- Email: [email protected]
A selective colorimetric fluorescent chemosensor for Hg2+ in aqueous medium and in the solid state
Journal of Luminescence 194 (2018) 279–283
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
A selective colorimetric fluorescent chemosensor for Hg2+ in aqueous medium and in the solid state Hsuan-Ju Tsaia, Yu-chun Sua, Chin-Feng Wanb, An-Tai Wua, a b
MARK
⁎
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan Department of Medical Applied Chemistry, Chung Shan Medical University, No.110, Section 1, Jianguo N. Rd., Taichung City 40201, Taiwan
A R T I C L E I N F O
A B S T R A C T
Keywords: Colorimetric Fluorescence Turn-on Rhodamine
A simple colorimetric and turn-on fluorescence receptor RN (Rhodamine-based derivative) was prepared and its cation-sensing properties were investigated. Receptor RN displayed a selective sensing of Hg2+ and showed an obvious colorimetric changing in aqueous solution (from colorless to pink color) and in the solid state (from red to pink color), respectively. The association constant of RN-Hg2+ complex was calculated to be 8.25×109 M−2, and the detection limit for Hg2+ was found to be 27 ppb.
1. Introduction The design and synthesis of new chemosensors for the efficient detection of trace metal ions are among the most important research topics in environmental chemistry and biology [1–32]. In particular, mercury is recognized as a significant environmental pollutant having ability to accumulate in plants, soil, and water. Ionic mercury is converted to a highly potent neurotoxin methyl mercury by certain bacteria in the marine environment. Methyl mercury toxins are further passed on to the food chain and bioaccumulate in human [33–35]. Accordingly, it is imperative to develop analytical methods for sensitive and selective detection of trace amounts of mercury ion [36,37]. Although a number of chemosensors specific for Hg2+ ion have been developed [38–44]; however, most of them required complicated synthesis, slow response, fluorescence quenching upon Hg2+ coordination and are aqueous unsolvable. In addition, only few fluorescent sensors could serve as ‘naked-eye’ indicators and were capable of detecting Hg2+ in aqueous solution. Therefore, for practical applications it is a great challenge to design aqueous soluble fluorescent sensors that provide a naked-eye detection and turn-on response of Hg2+. These properties are great advantages for the applications in the analysis of environmental sources and biological systems. The rhodamine family have emerged as the most effective functional groups for fluorescence signaling due to their excellent spectroscopic properties, high fluorescence quantum yield, large extinction coefficient and high stability to light [45–49]. Therefore, several rhodamine-based sensor for metal ions, such as Cu2+ [50–54], Hg2+ [55–61] have been studied. Generally, most of the sensors are suitable for detecting metal ions in solution samples but are rarely applicable to solid ⁎
samples. Herein, we synthesized a rhodamine-hydrazone derivative (receptor RN), receptor RN demonstrates highly selective colorimetric and fluorescent recognition toward Hg2+. Specially, it shows obvious color change in the presence of Hg2+ both in the solution and solid state. 2. Experimental 2.1. Apparatus and reagents All reagents were obtained from commercial suppliers and were used without further purification. 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 were given in ppm with residual THF 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 Fluorescene spectrophotometer. 3. Results and discussion 3.1. Synthesis and characterization Receptor RN was synthesized as shown in Scheme 1. The structure of receptor RN was confirmed by NMR spectra, Mass data (Figs. S1–S3). Rhodamine-hydrazone acetaldehyde (0.5 g, 1.0 mol) was dissolved in dry ethanol (30 mL). 4-Nitrobenzoic hydrazide (0.217 g, 0.0012 mol)
Corresponding author. E-mail address: [email protected] (A.-T. Wu).
http://dx.doi.org/10.1016/j.jlumin.2017.10.023 Received 16 August 2017; Received in revised form 5 October 2017; Accepted 10 October 2017 Available online 11 October 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Journal of Luminescence 194 (2018) 279–283
H.-J. Tsai et al.
