A highly selective fluorescent probe for Hg2+ based on a rhodamine–coumarin conjugate

Analytica Chimica Acta 663 (2010) 85–90

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A highly selective fluorescent probe for Hg2+ based on a rhodamine–coumarin conjugate Qiu-Juan Ma a,b , Xiao-Bing Zhang b,∗ , Xu-Hua Zhao b , Zhen Jin b , Guo-Jiang Mao b , Guo-Li Shen b , Ru-Qin Yu b,∗ a b

College of Pharmacology, Henan University of Traditional Chinese Medicine, Zhengzhou 450008, PR China State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan 410082, PR China

a r t i c l e

i n f o

Article history: Received 21 September 2009 Received in revised form 3 January 2010 Accepted 14 January 2010 Available online 22 January 2010 Keywords: Fluorescent probe Mercuric ions Rhodamine derivative Fluorescence enhancement

a b s t r a c t A fluorescent probe 1 for Hg2+ based on a rhodamine–coumarin conjugate was designed and synthesized. Probe 1 exhibits high sensitivity and selectivity for sensing Hg2+ , and about a 24-fold increase in fluorescence emission intensity is observed upon binding excess Hg2+ in 50% water/ethanol buffered at pH 7.24. The fluorescence response to Hg2+ is attributed to the 1:1 complex formation between probe 1 and Hg2+ , which has been utilized as the basis for the selective detection of Hg2+ . Besides, probe 1 was also found to show a reversible dual chromo- and fluorogenic response toward Hg2+ likely due to the chelation-induced ring opening of rhodamine spirolactam. The analytical performance characteristics of the proposed Hg2+ -sensitive probe were investigated. The linear response range covers a concentration range of Hg2+ from 8.0 × 10−8 to 1.0 × 10−5 mol L−1 and the detection limit is 4.0 × 10−8 mol L−1 . The determination of Hg2+ in both tap and river water samples displays satisfactory results. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Mercury is one of the most dangerous and ubiquitous of pollutants in the global environment, is derived from both natural sources and human enterprise, and exists in a variety of forms (elemental, inorganic, and organic mercury). Mercuric ion (Hg2+ ), much more common than mercurous ion (Hg+ ), is a caustic and carcinogenic material with high cellular toxicity [1]. Methylmercury, one primary form of organic mercury, can be formed naturally by biomethylation of mercuric ion in the aquatic environment, is bioaccumulated in human body through biological food chains, and can cause brain damage and other chronic diseases [2,3]. Therefore, it is important to monitor Hg2+ in many scientific fields, including medicine, environmental monitoring, etc. In the past few years, many analytical methods such as atomic absorption spectrometry [4], inductively coupled plasma-mass spectrometry [5,6], continuous flow cold vapor atomic fluorescence spectroscopy [7], inductively coupled plasma-atomic emission spectrometry [8], electrochemical methods [9–11], and UV–vis spectrometry [12] have been applied to detect the concentration of Hg2+ . Though these techniques are sensitive, selective, and accurate for Hg2+ assay, most of them are rather complicated, time-

∗ Corresponding authors. Tel.: +86 731 88821916; fax: +86 731 88821916. E-mail addresses: [email protected] (X.-B. Zhang), [email protected] (R.-Q. Yu). 0003-2670/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2010.01.029

consuming, and costly as well as inappropriate for on-line or field monitoring. Due to their advantages of simplicity, high sensitivity and low cost, a large number of fluorescent probes have been developed for the determination of mercuric ions so far. Hg2+ can cause fluorescence quenching of the fluorophores via the spin–orbit coupling effect [13], thus most of Hg2+ fluorescent probes were designed based on fluorescence quenching [14–19], which is not as sensitive as a fluorescence enhancement signal. Until now, only a few probes for Hg2+ with fluorescence enhancement have been proposed [20–23]. Therefore, searching for Hg2+ probes based on fluorescence enhancement is still an active field as well as a challenge for the analytical chemistry research effort. Rhodamine and its derivatives are employed extensively as fluorescence labeling reagents due to their favorable spectroscopic properties including large molar extinction coefficient (ε), relatively long excitation and emission wavelengths, and high fluorescence quantum yield (Ф) [24]. These make rhodamine and its derivatives potential fluorophores for several metal ions. Recently, rhodamine-based probes for various cations via chromogenical and fluorogenical signals have received increasing interests such as Cu2+ [25–28], Pb2+ [29], Cr3+ [30–32] and Fe3+ [32,33]. The cation sensing mechanism of these probes is based on the change in structure between spirocyclic and open-cycle forms. Without cations, these probes exist in a spirocyclic form and are non-fluorescent and colorless. Upon addition of metal cation, these probes undergo ring opening of the corresponding spirolactone or spirolactam via a reversible coordination or an irreversible chemical reaction and

