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A highly selective and sensitive chemosensor for instant detection cyanide via different channels in aqueous solution
Tetrahedron 70 (2014) 1889e1894
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Tetrahedron journal homepage: www.elsevier.com/locate/tet
A highly selective and sensitive chemosensor for instant detection cyanide via different channels in aqueous solution Peng Zhang a, b, BingBing Shi a, b, XingMei You a, b, YouMing Zhang a, b, Qi Lin a, b, Hong Yao a, b, TaiBao Wei a, b, * a Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, PR China b Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 October 2013 Received in revised form 12 January 2014 Accepted 20 January 2014 Available online 25 January 2014
A colorimetric and fluorescent cyanide probe bearing naphthol and imine group has been designed and synthesized. This structurally simple probe displays rapid response and high selectivity for cyanide over other common anions (F, Cl, Br, I, AcO, H2 PO2 4 ; HSO4 ; ClO4 ; and SCN ) in aqueous solution. The sensing of cyanide was performed via the nucleophilic attack of cyanide anion to imine groups of the probe with a 1:2 binding stoichiometry, and the fluorescence enhancement of the sensor is mainly due to the ICT process improvement. The detection limit for CN was 4.0107 M, which is far lower than the WHO guideline of 1.9106 M. Thus, the present probe should be applicable as a practical system for the monitoring of cyanide concentrations in aqueous samples. Ó 2014 Published by Elsevier Ltd.
Keywords: Cyanide (CN) Colorimetric fluorescence switch ICT Highly selectivity Imine group
1. Introduction In the past decade, the development of molecular probes for anions such as cyanide has been a subject of intense research interest, because such anions play important roles in biological systems and also constitute some pollutants in our environment.1 Cyanide (CN) is one of the most toxic anions and is deadly to humans, for it can affect many functions in the human body, including the vascular, visual, central nervous, cardiac, endocrine, and metabolic systems.2 It is known that 0.5e3.5 mg of cyanide per kg of body weight is fatal for humans.3 Nevertheless, cyanide is widely used in many chemical processes, such as electroplating, plastics manufacturing, gold and silver extraction, tanning, and metallurgy.4 Therefore sensitive, selective, simple, and affordable sensors for cyanide ion are in great demand for various applications. Up to now, a large number of chemosensors for CN have been invented,5 colorimetric and fluorimetric probes for naked eye detection have attracted considerable interest for their simple and fast implementation as well as their high sensitivity. Typically, a preeminent anion probe is usually constructed by combining * Corresponding author. Tel.: þ86 931 7973191; e-mail address: weitaibao@126. com (TaiBao Wei). 0040-4020/$ e see front matter Ó 2014 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.tet.2014.01.046
a luminophore and an anion binding unit. In many cases, the anion binding unit is primarily composed of H-bonding donors.6 Hbonding interactions between an anion and the H-bond donors in a sensing system can induce internal charge transfer (ICT), which cause changes in the absorption and emission of the chromogenic probe, thus allowing the naked eye chemosensing.7 ICT is one of the various signaling mechanisms by which the electronic transitions from the binding site to the fluorophore take place through a ‘pushepull’ mechanism, with binding affecting the donor/acceptor strength of the binding substituent and the resultant change in electronic structure leading to either emission enhancement or quenching.8 A variety of neutral receptors have been reported for selective anion recognition based on H-bonding owing to the strength and selectivity of this interaction.9 In view of this, as a part of our research interest in molecular recognition.10 We report a simple to synthesize, yet sensitive, and selective cyanide anion sensor S9, which was synthesized as show in Scheme 1. Our strategy as follows, firstly, in order to achieve intramolecular charge transfer (ICT) mechanisms, the sensor molecular should possess two hydrogen-bond donor groups, we introduced hydroxyl and imine groups to the sensor molecule as binding sites to detect CN by hydrogen-bonding interactions and nucleophilic additions. Secondly, in order to
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achieve high sensitivity for CN, the fluorescent signal report mode has been adopted, because the fluorescent sensors often provide higher sensitivity than other optic sensors. Therefore, we introduced naphthalene groups as the fluorescence signal group. Finally, the sensor molecular was designed easy to synthesis and low cost. As a result, sensor S9 could detect CN with specific selectivity and high sensitivity. Moreover, the detection limit of CN was 4.0107 M, which is quite low for the detection of CN ions found in many chemical and biological systems.
2.4. General procedure for fluorescence spectra experiments All the fluorescence spectroscopy was carried out in aqueous solution (0.01 M HEPES buffer, pH 7.24, 90% DMSO) on a Shimadzu RF-5301 spectrometer. Any changes in the fluorescence spectra of the synthesized compound were recorded on addition of tetrabutylammonium salts while keeping the ligand concentration constant (2.0105 M) in all experiments. Tetrabutylammonium salt of anions (F, Cl, Br, I, AcO, H2 PO 4 , CN , HSO4 , SCN , and ClO4 ) were used for the fluorescence experiments.
Scheme 1. Synthetic procedures for sensor S9.
