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A new fluorescent turn-on chemodosimeter for mercury(II) based on dithioacetal-substituted triphenylimidazole
Author’s Accepted Manuscript A new fluorescent turn-on chemodosimeter for mercury(II) based on dithioacetal-substituted triphenylimidazole Xiangzhu He, Shuangming Zhu, Hongbiao Chen, Yongpeng Wang, Huaming Li www.elsevier.com/locate/jlumin
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S0022-2313(15)30194-0 http://dx.doi.org/10.1016/j.jlumin.2016.01.025 LUMIN13822
To appear in: Journal of Luminescence Received date: 4 July 2015 Revised date: 27 December 2015 Accepted date: 19 January 2016 Cite this article as: Xiangzhu He, Shuangming Zhu, Hongbiao Chen, Yongpeng Wang and Huaming Li, A new fluorescent turn-on chemodosimeter for mercury(II) based on dithioacetal-substituted triphenylimidazole, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.01.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new fluorescent turn-on chemodosimeter for mercury(II) based on dithioacetal-substituted triphenylimidazole
Xiangzhu Hea, Shuangming Zhua, Hongbiao Chena*, Yongpeng Wang, Huaming Lia,b* a
College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, P. R. China
b
Key Laboratory of Polymeric Materials & Application Technology of Hunan Province, Key
Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, and Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan Province, P. R. China *
Corresponding author. Tel.: +86 731 58298572; Fax: +86 731 58293264.
E-mail address: [email protected] (H. Li), [email protected] (H. Chen)
Abstract In this paper, we demonstrate a new fluorescent turn-on chemodosimeter for Hg2+ determination. The chemodosimeter contains a triphenylimidazole fluorophore and an Hg2+-responsive dithioacetal moiety. In the presence of Hg2+, the probe displays a characteristic turn-on mode at 527 nm in its emission spectrum due to the irreversible Hg2+-promoted deprotection of the dithioacetal group. With the aid of the fluorescence spectrometer, the chemodosimeter in the dimethylformamide/H2O (7/3, v/v) mixed solvent (2.0 μM) exhibits a detection limit of 4.3 nM (S/N = 3). Interferences from other common cations associated with Hg2+ analysis are effectively inhibited.
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Keywords Mercury; chemodosimeter; triphenylimidazole; dithioacetal
1. Introduction The development of highly sensitive and selective fluorescent chemosensors for heavy metal ions has received considerable attention because of the inherent characteristic of chemosensory detection, such as high sensitivity, fast analysis, operational simplicity, low cost, and real-time monitoring on one hand [1-6], on the other hand, these metals usually play an important role in living systems and have an extremely toxic impact on the environment and human body [7-15]. As a highly toxic heavy metal, mercury and its compounds are a subject of worldwide concern due to their acute immune-, geno- and neuro-toxic effects on human, livestock, and marine mammals [16,17], since mercury can be absorbed through the skin and mucous membranes, especially mercury vapors can be inhaled [18]. Even a trace amount of mercury intake can lead to acute or chronic damage to the human body, in which toxic effects include damage to the brain, kidney, lung, muscle, and vision [19]. Therefore, the development of novel fluorescent chemosensors for the determination of Hg2+ in the environment and in industrial waste streams at innocuous levels is of great importance in both basic research and practical applications. Recently, extensive efforts have been devoted to developing fluorescent chemosensors for Hg2+ detection [20-22]. As a result, a large number of sensitive and selective chemosensors for Hg2+ in water or organic solvents have been exploited on the basis of Hg2+-ligand complexation or
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Hg2+-promoted irreversible reactions as in the case of chemodosimeter. Among them, chemodosimeters are particularly attractive because of their highly selective behavior together with a unique spectroscopic change resulted from Hg2+-induced specific reactions such as spirolactam ring opening [23,24], mercuration [25,26], and desulfurization [27,28]. Recently, chemodosimeters for Hg2+ determination based on irreversible Hg2+-promoted deprotection of the dithioacetal moiety have drawn much attention due to their sensitive and selective signaling with negligible background [29-41]. Taking advantage of the strong thiophilic affinity of Hg2+, several chemodosimeters for Hg2+ have been demonstrated based on dithioacetal recognition moiety, in which the chromogens contain triphenylamine [29], coumarin [30], perylenediimide [31], boron-dipyrromethene [32], triarylborane [33], β-carboline [34], azobenzene [35], pyrene [36], tetraphenylethylene [37,38], naphthalene [39], 1,8-naphthalimide [40], and donor-π-acceptor structures [41-43]. Among these reported chemodosimeters, however, there are only a few examples of fluorescent turn-on chemodosimeter for Hg2+ [29,40,41]. Therefore, it is still a challenge to fabricate new fluorescent turn-on chemodosimeter for Hg2+ adopting other fluorophores. Herein, we demonstrated a new fluorescent turn-on chemodosimeter for Hg2+ by the functionalization of 2,4,5-triphenylimidazole (TPI) core skeleton with a dithioacetal substituent. As is well known, arylimidazole derivatives have attracted considerable attention in recent years because of their unique properties and diverse applications such as photographic materials, luminescent materials, optical materials, and therapeutic agents [44-46]. In particularly, TPI is a typical fluorophore, which shows a maximum absorption wavelength at 308 nm (ε = 2.62 × 104 M−1 cm−1), fluorescence emission wavelength at 385 nm, and fluorescence quantum yield of 0.10 in
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CH3CN solution [47], anthracene in cyclohexane used as a standard (ΦF = 0.31, excitation wavelength 366 nm [48]). In addition, TPI contains an imidazole ring with dicoordinate nitrogen atoms, which can potentially build complexes assembled by hydrogen bonds with a molecule containing hydrogen-bond donor, such as a carboxylic acid. The chemical flexibility of this class of compounds allows the preparation of a large variety of related structures and, consequently, the tailoring of their optical properties. Therefore, one would expect that novel chemodosimeter based on dithioacetal-substituted TPI might result in new potential applications in Hg2+ detection. In the present work, we describe the synthesis and the spectroscopic evaluation of the fluorescent turn-on chemodosimeter in detail.