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give rise to strong fluorescence emission and a pink color. Based on the mechanism, several rhodamine-based fluorescent probes for Hg2+ have been also reported [34–43]. However, some of these Hg2+ fluorescent probes have shortcomings for practical application such as a narrow pH span [34], slow response time [35], and cross-sensitivities toward other metal cations [40]. Therefore, the demand for analytical techniques for the selective and sensitive determination of Hg2+ ions is of topical interest, especially in a wide pH span and real-time monitoring of Hg2+ in practical analysis. Coumarin dyes have been extensively used in fluorescent probes for their excellent photochemical and photophysical properties [44–47]. Moreover, the carbonyl oxygen atom in coumarins could participate in coordinating with metal ions [45–52], which thus effectively modulates their fluorescence. Accordingly, coumarins could play a role both as a fluorophore and a binding unit in probes. Taking into consideration the sensitivity and selectivity, an Hg2+ fluorescent probe (probe 1) based on a rhodamine–coumarin conjugate has been developed in this paper. Because N, S, and O binding sites might be a choice to be parts of a selective receptor for the selective recognition of Hg2+ [20,21,34,40,41,43,17], a sulfur-based functional group was considered and introduced to probe 1 in the present work. Recently, Zheng et al. reported a highly selective and sensitive Hg2+ probe based on rhodamine B thiohydrazide, in which the oxygen atom in carbonyl of rhodamine B was replaced by sulfur atom [34]. However, rhodamine B thiohydrazide only work well in strong acidic solution at pH 3.40. Liu et al. also described dithiolane linked thiorhodamine dimer for Hg2+ recognition based on chemodosimetric reaction, in which Hg2+ recognition is irreversible [43]. Herein, we report the design, synthesis and spectral characteristics of a mercuric probe 1 based on thiorhodamine. It shows fluorescence response upon the

addition of Hg2+ in 50% water/ethanol buffered at pH 7.24, which also provides a unique fluorescence response to Hg2+ in the presence of many other metal cations. Moreover, probe 1 shows the features of reversibility and real-time responses to Hg2+ . Besides, the feasibility of using probe 1 for the determination of Hg2+ in both tap and river water samples has been testified. 2. Experimental 2.1. Reagents Lawesson’s reagent (97%) was purchased from Aldrich. 7Diethylaminocoumarin was supplied by Fluka. Rhodamine B and hydrazine hydrate (85%) were obtained from Shanghai Chemical Reagents and used as received. Before being used, N,Ndimethylformamide (DMF) was freshly subjected to distillation from CaH2 and toluene was distilled at atmospheric pressure from sodium. Other chemicals were of analytical reagent grade and used without further purification except when specified. Doubly distilled water was used throughout all experiments. 2.2. Syntheses The synthetic procedure for fluorescence probe 1 is shown in Scheme 1. 7-Diethylaminocoumarin-3-aldehyde (compound 2) was prepared from 7-diethylaminocoumarin and POCl3 according to a previously reported procedure [50]. Compound 3 was synthesized according to Xiang’s method [53] by the reaction of rhodamine B and hydrazine hydrate (85%). Compound 4 was synthesized starting

Scheme 1. Synthesis of fluorescent probe 1: (a) POCl3 , DMF, 60 ◦ C, 30 min, 70.8%; (b) hydrazine hydrate (85%), ethanol, reflux, 2 h, 75%; (c) Lawesson’s reagent, toluene, reflux, 4 h, 18%; (d) ethanol, reflux, 12 h, 62%.

