Dinuclear Hg^II tetracarbene complex-triggered aggregation-induced emission for rapid and selective sensing of Hg²⁺ and organomercury species

Abstract

Rapid, reliable and highly selective detection of mercury species, including Hg²⁺ ions and organomercury, is of significant importance for environmental protection and human health. Herein, a new fluorescent dye 1,1,2,2-tetrakis[4-(3-methyl-1H-benzimidazol-1-yl)phenyl ethylene tetraiodide (Tmbipe) with aggregation-induced emission (AIE) potential was prepared and characterized. The presence of four positively charged methylated benzimidazole groups endows Tmbipe with excellent water solubility and almost undetectable background fluorescence. However, it can coordinate with two Hg²⁺ ions or two organomercury molecules (e.g. methylmercury and phenylmercury) to form a planar dinuclear Hg^II tetracarbene complex, which can then self-aggregate to turn on AIE fluorescence. Such a fluorescence turn-on process can be completed in 3 min. In addition, synergic rigidification of the tetraphenylethylene-bridged Tmbipe molecule by mercury-mediated chelate ring formation and subsequent aggregation results in obviously higher fluorescence enhancement than that given by the single aggregation-induced one. Low background, high fluorescence enhancement and rapid response time make Tmbipe a good fluorescent probe for reliable, sensitive and highly selective quantitation of both inorganic and organic mercury species. This probe was also demonstrated to work well for identification of mercury species accumulation in living cells.

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Introduction

Mercury is generally acknowledged as one of the most toxic and dangerous metal ions because of its well-characterized toxicological features.¹ Even at low concentrations, mercury is highly toxic. Mercury can spread through air, water and oil. And what's worse, it can accumulate through the food chain and atmospheric circulation.² Exposure to mercury can cause serious environmental and health threats. It has been found that mercury damages the kidneys, especially renal proximal tubular epithelial cells. Mercury can also cause immune dysfunction, nephrotic syndrome and glomerulonephritis. In addition, it can harm the brain, nerves and endocrine system.³

Great efforts have been made in the sensitive and selective detection of mercury. High performance liquid chromatography (HPLC),⁴ gas chromatography (GC),⁵ atomic absorption spectroscopy (AAS),⁶ capillary electrophoresis,⁷ inductively coupled plasma mass spectrometry (ICP-MS),⁸ neutron activation analysis,⁹ anodic stripping voltammetry,¹⁰ and X-ray fluorescence spectrophotometry¹¹ are the commonly used techniques for mercury detection. The main limitations of these methods lie in valuable, complex instruments, long data-acquisition time, and the requirement of professionally trained personnel. In the past decades, a great number of fluorescent probes and elegantly designed DNA-based biosensors have been reported for mercury detection.¹² Though highly sensitive and selective, most of these methods only give response to Hg²⁺ ions and cannot work for other mercury species.^12a,b

In fact, when mercury is released into the environment during various natural events or due to human activities, inorganic mercury can be converted into organic forms (e.g. methylmercury, MeHg⁺) by anaerobic microorganisms.¹³ In addition, organomercury is much more poisonous than its inorganic form.¹⁴ The reason is that the fat-soluble property enables organomercury compounds to easily pass through biofilms and act as deadly neurotoxins for many eukaryotes including fish, animals, and humans,¹⁵ leading to prenatal brain injury, cognitive and motor impairment, visual acuity, hearing loss, etc.¹⁶ It has been reported that more than 80% mercury in human blood exists in the methyl form.¹⁷ Therefore, the development of a cost- and labour-effective sensing platform showing highly sensitive and selective response to both Hg²⁺ and organomercury is still urgently required.