2. Experimental 2.1. Reagents and apparatus 4-(4,5-Diphenyl-1H-imidazol-2-yl)benzohydrazide (TPI-CHO) was synthesized according to the reported method [47]. All reagents were purchased from commercial source and used without further purification unless otherwise noted. Infrared (IR) spectra in cm–1 were recorded on a Bruker Equinox-55 spectrometer (Germany). The NMR spectra were recorded with a Bruker AV-400 NMR spectrometer. MALDI-TOF mass spectra were recorded on a Bruker BIFLEXeIII mass spectrometer using a nitrogen laser (337 nm) and an accelerating potential of 20 kV. UV-vis spectra were recorded with an Agilent Cary 100 UV-vis spectrometer. Photoluminescence emission spectra were recorded with an Agilent QM 100 luminescence spectrometer.
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2.2. Synthesis of dithioacetal-substituted 2,4,5-triphenylimidazole (TPI-S) To a solution of TPI-CHO (325 mg, 1.0 mmol) and ethanethiol (0.208 mL, 2.8 mmol) in dry THF (10 mL) was added BF3·Et2O (0.4 mL, 3.36 mmol). The reaction mixture was stirred at 0 °C under argon overnight and the progress of the reaction was monitored by TLC (THF/petroleum ether, 1/2, v/v) analysis. After completion, the reaction mixture was poured into water with stirring and later neutralized with NaHCO3 aqueous solution (0.1 M). After filtration, the obtained crude product was purified by silica gel chromatography (silica gel, gy-001-TY4194, 200-300 mesh) using THF/petroleum ether (1/4, v/v) as an eluent to isolate pure compound TPI-S (390 mg, 90.3%). 1H NMR (DMSO-d6, 400 MHz) δ (ppm): 12.67 (s, 1H, NH), 8.03 (d, 2H, J = 8.0 Hz, ArH), 7.20-7.52 (m, 12H, ArH), 5.20 (s, 1H, CH), 2.53 (m, 4H, CH2), 1.15 (t, 6H, CH3). 13C NMR (100 MHz, DMSO-d6) δ (ppm): 145.65, 141.34, 137.70, 135.64, 131.56, 130.17, 129.14, 128.93, 128.79, 128.66, 128.38, 128.28, 127.56, 127.01, 125.77, 51.63, 26.21, 14.89. MALDI-TOF MS (C26H26N2S2) m/z: calcd. for 430.154, found: 431.269 [M + H]+.
2.3. General spectroscopic procedures The solutions of metal ions were prepared from AgNO3, CoCl2, KCl, SnCl2·2H2O, ZnCl2, CuCl2, (CH3COO)2Ni·4H2O, (CH3COO)2Cd·2H2O, (CH3COO)2Pb·3H2O, NaCl, Ca(NO3)2·4H2O, MnCl2, Mg(NO3)2·6H2O, FeCl3 and HgCl2 with DMF/H2O (7/3, v/v) mixed solvent. A solution of TPI-S (2 μM or 10 μM) was prepared in DMF/H2O (7/3, v/v) mixed solvent. Then 3.0 mL of the solution of TPI-S was placed in a quartz cell (10.0 mm width) and the fluorescent and
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absorption spectra were recorded. The HgCl2 and other cation aqueous solutions were introduced in portions and the fluorescent and absorption changes were recorded at room temperature each time.
