A highly selective and sensitive 3-hydroxyflavone-based colorimetric and fluorescent probe for hydrogen sulfide with a large Stokes shift

Article abstract of DOI:10.1016/j.tet.2016.04.079

A 3-hydroxyflavone-based colorimetric and fluorescence probe for hydrogen sulfide (H₂S) has been developed with high sensitivity and excellent selectivity. This probe was designed based on the mechanism that H₂S selectively cleaved the NBD (7-nitro-2,1,3-benzoxadiazole) ether moiety in this probe and therefore release the fluorophore, 3-hydroxyflavone. The addition of H₂S to the solution of probe 1 resulted in a green fluorescence and an obvious color change from colorless to pink, indicating that this probe can serve as a colorimetric and fluorescent dual probe for H₂S. Furthermore, this probe displays a rapid response and reaches to a plateau within 5 min. It also exhibits a 146 nm Stokes shift and a low detection limit (20 nM, based on S/N=3) in detecting H₂S. Importantly, practical utility of this probe for the selective detection of H₂S in living cells has been successfully demonstrated.

Full text of DOI:10.1016/j.tet.2016.04.079

Accepted Manuscript
A highly selective and sensitive 3-hydroxyflavone-based colorimetric and fluorescent
probe for hydrogen sulfide with a large Stokes shift
Peng Hou, Hongmei Li, Song Chen
PII:
S0040-4020(16)30368-4
10.1016/j.tet.2016.04.079
TET 27722
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 15 March 2016
Revised Date: 26 April 2016
Accepted Date: 29 April 2016
Please cite this article as: Hou P, Li H, Chen S, A highly selective and sensitive 3-hydroxyflavone-based
colorimetric and fluorescent probe for hydrogen sulfide with a large Stokes shift, Tetrahedron (2016), doi:
10.1016/j.tet.2016.04.079.
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A highly selective and sensitive 3-
hydroxyflavone-based colorimetric and
fluorescent probe for hydrogen sulfide with a
large Stokes shift
Leave this area blank for abstract info.
Peng Hou,* Hongmei Li and Song Chen
College of Pharmacy, Qiqihar Medical University, Qiqihar, Heilongjiang Province, P. R. China, 161006.

1
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Tetrahedron
journal homepage: www.elsevier.com
A highly selective and sensitive 3-hydroxyflavone-based colorimetric and fluorescent
probe for hydrogen sulfide with a large Stokes shift
Peng Hou , Hongmei Li and Song Chen
College of Pharmacy, Qiqihar Medical University, Qiqihar, Heilongjiang Province, P. R. China, 161006.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A 3-hydroxyflavone-based colorimetric and fluorescence probe for hydrogen sulfide (H2S) has
been developed with high sensitivity and excellent selectivity. This probe was designed based on
the mechanism that H2S selectively cleaved the NBD (7-nitro-2,1,3-benzoxadiazole) ether
moiety in this probe and therefore release the fluorophore, 3-hydroxyflavone. The addition of
H2S to the solution of probe 1 resulted in a green fluorescence and an obvious color change from
colorless to pink, indicating that this probe can serve as a colorimetric and fluorescent dual
probe for H2S. Furthermore, this probe displays a rapid response and reaches to a plateau within
5 min. It also exhibits a 146 nm Stokes shift and a low detection limit (20 nM, based on S/N = 3)
in detecting H2S. Importantly, practical utility of this probe for the selective detection of H2S in
living cells has been successfully demonstrated.
Available online
Keywords:
Fluorescent probe
Flavone
Photo-induced electron transfer (PET)
Hydrogen sulfide
Colorimetric
2009 Elsevier Ltd. All rights reserved
.
dinitrophenyl ethers and NBD (7-nitro-2,1,3-benzoxadiazole).35-
38Due to their low cost, naked-eye mode and simple pretreatment,
colorimetric and fluorescent probes have received extensive
research attention. However, most of the current fluorescent
probes are still only respond to H2S with the changes in
fluorescence intensity. If a probe with color change and
fluorescence enhancement could be developed, the biological
roles of H2S would be expected to be clarified greatly.