2. Experimental section
2.5. General procedure for 1H NMR experiments
2.1. Materials and physical methods
For 1H NMR titrations, two stock solutions were prepared in DMSO-d6, one of them containing host only and the second one containing an appropriate concentration of guest. Aliquots of the two solutions were mixed directly in NMR tubes.
Fresh double distilled water was used throughout the experiments. All other reagents and solvents were commercially available at analytical grade and were used without further purification. 1H NMR and 13C NMR spectra were recorded on a Bruker AvanceIII 400 MHz spectrometer at 400 MHz and 100 MHz, respectively. Chemical shifts are reported in parts per milliom (ppm) downfield from tetramethylsilane (TMS, d scale with solvent resonances as internal standards). UVevis spectra were recorded on a Shimadzu UV-2550 spectrometer. Photoluminescence spectra were performed on a Shimadzu RF-5301 fluorescence spectrophotometer. Melting points were measured on an X-4 digital melting-point apparatus (uncorrected). Infrared spectra were performed on a Digilab FTS-3000 FT-IR spectrophotometer. 2.2. Synthesis of S9 2-Hydroxy-1-naphthaldehyde (0.378 g, 2.2 mmol), hydrazine monohydrate (0.100g, 2 mmol), and a catalytic amount of acetic acid (five drops of AcOH) were combined in absolute ethanol (30 mL). The solution was stirred under reflux for 4 h. After cooling to room temperature, the yellow precipitate was filtered, washed three times with hot absolute ethanol, then recrystallized with EtOH/DMF to give a luminous yellow powder product S9 (0.588 g) in 85% yield (mp>300 C), IR: (KBr, cm1) n: 3419 (OH), 3143 (C] NH), 3065 (ArH), 1620 (C]N), 1600 (C]C), 1577 (C]C), 1465 (C] C), 1312 (CeO). 1H NMR (DMSO-d6, 400 MHz): d 12.88 (s, 2H, ArOH), 10.00 (s, 2H, N]CH), 8.66e7.28 (m, 12H, ArH); 13C NMR (DMSO-d6, 100 MHz): d 151.73, 148.60, 145.97, 143.09, 131.07, 130.62, 130.02, 122.46, 117.99, 112.63, 110.69. Anal. Calcd for C22H16N2O2: C 77.62, H 4.70, N 8.23. Found: C, 77.63; H, 4.67; N, 8.25. ESI-MS: m/z calcd for C22H16N2O2, [MH]þ¼339.2, found [MH]¼339.9.
2.6. Determination of association constant The association constants (Ka) of S9 and CN were determined based on the absorbance titration curve using the equation as follows: where A1 and A0 represent the absorbance of host in the presence and absence of ions, respectively, Amax is the saturated absorbance of host in the presence of excess amount of ions; [G] is the concentration of CN added.
# " 1 1 1 ¼ þ1 Amax A0 A1 A0 k½G2 3. Results and discussion In this work, a chemosensor (S9) for CN bearing naphthol and imine group was designed and synthesized, as show in Scheme 1. Sensor S9 has been characterized by 1H NMR, 13C NMR, IR, ESI-MS, and elemental analyses. The structure of S9 was further confirmed by X-ray crystallography, as shown in Fig. 1. A summary of the crystallographic data and structural refinements for S9 and the hydrogen-bonding interactions in S9 are listed in Fig. 1, Tables S1 and S2 (see Supplementary data).
2.3. General procedure for UVevis experiments All the UVevis experiments were carried out in aqueous solution (0.01 M HEPES buffer, pH 7.24, 90% DMSO) on a Shimadzu UV2550 spectrometer. Any changes in the UVevis spectra of the synthesized compound were recorded on addition of tetrabutylammonium salts while keeping the ligand concentration constant (2.0105 M) in all experiments. Tetrabutylammonium salt of an ions (F, Cl, Br, I, AcO, H2 PO 4 , CN , HSO4 , SCN , and ClO4 ) were used for the UVevis experiments.
Fig. 1. The single crystal X-ray structure of S9.
Considering a report in the literature,4 sensor S9 would interact with anions by reversible covalent bonding, which is activated by
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an intramolecular hydrogen-bond to form adduct B1 and less efficiently by direct H-bonding to form species B2 (Scheme 2). Upon anion binding on its hydrazone receptor in chromogenic probe S9, the degree of internal charge transfer ICT is expected to be dependent less on the anion’s H-bonding ability, but may be dependent on the anion’s hydrazone carbon affinity. Hence, the usual guest selectivity pattern dependent on the anion’s H-bonding ability and/or basicity may be altered. In aqueous media, the anions with a weak H-bonding acceptor but with a strong carbon hydrazone affinity are expected to be differentiated from the other anions characterized by a strong H-bonding acceptor, particularly in aqueous media, because weak solvation is expected in the former case.