Scheme 1. Synthesis of receptor RN.
1.5
Hg
1.4
2+
5000
1.3 1.2 4000
host and other cations
1.0
Intensity (a.u.)
Absorbance (AU)
1.1 0.9 0.8 0.7 0.6
Cu
0.5
2+
3000
2000
0.4 1000
0.3
host and other cations
0.2 0.1 0.0 250
0 350
300
350
400
450
500
550
600
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Fig. 3. Fluorescence emission spectra (λex. = 319 nm) of receptor RN (20 μM) in the presence of 5.0 equiv. of various Metal ion in 90% THF/H2O.
Fig. 1. UV/vis spectra of receptor RN (20 μM) recorded in 90% THF/H2O after addition of 5.0 equiv. of various metal ion.
275 and 325 nm, respectively. In the presence of Cu2+, the absorption spectra of receptor RN in THF/H2O (9:1, v/v) solution showed a red shift. However, in the presence of Hg2+, the solution of receptor RN changed from colorless to pink which could easily be detected with naked eye (Fig. 2).
was added, and the mixture was stirred for 5 h at 80 °C. Ethanol was evaporated completely under reduced pressure, the crude product was column chromatographed on silica-gel (elution with hexanes/ EtOAc=2:1, v/v) to give RN as light yellow solid (0.6 g, 84%). M.p.: 206 °C; 1H NMR (d-THF, 300 MHz) δ:8.28-8.24 (m, 3 H), 8.02-7.99 (m, 3 H), 7.88 (d, J = 6.6 Hz, 1 H), 7.51-7.46 (m, 2 H), 7.08-7.05 (m, 1 H), 6.46-6.42 (m, 4 H), 6.31-6.27 (m, 2 H), 3.37 (d, J = 7.2 Hz, 8 H), 1.16 (t, J = 6.6 Hz, 12 H); 13C NMR (d-DMSO, 75 MHz) δ:164.3, 158.4, 152.1, 148.7, 147.0, 143.1, 134.6, 133.8, 128.9, 127.3, 126.9, 123.7, 123.3, 119.0, 117.1, 116.6, 108.2, 104.2, 97.5, 65.0, 43.7, 12.4; HRMS (EI): m/z cald for C37H37N7O5 [M+], 659.2856, found: 659.2851.
3.3. Fluorescence studies of receptor RN toward various metal ions The effect of Hg2+ on the fluorescence properties of receptor RN was also investigated in THF/H2O (9:1, v/v) solution. As shown in Fig. 3, the free receptor RN displayed very weak single fluorescence emission band at 400 nm. Upon addition of Hg2+, receptor RN exhibited a prominent fluorescence enhancement with a red shift relative to the band of 400 nm (Fig. 4). The fluorescent enhancement efficiency observed at 570 nm was 16 fold greater than that of the control in the absence of Hg2+ (Fig. 5). In the absence of Hg2+, receptor RN exists in the spirocyclic form which is colorless and non-fluorescent. Addition of Hg2+ leads the spirocycle unit open via coordination, resulting in color change and generation of strong fluorescence. The result showed that
3.2. Absorption studies of receptor RN toward various metal ions The UV–vis spectra of receptor RN were studied in the presence of various metal ions (as perchlorate salts): Li+, Na+, K+, Ca2+, Mn2+, Hg2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Pb2+, Cd2+, Zn2+ and Al3+. As shown in Fig. 1, receptor RN showed two major absorption bands at
Fig. 2. The color changes observed by naked eye of receptor RN (20 μM) upon addition of 5.0 equiv. of various Metal ion in 90% THF/H2O. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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H.-J. Tsai et al.
580 nm
5000
Intensity (a.u.)
4000
Fig. 6. The color changes observed by naked eye in solid state of (a) free host and (b) host + Hg2+. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3000
2000
principle of rhodamine spirolactam, as well our experimental results, we propose that the complex formation of RN-Hg2+ may induce the spirocycle opening and further chelate with Hg2+, causing the releasing of the color. The possible mechanism is shown in Scheme 2.