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from compound 3 and Lawesson’s reagent by the reported literature [34]. Synthesis of compound 1. In 25 mL of ethanol, compound 2 (0.245 g, 1.0 mmol) and compound 4 (0.473 g, 1.0 mmol) were dissolved to form a mixture. The reaction mixture was stirred and refluxed for 12 h under nitrogen atmosphere. After the solvent was evaporated under reduced pressure, the crude product was purified by column chromatography (petroleum ether/CH3 COOCH2 CH3 = 1:1) to give compound 1 as an orange-red solid (0.433 g, 62%). 1 H NMR (400 MHz, CDCl3 ), ı (ppm): 1.13–1.25 (m, 18H), 3.32 (q, 8H, J = 7.2 Hz), 3.42 (q, 4H, J = 7.2 Hz), 6.29–6.33 (m, 3H), 6.47 (d, 1H, J = 2.4 Hz), 6.56 (dd, 1H, J = 2.4 Hz, 8.8 Hz), 6.78 (d, 2H, J = 8.4 Hz), 7.12–7.14 (m, 1H), 7.25 (d, 2H, J = 8.0 Hz), 7.41–7.44 (m, 2H), 8.12–8.14 (m, 1H), 8.37 (s, 1H), 8.77 (s, 1H). 13 C NMR (100 MHz, CDCl ), ı (ppm): 12.50, 12.65, 44.39, 45.00, 3 63.05, 97.24, 97.51, 108.36, 108.93, 109.52, 110.66, 113.8 3, 122.51, 127.03, 127.88, 130.28, 130.55, 132.13, 135.40, 140.45, 148.26, 151.66, 151.84, 153.56, 155.06, 157.42, 161.60, 170.83. MS(ESI) m/z: 700.1 (M+H)+ . 2.3. Apparatus All fluorescence measurements were made on a Hitachi F-4500 Fluorescence Spectrometer (Tokyo, Japan) with excitation slit set at 5.0 nm and emission at 5.0 nm in 1 cm × 1 cm quartz cell. UV–vis absorption spectra were recorded on a UV-2450 spectrophotometer (Tokyo, Japan) in 1 cm path length quartz cuvette with a volume of 2 mL. 1 H NMR was acquired in CDCl3 on Varian INOVA-400 MHz NMR spectrometer using TMS as an internal standard. The measurements of pH were carried out on a Mettler-Toledo Delta 320 pH meter (Shanghai, China). Data processing was performed on a Pentium IV computer with software of SigmaPlot. 2.4. Measurement procedures A stock solution of 4.0 × 10−5 mol L−1 compound 1 was prepared by dissolving the requisite amount of 1 in absolute ethanol. The standard solutions of 8 × 10−7 –8 × 10−4 mol L−1 Hg2+ were obtained by serial dilution of 1.0 × 10−2 mol L−1 Hg2+ solution with pH 7.24 Tris–HCl buffer solution. The wide pH range solutions were prepared by adjustment of 0.05 mol L−1 Tris–HCl solution with HCl or NaOH solution. The complex solution of Hg2+ and compound 1 was obtained by mixing 12.5 mL of the stock solution of compound 1 and 2.5 mL of Hg2+ solution of the different concentrations in a 25 mL volumetric flask. Then the mixture was diluted to 25 mL with Tris–HCl buffer solution. In the solution thus obtained, the concentrations were 2 ×10−5 mol L−1 in compound 1 and 8 × 10−8 –8 × 10−5 mol L−1 in Hg2+ . Blank solution of compound 1 was prepared under the same conditions without Hg2+ . The fluorescence intensity was measured at excitation wavelength of 520 nm with the emission recorded over the wavelength range of 530–650 nm.

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Fig. 1. Changes of the fluorescence spectra of probe 1 (20 ␮M) in the presence of various concentrations of Hg2+ : 0, 0.08, 0.1, 0.4, 0.8, 2, 4, 6, 8, 10, 20, 40, 60, 80 ␮M from a to n (ex = 520 nm). These spectra were measured in 0.05 M Tris–HCl buffer (ethanol/water, 1:1, v/v, pH 7.24).

