- Email: [email protected]
A rigid conjugated pyridinylthiazole derivative and its nanoparticles for divalent copper fluorescent sensing in aqueous media
Tetrahedron Letters 52 (2011) 2710–2714
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
A rigid conjugated pyridinylthiazole derivative and its nanoparticles for divalent copper fluorescent sensing in aqueous media Jun Hou, Ling-Yu Wang, Dong-Hao Li, Xue Wu ⇑ Key Laboratory of Natural Resources of Changbai Mountain & Functional Molecules (Yanbian University), Ministry of Education, Yanji, Jilin 133002, China
a r t i c l e
i n f o
Article history: Received 19 January 2011 Revised 8 March 2011 Accepted 17 March 2011
Keywords: Organic nanoparticle Fluorescence sensing Copper sensor Quenching
a b s t r a c t A rigid conjugated pyridinylthiazole derivative (1) and two bithiazole derivatives with similar structures (2, 3) were synthesized and characterized. Their optical properties were investigated through spectral analysis. By applying the three compounds to Cu2+ ions detection, it was shown that compound 1 could be employed as a selective and sensitive Cu2+ ions fluorescent chemosensor. For aqueous assay, the nanoparticles of compound 1 were prepared in aqueous media. Compared to the monomer, 1 nanoparticles were more fluorescence sensitive to Cu2+ ions. Its binding mode with Cu2+ ions was correlated well with Langmuir equation. Compound 1 nanoparticles exhibit a dynamic working range for Cu2+ ions from 0.02 to 0.50 lM with a detection limit of 10 nM. The proposed chemosensor has been used for the direct measurement of Cu2+ content in drinking water samples with satisfying results. Ó 2011 Elsevier Ltd. All rights reserved.
The development of fluorescent chemosensors for metal ions is currently of significant importance for both chemistry and biology.1,2 Up to now, there are a number of fluorescent chemosensors that have been developed for the detection of Cu2+, Zn2+, Hg2+, Pb2+, Cd2+, Ag+, and other transition metal ions.3–9 Generally, the structure of a fluorescent chemosensor always consists of a fluorophore as the key component for generating a signal and an analyteresponsive receptor.10,11 Conjugated organic compounds are candidate fluorophores owing to their high fluorescence quantum yields,12 while their poor water-solubility limits their practical application in aqueous assay. Many efforts have been made in order to improve the watersolubility of organic compounds. Introduction of an assistant group into the fluorophore skeleton13 and coupling the organic chemosensor with an inorganic nano-material14,15 are two good examples. Recently, Fluorescent organic nanoparticles (FONs) are being widely investigated for their potential application in many areas.16–20 Better than organic–inorganic composite nano-materials, FONs show more benefits for their low cost and convenient preparation method. Generally, FONs could be facilely prepared and were stable in aqueous media. Therefore, FONs are being considered as possible fluorescent chemosensors in water. Some FONs have been reported as nucleobases and nucleic acids sensors in aqueous solutions.21–23 Pyridinylthiazoles, bithiazoles, and their derivatives are getting great attention because of their high chelating ability to a variety of transition elements.24,8 A series of 5-methoxy-2-pyridylthizole ⇑ Corresponding author. Tel.: +86 433 2732299; fax: +86 433 2732456. E-mail address: [email protected] (X. Wu). 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.03.082
derivatives have been used as fluorescent sensors for pH or Cu2+ ions.15,25 4-Diphenylaminostilbene and its derivatives are highly fluorescent molecules as a result of the extraordinary electronic effect of the N-phenyl substituents and their large conjugated structure.26–29 To obtain highly fluorescent compounds for Cu2+ ions detection, we are presenting a study utilizing the advantages of pyridinylthiazoles, bithiazoles, and 4-diphenylaminostilbene. A pyridinylthiazole derivative (1) and two bithiazole derivatives (2, 3) containing N-phenylstilbene moiety (Scheme 1) were synthesized and characterized (see Supplementary data) in this work. All compounds possess high fluorescence quantum yields. Compared to compounds 2 and 3, compound 1 showed more sensitive fluorescent sensing of Cu2+ ions. In order to perform the water analysis, the FONs of compound 1 were prepared in aqueous media, which were also Cu2+ ions sensitive. Absorption and fluorescence spectral data of 1–3 in different solvents are shown in Table 1. It was found that the polarity of
R
1 2 3
N S
R
N N
S R
S
N S
R=
∗
N
N S N
R
Scheme 1. The structure of compounds 1, 2 and 3.