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(0.01 M HEPES buffer, pH 7.24, 90% DMSO) are shown in Fig. S2. The absorption band at 410 nm decreased, while a new red-shifted band at 500 nm appeared. Other competing anions, such as F, Cl, Br, I, AcO, H2 PO 4 , CN , SCN , HSO4 , and ClO4 (50 equiv) are completely nonresponsive (Fig. 4). No interference of fluoride and acetate anions should arise from their high solvation energies in water as stated above. Only CN, which is a poor H-bond acceptor but it has a strong affinity toward the unsaturated carbon, can add to sensor S9, because of sensor S9 would interact with CN by reversible covalent bonding, which is activated by an intramolecular hydrogen-bond to form adduct A1 and less efficiently by direct Hbonding to form species A2. The blue shift of absorption of species (from 523 nm to 500 nm) formed by interaction of sensor S9 with
Scheme 2. Proposed sensing mechanism of sensor S9 for the detection of cyanide.
When the chemosensors based on the H-bonding interaction are tested with different anions such as F, Cl, Br, I, AcO, H2PO 4 , CN , SCN , HSO4 , and ClO4 , a strongly H-bonding F (pKa w3.15), AcO (pKa w4.76) or a relatively basic CN (pKa w9.2) can shift the equilibrium to the right side, forming specie B1 showing the most significant color change, as already observed in the literature.11 The color change may be reduced or negligible if the same experiment is carried out in aqueous media, due to the strong hydration effect, because of the high salvation energies for F (DHhyd¼505 kJ/mol) and AcO (DHhyd¼375 kJ/mol) in water. Indeed, when sensor S9 was treated with selected anions, such as CN, F and CH3COO (50 equiv) in DMSO, a new strong peak at 523 nm appeared along with a concomitant decrease at 410 nm. Other anions, such as Cl, Br, I, H2 PO 4 ; and HSO4 showed little change to sensor S9 as seen in Fig. 2. This result implied that the sensing of anions was probably performed via the H-bonding of anions to the OH of S9. This was further confirmed by the 1H NMR spectrum of S9. 1H NMR titration displayed the chemical shift changes of S9 upon the addition of CN, as shown in Fig. 3, sensor S9 showed a single peak at 12.88 ppm in DMSO-d6, which was confirmed to the proton of OH, after the addition of cyanide, the OH proton disappeared while all the aromatic protons exhibited an upfield shift to different extent, which suggest the increase in the electron density in naphthalene ring through charge delocalization in the conjugated system.15 This supports the deprotonation phenomena of S9. An even more striking feature results when the sensing experiment is carried out in aqueous media. The absorption spectral changes of sensor S9 on addition of cyanide in an aqueous solution
Fig. 2. UVevis absorbance spectra of S9 (20 mM) upon addition of different anions (50 equiv) in DMSO.
CN in aqueous media compared with that in DMSO also supported the nucleophilic addition mechanism as mentioned in Scheme 2. This highly selective detection of cyanide could be demonstrated even in the presence of other anions in the solution of DMSO/H2O (0.01M HEPES buffer, pH 7.24, 70% DMSO), as shown in Fig. 5.
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Fig. 3. Partial 1H NMR spectra of S9 (3 mg, DMSO-d6) and in the presence of varying amounts of CN.
Fig. 5. Absorbance spectra of S9 (20 mM) in the presence of various anions (50 equiv) in DMSO/H2O (8:2, v/v, containing 0.01 M HEPES, pH¼7.24) in response to CN (50 equiv).
to the coumarin-based pep* transition. On binding to the CN, the red-shifted and enhanced new band at 500 nm could be attributed to the adduct-based intramolecular proton transfer band. The variation of the absorbance at 500 nm was used to evaluate the binding constants of S9 with CN by assuming a 1:2 binding stoichiometry. The BenesieeHildebrand12 analysis of these changes gave a binding constant of 1.303107 M2 (R2: 0.997), see Fig. S3. Nice fittings supported the 1:2 binding stoichiometry.13
Fig. 4. (a) UVevis absorbance spectra of S9 (20 mM) upon addition of different anions (50 equiv) in DMSO/H2O (9:1, v/v, containing 0.01 M HEPES, pH¼7.24). Inset: color changes of S9 after addition of various anions (50 equiv). (b) Normalized of UVevis absorbance spectra of S9 (20 mM) upon addition of different anions.
To further investigate the interaction between sensor S9 and CN, UVevis absorption spectral variation of sensor S9 was monitored during titration with different concentrations of CN. As shown in Fig. 6, a sharp decrease in the UVevis absorption at 410 nm was observed after adding of CN, and then the absorption at 500 nm increased gradually. The isosbestic points at 320, 367, and 445 nm can be clearly observed with increasing concentrations of CN. Simultaneous appearance of isosbestic points signified the presence of two different species that remained in equilibrium. Absorption bands at 410 nm for sensor S9 was attributed primarily
Fig. 6. UVevis spectra of S9 (20 mM) in DMSO/H2O (9:1, v/v, containing 0.01 M HEPES, pH¼7.24) upon adding of an increasing concentration of CN.
