2+ in Aqueous Solution;kpubs;kpubs.org" /> 2+ in Aqueous Solution">
Advanced
A Simple Carbazole-based Schiff Base as Fluorescence "off-on" Probe for Highly Selective Recognition of Cu<sup>2+</sup> in Aqueous Solution
A Simple Carbazole-based Schiff Base as Fluorescence "off-on" Probe for Highly Selective Recognition of Cu2+ in Aqueous Solution
Bulletin of the Korean Chemical Society. 2014. Aug, 35(8): 2326-2330
Copyright © 2014, Korea Chemical Society
  • Received : April 04, 2014
  • Accepted : April 14, 2014
  • Published : August 20, 2014
Download
PDF
e-PUB
PubReader
PPT
Export by style
Article
Author
Metrics
Cited by
TagCloud
About the Authors
Lijun Tang
Di Wu
Shuhua Hou
Xin Wen
Xin Dai

Abstract
A carbazole-based Schiff base CB2 was synthesized and applied as a highly selective and sensitive fluorescent probe for Cu 2+ in H 2 O-DMSO (8/2, v/v, pH = 7.4) solution. CB2 exhibits an excellent selectivity to Cu 2+ over other examined metal ions with a prominent fluorescence “turn-on” at 475 nm. CB2 and Cu 2+ forms a 1:2 binding ratio complex with detection limit of 9.5 μM. In addition, the Cu 2+ recognition process is hardly interfered by other examined metal ions.
Keywords
Introduction
Selective detection of Cu 2+ has received considerable attention because it is not only an environmental pollutant at high concentrations 1 but also an essential trace element for many biological processes and systems. 2 Cu 2+ is the third most abundant metal after iron and zinc in human body, 3 it played pivotal biological roles as cofactor of many proteins, or as catalyst in oxido-reduction reactions. 4 However, excess Cu 2+ in human body has been reported to cause serious disease such as prion disease, 5 several neurodegenerative diseases including Menkes and Wilson diseases, 6 Alzheimer’s and Parkinson’s diseases. 7 In our daily lives, Cu 2+ is generated and accumulated in the environment and food chain with the development of various industries such as electroplating, wood, painting and paper industries. The limit of copper in drinking water is 1.3 ppm (~20 μM) set by US Environmental Protection Agency (EPA). Therefore, the development of fluorescence emission “off-on” probes for Cu 2+ sensing in aqueous media with high selectivity and sensitivity is still imperative.
To date, many excellent works associated with colorimetric or fluorescent probes for Cu 2+ sensing have been documented. 8 However, most of them displayed fluorescence “on-off” response to Cu 2+ owing to the paramagnetic nature of Cu 2+ . Recently, a number of fluorescence “off-on” Cu 2+ probes based on fluorophores such as rhodamine, 9 coumarin, 10 and 1,8-naphthalimide 11 have been reported. It is noteworthy that effective fluorescent probes derived from carbazole fluorophore are still rare.
Carbazole is a conjugated unit with interesting optical and electronic properties. A number of carbazole derivatives have been synthesized and employed as fluorescent probes for the recognition of Cd 2+ , 12 Hg 2+ , 13 and Pb 2+ 14 ions. Very recently, we have reported a carbazole-based fluorescent probe CB1 for Cu 2+ sensing in CH 3 CN-H 2 O (9:1, v/v) solution. 15 However, the Cu 2+ recognition by CB1 suffers from the influence of Co 2+ . Inspired by the previous work, in this work, we designed and synthesized a new carbazole-based fluorescent probe CB2 , which contains two picolinohydrazide imines as Cu 2+ chelators. Probe CB2 exhibits fluorescence “off-on” response to Cu 2+ in H 2 O-DMSO (8:2, v/v, pH = 7.4) solution with high selectivity and sensitivity.
PPT Slide
Lager Image
Synthesis of probe CB2 and the structures of CB1 and CB3.