2711
J. Hou et al. / Tetrahedron Letters 52 (2011) 2710–2714 Table 1 Absorption and emissiona of analogues 1–3 in different solvents Solvent
1 kab max /log
n-Hexane Toluene EAb THF Acetone Acetonitrile Methanol a b
emax nm/M
390/4.27 394/4.22 390/4.26 392/4.26 390/4.22 390/4.24 393/4.23
1
cm
2 -1
kflmax
nm
427 452 472 475 493 506 516
kab max /log
emax nm/M
394/4.50 401/4.42 400/4.50 399/4.57 397/4.46 399/4.43 399/4.51
1
3 -1
cm
kflmax 430 452 471 472 491 506 516
nm
kab max /log
emax nm/M
387/4.38 392/4.27 386/4.32 391/4.38 387/4.37 389/4.36 390/4.37
1
cm1
kflmax nm 426 449 471 477 494 505 517
Both the absorption and emission test concentration is 10 lM. EA: ethyl acetate.
the solvents does not show a big effect on the absorption wavelength. Instead, there is a significant red-shift of the fluorescence emission observed as the solvent polarity increased. This indicates that the polarity of the excited state is greater than that of the ground state, which may be explained by the charge-transfer nature of pull–push conjugated molecules.30 This observation is in accordance to a previous report showing that the fluorescence of pyridinylthiazoles originates from internal charge transfer.25 A similar electron-withdrawing ability of the bithiazole group is expected. All three compounds exhibit strong fluorescence with quantum yields of 0.41, 0.37, and 0.64 in THF, respectively, which was measured by using quinine sulfate as a reference.31 We studied the effect of the three compounds (1–3) in Cu2+ ions detection. Figure 1 shows the fluorescence spectra of these three compounds upon the addition of Cu2+ ions in methanol solvent. Clearly, the fluorescence intensity of the three compounds decreased with the increasing of Cu2+ concentration, while the spectral shape remained unchanged. Compared to compounds 2 and 3, the fluorescence of compound 1 was quenched most by the addition of Cu2+. The quenching efficiency (1-I/I0) of compound 1 was 86.3% after adding an equimolar amount of Cu2+ ions, which was much bigger than that of compound 2 or 3 (9.4% and 18.0%, respectively). It means that the binding ability of pyridinylthiazole to Cu2+ is stronger than that of bithiazole. It is mainly because the N atom electron density of pyridin is larger than that of thiazole. The selectivity of compound 1 to Cu2+ over other metal ions was also investigated to evaluate whether compound 1 could serve as a Cu2+-selective fluorescent chemosensor. With interaction given by equimolar amounts of different metal ions including Ag+, Zn2+, Co2+, Ni2+, Pb2+, Hg2+, Fe2+, and Cr3+ a slight fluorescence quenching of compound 1 was observed. However, the fluorescence quenching efficiencies were much lower than that given by Cu2+ ions. As shown in Figure 2, no interference in the detection of Cu2+ was observed during the detection process of Cu2+ ions in the presence of other metal ions. Thus, we conclude that compound 1 can be used as a Cu2+-selective sensor even with the presence of most competing cations. Absorption spectra change of compound 1 in methanol upon Cu2+ ions addition is shown in Figure 3. The absorption at 390 nm decreased sharply with the gradual Cu2+ ions addition. On the other hand, the low-energy band at 433 nm increased prominently with an isosbestic point at 413 nm. As illustrated in Figure 3a, with Cu2+ ions addition (up to 2.5 equiv), the absorbance of absorption band peaked at 433 nm had a linear increase at first, and reached its maximum with a Cu2+ ions dosage over 1.0 equiv. The nonlinear fit of the data demonstrates that Cu2+ compound 1 formed a 1:1 complex in solution. The Job’s curve (Fig. 3b) and MALDI–TOF Ms spectra (see Supplementary data) also suggest a 1:1 stoichiometry of the 1Cu2+ complex.6 Therefore, the 1:1 complex formation of Cu2+ and compound 1 is responsible for the fluorescence quenching which is mainly because of the paramagnetic nature of divalent copper
Figure 1. Fluorescence titration of analogues 1 (a), 2 (b), and 3 (c) with Cu(NO3)2 in methanol (the test concentration is 10 lM).
2712
J. Hou et al. / Tetrahedron Letters 52 (2011) 2710–2714
Figure 2. (a) Fluorescence spectra of compound 1 (10 lM) in methanol in the presence of several metal ions (10 lM) kex = 391 nm. (b) The fluorescence intensity (kex = 391 nm, kem = 517 nm) of compound 1 (10 lM) and interfering ions (10 lM) before and after the addition of 10 lM Cu2+.
ion.19 Based on the absorption spectra analysis, the binding constant of the complex was (2.14 ± 0.1) 106 M1, which was calculated by a strict 1:1 model.32 In order to use compound 1 in aqueous assay, FONs of compound 1 were prepared through a simple reprecipitation method.17,20 The absorption and fluorescence spectra of compound 1 in the THF–water mixture with different water fractions are shown in Figure 4. In all solutions, the concentration of compound 1 was constant (2 105 M). For the absorption spectra, when the water volume fraction is over 70%, a level-off tail could be observed in the visible spectral region (>450 nm). This tail which can be commonly found in nanoparticles suspensions, confirmed the formation of nanoparticles in solvent mixtures with a large water fraction.16 The size distribution of the nanoparticles was determined by scanning electron microscopy. As shown in Figure 5, the particle of compound 1 had an average diameter of 200 nm. A similar size was read by dynamic lighter scattering analysis (see Supplementary data). The fluorescence color of the solution showed a dramatic change from blue to green, while the emission intensity decreased as the water volume fraction increased from 0% to 70%. This was mainly because of the nature of the intramolecular charge transfer compound and the increase of the solvent polarity.30 Further increase of water content to 80% or 90% can result in a blue-shift in the emission peak. Based on the blue shift, we made a hypothesis that the aggregation of compound 1 is likely to be, to some extent, in the form of an H-type mode.33 For compounds 2 and 3, they also could form nanoparticles in THF/water mixture (1:9). The properties of 1, 2, and 3 nanoparticles are still stable after being kept in aqueous media for one month.