3. Results and discussion 3.1. Synthesis and photophysical properties of TPI-S The synthetic procedure for TPI-S is shown in Scheme 1. The target compound TPI-S was prepared conveniently through the general protection reaction between TPI-CHO and ethanethiol. The whole synthetic route was simple and the purification was easy. TPI-S exhibited good solubility in common organic solvents, such as DMF, DMSO, THF, etc. and its structure was fully characterized by 1H and 13C NMR spectroscopy and MALDI-TOF mass spectrometry (see Experimental Section and Figs. S1-S3, Supporting Information, SI). The UV–vis absorption spectra of TPI-CHO and TPI-S are shown in Fig. 1a. As can be seen, the compound TPI-CHO shows an intense absorption band at 363 nm in DMF/H2O (7/3, v/v) solution. After the reaction with ethanethiol, the obtained dithioacetal TPI-S displays an absorption band at 318 nm, around 45 nm blue-shifts compared to its precursor. To evaluate the applicability of TPI-S as an Hg2+ colorimetric probe, excess Hg2+ was added into the diluted solution of TPI-S, it was observed that the absorption band at 318 nm disappeared completely while a new absorption band at 363 nm appeared again. That is to say, the absorption profile of TPI-S became similar to that of its precursor, TPI-CHO, after the addition of excess Hg2+, indicating that the dithioacetal in TPI-S was completely converted to aldehyde in the presence of Hg2+ ions. Such a large red-shift (45 nm) can be easily distinguished by the naked eyes under a normal UV lamp as shown in the inset of Fig. 1a.
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The emission spectra of TPI-CHO and TPI-S recorded in DMF/H2O (7/3, v/v) solution are shown in Fig. 1b. As demonstrated in Fig. 1b, TPI-CHO exhibits a strong emission band centered at 525 with a quantum yield (ΦF) of 0.38 by using anthracene in cyclohexane as a standard (ΦF = 0.31, excitation wavelength 366 nm [48]). However, the fluorescence emission of TPI-S becomes almost nonfluorescent with a quantum yield of 0.054 due to the disruption of conjugation [29]. Upon the addition of excess Hg2+ ions, a strong emission band at 527 nm was observed due to the irreversible Hg2+-promoted deprotection of the dithioacetal group. Notably, the emission band upon the addition of Hg2+ ions red-shifted a little compared to the spectrum of TPI-CHO. This might be contributed to the interaction between the Hg2+ ions and the regenerated aldehyde [29], although it cannot be verified by 1H NMR spectra analysis. As shown in Fig. S4 (see SI), the 1H NMR spectrum of TIP-CHO is basically identical to the spectrum of TPI-S after addition of excess Hg2+ ions. On the basis of above observations, it possible to construct a fluorescent turn-on chemodosimeter for Hg2+ based on the platform of TPI-S.
3.2. Optical response of TPI-S to Hg2+ The absorption spectral titration of TPI-S (10 μM) was initially carried out by addition of increasing equivalents of Hg2+ ions (0–1 equivalents) in DMF/H2O (7/3, v/v) solution and the observed spectral changes are shown in Fig. 2. As shown in Fig. 2a, upon gradual addition of Hg2+ ions to TPI-S solution, the absorption band at 318 nm decreased and the hyperchromic peak at 363 nm appeared gradually with an isosbestic point at around 340 nm, indicating the formation of new species. In addition, a good linear relationship was observed between A363/A318 (the ratio of the absorption
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intensity at 363 and 318 nm) and Hg2+ ion concentrations at the 0–10 μM range, enabling the quantitative determination of Hg2+ ion concentrations (Fig. 2b). Obviously, the colorimetric Hg2+ ions sensing by naked eyes is also feasible owing to the dramatic color change from colorless to yellow under a normal UV lamp. To obtain a highly sensitive chemodosimeter for Hg2+ ions, the sensing behavior of TPI-S toward Hg2+ ions was investigated by fluorescence titration in DMF/H2O (7/3, v/v) solution (2.0 μM) at an excitation wavelength of 363 nm. Fig. 3 displays the fluorescence titration results at room temperature. As can be seen, the fluorescence emission intensity of TPI-S at 527 nm increased significantly with the gradual addition of Hg2+ ions and was saturated at 1 equiv of Hg2+ ions (Fig. 3a), suggesting that the chemodosimeter TPI-S responds to Hg2+ ions at a ratio of 1/1. Job′s plot of fluorescence obtained for TPI-S and Hg2+ ions obviously demonstrated the formation of 1/1 stoichiometry (Fig. S5; see SI) [39]. After the addition of 1 equiv of Hg2+ ions to the TPI-S solution, an increased 20-fold of the fluorescent intensity was observed at 527 nm. In the range of 0–2.0 μM Hg2+ ion concentrations, the plot of the fluorescence intensity at 527 nm as a function of the Hg2+ ion concentrations shows a good linear relationship (R = 0.9954, Fig. 3b), indicating that TPI-S can be used to determine Hg2+ ion concentrations quantitatively. The detection limit of TPI-S was determined to be 4.3 nM at a ratio of signal to noise of 3. Such a low detection limit is indeed competitive with most of the fluorescent chemodosimeters previously reported (Table S1, see SI). Considering the fact that TIP-S is insoluble in water, however, this probe is suboptimal for biological applications. As we know, Hg2+ as a high toxic pollutant to the environment and a lethal substance for human being, is usually found in water or biocompatible solutions.