1000
0 HOSTZn
Pb
Ni
Na Mn Mg
Li
K
Hg Fe
Fe Cu Co Cd Ca
3.4. Application in real samples
Al
Fig. 4. Variation of the fluorescence intensity at 540 nm (λex. = 319 nm) of receptor RN (40 μM) in the presence of 5.0 equiv. of various cations in 90% THF/H2O.
For practical applications, the real-time determination was also important. The time evolution of receptor RN in the presence of 5.0 equiv of Hg2+ in THF/H2O (9:1, v/v) solution was investigated. As shown in Fig. S6, the recognition interaction was almost completed after 20 min after addition of the Hg2+. It means the receptor RN could be used as a real-time Hg2+ chemosensor. Another interesting feature of receptor RN under investigation is that it is able to exhibit dramatic color change with the Hg2+ (in the form of Hg(ClO4)2) when simply ground in the solid state (Fig. 6). Also, other common cations do not produce any observable color change in the solid state. This selectivity would definitely render receptor RN as an excellent chemosensor for Hg2+ in solid samples in real life situations. This provides an effective and convenient tool for tracking Hg2+. The practical application of receptor RN for selective sensing of Hg2+ in different source of water such as running, lake, ground, tap, and ditch water have also been demonstrated. Hg2+ was dissolved in different source of water (40 μM). Addition of receptor RN into each water sample clearly showed color changes from colorless to pink by naked eye (Fig. 7). The result showed that receptor RN is a sensitive chemosensor and could be applied in different source of water analysis.
4000
Intensity (a.u.)
20 eq 3000
2000
0 eq
1000
0 350
400
450
500
550
600
650
Wavelength (nm) Fig. 5. Fluorescence spectra of receptor RN (20 μM) in 90% THF/H2O upon addition of increasing concentrations of Hg2+.
3.5. Reversible and competition experiments To examine the reversibility of receptor RN toward Hg2+ in THF/ H2O (9:1, v/v), a solution of EDTA (20 µM) was added to the complex solution of receptor RN with Hg2+. When EDTA was added to the receptor RN-Hg2+ solution, the intensity of fluorescence at 580 nm was recovered more than 80%. Meanwhile, the color of the solution changed back to the original colorless instantly. These changes were fully reversible as the addition of EDTA reversed the spectroscopic as well as color changes (Fig. S7). These results suggest that probe L has the potential to be fabricated into reversible devices to sense Hg2+. The selectivity toward Hg2+ was further ascertained by the competition experiment. As shown in Fig. S8, receptor RN was treated with 5 eqv. of
receptor RN was a sensitive sensor and could detect Hg2+ by naked eye in aqueous media. A Job plot indicated a 1:2 stoichiometric complexation of receptor RN with Hg2+ ion (Fig. S4). In addition, the formation of 1:2 complex between receptor RN and Hg2+ was further confirmed by the appearance of a peak at m/z 1099.3, assignable to [receptor RN + 2Hg2+ + 2H2O] (Fig. S5). The association constant for receptor RN-Hg2+ was determined as 8.25×109 M−2 by the Hill plot. The detection limit of receptor RN towards Hg2+ was determined to be 27 ppb. The detection limit value is sufficiently low to detect low concentration of the Hg2+. Based on the well-known ring-opening
Scheme 2. The proposed sensing mechanism of receptor RN for Hg2+.
281
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H.-J. Tsai et al.
Fig. 7. The color changes in different sources of water containing Hg2+, observed by naked eyes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. 1H NMR titration plots of receptor RN with Hg2+ in dTHF/D2O (9:1, v/v).
Hg2+ in the presence of other metal ions of the same concentration. Relatively low interference was observed for the detection of Hg2+ in the presence of other ions. Thus receptor RN can be used as a selective fluorescent sensor toward Hg2+ in the presence of most competing ions.
