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Colorimetric fluorescent cyanide chemodosimeter based on triphenylimidazole derivative
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 97–101
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Colorimetric fluorescent cyanide chemodosimeter based on triphenylimidazole derivative Wei Zheng a, Xiangzhu He a, Hongbiao Chen a,⇑, Yong Gao a, Huaming Li a,b,⇑ a
College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, PR China 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, PR China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A triarylimidazole-based colorimetric
chemodosimeter for CN was demonstrated. The chemodosimeter can recognize CN with obvious color and fluorescence change. The chemodosimeter exhibits a very low limit of detection (0.11 lM) for CN . The chemodosimeter allows naked eye detection of CN .
a r t i c l e
i n f o
Article history: Received 28 September 2013 Received in revised form 6 December 2013 Accepted 22 December 2013 Available online 4 January 2014 Keywords: Cyanide anion Chemodosimeter Colorimetry Intramolecular charge transfer
a b s t r a c t In this paper, we demonstrated a highly selective colorimetric chemodosimeter for cyanide anion detection. This chemodosimeter having a triphenylimidazole group as a fluorescent signal unit and a dicyano-vinyl group as a reaction unit was synthesized by the Knoevenagel condensation of 4-(4,5-diphenyl-1H-imidazol-2-yl)benzaldehyde with malononitrile in a reasonable yield. The probe exhibited an intramolecular charge transfer (ICT) absorption band at 420 nm and emission band at 620 nm, respectively. Upon the addition of cyanide anion, the probe displayed a blue-shifted spectrum and loss in color due to the disruption of conjugation. With the aid of the fluorescence spectrometer, the chemodosimeter exhibited a detection limit of 0.11 lM (S/N = 3). Interferences from other common anions associated with cyanide anion analysis were effectively inhibited. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Cyanide is considered the most toxic of all anions and can be absorbed through inhalation, ingestion, or skin contact [1]. Even a trace amount of cyanide intake will lead to lethal damage to the human body, since cyanide is a powerful inhibitor of the activity of cytochrome c oxidase. The binding of cyanide to this cyto-
⇑ Corresponding authors. Address: College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, PR China. Tel.: +86 731 58298572; fax: +86 731 58293264 (H. Li). E-mail addresses: [email protected] (H. Chen), [email protected] (H. Li). 1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.098
chrome can prevent transport of electrons from cytochrome c oxidase to oxygen, causing cytotoxic hypoxia in the presence of normal hemoglobin oxygenation [2–4]. Consequently, tissues that depend highly on aerobic respiration, such as the central nervous system and the heart, are particularly affected. Nowadays, the concern over the effect of cyanide on humans was heightened by the fact that the use of cyanide salts remained widespread, particularly in gold and silver mining, electroplating, plastics manufacturing, and metallurgy [5–8]. Unfortunately, accidental releases of cyanide into the environment do occur although safeguards and increasing levels of monitoring and control. In addition, certain plastics, especially those derived from acrylonitrile, release hydrogen cyanide
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when heated or burnt [9]. Therefore, the development of novel methods for the determination of cyanide at trace concentrations has become one of the most attractive subjects of investigation in analytical chemistry because of the practical applications. Traditionally, cyanide has been determined by using ion chromatography, electrochemical analysis, and spectroscopic techniques [10–13]. However, most of these strategies require either multiple experimental steps with tedious sample pretreatments or sophisticated instrumentation. In addition, these methods often suffer from interference by other anions, such as NO3 , AcO , H2 PO4 , HSO4 , ClO4 and halides. Specifically, the discrimination of cyanide from halides is rather problematic [14,15]. In this regard, analytical methods based on colorimetric chemosensors present many advantages, including high sensitivity, easy detection, inexpensive, and rapid in real-time monitoring. The sensing process is often accompanied by changes in absorption or fluorescence spectra that can be precisely monitored and sometimes detected by the naked eye [16–21]. Compared to the relatively well-developed cyanide chemosensors, colorimetric chemodosimeters based on the special nucleophilicity of cyanide have emerged as a research area of significant importance. Generally, colorimetric chemodosimeters are used to detect an analyte through a highly selective and irreversible chemical reaction between the dosimeter molecule and the target analyte, leading to signal changes in both the absorption wavelength and color that has an accumulative effect and hence is directly related to the concentration of the analyte. Taking advantage of the unique nucleophilicity of cyanide, various colorimetric cyanide chemodosimeters have been demonstrated, in which the chemodosimetric molecule contains a p-conjugated chromogenic unit and a reactive