Expermental
Reagents and Instruments. Unless otherwise stated, solvents and reagents were analytical grade and used without further purification. Doubly distilled water was used for spectral detection. 9-Ethyl-9 H -carbazole-3,6-dicarboxaldehyde ( 1 ) 16 was prepared by the literature method. 1 H NMR and 13 C NMR spectra were recorded on Agilent 400-MR spectrometer, chemical shifts ( δ ) were expressed in ppm and coupling constants ( J ) in Hertz. High-resolution mass spectroscopy (HRMS) was measured on a Bruker micrOTOF-Q mass spectrometer (Bruker Daltonik, Bremen, Germany). Low-resolution mass spectroscopy (LRMS) was measured on an Agilent 1100 series LC/MSD mass spectrometer. Fluorescence measurements were performed on a Sanco 970-CRT spectrofluorometer (Shanghai, China). The pH values were measured with a Model PHS-25B meter (Shanghai Dapu instruments Co., Ltd., China)
The salts used in stock solutions of metal ions are Ni(NO 3 ) 2 ·6H 2 O, Hg(NO 3 ) 2 , Ba(NO 3 ) 2 , Mg(NO 3 ) 2 ·6H 2 O, AgNO 3 , FeSO 4 ·7H 2 O, KNO 3 , NaCl, CaC l2 ·6H 2 O, Al(NO 3 ) 3 ·9H 2 O, Mn(NO 3 ) 2 , Pb(NO 3 ) 2 , Sr(NO 3 ) 2 , Cu(NO 3 ) 2 ·3H 2 O, Co(NO 3 ) 2 ·6H 2 O, Zn(NO 3 ) 2 ·6H 2 O, Cd(NO 3 ) 2 , CrCl 3 ·6H 2 O, Fe(NO 3 ) 3 ·9H 2 O, respectively.
- Synthetic Procedure.
Synthesis of CB2: A mixture of compound 1 (251 mg, 1 mmol) and picolinohydrazide ( 2 , 240 mg, 2.5 mmol) in 50 mL ethanol was stirred and heated at reflux for 4 h. After cooling to room temperature, the precipitates formed were collected and purified by silica gel column chromatography to give CB2 as yellow solids. Yield: 75%. mp > 250 ℃. 1 H NMR (400 MHz, DMSO- d 6 ) δ12.11 (s, 2H), 8.82 (s, 2H), 8.74 (d, J = 4.4 Hz, 2H), 8.55 (s, 2H), 8.16 (d, J = 7.6 Hz, 2H), 8.07 (t, J = 6.8 Hz, 2H), 7.98 (d, J = 8.0 Hz, 2H), 7.76 (d, J = 8.8 Hz, 2H), 7.68 (dd, J 1 = 6.8 Hz, J 2 = 4.8 Hz, 2H), 4.52 (dd, J = 7.2 Hz, 2H), 1.38 (t, J = 7.2 Hz, 3H). 13 C NMR (100 MHz, DMSO- d 6 ) δ160.63, 150.59, 150.21, 148.97, 141.55, 138.45, 129.80, 127.33, 126.44, 125.17, 123.05, 122.73, 121.48, 110.56, 37.89, 14.32. HRMS (ESI+), calcd for C 28 H 24 N 7 O 2 [M+H] + 490.1991, found 490.1981.
Synthesis of CB3: Control compound CB3 was prepared following the similar procedures as depicted for CB2 with the exception of benzoylhydrazine was used. Yield: 80%. mp 159-160 ℃. 1 H NMR (400 MHz, DMSO- d 6 ) δ 11.90 (s, 2H), 8.62 (s, 2H), 8.53 (s, 2H), 7.94 (m, 6H), 7.70 (d, J = 8.8 Hz, 2H), 7.54 (m, 6H), 4.46 (dd, 7.2 Hz, 2H), 1.32 (t, J = 7.2 Hz, 3H). 13 C NMR (100 MHz, DMSO- d 6 ) δ 163.48, 149.39, 141.55, 134.12, 132.06, 128.89, 128.14, 126.51, 125.20, 122.84, 121.34, 110.54, 14.33. LRMS (ESI+), calcd for C 28 H 24 N 7 O 2 [M+H] + 488, found 488.
General Spectroscopic Methods. Probe CB2 was dissolved in H 2 O-DMSO (8/2, v/v, pH = 7.4) to afford the test solution (10 μM). Titration experiments were carried out in 10-mm quartz curvettes at 25 ℃. Stock solutions of metal ions (as chloride or nitrate salts, 10 mM) were added to the host solution and used for the titration experiment. The excitation wavelength is 313 nm.