1. Introduction
Hydrogen sulfide (H2S) with unpleasant smell is well known
as toxic gas which affects the nervous, respiratory, and
cardiovascular system of mammals.1 Recently, it has been
demonstrated that H2S was the third gas signaling molecule after
NO and CO, and endogenous H2S can be formed from cysteine
and homocysteine by enzymes catalyzation. It plays vital roles in
many physiological and pathological processes because of its
remarkable cardio-, retina- and neuron-protective and other
biological properties.2-3 It was reported that the toxic gas can
induce the inhibition of cytochrome c oxidization in the
mitochondrial electron transport chain. Furthermore, the
abnormal levels of H2S are closely associated with many diseases
such as atherosclerosis, Alzheimer’s disease, ischemic stroke and
Down syndrome.4-7 Therefore, the selective and sensitive
detection of a trace amount of hydrogen sulfide to understand its
physiological properties and functions mechanism has gained
increasing attention.
Owing to the advantages of sensitivity, rapid response, non-
invasiveness and ease of operation,8-22 fluorescent probes has
been a powerful tool to detect and image H2S in biological
systems. The design of fluorescent probes for H2S is mainly
based on the reduction of azide to amine,23-26 nucleophilic
reactions,27-28 reduction of hydroxyl amine29, disulfide
Scheme 1 Synthetic route to probe 1 and the sensing reaction with NaHS.
exchange30-32, high affinity of S2- to Cu2+,33-34 the thiolysis of 2,4-
3-Hydroxyflavone processes many excellent optical properties
———
Corresponding author. Tel.: +86-452-2663159; fax: +86-452-2663159; e-mail: houpeng1982@163.com

2
Tetrahedron
ACCEPTED MANUSCRIPT
such as emission in green, good photostability , relatively high
quantum yield. Importantly, it features an ESIPT process with a
large Stokes shift upon being excitated, which was desirable for
the design of fluorescent probe.39 In this work, we designed and
synthesized a 3-hydroxyflavone-based dual probe (probe 1) for
the selective detection of H2S. The preparation of probe 1 was
outlined in Scheme 1. We reasoned that probe 1 was essentially
none-fluorescent due to the photoinduced electron transfer (PET)
process caused by the NBD (7-nitro-2,1,3-benzoxadiazole) ether
moiety. We anticipated that H2S would selectively cleaved the
NBD moiety in probe 1 and subsequenlty dye 2 was released, in
which process the solution of probe 1 displayed a green
fluorescence and an obvious colour change from colourless to
pink. As a consequence, probe 1 can serve a colorimetric and
fluorescent dual probe for real-time detection of H2S.
Results and discussion
max
max
Fig. 2 Fluorescence response (λex = 346 nm, λem = 516 nm) of probe
1 (10.0 ꢀM) upon the addition of NaHS (0.0 – 200.0 ꢀM) in PBS buffer. Inset:
Fluorescence images of probe 1 (10.0 ꢀM) in the absence (left) and presence
(right) of NaHS under a 365 nm UV lamp.
1.1. Absorption properties
NaHS was used as a standard H2S source to investigate the
absorption spectra changes of probe 1 (10 µM) in PBS buffer (50
mM, pH=7.40, 20% acetonitrile, v/v). As shown in Fig. 1, with
the increasing addition of H2S (0-50 equiv.), the absorption peaks
of probe 1 at 370 nm gradually decreased accompanying with a
new peak at 540 nm appeared with an isosbestic point at 416 nm
(red-shifted 170 nm). This large red-shift may be due to H2S-
specific cleavage of the NBD ether group in probe 1 to yield 7-
nitrobenzo-2, 1, 3-oxadiazole-4-thiol (NBD-SH). Upon treatment
with 50 equiv. H2S, the solution of probe 1 instantaneously
displayed a pink color, indicating the probe was able to detect
H2S with naked-eye.
to generate the 3-hydroxyflavone 2. Moreover, the reaction
mechanism was strongly supported by the mass spectrum
analysis on the reaction product of probe 1 with H2S. The MS
spectrum of the reaction product displayed a peak at m/z = 239.1
(Fig. S3), which was identical to the exact weight of reference
dye 2 ( [M + H]+ = 239. 1). Furthermore, a good linear
relationship was obtained between the fluorescence intensity and
the concentration of H2S from 1.0 to 10.0 ꢀM (Fig. 3). The linear
equation was found to be y = 4.0022 + 28.8395x (R = 0.9987)
and the detection limit was 20 nM based on S/N = 3. These
results suggested that probe 1 could quantitatively detect H2S.