subunit. Hitherto, dicyano-vinyl group [22–26], diketone group in benzyl [27], salicylaldehyde group [28–31], benzamide group [32–34], trifluoroacetyl group [19,35], N-acyl triazenes [17,36], and [1,3]oxazine ring [37,38] have been adopted as the reactive subunit. Among these, dicyano-vinyl group is a popular reaction counterpart for cyanide nucleophilic addition reactions owing to its high efficiency. Although the design of colorimetric cyanide chemodosimeters with dicyano-vinyl groups as the recognition site has currently attracted attention, there are only a few examples of fluorescent chemodosimeters for cyanide [22–26]. Therefore, it is still a challenge to fabricate new colorimetric cyanide chemodosimeters adopting dicyano-vinyl group as the reactive subunit. Herein, we demonstrated a highly sensitive colorimetric fluorescent chemodosimeter for the detection of cyanide by covalent linking triphenylimidazole and dicyano-vinyl units. 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 [39–41]. In particularly, 2,4,5-triphenylimidazole (TPI) is a typical fluorophore, which shows a maximum absorption wavelength at 308 nm (e = 2.62 104 M 1 cm 1), fluorescence emission wavelength at 385 nm, and fluorescence quantum yield of 0.10 in CH3CN solution, anthracene in cyclohexane used as a standard (UF = 0.31; excitation wavelength 366 nm) [49]. (Figs. S1 and S2, see Supporting Information, SI). Additionally, TPI contains an imidazole ring with dicoordinate nitrogen atom, 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 directly linked arylimidazole-dicyano-vinyl units might result in new potential applications in cyanide detection. In the present work, we describe the synthesis and the
spectroscopic evaluation of the new colorimetric fluorescent cyanide chemodosimeter in detail. Experimental Reagents and apparatus All reagents and solvents were purchased from commercial source and used without further purification unless otherwise noted. Triethylamine (Et3N) was distilled and kept over potassium hydroxide. 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 a Perkin–Elmer Lamda-25 UV–vis spectrometer. Photoluminescence emission spectra were recorded with a Perkin–Elmer LS-50b luminescence spectrometer. Synthesis of 4-(4,5-diphenyl-1H-imidazol-2-yl)benzaldehyde (TPI-0) Under an atmosphere of dry argon, benzil (2.10 g, 10 mmol), terephthalaldehyde (1.34 g 10 mmol) and ammonium acetate (6.16 g, 80 mmol) in 50 mL acetic acid were refluxing for 6 h. After cooling to room temperature, the reaction mixture was poured into water, filtered, and dried in vacuo. The crude product was purified by silica gel chromatography using ethyl acetate/dichloromethane (1/10, v/v) as an eluent to isolate pure compound TPI-0 (2.60 g, 80%). 1H NMR (400 MHz, DMSO-d6) d (ppm): 12.99 (s, 1H, NH), 10.01 (s, 1H, CHO), 8.29-8.27 (d, 2H, ArH), 8.01-7.99 (d, 2H, ArH), 7.55-7.21 (m, 10H, ArH). 13C NMR (100 MHz, DMSO-d6) d (ppm): 192.8, 144.7, 138.6, 135.9, 131.2, 130.5, 129.1, 128.7, 127.6, 125.9. MALDI-TOF MS (C22H16N2O) m/z: calcd for 324.12, found: 325.20 [M + H]+. Synthesis of 2-(4-(4,5-diphenyl-1H-imidazol-2yl)benzylidene)malononitrile (TPI-1) Under an atmosphere of dry argon, compound TPI-0 (325 mg, 1.0 mmol) and malononitrile (132 mg, 2.0 mmol) in 20 mL absolute ethanol were refluxing overnight with trace Et3N as catalyst. After cooling to room temperature, the reaction mixture was poured into water, filtered, and dried in vacuo. The crude product was purified by silica gel chromatography using ethyl acetate/ dichloromethane (1/20, v/v) as an eluent to isolate pure compound TPI-1 (240 mg, 65%). 1H NMR (400 MHz, DMSO-d6) d (ppm): 13.05 (s, 1H, NH), 8.51 (s, 1H, CH@C), 8.27-8.25 (d, 2H, ArH), 8.06-8.04 (d, 2H, ArH), 7.5-7.2 (m, 10H, ArH). 13C NMR (100 MHz, DMSO-d6) d (ppm): 160.8, 144.4, 135.7, 131.7, 130.5, 128.9, 126.1, 114.9, 114.0, 80.8. MALDI-TOF MS (C25H16N4) m/z: calcd for 372.13, found: 373.21 [M + H]+. General spectroscopic procedures A solution of TPI-1 (10 lM and 20 lM) was prepared in CH3CN solution. Titration experiments were carried out in 10-mm quartz cell at room temperature. Anions (as the tetrabutylammonium salt) in CH3CN were added to the host solution and used for the titration experiments. Binding stoichiometry The binding stoichiometry of TPI-1 with cyanide ion was investigated through the Job’s plot. For the Job’s plot analyses, a series of solutions with varying mole fraction of cyanide ion were prepared by keeping the total concentration of TPI-1 and cyanide ion
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constant (10 lM). Fluorescence spectra of these solutions were measured for each sample at an excitation wavelength of 420 nm. The fluorescence intensity at 620 nm was monitored for each spectrum. Results and discussion The synthetic procedure for TPI-1 is shown in Scheme 1. Initially, the precursor TPI-0 was synthesized by a one-step reaction from benzil, terephthalaldehyde, and ammonium acetate in acetic acid with a yield of 81%. Subsequently, TPI-0 was further condensed with malononitrile by using a Et3N-mediated Knoevenagel condensation reaction [23,42] to afford the TPI-1 in 65% yield. The whole synthetic route was simple and the purification was easy. Both TPI-0 and TPI-1 were carefully purified and characterized by spectroscopic methods, from which satisfactory analysis data corresponding to their molecular