Figure 3. (a) UV–vis titration of compound 1 (10 lM) with Cu(NO3)2 in methanol. The insert shows the absorbance of compound 1 at 431 nm with varying amounts of Cu2+. (b) Job’s plot of the complex formed by compound 1 and Cu2+ (the total concentration of compound 1 and Cu2+ is 20 lM).
The utilization of 1, 2, and 3 nanoparticles in Cu2+ ions detection in aqueous media (water/THF, 9:1) was investigated. The emission of 1 nanoparticles was remarkably quenched upon the addition of Cu2+ ions (Fig. 6a). For compounds 2 and 3, like the case in methanol solution, their nanoparticles were not sensitive to divalent copper (see Supplementary data). We presumed that the fluorescence quenching could be caused by the adsorption of Cu2+ ions to the surface of the nanoparticles34 and the quenching efficiency was directly related with adsorption efficiency. The quenching curve (Fig. 6b) of 1 nanoparticles can be described by Langmuir equation, with a correlation coefficient (R2) of 0.98.
h¼
bc 1 þ bc
where h is adsorption efficiency, c is the concentration of Cu2+ ions and b is the Langmuir adsorption equilibrium constant. According to the fitting curve, the adsorption constant was 3.34 105 M1, which could be considered as the quenching constant. This quenching constant of 1 nanoparticles was larger than that of the monomer in methanol or THF based on Stern–Volumer equation (1.95 105 M1 and 7.12 104 M1, respectively).13 This result demonstrated that the fluorescence response of 1 nanoparticles was more sensitive than that of the monomer molecule to the presence of Cu2+ ions. In addition, when the solvent was changed to a water/THF (5:5) mixture in which there is no nanoparticle formation, a quenching efficiency of 48% was obtained by adding an equimolar amount of Cu2+ ions. This quenching efficiency was much smaller than that of the nanoparticles in aqueous media (85%). In this case, the
2713
J. Hou et al. / Tetrahedron Letters 52 (2011) 2710–2714
Figure 4. Absorption (a) and fluorescence (b) spectra of compound 1 (20 lM) in THF–water mixtures with different water contents.
Figure 5. SEM images of 1 nanoparticles obtained from nanoparticles’ suspension containing 10% volume fraction of THF in water.
quenching constant was 4.52 104 M1 based on Stern–Volumer equation, which was also smaller than that of the nanoparticles. The results indicated that the nanoparticles can improve Cu2+ ions detection in aqueous media. By the way, 1 nanoparticles were also selective toward Cu2+ ions. The fluorescence quenching of 1 nanoparticles could quantitatively reflect Cu2+ addition. A good linear correlation (Y = 0.39914X, R2 = 0.9942) between the value of quenching efficiency and the Cu2+ ions concentration within the range of 0.02–0.50 lM was obtained. The detection limit, based on the definition by IUPAC
Figure 6. (a) Fluorescence titration of compound 1 nanoparticles with Cu(NO3)2 in a THF/water mixture (1:9) (the test concentration is 20 lM). (b) Quenching efficiency plot of compound 1 in methanol, THF/water mixture (1:1 and 1:9) in the present of Cu2+ ions.
(CDL = 3 Sb/m),35 was found to be 10 nM from 10 blank solutions. This value is much lower than that obtained from reported pyridinylthiazole type Cu2+ ions sensor.14 With the water/THF (9:1) mixture containing 20 lM 1 nanoparticles, a drinking water sample was analyzed by the proposed fluorometric method. The results were summarized in Table 2, which showed satisfactory recovery and R.S.D. values for all of the samples, suggesting a potential application of 1 nanoparticles to real sample analysis. In summary, a novel synthesized high fluorescence 4-pyridinlythizole derivative showed high selectivity and sensitivity to Cu2+ ions. Its corresponding FONs were prepared by reprecipitation method and applied to aqueous Cu2+ ions detection. The binding mode of FONs with Cu2+ ions was reasonably described by Langmuir equation. The fluorescence response of FONs to Cu2+ ions was more sensitive than that of the monomer and allowed a determination of Cu2+ concentration as low as 10 nM. Test with drinking water demonstrated that FONs of compound 1 can be applied to monitor low level Cu2+ contamination in the environment.
Table 2 Determination of Cu2+ in water sample
a
Sample
Cu2+ added (lM)
Cu2+ found (lM)
Recovery (%)
R.S.D.a (%)
Water 1 Water 2 Water 3
0.00 0.05 0.25
0.034 0.081 0.264
— 94 92
5.4 4.9 6.1
RSD: relative standard deviation, n = 5.
