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A new multifunctional Schiff base as a fluorescence sensor for Fe2+ and F− ions, and a colorimetric sensor for Fe3+
Journal of Luminescence 178 (2016) 115–120
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
A new multifunctional Schiff base as a fluorescence sensor for Fe2 þ and F ions, and a colorimetric sensor for Fe3 þ Chin-Feng Wan b, Ya-Ju Chang a, Chih-Yu Chien a, Yi-Wun Sie a, Ching-Han Hu a, An-Tai Wu a,n a b
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan School of Medical Applied Chemistry, Chung Shan Medical University, Taichung City 40201, Taiwan
art ic l e i nf o
a b s t r a c t
Article history: Received 12 April 2016 Received in revised form 17 May 2016 Accepted 24 May 2016 Available online 1 June 2016
A multifunctional Schiff base fluorescent sensor (receptor L) was prepared and its metal ions and anions sensing properties were investigated. Receptor L exhibited an excellent selective fluorescence response toward Fe2 þ and F . It also showed colorimetric response (from colorless to yellow) toward Fe3 þ among a series of ions. Moreover, the detection limits of receptor L for Fe2 þ and F were determined to be 0.3 ppm and 25.7 ppb, respectively. The two detection limit values were sufficiently low to detect nanomolar concentration of Fe2 þ and F . & 2016 Published by Elsevier B.V.
Keywords: Chemosensor Fluorescence Turn-on Colorimetric
1. Introduction Development of new receptors for the detection of different analytes simultaneously is emerging as an area of great interest [1–4], since such system would lead to faster analytical processing and potential cost reductions. However most of the reported sensors are effective in selective recognition of a particular analyte only [5–11]. Since the recognition units of the sensors are distinctive in sensing cations or anions behavior, thus, developing such sensors with multiple analyst recognition capability is a challenging task. Among various important analytes, iron is the most abundant transition-metal ion. Both Fe2 þ and Fe3 þ play important roles in various biological systems [12]. Its deficiency leads to anemia, liver and kidney damages, diabetes, and heart diseases [13–15]. Therefore, detection of Fe2 þ or Fe3 þ is crucial in controlling its concentration levels in the biosphere and its direct impact on human health. However, many of the reported Fe2 þ sensors suffer from disturbance by Fe3 þ . Up to know, there are only few reports where sensors could allow selective detection of Fe2 þ and Fe3 þ simultaneously by distinct color changes. In addition, due to the paramagnetic behavior of Fe2 þ and Fe3 þ , that chemosensors showed enhanced fluorescence in the presence of Fe2 þ or Fe3 þ are still scarce [16,17]. The discrimination of Fe2 þ from Fe3 þ is very n
Correspondence to: Fax: þ 886 4 7211 19. E-mail address: [email protected] (A.-T. Wu).
http://dx.doi.org/10.1016/j.jlumin.2016.05.039 0022-2313/& 2016 Published by Elsevier B.V.
important in order to understand the biological functions regulated by iron, because their ferrous/ferric states are one of the important redox pairs in biological systems [18–25]. Therefore, developing sensors capable of determining both Fe2 þ and Fe3 þ is very valuable and desirable. On the other hand, fluoride ions (F ) are significant due to their role in dental care and treatment of osteoporosis [26]. In addition, high concentration of F after a certain level is toxic, leads to severe health risks. F usually forms the strongest H-bond interaction with NH or OH fragment of an artificial receptor. Such features have been observed in few amide-, phenol-, and ureabased receptors containing electron-withdrawing chromogenic substituents. Although such of these type of receptors for F detection have been developed [27–38], but most of them usually require complicated syntheses involving severe reaction conditions and expensive chemicals. Thus, for practical applications, it is necessary to develop the F sensor that can be easily prepared, and possess selective and sensitive signaling mechanisms. As a result, intense research has been focused on the development of sensitive and selective receptors for the qualitative and quantitative recognition of Fe2 þ , Fe3 þ and F . Interestingly, the various reported sensors are quite specific, either for Fe2 þ and F [39], or for Fe2 þ , Fe3 þ and Cu2 þ [40], or for Fe2 þ , Fe3 þ and H2PO4 [41] or for Fe2 þ and Fe3 þ [42,43], but to the best of our knowledge, a fluorescent sensor for the simultaneous detection of Fe2 þ , Fe3 þ and F remains unreported. Herein, the Schiff base fluorescent sensor (receptor L) has been developed for the selective and sensitive detection of Fe2 þ , Fe3 þ and F . Receptor L
116
C.-F. Wan et al. / Journal of Luminescence 178 (2016) 115–120
exhibits high fluorescence sensitivity and selectivity toward Fe2 þ and F among a series of ions and show the naked-eyes detection toward Fe3 þ .