structures were obtained (see Experimental section and SI, Figs. S3–S6). Similar to compound TPI-0, TPI-1 was soluble in common organic solvents such as CH3CN, THF, EtOAc, DMF, acetone, etc. The UV–vis absorption spectra of TPI-0 and TPI-1 are shown in Fig. 1a. As can be seen, compound TPI-0 showed an intense absorption band centering at 357 nm (e = 2.55 104 M 1 cm 1), giving a colorless solution in CH3CN (20 lM). Upon condensation with malononitrile, the resultant compound TPI-1 displayed two main absorption bands centering at 295 nm (e = 2.09 104 M 1 cm 1) and 420 nm (e = 2.35 10 4 M 1 cm 1), which can be assigned to the p–p transition and the intramolecular charge transfer (ICT) bands, respectively, giving a brilliant yellow solution in CH3CN (20 lM). Fig. 1b presents the fluorescence emission spectra of TPI-0 and TPI-1 in their CH3CN solution (10 lM). As demonstrated in Fig. 1b, both of TPI-0 and TPI-1 exhibited strong luminescence in diluted solutions with the maximum emission wavelength at 490 and 620 nm, respectively. Due to the fact that the electronic property of triarylimidazole derivatives changed before and after the condensation reaction with malononitrile, these compounds thus exhibited different absorption behaviors and obvious color changes. As mentioned previously, nucleophilic attack by cyanide anion was expected to occur toward the a-position of the dicyano-vinyl group in compound TPI-1 as shown in Scheme 1. Upon addition of equivalent cyanide anion, the electron-withdrawing dicyanovinyl group was expected to be transformed into an anionic electron-pushing group due to the destruction of the dicyano-vinyl group. Therefore, the dicyano-vinyl acceptor in TPI-1 became donor in TPI-2 after cyanide addition. As a result, the extent of effective conjugation in the TPI-1 molecule was broken, which could affect the ICT efficiency and optical properties of the sensing system. Indeed, the solution of stabilized anionic specie TPI-2 in CH3CN (20 lM) was colorless and exhibited a new absorption band centering at 310 nm (Fig. 1a), which was originated by the decrease and
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hypsochromic shift of the absorption band at 420 nm together with the bathochromic shift of the absorbance at 298 nm. Absorption spectra changes with different concentrations of cyanide anion further displayed the shift in the absorption bands, and showed an isosbestic point at about 358 nm, indicating the formation of new species (Figs. S7, SI). Meanwhile, the fluorescence emission of TPI2 in CH3CN (10 lM) was completely quenched upon excitation at 420 nm (Fig. 1b), making it possible to construct a colorimetric chemodosimeter based on the platform of TPI-1. Considering that the optical signal changes depended on the rate of nucleophilic addition reaction between compound TPI-1 and cyanide anion, the influence of the reaction time on the probing results was thus investigated by monitoring the changes in fluorescence intensity at 620 nm (I620). As can be seen in Fig. 2, there are gradual changes in the fluorescence intensity from 0 to 15 min when the concentration of cyanide anion is lower than 8.0 lM, however, the changes begin to level off after prolonging reaction time to 10 min. With an increase in the concentration of cyanide anion to 15 lM, less than 4 min are needed to achieve a plateau. That is to say, the nucleophilic addition reaction almost completes within 4 min in high cyanide concentrations, whereas the reaction time is longer in low concentrations. To obtain a highly sensitive probe for cyanide anion, the sensing behavior of TPI-1 toward cyanide anion was investigated by fluorescence titration in CH3CN solvent (10 lM) at an excitation wavelength of 420 nm. It is important to note that the globular shape of TPI-1 and the spherical hindrance cause this compound to solubilize with different aggregates of the solute in some solvents [43]. In the current case, we choose CH3CN as the solvent for cyanide anion determination since a typical absorption spectrum of monomeric TPI-1 is observed in this solvent as evidenced by UV–vis measurement. Fig. 3 displays the fluorescence titration results at room temperature. As can be seen, upon addition of cyanide anions, the fluorescence emission intensity of the probe at 620 nm was decreased gradually and was saturated at 1.5 equiv of cyanide anion (Fig. 3a). In the range of 0.5–10 lM cyanide concentration, the plot of the fluorescence intensity at 620 nm as a function of the cyanide concentrations showed a good linear relationship (R = 0.998, Fig. 3b), indicating that TPI-1 could be used to detect cyanide concentration quantitatively. The detection limit of TPI-1 was determined to be 0.11 lM at a ratio of signal to noise of 3 [28,44], which was lower than those reported previously [22,23,26,28,45,46]. Moreover, this detection limit was also much lower than the maximum contaminant level (MCL) for cyanide anion in drinking water (2.7 lM) set by the World Health Organization (WHO) [47]. Considering the fact that cyanide anion is usually found in water or biocompatible solutions, we further studied the influence of trace water on the sensing process. The absorption of TPI-1 (10 lM, 10 mL) changed at a very limited degree after 15 lL of H2O was introduced (Fig. S10, SI), indicating that the water in the
Scheme 1. Structure of TPI-1 and the sensing mechanism.