Compound S9 alone displays a weak, single fluorescence emission band at 588 nm when excited at 500 nm, because the electron cloud of the naphthalene group was reduced by an electronwithdrawing imine group, which induced fluorescence quenching. The addition of CN to the aqueous solution (0.01 M HEPES buffer, pH 7.24, 90% DMSO) of S9 led to a prominent fluorescence enhancement (Fig. S4), the fluorescence enhancement can be explained by intramolecular H-bonding stabilization of a cyanide adduct, through which an ICT process (from aminemethyl to naphthalene group) can be improved. However, other anions, such as F, Cl, Br, I, AcO, H2 PO 4 , CN , SCN , HSO4 , and ClO4 did not
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cause any significant changes in the fluorescence emission intensity. The fluorescence profiles at 588 nm showed a high selectivity for cyanide over other various anions (Fig. 7). A large increase in the fluorescence intensity could be distinguished visually as shown in Fig. 7a, where photos of solutions of S9 in the absence and presence of CN under UV light (365 nm) illumination are displayed, this also could be applied to the detection of cyanide anions by the naked eye. This highly selective fluorescent detection of cyanide could be demonstrated even in the presence of other anions in the solution of DMSO/H2O (0.01 M HEPES buffer, pH 7.24, 70% DMSO), as shown in Fig. S5.
Fig. 8. Fluorescence titration spectra of S9 (20 mM) in DMSO/H2O (9:1, v/v, containing 0.01 M HEPES, pH¼7.24) upon adding of an increasing concentration of CN (lex¼498 nm).
S9/CN adduct (Fig. 9), from the ESI-MS spectrum, we can also see an obvious peak at m/z 392.1 assignable to [S9þ2CN] (m/ zcalcd¼392.2). The recognition mechanism of the sensor S9 with CN was also certified by FTIR spectroscopy (Fig. S7). Compared with S9 the S9/CN adduct showed a new peak at 2252 cm1, when 2 equiv of CN was added, this also indicated the nucleophilic addition of CN, the peaks of OH and NH at 3602 cm1 and 3532 cm1 can be explained by intramolecular H-bonding stabilization of a cyanide adduct, through an ICT process. These spectra indicate that the cyanide anion was indeed added to the carbon of imine groups of sensor S9.
Fig. 7. (a) Fluorescence spectra response of S9 (20 mM) upon addition of different anions (50 equiv) in DMSO/H2O (9:1, v/v, containing 0.01 M HEPES, pH¼7.24). Inset: photograph of S9 (20 mM) upon adding 50 equiv of anions, which was taken under an UV-lamp (365 nm). (b) Normalized of fluorescence absorbance spectra of S9 (20 mM) upon addition of different anions.
Fig. 8 shows the fluorescence changes of S9 after addition of different amounts of CN. As anticipated, the fluorescence started to increase gradually after addition of CN. For instance, the fluorescence intensity increased by more than 200 times when the concentration of CN reached 20.0 mM in the solution of S9. On the basis of the 1:2 stoichiometry and fluorescence titration data, the binding constant of S9 for CN was calculated to be 1.368107 M2 (R2: 0.997), see Fig. S6. In the meantime, the detection limit of the fluorescence spectra changes calculated on the basis of 3SB/S was 0.4 mM for CN anion in aqueous solution,14 which is far lower than the WHO guideline of 1.9 mM cyanide. As expected nucleophilic attack on the unsaturated bond groups of sensor S9 should satisfy 1:2 probe and cyanide stoichiometry. The result obtained from a Job’s plot supports the formation of a 1:2
Fig. 9. The Job’s plot examined between CN and S9, indicating the 1:2 stoichiometry.
4. Conclusion In conclusion, we have presented a facile, low-cost, and efficient Schiff base example of a highly selective chemosensor for CN through reversible covalent bonding between an ionophore and an anion. The sensor gives an immediate response to the cyanide ion both by visible color changes as well as fluorescence turn-on response. Cyanide anions are detectable by its nucleophilic attack toward the imine groups and the mechanism of the reaction was investigated by various means (such as, 1H NMR spectroscopic
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titration, FTIR spectra, and mass spectral analysis). A plausible mechanism of the sensing reaction sequence is presented, which is shown to operate by means of a nucleophilic addition adduct formation under ambient conditions of pH and temperature in aqueous solution.
5.
6.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21064006, 21262032, and 21161018), the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (No. IRT1177), the Natural Science Foundation of Gansu Province (No. 1010RJZA018), the Youth Foundation of Gansu Province (No. 2011GS04735), and NWNU-LKQN-11-32.
7. 8. 9.
Supplementary data 10.
Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.tet.2014.01.046. References and notes 1. (a) Xu, Z. C.; Kim, S. K.; Yoon, J. Y. Chem. Soc. Rev. 2010, 39, 1457e1466; (b) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107, 1339e1387; (c) Yoon, J. Y.; Kim, S. K.; Singh, N. J.; Kim, K. S. Chem. Soc. Rev. 2006, 35, 355e360. 2. Matsubara, K.; Akane, A.; Maseda, C.; Shiono, H. Forensic Sci. Int. 1990, 46, 203e208. 3. Huang, X. H.; Gu, X. G.; Zhang, G. X.; Zhang, D. Q. Chem. Commun. 2012, 12195e12197. 4. (a) Qian, G.; Li, X. Z.; Wang, Z. Y. J. Mater. Chem. 2009, 19, 522e530; (b) Peng, L. H.; Wang, M.; Zhang, G. X.; Zhang, D. Q.; Zhu, D. B. Org. Lett. 2009, 11, 1943e1946; (c) Xu, Z. C.; Chen, X. Q.; Kim, H. N.; Yoon, J. Y. Chem. Soc. Rev. 2010, 39, 127e137; (d) Kim, D. S.; Chung, Y. M.; Jun, M.; Ahn, K. H. J. Org. Chem. 2009,
11. 12. 13. 14. 15.
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Contents lists available at ScienceDirect
Tetrahedron journal homepage: www.elsevier.com/locate/tet
A highly selective and sensitive chemosensor for instant detection cyanide via different channels in aqueous solution Peng Zhang a, b, BingBing Shi a, b, XingMei You a, b, YouMing Zhang a, b, Qi Lin a, b, Hong Yao a, b, TaiBao Wei a, b, * a Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, PR China b Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 October 2013 Received in revised form 12 January 2014 Accepted 20 January 2014 Available online 25 January 2014
A colorimetric and fluorescent cyanide probe bearing naphthol and imine group has been designed and synthesized. This structurally simple probe displays rapid response and high selectivity for cyanide over other common anions (F, Cl, Br, I, AcO, H2 PO2 4 ; HSO4 ; ClO4 ; and SCN ) in aqueous solution. The sensing of cyanide was performed via the nucleophilic attack of cyanide anion to imine groups of the probe with a 1:2 binding stoichiometry, and the fluorescence enhancement of the sensor is mainly due to the ICT process improvement. The detection limit for CN was 4.0107 M, which is far lower than the WHO guideline of 1.9106 M. Thus, the present probe should be applicable as a practical system for the monitoring of cyanide concentrations in aqueous samples. Ó 2014 Published by Elsevier Ltd.
Keywords: Cyanide (CN) Colorimetric fluorescence switch ICT Highly selectivity Imine group
1. Introduction In the past decade, the development of molecular probes for anions such as cyanide has been a subject of intense research interest, because such anions play important roles in biological systems and also constitute some pollutants in our environment.1 Cyanide (CN) is one of the most toxic anions and is deadly to humans, for it can affect many functions in the human body, including the vascular, visual, central nervous, cardiac, endocrine, and metabolic systems.2 It is known that 0.5e3.5 mg of cyanide per kg of body weight is fatal for humans.3 Nevertheless, cyanide is widely used in many chemical processes, such as electroplating, plastics manufacturing, gold and silver extraction, tanning, and metallurgy.4 Therefore sensitive, selective, simple, and affordable sensors for cyanide ion are in great demand for various applications. Up to now, a large number of chemosensors for CN have been invented,5 colorimetric and fluorimetric probes for naked eye detection have attracted considerable interest for their simple and fast implementation as well as their high sensitivity. Typically, a preeminent anion probe is usually constructed by combining * Corresponding author. Tel.: þ86 931 7973191; e-mail address: weitaibao@126. com (TaiBao Wei). 0040-4020/$ e see front matter Ó 2014 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.tet.2014.01.046
a luminophore and an anion binding unit. In many cases, the anion binding unit is primarily composed of H-bonding donors.6 Hbonding interactions between an anion and the H-bond donors in a sensing system can induce internal charge transfer (ICT), which cause changes in the absorption and emission of the chromogenic probe, thus allowing the naked eye chemosensing.7 ICT is one of the various signaling mechanisms by which the electronic transitions from the binding site to the fluorophore take place through a ‘pushepull’ mechanism, with binding affecting the donor/acceptor strength of the binding substituent and the resultant change in electronic structure leading to either emission enhancement or quenching.8 A variety of neutral receptors have been reported for selective anion recognition based on H-bonding owing to the strength and selectivity of this interaction.9 In view of this, as a part of our research interest in molecular recognition.10 We report a simple to synthesize, yet sensitive, and selective cyanide anion sensor S9, which was synthesized as show in Scheme 1. Our strategy as follows, firstly, in order to achieve intramolecular charge transfer (ICT) mechanisms, the sensor molecular should possess two hydrogen-bond donor groups, we introduced hydroxyl and imine groups to the sensor molecule as binding sites to detect CN by hydrogen-bonding interactions and nucleophilic additions. Secondly, in order to
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achieve high sensitivity for CN, the fluorescent signal report mode has been adopted, because the fluorescent sensors often provide higher sensitivity than other optic sensors. Therefore, we introduced naphthalene groups as the fluorescence signal group. Finally, the sensor molecular was designed easy to synthesis and low cost. As a result, sensor S9 could detect CN with specific selectivity and high sensitivity. Moreover, the detection limit of CN was 4.0107 M, which is quite low for the detection of CN ions found in many chemical and biological systems.