2714
J. Hou et al. / Tetrahedron Letters 52 (2011) 2710–2714
Acknowledgments This work was financially supported by the Ph.D. Programs Foundation of Ministry of Education of China (No. 2009220 1110001) and the Science and Technology Committee of Jilin Province (No. 200705425). Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tetlet.2011.03.082. References and notes 1. Tian, H.; Wang, Q. C. Chem. Soc. Rev. 2006, 35, 361. 2. de Silva, A. P. H.; Gunaratne, Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515. 3. Guo, Z.; Zhu, W.; Tian, H. Macromolecules 2010, 43, 739. 4. Ranyuk, E.; Douaihy, C. M.; Bessmertnykh, A.; Denat, F.; Averin, A.; Beletskaya, I.; Guilard, R. Org. Lett. 2009, 11, 987. 5. Xu, Z.; Baek, K.-H.; Kim, H. N.; Cui, J.; Qian, X.; Spring, D. R.; Shin, I.; Yoon, J. J. Am. Chem. Soc. 2010, 132, 601–610. 6. Xiang, Y.; Li, Z.; Chen, X.; Tong, A. Talanta 2008, 74, 1148. 7. Luo, H.-Y.; Jiang, J.-H.; Zhang, X.-B.; Li, C.-Y.; Shen, G.-L.; Yu, R.-Q. Talanta 2007, 72, 575. 8. Helal, A.; Kim, H.-S. Tetrahedron Lett. 2009, 50, 5510. 9. Jágerszki, G.; Grün, A.; Bitter, I.; Tóth, K.; Gyurcsányi, R. E. Chem. Commun. 2010, 46, 607. 10. Jiang, P.; Guo, Z. Coord. Chem. Rev. 2004, 248, 205.
11. Rurack, K.; Resch-Genger, U. Chem. Soc. Rev. 2002, 31, 116. 12. Li, H.-C.; Lin, Y.-P.; Chou, P.-T.; Cheng, Y.-M.; Liu, R.-S. Adv. Funct. Mater. 2007, 17, 520. 13. Xing, C.; Shi, Z.; Yu, M.; Wang, S. Polymer 2008, 49, 2698. 14. Wang, B.; Hai, J.; Liu, Z.; Wang, Q.; Yang, Z.; Sun, S. Angew. Chem., Int. Ed. 2010, 49, 4576. 15. Li, L.-L.; Sun, H.; Fang, C.-J.; Xu, J.; Jin, J.-Y.; Yan, C.-H. J. Mater. Chem. 2007, 17, 4492. 16. Hagfeldt, A.; Grätzel, M. Chem. Rev. 1995, 95, 49. 17. Xiao, D.; Xi, L.; Yang, W.; Fu, H.; Shuai, Z.; Fang, Y.; Yao, J. J. Am. Chem. Soc. 2003, 125, 6740. 18. An, B.-K.; Kwon, S.-K.; Park, S. Y. Angew. Chem., Int. Ed. 2007, 46, 1978. 19. An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410. 20. Palayangoda, S. S.; Cai, X.; Adhikari, R. M.; Neckers, D. C. Org. Lett. 2008, 10, 281. 21. Xu, X.; Chen, S.; Li, L.; Yu, G.; Di, C.; Liu, Y. J. Mater. Chem. 2008, 18, 2555. 22. Zhou, Y.; Bian, G.; Wang, L.; Kan, J. Spectrochim. Acta, Part A 2005, 61, 1841. 23. Xia, T.; Wang, L.; Bian, G.; Dong, L.; Hong, S. Mircrochim. Acta 2006, 154, 309. 24. Mathieu, J.; Ghermani, N.; Bouhmaida, N.; Fenet, B.; Marsura, A. Eur. J. Inorg. Chem. 2004, 3172. 25. Zheng, M.-H.; Jin, J.-Y.; Sun, W.; Yan, C.-H. New J. Chem. 2006, 30, 1192. 26. Yang, J.-S.; Chiuo, S.-Y.; Liau, K.-L. J. Am. Chem. Soc. 2002, 124, 2518. 27. Létard, J.-F.; Lapouyade, R.; Retting, W. J. Am. Chem. Soc. 1993, 115, 2441. 28. Waldeck, D. H. Chem. Rev. 1991, 91, 415–436. 29. Ning, Z.; Chen, Z.; Zhang, Q.; Yan, Y.; Qian, S.; Cao, Y.; Tian, H. Adv. Funct. Mater. 2007, 17, 3799. 30. Laage, D.; Thompson, W. H.; Blanchard-Desce, M.; Hynes, J. T. J. Phys. Chem. A 2003, 107, 6032. 31. Dawson, W. R.; Windsor, M. W. J. Phys. Chem. 1968, 72, 3251. 32. Yuasa, J.; Fukuzumi, S. J. Am. Chem. Soc. 2008, 130, 566. 33. Peng, A-D.; Xiao, D-B.; Ma, Y.; Yang, W-S.; Yao, J-N. Adv. Mater. 2005, 17, 2070. 34. Wang, J-W.; Kuo, Y-M. J. Appl. Polym. Sci. 2008, 107, 2333. 35. IUPAC Compendium of Analytical Nomenclature, Definitive Rules; Irving, H. M. N. H., Freiser, H., West, T. S., Eds.; Pergamon Press: Oxford, 1981.
