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A novel colorimetric and “turn-on” fluorimetric chemosensor for selective recognition of CN− ions based on asymmetric azine derivatives in aqueous media
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 198 (2018) 182–187
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A novel colorimetric and “turn-on” fluorimetric chemosensor for selective recognition of CN− ions based on asymmetric azine derivatives in aqueous media Peng-Xiang Pei, Jing-Han Hu ⁎, Chen Long, Peng-Wei Ni College of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, PR China
a r t i c l e
i n f o
Article history: Received 11 April 2017 Received in revised form 28 February 2018 Accepted 8 March 2018 Available online 10 March 2018 Keywords: Anion sensor Colorimetric Fluorescence Cyanide Test strip Bitter almond DFT
a b s t r a c t A novel chemosensor 2-((Z)-(((E)-quinolin-2-ylmethylene)hydrazono)methyl)phenol PX has been successfully designed and synthesized, which showed both colorimetric and “turn-on” fluorescence responses for CN− in DMSO/H2O (3:2, v/v; pH = 7.20) solution. The sensor could respond effectively to the stimulation of CN− ions via deprotonation and sensing mechanism of intramolecular charge transfer (ICT). Moreover, the sensor PX was successfully utilized to detect CN− in bitter almond, and the detection limit on fluorescence response of PX towards CN− was down to 4.5 × 10−7 M. Test strips containing PX were also prepared, which could act as a practical colorimetric tool to detect CN− in aqueous media. © 2018 Elsevier B.V. All rights reserved.
1. Introduction The development of anions sensors have been an interesting and emerging field of research due to the important roles of anions in industrial and biological processes [1,2]. It's well that CN− ion is one of the most toxic anions in various anions, and its toxicity can damage many functions including the vascular, visual, central nervous, cardiac, endocrine, and metabolic systems [3–6] in human body. According to the World Health Organization (WHO), maximum acceptable amount of cyanide ion in drinking water is 1.9 μM [7]. However, widespread utilization of cyanide salts in various industries such as gold extraction, synthetic fibers, electroplating and so on [8–10] have caused a serious environmental concern. Hence, it is necessary for designing reasonable chemosensor to monitor cyanide concentration in environment. In the past few decades, a variety of techniques such as ion chromatography, potentiometric, electrochemical methods and titrimetric [11–14] have been reported for CN− detection. However, those methods usually suffer from a series of problem such as high cost, long response time and sophisticated equipment and so on, which seriously limits its practical application. In contrast, fluorimetric and
⁎ Corresponding author. E-mail address: [email protected] (J.-H. Hu).
https://doi.org/10.1016/j.saa.2018.03.022 1386-1425/© 2018 Elsevier B.V. All rights reserved.
colorimetric sensors due to low cost, simplicity, high selectivity and sensitivity and fast response [15] have gained more attention. Currently, a great deal of fluorimetric and colorimetric sensors based on various mechanisms (intramolecular charge transfer (ICT) [16–20], excited state intramolecular proton transfer (ESIPT) [21], photoinduced electron transfer (PET) [22], and metal-ligand charge transfer (MLCT) [23,24]) have been synthesized. Among different intelligent strategies in designing sensors, the deprotonation approach is often used to detect CN− due to easy design and comprehension. In many cases, sensors usually containing groups such as amide, thiourea, pyrrole and hydroxyl could detect CN− [25]. Our research group has a longstanding interest in molecular recognition [26–34]. Herein, on the basis of the previous work, we designed a novel chemosensor PX based on asymmetric azine derivatives, which showed dual-channel response for CN− in DMSO/H2O (3:2, v/v) solution. To the best of our knowledge, the azine chemosensors-based colorimetric and fluorescence “turn-on” process reported so far are very few. Here, the function of sensor PX for recognizing CN− is based on the appended salicylaldehyde hydrazone and quinoline that act as respectively binding sites as well as fluorescent signal group. Moreover, this probe could successfully detect CN− in bitter almond, and test strips containing PX could act as a practical colorimetric tool to detect CN− in aqueous solution. The recognition mechanism was investigated by mass spectrometry and 1H NMR.
P.-X. Pei et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 198 (2018) 182–187
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Scheme 1. The synthetic procedure for sensor PX.