Results and Discussion
Probe CB2 was prepared by the condensation of 1 and 2 in absolute ethanol and was structurally characterized by 1 H NMR, 13 C NMR and HRMS spectroscopy. Then, the metal ion selectivity of CB2 was investigated in H 2 O-DMSO (8:2, v/v, pH = 7.4) solution ( Fig. 1 ). Free CB2 solution (10 μM) exhibited a very weak fluorescence emission at 475 nm, this may attributed to the C=N isomerization in the excited state. On addition of 2.0 equiv. of Cu 2+ , a remarkable fluorescence enhancement at 475 nm was observed. This fluorescence enhancement can be mainly attributed to the inhibition of C=N isomerization after binding with Cu 2+ . The chelation of CB2 with Cu 2+ also can induce increasing in rigidity of the receptor and leads to chelation-enhanced fluorescence (CHEF) of CB2 . Whereas, addition of other transition and heavy metal ions including Ag + , Ba 2+ , Ca 2+ , Co 2+ , Cd 2+ , K + , Mg 2+ , Mn 2+ , Na + , Ni 2+ , Pb 2+ , Sr 2+ , Zn 2+ , Hg 2+ , Fe 3+ , Cr 3+ , Al 3+ , and Fe 2+ promoted no significant fluorescence enhancement, which led to greatly improved selectivity over the previous probe CB1 . Additionally, the fluorescence color changes of CB2 before and after addition of Cu 2+ was naked eye detectable ( Fig. 1 , inset). These results demonstrate that CB2 has an excellent selectivity toward Cu 2+ over other interested metal ions.
PPT Slide
Lager Image
Fluorescence spectra of CB2 (10 μM) in H2O-DMSO (8/ 2, v/v, pH = 7.4) on addition of various metal ions. Inset: Fluorescence color changes of CB2 before and after addition of Cu2+.
Besides the high selectivity of probe to the target metal ion, its anti-interference ability to other potential competitive metal ions is also important. Thus, the competition experiments were subsequently carried out. As shown in Figure 2 , except Cu 2+ , other tested metal ions (each cation was used as 2.0 equiv. to CB2 ) did not induce significant fluorescence changes. Nevertheless, a dramatic fluorescence emission enhancement was observed on further addition 2.0 equiv. of Cu 2+ to the above competitive metal ion containing solutions. These results show that the Cu 2+ recognition event by CB2 has a good anti-influence ability to other potential competitive metal ions.
PPT Slide
Lager Image
Fluorescence intensity of CB2 (10 μM) in H2O-DMSO (8/2, v/v, pH = 7.4) at 475 nm. The black bars represent the emission intensity of CB2 in the presence of 2.0 equiv. of competing metal ion; the red bars represent the emission intensity of the above solution upon addition of 2.0 equiv. of Cu2+.
To further investigate the interaction of probe CB2 and Cu 2+ , fluorescence titration experiment was conducted ( Fig. 3 ). Upon addition of increasing amounts of Cu 2+ ions, the fluorescence emission band centered at 475 nm gradually increased. The maximum emission intensity was obtained when 2.0 equiv. of Cu 2+ ions was employed. Based on the titration data, the detection limit of CB2 to Cu 2+ was estimated to be 9.5 μM ( Fig. 4 ), 17 indicating that CB2 is sufficiently sensitive to detect Cu 2+ concentration in some aqueous system.
PPT Slide
Lager Image
Fluorescence spectra changes of CB2 (10 μM) in H2O-DMSO (8/2, v/v, pH = 7.4) upon addition of Cu2+ (0 to 2.0 equiv.).
PPT Slide
Lager Image
Normalized fluorescence intensity of CB2 (10 μM) as a function of Log[Cu2+]. λem = 475 nm.
To get insight into the binding ratio of probe CB2 with Cu 2+ , the Job’s plot was examined with a total concentration of CB2 and Cu 2+ as 20 μM. An emission turning point was observed when the molar fraction of Cu 2+ is about 0.67, suggesting the formation of a 2:1 binding stoichiometry between Cu 2+ and probe CB2 ( Fig. 5 ). Linear fitting of the titration profiles using Benesi-Hildebrand plot based on a 2:1 binding mode results in a good linearity (correlation coefficient is over 0.99) ( Fig. 6 ), which also strongly support the 2:1 binding stoicheiometry of Cu 2+ and CB2 , and the apparent association constant ( K a ) is evaluated to be 3.7 × 10 9 M −2 .