Appendix A. Supporting information
3.6. 1H NMR titration and IR Experiments
References
To better understand the complexation behavior of receptor RN with Hg2+, 1H NMR experiments were carried out in THF/D2O (9:1, v/ v). The spectral differences are depicted in Fig. 8. With the addition of Hg2+, the protons of aromatic rings showed a slight difference. Most of the aromatic protons displayed downfield shift compared to those of receptor RN alone. Obvious changes of the chemical shifts indicated that receptor RN could form a stable complex with Hg2+. In order to elucidate the formation of the RN–Hg2+ complex, we also performed the IR experiments of receptor RN and receptor RN–Hg2+ complex (Fig. S9). The typical IR spectra are shown in Fig. S8. The IR spectrum of the receptor RN indicated a sharp peak at 1614 cm−1, which was assigned to C˭N stretching of imine group. In addition, receptor RN displayed a C˭O and NH stretching band in the 1691 and 3350 cm−1 region. The IR spectrum of RN-Hg2+ complex exhibited a broad peak at 1610 cm−1 (C˭N stretching band) and the doublet NH stretching band in the 3525 cm−1 region. The result showed the possibility that Hg2+ induced the spiro ring opening of the receptor RN.
[1] A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T. Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515–1566. [2] V.K. Gupta, S. Chandra, R. Mangla, Electrochim. Acta 47 (2002) 1579. [3] R. Jain, V.K. Gupta, N. Jadon, K. Radhapyari, Anal. Biochem. 407 (2010) 79. [4] V.K. Gupta, M.R. Ganjali, P. Norouzi, H. Khani, A. Nayak, S. Agarwal, Crit. Rev. Anal. Chem. 41 (2011) 282. [5] V.K. Gupta, A.K. Jain, Pankaj Kumar, Sens. Actuators B 120 (2006) 259. [6] V.K. Guptu, S. Juin, U. Khuranu, Electroanalysis 9 (1997) 478. [7] A.K. Jain, V.K. Gupta, L.P. Singh, J.R. Raisoni, Electrochim. Acta 51 (2006) 2547. [8] V.K. Gupta, A.K. Singh, M. Al Khayat, B. Gupta, Anal. Chim. Acta 590 (2007) 81. [9] V.K. Gupta, S. Jain, S. Chandra, Anal. Chim. Acta 486 (2003) 199. [10] A.K. Jain, V.K. Gupta, L.P. Singh, U. Khurana, Analyst 122 (1997) 583. [11] V.K. Gupta, L.P. Singh, R. Singh, N. Upadhyay, S.P. Kaur, B. Sethi, J. Mol. Liq. 174 (2012) 11. [12] V.K. Gupta, A.K. Jain, G. Maheshwari, Talanta 72 (2007) 1469. [13] V.K. Gupta, A.K. Jain, S. Agarwal, G. Maheshwari, Talanta 71 (2007) 1964. [14] V.K. Gupta, A.K. Jain, L.P. Singh, U. Khurana, Anal. Chim. Acta 355 (1997) 33. [15] S.K. Srivastava, V.K. Gupta, S. Jain, Analyst 120 (1995) 495. [16] V.K. Gupta, R.N. Goyal, R.A. Sharma, Talanta 76 (2008) 859. [17] V.K. Gupta, N. Mergu, L.K. Kumawat, A.K. Singh, Talanta 144 (2015) 80. [18] V.K. Gupta, A.K. Singh, L.K. Kumawat, Sens. Actuators B 195 (2014) 98. [19] S.K. Srivastava, V.K. Gupta, S. Jain, Anal. Chem. 68 (1996) 1272. [20] V.K. Gupta, A.K. Jain, G. Maheshwari, H. Lang, Z. Ishtaiwi, Sens. Actuators