2. Experimetal 2.1. Reagents and Apparatus All reagents and solvents were obtained from the commercial sources without further purification. Double-distilled water was used throughout the experiment. Tetrabutylammonium salts of anions (F−, 2− − − − − − Cl−, Br−, I−, CO2− , PO3− 3 , S 4 , NO2 , AcO , H2PO4 , HSO4 and ClO4 ) and sodium salts of anions (CN− and SCN−) were purchased from Alfa-Aesar Chemical Reagent Co. and stored in vacuum desiccators. 1H NMR and 13C NMR spectra were respectively recorded on a Mercury400BB spectrometer at 400 MHz and 100 MHz, and chemical shifts were recorded in ppm (DMSO d6 as solvent). Melting points were performed on an X-4 digital melting point apparatus and were uncorrected. Fluorescence measurements were made using a Shimadzu RF-5301 fluorescence spectrophotometer. UV–Vis absorption spectra were measured on a Shimadzu UV-2550 spectrometer. ESI-MS was measured on an Agilent 1100 LC-MSD-Trap-VL system. 2.2. Synthesis of Chemosensor PX The synthesis of chemosensor PX is shown in Scheme 1. 2quinolinecarbaldehyde (0.236 g, 1.5 mmol) was added to the DMF solution (10 mL) of salicylaldehyde hydrazone (0.204 g, 1.5 mmol). The reaction mixture was stirred and refluxed for 6 h with acetic acid as catalyst. After cooling to room temperature, the precipitate was filtered, and recrystallized with DMF to get pale yellow compound PX. Color: yellow solid, mp: 164–166 °C·1H NMR (DMSO d6, 400 MHz, ppm) δ: 11.07 (s, 1H), 9.03 (s, 1H), 8.75 (s, 1H), 8.48(t, J = 8.62 Hz, 1H), 8.22 (dd, J = 8.33 Hz, 1H), 8.09 (m, 1H), 8.04(t, J = 8.33 Hz, 1H), 7.82(t, J = 6.45 Hz 1H), 7.73(t, J = 7.45 Hz, 1H), 7.67(t, J = 6.97 Hz, 1H), 7.41(t, J = 7.24 Hz, 1H), 6.97(dd, J = 7.99 Hz, 2H). 13C NMR (DMSO d6,
Fig. 1. UV–vis spectra of PX (20 μM) in the presence of 50 equiv. of various anions in DMSO/H2O (3:2, v/v; pH = 7.20) solution. Inset: Color changes of PX (20 μM) with various anions (50 equiv.) in DMSO/H2O (3:2, v/v; pH = 7.20) solution.
100 MHz, ppm) δ: 164.39, 162.47, 157.30, 153.25, 147.96, 137.41, 134.01, 131.43, 130.72, 129.68, 128.77, 128.50, 128.40, 120.01, 118.80, 118.62, 117.03. 3. Results and Discussion The colorimetric and fluorimetric sensing ability of PX were 2− investgated with various anions (F−, Cl−, Br−, I−, CO2− , PO3− 3 , S 4 , − − − − − − NO− , AcO , H PO , HSO , ClO , SCN and CN ) in DMSO/H 2 2 4 4 4 2O (3:2, v/v; pH = 7.20) solution. When 50 equiv. various anions were respectively added into the solution of PX (2.0 × 10−5 M), only CN− induced a dramatic color change from colorless to yellow, which was easily distinguished by naked eyes. In the corresponding UV–Vis spectra, an obvious red shift from 350 nm to 435 nm was observed (Fig. 1). However, 2− − − other examined anions (F−, Cl−, Br−, I−, CO2− , PO3− 3 ,S 4 , NO2 , AcO , − − − − H2PO4 , HSO4 , ClO4 and SCN ) didn't induce any dramatic color and spectra change. These results suggested that PX displayed an excellent selectivity for CN− over all other anions tested. Compound PX alone displayed a weak fluorescence emission band at 440 nm in DMSO/H2O (3:2, v/v; pH = 7.20). Upon addition of 50 equiv. CN− in aqueous solution, PX produced a significant fluorescence enhancement response and shifted towards 516 nm, which responded with a color change from colorless to green under UV lamp. Moreover, 2− − − − other anions (F−, Cl−, Br−, I−, CO2− , PO3− 3 , S 4 , NO2 , AcO , H2PO4 , − − HSO− , ClO and SCN ) produced a negligible change in fluorescence 4 4 color and intensity, which indicated that PX could detect CN− with specific selectivity and high sensitivity in DMSO/H2O (v/v = 3:2; pH = 7.20) solution (Fig. 2). To get insight into the CN− (0.01 M) sensing property of sensor PX, UV–Vis absorption titration experiment was carried out in DMSO/H2O
Fig. 2. Fluorescence spectra of PX (20 μM) with various anions (50 equiv.) in DMSO/H2O (3:2, v/v; pH = 7.20). Inset: Color changes for PX (20 μM) with various anions (50 equiv.) in DMSO/H2O (3:2, v/v; pH = 7.20) solution under the UV lamp.