NMR spectrum. Similar to other rhodamine spirolactam derivatives, compound 1 forms a fluorescence inactive solution in either Tris–HCl aqueous buffer solution or pure organic solvent, indicating that the spirolactam form exists predominantly. This is also confirmed by a distinctive spirocycle carbon chemical shift at 65.05 ppm in the 13 C NMR spectrum of 1 [36]. Fig. 1 shows fluorescence emission changes of probe 1 (ex = 520 nm) in pH 7.24 Tris–HCl buffer solution (ethanol/water, 1:1, v/v) upon addition of different Hg2+ concentrations. Free 1 exhibits very slight fluorescence in the range from 530 to 650 nm (Fig. 1). Upon the addition of Hg2+ , the fluorescence intensities of probe 1 increase at around 580 nm with increasing concentration of Hg2+ to 1 equiv., but remained unchanged with further addition of Hg2+ . In the presence of 20 ␮M Hg2+ , an approximately 24-fold enhancement in the fluorescence emission intensity was observed and the emission quantum yield (˚) was found to be 0.12 relative to rhodamine 6G in EtOH (˚F = 0.94) [54]. These results constitute the basis for the determination of Hg2+ concentration with probe 1 proposed in this paper. In order to obtain a better insight into the response mechanism of 1 toward Hg2+ , the absorption spectra of 1 in the absence and presence of Hg2+ were recorded (Fig. 2). The UV–vis spectrum of probe 1 exhibits an absorption peak at 459 nm, which

3. Results and discussion 3.1. Synthesis and spectral characteristics of probe 1 Compound 1 was readily synthesized by the route as outlined in Scheme 1. Compound 2 was prepared from 7diethylaminocoumarin and POCl3 via Vilsmeier reaction [50]. Compound 3 was synthesized by the reaction of rhodamine B and hydrazine hydrate (85%) [53], which was then treated with Lawesson’s reagent to give compound 4 [34]. Finally, probe 1 was synthesized by the condensation reaction between 2 and 4. Its structure was confirmed by MS data, 1 H NMR and 13 C

Fig. 2. UV spectra of probe 1 (20 ␮M) before (···) and after (—) the addition of Hg2+ (20 ␮M) in pH 7.24 Tris–HCl buffer (ethanol/water, 1:1, v/v).

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Fig. 3. Job’s plot for probe 1 in 0.05 M Tris–HCl buffer (ethanol/water, 1:1, v/v, pH 7.24). The total concentration of 1 and Hg2+ was 40 ␮M (ex = 520 nm).

Scheme 2. Possible coordination mode for probe 1 with Hg2+ .

corresponds to absorption of the coumarin group [50] and a very weak band above 565 nm, which could be ascribed to the presence of a trace amount of the ring-opened form of 1. The above results indicated that the closed spirolactam form was the predominant species in the absence of Hg2+ . When 20 ␮M Hg2+ was added to a solution (ethanol/water, 1:1, v/v) of 1, a significant absorption at 565 nm (ε = 5.16 × 104 L mol−1 cm−1 ) was noticeable due to the ring-opened amide form of 1 upon Hg2+ binding [41] and the absorption peak at 459 nm disappeared which indicated the carbonyl group on coumarin moiety might participate in the coordination of Hg2+ [52]. The FT-IR spectra of compound 1 and 1–Hg2+ complex were also studied to determine the binding mode of probe 1 and Hg2+ . When probe 1 is completely coordinated with Hg2+ , the peak at 1714 cm−1 , which corresponds to the absorption of carbonyl group on coumarin moiety, was shifted to 1630 cm−1 . The IR data support that a binding participation of the carboxyl group occurs with mercury ions. Thus, a possible coordination mode for probe 1 with Hg2+ was proposed (Scheme 2). Besides, the ring opening process of spirolactam can also enable the colorimetric change upon the addition of Hg2+ and an obvious color change from orange to pink was observed by the naked eye in the presence of Hg2+ .

sents the concentration of Hg2+ added. The detection limit is 4.0 × 10−8 mol L−1 (calculated as three times standard deviation of blank solution) [55]. Therefore, our proposed probe 1 was sensitive enough to detect Hg2+ in industrial wastewater, which has a discharge limit of 0.25 ␮M (50 ppb) defined by Standardization Administration (SA) of the People’s Republic of China [56]. 3.3. Effect of pH The effects of pH on the fluorescence intensity of probe 1 in the absence of Hg2+ were investigated in the pH range from 1.43 to 12.0 (Fig. 5). Probe 1 did not display any obvious and characteristic fluorescence (excitation at 520 nm) at a pH range from 4.00 to 12.0, suggesting that it was stable over the pH range of 4.00–12.0 and could work in real samples with very low background fluorescence (Fig. 5). When pH was lower than 4.00, the fluorescence intensity increased with decreasing pH values, which might be caused by the ring-opened form of 1. From the view of sensitivity and the speed time, the Tris–HCl buffer solution at pH 7.24 (ethanol/water, 1:1, v/v) was chosen as optimum experimental condition.