Fig. 1 The absorption spectra of probe 1 (10.0 ꢀM) upon the addition of
NaHS (0.0 – 500.0 ꢀM) in PBS buffer. Inset: The visual color changes of
probe 1 (10.0 ꢀM) in the absence (right) and presence (left) of NaHS under
natural light.
Fig. 3 Fluorescence intensity of probe 1 (10.0 ꢀM) at 516 nm as a function of
NaHS concentration in PBS buffer. Inset: The linear relationship between
fluorescence intensity and NaHS at low concentrations.
1.2. Fluorescence properties and linearity
The changes in the fluorescence spectra of probe 1 (10 ꢀM) in
the absence or presence of H2S (0-200 ꢀM) was investigated in
PBS buffer and the results were displayed in Fig. 2. The probe 1
was essentially non-fluorescent in the absence of H2S due to the
PET process. However, the addition of H2S caused a dramatic
change in the fluorescence spectra. A strong new emission peak
at 516 nm appeared, and an enhancement of the fluorescence
intensity (25-fold) was observed with a concomitant appearance
of green emission color under illumination with a 365 nm UV
lamp. This may attribute to H2S-induced the cleavage of the NBD
ether group in probe 1 and thereby eliminating the PET process
1.3. Time-dependent fluorescence spectra
The time-dependent fluorescence spectra of probe 1 with H2S
were investigated by monitoring the fluorescence intensity at 516
nm as a function of time. As shown in Fig. 4, fluorescence
intensity increased with the increase of reaction time after the
addition of different concentration of H2S (0.7, 2, 5, 10, 20 equiv.)
to the solution of probe 1. Upon the addition of 20 equiv. of H2S,
the fluorescent enhancement instantaneously initiated and
reached to plateau within 5 min, indicating that probe 1 was
capable of rapidly detecting H2S in aqueous media.

3
-
Cu2+, P. Mg2+, Q. Zn2+, R. H O , S. ClO-, T. CN-, U. S O 2-, V. N3 ) in PBS
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2
2
2
4
buffer.
-
-
-
2-
Cl-, NO2 , HCO3 , SCN-, citrate, Cys, GSH, H2PO4 , SO4 , Ca2+,
Cu2+, Mg2+, Zn2+, H2O2, ClO-, CN-, S2O4 , N3 . As shown in Fig.
5, only H2S induced a prominent fluorescence enhancement of
the probe at 516 nm, whereas no obvious fluorescence changes
were observed for other relevant analytes. It’s noteworthy that
this probe was able to detect H2S over Cys and GSH which was
the main competitive species in biological systems.
However, there are still many challenges in optical analysis of
H2S in real biological systems.40 Moreover, the fluorescence
responses of probe 1 to H2S in the presence of typical
competition analytes were studied to confirm the effective
application of the probe 1. The test result was displayed in Fig. 6.
All of competing analytes had little interference in the detection
of H2S. This suggested that probe 1 was a highly selective H2S
probe over other relevant analytes.
2-
-
Fig. 4 Time-dependent fluorescence intensity of probe 1 (10.0 ꢀM) at 516 nm
with different concentration of NaHS (7, 20, 50, 100, 200 ꢀM) in PBS buffer.
1.4. Selectivity and competition studies
1.5. Fluorescent imaging of living A431 cells
Next, we evaluated the selectivity of probe 1 (10.0 µM) for
H2S over other relevant analytes including F-, NO3 , OAc-, S2O3 ,
-
2-
To explore the application ability of the probe 1 towards H2S
in biological samples, the living A431 cells were determined to
monitor the behavior of the probe 1 recognized with H2S using
fluorescence microscopy. It can be seen in Fig. 7 that only very
weak fluorescence was observed when cells were incubated with
probe 1 for 30 min at 37℃. In contrast, an intense yellow
fluorescence was produced when the cells were pre-treated with
100 ꢀM H2S for 30 min at 37℃ and then incubated with 5.0 ꢀM
probe 1 for another 30 min. The above results demonstrate that
probe 1 could be used as a reliability and practicality possible
sensor to image H2S in living cells.