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Fig. 3. Absorption spectra titration of sensor PX (20 μM) in presence of different concentration of CN− (0–34.5 equiv.) in DMSO/H2O (3:2, v/v; pH = 7.20) solution. Inset: photograph showing color change of PX after the addition of CN−.
Fig. 5. Fluorescence spectra of PX in the presence of different concentration of CN− (0.0–52.75 equiv.) in DMSO/H2O (3:2, v/v; pH = 7.20) solution. Inset: the change of fluorescence 516 nm depending on the concentrations of CN−.
(3:2, v/v; pH = 7.20) solution. As shown in Fig. 3, with the increasing amount of CN− from 0 equiv. to 34.5 equiv, the absorption band at 320 nm decreased gradually whereas the absorption band at 435 nm increased gradually, concomitantly, the well-defined isosbestic point appeared at 385 nm, which clearly indicated that an interconversion into single discrete chemical species during the titration process. Furthermore, we have also carried out a series of anti-interference experiments to validate the selectivity of sensor PX, as shown in Fig. 4, results clearly showed that other coexisting anions had no or little influence on detecting CN− in aqueous solution. Similarly, the fluorescence spectra of PX after addition of various amounts of CN− (0.01 M) were shown in Fig. 5, with gradual addition of CN−, the emission intensity at 516 nm remarkably increased, and the fluorescence intensity increased to the maximum after the addition of 52.75 equiv CN−. Furthermore, the fluorescence intensities at 516 nm were plotted to obtain a calibration graph, which clearly showed an
excellent linear relationship between fluorescence intensity and CN− concentration. Moreover, we have also validated the selectivity of sensor PX towards CN− in the context of coexisting anions (F−, Cl−, Br−, 2− − − − − − − I−, CO2− , PO3− 3 , S 4 , NO2 , AcO , H2PO4 , HSO4 , ClO4 and SCN ), results clearly showed that other examined anions had little impact on CN− detection (Fig. 6). The reversibility of sensor PX was measured by alternating addition of H+ and CN−. As shown in Fig. Fig. 7, upon addition of H+ to the solution of PX-CN− system, the fluorescence intensity clearly showed “OFF” behavior through the regeneration of PX. On further addition of CN−, the fluorescence intensity was revived again. This indicated that sensor PX showed an excellent reversibility for CN− ions, and the “OFF-ONOFF” switching process could be repeated seven times without larger fluorescence loss. Since the pH value may affect the charge distribution of PX and change its inherent fluorescence properties, the pH dependence of PX
Fig. 4. Absorption spectra changes for PX-CN− upon addition of 50 equiv. of other examined anions in DMSO/H2O (3:2, v/v; pH = 7.20) solution.
Fig. 6. Fluorescence spectra of PX (20 μM) in presence of 50 equiv. of CN− and 50 equiv. of various interference anions in DMSO/H2O (3:2, v/v; pH = 7.20) solution.
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Fig. 7. The reversible and reproducible fluorimetric switch controlled by alternating addition of H+ and CN− into the solution of PX.
in DMSO/H2O (3:2, v/v; pH = 7.20) system was studied by fluorescence spectroscopy. As shown in Fig. S1, the PX-CN− system showed a significant fluorescence response within the basic pH range from 7.0 and 9.0, indicating clearly that PX showed an excellent fluorescence response for CN− within the basic pH range in DMSO/H2O (3:2, v/v; pH = 7.20) solution.(pH = 7.0–9.0). The detection limit of the sensor is one of an important parameter in ion recognition. It is of great importance for practical application to detect the analytes at low concentrations. Herein, the fluorimetric detection limits of PX towards CN− were also carried out on the basis of 3SB/S (where SB is the standard deviation of the blank solution and S is the slope of the calibration curve). As shown in Fig. 8, the fluorescence detection limits on sensor PX for CN− were down to 4.5 × 10−7 M, which indicated that PX could detect the analytes at very low concentration of CN− in practical application.
Fig. 8. Fluorescence detection limit of PX (20 μM) towards CN−in DMSO/H2O (3:2, v/v) solution.
185
Fig. 9. Job plots of PX and CN−.