B 117 (2006) 99. [21] V.K. Gupta, A.K. Jain, P. Kumar, Sens. Actuators B 120 (2006) 259. [22] V.K. Gupta, A.K. Jain, P. Kumar, S. Agarwal, G. Maheshwari, Sens. Actuators B 113 (2006) 182. [23] R.N. Goyal, V.K. Gupta, S. Chatterjee, Sens. Actuators B 149 (2010) 252. [24] V.K. Gupta, N. Mergu, L.K. Kumawat, A.K. Singh, Sens. Actuators B 207 (2015) 216. [25] V.K. Gupta, A.K. Singh, S. Mehtab, B. Gupta, Anal. Chim. Acta 566 (2006) 5. [26] V.K. Gupta, R. Prasad, P. Kumar, R. Mangla, Anal. Chim. Acta 420 (2000) 19. [27] R. Prasad, V.K. Gupta, A. Kumar, Anal. Chim. Acta 61 (2004) 61. [28] V.K. Gupta, R. Mangla, U. Khurana, P. Kumar, Electroanalysis 11 (1999) 573. [29] V.K. Gupta, S. Jain, U. Khurana, Electroanalysis 9 (1997) 478. [30] A.K. Jain, V.K. Gupta, U. Khurana, L.P. Singh, Electroanalysis 9 (1997) 857. [31] R. Jain, V.K. Gupta, N. Jadon, K. Radhapyari, Anal. Biochem. 407 (2010) 79. [32] V.K. Gupta, B. Sethi, R.A. Sharma, S. Agarwal, A. Bharti, J. Mol. Liq. 177 (2013) 114. [33] G.E. McKeown-Eyssen, J. Ruedy, A. Neims, Am. J. Epidemol. 118 (1983) 470. [34] P.W. Davidson, G.J. Myers, C. Cox, C.F. Shamlaye, D.O. Marsh, M.A. Tanner,
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2017.10.023.
4. Conclusion In summary, we have successfully synthesized a simple, fluorescent and colorimetric receptor RN. The receptor RN showed a high selectivity for Hg2+ in aqueous solution and in the solid state. The fluorescent and colorimetric response to Hg2+ can be easily detected by naked eye, thereby providing a simple method for detecting Hg2+ in aqueous solution and in the solid state. Acknowledgement Ministry of Science and Technology, R.O.C. (grant numbers: 1042113-M-018-002). 282
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283
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
A selective colorimetric fluorescent chemosensor for Hg2+ in aqueous medium and in the solid state Hsuan-Ju Tsaia, Yu-chun Sua, Chin-Feng Wanb, An-Tai Wua, a b
MARK
⁎
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan Department of Medical Applied Chemistry, Chung Shan Medical University, No.110, Section 1, Jianguo N. Rd., Taichung City 40201, Taiwan
A R T I C L E I N F O
A B S T R A C T
Keywords: Colorimetric Fluorescence Turn-on Rhodamine
A simple colorimetric and turn-on fluorescence receptor RN (Rhodamine-based derivative) was prepared and its cation-sensing properties were investigated. Receptor RN displayed a selective sensing of Hg2+ and showed an obvious colorimetric changing in aqueous solution (from colorless to pink color) and in the solid state (from red to pink color), respectively. The association constant of RN-Hg2+ complex was calculated to be 8.25×109 M−2, and the detection limit for Hg2+ was found to be 27 ppb.