max
max
Fig. 5 Fluorescence response (λex = 346 nm, λem = 516 nm) of probe
1 (10.0 ꢀM) upon the addition of different tested analytes (NaHS: 200.0 ꢀM,
-
-
-
other tested analytes: 500.0 ꢀM (F-, NO3 , OAc-, S2O32-, Cl-, NO2 , HCO3 ,
SCN-, citrate, Cys, GSH, H2PO4 , SO42-, Ca2+, Cu2+, Mg2+, Zn2+, H2O2,
-
-
ClO-, CN-, S2O42-, N3 ) in PBS buffer.
Fig. 7 Fluorescence images of A431 cells incubated with 5.0 ꢀM probe
1
for 30 min at 37
℃
(bottom row) and pre-treated with 100.0 ꢀM
for 30 min at 37 (top
NaHS, followed by incubation with probe
1
℃
row). (a, c) Bright field images; (b, d) Fluorescence images.
2. Conclusions
In conclusion, we have developed a 3-hydroxyflavone-based
fluorescent probe, 1, for the detection of H2S with high
sensitivity and selectivity. This probe can serve as a colorimetric
and fluorescent dual probe for H2S due to the colour and
fluorescence induced by the interaction of probe 1 with H2S. It
also displays a rapid response, a low detection limit (20 nM), and
a large Stokes shift (146 nm). Importantly, the probe was
Fig. 6 The fluorescence response of probe 1 (10.0 ꢀM) in response to the
tested analytes (black bars) and NaHS (200.0 ꢀM) with the competition
analytes (500.0 ꢀM A. F-, B. NO3 , C. OAc-, D. S2O32-, E. Cl-, F. NO2 , G.
-
-
-
-
HCO3 , H. SCN-, I. citrate, J. Cys, K. GSH, L. H2PO4 , M. SO42-, N. Ca2+, O.

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3. Kimura, H.; Nagai, Y.; Umemura K.; Kimura, Y. Antioxid. Redox Sign.
successfully demonstrated to the practical utility in the imaging
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2005, 7, 795.
4. Hu, L. F.; Lu, M.; Wu, Z.; Wong, Y. P. T. H.; Bian, J. S. Mol.
of H2S in living A431 cells.
3. Experimental section
3.1. Instruments and Materials
Pharmacol. 2009, 75, 27.
5. Yang, G.; Wu, L.; Jiang, B.; Yang, W.; Qi, J.; Cao, K.; Meng, Q.;
Mustafa, A. K.; Mu, W.; Zhang, S. S.; Snyder, H.; Wang, R. Science
2008, 322, 587.
Unless otherwise stated, solvents were purified by standard
methods prior to use. All reagents were purchased from
commercial suppliers and used as received. Twice-distilled water
was used throughout all experiments. NMR spectra were
recorded on a BRUKER 500 or 600 spectrometer, All accurate
6. Kimura, H. Amino Acids 2011, 41, 113.
7. Kulkarni, K. H.; Monjok, E. M.; Zeyssig,mR.; Kouamou, G.; Bongmba,
O. N.; Opere, C. A.; Njie, Y. F.; Ohia, S. E. Neurochem. Res. 2009, 34,
400.
8. Ding, Y. B.; Tang, Y. Y.; Zhu, W. H.; Xie, Y. S. Chem. Soc. Rev. 2015,
44, 1101.
9. Guo, Z. Q.; Park, S.; Yoon, J.; Shin, I. Chem. Soc. Rev. 2014, 43, 16.
10. Xiong, B.; Peng, L.; Cao, X.; He, Y.; Yeung, E. S. Analyst 2015, 140,
1763.
11. Lin, V. S.; Chen, W.; Xian, M.; Chang, C. J. Chem. Soc. Rev. 2015, 44,
4596.
12. Li, J.; Yin, C.; Huo, F. RSC Adv. 2015, 5, 2191.
13. Wang, F. Y.; Zhou, L.; Zhao, C. C.; Wang, R.; Fei, Q.; Luo, S. H.; Guo,
Z. Q.; Tian, H.; Zhu, W. H. Chem. Sci. 2015, 6, 2584.
14. Xie, H. F.; Jiang, X. H.; Zeng, F.; Yu, C. M.; Wu, S. Z. Sens. Actuators
B: Chem. 2014, 220, 504.