PPT Slide
Lager Image
Job’s plot showing the 2:1 binding of Cu2+ to CB2.
PPT Slide
Lager Image
Benesi-Hildebrand plot CB2 assuming 1:2 stoicheiometry with Cu2+. λem = 475 nm.
For practical application, the fluorescence intensity (at 475nm) of free probe CB2 at different pH values were explored ( Fig. 7 ). The results showed that CB2 displayed weak fluorescence emission from pH 5.0 to 11.0. Under strong acidic (pH < 5.0) and strong basic (pH > 11) conditions, the solutions displayed greatly enhanced fluorescence emissions. These results demonstrate that CB2 can be used for Cu 2+ recognition in a wide pH range.
PPT Slide
Lager Image
Influences of pH on the fluorescence intensity (at 475 nm) of CB2 (10 μM) in H2O-DMSO (8/2, v/v) solution.
To validate the binding function of pyridine nitrogen atom in CB2 , a carbazole based Schiff-base CB3 was prepared as a control compound, and its fluorescence responses to Cu 2+ was also examined. Compound CB3 (10 μM) in H 2 O-DMSO (8/2, v/v, pH = 7.4) solution emitted a relative weak fluorescence. In the presence of Cu 2+ , the fluorescence emission was greatly quenched rather than increased ( Fig. 8 ). This response of CB3 to Cu 2+ is quite different from that of CB2 , suggesting the pyridine moieties in CB2 are requisite for its fluorescence “off-on” Cu 2+ selectivity. The proposed binding mode of CB2 and Cu 2+ is illustrated in Scheme 2 .
PPT Slide
Lager Image
Different fluorescence responses of probe CB2 and control compound CB3 to Cu2+.
PPT Slide
Lager Image
roposed bind mode of CB2 with Cu2+.
Conclusions
In summary, we have developed a simple carbazole-based Schiff base CB2 as a fluorescence “turn-on” probe for Cu 2+ recognition in aqueous solution. Probe CB2 displayed an extremely high selectivity toward Cu 2+ over other interested metal ions with a 1:2 binding ratio. The low detection limit of CB2 to Cu 2+ makes it have a potential applicability in real water sample Cu 2+ detection.
Acknowledgements
We are grateful to the NSFC (No. 21176029), the Natural Science Foundation of Liaoning Province (No. 20102004) and the Program for Liaoning Excellent Talents in University (LJQ2012096) for financial support.
References
High B. , Bruce D. , Richter M. M. 2001 Anal. Chim. Acta 449 17 - 22    DOI : 10.1016/S0003-2670(01)01357-5
Waggoner D. J. , Bartnikas T. B. , Gitlin J. D. 1999 Neurobiol. Dis. 6 221 - 230    DOI : 10.1006/nbdi.1999.0250
Boal A. K. , Rosenzweig A. C. 2009 Chem. Rev. 109 4760 - 4779    DOI : 10.1021/cr900104z
Mei Y. , Bentley P. A. , Wang W. 2006 Tetrahedron Lett. 47 2447 - 2449    DOI : 10.1016/j.tetlet.2006.01.091
Chandrasekhar V. , Bag P. , Pandey M. D. 2009 Tetrahedron 65 9876 - 9883    DOI : 10.1016/j.tet.2009.09.040
Zou Y. , Wan M. , Sang G. , Ye M. , Li Y. 2008 Adv. Funct. Mater. 18 2724 - 2732    DOI : 10.1002/adfm.200800567
Güney O. , Yılmaz Y. , Pekcan Ö. 2002 Sens. Actuators, B 85 86 - 89    DOI : 10.1016/S0925-4005(02)00057-6
Tang L. , Zhao G. , Wang N. 2013 Chem. Res. Appl. 25 946 - 951
Zhang Y. , Wang J. , Jia P. , Yu X. , Liu H. , Liu X. , Zhao N. , Huang B. 2010 Org. Biomol. Chem. 8 4582 - 4588    DOI : 10.1039/c0ob00030b
Lin W. , Yuan L. , Cao Z. , Feng Y. , Long L. 2009 Chem.-Eur. J. 15 5096 - 5103    DOI : 10.1002/chem.200802751