3.2. Principle of operation Probe 1 coordinates with Hg2+ in a 1:1 stoichiometry. This is confirmed by a Job’s plot (Fig. 3). A maximum emission intensity was showed when the molecular fraction of Hg2+ was close to 0.5, which indicated the 1:1 complex formation between 1 and Hg2+ with a total concentration of 40 ␮M (Fig. 3). On the basis of 1:1 stoichiometry, the association constant for Hg2+ was estimated to be 1.18 × 106 M−1 in EtOH/H2 O (1:1, v/v) solution by a nonlinear curve fitness [41] (Fig. 4). The linear response of the fluorescence emission intensity toward [Hg2+ ] was obtained in Hg2+ concentration range of 8.0 × 10−8 –1.0 × 10−5 mol L−1 (insert in Fig. 4), and can be expressed by the following Eq. (1) of the calibration line: F = 18.0261 + 1.6759 × 107 × [Hg2+ ]

(R = 0.9950)

(1)

Here F is the fluorescence emission intensity of probe 1 actually measured at a given metal concentration, and [Hg2+ ] repre-

Fig. 4. The curve fitting according to the method reported in the literature [41]. Insert: the plot of fluorescence intensity of probe 1 (20 ␮M) as a function of the [Hg2+ ] from 0.08 to 10 ␮M. These data were measured in pH 7.24 Tris–HCl buffer (ethanol/water, 1:1, v/v). The excitation wavelength was 520 nm.

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Fig. 5. pH dependence of the fluorescence intensity of 20 ␮M probe 1 in the absence of Hg2+ . All data were obtained at various pH values (pH 1.43–12.0) and the excitation wavelength was 520 nm.

3.4. Selectivity The fluorescence responses of probe 1 to various cations and its selectivity for Hg2+ are illustrated in Fig. 6. The experiments were carried out by fixing the concentration of Hg2+ at 1.0 × 10−5 mol L−1 , cations were added as chlorides, nitrates, acetate and sulfates. As can be seen from the black bars in Fig. 6, fluorescence almost did not change in the 10−3 mol L−1 Li+ , Na+ , K+ , Mg2+ , Ca2+ , Mn2+ , Fe3+ and Al3+ solutions. Furthermore, the fluorescence inferences were negligible in the 10−4 mol L−1 Ag+ and Pb2+ solutions. The fluorescence was affected to some extent in the 10−3 mol L−1 Zn2+ , Cd2+ , Co2+ , Ni2+ , and Cu2+ solutions. However, when the concentrations of Zn2+ , Cd2+ , Co2+ , and Ni2+ solutions were lowered to 10−4 mol L−1 and the concentration of Cu2+ solution was lowered to 10−5 mol L−1 , these ions did not induce any obvious fluorescence interferences, which were probably due to several combined influences cooperating to achieve the unique selectivity for the Hg2+ ion. In order to further test the interference for other common cations on the determination of Hg2+ , competition experiments were performed in which the fluorescent probe was added to

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Fig. 7. The fluorescence titration of EDTA (top to bottom: 0, 0.5, 1.0, 1.5 and 2.0 equiv.) in the presence of equimolar 1/Hg2+ (20 ␮M) at pH 7.24 Tris–HCl buffer (ethanol/water, 1:1, v/v). Excitation was performed at 520 nm.