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To evaluate the selectivity of chemodosimeter TPI-S for Hg2+ ions detection, fluorescence spectral changes upon addition of various cations including Ag+, Co2+, K+, Sn2+, Zn2+, Cu2+, Ni2+, Mn2+, Na+, Ca2+, Mg2+, Pb2+, Fe3+ and Cd2+ were studied. Each spectrum was obtained after addition of various analytes at room temperature for 1 h. As shown in Fig. 4 and Fig. S6 (see SI), the selectivity observed by fluorescence monitoring was matched when TPI-S was used as a chemosensor for Hg2+ ions. That is to say, the Hg2+-promoted deprotection of the dithioacetal group in TPI-S gave a dramatic increase of the fluorescence intensity at 527 nm. In contrast, no significant fluorescence spectral changes were promoted by addition of other cations. The mechanism of TPI-S in signaling of Hg2+ ions was also proved by 1H NMR, MS and IR spectroscopic analysis. As shown in Fig. 5, the signals at δ 1.13-1.48, 2.47-2.64, and 5.20 ppm can be ascribed to the the methyl (Ha), methylene (Hb) and methine (Hc) protons of dithioacetal moiety in TPI-S. After the addition of excess Hg2+ ions, a new signal at 10.01 ppm corresponding to the aldehyde (Hd) appeared, and the the methyl (Ha), methylene (Hb) and methine (Hc) protons for dithioacetal vanished completely, suggesting the irreversible Hg2+-promoted deprotection of the dithioacetal group to aldehyde group (Scheme S1, see SI) [39,40]. The MS spectral analysis shows peak at m/z = 431.269 for pure TPI-S and at m/z = 325.092 for TPI-S upon addition of excess Hg2+ ions (Fig. S7, see SI), also giving a more direct evidence for the Hg2+-promoted deprotection reaction of the dithioacetal. The IR spectra of TPI-S before and after the addition of Hg2+ ions also gave some proof, no peak for aldehyde group was observed in the IR spectrum of TPI-S. But after the addition of excess Hg2+ ions, a typical vibration absorption peak for aldehyde group at 1695 cm−1 appeared owing to the deprotection reaction of TPI-S induced by Hg2+ ions (Fig. S8, see SI).
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4. Conclusion In summary, we have designed and synthesized a novel fluorescent turn-on chemodosimeter for Hg2+ ions based on dithioacetal-substituted triphenylimidazole (TPI-S). Due to the irreversible Hg2+-promoted deprotection of the dithioacetal group, TPI-S showed high selectivity for Hg2+ ions detection over other common cations and could recognize Hg2+ ions by naked eye under a normal UV lamp. With the aid of the fluorescence spectrometer, the TPI-S in DMF/H2O (7/3, v/v) mixed solvent (2.0 μM) exhibited a very low detection limit of 4.3 nM (S/N = 3) Hg2+ ions.
Acknowledgments Financial support from Program for NSFC (51273170), RFDP (20124301110006), and the Construct Program of the Key Discipline in Hunan Province is greatly acknowledged.
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Figure Captions Scheme 1. Synthesis of TPI-S. Fig. 1. (a) Absorption spectra of TPI-S, TPI-S + Hg2+, and TPI-CHO in DMF/H2O (7/3, v/v) (each 10.0 μM). (b) Fluorescence spectra (λex = 363 nm) of TPI-S, TPI-S + Hg2+, and TPI-CHO in DMF/H2O (7/3, v/v) (each 2.0 μM). Inset a: photographs of TPI-S (40.0 μM) in the absence (left) and presence of Hg2+ (40.0 μM, right) under a UV lamp at 365 nm.
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Fig. 2. (a) Absorption spectra of TPI-S (10.0 µM) in DMF/H2O (7/3, v/v) solution responding to various concentration of Hg2+: 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 8.0, 10.0 µM. (b) Ratiometric calibration curve A363/A318 of TPI-S (10.0 µM) in DMF/H2O (7/3, v/v) solution as a function of the Hg2+ concentration (0-10.0 µM). Fig. 3. (a) Fluorescence spectra (λex = 363 nm) of TPI-S (2.0 μM) in DMF/H2O (7/3, v/v) solution responding to various concentrations of Hg2+, 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 3.0, 4.0, 5.0 µM. (b) Fluorescence response at 527 nm (I527) of TPI-S as a function of Hg2+ concentration (0-5.0 µM). Fig. 4. Fluorescent intensity changes at 527 nm of TPI-S (λex = 363 nm, 2.0 μM, DMF/H2O, 7/3, v/v) in the presence of different anions (Hg2+ = 5.0 µM; other cations 10.0 µM). Fig. 5. 1H NMR spectra of TPI-S and the product of Hg2+-promoted deprotection reaction in DMSO-d6.