15. Li, Y. H.; Sun, Y.; Li, J. C.; Su, Q. Q.; Yuan, W.; Dai, Y.; Han, C. M.;
Wang, Q. H.; Feng, W.; Li, F. Y. J. Am. Chem. Soc. 2015, 137, 6407.
16. Li, P.; Xiao, H. B.; Cheng, Y. F.; Zhang, W.; Huang, F.; Zhang, W.;
Wang, H.; Tang, B. Chem. Commun. 2014, 50, 7184.
17. Chen, Y. C.; Bai, Y.; Han, Z. W.; He, J.; Guo, Z. J. Chem. Soc. Rev.
2015, 44, 4517.
18. Xu, S. Y.; Sun, X. L.; Ge, H. B.; Arrowsmith, R. L.; Fossey, J. S.;
Pascu, S. I.; Jiang, Y. B.; James, T. D. Org, Biomol. Chem. 2015, 13,
4143.
19. Yuan, L.; Lin,W.; Zheng, K.; He, L.; Huang, W. Chem. Soc. Rev. 2013,
42, 622.
mass spectrometric experiments were performed on
a
micrOTOF-Q II mass spectrometer (BrukerDaltonik, Germany).
UV-Vis absorption spectra were measured using a Shimadzu
UV-2450 spectrophotometer. Uncorrected emission spectra were
recorded at room temperature on a HITACHI F4600 fluorescence
spectrophotometer with both the excitation and emission slit
widths set at 5.0 nm. Cell imaging was performed with an
Olympus 71X microscope. TLC analysis was performed on silica
gel plates and column chromatography was conducted over silica
gel (mesh 200–300), both of which were obtained from Qingdao
Ocean Chemicals.
3.2. Synthesis of Probe 1
41
A mixture of 3-hydroxyflavone 2 (238 mg, 1.0 mmol) and
NBD-Cl (199 mg, 1.0 mmol) was dissolved in 20 mL ethanol.
Triethylamine (121 mg, 1.2 mmol) was added to the reaction
mixture, and the resultant reaction mixture was stirred at 45℃
overnight. The obtained solid was isolated by filtration, washed
with 10 mL ethanol and yielding a brown solid 1 (248 mg, 62%).
1H NMR (500 MHz, DMSO) δ 8.61 (d, J = 8.4 Hz, 1H), 8.12 (dd,
J = 7.9, 1.3 Hz, 1H), 7.95 (ddd, J = 15.8, 11.6, 7.4 Hz, 4H), 7.65
– 7.50 (m, 4H), 7.34 (d, J = 8.4 Hz, 1H). 13C NMR (150 MHz,
DMSO) 171.37, 157.71, 155.82, 151.37, 145.18, 144.81, 135.49,
135.44, 135.25, 132.45, 131.48, 129.51, 128.82, 126.33, 125.66,
123.81, 119.35, 110.71. HRMS (EI) m/z calcd for [C21H11N3O6 +
Na]+: 424.0648, Found : 424.0549.
20. Wu, B. Y.; Yan, X. P. Chem. Commun. 2015, 51, 3903.
21. Lim, S. Y.; Hong, K. H.; Kim, D. I.; Kwon, H.; Kim, H. J. J. Am. Chem.
Soc. 2014, 136, 7018.
22. Zhang, P. S.; Li, J.; Li, B. W.; Xu, J. S.; Zeng, F.; Lv, J.; Wu, S. Z.
Chem. Commun. 2015, 51, 4414.
23. Zheng, Y.; Zhao, M.; Qiao, Q.; Liu, H.; Lang, H.; Xu, Z. Dyes Pigm.
2013, 98, 367.
24. Li, W.; Sun,W.; Yu, X.; Du, L.; Li, M. J. Fluoresc. 2013, 23, 181.
25. Zhang, J.; Guo, W. Chem. Commun. 2014, 50, 4214.
26. Chen, B.; Li, W.; Lv, C.; Zhao, M.; Jin, H.; Jin, H.; Du, J.; Zhang L.;
Tang, X. Analyst 2013, 138, 946.