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Fig. 1. (a) Absorption spectra of TPI-0, TPI-1, and TPI-2 in CH3CN (20 lM); (b) fluorescence spectra of TPI-0, TPI-1, and TPI-2 in CH3CN (10 lM). Inset a: the fluorescence images of TPI-1 in CH3CN (100 lM) in the absence and presence of cyanide under a UV lamp at 365 nm.
Fig. 2. Reaction time profile of probe TPI-1 (10 lM, in CH3CN) in the presence of different concentrations of cyanide anion.
cyanide solutions did not affect the results of the titration experiment. Indeed, the plot of fluorescence intensity at 620 nm as a function of cyanide concentration showed a good linear relationship (R = 0.9958, Fig. S11, SI) when the cyanide aqueous solution was used instead of cyanide/CH3CN solution, demonstrating that TPI-1 could be used to detect real cyanide samples. To evaluate the selectivity of probe TPI-1 for cyanide anion detection, fluorescence spectral changes upon addition 3 equiv of various anions including NO3 , AcO , H2 PO4 , HSO4 , ClO4 , F , Cl , Br , and I were studied. Each spectrum was obtained after addition of various analytes at room temperature for 25 min. As shown in Fig. 4 and Fig. S12 (see SI), the selectivity observed by fluorescence monitoring was matched when TPI-1 was used as a chemodosimeter for cyanide anion. That is to say, the reaction of TPI-1 with cyanide anion gave a dramatic decrease of the fluorescence intensity at 620 nm. In contrast, no significant fluorescence spectral changes were promoted by addition of other anions (Fig. 4). Moreover, the significant color changes of TPI-1 solution could be applied for the detection of cyanide anion upon the addition of cyanide anion as shown in Fig. S13 (see SI). In contrast, other anions did not induce any significant color changes. In addition, the color changes of TPI-1 solution with different concentrations of cyanide anion were recorded in Figs. S8 and S9 (see SI). These results further confirm that TPI-1 can act as a cyanidespecific colorimetric fluorescent sensor. The mechanism of TPI-1 in sensing of cyanide anion was monitored the 1H NMR spectral changes induced via the addition of cyanide anion in DMSO-d6 at room temperature. As shown in
Fig. 3. (a) Fluorescence spectra of TPI-1 (10 lM, in CH3CN) in the presence of different concentrations of cyanide anion upon excitation at 420 nm. (b) Fluorescence intensity at 620 nm of TPI-1 (10 lM, in CH3CN) versus the concentration of cyanide anion at low concentration range of 0.5–10 lM.
Fig. 5, the signals at 8.51 ppm and 13.05 ppm can be ascribed to the vinylic proton (Ha) and imidazole proton (Hc), respectively, for the free TPI-1. After the addition of excess cyanide anion, a new signal at 4.62 ppm corresponding to the a-proton (Hb) was appeared, and the vinylic proton (Ha) at 8.51 ppm vanished, suggesting that the dicyano-vinyl moiety of TPI-1 had reacted with the cyanide anion through breaking of the C@C double bond and
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Hunan Province (2013WK3036), Open Project of Hunan Provincial University Innovation Platform (12K050), and the Construct Program of the Key Discipline in Hunan Province is greatly acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2013.12.098. References
Fig. 4. Fluorescence intensity changes at 620 nm of TPI-1 (10 lM, in CH3CN) in the presence of different anions (cyanide anion, 15 lM; others, 30 lM).
Fig. 5. Partial 1H NMR spectra of TPI-1 (a) and TPI-2 (b) in DMSO-d6.
the formation of the new CAC single bond. In addition, the imidazole proton (Hc) was upfield shifted to 12.71 ppm. The evidence for the 1:1 adduct of TPI-1 and cyanide anion was found in Job analysis [48] (Fig. S14, SI). Moreover, the TOF-MS peak at m/z 373.211 was ascribe to the compound TPI-1, while a peak at 398.231 corresponding to [TPI-1 + CN + H]+ appeared after the addition of excess cyanide anion into the solution of TPI-1, indicating the formation of the stabilized anionic species TPI-2. Conclusion In summary, we have designed and synthesized a novel fluorescent triarylimidazole-based colorimetric chemodosimeter TPI-1 for cyanide anion. Based on the nucleophilic addition reaction between the dicyano-vinyl group and cyanide anion, TPI-1 showed high selectivity for cyanide detection over other common anions and could recognize cyanide by naked eye. With the aid of the fluorescence spectrometer, the TPI-1 in ethanenitrile (10 lM) exhibited linearity range from 0.5 lM to 10 lM cyanide anion with a detection limit of 0.11 lM (S/N = 3), which was among the best results for cyanide anion sensing by the fluorescence dicyano-vinyl-based sensors. Acknowledgments Financial support from Program for NSFC (51273170), RFDP (20124301110006), International S&T Cooperation Program of
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Colorimetric fluorescent cyanide chemodosimeter based on triphenylimidazole derivative Wei Zheng a, Xiangzhu He a, Hongbiao Chen a,⇑, Yong Gao a, Huaming Li a,b,⇑ a
College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, PR China 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, PR China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A triarylimidazole-based colorimetric
chemodosimeter for CN was demonstrated. The chemodosimeter can recognize CN with obvious color and fluorescence change. The chemodosimeter exhibits a very low limit of detection (0.11 lM) for CN . The chemodosimeter allows naked eye detection of CN .