As a newly discovered photophysical phenomenon, aggregation-induced emission (AIE) has attracted more and more attention in recent years, and has great promising potential for sensing and bioimaging applications.¹⁸ In this work, we prepared a new water soluble AIE dye (1,1,2,2-tetrakis[4-(3-methyl-1H-benzimidazol-1-yl)phenyl]ethylene tetraiodide, Tmbipe, Scheme 1) that could rapidly coordinate with Hg(II) species to form a dinuclear Hg^II tetracarbene complex, and thus reported a novel sensing strategy for the sensitive and selective detection of both Hg²⁺ ions and organic mercury (including methylmercury and phenylmercury (PhHg⁺)). Different from traditional AIE-active mechanisms, the fluorescence response of Tmbipe to Hg(II) species relies on two successive processes: mercury-mediated chelate ring formation and subsequent aggregation. Synergic rigidification of Tmbipe molecules by these two processes could give significant fluorescence enhancement and thus improved Hg²⁺ detection sensitivity. This probe was also demonstrated to work well for imaging detection of mercury species in living cells.

Scheme 1 Structure of Tmbipe and its use in the detection of inorganic and organic mercury.

Results and discussion

Tmbipe preparation and its interaction with Hg(II) species

To achieve the sensitive detection of both Hg²⁺ ions and organomercury, a water-soluble AIE dye Tmbipe was designed and prepared following the synthetic route shown in Scheme S1.‡ Briefly, the precursor 1,1,2,2-tetrakis[4-(1H-benzimidazol-1-yl)phenyl]ethylene (Tbipe) was synthesized according to the reported method (Fig. S1‡).¹⁹ That is, the symmetric McMurry coupling reaction was used to prepare Tbipe in high yield. Then simple Hofmann alkylation between Tbipe and methyl iodide was carried out in dichloromethane to obtain the target probe Tmbipe. The details of the synthesis and characterization of Tmbipe are available in the ESI (Fig. S2 and S3‡). The results of both ¹H-nuclear magnetic resonance (¹H-NMR) and high resolution mass spectrometry (HRMS) characterization confirm the successful preparation of high purity Tbipe and Tmbipe with a chemical structure as shown in Scheme 1. Due to the presence of four positively charged methylated benzimidazole groups, Tmbipe shows excellent water solubility.

The tetraphenylethylene skeleton endows Tmbipe with typical AIE-based fluorescence features.²⁰ Thus, the fluorescence property of Tmbipe was investigated. Both the C–C bonds between phenyl rings and C=C bonds and the C–N bonds between phenyl rings and benzimidazoles show high rotation tendencies (Scheme 1), which not only reduces the molecular planarity but also increases the possibility of radiationless relaxation, thus conferring Tmbipe with negligible fluorescence emission in aqueous solution. However, we were pleased to find that addition of Hg²⁺ and organomercury (e.g. MeHg⁺ and PhHg⁺) could result in significant enhancement of the fluorescence emission (Fig. 1), thus suggesting that Tmbipe might be used as a fluorescent probe for convenient detection of both Hg²⁺ and organomercury.

Fig. 1 Fluorescence spectra of Tmbipe in the absence and presence of Hg²⁺, MeHg⁺ or PhHg⁺. [Tmbipe] = 10 μM; [Hg²⁺] = [MeHg⁺] = [PhHg⁺] = 20 μM. The inset shows the aqueous solution photograph of Tmbipe in the absence and presence of Hg²⁺ under UV light irradiation.

To understand why Hg²⁺ and organomercury can turn on the fluorescence of Tmbipe, the interactions between Tmbipe and Hg²⁺ (and organomercury) were investigated. First of all, Job's plot analysis was used to determine the binding stoichiometry between them. The results showed that Tmbipe bound to Hg²⁺, MeHg⁺ and PhHg⁺ with the same stoichiometry of 1 : 2 (Fig. S4‡), indicating that one Tmbipe molecule can bind with two molecules of Hg(II) species. The above results suggested that Tmbipe might bind to Hg²⁺ ions and organomercury in a similar manner.