2. Materials and instrumental methods 2.1. Experimental section All reagents were obtained from commercial suppliers and were used without further purification. MeOH was distilled over magnesium and iodine. Analytical thin-layer chromatography was
performed using silica gel 60 F254 plates (Merck). The 1H and 13C NMR spectra were recorded with a Bruker AM 300 spectrometer. Chemical shifts are given in ppm with residual DMSO as reference. Mass spectra were recorded under electron impact (EI) or electron spray interface (ESI) conditions. UV–vis spectra were recorded by using Jasco V630 spectrophotometer with a diode array detector, and the resolution was set at 1 nm. Fluorescence spectra were recorded on a Jasco FP-8300 Fluorescence spectrophotometer. 2.2. Synthesis of compound (E)-2-((pyren-1-ylmethylene)amino) benzenethiol 2-Aminobenzenethiol (300 mg, 2.5 mmol) was added to a stirring solution of pyrene-1-carbaldehyde (500 mg, 2.5 mmol) in dry methanol (100 mL). The reaction mixture was stirred at 70 °C for 12 h. The resulting canary yellow precipitate was filtered and washed with cold methanol. The resulting residue was dried under vacuum to give L as a yellow solid. (86%); mp: 165 °C; H NMR (300 MHz, THF-d8) δ: 6.36 (s, 1H), 6.65 (t, J ¼7.50 Hz, 1H), 6.73 (d, J¼ 3.90 Hz, 1H), 6.96 (t, J¼ 7.2 Hz, 1H), 7.56 (d, J ¼2.7 Hz, 1H), 8.01
Scheme 1. Synthesis of receptor L.
1800 5000
host F Cl Br I HSO4
1600
Fe 1400
Intensity(AU)
Intensity(A.U.)
4000
3000
2000
host and other cations
-
NO3
1000
-
H2 PO4
800
-
CH3COO 600
Fe
1000
1200
NaN3 NaCN
400 200
0
0 400
450
500
550
600
wavelength nm Fig. 1. Fluorescence emission spectra (λex. ¼331 nm) of receptor L (20 μM) in the presence of 10.0 equiv. of various metal ions in MeCN.
400
450
500
550
600
Wavelength(nm) Fig. 4. Fluorescence emission spectra (λex. ¼ 316 nm) of receptor L (20 μM) in the presence of 10.0 equiv. of various anions in MeCN.
Fig. 2. The color changes observed by naked eye of receptor L (20 μM) upon addition of 10.0 equiv. of various metals in CH3CN. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 3. The color changes of receptor L (20 μM) under UV light upon addition of 10.0 equiv. of various metals in CH3CN. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
C.-F. Wan et al. / Journal of Luminescence 178 (2016) 115–120
(t, J ¼7.5 Hz, 1H), 8.09 (s, 2H), 8.22 (m, 4H), 8.39 (d, J¼ 9.6 Hz, 1H), 8.55 (d, J ¼8.1 Hz, 1H); C NMR (75 MHz, THF-d8) δ: 109.90, 120.13, 122.17, 123.26, 125.76, 125.85, 126.17, 126.20, 126.35, 126.94, 127.23, 128.34, 128.63, 128.83, 131.85, 135.28, 132.48, 136.54, 149.12; HRMS (EI): Calcd for C23H15NS (M þ ), m/z 337.0925; found m/z 337.0930. Scheme 1.
3000
20 eq 2500
Intensity (a.u.)
117
2000
0 eq 1500
3. Results and discussion 1000
3.1. Fluorescence and absorption studies of receptor L toward various anions and metal ions
500
0 400
450
500
550
Wavelength (nm) Fig. 5. Fluorescence spectra of receptor L (20 μM) in CH3CN upon addition of increasing concentrations Fe2 þ .
1600 1400 1200
Intensity(AU)
40 eq 1000 800 600
0 eq
400 200 0 400
450
500
550
Wavelength(nm) Fig. 6. Fluorescence spectra of receptor L (20 μM) in CH3CN upon addition of increasing concentrations F .