2.4. General procedure for fluorescence spectra experiments All the fluorescence spectroscopy was carried out in aqueous solution (0.01 M HEPES buffer, pH 7.24, 90% DMSO) on a Shimadzu RF-5301 spectrometer. Any changes in the fluorescence spectra of the synthesized compound were recorded on addition of tetrabutylammonium salts while keeping the ligand concentration constant (2.0105 M) in all experiments. Tetrabutylammonium salt of anions (F, Cl, Br, I, AcO, H2 PO 4 , CN , HSO4 , SCN , and ClO4 ) were used for the fluorescence experiments.
Scheme 1. Synthetic procedures for sensor S9.
2. Experimental section
2.5. General procedure for 1H NMR experiments
2.1. Materials and physical methods
For 1H NMR titrations, two stock solutions were prepared in DMSO-d6, one of them containing host only and the second one containing an appropriate concentration of guest. Aliquots of the two solutions were mixed directly in NMR tubes.
Fresh double distilled water was used throughout the experiments. All other reagents and solvents were commercially available at analytical grade and were used without further purification. 1H NMR and 13C NMR spectra were recorded on a Bruker AvanceIII 400 MHz spectrometer at 400 MHz and 100 MHz, respectively. Chemical shifts are reported in parts per milliom (ppm) downfield from tetramethylsilane (TMS, d scale with solvent resonances as internal standards). UVevis spectra were recorded on a Shimadzu UV-2550 spectrometer. Photoluminescence spectra were performed on a Shimadzu RF-5301 fluorescence spectrophotometer. Melting points were measured on an X-4 digital melting-point apparatus (uncorrected). Infrared spectra were performed on a Digilab FTS-3000 FT-IR spectrophotometer. 2.2. Synthesis of S9 2-Hydroxy-1-naphthaldehyde (0.378 g, 2.2 mmol), hydrazine monohydrate (0.100g, 2 mmol), and a catalytic amount of acetic acid (five drops of AcOH) were combined in absolute ethanol (30 mL). The solution was stirred under reflux for 4 h. After cooling to room temperature, the yellow precipitate was filtered, washed three times with hot absolute ethanol, then recrystallized with EtOH/DMF to give a luminous yellow powder product S9 (0.588 g) in 85% yield (mp>300 C), IR: (KBr, cm1) n: 3419 (OH), 3143 (C] NH), 3065 (ArH), 1620 (C]N), 1600 (C]C), 1577 (C]C), 1465 (C] C), 1312 (CeO). 1H NMR (DMSO-d6, 400 MHz): d 12.88 (s, 2H, ArOH), 10.00 (s, 2H, N]CH), 8.66e7.28 (m, 12H, ArH); 13C NMR (DMSO-d6, 100 MHz): d 151.73, 148.60, 145.97, 143.09, 131.07, 130.62, 130.02, 122.46, 117.99, 112.63, 110.69. Anal. Calcd for C22H16N2O2: C 77.62, H 4.70, N 8.23. Found: C, 77.63; H, 4.67; N, 8.25. ESI-MS: m/z calcd for C22H16N2O2, [MH]þ¼339.2, found [MH]¼339.9.
2.6. Determination of association constant The association constants (Ka) of S9 and CN were determined based on the absorbance titration curve using the equation as follows: where A1 and A0 represent the absorbance of host in the presence and absence of ions, respectively, Amax is the saturated absorbance of host in the presence of excess amount of ions; [G] is the concentration of CN added.
# " 1 1 1 ¼ þ1 Amax A0 A1 A0 k½G2 3. Results and discussion In this work, a chemosensor (S9) for CN bearing naphthol and imine group was designed and synthesized, as show in Scheme 1. Sensor S9 has been characterized by 1H NMR, 13C NMR, IR, ESI-MS, and elemental analyses. The structure of S9 was further confirmed by X-ray crystallography, as shown in Fig. 1. A summary of the crystallographic data and structural refinements for S9 and the hydrogen-bonding interactions in S9 are listed in Fig. 1, Tables S1 and S2 (see Supplementary data).
2.3. General procedure for UVevis experiments All the UVevis experiments were carried out in aqueous solution (0.01 M HEPES buffer, pH 7.24, 90% DMSO) on a Shimadzu UV2550 spectrometer. Any changes in the UVevis spectra of the synthesized compound were recorded on addition of tetrabutylammonium salts while keeping the ligand concentration constant (2.0105 M) in all experiments. Tetrabutylammonium salt of an ions (F, Cl, Br, I, AcO, H2 PO 4 , CN , HSO4 , SCN , and ClO4 ) were used for the UVevis experiments.
Fig. 1. The single crystal X-ray structure of S9.
Considering a report in the literature,4 sensor S9 would interact with anions by reversible covalent bonding, which is activated by
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an intramolecular hydrogen-bond to form adduct B1 and less efficiently by direct H-bonding to form species B2 (Scheme 2). Upon anion binding on its hydrazone receptor in chromogenic probe S9, the degree of internal charge transfer ICT is expected to be dependent less on the anion’s H-bonding ability, but may be dependent on the anion’s hydrazone carbon affinity. Hence, the usual guest selectivity pattern dependent on the anion’s H-bonding ability and/or basicity may be altered. In aqueous media, the anions with a weak H-bonding acceptor but with a strong carbon hydrazone affinity are expected to be differentiated from the other anions characterized by a strong H-bonding acceptor, particularly in aqueous media, because weak solvation is expected in the former case.