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
A rigid conjugated pyridinylthiazole derivative and its nanoparticles for divalent copper fluorescent sensing in aqueous media Jun Hou, Ling-Yu Wang, Dong-Hao Li, Xue Wu ⇑ Key Laboratory of Natural Resources of Changbai Mountain & Functional Molecules (Yanbian University), Ministry of Education, Yanji, Jilin 133002, China
a r t i c l e
i n f o
Article history: Received 19 January 2011 Revised 8 March 2011 Accepted 17 March 2011
Keywords: Organic nanoparticle Fluorescence sensing Copper sensor Quenching
a b s t r a c t A rigid conjugated pyridinylthiazole derivative (1) and two bithiazole derivatives with similar structures (2, 3) were synthesized and characterized. Their optical properties were investigated through spectral analysis. By applying the three compounds to Cu2+ ions detection, it was shown that compound 1 could be employed as a selective and sensitive Cu2+ ions fluorescent chemosensor. For aqueous assay, the nanoparticles of compound 1 were prepared in aqueous media. Compared to the monomer, 1 nanoparticles were more fluorescence sensitive to Cu2+ ions. Its binding mode with Cu2+ ions was correlated well with Langmuir equation. Compound 1 nanoparticles exhibit a dynamic working range for Cu2+ ions from 0.02 to 0.50 lM with a detection limit of 10 nM. The proposed chemosensor has been used for the direct measurement of Cu2+ content in drinking water samples with satisfying results. Ó 2011 Elsevier Ltd. All rights reserved.
The development of fluorescent chemosensors for metal ions is currently of significant importance for both chemistry and biology.1,2 Up to now, there are a number of fluorescent chemosensors that have been developed for the detection of Cu2+, Zn2+, Hg2+, Pb2+, Cd2+, Ag+, and other transition metal ions.3–9 Generally, the structure of a fluorescent chemosensor always consists of a fluorophore as the key component for generating a signal and an analyteresponsive receptor.10,11 Conjugated organic compounds are candidate fluorophores owing to their high fluorescence quantum yields,12 while their poor water-solubility limits their practical application in aqueous assay. Many efforts have been made in order to improve the watersolubility of organic compounds. Introduction of an assistant group into the fluorophore skeleton13 and coupling the organic chemosensor with an inorganic nano-material14,15 are two good examples. Recently, Fluorescent organic nanoparticles (FONs) are being widely investigated for their potential application in many areas.16–20 Better than organic–inorganic composite nano-materials, FONs show more benefits for their low cost and convenient preparation method. Generally, FONs could be facilely prepared and were stable in aqueous media. Therefore, FONs are being considered as possible fluorescent chemosensors in water. Some FONs have been reported as nucleobases and nucleic acids sensors in aqueous solutions.21–23 Pyridinylthiazoles, bithiazoles, and their derivatives are getting great attention because of their high chelating ability to a variety of transition elements.24,8 A series of 5-methoxy-2-pyridylthizole ⇑ Corresponding author. Tel.: +86 433 2732299; fax: +86 433 2732456. E-mail address: [email protected] (X. Wu). 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.03.082
derivatives have been used as fluorescent sensors for pH or Cu2+ ions.15,25 4-Diphenylaminostilbene and its derivatives are highly fluorescent molecules as a result of the extraordinary electronic effect of the N-phenyl substituents and their large conjugated structure.26–29 To obtain highly fluorescent compounds for Cu2+ ions detection, we are presenting a study utilizing the advantages of pyridinylthiazoles, bithiazoles, and 4-diphenylaminostilbene. A pyridinylthiazole derivative (1) and two bithiazole derivatives (2, 3) containing N-phenylstilbene moiety (Scheme 1) were synthesized and characterized (see Supplementary data) in this work. All compounds possess high fluorescence quantum yields. Compared to compounds 2 and 3, compound 1 showed more sensitive fluorescent sensing of Cu2+ ions. In order to perform the water analysis, the FONs of compound 1 were prepared in aqueous media, which were also Cu2+ ions sensitive. Absorption and fluorescence spectral data of 1–3 in different solvents are shown in Table 1. It was found that the polarity of
R
1 2 3
N S
R
N N
S R
S
N S
R=
∗
N
N S N
R
Scheme 1. The structure of compounds 1, 2 and 3.