Then, UV-vis and infrared (IR) absorption spectroscopies were used to further elucidate the binding interactions between Tmbipe and mercury species. Since the Hg²⁺ ion itself gives much simpler UV-vis and IR absorption signals than organomercury species, the IR and UV-vis spectra of the Tmbipe/Hg²⁺ mixture were recorded and compared with that of Tmbipe (Fig. 2). The UV-vis spectrum of Tmbipe showed an absorption peak at 265 nm. With the addition of increasing concentrations of Hg²⁺, the peak intensity continuously increased, which might be attributed to the coordination of mercury ions with Tmbipe.²¹ Interestingly, when the concentration ratio of Hg²⁺ to Tmbipe exceeded 1.0, an isosbestic point was observed at 340 nm, indicating that two Tmbipe/Hg²⁺ complexes existed in the mixture: one might be the complex with a stoichiometric ratio of 1 : 1, and the other might be the one with a stoichiometric ratio of 1 : 2. That is to say, in the low Hg²⁺ concentration range, one Tmbipe molecule preferentially binds with one Hg²⁺ ion to form a 1 : 1 complex. With the further addition of Hg²⁺ ions, the 1 : 1 complex will bind with a second Hg²⁺ ion to form the 1 : 2 complex. The IR spectrum of the Tmbipe/Hg²⁺ mixture clearly shows a new band centered at 1445 cm⁻¹ compared to that of free Tmbipe, suggesting the existence of C–Hg bonds in the complex formed by Hg²⁺ and Tmbipe.^18b Therefore, it can be speculated that one mercury atom simultaneously coordinates with two benzimidazole groups to form a dinuclear Hg^II tetracarbene complex Hg₂Tmbipe, in which two 15-membered macrocycles are contained (Scheme 1).

Fig. 2 UV-vis (a) and IR (b) spectra of Tmbipe in the absence or presence of Hg²⁺.

Formation of the closed macrocycles could “lock” the free rotation of both C–N and C–C bonds in Tmbipe, resulting in a great increase of molecular planarity. Such a rigidification of the tetraphenylethylene-bridged Tmbipe molecule suppresses the radiationless deactivation pathways. As a result, the fluorescence of Tmbipe is turned on. A similar complexation reaction has recently been used by Sinha et al. to prepare dinuclear Ag^I and Au^I tetracarbene complexes using two tetraphenylethylene-bridged tetraimidazolium salts as precursors.^21a Different from the fact that a relatively long reaction time (2 days) and high temperature (60 °C) are required for Ag^I and Au^I tetracarbene complex formation, our Hg₂Tmbipe can be readily formed at ambient temperature, which is reflected by the fact that a stable and strong fluorescence signal output can be obtained within 3 min after Hg²⁺ addition (Fig. S5‡). Such a rapid signal response makes Tmbipe an excellent candidate for fluorescence sensing of mercury species.

Since Tmbipe shares common structural characteristics of AIE dyes, we want to determine if the mercury species-induced fluorescence enhancement is related with aggregation of Tmbipe or not. To answer this question, several techniques, including transmission electron microscopy (TEM), dynamic light scattering (DLS) and resonance light scattering (RLS), were used to characterize the morphology and size changes of Tmbipe before and after Hg²⁺ addition. DLS results (Fig. 3a) showed that free Tmbipe had an average size about 1.3 nm, indicating that it was monodisperse in solution. Addition of 2.0 equiv. of Hg²⁺ resulted in a sharp increase of the hydrodynamic size to around 1000 nm, suggesting the formation of large aggregates. The results of TEM characterization also demonstrated the Hg²⁺-induced aggregation of Tmbipe. As shown in Fig. 3b and c, almost no observable nanoparticles are shown in the TEM image of free Tmbipe. After addition of 2.0 equiv. of Hg²⁺, however, big clumps are clearly observed, thus giving direct evidence for Tmbipe aggregation in the presence of Hg²⁺. RLS is also an effective technique for studying the aggregation behavior of dyes. As shown in Fig. 3d, the RLS intensities of Tmbipe continuously increased with Hg²⁺ concentration, further demonstrating the size increase caused by Hg²⁺-induced aggregation. Collectively, when Tmbipe is mixed with Hg²⁺ ions, it will rapidly coordinate with two Hg²⁺ ions to form a highly planar dinuclear Hg^II tetracarbene complex, Hg₂Tmbipe. The increase in molecular planarity can then promote the self-aggregation of Hg₂Tmbipevia hydrophobic and π–π interactions. The formation of aggregates might further restrict the skeletal vibration of Tmbipe molecules, thus giving strong AIE fluorescence.