1. Introduction The design and synthesis of new chemosensors for the efficient detection of trace metal ions are among the most important research topics in environmental chemistry and biology [1–32]. In particular, mercury is recognized as a significant environmental pollutant having ability to accumulate in plants, soil, and water. Ionic mercury is converted to a highly potent neurotoxin methyl mercury by certain bacteria in the marine environment. Methyl mercury toxins are further passed on to the food chain and bioaccumulate in human [33–35]. Accordingly, it is imperative to develop analytical methods for sensitive and selective detection of trace amounts of mercury ion [36,37]. Although a number of chemosensors specific for Hg2+ ion have been developed [38–44]; however, most of them required complicated synthesis, slow response, fluorescence quenching upon Hg2+ coordination and are aqueous unsolvable. In addition, only few fluorescent sensors could serve as ‘naked-eye’ indicators and were capable of detecting Hg2+ in aqueous solution. Therefore, for practical applications it is a great challenge to design aqueous soluble fluorescent sensors that provide a naked-eye detection and turn-on response of Hg2+. These properties are great advantages for the applications in the analysis of environmental sources and biological systems. The rhodamine family have emerged as the most effective functional groups for fluorescence signaling due to their excellent spectroscopic properties, high fluorescence quantum yield, large extinction coefficient and high stability to light [45–49]. Therefore, several rhodamine-based sensor for metal ions, such as Cu2+ [50–54], Hg2+ [55–61] have been studied. Generally, most of the sensors are suitable for detecting metal ions in solution samples but are rarely applicable to solid ⁎
samples. Herein, we synthesized a rhodamine-hydrazone derivative (receptor RN), receptor RN demonstrates highly selective colorimetric and fluorescent recognition toward Hg2+. Specially, it shows obvious color change in the presence of Hg2+ both in the solution and solid state. 2. Experimental 2.1. Apparatus and reagents All reagents were obtained from commercial suppliers and were used without further purification. 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 were given in ppm with residual THF 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 Fluorescene spectrophotometer. 3. Results and discussion 3.1. Synthesis and characterization Receptor RN was synthesized as shown in Scheme 1. The structure of receptor RN was confirmed by NMR spectra, Mass data (Figs. S1–S3). Rhodamine-hydrazone acetaldehyde (0.5 g, 1.0 mol) was dissolved in dry ethanol (30 mL). 4-Nitrobenzoic hydrazide (0.217 g, 0.0012 mol)
Corresponding author. E-mail address: [email protected] (A.-T. Wu).
http://dx.doi.org/10.1016/j.jlumin.2017.10.023 Received 16 August 2017; Received in revised form 5 October 2017; Accepted 10 October 2017 Available online 11 October 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Journal of Luminescence 194 (2018) 279–283
H.-J. Tsai et al.
Scheme 1. Synthesis of receptor RN.
1.5
Hg
1.4
2+
5000
1.3 1.2 4000
host and other cations
1.0
Intensity (a.u.)
Absorbance (AU)
1.1 0.9 0.8 0.7 0.6
Cu
0.5
2+
3000
2000
0.4 1000
0.3
host and other cations
0.2 0.1 0.0 250
0 350
300
350
400
450
500
550
600
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Fig. 3. Fluorescence emission spectra (λex. = 319 nm) of receptor RN (20 μM) in the presence of 5.0 equiv. of various Metal ion in 90% THF/H2O.
Fig. 1. UV/vis spectra of receptor RN (20 μM) recorded in 90% THF/H2O after addition of 5.0 equiv. of various metal ion.
275 and 325 nm, respectively. In the presence of Cu2+, the absorption spectra of receptor RN in THF/H2O (9:1, v/v) solution showed a red shift. However, in the presence of Hg2+, the solution of receptor RN changed from colorless to pink which could easily be detected with naked eye (Fig. 2).
was added, and the mixture was stirred for 5 h at 80 °C. Ethanol was evaporated completely under reduced pressure, the crude product was column chromatographed on silica-gel (elution with hexanes/ EtOAc=2:1, v/v) to give RN as light yellow solid (0.6 g, 84%). M.p.: 206 °C; 1H NMR (d-THF, 300 MHz) δ:8.28-8.24 (m, 3 H), 8.02-7.99 (m, 3 H), 7.88 (d, J = 6.6 Hz, 1 H), 7.51-7.46 (m, 2 H), 7.08-7.05 (m, 1 H), 6.46-6.42 (m, 4 H), 6.31-6.27 (m, 2 H), 3.37 (d, J = 7.2 Hz, 8 H), 1.16 (t, J = 6.6 Hz, 12 H); 13C NMR (d-DMSO, 75 MHz) δ:164.3, 158.4, 152.1, 148.7, 147.0, 143.1, 134.6, 133.8, 128.9, 127.3, 126.9, 123.7, 123.3, 119.0, 117.1, 116.6, 108.2, 104.2, 97.5, 65.0, 43.7, 12.4; HRMS (EI): m/z cald for C37H37N7O5 [M+], 659.2856, found: 659.2851.