The fluorescence spectra of receptor L were investigated in CH3CN by the presence of various metal ions (as perchlorate salts): Li þ , Na þ , K þ , Ca2 þ , Mn2 þ , Hg2 þ , Fe2 þ , Fe3 þ , Co2 þ , Ni2 þ , Cu2 þ , Pb2 þ , Cd2 þ , Zn2 þ and Al3 þ . From the fluorescence spectra of receptor L (Fig. 1), receptor L alone and other cations all displayed very weak single fluorescence emission band at 448 nm except for Fe2 þ and Fe3 þ . Upon addition of Fe2 þ , receptor L exhibited a prominent fluorescence enhancement. The fluorescent enhancement efficiency observed at 448 nm was 16 fold greater than that of the control in the absence of Fe2 þ (Fig. S5). However, upon addition of Fe3 þ , receptor L exhibited a weak fluorescence enhancement and accompanied by a red shift of 115 nm from 415 to 530 nm. The fluorescent enhancement efficiency observed at 530 nm was 1 fold greater than that of the control in the absence of Fe3 þ (Fig. S5). In addition, the solution of receptor L with Fe3 þ showed a dramatic color change from colorless to yellow which could easily be detected by the naked eye (Fig. 2). Under UV light, the solution of receptor L with Fe2 þ showed a bright blue color. On the other hand, the solution of receptor L with Fe3 þ showed a bright green color (Fig. 3). The observed fluorescent enhancement may be attributed to the formation of a rigid system after binding with Fe2 þ , causing the chelation-enhanced fluorescence (CHEF) effect [44–46]. The result showed that receptor L was a sensitive sensor and could be discriminating detection between Fe2 þ and Fe3 þ by naked eye in environmental analysis. On the other hand, we also performed the screening of receptor L toward anions (as TBA salts): (F , Cl , Br , I , NO3 , HSO4 ,
Fig. 7. Digital images of Fe2 þ induced turn-on fluorescence in different source of water. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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H2PO4 , AcO , CN , N3 ). From the fluorescence spectra (Fig. 4), receptor L alone and other anions all displayed weak fluorescence emission bands except for F . Upon addition of F , receptor L exhibited a prominent fluorescence enhancement. The fluorescent enhancement efficiency observed at 422 nm was 3.5 fold greater than that of the control in the absence of F (Fig. S6). Meanwhile, the solution of receptor L showed a dramatic color change from dark blue to bright blue which could easily be detected under UV light (Fig. S7). 3.2. Fluorescence titration and binding studies
Fig. 8. Photographs of test paper for detecting F in water with different concentrations. From left to right: (a) 0 ppm (b) 1 ppm (c) 10 ppm (d) 100 ppm (e) 1000 ppm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
From the fluorescence titration profiles (Figs. 5 and 6), the association constants for receptor L-Fe2 þ and L-F in CH3CN were determined as 3.04 109 M 1 and 3.97 106 M 1, respectively, by the Hill equation (Figs. S8 and S9). A Job plot indicated a 1:1 stoichiometric complexation of receptor L with Fe2 þ and F ion (Figs. S10 and S11). In addition, the formation of 1:1 complex between receptor L and Fe2 þ was further confirmed by the appearance of a peak at m/z 608, assignable to [receptor Lþ Fe2 þ þH2O þ2ClO4 ] (Fig. S12). Similarly, for complex between receptor L and F ion a peak at m/z 520 was assignable to [receptor L þF þH2O þCH3CNþ H þ ] (Fig. S13). The detection limit of receptor L for the analysis of Fe2 þ ion was determined as 0.3 ppm, for F ion was determined as 25.7 ppb. 3.3. pH effect and application in real samples
Fig. 9. Fluorescence emission spectra of receptor L in the presence of Fe2 þ (10.0 equiv.) or TMEDA (10.0 equiv.) in MeCN.
To check the pH effect, we also evaluated the effect of pH on the emission bands of receptor L in CH3CN aqueous. It was found that fluorescence intensity of receptor L remained unaffected over a wide pH span of 3–12 (Fig. S14). The result showed that receptor L could be applied in some environmental analysis. For practical applications, the real-time determination was also important. Therefore, the time evolution of receptor L in the presence of 10.0 equiv. of Fe2 þ in CH3CN was investigated. As shown in Figs. S15 and S16, the recognition interaction was almost completed in 1 min and 5 min after addition of the Fe2 þ and F , respectively. The practical application of receptor L for selective sensing of Fe2 þ in different source of water such as lake, ground and tap water has also been demonstrated. Fe2 þ was dissolved in different source of water (300 μM). Addition of receptor L into each sample water clearly showed changing color from blue to bright blue under UV light (Fig. 7). The result showed that receptor L is a sensitive sensor and could be applied in environmental analysis. On the other hand, the indicator paper experiments were done using filter paper coated with the solution of receptor L. When this coated test paper was dropped in F water solution, the color of the paper was changed from purple to light yellow instantly under UV light, as shown in Fig. 8. As the concentration of F increased, the color of the test paper gradually deepened and finally changed to gray. This indicated that the change in color on the test paper was caused by the interaction of receptor L and F , implying that the visually test paper showed specific recognition to F . Thus receptor L can be made into a ready to use paper for detecting the presence of F in aqueous media. 3.4. Reversible and competition experiments
Fig. 10. Fluorescence emission spectra of receptor L in the presence of F (10.0 equiv.) or Ca(NO3)2 (10.0 equiv.) in MeCN.
Reversibility is a prerequisite in developing novel sensor for practical application. To examine the reversibility of receptor L toward Fe2 þ in CH3CN solution of TMEDA (20 mM), was added to the complex solution of receptor L (10 equiv.) and Fe2 þ (10 equiv.). As expected, a fluorescence signal at 420 nm was completely quenched, which indicated the regeneration of the free receptor L (Fig. 9). On the other hand, for F when CH3CN solution of Ca(NO3)2
C.-F. Wan et al. / Journal of Luminescence 178 (2016) 115–120
119
Fig. 11. 1H NMR titration plots of receptor L with Fe2 þ in THF.