a r t i c l e
i n f o
Article history: Received 28 September 2013 Received in revised form 6 December 2013 Accepted 22 December 2013 Available online 4 January 2014 Keywords: Cyanide anion Chemodosimeter Colorimetry Intramolecular charge transfer
a b s t r a c t In this paper, we demonstrated a highly selective colorimetric chemodosimeter for cyanide anion detection. This chemodosimeter having a triphenylimidazole group as a fluorescent signal unit and a dicyano-vinyl group as a reaction unit was synthesized by the Knoevenagel condensation of 4-(4,5-diphenyl-1H-imidazol-2-yl)benzaldehyde with malononitrile in a reasonable yield. The probe exhibited an intramolecular charge transfer (ICT) absorption band at 420 nm and emission band at 620 nm, respectively. Upon the addition of cyanide anion, the probe displayed a blue-shifted spectrum and loss in color due to the disruption of conjugation. With the aid of the fluorescence spectrometer, the chemodosimeter exhibited a detection limit of 0.11 lM (S/N = 3). Interferences from other common anions associated with cyanide anion analysis were effectively inhibited. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Cyanide is considered the most toxic of all anions and can be absorbed through inhalation, ingestion, or skin contact [1]. Even a trace amount of cyanide intake will lead to lethal damage to the human body, since cyanide is a powerful inhibitor of the activity of cytochrome c oxidase. The binding of cyanide to this cyto-
⇑ Corresponding authors. Address: College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, PR China. Tel.: +86 731 58298572; fax: +86 731 58293264 (H. Li). E-mail addresses: [email protected] (H. Chen), [email protected] (H. Li). 1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.098
chrome can prevent transport of electrons from cytochrome c oxidase to oxygen, causing cytotoxic hypoxia in the presence of normal hemoglobin oxygenation [2–4]. Consequently, tissues that depend highly on aerobic respiration, such as the central nervous system and the heart, are particularly affected. Nowadays, the concern over the effect of cyanide on humans was heightened by the fact that the use of cyanide salts remained widespread, particularly in gold and silver mining, electroplating, plastics manufacturing, and metallurgy [5–8]. Unfortunately, accidental releases of cyanide into the environment do occur although safeguards and increasing levels of monitoring and control. In addition, certain plastics, especially those derived from acrylonitrile, release hydrogen cyanide
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when heated or burnt [9]. Therefore, the development of novel methods for the determination of cyanide at trace concentrations has become one of the most attractive subjects of investigation in analytical chemistry because of the practical applications. Traditionally, cyanide has been determined by using ion chromatography, electrochemical analysis, and spectroscopic techniques [10–13]. However, most of these strategies require either multiple experimental steps with tedious sample pretreatments or sophisticated instrumentation. In addition, these methods often suffer from interference by other anions, such as NO3 , AcO , H2 PO4 , HSO4 , ClO4 and halides. Specifically, the discrimination of cyanide from halides is rather problematic [14,15]. In this regard, analytical methods based on colorimetric chemosensors present many advantages, including high sensitivity, easy detection, inexpensive, and rapid in real-time monitoring. The sensing process is often accompanied by changes in absorption or fluorescence spectra that can be precisely monitored and sometimes detected by the naked eye [16–21]. Compared to the relatively well-developed cyanide chemosensors, colorimetric chemodosimeters based on the special nucleophilicity of cyanide have emerged as a research area of significant importance. Generally, colorimetric chemodosimeters are used to detect an analyte through a highly selective and irreversible chemical reaction between the dosimeter molecule and the target analyte, leading to signal changes in both the absorption wavelength and color that has an accumulative effect and hence is directly related to the concentration of the analyte. Taking advantage of the unique nucleophilicity of cyanide, various colorimetric cyanide chemodosimeters have been demonstrated, in which the chemodosimetric molecule contains a p-conjugated chromogenic unit and a reactive subunit. Hitherto, dicyano-vinyl group [22–26], diketone group in benzyl [27], salicylaldehyde group [28–31], benzamide group [32–34], trifluoroacetyl group [19,35], N-acyl triazenes [17,36], and [1,3]oxazine ring [37,38] have been adopted as the reactive subunit. Among these, dicyano-vinyl group is a popular reaction counterpart for cyanide nucleophilic addition reactions owing to its high efficiency. Although the design of colorimetric cyanide chemodosimeters with dicyano-vinyl groups as the recognition site has currently attracted attention, there are only a few examples of fluorescent chemodosimeters for cyanide [22–26]. Therefore, it is still a challenge to fabricate new colorimetric cyanide chemodosimeters adopting dicyano-vinyl group as the reactive subunit. Herein, we demonstrated a highly sensitive colorimetric fluorescent chemodosimeter for the detection of cyanide by covalent linking triphenylimidazole and dicyano-vinyl units. 