To confirm the binding stoichiometry between PX and CN− in aqueous solution, job plots were carried out. As shown in Fig. 9, the results demonstrated that a stoichiometry for PX-CN− was 1:1. The recognition mechanism of sensor PX with CN− was finally investigated by 1H NMR titration and mass spectrometry. As shown in Fig. 10, sensor PX showed a strong peak at 11.15 ppm in DMSO d6, we confirmed that which correspond to the protons of –OH, upon the addition of 0.5 equiv. CN− to the DMSO d6 solution of sensor PX, The –OH peak at 11.15 weakened, with the continuous addition of CN−, we found that the proton peak of benzene ring gradually showed upfield shift, which indicated that the deprotonation of hydroxyl made sensor PX shielded by the increase of electron density through charge delocalization in the conjugated system. Further evidence for the deprotonation of PX was obtained by ESIMS, as shown in Fig. S3, PX ion peak was detected at m/z 274.1250 (ESI. PX-H+), and it appeared at m/z 298.1384(ESI. PX-H+ + Na+ + H+) when the addition of CN− to the solution of PX (Fig. S4), which indicated that PX did undergo deprotonation and lost a proton. Further analysis of frontier molecular orbital (FMO) plots of PX and PX-CN− complex (Fig. 11) indicate that the HOMO–LUMO energy band gap of PX and PX-CN− complex were 162.65 kcal/mol and 83.32 kcal/mol respectively. Obviously, the HOMO–LUMO energy band gap of PX was higher than the PX-CN− system, which caused red– shift in the absorption band of PX. Furthermore, we could observe that the electron cloud density of the HOMO–LUMO levels of PX was less than the PX-CN− complex, which clearly indicated that the addition of CN− induced intramolecular charge transfer (ICT) in PX. This deprotonation caused a red shift in the absorption band with Δλ = 85 nm, and the fluorescence enhancement at 516 nm was possibly caused by a large charge separation resulting in a strong intramolecular charge transfer (ICT) in PX, simultaneously, we also confirmed that the stoichiometry of PX-CN− was 1:1. Based on the above findings, we proposed that a possible interaction mechanism between PX and CN− in this system which may proceed through the route depicted in Scheme 2. To facilitate the use of PX for the detection of cyanide in practical application, test strips were prepared by immersing filter papers into the DMSO solution of PX (0.01 M) and then drying in air, as shown in Fig. 12, obviously, the color of test strip changed from colorless to yellow under visible light, and changed from colorless to green under UV lamp, which indicated that test strips containing PX could act as a practical colorimetric tool to detect CN−.
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Fig. 10. 1H NMR spectra (400 MHz, DMSO d6) of free PX and in the presence of CN−.
Fig. 11. The DFT computed HOMO and LUMO diagram of PX and PX-CN− system.
Scheme 2. A possible mechanism of PX and CN−.
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fluorescence loss. On account of those advantages, we believe that PX as a CN−sensor makes it more conspicuous for its potential applications. Acknowledgment We gratefully acknowledge the support of the National Natural Science Foundation of China (No. 21467012). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2018.03.022.
Fig. 12. Photographs of test strips (A) only PX (B) PX with CN− by naked eyes, (C) PX with CN− under UV lamp.
To further investigate the practical application of sensor PX in our lives, we used it to detect CN− in the bitter almond. 25 g of crushed bitter almond were put into a round flask containing 100 mL of water and 0.5 g of NaOH, the mixture solution was stirred for 20 min before filtrating. Then the filtrate was adjusted to pH = 9 with fresh double water, and the solution (2.0 mL) was added to a solution of PX (0.5 mL, 1.0 × 10−4 M), we found that the filtrate induced a remarkable “turn-on” fluorescence enhancement response in fluorescence intensity at 516 nm, which clearly indicated that PX was able to be applied successfully for detecting CN− in bitter almond (Fig. 13). In order to show excellent properties of PX, we also compared detection limits, solvent medium, and applications of reported sensors with compound PX, as shown in Table S1.
4. Conclusion In summary, we have designed and synthesized a novel chemosensor PX, which showed special selective and highly sensitivity for CN− in DMSO/H2O (3:2, v/v; pH = 7.20) solution within the basic pH range (pH = 7.0–9.0). Notably, the simple but efficient sensor PX was successfully applied to the detection of cyanide in bitter almond, and the detection limit on fluorescence response of sensor PX to CN− was down to 4.5 × 10−7 M. Moreover, test trips based on sensor PX could be utilized to detect CN− in aqueous solution. The CN− induced fluorescence process could be reversed by adding H+ and the switching process could be repeated more than seven times without a large
Fig. 13. Fluorescence spectral response of PX (20 μM) in diluted bitter almond.