Fig. 12. 1H NMR titration plots of receptor L with F in CH3CN.
(40 mM) was added to the complex solution of receptor L (10 equiv.) and F (10 equiv.), as expected, a fluorescence signal at 442 nm was completely quenched, which indicated the regeneration of the free receptor L (Fig. 10). From the results, such reversibility is important for the fabrication of devices to sense the Fe2 þ or F . The selectivity toward Fe2 þ and F was further ascertained by the competition experiment. As shown in Fig. S17 and S18, receptor L was treated with 10 equiv. of Fe2 þ or F in the presence of other metal ions or anions of the same concentration. Relatively low interference was observed for the detection of Fe2 þ or F in the presence of other ions. Thus receptor L can be used as a selective fluorescent sensor toward Fe2 þ or F in the presence of most competing ions. 3.5.
and 12, respectively. Upon addition of Fe2 þ , the protons of benzenethiol rings and imine all displayed downfield shift compared to those of receptor L alone. In addition, the proton of imine displayed two singlet peaks. These observations obviously indicated that the presence of two receptor L isomers. Upon the addition of 1.0 equiv. of F , the imine proton at 8.42 ppm was shifted downfield toward 9.58 ppm. Meanwhile, most of the aromatic protons displayed downfield shift compared to those of receptor L alone. In addition, a sharp peak was appeared at 10.8 ppm. The sharp peak at 10.8 ppm is assigned to the hydroxyl proton of thiol. This might be due to the formation of hydrogen bonds between F and the hydroxyl group of receptor L.
1
H NMR titration experiments 4. Conclusion
To better understand the complexation behavior of receptor L with Fe2 þ or F , 1H NMR experiments were carried out in THF. The spectral differences for Fe2 þ and F are depicted in Figs. 11
In summary, we have successfully designed and synthesized a simple, fluorescent and colorimetric receptor L, capable of
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recognizing both cations and anions in CH3CN solution. Receptor L exhibited an excellent selectivity and sensitivity towards Fe2 þ and F by fluorescent intensity enhancement. In addition, receptor L also showed a good selectivity towards Fe3 þ by inducing a rapid color change from colorless to yellow.
Acknowledgment We thank the National Science Council of Taiwan for financial support (grant number: 104-2113-M-018-002).
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.jlumin.2016.05. 039.
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[13] B.F. Matzanke, G. Muller-Matzanke, K.N. Raymond, in: T.M. Loehr (Ed.), Iron Carriers and Iron Proteins, vol. 5, VCH, New York, 1989. [14] H.B. Gray, J.R. Winkler, Annu. Rev. Biochem. 65 (1996) 537. [15] C.D. Kaplan, J. Kaplan, Chem. Rev. 109 (2009) 4536. [16] I. Grabchev, J.-M. Chevelon, X. Qian, New J. Chem. 27 (2003) 337. [17] S. Gandhi, S. Vijayaraj, C. Duraisamy, J. Lumin. 145 (2014) 480. [18] J. Zhan, L. Wen, F. Miao, D. Tian, X. Zhu, H. Li, New J. Chem. 36 (2012) 656. [19] S.L. Hu, N.F. She, G.D. Yin, H.Z. Guo, A.X. Wu, C.L. Yang, Tetrahedron Lett. 48 (2007) 1591. [20] Y. Xiang, A. Tong, Org. Lett. 8 (2006) 1549. [21] J. Mao, L.N. Wang, W. Dou, X.L. Tang, Y. Yan, W.S. Liu, Org. Lett. 9 (2007) 4567. [22] X.B. Zhang, G. Cheng, W.J. Zhang, G.L. Shen, R.Q. Yu, Talanta 71 (2007) 171. [23] O. Oter, K. Ertekin, C. Kirilmis, M. Koca, M. Ahmedzade, Sens. Actuators B 122 (2007) 450. [24] G.E. Tumambac, C.M. Rosencrance, C. Wolf, Tetrahedron 60 (2004) 11293. [25] J.L. Bricks, A. Kovalchuk, C. Trieflinger, M. Nofz, M. Buschel, A.I. Tolmachev, J. Daub, K. Rurack, J. Am. Chem. Soc. 127 (2005) 13522. [26] Z. Lin, S. Ou, C. Duan, B. Zhang, Z. Bai, Chem. Commun. 6 (2006) 624. [27] T. Zhang, E.V. Anslyn, Org. Lett. 8 (2006) 1649. [28] D.E. Gomez, L. Fabbrizzi, M. Licchelli, J. Org. Chem. 70 (2005) 5717. [29] S. Xu, K.C. Chen, H. Tian, J. Mater. Chem. 15 (2005) 2676. [30] E.J. Cho, B.J. Ryu, Y.J. Lee, K.C. Nam, Org. Lett. 7 (2005) 2607. [31] B. Chetia, P.K. Iyer, Tetrahedron Lett. 49 (2008) 94. [32] E.J. Cho, J.W. Moon, S.W. Ko, J.Y. Lee, S.K. Kim, J. Yoon, K.C. Nam, J. Am. Chem. Soc. 125 (2003) 12376. [33] G.X. Xu, M.A. Tarr, Chem. Commun. 