Figures
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Scheme 1
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PII: DOI: Reference:
S0022-2313(15)30194-0 http://dx.doi.org/10.1016/j.jlumin.2016.01.025 LUMIN13822
To appear in: Journal of Luminescence Received date: 4 July 2015 Revised date: 27 December 2015 Accepted date: 19 January 2016 Cite this article as: Xiangzhu He, Shuangming Zhu, Hongbiao Chen, Yongpeng Wang and Huaming Li, A new fluorescent turn-on chemodosimeter for mercury(II) based on dithioacetal-substituted triphenylimidazole, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.01.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new fluorescent turn-on chemodosimeter for mercury(II) based on dithioacetal-substituted triphenylimidazole
Xiangzhu Hea, Shuangming Zhua, Hongbiao Chena*, Yongpeng Wang, Huaming Lia,b* a
College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, P. R. China
b
Key Laboratory of Polymeric Materials & Application Technology of Hunan Province, Key
Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, and Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan Province, P. R. China *
Corresponding author. Tel.: +86 731 58298572; Fax: +86 731 58293264.
E-mail address: [email protected] (H. Li), [email protected] (H. Chen)
Abstract In this paper, we demonstrate a new fluorescent turn-on chemodosimeter for Hg2+ determination. The chemodosimeter contains a triphenylimidazole fluorophore and an Hg2+-responsive dithioacetal moiety. In the presence of Hg2+, the probe displays a characteristic turn-on mode at 527 nm in its emission spectrum due to the irreversible Hg2+-promoted deprotection of the dithioacetal group. With the aid of the fluorescence spectrometer, the chemodosimeter in the dimethylformamide/H2O (7/3, v/v) mixed solvent (2.0 μM) exhibits a detection limit of 4.3 nM (S/N = 3). Interferences from other common cations associated with Hg2+ analysis are effectively inhibited.
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Keywords Mercury; chemodosimeter; triphenylimidazole; dithioacetal
1. Introduction The development of highly sensitive and selective fluorescent chemosensors for heavy metal ions has received considerable attention because of the inherent characteristic of chemosensory detection, such as high sensitivity, fast analysis, operational simplicity, low cost, and real-time monitoring on one hand [1-6], on the other hand, these metals usually play an important role in living systems and have an extremely toxic impact on the environment and human body [7-15]. As a highly toxic heavy metal, mercury and its compounds are a subject of worldwide concern due to their acute immune-, geno- and neuro-toxic effects on human, livestock, and marine mammals [16,17], since mercury can be absorbed through the skin and mucous membranes, especially mercury vapors can be inhaled [18]. Even a trace amount of mercury intake can lead to acute or chronic damage to the human body, in which toxic effects include damage to the brain, kidney, lung, muscle, and vision [19]. Therefore, the development of novel fluorescent chemosensors for the determination of Hg2+ in the environment and in industrial waste streams at innocuous levels is of great importance in both basic research and practical applications. Recently, extensive efforts have been devoted to developing fluorescent chemosensors for Hg2+ detection [20-22]. As a result, a large number of sensitive and selective chemosensors for Hg2+ in water or organic solvents have been exploited on the basis of Hg2+-ligand complexation or
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Hg2+-promoted irreversible reactions as in the case of chemodosimeter. Among them, chemodosimeters are particularly attractive because of their highly selective behavior together with a unique spectroscopic change resulted from Hg2+-induced specific reactions such as spirolactam ring opening [23,24], mercuration [25,26], and desulfurization [27,28]. Recently, chemodosimeters for Hg2+ determination based on irreversible Hg2+-promoted deprotection of the dithioacetal moiety have drawn much attention due to their sensitive and selective signaling with negligible background [29-41]. Taking advantage of the strong thiophilic affinity of Hg2+, several chemodosimeters for Hg2+ have been demonstrated based on dithioacetal recognition moiety, in which the chromogens contain triphenylamine [29], coumarin [30], perylenediimide [31], boron-dipyrromethene [32], triarylborane [33], β-carboline [34], azobenzene [35], pyrene [36], tetraphenylethylene [37,38], naphthalene [39], 1,8-naphthalimide [40], and donor-π-acceptor structures [41-43]. Among these reported chemodosimeters, however, there are only a few examples of fluorescent turn-on chemodosimeter for Hg2+ [29,40,41]. Therefore, it is still a challenge to fabricate new fluorescent turn-on chemodosimeter for Hg2+ adopting other fluorophores. Herein, we demonstrated a new fluorescent turn-on chemodosimeter for Hg2+ by the functionalization of 2,4,5-triphenylimidazole (TPI) core skeleton with a dithioacetal substituent. As is well known, arylimidazole derivatives have attracted considerable attention in recent years because of their unique properties and diverse applications such as photographic materials, luminescent materials, optical materials, and therapeutic agents [44-46]. In particularly, TPI is a typical fluorophore, which shows a maximum absorption wavelength at 308 nm (ε = 2.62 × 104 M−1 cm−1), fluorescence emission wavelength at 385 nm, and fluorescence quantum yield of 0.10 in
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CH3CN solution [47], anthracene in cyclohexane used as a standard (ΦF = 0.31, excitation wavelength 366 nm [48]). In addition, TPI contains an imidazole ring with dicoordinate nitrogen atoms, which can potentially build complexes assembled by hydrogen bonds with a molecule containing hydrogen-bond donor, such as a carboxylic acid. The chemical flexibility of this class of compounds allows the preparation of a large variety of related structures and, consequently, the tailoring of their optical properties. Therefore, one would expect that novel chemodosimeter based on dithioacetal-substituted TPI might result in new potential applications in Hg2+ detection. In the present work, we describe the synthesis and the spectroscopic evaluation of the fluorescent turn-on chemodosimeter in detail.