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A novel colorimetric and “turn-on” fluorimetric chemosensor for selective recognition of CN− ions based on asymmetric azine derivatives in aqueous media Peng-Xiang Pei, Jing-Han Hu ⁎, Chen Long, Peng-Wei Ni College of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, PR China
a r t i c l e
i n f o
Article history: Received 11 April 2017 Received in revised form 28 February 2018 Accepted 8 March 2018 Available online 10 March 2018 Keywords: Anion sensor Colorimetric Fluorescence Cyanide Test strip Bitter almond DFT
a b s t r a c t A novel chemosensor 2-((Z)-(((E)-quinolin-2-ylmethylene)hydrazono)methyl)phenol PX has been successfully designed and synthesized, which showed both colorimetric and “turn-on” fluorescence responses for CN− in DMSO/H2O (3:2, v/v; pH = 7.20) solution. The sensor could respond effectively to the stimulation of CN− ions via deprotonation and sensing mechanism of intramolecular charge transfer (ICT). Moreover, the sensor PX was successfully utilized to detect CN− in bitter almond, and the detection limit on fluorescence response of PX towards CN− was down to 4.5 × 10−7 M. Test strips containing PX were also prepared, which could act as a practical colorimetric tool to detect CN− in aqueous media. © 2018 Elsevier B.V. All rights reserved.
1. Introduction The development of anions sensors have been an interesting and emerging field of research due to the important roles of anions in industrial and biological processes [1,2]. It's well that CN− ion is one of the most toxic anions in various anions, and its toxicity can damage many functions including the vascular, visual, central nervous, cardiac, endocrine, and metabolic systems [3–6] in human body. According to the World Health Organization (WHO), maximum acceptable amount of cyanide ion in drinking water is 1.9 μM [7]. However, widespread utilization of cyanide salts in various industries such as gold extraction, synthetic fibers, electroplating and so on [8–10] have caused a serious environmental concern. Hence, it is necessary for designing reasonable chemosensor to monitor cyanide concentration in environment. In the past few decades, a variety of techniques such as ion chromatography, potentiometric, electrochemical methods and titrimetric [11–14] have been reported for CN− detection. However, those methods usually suffer from a series of problem such as high cost, long response time and sophisticated equipment and so on, which seriously limits its practical application. In contrast, fluorimetric and
⁎ Corresponding author. E-mail address: [email protected] (J.-H. Hu).
https://doi.org/10.1016/j.saa.2018.03.022 1386-1425/© 2018 Elsevier B.V. All rights reserved.
colorimetric sensors due to low cost, simplicity, high selectivity and sensitivity and fast response [15] have gained more attention. Currently, a great deal of fluorimetric and colorimetric sensors based on various mechanisms (intramolecular charge transfer (ICT) [16–20], excited state intramolecular proton transfer (ESIPT) [21], photoinduced electron transfer (PET) [22], and metal-ligand charge transfer (MLCT) [23,24]) have been synthesized. Among different intelligent strategies in designing sensors, the deprotonation approach is often used to detect CN− due to easy design and comprehension. In many cases, sensors usually containing groups such as amide, thiourea, pyrrole and hydroxyl could detect CN− [25]. Our research group has a longstanding interest in molecular recognition [26–34]. Herein, on the basis of the previous work, we designed a novel chemosensor PX based on asymmetric azine derivatives, which showed dual-channel response for CN− in DMSO/H2O (3:2, v/v) solution. To the best of our knowledge, the azine chemosensors-based colorimetric and fluorescence “turn-on” process reported so far are very few. Here, the function of sensor PX for recognizing CN− is based on the appended salicylaldehyde hydrazone and quinoline that act as respectively binding sites as well as fluorescent signal group. Moreover, this probe could successfully detect CN− in bitter almond, and test strips containing PX could act as a practical colorimetric tool to detect CN− in aqueous solution. The recognition mechanism was investigated by mass spectrometry and 1H NMR.
P.-X. Pei et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 198 (2018) 182–187
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Scheme 1. The synthetic procedure for sensor PX.