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(0.01 M HEPES buffer, pH 7.24, 90% DMSO) are shown in Fig. S2. The absorption band at 410 nm decreased, while a new red-shifted band at 500 nm appeared. Other competing anions, such as F, Cl, Br, I, AcO, H2 PO 4 , CN , SCN , HSO4 , and ClO4 (50 equiv) are completely nonresponsive (Fig. 4). No interference of fluoride and acetate anions should arise from their high solvation energies in water as stated above. Only CN, which is a poor H-bond acceptor but it has a strong affinity toward the unsaturated carbon, can add to sensor S9, because of sensor S9 would interact with CN by reversible covalent bonding, which is activated by an intramolecular hydrogen-bond to form adduct A1 and less efficiently by direct Hbonding to form species A2. The blue shift of absorption of species (from 523 nm to 500 nm) formed by interaction of sensor S9 with
Scheme 2. Proposed sensing mechanism of sensor S9 for the detection of cyanide.
When the chemosensors based on the H-bonding interaction are tested with different anions such as F, Cl, Br, I, AcO, H2PO 4 , CN , SCN , HSO4 , and ClO4 , a strongly H-bonding F (pKa w3.15), AcO (pKa w4.76) or a relatively basic CN (pKa w9.2) can shift the equilibrium to the right side, forming specie B1 showing the most significant color change, as already observed in the literature.11 The color change may be reduced or negligible if the same experiment is carried out in aqueous media, due to the strong hydration effect, because of the high salvation energies for F (DHhyd¼505 kJ/mol) and AcO (DHhyd¼375 kJ/mol) in water. Indeed, when sensor S9 was treated with selected anions, such as CN, F and CH3COO (50 equiv) in DMSO, a new strong peak at 523 nm appeared along with a concomitant decrease at 410 nm. Other anions, such as Cl, Br, I, H2 PO 4 ; and HSO4 showed little change to sensor S9 as seen in Fig. 2. This result implied that the sensing of anions was probably performed via the H-bonding of anions to the OH of S9. This was further confirmed by the 1H NMR spectrum of S9. 1H NMR titration displayed the chemical shift changes of S9 upon the addition of CN, as shown in Fig. 3, sensor S9 showed a single peak at 12.88 ppm in DMSO-d6, which was confirmed to the proton of OH, after the addition of cyanide, the OH proton disappeared while all the aromatic protons exhibited an upfield shift to different extent, which suggest the increase in the electron density in naphthalene ring through charge delocalization in the conjugated system.15 This supports the deprotonation phenomena of S9. An even more striking feature results when the sensing experiment is carried out in aqueous media. The absorption spectral changes of sensor S9 on addition of cyanide in an aqueous solution
Fig. 2. UVevis absorbance spectra of S9 (20 mM) upon addition of different anions (50 equiv) in DMSO.
CN in aqueous media compared with that in DMSO also supported the nucleophilic addition mechanism as mentioned in Scheme 2. This highly selective detection of cyanide could be demonstrated even in the presence of other anions in the solution of DMSO/H2O (0.01M HEPES buffer, pH 7.24, 70% DMSO), as shown in Fig. 5.
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Fig. 3. Partial 1H NMR spectra of S9 (3 mg, DMSO-d6) and in the presence of varying amounts of CN.
Fig. 5. Absorbance spectra of S9 (20 mM) in the presence of various anions (50 equiv) in DMSO/H2O (8:2, v/v, containing 0.01 M HEPES, pH¼7.24) in response to CN (50 equiv).
to the coumarin-based pep* transition. On binding to the CN, the red-shifted and enhanced new band at 500 nm could be attributed to the adduct-based intramolecular proton transfer band. The variation of the absorbance at 500 nm was used to evaluate the binding constants of S9 with CN by assuming a 1:2 binding stoichiometry. The BenesieeHildebrand12 analysis of these changes gave a binding constant of 1.303107 M2 (R2: 0.997), see Fig. S3. Nice fittings supported the 1:2 binding stoichiometry.13
Fig. 4. (a) UVevis absorbance spectra of S9 (20 mM) upon addition of different anions (50 equiv) in DMSO/H2O (9:1, v/v, containing 0.01 M HEPES, pH¼7.24). Inset: color changes of S9 after addition of various anions (50 equiv). (b) Normalized of UVevis absorbance spectra of S9 (20 mM) upon addition of different anions.
