of M2 (2.0 × 10⁻⁴ M) upon addition of different cations (10 equiv.).

To further investigate the interaction between sensor M2 and Hg²⁺, the fluorescence emission spectral variation of sensor M2 (2.0 × 10⁻⁴ M) in water (buffered with HEPES, pH = 7.2) was recorded during titrations with different concentrations of Hg²⁺. As shown in Fig. 2, with the addition of increasing concentrations of Hg²⁺ from 0 to 3.26 equiv., the emission peak at 425 nm gradually enhanced. Furthermore, in order to determine the detection limits of M2 for Hg²⁺, the fluorescence spectra of blank tests were measured 20 times and the standard deviation of the blank measurements was determined (Fig. S4†). The fluorescence quantum yield (Φ) of sensor M2 in water is 0.03 with quinine hemisulfate salt as a reference while the Φ increased to 0.32 when sensor M2 reacts with Hg²⁺ (see ESI†). The linear fitting was performed according to the titration curves, and the mean intensity was calculated to determine the slope. The limit of detection (LOD) was determined from the equation LOD = K × σ/S,³⁶ where K = 3, σ is the standard deviation of the emission intensity of M2 in the presence of Hg²⁺, and S is the slope of the calibration curve of the fluorescence emission. The detection limit of the fluorescence spectrum was 1.18 × 10⁻⁶ M. This data indicate that the sensor can detect Hg²⁺ at very low concentrations in the environment (Fig. S5†). Moreover, the binding constant (K) derived from the fluorescence titration data was found to be 2.65 × 10⁹ M (see ESI†) using a Benesi–Hildebrand plot,³⁷ which indicates a high detection sensitivity.

Fig. 2 Fluorescence titration spectra of Hg²⁺ in water solution (buffered with HEPES, pH = 7.2) upon addition of increasing concentration of Hg²⁺.

Then, the competition experiments were also measured by the addition of 10 equiv. of Hg²⁺ to the water solution of M2 in the presence of 10 equiv. of other metal ions, such as Fe³⁺, Ca²⁺, Cu²⁺, Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Zn²⁺, Cr³⁺, Ag⁺ and Mg²⁺. As shown in Fig. 3, the effect on emission intensity of M2 upon the addition of higher concentrations of various cations was almost negligible. These results revealed that M2 had a remarkable selectivity towards Hg²⁺ over competitive ions, and moreover, the detection of Hg²⁺ by M2 was hardly affected by these common coexisting cations in water (buffered with HEPES, pH = 7.2). Moreover, in order to investigate the influence of coexisting anions on the Hg²⁺ sensing process, the competition experiments were also carried out by adding various anions into the M2–Hg²⁺ water solution (Fig. 4). As a result, coexisting anion ions, such as F⁻, Cl⁻, Br⁻, I⁻, Ac⁻, H₂PO₄⁻, HSO₄⁻, HClO₄⁻, SCN⁻ and CN⁻, could not interfere in the Hg²⁺ sensing process of M2.

Fig. 3 Fluorescent emission (λ_ex = 365 nm) spectra of M2 (2 × 10⁻⁴ M) to Hg²⁺ (10 equiv.) in the presence of various competing cations in water (buffered with HEPES, pH = 7.2). Form 1 to 12: M2, M2 + Hg²⁺, M2 + Hg²⁺ + Fe³⁺, M2 + Hg²⁺ + Ca²⁺, M2 + Hg²⁺ + Cu²⁺, M2 + Hg²⁺ + Co²⁺, M2 + Hg²⁺ + Ni²⁺, M2 + Hg²⁺ + Cd²⁺, M2 + Hg²⁺ + Pb²⁺, M2 + Hg²⁺ + Zn²⁺, M2 + Hg²⁺ + Ag⁺, M2 + Hg²⁺ + Cr³⁺, M2 + Hg²⁺ + Mg²⁺.

Fig. 4 Fluorescent emission (λ_ex = 365 nm) spectra of M2 (2 × 10⁻⁴ M) to Hg²⁺ (10 equiv.) in the presence of various anion ions (10 equiv.) in water (buffered with HEPES, pH = 7.2). Form 1 to 12: M2, M2 + Hg²⁺, M2 + Hg²⁺ + F⁻, M2 + Hg²⁺ + Cl⁻, M2 + Hg²⁺ + Br⁻, M2 + Hg²⁺ + I⁻, M2 + Hg²⁺ + Ac⁻, M2 + Hg²⁺ + H₂PO₄⁻, M2 + Hg²⁺ + HSO₄⁻, M2 + Hg²⁺ + ClO₄⁻, M2 + Hg²⁺ + SCN⁻, M2 + Hg²⁺ + CN⁻.