2. Experimetal 2.1. Reagents and Apparatus All reagents and solvents were obtained from the commercial sources without further purification. Double-distilled water was used throughout the experiment. Tetrabutylammonium salts of anions (F−, 2− − − − − − Cl−, Br−, I−, CO2− , PO3− 3 , S 4 , NO2 , AcO , H2PO4 , HSO4 and ClO4 ) and sodium salts of anions (CN− and SCN−) were purchased from Alfa-Aesar Chemical Reagent Co. and stored in vacuum desiccators. 1H NMR and 13C NMR spectra were respectively recorded on a Mercury400BB spectrometer at 400 MHz and 100 MHz, and chemical shifts were recorded in ppm (DMSO d6 as solvent). Melting points were performed on an X-4 digital melting point apparatus and were uncorrected. Fluorescence measurements were made using a Shimadzu RF-5301 fluorescence spectrophotometer. UV–Vis absorption spectra were measured on a Shimadzu UV-2550 spectrometer. ESI-MS was measured on an Agilent 1100 LC-MSD-Trap-VL system. 2.2. Synthesis of Chemosensor PX The synthesis of chemosensor PX is shown in Scheme 1. 2quinolinecarbaldehyde (0.236 g, 1.5 mmol) was added to the DMF solution (10 mL) of salicylaldehyde hydrazone (0.204 g, 1.5 mmol). The reaction mixture was stirred and refluxed for 6 h with acetic acid as catalyst. After cooling to room temperature, the precipitate was filtered, and recrystallized with DMF to get pale yellow compound PX. Color: yellow solid, mp: 164–166 °C·1H NMR (DMSO d6, 400 MHz, ppm) δ: 11.07 (s, 1H), 9.03 (s, 1H), 8.75 (s, 1H), 8.48(t, J = 8.62 Hz, 1H), 8.22 (dd, J = 8.33 Hz, 1H), 8.09 (m, 1H), 8.04(t, J = 8.33 Hz, 1H), 7.82(t, J = 6.45 Hz 1H), 7.73(t, J = 7.45 Hz, 1H), 7.67(t, J = 6.97 Hz, 1H), 7.41(t, J = 7.24 Hz, 1H), 6.97(dd, J = 7.99 Hz, 2H). 13C NMR (DMSO d6,
Fig. 1. UV–vis spectra of PX (20 μM) in the presence of 50 equiv. of various anions in DMSO/H2O (3:2, v/v; pH = 7.20) solution. Inset: Color changes of PX (20 μM) with various anions (50 equiv.) in DMSO/H2O (3:2, v/v; pH = 7.20) solution.
100 MHz, ppm) δ: 164.39, 162.47, 157.30, 153.25, 147.96, 137.41, 134.01, 131.43, 130.72, 129.68, 128.77, 128.50, 128.40, 120.01, 118.80, 118.62, 117.03. 3. Results and Discussion The colorimetric and fluorimetric sensing ability of PX were 2− investgated with various anions (F−, Cl−, Br−, I−, CO2− , PO3− 3 , S 4 , − − − − − − NO− , AcO , H PO , HSO , ClO , SCN and CN ) in DMSO/H 2 2 4 4 4 2O (3:2, v/v; pH = 7.20) solution. When 50 equiv. various anions were respectively added into the solution of PX (2.0 × 10−5 M), only CN− induced a dramatic color change from colorless to yellow, which was easily distinguished by naked eyes. In the corresponding UV–Vis spectra, an obvious red shift from 350 nm to 435 nm was observed (Fig. 1). However, 2− − − other examined anions (F−, Cl−, Br−, I−, CO2− , PO3− 3 ,S 4 , NO2 , AcO , − − − − H2PO4 , HSO4 , ClO4 and SCN ) didn't induce any dramatic color and spectra change. These results suggested that PX displayed an excellent selectivity for CN− over all other anions tested. Compound PX alone displayed a weak fluorescence emission band at 440 nm in DMSO/H2O (3:2, v/v; pH = 7.20). Upon addition of 50 equiv. CN− in aqueous solution, PX produced a significant fluorescence enhancement response and shifted towards 516 nm, which responded with a color change from colorless to green under UV lamp. Moreover, 2− − − − other anions (F−, Cl−, Br−, I−, CO2− , PO3− 3 , S 4 , NO2 , AcO , H2PO4 , − − HSO− , ClO and SCN ) produced a negligible change in fluorescence 4 4 color and intensity, which indicated that PX could detect CN− with specific selectivity and high sensitivity in DMSO/H2O (v/v = 3:2; pH = 7.20) solution (Fig. 2). To get insight into the CN− (0.01 M) sensing property of sensor PX, UV–Vis absorption titration experiment was carried out in DMSO/H2O
Fig. 2. Fluorescence spectra of PX (20 μM) with various anions (50 equiv.) in DMSO/H2O (3:2, v/v; pH = 7.20). Inset: Color changes for PX (20 μM) with various anions (50 equiv.) in DMSO/H2O (3:2, v/v; pH = 7.20) solution under the UV lamp.