2. Experimental 2.1. Reagents and apparatus 4-(4,5-Diphenyl-1H-imidazol-2-yl)benzohydrazide (TPI-CHO) was synthesized according to the reported method [47]. All reagents were purchased from commercial source and used without further purification unless otherwise noted. Infrared (IR) spectra in cm–1 were recorded on a Bruker Equinox-55 spectrometer (Germany). The NMR spectra were recorded with a Bruker AV-400 NMR spectrometer. MALDI-TOF mass spectra were recorded on a Bruker BIFLEXeIII mass spectrometer using a nitrogen laser (337 nm) and an accelerating potential of 20 kV. UV-vis spectra were recorded with an Agilent Cary 100 UV-vis spectrometer. Photoluminescence emission spectra were recorded with an Agilent QM 100 luminescence spectrometer.
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2.2. Synthesis of dithioacetal-substituted 2,4,5-triphenylimidazole (TPI-S) To a solution of TPI-CHO (325 mg, 1.0 mmol) and ethanethiol (0.208 mL, 2.8 mmol) in dry THF (10 mL) was added BF3·Et2O (0.4 mL, 3.36 mmol). The reaction mixture was stirred at 0 °C under argon overnight and the progress of the reaction was monitored by TLC (THF/petroleum ether, 1/2, v/v) analysis. After completion, the reaction mixture was poured into water with stirring and later neutralized with NaHCO3 aqueous solution (0.1 M). After filtration, the obtained crude product was purified by silica gel chromatography (silica gel, gy-001-TY4194, 200-300 mesh) using THF/petroleum ether (1/4, v/v) as an eluent to isolate pure compound TPI-S (390 mg, 90.3%). 1H NMR (DMSO-d6, 400 MHz) δ (ppm): 12.67 (s, 1H, NH), 8.03 (d, 2H, J = 8.0 Hz, ArH), 7.20-7.52 (m, 12H, ArH), 5.20 (s, 1H, CH), 2.53 (m, 4H, CH2), 1.15 (t, 6H, CH3). 13C NMR (100 MHz, DMSO-d6) δ (ppm): 145.65, 141.34, 137.70, 135.64, 131.56, 130.17, 129.14, 128.93, 128.79, 128.66, 128.38, 128.28, 127.56, 127.01, 125.77, 51.63, 26.21, 14.89. MALDI-TOF MS (C26H26N2S2) m/z: calcd. for 430.154, found: 431.269 [M + H]+.
2.3. General spectroscopic procedures The solutions of metal ions were prepared from AgNO3, CoCl2, KCl, SnCl2·2H2O, ZnCl2, CuCl2, (CH3COO)2Ni·4H2O, (CH3COO)2Cd·2H2O, (CH3COO)2Pb·3H2O, NaCl, Ca(NO3)2·4H2O, MnCl2, Mg(NO3)2·6H2O, FeCl3 and HgCl2 with DMF/H2O (7/3, v/v) mixed solvent. A solution of TPI-S (2 μM or 10 μM) was prepared in DMF/H2O (7/3, v/v) mixed solvent. Then 3.0 mL of the solution of TPI-S was placed in a quartz cell (10.0 mm width) and the fluorescent and
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absorption spectra were recorded. The HgCl2 and other cation aqueous solutions were introduced in portions and the fluorescent and absorption changes were recorded at room temperature each time.