Fig. 3 (a) DLS, (b and c) TEM and (d) RLS characterization of Tmbipe in the absence and presence of Hg²⁺. In (a) and (c), 2.0 equiv. of Hg²⁺ was added.

Selection of solvent for sensitive mercury species quantitation

Tmbipe is water soluble. This makes mercury species detection in pure aqueous media possible. However, we noticed that a very weak background fluorescence could also be observed without mercury species addition in pure aqueous solution (Fig. 4). To give a high signal-to-noise ratio in mercury species detection, it is best to decrease the background to a negligible level. Therefore, a suitable solvent system was screened. Fig. 4 shows the fluorescence change of Tmbipe in water/tetrahydrofuran (THF)-mixed solvents. In 100% aqueous solution, Tmbipe showed a very weak fluorescence, which might be attributed to the slight self-aggregation due to the hydrophobic and π–π interactions of the tetraphenylethene skeleton of Tmbipe. This self-aggregation was easily destroyed when a small amount of THF (e.g. 5%) was added, accompanied by the decrease of the fluorescence to a negligible level. The low background was maintained when the fraction of THF was increased to 80%. After that, the fluorescence significantly increased with the THF volume fraction, which can be rationally interpreted by the emergence of the AIE signal due to the aggregation of water-soluble Tmbipe in organic solvent. Therefore, a mixed solvent (5% THF–water) was used as the working solvent system for subsequent ultralow background detection of mercury species.

Fig. 4 Fluorescence intensity of Tmbipe at 468 nm in different solvents.

It should be noted that Hg²⁺-induced fluorescence signal enhancement (Fig. 1 and 5) was much higher than that induced by solvent. This can be reasonably interpreted by their different working mechanisms. That is, in our Hg²⁺-induced aggregation system, the radiationless deactivation of Tmbipe can be overcome in two ways: formation of a planar dinuclear Hg^II tetracarbene complex with two chelate rings and subsequent aggregation of the tetracarbene complex. Such a synergic rigidification of tetraphenylethylene-bridged Tmbipe molecules can give significantly enhanced fluorescence. Such a highly efficient fluorescence enhancement mechanism is beneficial for the sensitive detection of Hg²⁺ ions. In a solvent-induced aggregation system, however, only aggregation but no chelate ring formation-induced probe molecule rigidification is used to turn on fluorescence. The effects of pH on the fluorescence response of Tmbipe to Hg²⁺ and organomercury species were also investigated. The results (Fig. S6‡) showed that pH had little effect on the fluorescence signals of the sensing systems, indicating that Tmbipe can be used for the detection of Hg²⁺ and organomercury species in a broad pH range.

Fig. 5 Quantitation of Hg²⁺. (a) Fluorescence spectra of the sensing systems containing different concentrations of Hg²⁺. (b) Hg²⁺ concentration-dependent changes in fluorescence intensity at 468 nm. The inset shows the linear relationship between the fluorescence intensity and Hg²⁺ concentration in the range of 200 nM–20 μM. (c) Selectivity of the Hg²⁺-sensing system. The red bars represent the fluorescence signals of the sensing systems in the presence of 100 μM other metal ions. The green bars represent the fluorescence signals of the sensing systems containing 20 μM Hg²⁺ and 100 μM other metal ions.