9 (2004) 1050. [34] E.J. Jun, M.K. Swamy, H. Bang, S. Kim, J. Yoon, Tetrahedron Lett. 47 (2006) 3103. [35] Y. Peng, Y.-M. Dong, M. Dong, Y.-W. Wang, J. Org. Chem. 77 (2012) 9072. [36] Y. Zhou, J.F. Zhang, J. Yoon, Chem. Rev. 114 (2014) 5511. [37] S. Gandhi, C. Duraisamy, J. Mater. Chem. B 1 (2013) 5768. [38] A. Thangaraj, S. Gandhi, I. Murugan, S. Ayyanar, C. Duraisamy, Anal. Chim. Acta 876 (2015) 1. [39] B. Chanchal, D. Shyamal, M. Dinesh, B. Sujoy, Dalton Trans. 40 (2011) 11795. [40] N. Jitendra, P. Samadhan, P. Prashant, S. Suban, R. Carl, M. Pramod, P. Umesh, J. Fluoresc. 24 (2014) 1563. [41] S. Gurjaspreet, S. Jandeep, S. Jasbhinder, S.M. Satinderpal, J. Lumin. 165 (2015) 123. [42] Z.Q. Liang, C.X. Wang, J.X. Yang, H.W. Gao, Y.P. Tian, X.T. Tao, M.H. Jiang, New J. Chem. 31 (2007) 906. [43] S. Sen, S. Sarkar, B. Chattopadhyay, A. Moirangthem, A. Basu, K. Dharad, P. Chattopadhyay, Analyst 137 (2012) 3335. [44] M. Shellaiah, Y.-H. Wu, H.-C. Lin, Analyst 138 (2013) 2931. [45] J.-C. Qin, T.-R. Li, B.-D. Wang, Z.-Y. Yang, L. Fan, Spectrochim. Acta Part A 133 (2014) 38. [46] S. Kim, J.Y. Noh, K.Y. Kim, J.H. Kim, H.K. Kang, S.-W. Nam, S.H. Kim, S. Park, C. Kim, J. Kim, Inorg. Chem. 51 (2012) 3597.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
A new multifunctional Schiff base as a fluorescence sensor for Fe2 þ and F ions, and a colorimetric sensor for Fe3 þ Chin-Feng Wan b, Ya-Ju Chang a, Chih-Yu Chien a, Yi-Wun Sie a, Ching-Han Hu a, An-Tai Wu a,n a b
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan School of Medical Applied Chemistry, Chung Shan Medical University, Taichung City 40201, Taiwan
art ic l e i nf o
a b s t r a c t
Article history: Received 12 April 2016 Received in revised form 17 May 2016 Accepted 24 May 2016 Available online 1 June 2016
A multifunctional Schiff base fluorescent sensor (receptor L) was prepared and its metal ions and anions sensing properties were investigated. Receptor L exhibited an excellent selective fluorescence response toward Fe2 þ and F . It also showed colorimetric response (from colorless to yellow) toward Fe3 þ among a series of ions. Moreover, the detection limits of receptor L for Fe2 þ and F were determined to be 0.3 ppm and 25.7 ppb, respectively. The two detection limit values were sufficiently low to detect nanomolar concentration of Fe2 þ and F . & 2016 Published by Elsevier B.V.
Keywords: Chemosensor Fluorescence Turn-on Colorimetric
1. Introduction Development of new receptors for the detection of different analytes simultaneously is emerging as an area of great interest [1–4], since such system would lead to faster analytical processing and potential cost reductions. However most of the reported sensors are effective in selective recognition of a particular analyte only [5–11]. Since the recognition units of the sensors are distinctive in sensing cations or anions behavior, thus, developing such sensors with multiple analyst recognition capability is a challenging task. Among various important analytes, iron is the most abundant transition-metal ion. Both Fe2 þ and Fe3 þ play important roles in various biological systems [12]. Its deficiency leads to anemia, liver and kidney damages, diabetes, and heart diseases [13–15]. Therefore, detection of Fe2 þ or Fe3 þ is crucial in controlling its concentration levels in the biosphere and its direct impact on human health. However, many of the reported Fe2 þ sensors suffer from disturbance by Fe3 þ . Up to know, there are only few reports where sensors could allow selective detection of Fe2 þ and Fe3 þ simultaneously by distinct color changes. In addition, due to the paramagnetic behavior of Fe2 þ and Fe3 þ , that chemosensors showed enhanced fluorescence in the presence of Fe2 þ or Fe3 þ are still scarce [16,17]. The discrimination of Fe2 þ from Fe3 þ is very n
Correspondence to: Fax: þ 886 4 7211 19. E-mail address: [email protected] (A.-T. Wu).