3.3. Fluorescence studies of receptor RN toward various metal ions The effect of Hg2+ on the fluorescence properties of receptor RN was also investigated in THF/H2O (9:1, v/v) solution. As shown in Fig. 3, the free receptor RN displayed very weak single fluorescence emission band at 400 nm. Upon addition of Hg2+, receptor RN exhibited a prominent fluorescence enhancement with a red shift relative to the band of 400 nm (Fig. 4). The fluorescent enhancement efficiency observed at 570 nm was 16 fold greater than that of the control in the absence of Hg2+ (Fig. 5). In the absence of Hg2+, receptor RN exists in the spirocyclic form which is colorless and non-fluorescent. Addition of Hg2+ leads the spirocycle unit open via coordination, resulting in color change and generation of strong fluorescence. The result showed that
3.2. Absorption studies of receptor RN toward various metal ions The UV–vis spectra of receptor RN were studied in the presence of various metal ions (as perchlorate salts): Li+, Na+, K+, Ca2+, Mn2+, Hg2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Pb2+, Cd2+, Zn2+ and Al3+. As shown in Fig. 1, receptor RN showed two major absorption bands at
Fig. 2. The color changes observed by naked eye of receptor RN (20 μM) upon addition of 5.0 equiv. of various Metal ion in 90% THF/H2O. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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580 nm
5000
Intensity (a.u.)
4000
Fig. 6. The color changes observed by naked eye in solid state of (a) free host and (b) host + Hg2+. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3000
2000
principle of rhodamine spirolactam, as well our experimental results, we propose that the complex formation of RN-Hg2+ may induce the spirocycle opening and further chelate with Hg2+, causing the releasing of the color. The possible mechanism is shown in Scheme 2.
1000
0 HOSTZn
Pb
Ni
Na Mn Mg
Li
K
Hg Fe
Fe Cu Co Cd Ca
3.4. Application in real samples
Al
Fig. 4. Variation of the fluorescence intensity at 540 nm (λex. = 319 nm) of receptor RN (40 μM) in the presence of 5.0 equiv. of various cations in 90% THF/H2O.
For practical applications, the real-time determination was also important. The time evolution of receptor RN in the presence of 5.0 equiv of Hg2+ in THF/H2O (9:1, v/v) solution was investigated. As shown in Fig. S6, the recognition interaction was almost completed after 20 min after addition of the Hg2+. It means the receptor RN could be used as a real-time Hg2+ chemosensor. Another interesting feature of receptor RN under investigation is that it is able to exhibit dramatic color change with the Hg2+ (in the form of Hg(ClO4)2) when simply ground in the solid state (Fig. 6). Also, other common cations do not produce any observable color change in the solid state. This selectivity would definitely render receptor RN as an excellent chemosensor for Hg2+ in solid samples in real life situations. This provides an effective and convenient tool for tracking Hg2+. The practical application of receptor RN for selective sensing of Hg2+ in different source of water such as running, lake, ground, tap, and ditch water have also been demonstrated. Hg2+ was dissolved in different source of water (40 μM). Addition of receptor RN into each water sample clearly showed color changes from colorless to pink by naked eye (Fig. 7). The result showed that receptor RN is a sensitive chemosensor and could be applied in different source of water analysis.
4000
Intensity (a.u.)