Sensitivity and selectivity of Hg²⁺ quantitation

Having demonstrated the feasibility of Hg²⁺ detection using Tmbipe as the AIE probe, the detection sensitivity was evaluated by recording the fluorescence response of the proposed sensing platform towards different concentrations of Hg²⁺. As shown in Fig. 5, Hg²⁺ concentration-dependent fluorescence signal increase was given by the sensing system, and a linear relationship (I = 6.44 + 9.50C (μM), R² = 0.9962) between the fluorescence intensity and Hg²⁺ concentration was observed in the range of 200 nM–20 μM. Based on the rule of 3σ/S, the detection limit was calculated to be 63 nM.

The prepared Hg²⁺ probe exhibits good detection selectivity, which was demonstrated by testing the response of Tmbipe to other metal ions with 5 times the concentration of Hg²⁺ (Fig. 5c). None of these competitive metal ions, including Co²⁺, Ni²⁺, Mg²⁺, Zn²⁺, Fe³⁺, Fe²⁺, Pb²⁺, Mn²⁺, Cu²⁺, La³⁺, K⁺, Cd²⁺, Al³⁺ and Ba²⁺, could give an observable increase of fluorescence intensity, demonstrating the high selectivity of the probe for Hg²⁺ quantitation. More interestingly, the selectivity of Tmbipe was so high that it enabled selective Hg²⁺ detection even in the mixture of Hg²⁺ and other challenging metal ions. That is, when Hg²⁺ and another metal ion (5 times the Hg²⁺ concentration) were both added to the sensing system, the fluorescence signal increased to a level similar to that when Hg²⁺ alone was added, thus indicating that Hg²⁺ detection would not be interfered with by the presence of other metal ions. Such a high selectivity might be attributed to the formation of dinuclear Hg^II tetracarbene complexes, which requires the coordination of Hg²⁺ with carbon atoms in benzimidazole groups. Compared to other metal ions, Hg²⁺ has a much higher tendency to coordinate with carbon atoms, which can be demonstrated by the high conversion rate of Hg²⁺ to MeHg⁺ in the natural environment and extraordinary stability of MeHg⁺ in a hot sulfuric acid solution.²²

Quantitation of organomercury compounds

Considering that organomercury is much more poisonous than Hg²⁺ due to its good lipid solubility, the sensitive and reliable detection of organomercury is more striking. Therefore, the feasibility of organomercury quantitation using the proposed sensing system was investigated by recording the changes in fluorescence intensity of Tmbipe (10 μM) as a function of organomercury concentration (Fig. 6). First, the applicability for MeHg⁺ detection was tested. Encouragingly, Tmbipe could also give a rapid fluorescence response to MeHg⁺, and about 16-fold fluorescence intensity increase was observed after addition of 2.0 equiv. of MeHg⁺. A good relationship (I = 8.57 + 6.31C (μM), R² = 0.9902) was observed between fluorescence intensity and MeHg⁺ concentration in the range of 200 nM–20 μM. The detection limit was calculated to be 94 nM. To date, only a few fluorescent probes that can work for both Hg²⁺ and MeHg⁺ have been reported.^18b,23 Compared to these probes, our Tmbipe shows comparable or better detection sensitivity (Table S1‡). Besides MeHg⁺, PhHg⁺, another organomercury species could also be easily detected by Tmbipe in the same way. The linear detection range was 200 nM–20 μM (I = 7.68 + 7.62C (μM), R² = 0.9961) and the detection limit was 78 nM. These results, combined with those of Job plot analysis, suggested that both Hg²⁺ ions and organomercury can coordinate with Tmbipe to form dinuclear Hg^II tetracarbene complexes and AIE-active aggregates, thus making the simultaneous detection of inorganic and organic mercury species possible.