As we all know, the pH has a strong influence on the coordination properties of ligands and metal ions. Therefore, the effects of pH on the Hg²⁺ sensing process were also investigated (Fig. 5). Over the tested pH range, sensor M2 alone had no fluorescence and was stable in the pH range 4.0–13.0. However, the M2–Hg²⁺ complex showed a significant fluorescence response between pH 4.0 and 8.0. These results indicate that Hg²⁺ could be clearly detected by the fluorescence spectral measurement using M2 over a pH range from 4.0 to 8.0. In acidic conditions, carboxylate groups in M2 changed to carboxylic acids, whereas, in alkaline conditions, the Hg²⁺ could form Hg(OH)₂. In either case, the coordination abilities of M2 and Hg²⁺ were restrained. Moreover, in strongly acidic (pH < 1) or strongly alkaline conditions (pH > 13), the sensor M2 showed unusual fluorescence, which indicated the M2 is unstable in these conditions.

Fig. 5 Effect of pH on the fluorescence spectra of M2 (20 μM) in response to Hg²⁺ (10 equiv.) from pH 2.0 to 13.0 in water solution.

The recognition mechanism of the sensor M2 with Hg²⁺ was primarily investigated by IR spectroscopy. In the IR spectrum of M2 (Fig. 6), the carboxyl groups show the stretching vibrations absorption peak at 1715 cm⁻¹. However, after the addition of Hg²⁺, this peak shifts to 1663 cm⁻¹, which indicates that M2 bonds to Hg²⁺ via carboxyl groups. In addition, the mass spectroscopy results also supported this presumption. The ion peak at m/z 582.60 demonstrated the presence of [M2 + Hg²⁺] (Fig. S7†).

Fig. 6 IR spectra of compound M2 and M2–Hg²⁺ in KBr disks.

The recognition mechanism of the sensor M2 with Hg²⁺ was also investigated by ¹H NMR titration. As shown in Fig. 7, sensor M2 shows two single peaks at 8.55 and 4.58 ppm in D₂O solution, which correspond to the protons of the naphthalene rings (H_a) and –CH₂ (H_b), respectively. With the addition of Hg²⁺, the signal of –CH₂ (H_b) showed a slight upfield shift, indicating that M2 combined with Hg²⁺ via the carboxyl group. Moreover, the signal of the naphthalene rings (H_a) also showed a slight upfield shift, indicating that the π–π stacking interactions between the naphthalene rings were involved in the detection process.³⁸ Moreover, the XRD patterns (Fig. S6†) of the M2–Hg²⁺ complex showed a peak at 2θ = 25.67° corresponding to d spacings of 3.53 Å, which also suggested that π–π stacking existed in the naphthyl groups.³⁹ Interestingly, after adding 2.0 equiv. Hg²⁺, all the ¹H NMR signal disappeared. According to these ¹H NMR, XRD and IR experiments, we presumed that M2 formed a coordination polymer with Hg²⁺ and, simultaneously, each coordination polymer chain stacked together through the π–π interactions (Scheme 2).

Fig. 7 Partial ¹H NMR spectra (600 MHz, D-H₂O) of free M2 (0.1 M) and in the different concentration of Hg²⁺ respectively.

Scheme 2 Possible mechanism for the sensing and removal of Hg²⁺ by the sensor M2 in water.

Then, we further measured the capacity of the removal of Hg²⁺ by sensor M2 via inductively coupled plasma (ICP) analysis in aqueous solution (see ESI†). The ICP analysis identified an efficient removal of Hg²⁺. In order to assess the ingestion capacity of the sensor for Hg²⁺ in water for potential practical applications, M2 (0.19 mg) was suspended in a dilute aqueous solution of Hg(ClO₄)₂ (0.20 mg in 5.0 mL), and precipitation was found to occur. We separated the precipitate by centrifugation (20 min) and gained the supernatant liquor (5 mL). The ICP analysis verified that the concentration of the residual Hg²⁺ in water was less than 0.2 ppm, indicating that over 98% of Hg²⁺ could be effectively removed. This data indicated that Hg²⁺ could be effectively removed even in extremely dilute solutions.

In summary, a novel water-soluble Hg²⁺ sensor M2 was designed and synthesized via an easy to make method. The sensor M2 employs carboxyl groups as hydrophilic groups and Hg²⁺ binding sites, while NDI moieties act as signal groups and π–π stacking sites. Interestingly, the sensor M2 could fluorescently “turn-on” when it detected Hg²⁺ in water with high selectivity and sensitivity. Moreover and more meaningfully, the sensor M2 can remove Hg²⁺ from a water solution effectively via the formation of a M2–Hg²⁺ coordination polymer. Therefore, the sensor M2 can be used as an easy to make and efficient sensor for fluorescence detection as well as for the removal of Hg²⁺ in water.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (NSFC) (No. 21662031; 21661028; 21574104; 21262032), the Natural Science Foundation of Gansu Province (1506RJZA273) and the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (IRT 15R56).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, synthesis of M2, NMR spectra, and other materials. See DOI: 10.1039/c6ra28419a

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