a solution of Hg2+ in the presence of other metal ions (white bars in Fig. 6). Experimental results indicate that these ions show no obvious interference for Hg2+ detection. It can be found that the relative error of common interference such as alkali, alkaline earth and transitional metal ions was less than ±5%, which is considered as tolerated. Thus, the probe 1 exhibited excellent selectivity toward Hg2+ , which makes it feasible for practical applications. 3.5. Reversibility and response time As is well known, the reversibility is an important matter to obtain an excellent chemical sensor. Thus, the EDTA-adding experiments were conducted to examine the reversibility of the probe 1 (Fig. 7). It is shown clearly that the fluorescence intensities of solution containing 1 and Hg2+ decrease with increasing EDTA concentration. Besides, the color also gradually changed from pink to orange. When Hg2+ was added to the system again, the fluorescence could be reproduced and the orange solution turned to pink. These findings indicated that probe 1 reversibly coordinated with Hg2+ , a similar result was reported by the literature [42]. The above results also further elicit that the spectral response of probe 1 to Hg2+ is likely due to the chelation-induced ring opening of rhodamine spirolactam, rather than other possible reactions [40,43]. We also investigated the time course of the response of probe 1 (20 ␮M) to 6.0 × 10−6 mol L−1 Hg2+ in pH 7.24 Tris–HCl buffer solution (ethanol/water, 1:1, v/v). The results indicate the recognition interaction was completed immediately after addition of Hg2+ without any detectable time-delay. Thus, this system might be used for the real-time monitoring of Hg2+ in practical analysis. 3.6. Preliminary analytical application

Fig. 6. Metal ion selectivity of probe 1 (20 ␮M). All data were obtained at pH 7.24 Tris–HCl buffer (ethanol/water, 1:1, v/v). The concentration of ions added to probe 1 was 1.0 × 10−5 M for Cu2+ and Hg2+ , 1.0 × 10−4 M for Ag+ , Pb2+ , Cd2+ , Co2+ , Ni2+ , Zn2+ and 1.0 × 10−3 M for all remaining ions. The excitation wavelength was 520 nm. Black bars: different metal ions were added. White bars: different metal ions in the presence of Hg2+ were added.

In order to examine the applicability of the proposed method in practical sample analysis, the probe 1 was applied in the determination of Hg2+ in both tap and river water samples. The river water samples were obtained from Xiang River and simply filtered. No Hg2+ was found in these samples. All these water samples were spiked with standard Hg2+ solutions and then analyzed with proposed probe. The recovery study of spiked Hg2+ determined by the sensor shows satisfactory results (Table 1). Thus, the present probe seems useful for the determination of Hg2+ in real samples.

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Table 1 Determination of Hg2+ in tap and river water samples with probe 1. Sample

Hg2+ spiked (mol L−1 )

Hg2+ recovered (mol L−1 )

Recovery (%)

River water

1 2 3

0 6 × 10−6 −5 1 × 10

Not detected (6.17a ± 0.15b ) × 10−6 (1.05a ± 0.03b ) × 10−5

– 102.8 105.0

Tap water

1 2 3

0 6 × 10−6 1 × 10−5

Not detected (6.25a ± 0.23b ) × 10−6 (0.98a ± 0.04b ) × 10−5

– 104.2 98.0

a b

Mean values of the three determinations. Standard deviation.

4. Conclusion In conclusion, a fluorescent probe for mercuric ions based on a rhodamine–coumarin conjugate has been developed, in which the signal was transduced through simply opening the spirolactam ring upon Hg2+ binding. Probe 1 exhibits the feature of a reversible dual-responsive colorimetric and fluorescent response to Hg2+ , and shows a high sensitivity and selectivity for Hg2+ sensing in comparison to other cations in 50% water/ethanol buffered at pH 7.24. The proposed probe can be applied to the quantification of Hg2+ with a linear range covering from 8.0 × 10−8 to 1.0 × 10−5 mol L−1 and the detection limit is 4.0 × 10−8 mol L−1 . The proposed probe has been used for the determination of Hg2+ in both tap and river water samples and shows satisfactory results. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant nos. J0830415, 20505008, 20675028, 20875027 and 20775023), “973” National Key Basic Research Program of China (2007CB310500), Doctoral Research Fund of Henan Chinese Medicine (BSJJ2009-27), and Hunan Natural Science Foundation (07JJ3025). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

[12] [13] [14] [15] [16] [17] [18]