Fig. 6 Quantitation of (a and b) MeHg⁺ or (c and d) PhHg⁺ using the Tmbipe probe. (a and c) Fluorescence spectra of the sensing systems containing different concentrations of (a) MeHg⁺ or (c) PhHg⁺. (b and d) MeHg⁺ or PhHg⁺ concentration-dependent changes in fluorescence intensity at 468 nm. The insets show the linear relationship between the fluorescence intensity and MeHg⁺ or PhHg⁺ concentration in the range of 200 nM–20 μM.

Fluorescence imaging in living cells

Having demonstrated the ability of Tmbipe to quantitatively detect Hg²⁺ ions and organomercury, its bioimaging application in detecting Hg(II) species in living cells was then explored. To achieve this, human cervical carcinoma HeLa cells were divided into three groups: one group was incubated with 10 μM Tmbipe probe for 30 min and the other two groups were treated with 10 μM or 20 μM Hg²⁺ for 1 h, respectively, followed by incubation with 10 μM Tmbipe for 30 min after removing non-accumulated Hg²⁺. As shown in Fig. 7, no evidence of fluorescence appeared in the HeLa cells without Hg²⁺ treatment, indicating that Tmbipe remained in a dispersed state in living cells containing no Hg(II) species. In contrast, blue fluorescence was observed from the cells contaminated with 1.0 equiv. of Hg²⁺, suggesting that probe molecules can efficiently pass through cell membranes and coordinate with intracellular Hg(II) species, giving an active fluorescence signal. When the Hg²⁺ concentration was increased to 2.0 equivalents, the fluorescence intensity was significantly enhanced, demonstrating that the observed fluorescence intensity is concentration-dependent. Similar results were observed for Hg(II)-imaging in HL7704 human adult hepatocyte cells and MCF-7 human breast cancer cells (Fig. S7‡). These results suggest that the prepared Tmbipe probe is able to detect the accumulation of mercury species in living cells.

Fig. 7 Detection of Hg(II) species in HeLa cells.

Conclusions

In summary, a new water-soluble AIE-based fluorescent probe, Tmbipe, which contains a tetraphenylethylene-bridged skeleton and four positively charged benzimidazole groups, was designed, synthesized and well characterized. This probe could coordinate with both Hg²⁺ and organomercury to form planar dinuclear Hg^II tetracarbene complexes, which would self-aggregate to give active AIE fluorescence. Synergic rigidification of the tetraphenylethylene-bridged Tmbipe molecule by mercury-mediated chelate ring formation and subsequent aggregation could result in higher fluorescence enhancement than that given by single aggregation-induced AIE activation. The fluorescence response of Tmbipe to Hg(II) species was so rapid that it could reach a maximum in 3 min. The unique dinuclear Hg^II tetracarbene formation-based working mechanism endows Tmbipe with highly specific response towards Hg²⁺ and organomercury species. These attractive characteristics endow Tmbipe with the ability to sensitively and selectively detect both Hg²⁺ ion and organomercury species. This probe was also demonstrated to work well for the detection of mercury species accumulation in living cells.

Experimental

Materials and reagents

Methylmercury, phenylmercury, iodomethane, 1H-benzimidazole and 4,4′-difluorobenzophenone were obtained from Sigma. TiCl₄, tetrahydrofuran (THF), copper power, dimethyl sulfoxide (DMSO), zinc powder, ethyl acetate, diethyl ether and other metal salts (Hg(Ac)₂, Co(Ac)₂, Ni(NO₃)₂, Mg(NO₃)₂, Zn(Ac)₂, Fe(NO₃)₃, Fe(NO₃)₂, Pb(NO₃)₂, Mn(Ac)₂, Cu(NO₃)₂, La(NO₃)₃, KAc, Cd(NO₃)₂, Al(NO₃)₃ and Ba(NO₃)₂) were obtained from Jiangtian Co. Ltd. (Tianjin, China). Deionized and sterilized water (resistance > 18 MΩ cm⁻¹) was used in all the experiments. All chemical reagents were of analytical grade and used without further purification.