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Fig. 3. Absorption spectra titration of sensor PX (20 μM) in presence of different concentration of CN− (0–34.5 equiv.) in DMSO/H2O (3:2, v/v; pH = 7.20) solution. Inset: photograph showing color change of PX after the addition of CN−.
Fig. 5. Fluorescence spectra of PX in the presence of different concentration of CN− (0.0–52.75 equiv.) in DMSO/H2O (3:2, v/v; pH = 7.20) solution. Inset: the change of fluorescence 516 nm depending on the concentrations of CN−.
(3:2, v/v; pH = 7.20) solution. As shown in Fig. 3, with the increasing amount of CN− from 0 equiv. to 34.5 equiv, the absorption band at 320 nm decreased gradually whereas the absorption band at 435 nm increased gradually, concomitantly, the well-defined isosbestic point appeared at 385 nm, which clearly indicated that an interconversion into single discrete chemical species during the titration process. Furthermore, we have also carried out a series of anti-interference experiments to validate the selectivity of sensor PX, as shown in Fig. 4, results clearly showed that other coexisting anions had no or little influence on detecting CN− in aqueous solution. Similarly, the fluorescence spectra of PX after addition of various amounts of CN− (0.01 M) were shown in Fig. 5, with gradual addition of CN−, the emission intensity at 516 nm remarkably increased, and the fluorescence intensity increased to the maximum after the addition of 52.75 equiv CN−. Furthermore, the fluorescence intensities at 516 nm were plotted to obtain a calibration graph, which clearly showed an
excellent linear relationship between fluorescence intensity and CN− concentration. Moreover, we have also validated the selectivity of sensor PX towards CN− in the context of coexisting anions (F−, Cl−, Br−, 2− − − − − − − I−, CO2− , PO3− 3 , S 4 , NO2 , AcO , H2PO4 , HSO4 , ClO4 and SCN ), results clearly showed that other examined anions had little impact on CN− detection (Fig. 6). The reversibility of sensor PX was measured by alternating addition of H+ and CN−. As shown in Fig. Fig. 7, upon addition of H+ to the solution of PX-CN− system, the fluorescence intensity clearly showed “OFF” behavior through the regeneration of PX. On further addition of CN−, the fluorescence intensity was revived again. This indicated that sensor PX showed an excellent reversibility for CN− ions, and the “OFF-ONOFF” switching process could be repeated seven times without larger fluorescence loss. Since the pH value may affect the charge distribution of PX and change its inherent fluorescence properties, the pH dependence of PX
Fig. 4. Absorption spectra changes for PX-CN− upon addition of 50 equiv. of other examined anions in DMSO/H2O (3:2, v/v; pH = 7.20) solution.
Fig. 6. Fluorescence spectra of PX (20 μM) in presence of 50 equiv. of CN− and 50 equiv. of various interference anions in DMSO/H2O (3:2, v/v; pH = 7.20) solution.
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Fig. 7. The reversible and reproducible fluorimetric switch controlled by alternating addition of H+ and CN− into the solution of PX.
in DMSO/H2O (3:2, v/v; pH = 7.20) system was studied by fluorescence spectroscopy. As shown in Fig. S1, the PX-CN− system showed a significant fluorescence response within the basic pH range from 7.0 and 9.0, indicating clearly that PX showed an excellent fluorescence response for CN− within the basic pH range in DMSO/H2O (3:2, v/v; pH = 7.20) solution.(pH = 7.0–9.0). The detection limit of the sensor is one of an important parameter in ion recognition. It is of great importance for practical application to detect the analytes at low concentrations. Herein, the fluorimetric detection limits of PX towards CN− were also carried out on the basis of 3SB/S (where SB is the standard deviation of the blank solution and S is the slope of the calibration curve). As shown in Fig. 8, the fluorescence detection limits on sensor PX for CN− were down to 4.5 × 10−7 M, which indicated that PX could detect the analytes at very low concentration of CN− in practical application.
Fig. 8. Fluorescence detection limit of PX (20 μM) towards CN−in DMSO/H2O (3:2, v/v) solution.
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Fig. 9. Job plots of PX and CN−.