2711
J. Hou et al. / Tetrahedron Letters 52 (2011) 2710–2714 Table 1 Absorption and emissiona of analogues 1–3 in different solvents Solvent
1 kab max /log
n-Hexane Toluene EAb THF Acetone Acetonitrile Methanol a b
emax nm/M
390/4.27 394/4.22 390/4.26 392/4.26 390/4.22 390/4.24 393/4.23
1
cm
2 -1
kflmax
nm
427 452 472 475 493 506 516
kab max /log
emax nm/M
394/4.50 401/4.42 400/4.50 399/4.57 397/4.46 399/4.43 399/4.51
1
3 -1
cm
kflmax 430 452 471 472 491 506 516
nm
kab max /log
emax nm/M
387/4.38 392/4.27 386/4.32 391/4.38 387/4.37 389/4.36 390/4.37
1
cm1
kflmax nm 426 449 471 477 494 505 517
Both the absorption and emission test concentration is 10 lM. EA: ethyl acetate.
the solvents does not show a big effect on the absorption wavelength. Instead, there is a significant red-shift of the fluorescence emission observed as the solvent polarity increased. This indicates that the polarity of the excited state is greater than that of the ground state, which may be explained by the charge-transfer nature of pull–push conjugated molecules.30 This observation is in accordance to a previous report showing that the fluorescence of pyridinylthiazoles originates from internal charge transfer.25 A similar electron-withdrawing ability of the bithiazole group is expected. All three compounds exhibit strong fluorescence with quantum yields of 0.41, 0.37, and 0.64 in THF, respectively, which was measured by using quinine sulfate as a reference.31 We studied the effect of the three compounds (1–3) in Cu2+ ions detection. Figure 1 shows the fluorescence spectra of these three compounds upon the addition of Cu2+ ions in methanol solvent. Clearly, the fluorescence intensity of the three compounds decreased with the increasing of Cu2+ concentration, while the spectral shape remained unchanged. Compared to compounds 2 and 3, the fluorescence of compound 1 was quenched most by the addition of Cu2+. The quenching efficiency (1-I/I0) of compound 1 was 86.3% after adding an equimolar amount of Cu2+ ions, which was much bigger than that of compound 2 or 3 (9.4% and 18.0%, respectively). It means that the binding ability of pyridinylthiazole to Cu2+ is stronger than that of bithiazole. It is mainly because the N atom electron density of pyridin is larger than that of thiazole. The selectivity of compound 1 to Cu2+ over other metal ions was also investigated to evaluate whether compound 1 could serve as a Cu2+-selective fluorescent chemosensor. With interaction given by equimolar amounts of different metal ions including Ag+, Zn2+, Co2+, Ni2+, Pb2+, Hg2+, Fe2+, and Cr3+ a slight fluorescence quenching of compound 1 was observed. However, the fluorescence quenching efficiencies were much lower than that given by Cu2+ ions. As shown in Figure 2, no interference in the detection of Cu2+ was observed during the detection process of Cu2+ ions in the presence of other metal ions. Thus, we conclude that compound 1 can be used as a Cu2+-selective sensor even with the presence of most competing cations. Absorption spectra change of compound 1 in methanol upon Cu2+ ions addition is shown in Figure 3. The absorption at 390 nm decreased sharply with the gradual Cu2+ ions addition. On the other hand, the low-energy band at 433 nm increased prominently with an isosbestic point at 413 nm. As illustrated in Figure 3a, with Cu2+ ions addition (up to 2.5 equiv), the absorbance of absorption band peaked at 433 nm had a linear increase at first, and reached its maximum with a Cu2+ ions dosage over 1.0 equiv. The nonlinear fit of the data demonstrates that Cu2+ compound 1 formed a 1:1 complex in solution. The Job’s curve (Fig. 3b) and MALDI–TOF Ms spectra (see Supplementary data) also suggest a 1:1 stoichiometry of the 1Cu2+ complex.6 Therefore, the 1:1 complex formation of Cu2+ and compound 1 is responsible for the fluorescence quenching which is mainly because of the paramagnetic nature of divalent copper
Figure 1. Fluorescence titration of analogues 1 (a), 2 (b), and 3 (c) with Cu(NO3)2 in methanol (the test concentration is 10 lM).
2712
J. Hou et al. / Tetrahedron Letters 52 (2011) 2710–2714
Figure 2. (a) Fluorescence spectra of compound 1 (10 lM) in methanol in the presence of several metal ions (10 lM) kex = 391 nm. (b) The fluorescence intensity (kex = 391 nm, kem = 517 nm) of compound 1 (10 lM) and interfering ions (10 lM) before and after the addition of 10 lM Cu2+.
ion.19 Based on the absorption spectra analysis, the binding constant of the complex was (2.14 ± 0.1) 106 M1, which was calculated by a strict 1:1 model.32 In order to use compound 1 in aqueous assay, FONs of compound 1 were prepared through a simple reprecipitation method.17,20 The absorption and fluorescence spectra of compound 1 in the THF–water mixture with different water fractions are shown in Figure 4. In all solutions, the concentration of compound 1 was constant (2 105 M). For the absorption spectra, when the water volume fraction is over 70%, a level-off tail could be observed in the visible spectral region (>450 nm). This tail which can be commonly found in nanoparticles suspensions, confirmed the formation of nanoparticles in solvent mixtures with a large water fraction.16 The size distribution of the nanoparticles was determined by scanning electron microscopy. As shown in Figure 5, the particle of compound 1 had an average diameter of 200 nm. A similar size was read by dynamic lighter scattering analysis (see Supplementary data). The fluorescence color of the solution showed a dramatic change from blue to green, while the emission intensity decreased as the water volume fraction increased from 0% to 70%. This was mainly because of the nature of the intramolecular charge transfer compound and the increase of the solvent polarity.30 Further increase of water content to 80% or 90% can result in a blue-shift in the emission peak. Based on the blue shift, we made a hypothesis that the aggregation of compound 1 is likely to be, to some extent, in the form of an H-type mode.33 For compounds 2 and 3, they also could form nanoparticles in THF/water mixture (1:9). The properties of 1, 2, and 3 nanoparticles are still stable after being kept in aqueous media for one month.