3. Results and discussion 3.1. Synthesis and photophysical properties of TPI-S The synthetic procedure for TPI-S is shown in Scheme 1. The target compound TPI-S was prepared conveniently through the general protection reaction between TPI-CHO and ethanethiol. The whole synthetic route was simple and the purification was easy. TPI-S exhibited good solubility in common organic solvents, such as DMF, DMSO, THF, etc. and its structure was fully characterized by 1H and 13C NMR spectroscopy and MALDI-TOF mass spectrometry (see Experimental Section and Figs. S1-S3, Supporting Information, SI). The UV–vis absorption spectra of TPI-CHO and TPI-S are shown in Fig. 1a. As can be seen, the compound TPI-CHO shows an intense absorption band at 363 nm in DMF/H2O (7/3, v/v) solution. After the reaction with ethanethiol, the obtained dithioacetal TPI-S displays an absorption band at 318 nm, around 45 nm blue-shifts compared to its precursor. To evaluate the applicability of TPI-S as an Hg2+ colorimetric probe, excess Hg2+ was added into the diluted solution of TPI-S, it was observed that the absorption band at 318 nm disappeared completely while a new absorption band at 363 nm appeared again. That is to say, the absorption profile of TPI-S became similar to that of its precursor, TPI-CHO, after the addition of excess Hg2+, indicating that the dithioacetal in TPI-S was completely converted to aldehyde in the presence of Hg2+ ions. Such a large red-shift (45 nm) can be easily distinguished by the naked eyes under a normal UV lamp as shown in the inset of Fig. 1a.
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The emission spectra of TPI-CHO and TPI-S recorded in DMF/H2O (7/3, v/v) solution are shown in Fig. 1b. As demonstrated in Fig. 1b, TPI-CHO exhibits a strong emission band centered at 525 with a quantum yield (ΦF) of 0.38 by using anthracene in cyclohexane as a standard (ΦF = 0.31, excitation wavelength 366 nm [48]). However, the fluorescence emission of TPI-S becomes almost nonfluorescent with a quantum yield of 0.054 due to the disruption of conjugation [29]. Upon the addition of excess Hg2+ ions, a strong emission band at 527 nm was observed due to the irreversible Hg2+-promoted deprotection of the dithioacetal group. Notably, the emission band upon the addition of Hg2+ ions red-shifted a little compared to the spectrum of TPI-CHO. This might be contributed to the interaction between the Hg2+ ions and the regenerated aldehyde [29], although it cannot be verified by 1H NMR spectra analysis. As shown in Fig. S4 (see SI), the 1H NMR spectrum of TIP-CHO is basically identical to the spectrum of TPI-S after addition of excess Hg2+ ions. On the basis of above observations, it possible to construct a fluorescent turn-on chemodosimeter for Hg2+ based on the platform of TPI-S.
3.2. Optical response of TPI-S to Hg2+ The absorption spectral titration of TPI-S (10 μM) was initially carried out by addition of increasing equivalents of Hg2+ ions (0–1 equivalents) in DMF/H2O (7/3, v/v) solution and the observed spectral changes are shown in Fig. 2. As shown in Fig. 2a, upon gradual addition of Hg2+ ions to TPI-S solution, the absorption band at 318 nm decreased and the hyperchromic peak at 363 nm appeared gradually with an isosbestic point at around 340 nm, indicating the formation of new species. In addition, a good linear relationship was observed between A363/A318 (the ratio of the absorption
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intensity at 363 and 318 nm) and Hg2+ ion concentrations at the 0–10 μM range, enabling the quantitative determination of Hg2+ ion concentrations (Fig. 2b). Obviously, the colorimetric Hg2+ ions sensing by naked eyes is also feasible owing to the dramatic color change from colorless to yellow under a normal UV lamp. To obtain a highly sensitive chemodosimeter for Hg2+ ions, the sensing behavior of TPI-S toward Hg2+ ions was investigated by fluorescence titration in DMF/H2O (7/3, v/v) solution (2.0 μM) at an excitation wavelength of 363 nm. Fig. 3 displays the fluorescence titration results at room temperature. As can be seen, the fluorescence emission intensity of TPI-S at 527 nm increased significantly with the gradual addition of Hg2+ ions and was saturated at 1 equiv of Hg2+ ions (Fig. 3a), suggesting that the chemodosimeter TPI-S responds to Hg2+ ions at a ratio of 1/1. Job′s plot of fluorescence obtained for TPI-S and Hg2+ ions obviously demonstrated the formation of 1/1 stoichiometry (Fig. S5; see SI) [39]. After the addition of 1 equiv of Hg2+ ions to the TPI-S solution, an increased 20-fold of the fluorescent intensity was observed at 527 nm. In the range of 0–2.0 μM Hg2+ ion concentrations, the plot of the fluorescence intensity at 527 nm as a function of the Hg2+ ion concentrations shows a good linear relationship (R = 0.9954, Fig. 3b), indicating that TPI-S can be used to determine Hg2+ ion concentrations quantitatively. The detection limit of TPI-S was determined to be 4.3 nM at a ratio of signal to noise of 3. Such a low detection limit is indeed competitive with most of the fluorescent chemodosimeters previously reported (Table S1, see SI). Considering the fact that TIP-S is insoluble in water, however, this probe is suboptimal for biological applications. As we know, Hg2+ as a high toxic pollutant to the environment and a lethal substance for human being, is usually found in water or biocompatible solutions.