To further investigate the interaction between sensor S9 and CN, UVevis absorption spectral variation of sensor S9 was monitored during titration with different concentrations of CN. As shown in Fig. 6, a sharp decrease in the UVevis absorption at 410 nm was observed after adding of CN, and then the absorption at 500 nm increased gradually. The isosbestic points at 320, 367, and 445 nm can be clearly observed with increasing concentrations of CN. Simultaneous appearance of isosbestic points signified the presence of two different species that remained in equilibrium. Absorption bands at 410 nm for sensor S9 was attributed primarily
Fig. 6. UVevis spectra of S9 (20 mM) in DMSO/H2O (9:1, v/v, containing 0.01 M HEPES, pH¼7.24) upon adding of an increasing concentration of CN.
Compound S9 alone displays a weak, single fluorescence emission band at 588 nm when excited at 500 nm, because the electron cloud of the naphthalene group was reduced by an electronwithdrawing imine group, which induced fluorescence quenching. The addition of CN to the aqueous solution (0.01 M HEPES buffer, pH 7.24, 90% DMSO) of S9 led to a prominent fluorescence enhancement (Fig. S4), the fluorescence enhancement can be explained by intramolecular H-bonding stabilization of a cyanide adduct, through which an ICT process (from aminemethyl to naphthalene group) can be improved. However, other anions, such as F, Cl, Br, I, AcO, H2 PO 4 , CN , SCN , HSO4 , and ClO4 did not
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cause any significant changes in the fluorescence emission intensity. The fluorescence profiles at 588 nm showed a high selectivity for cyanide over other various anions (Fig. 7). A large increase in the fluorescence intensity could be distinguished visually as shown in Fig. 7a, where photos of solutions of S9 in the absence and presence of CN under UV light (365 nm) illumination are displayed, this also could be applied to the detection of cyanide anions by the naked eye. This highly selective fluorescent detection of cyanide could be demonstrated even in the presence of other anions in the solution of DMSO/H2O (0.01 M HEPES buffer, pH 7.24, 70% DMSO), as shown in Fig. S5.
Fig. 8. Fluorescence titration spectra of S9 (20 mM) in DMSO/H2O (9:1, v/v, containing 0.01 M HEPES, pH¼7.24) upon adding of an increasing concentration of CN (lex¼498 nm).
S9/CN adduct (Fig. 9), from the ESI-MS spectrum, we can also see an obvious peak at m/z 392.1 assignable to [S9þ2CN] (m/ zcalcd¼392.2). The recognition mechanism of the sensor S9 with CN was also certified by FTIR spectroscopy (Fig. S7). Compared with S9 the S9/CN adduct showed a new peak at 2252 cm1, when 2 equiv of CN was added, this also indicated the nucleophilic addition of CN, the peaks of OH and NH at 3602 cm1 and 3532 cm1 can be explained by intramolecular H-bonding stabilization of a cyanide adduct, through an ICT process. These spectra indicate that the cyanide anion was indeed added to the carbon of imine groups of sensor S9.
Fig. 7. (a) Fluorescence spectra response of S9 (20 mM) upon addition of different anions (50 equiv) in DMSO/H2O (9:1, v/v, containing 0.01 M HEPES, pH¼7.24). Inset: photograph of S9 (20 mM) upon adding 50 equiv of anions, which was taken under an UV-lamp (365 nm). (b) Normalized of fluorescence absorbance spectra of S9 (20 mM) upon addition of different anions.
Fig. 8 shows the fluorescence changes of S9 after addition of different amounts of CN. As anticipated, the fluorescence started to increase gradually after addition of CN. For instance, the fluorescence intensity increased by more than 200 times when the concentration of CN reached 20.0 mM in the solution of S9. On the basis of the 1:2 stoichiometry and fluorescence titration data, the binding constant of S9 for CN was calculated to be 1.368107 M2 (R2: 0.997), see Fig. S6. In the meantime, the detection limit of the fluorescence spectra changes calculated on the basis of 3SB/S was 0.4 mM for CN anion in aqueous solution,14 which is far lower than the WHO guideline of 1.9 mM cyanide. As expected nucleophilic attack on the unsaturated bond groups of sensor S9 should satisfy 1:2 probe and cyanide stoichiometry. The result obtained from a Job’s plot supports the formation of a 1:2
Fig. 9. The Job’s plot examined between CN and S9, indicating the 1:2 stoichiometry.
4. Conclusion In conclusion, we have presented a facile, low-cost, and efficient Schiff base example of a highly selective chemosensor for CN through reversible covalent bonding between an ionophore and an anion. The sensor gives an immediate response to the cyanide ion both by visible color changes as well as fluorescence turn-on response. Cyanide anions are detectable by its nucleophilic attack toward the imine groups and the mechanism of the reaction was investigated by various means (such as, 1H NMR spectroscopic
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titration, FTIR spectra, and mass spectral analysis). A plausible mechanism of the sensing reaction sequence is presented, which is shown to operate by means of a nucleophilic addition adduct formation under ambient conditions of pH and temperature in aqueous solution.
5.
6.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21064006, 21262032, and 21161018), the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (No. IRT1177), the Natural Science Foundation of Gansu Province (No. 1010RJZA018), the Youth Foundation of Gansu Province (No. 2011GS04735), and NWNU-LKQN-11-32.
7. 8. 9.
Supplementary data 10.
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