20 eq 3000
2000
0 eq
1000
0 350
400
450
500
550
600
650
Wavelength (nm) Fig. 5. Fluorescence spectra of receptor RN (20 μM) in 90% THF/H2O upon addition of increasing concentrations of Hg2+.
3.5. Reversible and competition experiments To examine the reversibility of receptor RN toward Hg2+ in THF/ H2O (9:1, v/v), a solution of EDTA (20 µM) was added to the complex solution of receptor RN with Hg2+. When EDTA was added to the receptor RN-Hg2+ solution, the intensity of fluorescence at 580 nm was recovered more than 80%. Meanwhile, the color of the solution changed back to the original colorless instantly. These changes were fully reversible as the addition of EDTA reversed the spectroscopic as well as color changes (Fig. S7). These results suggest that probe L has the potential to be fabricated into reversible devices to sense Hg2+. The selectivity toward Hg2+ was further ascertained by the competition experiment. As shown in Fig. S8, receptor RN was treated with 5 eqv. of
receptor RN was a sensitive sensor and could detect Hg2+ by naked eye in aqueous media. A Job plot indicated a 1:2 stoichiometric complexation of receptor RN with Hg2+ ion (Fig. S4). In addition, the formation of 1:2 complex between receptor RN and Hg2+ was further confirmed by the appearance of a peak at m/z 1099.3, assignable to [receptor RN + 2Hg2+ + 2H2O] (Fig. S5). The association constant for receptor RN-Hg2+ was determined as 8.25×109 M−2 by the Hill plot. The detection limit of receptor RN towards Hg2+ was determined to be 27 ppb. The detection limit value is sufficiently low to detect low concentration of the Hg2+. Based on the well-known ring-opening
Scheme 2. The proposed sensing mechanism of receptor RN for Hg2+.
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Fig. 7. The color changes in different sources of water containing Hg2+, observed by naked eyes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. 1H NMR titration plots of receptor RN with Hg2+ in dTHF/D2O (9:1, v/v).
Hg2+ in the presence of other metal ions of the same concentration. Relatively low interference was observed for the detection of Hg2+ in the presence of other ions. Thus receptor RN can be used as a selective fluorescent sensor toward Hg2+ in the presence of most competing ions.
Appendix A. Supporting information
3.6. 1H NMR titration and IR Experiments
References
To better understand the complexation behavior of receptor RN with Hg2+, 1H NMR experiments were carried out in THF/D2O (9:1, v/ v). The spectral differences are depicted in Fig. 8. With the addition of Hg2+, the protons of aromatic rings showed a slight difference. Most of the aromatic protons displayed downfield shift compared to those of receptor RN alone. Obvious changes of the chemical shifts indicated that receptor RN could form a stable complex with Hg2+. In order to elucidate the formation of the RN–Hg2+ complex, we also performed the IR experiments of receptor RN and receptor RN–Hg2+ complex (Fig. S9). The typical IR spectra are shown in Fig. S8. The IR spectrum of the receptor RN indicated a sharp peak at 1614 cm−1, which was assigned to C˭N stretching of imine group. In addition, receptor RN displayed a C˭O and NH stretching band in the 1691 and 3350 cm−1 region. The IR spectrum of RN-Hg2+ complex exhibited a broad peak at 1610 cm−1 (C˭N stretching band) and the doublet NH stretching band in the 3525 cm−1 region. The result showed the possibility that Hg2+ induced the spiro ring opening of the receptor RN.
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Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2017.10.023.
4. Conclusion In summary, we have successfully synthesized a simple, fluorescent and colorimetric receptor RN. The receptor RN showed a high selectivity for Hg2+ in aqueous solution and in the solid state. The fluorescent and colorimetric response to Hg2+ can be easily detected by naked eye, thereby providing a simple method for detecting Hg2+ in aqueous solution and in the solid state. Acknowledgement Ministry of Science and Technology, R.O.C. (grant numbers: 1042113-M-018-002). 282
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