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 [39–41]. In particularly, 2,4,5-triphenylimidazole (TPI) is a typical fluorophore, which shows a maximum absorption wavelength at 308 nm (e = 2.62 104 M 1 cm 1), fluorescence emission wavelength at 385 nm, and fluorescence quantum yield of 0.10 in CH3CN solution, anthracene in cyclohexane used as a standard (UF = 0.31; excitation wavelength 366 nm) [49]. (Figs. S1 and S2, see Supporting Information, SI). Additionally, TPI contains an imidazole ring with dicoordinate nitrogen atom, 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 directly linked arylimidazole-dicyano-vinyl units might result in new potential applications in cyanide detection. In the present work, we describe the synthesis and the
spectroscopic evaluation of the new colorimetric fluorescent cyanide chemodosimeter in detail. Experimental Reagents and apparatus All reagents and solvents were purchased from commercial source and used without further purification unless otherwise noted. Triethylamine (Et3N) was distilled and kept over potassium hydroxide. 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 a Perkin–Elmer Lamda-25 UV–vis spectrometer. Photoluminescence emission spectra were recorded with a Perkin–Elmer LS-50b luminescence spectrometer. Synthesis of 4-(4,5-diphenyl-1H-imidazol-2-yl)benzaldehyde (TPI-0) Under an atmosphere of dry argon, benzil (2.10 g, 10 mmol), terephthalaldehyde (1.34 g 10 mmol) and ammonium acetate (6.16 g, 80 mmol) in 50 mL acetic acid were refluxing for 6 h. After cooling to room temperature, the reaction mixture was poured into water, filtered, and dried in vacuo. The crude product was purified by silica gel chromatography using ethyl acetate/dichloromethane (1/10, v/v) as an eluent to isolate pure compound TPI-0 (2.60 g, 80%). 1H NMR (400 MHz, DMSO-d6) d (ppm): 12.99 (s, 1H, NH), 10.01 (s, 1H, CHO), 8.29-8.27 (d, 2H, ArH), 8.01-7.99 (d, 2H, ArH), 7.55-7.21 (m, 10H, ArH). 13C NMR (100 MHz, DMSO-d6) d (ppm): 192.8, 144.7, 138.6, 135.9, 131.2, 130.5, 129.1, 128.7, 127.6, 125.9. MALDI-TOF MS (C22H16N2O) m/z: calcd for 324.12, found: 325.20 [M + H]+. Synthesis of 2-(4-(4,5-diphenyl-1H-imidazol-2yl)benzylidene)malononitrile (TPI-1) Under an atmosphere of dry argon, compound TPI-0 (325 mg, 1.0 mmol) and malononitrile (132 mg, 2.0 mmol) in 20 mL absolute ethanol were refluxing overnight with trace Et3N as catalyst. After cooling to room temperature, the reaction mixture was poured into water, filtered, and dried in vacuo. The crude product was purified by silica gel chromatography using ethyl acetate/ dichloromethane (1/20, v/v) as an eluent to isolate pure compound TPI-1 (240 mg, 65%). 1H NMR (400 MHz, DMSO-d6) d (ppm): 13.05 (s, 1H, NH), 8.51 (s, 1H, CH@C), 8.27-8.25 (d, 2H, ArH), 8.06-8.04 (d, 2H, ArH), 7.5-7.2 (m, 10H, ArH). 13C NMR (100 MHz, DMSO-d6) d (ppm): 160.8, 144.4, 135.7, 131.7, 130.5, 128.9, 126.1, 114.9, 114.0, 80.8. MALDI-TOF MS (C25H16N4) m/z: calcd for 372.13, found: 373.21 [M + H]+. General spectroscopic procedures A solution of TPI-1 (10 lM and 20 lM) was prepared in CH3CN solution. Titration experiments were carried out in 10-mm quartz cell at room temperature. Anions (as the tetrabutylammonium salt) in CH3CN were added to the host solution and used for the titration experiments. Binding stoichiometry The binding stoichiometry of TPI-1 with cyanide ion was investigated through the Job’s plot. For the Job’s plot analyses, a series of solutions with varying mole fraction of cyanide ion were prepared by keeping the total concentration of TPI-1 and cyanide ion
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constant (10 lM). Fluorescence spectra of these solutions were measured for each sample at an excitation wavelength of 420 nm. The fluorescence intensity at 620 nm was monitored for each spectrum. Results and discussion The synthetic procedure for TPI-1 is shown in Scheme 1. Initially, the precursor TPI-0 was synthesized by a one-step reaction from benzil, terephthalaldehyde, and ammonium acetate in acetic acid with a yield of 81%. Subsequently, TPI-0 was further condensed with malononitrile by using a Et3N-mediated Knoevenagel condensation reaction [23,42] to afford the TPI-1 in 65% yield. The whole synthetic route was simple and the purification was easy. Both TPI-0 and TPI-1 were carefully purified and characterized by spectroscopic methods, from which satisfactory analysis data corresponding to their molecular structures were obtained (see Experimental section and SI, Figs. S3–S6). Similar to compound TPI-0, TPI-1 was soluble in common organic solvents such as CH3CN, THF, EtOAc, DMF, acetone, etc. The UV–vis absorption spectra of TPI-0 and TPI-1 are shown in Fig. 1a. As can be seen, compound TPI-0 showed an