http://dx.doi.org/10.1016/j.jlumin.2016.05.039 0022-2313/& 2016 Published by Elsevier B.V.
important in order to understand the biological functions regulated by iron, because their ferrous/ferric states are one of the important redox pairs in biological systems [18–25]. Therefore, developing sensors capable of determining both Fe2 þ and Fe3 þ is very valuable and desirable. On the other hand, fluoride ions (F ) are significant due to their role in dental care and treatment of osteoporosis [26]. In addition, high concentration of F after a certain level is toxic, leads to severe health risks. F usually forms the strongest H-bond interaction with NH or OH fragment of an artificial receptor. Such features have been observed in few amide-, phenol-, and ureabased receptors containing electron-withdrawing chromogenic substituents. Although such of these type of receptors for F detection have been developed [27–38], but most of them usually require complicated syntheses involving severe reaction conditions and expensive chemicals. Thus, for practical applications, it is necessary to develop the F sensor that can be easily prepared, and possess selective and sensitive signaling mechanisms. As a result, intense research has been focused on the development of sensitive and selective receptors for the qualitative and quantitative recognition of Fe2 þ , Fe3 þ and F . Interestingly, the various reported sensors are quite specific, either for Fe2 þ and F [39], or for Fe2 þ , Fe3 þ and Cu2 þ [40], or for Fe2 þ , Fe3 þ and H2PO4 [41] or for Fe2 þ and Fe3 þ [42,43], but to the best of our knowledge, a fluorescent sensor for the simultaneous detection of Fe2 þ , Fe3 þ and F remains unreported. Herein, the Schiff base fluorescent sensor (receptor L) has been developed for the selective and sensitive detection of Fe2 þ , Fe3 þ and F . Receptor L
116
C.-F. Wan et al. / Journal of Luminescence 178 (2016) 115–120
exhibits high fluorescence sensitivity and selectivity toward Fe2 þ and F among a series of ions and show the naked-eyes detection toward Fe3 þ .
2. Materials and instrumental methods 2.1. Experimental section All reagents were obtained from commercial suppliers and were used without further purification. MeOH was distilled over magnesium and iodine. Analytical thin-layer chromatography was
performed using silica gel 60 F254 plates (Merck). The 1H and 13C NMR spectra were recorded with a Bruker AM 300 spectrometer. Chemical shifts are given in ppm with residual DMSO as reference. Mass spectra were recorded under electron impact (EI) or electron spray interface (ESI) conditions. UV–vis spectra were recorded by using Jasco V630 spectrophotometer with a diode array detector, and the resolution was set at 1 nm. Fluorescence spectra were recorded on a Jasco FP-8300 Fluorescence spectrophotometer. 2.2. Synthesis of compound (E)-2-((pyren-1-ylmethylene)amino) benzenethiol 2-Aminobenzenethiol (300 mg, 2.5 mmol) was added to a stirring solution of pyrene-1-carbaldehyde (500 mg, 2.5 mmol) in dry methanol (100 mL). The reaction mixture was stirred at 70 °C for 12 h. The resulting canary yellow precipitate was filtered and washed with cold methanol. The resulting residue was dried under vacuum to give L as a yellow solid. (86%); mp: 165 °C; H NMR (300 MHz, THF-d8) δ: 6.36 (s, 1H), 6.65 (t, J ¼7.50 Hz, 1H), 6.73 (d, J¼ 3.90 Hz, 1H), 6.96 (t, J¼ 7.2 Hz, 1H), 7.56 (d, J ¼2.7 Hz, 1H), 8.01
Scheme 1. Synthesis of receptor L.
1800 5000
host F Cl Br I HSO4
1600
Fe 1400
Intensity(AU)
Intensity(A.U.)
4000
3000
2000
host and other cations
-
NO3
1000
-
H2 PO4
800
-
CH3COO 600
Fe
1000
1200
NaN3 NaCN
400 200
0
0 400
450
500
550
600
wavelength nm Fig. 1. Fluorescence emission spectra (λex. ¼331 nm) of receptor L (20 μM) in the presence of 10.0 equiv. of various metal ions in MeCN.
400
450
500
550
600
Wavelength(nm) Fig. 4. Fluorescence emission spectra (λex. ¼ 316 nm) of receptor L (20 μM) in the presence of 10.0 equiv. of various anions in MeCN.