Instruments and measurements

The ¹H-nuclear magnetic resonance (¹H-NMR) spectra were recorded in DMSO-d₆ on a Bruker Spectrospin Avance 400 MHz NMR spectrophotometer. Chemical shifts are given in ppm downfield from tetramethylsilane. Infrared (IR) spectra were measured on a Perkin Elmer Spectrum One spectrophotometer. UV-vis absorption spectra were recorded on a Cary 60 UV-vis spectrophotometer (Agilent Technologies). High resolution mass spectrometry (HRMS) was carried out using an Agilent 6520 Q-TOF LC/MS mass spectrometer. Elemental analysis was performed on a Perkin-Elmer 240 elemental analyzer. Fluorescence spectra and resonance light scattering (RLS) spectra were measured at room temperature using a SHIMADZU RF-5301PC spectrofluorimeter. The transmission electron micrograph (TEM) images were recorded on a German Leica TCS-SP8 transmission electron microscope. A Malvern Zetasizer Nano ZS90 particle size analyzer was used for dynamic light scattering (DLS) studies.

Synthesis and characterization of Tmbipe

The synthetic route to Tmbipe is shown in Scheme S1.‡ The precursor Tbipe was synthesized according to the reported method.¹⁹ Its ¹H-NMR spectrum is given in Fig. S1.‡ Then, the compound Tbipe (0.500 g, 0.630 mmol) and an excess of iodomethane (0.716 g, 5.040 mmol) were dissolved in 10 mL of dry dichloromethane. The obtained reaction mixture was heated to reflux for 48 h. During this period, a pale yellow precipitate was generated. After cooling of the reaction mixture to ambient temperature, the precipitate was isolated by filtration, washed with diethyl ether and then dried in vacuo to give a pale yellow solid. ¹H-NMR (400 MHz, DMSO-d₆), δ = 10.18 (s, 4H), 8.16 (d, 4H), 7.79–7.83 (m, 16H), 7.70–7.72 (d, 4H), 7.61–7.63 (d, 8H), 4.18 ppm (s, 12H); HRMS (ESI, positive ions): m/z = 214.0998 (calcd for Tmbipe⁴⁺ = 214.2682). Elemental analysis calcd (%) for C₅₈H₄₈N₈I₄: C, 51.05; H, 3.55; N, 8.21; found: C, 51.15; H, 3.42; N, 8.09.

Detection of Hg²⁺ and organomercury

A stock solution of Tmbipe (1 mM) was prepared in deionized water. The mixture of 10 μM Tmbipe and different concentrations of Hg²⁺, methylmercury or phenylmercury was prepared in a mixed solvent (THF : H₂O = 5 : 95). After incubation at ambient temperature for 3 min, the fluorescence spectrum in the range of 400–550 nm was recorded under the excitation of 385 nm (excitation slit = emission slit = 5 nm), and the fluorescence intensity at 468 nm was used for mercury species quantitation.

Detection of Hg(II) species in living cells

HeLa cells and MCF-7 cells were cultured in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS). HL 7702 cells were cultured in RPMI 1640 medium supplemented with 10% FBS. Cells in the exponential phase of growth were seeded into 24 well plates and then incubated with 10 μM or 20 μM Hg²⁺ for 1 h at 37 °C. After being washed three times with PBS, the cells were incubated with 10 μM Tmbipe for another 30 min. Then, the cells were washed three times with PBS buffer. Fluorescence imaging of living cells was performed on an Olympus IX-81 microscope.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 21371130 and 21728801), the Natural Science Foundation of Tianjin (no. 15JCYBJC48300 and 16JCYBJC19900), and the Innovation Fund of Tianjin University.

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Footnotes

† This work is dedicated to Professor Dai-Zheng Liao on the occasion of his 80th birthday.

‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/c8sc05714a

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