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To evaluate the selectivity of chemodosimeter TPI-S for Hg2+ ions detection, fluorescence spectral changes upon addition of various cations including Ag+, Co2+, K+, Sn2+, Zn2+, Cu2+, Ni2+, Mn2+, Na+, Ca2+, Mg2+, Pb2+, Fe3+ and Cd2+ were studied. Each spectrum was obtained after addition of various analytes at room temperature for 1 h. As shown in Fig. 4 and Fig. S6 (see SI), the selectivity observed by fluorescence monitoring was matched when TPI-S was used as a chemosensor for Hg2+ ions. That is to say, the Hg2+-promoted deprotection of the dithioacetal group in TPI-S gave a dramatic increase of the fluorescence intensity at 527 nm. In contrast, no significant fluorescence spectral changes were promoted by addition of other cations. The mechanism of TPI-S in signaling of Hg2+ ions was also proved by 1H NMR, MS and IR spectroscopic analysis. As shown in Fig. 5, the signals at δ 1.13-1.48, 2.47-2.64, and 5.20 ppm can be ascribed to the the methyl (Ha), methylene (Hb) and methine (Hc) protons of dithioacetal moiety in TPI-S. After the addition of excess Hg2+ ions, a new signal at 10.01 ppm corresponding to the aldehyde (Hd) appeared, and the the methyl (Ha), methylene (Hb) and methine (Hc) protons for dithioacetal vanished completely, suggesting the irreversible Hg2+-promoted deprotection of the dithioacetal group to aldehyde group (Scheme S1, see SI) [39,40]. The MS spectral analysis shows peak at m/z = 431.269 for pure TPI-S and at m/z = 325.092 for TPI-S upon addition of excess Hg2+ ions (Fig. S7, see SI), also giving a more direct evidence for the Hg2+-promoted deprotection reaction of the dithioacetal. The IR spectra of TPI-S before and after the addition of Hg2+ ions also gave some proof, no peak for aldehyde group was observed in the IR spectrum of TPI-S. But after the addition of excess Hg2+ ions, a typical vibration absorption peak for aldehyde group at 1695 cm−1 appeared owing to the deprotection reaction of TPI-S induced by Hg2+ ions (Fig. S8, see SI).
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4. Conclusion In summary, we have designed and synthesized a novel fluorescent turn-on chemodosimeter for Hg2+ ions based on dithioacetal-substituted triphenylimidazole (TPI-S). Due to the irreversible Hg2+-promoted deprotection of the dithioacetal group, TPI-S showed high selectivity for Hg2+ ions detection over other common cations and could recognize Hg2+ ions by naked eye under a normal UV lamp. With the aid of the fluorescence spectrometer, the TPI-S in DMF/H2O (7/3, v/v) mixed solvent (2.0 μM) exhibited a very low detection limit of 4.3 nM (S/N = 3) Hg2+ ions.
Acknowledgments Financial support from Program for NSFC (51273170), RFDP (20124301110006), and the Construct Program of the Key Discipline in Hunan Province is greatly acknowledged.
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Figure Captions Scheme 1. Synthesis of TPI-S. Fig. 1. (a) Absorption spectra of TPI-S, TPI-S + Hg2+, and TPI-CHO in DMF/H2O (7/3, v/v) (each 10.0 μM). (b) Fluorescence spectra (λex = 363 nm) of TPI-S, TPI-S + Hg2+, and TPI-CHO in DMF/H2O (7/3, v/v) (each 2.0 μM). Inset a: photographs of TPI-S (40.0 μM) in the absence (left) and presence of Hg2+ (40.0 μM, right) under a UV lamp at 365 nm.
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Fig. 2. (a) Absorption spectra of TPI-S (10.0 µM) in DMF/H2O (7/3, v/v) solution responding to various concentration of Hg2+: 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 8.0, 10.0 µM. (b) Ratiometric calibration curve A363/A318 of TPI-S (10.0 µM) in DMF/H2O (7/3, v/v) solution as a function of the Hg2+ concentration (0-10.0 µM). Fig. 3. (a) Fluorescence spectra (λex = 363 nm) of TPI-S (2.0 μM) in DMF/H2O (7/3, v/v) solution responding to various concentrations of Hg2+, 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 3.0, 4.0, 5.0 µM. (b) Fluorescence response at 527 nm (I527) of TPI-S as a function of Hg2+ concentration (0-5.0 µM). Fig. 4. Fluorescent intensity changes at 527 nm of TPI-S (λex = 363 nm, 2.0 μM, DMF/H2O, 7/3, v/v) in the presence of different anions (Hg2+ = 5.0 µM; other cations 10.0 µM). Fig. 5. 1H NMR spectra of TPI-S and the product of Hg2+-promoted deprotection reaction in DMSO-d6.
Figures
Fig. 1
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Fig. 2
Fig. 3
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Fig. 4
Fig. 5
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Scheme 1
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