Fig. 2. The color changes observed by naked eye of receptor L (20 μM) upon addition of 10.0 equiv. of various metals in CH3CN. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 3. The color changes of receptor L (20 μM) under UV light upon addition of 10.0 equiv. of various metals in CH3CN. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
C.-F. Wan et al. / Journal of Luminescence 178 (2016) 115–120
(t, J ¼7.5 Hz, 1H), 8.09 (s, 2H), 8.22 (m, 4H), 8.39 (d, J¼ 9.6 Hz, 1H), 8.55 (d, J ¼8.1 Hz, 1H); C NMR (75 MHz, THF-d8) δ: 109.90, 120.13, 122.17, 123.26, 125.76, 125.85, 126.17, 126.20, 126.35, 126.94, 127.23, 128.34, 128.63, 128.83, 131.85, 135.28, 132.48, 136.54, 149.12; HRMS (EI): Calcd for C23H15NS (M þ ), m/z 337.0925; found m/z 337.0930. Scheme 1.
3000
20 eq 2500
Intensity (a.u.)
117
2000
0 eq 1500
3. Results and discussion 1000
3.1. Fluorescence and absorption studies of receptor L toward various anions and metal ions
500
0 400
450
500
550
Wavelength (nm) Fig. 5. Fluorescence spectra of receptor L (20 μM) in CH3CN upon addition of increasing concentrations Fe2 þ .
1600 1400 1200
Intensity(AU)
40 eq 1000 800 600
0 eq
400 200 0 400
450
500
550
Wavelength(nm) Fig. 6. Fluorescence spectra of receptor L (20 μM) in CH3CN upon addition of increasing concentrations F .
The fluorescence spectra of receptor L were investigated in CH3CN by the presence of various metal ions (as perchlorate salts): Li þ , Na þ , K þ , Ca2 þ , Mn2 þ , Hg2 þ , Fe2 þ , Fe3 þ , Co2 þ , Ni2 þ , Cu2 þ , Pb2 þ , Cd2 þ , Zn2 þ and Al3 þ . From the fluorescence spectra of receptor L (Fig. 1), receptor L alone and other cations all displayed very weak single fluorescence emission band at 448 nm except for Fe2 þ and Fe3 þ . Upon addition of Fe2 þ , receptor L exhibited a prominent fluorescence enhancement. The fluorescent enhancement efficiency observed at 448 nm was 16 fold greater than that of the control in the absence of Fe2 þ (Fig. S5). However, upon addition of Fe3 þ , receptor L exhibited a weak fluorescence enhancement and accompanied by a red shift of 115 nm from 415 to 530 nm. The fluorescent enhancement efficiency observed at 530 nm was 1 fold greater than that of the control in the absence of Fe3 þ (Fig. S5). In addition, the solution of receptor L with Fe3 þ showed a dramatic color change from colorless to yellow which could easily be detected by the naked eye (Fig. 2). Under UV light, the solution of receptor L with Fe2 þ showed a bright blue color. On the other hand, the solution of receptor L with Fe3 þ showed a bright green color (Fig. 3). The observed fluorescent enhancement may be attributed to the formation of a rigid system after binding with Fe2 þ , causing the chelation-enhanced fluorescence (CHEF) effect [44–46]. The result showed that receptor L was a sensitive sensor and could be discriminating detection between Fe2 þ and Fe3 þ by naked eye in environmental analysis. On the other hand, we also performed the screening of receptor L toward anions (as TBA salts): (F , Cl , Br , I , NO3 , HSO4 ,
Fig. 7. Digital images of Fe2 þ induced turn-on fluorescence in different source of water. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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H2PO4 , AcO , CN , N3 ). From the fluorescence spectra (Fig. 4), receptor L alone and other anions all displayed weak fluorescence emission bands except for F . Upon addition of F , receptor L exhibited a prominent fluorescence enhancement. The fluorescent enhancement efficiency observed at 422 nm was 3.5 fold greater than that of the control in the absence of F (Fig. S6). Meanwhile, the solution of receptor L showed a dramatic color change from dark blue to bright blue which could easily be detected under UV light (Fig. S7). 3.2. Fluorescence titration and binding studies
Fig. 8. Photographs of test paper for detecting F in water with different concentrations. From left to right: (a) 0 ppm (b) 1 ppm (c) 10 ppm (d) 100 ppm (e) 1000 ppm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
From the fluorescence titration profiles (Figs. 5 and 6), the association constants for receptor L-Fe2 þ and L-F in CH3CN were determined as 3.04 109 M 1 and 3.97 106 M 1, respectively, by the Hill equation (Figs. S8 and S9). A Job plot indicated a 1:1 stoichiometric complexation of receptor L with Fe2 þ and F ion (Figs. S10 and S11). In addition, the formation of 1:1 complex between receptor L and Fe2 þ was further confirmed by the appearance of a peak at m/z 608, assignable to [receptor Lþ Fe2 þ þH2O þ2ClO4 ] (Fig. S12). Similarly, for complex between receptor L and F ion a peak at m/z 520 was assignable to [receptor L þF þH2O þCH3CNþ H þ ] (Fig. S13). The detection limit of receptor L for the analysis of Fe2 þ ion was determined as 0.3 ppm, for F ion was determined as 25.7 ppb. 3.3. pH effect and application in real samples
Fig. 9. Fluorescence emission spectra of receptor L in the presence of Fe2 þ (10.0 equiv.) or TMEDA (10.0 equiv.) in MeCN.