Figure 3. (a) UV–vis titration of compound 1 (10 lM) with Cu(NO3)2 in methanol. The insert shows the absorbance of compound 1 at 431 nm with varying amounts of Cu2+. (b) Job’s plot of the complex formed by compound 1 and Cu2+ (the total concentration of compound 1 and Cu2+ is 20 lM).
The utilization of 1, 2, and 3 nanoparticles in Cu2+ ions detection in aqueous media (water/THF, 9:1) was investigated. The emission of 1 nanoparticles was remarkably quenched upon the addition of Cu2+ ions (Fig. 6a). For compounds 2 and 3, like the case in methanol solution, their nanoparticles were not sensitive to divalent copper (see Supplementary data). We presumed that the fluorescence quenching could be caused by the adsorption of Cu2+ ions to the surface of the nanoparticles34 and the quenching efficiency was directly related with adsorption efficiency. The quenching curve (Fig. 6b) of 1 nanoparticles can be described by Langmuir equation, with a correlation coefficient (R2) of 0.98.
h¼
bc 1 þ bc
where h is adsorption efficiency, c is the concentration of Cu2+ ions and b is the Langmuir adsorption equilibrium constant. According to the fitting curve, the adsorption constant was 3.34 105 M1, which could be considered as the quenching constant. This quenching constant of 1 nanoparticles was larger than that of the monomer in methanol or THF based on Stern–Volumer equation (1.95 105 M1 and 7.12 104 M1, respectively).13 This result demonstrated that the fluorescence response of 1 nanoparticles was more sensitive than that of the monomer molecule to the presence of Cu2+ ions. In addition, when the solvent was changed to a water/THF (5:5) mixture in which there is no nanoparticle formation, a quenching efficiency of 48% was obtained by adding an equimolar amount of Cu2+ ions. This quenching efficiency was much smaller than that of the nanoparticles in aqueous media (85%). In this case, the
2713
J. Hou et al. / Tetrahedron Letters 52 (2011) 2710–2714
Figure 4. Absorption (a) and fluorescence (b) spectra of compound 1 (20 lM) in THF–water mixtures with different water contents.
Figure 5. SEM images of 1 nanoparticles obtained from nanoparticles’ suspension containing 10% volume fraction of THF in water.
quenching constant was 4.52 104 M1 based on Stern–Volumer equation, which was also smaller than that of the nanoparticles. The results indicated that the nanoparticles can improve Cu2+ ions detection in aqueous media. By the way, 1 nanoparticles were also selective toward Cu2+ ions. The fluorescence quenching of 1 nanoparticles could quantitatively reflect Cu2+ addition. A good linear correlation (Y = 0.39914X, R2 = 0.9942) between the value of quenching efficiency and the Cu2+ ions concentration within the range of 0.02–0.50 lM was obtained. The detection limit, based on the definition by IUPAC
Figure 6. (a) Fluorescence titration of compound 1 nanoparticles with Cu(NO3)2 in a THF/water mixture (1:9) (the test concentration is 20 lM). (b) Quenching efficiency plot of compound 1 in methanol, THF/water mixture (1:1 and 1:9) in the present of Cu2+ ions.
(CDL = 3 Sb/m),35 was found to be 10 nM from 10 blank solutions. This value is much lower than that obtained from reported pyridinylthiazole type Cu2+ ions sensor.14 With the water/THF (9:1) mixture containing 20 lM 1 nanoparticles, a drinking water sample was analyzed by the proposed fluorometric method. The results were summarized in Table 2, which showed satisfactory recovery and R.S.D. values for all of the samples, suggesting a potential application of 1 nanoparticles to real sample analysis. In summary, a novel synthesized high fluorescence 4-pyridinlythizole derivative showed high selectivity and sensitivity to Cu2+ ions. Its corresponding FONs were prepared by reprecipitation method and applied to aqueous Cu2+ ions detection. The binding mode of FONs with Cu2+ ions was reasonably described by Langmuir equation. The fluorescence response of FONs to Cu2+ ions was more sensitive than that of the monomer and allowed a determination of Cu2+ concentration as low as 10 nM. Test with drinking water demonstrated that FONs of compound 1 can be applied to monitor low level Cu2+ contamination in the environment.
Table 2 Determination of Cu2+ in water sample
a
Sample
Cu2+ added (lM)
Cu2+ found (lM)
Recovery (%)
R.S.D.a (%)
Water 1 Water 2 Water 3
0.00 0.05 0.25
0.034 0.081 0.264
— 94 92
5.4 4.9 6.1
RSD: relative standard deviation, n = 5.