intense absorption band centering at 357 nm (e = 2.55 104 M 1 cm 1), giving a colorless solution in CH3CN (20 lM). Upon condensation with malononitrile, the resultant compound TPI-1 displayed two main absorption bands centering at 295 nm (e = 2.09 104 M 1 cm 1) and 420 nm (e = 2.35 10 4 M 1 cm 1), which can be assigned to the p–p transition and the intramolecular charge transfer (ICT) bands, respectively, giving a brilliant yellow solution in CH3CN (20 lM). Fig. 1b presents the fluorescence emission spectra of TPI-0 and TPI-1 in their CH3CN solution (10 lM). As demonstrated in Fig. 1b, both of TPI-0 and TPI-1 exhibited strong luminescence in diluted solutions with the maximum emission wavelength at 490 and 620 nm, respectively. Due to the fact that the electronic property of triarylimidazole derivatives changed before and after the condensation reaction with malononitrile, these compounds thus exhibited different absorption behaviors and obvious color changes. As mentioned previously, nucleophilic attack by cyanide anion was expected to occur toward the a-position of the dicyano-vinyl group in compound TPI-1 as shown in Scheme 1. Upon addition of equivalent cyanide anion, the electron-withdrawing dicyanovinyl group was expected to be transformed into an anionic electron-pushing group due to the destruction of the dicyano-vinyl group. Therefore, the dicyano-vinyl acceptor in TPI-1 became donor in TPI-2 after cyanide addition. As a result, the extent of effective conjugation in the TPI-1 molecule was broken, which could affect the ICT efficiency and optical properties of the sensing system. Indeed, the solution of stabilized anionic specie TPI-2 in CH3CN (20 lM) was colorless and exhibited a new absorption band centering at 310 nm (Fig. 1a), which was originated by the decrease and
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hypsochromic shift of the absorption band at 420 nm together with the bathochromic shift of the absorbance at 298 nm. Absorption spectra changes with different concentrations of cyanide anion further displayed the shift in the absorption bands, and showed an isosbestic point at about 358 nm, indicating the formation of new species (Figs. S7, SI). Meanwhile, the fluorescence emission of TPI2 in CH3CN (10 lM) was completely quenched upon excitation at 420 nm (Fig. 1b), making it possible to construct a colorimetric chemodosimeter based on the platform of TPI-1. Considering that the optical signal changes depended on the rate of nucleophilic addition reaction between compound TPI-1 and cyanide anion, the influence of the reaction time on the probing results was thus investigated by monitoring the changes in fluorescence intensity at 620 nm (I620). As can be seen in Fig. 2, there are gradual changes in the fluorescence intensity from 0 to 15 min when the concentration of cyanide anion is lower than 8.0 lM, however, the changes begin to level off after prolonging reaction time to 10 min. With an increase in the concentration of cyanide anion to 15 lM, less than 4 min are needed to achieve a plateau. That is to say, the nucleophilic addition reaction almost completes within 4 min in high cyanide concentrations, whereas the reaction time is longer in low concentrations. To obtain a highly sensitive probe for cyanide anion, the sensing behavior of TPI-1 toward cyanide anion was investigated by fluorescence titration in CH3CN solvent (10 lM) at an excitation wavelength of 420 nm. It is important to note that the globular shape of TPI-1 and the spherical hindrance cause this compound to solubilize with different aggregates of the solute in some solvents [43]. In the current case, we choose CH3CN as the solvent for cyanide anion determination since a typical absorption spectrum of monomeric TPI-1 is observed in this solvent as evidenced by UV–vis measurement. Fig. 3 displays the fluorescence titration results at room temperature. As can be seen, upon addition of cyanide anions, the fluorescence emission intensity of the probe at 620 nm was decreased gradually and was saturated at 1.5 equiv of cyanide anion (Fig. 3a). In the range of 0.5–10 lM cyanide concentration, the plot of the fluorescence intensity at 620 nm as a function of the cyanide concentrations showed a good linear relationship (R = 0.998, Fig. 3b), indicating that TPI-1 could be used to detect cyanide concentration quantitatively. The detection limit of TPI-1 was determined to be 0.11 lM at a ratio of signal to noise of 3 [28,44], which was lower than those reported previously [22,23,26,28,45,46]. Moreover, this detection limit was also much lower than the maximum contaminant level (MCL) for cyanide anion in drinking water (2.7 lM) set by the World Health Organization (WHO) [47]. Considering the fact that cyanide anion is usually found in water or biocompatible solutions, we further studied the influence of trace water on the sensing process. The absorption of TPI-1 (10 lM, 10 mL) changed at a very limited degree after 15 lL of H2O was introduced (Fig. S10, SI), indicating that the water in the
Scheme 1. Structure of TPI-1 and the sensing mechanism.