27. Yang, Y.; Yin, C.; Huo, F.; Zhang, Y.; Chao, J. Sens. Actuators, B
2014, 203, 596.
28. Fu, L.; Tian, F. F.; Lai, L.; Liu, Y.; Harvey, P. D.; Jiang, F. L. Sens.
Actuators, B 2014, 193, 701.
29. Xuan, W.; Pan, R.; Cao, Y.; Liu, K.; Wang, W. Chem. Commun. 2012,
48, 10669.
30. Liu, C.; Pan, J.; Li, S.; Zhao, Y.; Wu, L. Y.; Berkman, C. E.; Whorton,
A. R.; Xian M. Angew. Chem., Int. Ed. 2011, 50, 10327.
31. Peng, B.; Chen, W.; Liu, C.; Rosser, E. W.; Pacheco, A.; Zhao, Y.;
Aguilar, H. C.; Xian, M. Chem. – Eur. J. 2014, 20, 1010.
32. Xu, Z.; Xu, L.; Zhou, J.; Xu, Y.; Zhu, W.; Qian, X. Chem.Commun.
2012, 48, 10871.
33. Hou, F.; Huang, L.; Xi, P.; Cheng, J.; Zhao, X.; Xie, G.; Shi, Y.; Chen,
F.; Yao, X.; Bai D.; Zeng, Z. Inorg. Chem. 2012, 51, 2454.
34. Liu, T.; Xu, Z.; Spring D. R.; Cui, J. Org. Lett. 2013, 15, 2310.
35. Zheng, K.; Lin, W.; Tan, L.; Cheng, D. Anal. Chim. Acta 2015, 853,
548.
36. Wei, C.; Zhu, Q.; Liu, W.; Chen, W.; Xi, Z.; Yi, L. Org. Biomol. Chem.
2014, 12, 479.
37. Wei, C.; Wei, L.; Xi, Z.; Yi, L. Tetrahedron Lett. 2013, 54, 6937.
38. Wang, J.; Yu, H.; Li, Q.; Shao, S. Talanta 2015, 144, 763.
39. Buffa, M.; Carturan, S.; Quaranta, A.; Maggioni, G.; Mea, G. D. Opt.
Mater. 2012, 34, 1219.
3.3. Imaging of A431 cells
A431 cells were seeded in a 12-well plate in Dulbecco’s
modified Eagle’s medium (DMEM) supplemented with 10%
fetal bovine serum and 1% penicillin, incubated under the
atmosphere of 5% CO2 and 95% air at 37 ℃ for 24 h.
Immediately before the experiments, the cells were washed with
PBS buffer. the cells were pre-incubated with NaHS (100.0 ꢀM)
for 30 min. After washing with PBS three times, A431 cells were
then incubated with the probe (5.0 ꢀM) for 30 min at 37 °C. For
the control experiment, A431 cells were incubated with the probe
(5.0 ꢀM) for 30 min at 37 °C, and Fluorescence imaging was
performed after washing the cells three times with PBS buffer.
The fluorescence images were obtained using Olympus 71X
microscope for collecting experiment images.
Acknowledgements
The research was supported by Item of Scientific Research
Fund for Doctor of Qiqihar Medical University (No. QY2016B-
18).
References and notes
†
Electronic supplementary information (ESI) available. See DOI:
10.1016/j.tet.2015.xx.xxx.
40. For example, the concentration of H2S could be as low as low ꢀM or
even pM, while common thiols such as GSH could be up to 5 mM high;
catalytic cysteine in proteins could have a pka much lower than 7,
which should then compete strongly with H2S to react with the probe.
1. Li, L.; Rose, P.; Moore, P. K. Annu. Rev. Pharmacol. 2011, 51, 169.
2. d’Emmanuele, R.; Bianca, di Villa, Sorrentino, R.; Mirone, V.; Cirino,
G. Nat. Rev. 2011, 8, 286.

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41. Liu, B.; Wang, H.; Wang, T.; Bao, Y.; Du, F.; Tian, J.; Li Q.; Bai, R.
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Chem. Commun. 2012, 48, 2867.