J.S. Lee, M.S. Han, C.A. Mirkin, Angew. Chem. Int. Ed. 46 (2007) 4093–4096. D.W. Boening, Chemosphere 40 (2000) 1335–1351. I. Onyido, A.R. Norris, E. Buncel, Chem. Rev. 104 (2004) 5911–5929. C.M. Tseng, A.D. Diego, F.M. Martin, D. Amouroux, O.F.X. Donard, J. Anal. Atom. Spectrom. 12 (1997) 743–750. Y.F. Li, C.Y. Chen, B. Li, J. Sun, J.X. Wang, Y.X. Gao, Y.L. Zhao, Z.F. Chai, J. Anal. Atom. Spectrom. 21 (2006) 94–96. B. Vallant, R. Kadnar, W. Goessler, J. Anal. Atom. Spectrom. 22 (2007) 322–325. Y.W. Chen, J. Tong, A. D’Ulivo, N. Belzile, Analyst 127 (2002) 1541–1546. A.N. Anthemidis, G.A. Zachariadis, C.E. Michos, J.A. Stratis, Anal. Bioanal. Chem. 379 (2004) 764–769. L. Pérez-Marín, E. Otazo-Sánchez, G. Macedo-Miranda, P. Avila-Pérez, J. Alonso Chamaro, H. López-Valdivia, Analyst 125 (2000) 1787–1790. W. Yantasee, Y.H. Lin, T.S. Zemanian, G.E. Fryxell, Analyst 128 (2003) 467–472. A. Caballero, V. Lloveras, D. Curiel, A. Tárrage, A. Espinosa, R. Garcia, J. VidalGancedo, C. Rovira, K. Wurst, P. Molina, J. Veciana, Inorg. Chem. 46 (2007) 825–838. J. Tan, X.P. Yan, Talanta 76 (2008) 9–14. D.S. McClure, J. Chem. Phys. 20 (1952) 682–686. R. Métivier, I. Leray, B. Valeur, Chem. Eur. J. 10 (2004) 4480–4490. S.Y. Moon, N.J. Youn, S.M. Park, S.K. Chang, J. Org. Chem. 70 (2005) 2394–2397. X.J. Zhu, S.T. Fu, W.K. Wong, J.P. Guo, W.Y. Wong, Angew. Chem. 45 (2006) 3222–3226. S.H. Kim, K.C. Song, S. Ahn, Y.S. Kang, S.K. Chang, Tetrahedron Lett. 47 (2006) 497–500. Y. Yu, L.R. Lin, K.B. Yang, X. Zhong, R.B. Huang, L.S. Zheng, Talanta 69 (2006) 103–106.

