Highly selective fluorescent probe for detecting mercury ions in water

Abstract

Mercury ion (Hg²⁺) is a well-known toxic heavy metal. It has become one of the most significant environmental pollutants in the world because of its serious physiological toxicity, persistence, easy migration, and high bioconcentration. Thus, the development of methods for monitoring Hg²⁺ is indispensable. Herein, we have designed and synthesized a new fluorescent probe, TPH, for the detection of Hg²⁺ in the water environment. The TPH probe could quantitatively detect Hg²⁺ between 0 and 5 μM (LOD = 16 nM), with a linear range of 0–2.5 μM. In addition, the TPH probe was used to monitor Hg²⁺ in water samples successfully. Thus, this probe is suitable for monitoring Hg²⁺ in the actual water environment.

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1 Introduction

Mercury has attracted attention from researchers owing to its strong toxicity and bioaccumulation.^1–4 With the development of industry, especially gold mining, the burning of fossil and oil refining, mercury ion pollution is widely distributed in the environment.^5–7 Through biological enrichment, mercury ions in water can be transformed into organic mercury ions with stronger toxicity and then enter the human body via the food chain.^8–10 Even very small amounts of mercury ions have a severe impact on the human body, including the digestive system and kidneys, cognitive disorders, and even central nervous system damage.^11–16 Consequently, the development of methods for the detection of mercury ions with simple synthesis and high selectivity and sensitivity is significant.

Recently, many methods have been reported for the detection of mercury ions (Hg²⁺), including gas chromatography, inductive coupled plasma mass spectroscopy, and atomic absorption spectrometry.^17–21 However, most of the above-mentioned methods have some disadvantages, such as complex pretreatment, use of expensive instruments, and difficulty in realizing real-time and on-site monitoring.^22,23 In contrast, the use of fluorescent probes has attracted significant attention due to their simple operation and high selectivity and sensitivity.^24–30 Thus, an increasing number of fluorescent probes has been used to detect environmental heavy metal pollutants including Hg²⁺. However, the reported probes for monitoring Hg²⁺ still have some shortcomings, such as poor selectivity and water solubility and high detection limits (LODs).^31–35 Therefore, new fluorescent probes need to be developed for monitoring Hg²⁺ in the environment with excellent selectivity and sensitivity and good water solubility.

Accordingly, herein, we synthesized the TPH probe, which was based on the TPC-OH dye as the fluorophore³⁶ and phenyl thiochloroformate as the recognition receptor of Hg²⁺.^37,38 Phenyl thiochloroformate possesses high selectivity for the detection of Hg²⁺, and thus the TPH probe could also achieve the specific and sensitive detection of Hg²⁺. The TPH probe exhibited the following excellent properties: (1) good water solubility, (2) excellent sensitivity (LOD = 16 nM), (3) high selectivity, and (4) excellent application in the environment. Thus, this probe will have a wide application prospect for monitoring Hg²⁺ in the environment.

2 Experimental

2.1 Materials and instruments

All chemical reagents were obtained from commercial sources and used without further purification. Absorption and fluorescence spectra were recorded on a UV-3101PC spectrophotometer and Horiba FluoroMax-4 spectrophotometer, respectively.

2.2 Synthesis of TPH probe

TPC-OH dye (236 mg, 1 mmol) was dissolved in dry CH₂Cl₂ (15 mL), and then phenyl thiochloroformate (259 mg, 1.5 mmol) and N,N-diisopropylethylamine (DIPEA) (194 mg, 1.5 mmol) were added (Scheme 1). The mixed solution was stirred at 25 °C for 2 h.

Scheme 1 Synthesis of TPH probe.

The crude product was purified by column silica chromatography over silica gel using dichloromethane/petroleum ether (14 : 5) as the eluent to provide a faint-yellow pure solid product. ¹H NMR (400 MHz, DMSO) δ (ppm): 7.94 (d, J = 5.6 Hz, 1H), 7.81 (d, J = 8.0 Hz, 2H), 7.68 (s, 1H), 7.58 (s, 1H), 7.55–5.52 (m, 4H), 7.48 (t, J = 8.0 Hz, 2H), 7.39 (t, J = 8.0 Hz, 3H), 4.21 (s, 2H). ¹³C NMR (100 MHz, DMSO) δ (ppm): 193.95, 192.55, 158.01, 153.62, 152.68, 136.23, 135.34, 135.21, 133.66, 131.32, 130.46, 130.42, 129.52, 127.57, 125.82, 122.74, 122.20, 120.54, 32.44.

3 Results and discussion

3.1 Spectral response of TPH probe

All the reactions were carried out in aqueous solution (HEPES 5 mM, pH = 7.4). The fluorescence spectra of the TPH probe for monitoring Hg²⁺ was investigated. When Hg²⁺ (20 μM) was added, the fluorescence intensity displayed a significant enhancement at 505 nm (Fig. 1a). The quantum yield of the TPH probe was calculated to be 0.07. Then, its absorption spectrum was also studied. According to the results, the absorption peak changed from 325 nm to 350 nm (Fig. 1b), implying that Hg²⁺ could promote the splitting of the carbonothioate moiety (Scheme 2). Furthermore, we conducted HRMS and NMR to explore the reaction mechanism of the TPH probe and Hg²⁺ (Fig. S1–S4 in the ESI†).