2714
J. Hou et al. / Tetrahedron Letters 52 (2011) 2710–2714
Acknowledgments This work was financially supported by the Ph.D. Programs Foundation of Ministry of Education of China (No. 2009220 1110001) and the Science and Technology Committee of Jilin Province (No. 200705425). Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tetlet.2011.03.082. References and notes 1. Tian, H.; Wang, Q. C. Chem. Soc. Rev. 2006, 35, 361. 2. de Silva, A. P. H.; Gunaratne, Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515. 3. Guo, Z.; Zhu, W.; Tian, H. Macromolecules 2010, 43, 739. 4. Ranyuk, E.; Douaihy, C. M.; Bessmertnykh, A.; Denat, F.; Averin, A.; Beletskaya, I.; Guilard, R. Org. Lett. 2009, 11, 987. 5. Xu, Z.; Baek, K.-H.; Kim, H. N.; Cui, J.; Qian, X.; Spring, D. R.; Shin, I.; Yoon, J. J. Am. Chem. Soc. 2010, 132, 601–610. 6. Xiang, Y.; Li, Z.; Chen, X.; Tong, A. Talanta 2008, 74, 1148. 7. Luo, H.-Y.; Jiang, J.-H.; Zhang, X.-B.; Li, C.-Y.; Shen, G.-L.; Yu, R.-Q. Talanta 2007, 72, 575. 8. Helal, A.; Kim, H.-S. Tetrahedron Lett. 2009, 50, 5510. 9. Jágerszki, G.; Grün, A.; Bitter, I.; Tóth, K.; Gyurcsányi, R. E. Chem. Commun. 2010, 46, 607. 10. Jiang, P.; Guo, Z. Coord. Chem. Rev. 2004, 248, 205.
11. Rurack, K.; Resch-Genger, U. Chem. Soc. Rev. 2002, 31, 116. 12. Li, H.-C.; Lin, Y.-P.; Chou, P.-T.; Cheng, Y.-M.; Liu, R.-S. Adv. Funct. Mater. 2007, 17, 520. 13. Xing, C.; Shi, Z.; Yu, M.; Wang, S. Polymer 2008, 49, 2698. 14. Wang, B.; Hai, J.; Liu, Z.; Wang, Q.; Yang, Z.; Sun, S. Angew. Chem., Int. Ed. 2010, 49, 4576. 15. Li, L.-L.; Sun, H.; Fang, C.-J.; Xu, J.; Jin, J.-Y.; Yan, C.-H. J. Mater. Chem. 2007, 17, 4492. 16. Hagfeldt, A.; Grätzel, M. Chem. Rev. 1995, 95, 49. 17. Xiao, D.; Xi, L.; Yang, W.; Fu, H.; Shuai, Z.; Fang, Y.; Yao, J. J. Am. Chem. Soc. 2003, 125, 6740. 18. An, B.-K.; Kwon, S.-K.; Park, S. Y. Angew. Chem., Int. Ed. 2007, 46, 1978. 19. An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410. 20. Palayangoda, S. S.; Cai, X.; Adhikari, R. M.; Neckers, D. C. Org. Lett. 2008, 10, 281. 21. Xu, X.; Chen, S.; Li, L.; Yu, G.; Di, C.; Liu, Y. J. Mater. Chem. 2008, 18, 2555. 22. Zhou, Y.; Bian, G.; Wang, L.; Kan, J. Spectrochim. Acta, Part A 2005, 61, 1841. 23. Xia, T.; Wang, L.; Bian, G.; Dong, L.; Hong, S. Mircrochim. Acta 2006, 154, 309. 24. Mathieu, J.; Ghermani, N.; Bouhmaida, N.; Fenet, B.; Marsura, A. Eur. J. Inorg. Chem. 2004, 3172. 25. Zheng, M.-H.; Jin, J.-Y.; Sun, W.; Yan, C.-H. New J. Chem. 2006, 30, 1192. 26. Yang, J.-S.; Chiuo, S.-Y.; Liau, K.-L. J. Am. Chem. Soc. 2002, 124, 2518. 27. Létard, J.-F.; Lapouyade, R.; Retting, W. J. Am. Chem. Soc. 1993, 115, 2441. 28. Waldeck, D. H. Chem. Rev. 1991, 91, 415–436. 29. Ning, Z.; Chen, Z.; Zhang, Q.; Yan, Y.; Qian, S.; Cao, Y.; Tian, H. Adv. Funct. Mater. 2007, 17, 3799. 30. Laage, D.; Thompson, W. H.; Blanchard-Desce, M.; Hynes, J. T. J. Phys. Chem. A 2003, 107, 6032. 31. Dawson, W. R.; Windsor, M. W. J. Phys. Chem. 1968, 72, 3251. 32. Yuasa, J.; Fukuzumi, S. J. Am. Chem. Soc. 2008, 130, 566. 33. Peng, A-D.; Xiao, D-B.; Ma, Y.; Yang, W-S.; Yao, J-N. Adv. Mater. 2005, 17, 2070. 34. Wang, J-W.; Kuo, Y-M. J. Appl. Polym. Sci. 2008, 107, 2333. 35. IUPAC Compendium of Analytical Nomenclature, Definitive Rules; Irving, H. M. N. H., Freiser, H., West, T. S., Eds.; Pergamon Press: Oxford, 1981.