To confirm the binding stoichiometry between PX and CN− in aqueous solution, job plots were carried out. As shown in Fig. 9, the results demonstrated that a stoichiometry for PX-CN− was 1:1. The recognition mechanism of sensor PX with CN− was finally investigated by 1H NMR titration and mass spectrometry. As shown in Fig. 10, sensor PX showed a strong peak at 11.15 ppm in DMSO d6, we confirmed that which correspond to the protons of –OH, upon the addition of 0.5 equiv. CN− to the DMSO d6 solution of sensor PX, The –OH peak at 11.15 weakened, with the continuous addition of CN−, we found that the proton peak of benzene ring gradually showed upfield shift, which indicated that the deprotonation of hydroxyl made sensor PX shielded by the increase of electron density through charge delocalization in the conjugated system. Further evidence for the deprotonation of PX was obtained by ESIMS, as shown in Fig. S3, PX ion peak was detected at m/z 274.1250 (ESI. PX-H+), and it appeared at m/z 298.1384(ESI. PX-H+ + Na+ + H+) when the addition of CN− to the solution of PX (Fig. S4), which indicated that PX did undergo deprotonation and lost a proton. Further analysis of frontier molecular orbital (FMO) plots of PX and PX-CN− complex (Fig. 11) indicate that the HOMO–LUMO energy band gap of PX and PX-CN− complex were 162.65 kcal/mol and 83.32 kcal/mol respectively. Obviously, the HOMO–LUMO energy band gap of PX was higher than the PX-CN− system, which caused red– shift in the absorption band of PX. Furthermore, we could observe that the electron cloud density of the HOMO–LUMO levels of PX was less than the PX-CN− complex, which clearly indicated that the addition of CN− induced intramolecular charge transfer (ICT) in PX. This deprotonation caused a red shift in the absorption band with Δλ = 85 nm, and the fluorescence enhancement at 516 nm was possibly caused by a large charge separation resulting in a strong intramolecular charge transfer (ICT) in PX, simultaneously, we also confirmed that the stoichiometry of PX-CN− was 1:1. Based on the above findings, we proposed that a possible interaction mechanism between PX and CN− in this system which may proceed through the route depicted in Scheme 2. To facilitate the use of PX for the detection of cyanide in practical application, test strips were prepared by immersing filter papers into the DMSO solution of PX (0.01 M) and then drying in air, as shown in Fig. 12, obviously, the color of test strip changed from colorless to yellow under visible light, and changed from colorless to green under UV lamp, which indicated that test strips containing PX could act as a practical colorimetric tool to detect CN−.
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Fig. 10. 1H NMR spectra (400 MHz, DMSO d6) of free PX and in the presence of CN−.
Fig. 11. The DFT computed HOMO and LUMO diagram of PX and PX-CN− system.
Scheme 2. A possible mechanism of PX and CN−.
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fluorescence loss. On account of those advantages, we believe that PX as a CN−sensor makes it more conspicuous for its potential applications. Acknowledgment We gratefully acknowledge the support of the National Natural Science Foundation of China (No. 21467012). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2018.03.022.
Fig. 12. Photographs of test strips (A) only PX (B) PX with CN− by naked eyes, (C) PX with CN− under UV lamp.
To further investigate the practical application of sensor PX in our lives, we used it to detect CN− in the bitter almond. 25 g of crushed bitter almond were put into a round flask containing 100 mL of water and 0.5 g of NaOH, the mixture solution was stirred for 20 min before filtrating. Then the filtrate was adjusted to pH = 9 with fresh double water, and the solution (2.0 mL) was added to a solution of PX (0.5 mL, 1.0 × 10−4 M), we found that the filtrate induced a remarkable “turn-on” fluorescence enhancement response in fluorescence intensity at 516 nm, which clearly indicated that PX was able to be applied successfully for detecting CN− in bitter almond (Fig. 13). In order to show excellent properties of PX, we also compared detection limits, solvent medium, and applications of reported sensors with compound PX, as shown in Table S1.
4. Conclusion In summary, we have designed and synthesized a novel chemosensor PX, which showed special selective and highly sensitivity for CN− in DMSO/H2O (3:2, v/v; pH = 7.20) solution within the basic pH range (pH = 7.0–9.0). Notably, the simple but efficient sensor PX was successfully applied to the detection of cyanide in bitter almond, and the detection limit on fluorescence response of sensor PX to CN− was down to 4.5 × 10−7 M. Moreover, test trips based on sensor PX could be utilized to detect CN− in aqueous solution. The CN− induced fluorescence process could be reversed by adding H+ and the switching process could be repeated more than seven times without a large
Fig. 13. Fluorescence spectral response of PX (20 μM) in diluted bitter almond.
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