[19] S. Pandey, A. Azam, S. Pandey, H.M. Chawla, Org. Biomol. Chem. 7 (2009) 269–279. [20] E.M. Nolan, S.J. Lippard, J. Am. Chem. Soc. 125 (2003) 14270–14271. [21] E.M. Nolan, S.J. Lippard, J. Am. Chem. Soc. 129 (2007) 5910–5918. [22] R.R. Avirah, K. Jyothish, D. Ramaiah, Org. Lett. 9 (2007) 121–124. [23] G.J. He, Y.G. Zhao, C. He, Y. Liu, C.Y. Duan, Inorg. Chem. 47 (2008) 5169–5176. [24] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, third ed., Springer, New York, 2006. [25] M.H. Lee, H.J. Kim, S. Yoon, N. Park, J.S. Kim, Org. Lett. 10 (2008) 213–216. [26] K.M.K. Swamy, S.K. Ko, S.K. Kwon, H.N. Lee, C. Mao, J.M. Kim, K.H. Lee, J. Kim, I. Shin, J. Yoon, Chem. Commun. (2008) 5915–5917. [27] X. Chen, M.J. Jou, H. Lee, S. Kou, J. Lim, S.W. Nam, S. Park, K.M. Kim, J. Yoon, Sens. Actuators B: Chem. 137 (2009) 597–602. [28] X.Q. Chen, J. Jia, H.M. Ma, S.J. Wang, X.C. Wang, Anal. Chim. Acta 632 (2009) 9–14. [29] J.Y. Kwon, Y.J. Jang, Y.J. Lee, K.M. Kim, M.S. Seo, W. Nam, J.Y. Yoon, J. Am. Chem. Soc. 127 (2005) 10107–10111. [30] K. Huang, H. Yang, Z. Zhou, M. Yu, F. Li, X. Gao, T. Yi, C. Huang, Org. Lett. 10 (2008) 2557–2560. [31] Z.G. Zhou, M.X. Yu, H. Yang, K. Huang, F.Y. Li, T. Yi, C.H. Huang, Chem. Commun. (2008) 3387–3389. [32] J. Mao, L. Wang, W. Dou, X.L. Tang, Y. Yan, W.S. Liu, Org. Lett. 9 (2007) 4567–4570. [33] S. Bae, J. Tae, Tetrahedron Lett. 48 (2007) 5389–5392. [34] H. Zheng, Z.H. Qian, L. Xu, F.F. Yuan, L.D. Lan, J.G. Xu, Org. Lett. 8 (2006) 859–861. [35] J.S. Wu, I.C.H. Wang, K.S. Kim, J.S. Kim, Org. Lett. 9 (2007) 907–910. [36] M. Suresh, A. Shrivastav, S. Mishra, E. Suresh, A. Das, Org. Lett. 10 (2008) 3013–3016. [37] M.J. Yuan, W.D. Zhou, X.F. Liu, M. Zhu, J.B. Li, X. Yin, H. Zheng, Z.Z. Zuo, C.O. Yang, H.B. Liu, Y.L. Li, D. Zhu, J. Org. Chem. 73 (2008) 5008–5014. [38] D. Wu, W. Huang, Z.H. Lin, C.Y. Duan, C. He, S. Wu, D.H. Wang, Inorg. Chem. 47 (2008) 7190–7201. [39] Y. Shiraishi, S. Sumiya, Y. Kohno, T. Hirai, J. Org. Chem. 73 (2008) 8571–8574. [40] W. Shi, H.M. Ma, Chem. Commun. (2008) 1856–1858. [41] J.H. Huang, Y.F. Xu, X.H. Qian, J. Org. Chem. 74 (2009) 2167–2170. [42] W. Huang, P. Zhou, W. Yan, C. He, L.Q. Xiong, F.Y. Li, C. Duan, J. Environ. Monit. 11 (2009) 330–335. [43] W. Liu, L.W. Xu, H.Y. Zhang, J.J. You, X.L. Zhang, R.L. Sheng, H. Li, S. Wu, P.F. Wang, Org. Biomol. Chem. 7 (2009) 660–664. [44] W.Y. Lin, L. Yuan, X.W. Cao, W. Tan, Y.M. Feng, Eur. J. Org. Chem. (2008) 4981–4987. [45] W.Y. Lin, L. Yuan, J.B. Feng, Eur. J. Org. Chem. (2008) 3821–3825. [46] R.L. Sheng, P.F. Wang, W.M. Liu, X.H. Wu, S.K. Wu, Sens. Actuators B: Chem. 128 (2008) 507–511. [47] H.S. Jung, P.S. Kwon, J.W. Lee, J.I. Kim, C.S. Hong, J.W. Kim, S.H. Yan, J.Y. Lee, J.H. Lee, T.H. Joo, J.S. Kim, J. Am. Chem. Soc. 131 (2009) 2008–2012. [48] N.C. Lim, C. Brückner, Chem. Commun. (2004) 1094–1095. [49] N.C. Lim, J.V. Schuster, M.C. Porto, M.A. Tanudra, L.L. Yao, H.C. Freake, C. Brckner, Inorg. Chem. 44 (2005) 2018–2030. [50] J.S. Wu, W.M. Liu, X.Q. Zhuang, F. Wang, P.F. Wang, S.L. Tao, X.H. Zhang, S.K. Wu, S.T. Lee, Org. Lett. 9 (2007) 33–36. [51] R. Sheng, P.F. Wang, Y.H. Gao, Y. Wu, W. Liu, J.J. Ma, H.P. Li, S.K. Wu, Org. Lett. 10 (2008) 5015–5018. [52] S.Y. Lee, H.J. Kim, J.S. Wu, K. No, J.S. Kim, Tetrahedron Lett. 49 (2008) 6141–6144. [53] Y. Xiang, A. Tong, P. Jin, Y. Ju, Org. Lett. 8 (2006) 2863–2866. [54] M. Fischer, J. Georges, Chem. Phys. Lett. 260 (1996) 115–118. [55] Q.P. Guo, X.H. Yang, K.M. Wang, W.H. Tan, W. Li, H.X. Tang, H.M. Li, Nucleic Acids Res. 37 (2009) e20. [56] Standardization Administration of the People’s Republic of China, Integrated wastewater discharge standard, GB 8978-1996.