Fig. 1 (a) Fluorescence and (b) absorption spectrum changes of TPH probe (5 μM and 20 μM, respectively) before and after the addition of Hg²⁺ (20 μM). λ_ex = 400 nm and λ_em = 505 nm. Conditions: HEPES (5 mM, pH 7.4).

Scheme 2 Recognition mechanism of TPH probe for Hg²⁺.

3.2 Quantification of Hg²⁺

The TPH probe showed good water solubility, and thus the influence of the concentration of Hg²⁺ on its fluorescence intensity in pure water was investigated. With an increase in Hg²⁺ concentration (0–5 μM), the fluorescence intensity of the TPH probe at 505 nm increased accordingly (Fig. 2a). In addition, when the concentration of Hg²⁺ was 0–2.5 μM, it was linearly correlated with the fluorescence intensity (y = 201 921 [Hg²⁺] (μM) + 85 603, R² = 0.9828) (Fig. 2b), and the LOD was 16 nM (3σ/k). Thus, all the above-mentioned results indicate that the TPH probe can provide a sensitive detection tool for Hg²⁺ in the actual water environment.

Fig. 2 (a) Fluorescence spectra of TPH probe (5 μM) for Hg²⁺ (0–5 μM). (b) Linear plot of fluorescence intensity (505 nm) to Hg²⁺ (0–2.5 μM). λ_ex = 400 nm and λ_em = 505 nm. Conditions: in HEPES (5 mM, pH 7.4).

3.3 Specificity for Hg²⁺

The specificity of the TPH probe toward Hg²⁺ and other various relevant analytes including Cd²⁺, Cu²⁺, Fe²⁺, Ca²⁺, K⁺, Mg²⁺, Na⁺, Ni²⁺, Pb²⁺, Sn²⁺, Zn²⁺, Co²⁺, Fe³⁺, NO³⁻, SO₄²⁻ and Cl⁻ was analyzed. The concentration of Hg²⁺ and the other analytes was 5 μM and 50 μM. Only Hg²⁺ caused a fluorescence response at 505 nm, while the other relevant analytes did not cause obvious fluorescence changes (Fig. 3a). Besides, interference experiments were also conducted. The fluorescence intensity response values exhibited a slight change at 505 nm (Fig. 3b). Thus, all these results strongly suggest that the TPH probe can specifically recognize Hg²⁺.

Fig. 3 (a) Fluorescence response of TPH probe (5 μM) toward Hg²⁺ (5 μM) and other ions (50 μM). (b) Fluorescence response of TPH probe (5 μM) toward Hg²⁺ (5 μM) in the presence of other ions (50 μM). λ_ex = 400 nm and λ_em = 505 nm. Conditions: in HEPES (5 mM, pH 7.4).

3.4 Analytical applications in real water samples

Then, the analytical application of the TPH probe in three water samples (lake water, underground water and river water) for the detection of Hg²⁺ was investigated. Firstly, no Hg²⁺ was found in the samples. Then, after 5 μM TPH probe was added to the test water samples, 1 and 2 μM Hg²⁺ were also respectively added. Each sample was tested three times. As can be seen in Table 1, the recoveries of the three water samples were 84.07–109.20%, further confirming that this newly synthesized probe could effectively detect Hg²⁺ in the real water environment.

Table 1 Application of TPH probe in three water samples^a

Real water samples	Found Hg^2+	Addition Hg^2+ (μM)	Found (μM)	Recovery (%)	RSD (n = 3) (%)
a ND: not detected. Sample A from JiaZi Lake, University of Jinan and samples B and C from Jinyun River and Jinyang River in Jinan, China.
Sample A	ND	1	0.97 ± 0.03	96.93	3.08
		2	2.01 ± 0.13	100.38	6.40
Sample B	ND	1	1.09 ± 0.06	109.20	6.42
		2	1.68 ± 0.13	84.07	6.64
Sample C	ND	1	0.96 ± 0.08	95.97	8.41
		2	1.72 ± 0.11	85.81	5.40

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4 Conclusions

The fluorescent TPH probe with phenyl thiochloroformate as the Hg²⁺ recognition site was synthesized in this study. This probe could specifically recognize Hg²⁺ and quantitatively detect Hg²⁺ in aqueous solution. According to the experimental results, we calculated that its detection limit is 16 nM. Meanwhile, the TPH probe has excellent water solubility, which is conducive for its application in the actual environment.

Data availability

The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of interest

The authors have no conflicts of interest to declare.

Acknowledgements

We gratefully acknowledge financial support from the NSFC (42007176, 52070122), and Natural Science Foundation of Shandong Province (ZR2020QD125, ZR2020ME222), Key Scientific and Technological Innovation Foundation of Shandong Province (No. 2020CXGC011404).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra02791k

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