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Fig. 1. (a) Absorption spectra of TPI-0, TPI-1, and TPI-2 in CH3CN (20 lM); (b) fluorescence spectra of TPI-0, TPI-1, and TPI-2 in CH3CN (10 lM). Inset a: the fluorescence images of TPI-1 in CH3CN (100 lM) in the absence and presence of cyanide under a UV lamp at 365 nm.
Fig. 2. Reaction time profile of probe TPI-1 (10 lM, in CH3CN) in the presence of different concentrations of cyanide anion.
cyanide solutions did not affect the results of the titration experiment. Indeed, the plot of fluorescence intensity at 620 nm as a function of cyanide concentration showed a good linear relationship (R = 0.9958, Fig. S11, SI) when the cyanide aqueous solution was used instead of cyanide/CH3CN solution, demonstrating that TPI-1 could be used to detect real cyanide samples. To evaluate the selectivity of probe TPI-1 for cyanide anion detection, fluorescence spectral changes upon addition 3 equiv of various anions including NO3 , AcO , H2 PO4 , HSO4 , ClO4 , F , Cl , Br , and I were studied. Each spectrum was obtained after addition of various analytes at room temperature for 25 min. As shown in Fig. 4 and Fig. S12 (see SI), the selectivity observed by fluorescence monitoring was matched when TPI-1 was used as a chemodosimeter for cyanide anion. That is to say, the reaction of TPI-1 with cyanide anion gave a dramatic decrease of the fluorescence intensity at 620 nm. In contrast, no significant fluorescence spectral changes were promoted by addition of other anions (Fig. 4). Moreover, the significant color changes of TPI-1 solution could be applied for the detection of cyanide anion upon the addition of cyanide anion as shown in Fig. S13 (see SI). In contrast, other anions did not induce any significant color changes. In addition, the color changes of TPI-1 solution with different concentrations of cyanide anion were recorded in Figs. S8 and S9 (see SI). These results further confirm that TPI-1 can act as a cyanidespecific colorimetric fluorescent sensor. The mechanism of TPI-1 in sensing of cyanide anion was monitored the 1H NMR spectral changes induced via the addition of cyanide anion in DMSO-d6 at room temperature. As shown in
Fig. 3. (a) Fluorescence spectra of TPI-1 (10 lM, in CH3CN) in the presence of different concentrations of cyanide anion upon excitation at 420 nm. (b) Fluorescence intensity at 620 nm of TPI-1 (10 lM, in CH3CN) versus the concentration of cyanide anion at low concentration range of 0.5–10 lM.
Fig. 5, the signals at 8.51 ppm and 13.05 ppm can be ascribed to the vinylic proton (Ha) and imidazole proton (Hc), respectively, for the free TPI-1. After the addition of excess cyanide anion, a new signal at 4.62 ppm corresponding to the a-proton (Hb) was appeared, and the vinylic proton (Ha) at 8.51 ppm vanished, suggesting that the dicyano-vinyl moiety of TPI-1 had reacted with the cyanide anion through breaking of the C@C double bond and
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Hunan Province (2013WK3036), Open Project of Hunan Provincial University Innovation Platform (12K050), and the Construct Program of the Key Discipline in Hunan Province is greatly acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2013.12.098. References
Fig. 4. Fluorescence intensity changes at 620 nm of TPI-1 (10 lM, in CH3CN) in the presence of different anions (cyanide anion, 15 lM; others, 30 lM).
Fig. 5. Partial 1H NMR spectra of TPI-1 (a) and TPI-2 (b) in DMSO-d6.
the formation of the new CAC single bond. In addition, the imidazole proton (Hc) was upfield shifted to 12.71 ppm. The evidence for the 1:1 adduct of TPI-1 and cyanide anion was found in Job analysis [48] (Fig. S14, SI). Moreover, the TOF-MS peak at m/z 373.211 was ascribe to the compound TPI-1, while a peak at 398.231 corresponding to [TPI-1 + CN + H]+ appeared after the addition of excess cyanide anion into the solution of TPI-1, indicating the formation of the stabilized anionic species TPI-2. Conclusion In summary, we have designed and synthesized a novel fluorescent triarylimidazole-based colorimetric chemodosimeter TPI-1 for cyanide anion. Based on the nucleophilic addition reaction between the dicyano-vinyl group and cyanide anion, TPI-1 showed high selectivity for cyanide detection over other common anions and could recognize cyanide by naked eye. With the aid of the fluorescence spectrometer, the TPI-1 in ethanenitrile (10 lM) exhibited linearity range from 0.5 lM to 10 lM cyanide anion with a detection limit of 0.11 lM (S/N = 3), which was among the best results for cyanide anion sensing by the fluorescence dicyano-vinyl-based sensors. Acknowledgments Financial support from Program for NSFC (51273170), RFDP (20124301110006), International S&T Cooperation Program of
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