To check the pH effect, we also evaluated the effect of pH on the emission bands of receptor L in CH3CN aqueous. It was found that fluorescence intensity of receptor L remained unaffected over a wide pH span of 3–12 (Fig. S14). The result showed that receptor L could be applied in some environmental analysis. For practical applications, the real-time determination was also important. Therefore, the time evolution of receptor L in the presence of 10.0 equiv. of Fe2 þ in CH3CN was investigated. As shown in Figs. S15 and S16, the recognition interaction was almost completed in 1 min and 5 min after addition of the Fe2 þ and F , respectively. The practical application of receptor L for selective sensing of Fe2 þ in different source of water such as lake, ground and tap water has also been demonstrated. Fe2 þ was dissolved in different source of water (300 μM). Addition of receptor L into each sample water clearly showed changing color from blue to bright blue under UV light (Fig. 7). The result showed that receptor L is a sensitive sensor and could be applied in environmental analysis. On the other hand, the indicator paper experiments were done using filter paper coated with the solution of receptor L. When this coated test paper was dropped in F water solution, the color of the paper was changed from purple to light yellow instantly under UV light, as shown in Fig. 8. As the concentration of F increased, the color of the test paper gradually deepened and finally changed to gray. This indicated that the change in color on the test paper was caused by the interaction of receptor L and F , implying that the visually test paper showed specific recognition to F . Thus receptor L can be made into a ready to use paper for detecting the presence of F in aqueous media. 3.4. Reversible and competition experiments
Fig. 10. Fluorescence emission spectra of receptor L in the presence of F (10.0 equiv.) or Ca(NO3)2 (10.0 equiv.) in MeCN.
Reversibility is a prerequisite in developing novel sensor for practical application. To examine the reversibility of receptor L toward Fe2 þ in CH3CN solution of TMEDA (20 mM), was added to the complex solution of receptor L (10 equiv.) and Fe2 þ (10 equiv.). As expected, a fluorescence signal at 420 nm was completely quenched, which indicated the regeneration of the free receptor L (Fig. 9). On the other hand, for F when CH3CN solution of Ca(NO3)2
C.-F. Wan et al. / Journal of Luminescence 178 (2016) 115–120
119
Fig. 11. 1H NMR titration plots of receptor L with Fe2 þ in THF.
Fig. 12. 1H NMR titration plots of receptor L with F in CH3CN.
(40 mM) was added to the complex solution of receptor L (10 equiv.) and F (10 equiv.), as expected, a fluorescence signal at 442 nm was completely quenched, which indicated the regeneration of the free receptor L (Fig. 10). From the results, such reversibility is important for the fabrication of devices to sense the Fe2 þ or F . The selectivity toward Fe2 þ and F was further ascertained by the competition experiment. As shown in Fig. S17 and S18, receptor L was treated with 10 equiv. of Fe2 þ or F in the presence of other metal ions or anions of the same concentration. Relatively low interference was observed for the detection of Fe2 þ or F in the presence of other ions. Thus receptor L can be used as a selective fluorescent sensor toward Fe2 þ or F in the presence of most competing ions. 3.5.
and 12, respectively. Upon addition of Fe2 þ , the protons of benzenethiol rings and imine all displayed downfield shift compared to those of receptor L alone. In addition, the proton of imine displayed two singlet peaks. These observations obviously indicated that the presence of two receptor L isomers. Upon the addition of 1.0 equiv. of F , the imine proton at 8.42 ppm was shifted downfield toward 9.58 ppm. Meanwhile, most of the aromatic protons displayed downfield shift compared to those of receptor L alone. In addition, a sharp peak was appeared at 10.8 ppm. The sharp peak at 10.8 ppm is assigned to the hydroxyl proton of thiol. This might be due to the formation of hydrogen bonds between F and the hydroxyl group of receptor L.
1
H NMR titration experiments 4. Conclusion
To better understand the complexation behavior of receptor L with Fe2 þ or F , 1H NMR experiments were carried out in THF. The spectral differences for Fe2 þ and F are depicted in Figs. 11
In summary, we have successfully designed and synthesized a simple, fluorescent and colorimetric receptor L, capable of
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recognizing both cations and anions in CH3CN solution. Receptor L exhibited an excellent selectivity and sensitivity towards Fe2 þ and F by fluorescent intensity enhancement. In addition, receptor L also showed a good selectivity towards Fe3 þ by inducing a rapid color change from colorless to yellow.
Acknowledgment We thank the National Science Council of Taiwan for financial support (grant number: 104-2113-M-018-002).
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.jlumin.2016.05. 039.
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