A Coumarin-Based Fluorescent Probe for Selective ... - ACS Publications

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A Coumarin-Based Fluorescent Probe for Selective and Sensitive Detection of Thiophenols and Its Application Jun Li,†,§ Chun-Fang Zhang,†,§ Shu-Hou Yang,† Wen-Chao Yang,*,† and Guang-Fu Yang*,†,‡ †

Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, P.R. China ‡ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 30071, P.R. China S Supporting Information *

ABSTRACT: The development of probes for specific thiophenol detection is of great importance, due to the toxicity of thiophenols and their derivatives in the environment. In the present study, a novel fluorescent probe was rationally designed for detecting thiophenols via an intramolecular charge transfer mechanism. The developed probe selectively and sensitively distinguished thiophenols from aliphatic thiols. It displayed a large Stokes shift (145 nm) and >280-fold fluorescence enhancement. Moreover, the new probe not only displayed excellent cell permeability for the successful detection of thiophenol in HEK293 cells but also quantitatively measured thiophenols in water samples with good recovery (more than 90%), indicating that it has promising prospects for application for thiophenol sensing in environmental and biological sciences.

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selective, sensitive thiophenol probe, with a mechanism based on an ICT pathway. This probe has the advantages of (1) a larger Stokes shift (145 nm) than other reported thiophenol probes, (2) a >280-fold fluorescence intensity enhancement in aqueous solution, and (3) feasibility in living cells and aqueous samples. Coumarins are among the most favorable fluorophores. They are attractive starting materials for fluorogenic probes due to their high fluorescence intensity, excellent solubility, efficient cell permeation, and ease of preparation.19−25 We therefore designed a novel coumarin-based fluorescent probe 1 for thiophenols based on the ICT mechanism (see Scheme 1). In this probe, we introduced the strong electron-donating N,Ndiethyl group at the 7 position and the electron-withdrawing 2,4-dinitrobenzenesulfonic amide group in the 3 position of coumarin, which function together as a push−pull system to quench the fluorescence of the fluorophore. We also envisioned that, under physiological pH, the sulfonamide could be readily cleaved by thiophenols (pKa = 6.5), but not by aliphatic thiols (pKa = 8.5), due to the different states determined by their pKa values. To investigate the feasibility of our design concept, probe 1 was synthesized and further evaluated for thiophenol detection in a pure aqueous system.

hiophenols, also called benzenethiols, play an important role in organic synthesis and are widely used in preparing agrochemicals, pharmaceuticals, and various industrial products. However, thiophenols are more toxic than aliphatic thiols; in fish, thiophenols possess a median lethal dose (LC50) of 0.01− 0.4 mM.1−3 In addition, prolonged exposure to thiophenols in water or soil can cause a series of serious health problems, including central nervous system damage, increased respiration, muscle weakness, hind limb paralysis, coma, and even death. Thus, an efficient, simple method for determining thiophenol levels is urgently needed. The development of fluorescent probes for specific analytes has attracted intense interest. This type of probe reliably provides a clear indication of target levels in biological settings.4−10 The first thiophenol probe reported by Wang and colleagues was based on an intramolecular charge transfer (ICT) mechanism, and 4-amino-7-nitro-2,1,3-benzoxadiazole (NBD) was utilized as the fluorophore.11 The probe showed good water solubility and selectivity, but the fluorophore released from the nucleophilic aromatic substitution (SNAr) showed low fluorescence quantum yield (Φ = 0.02). Recently, several other fluorescent probes for thiophenol detection have been developed through either ICT or photoinduced electron transfer (PET) processes. 12−18 Among these reported fluorescent thiophenol probes, some displayed relatively weak fluorescence intensity, some needed organic compound as a cosolvent, some suffered from the need to use high pH values for practical detection, and only a few of them could be applied to monitoring thiophenol in a real environment and in biological samples. Thus, an applicable fluorescent thiophenol probe is still needed. The present work describes a highly © 2014 American Chemical Society

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EXPERIMENTAL SECTION Materials and Chemicals. Unless otherwise noted, all chemical reagents were commercially available and treated with

Received: November 30, 2013 Accepted: February 10, 2014 Published: February 10, 2014 3037

dx.doi.org/10.1021/ac403885n | Anal. Chem. 2014, 86, 3037−3042

Analytical Chemistry

Article

Scheme 1. Diagram of the Probe Design for Thiophenol Detection

of CH2Cl2. The mixture was stirred overnight at room temperature. After the reaction was completed, the mixture was washed in turn by 50 mL of water and saturated brine and dried with anhydrous Na2SO4. After removing the solvent under reduced pressure, the residue was purified by flash column chromatography to give the title compound as a red solid (138 mg). Yield: 30%. 1H NMR (400 MHz, DMSO-d6): δ 10.58 (s, 1H), 8.87 (s, 1H), 8.64 (d, J = 8.0 Hz, 1H), 8.31 (d, J = 8.4 Hz, 1H), 7.83 (s, 1H), 7.50 (d, J = 8.8 Hz, 1H), 6.74 (d, J = 8.8 Hz, 1H), 6.50 (s, 1H), 3.42 (q, J = 6.4 Hz, 4H), 1.11 (t, J = 6.0 Hz, 6H). 13C NMR (100 MHz, DMSO-d6): δ 159.12, 154.99, 150.64, 149.70, 147.33, 140.22, 137.83, 132.02, 129.70, 126.83, 119.62, 114.01, 109.35, 107.07, 96.28, 43.99, 12.18. HRMS cacld for [M + K+]: 501.0482. Found: 501.0480. Chemical Reaction of Probe 1 with Thiophenol. Probe 1 (50 mg, 0.11 mmol) was dissolved in 5 mL of DMSO-PBS (1:1, V/V, pH 7.4) solution, and thiophenol (20 mg, 0.17 mmol) was then added into the solution. After stirring overnight at 50 °C, the mixture was cooled to room temperature, washed with water, and extracted by dichloromethane. The organic layer was evaporated under reduced pressure, and the crude product was purified by flash column chromatography to give the compounds 2 and 3. Compound 2 (3-Amino-7-diethylamino-chromene-2-one). 1 H NMR (400 MHz, CD3CN-d3): δ 7.10 (d, J = 8.8 Hz, 1H), 6.58 (s, 1H), 6.57 (dd, J1 = 2.8 Hz, J2 = 2.4 Hz, 1H), 6.47 (d, J = 2.4 Hz, 1H), 4.16 (s, 2H), 3.43 (q, J = 7.2 Hz, 4H), 1.08 (t, J = 7.2 Hz, 6H). 13C NMR (100 MHz, CD3CN-d3): δ 160.86, 152.30, 148.25, 129.47, 126.93, 113.51, 110.95, 110.35, 98.64, 45.14, 12.77. Compound 3 ((2,4-Dinitrophenyl)(phenyl)sulfane). 1H NMR (400 MHz, CDCl3): δ 9.12 (s, 1H), 8.13 (d, J = 6.8 Hz, 1H), 7.60 (m, 5H), 6.99 (d, J = 9.2 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 148.35, 144.31, 143.87, 135.91, 131.09, 130.71, 129.06, 128.81, 126.85, 121.44. Determination of Quantum Yield. The quantum yields of fluorescence were determined by comparison of the integrated area of the corrected emission spectrum of samples with a reference. Specifically, using fluorescein (Φ = 0.98, 0.1 M NaOH) as a reference, probe 1 (1 mM) was prepared in 0.01 M phosphate (pH 7.4) buffer and diluted to a concentration with the maximum absorption at 370 nm less than 0.05. Then, the UV−vis absorption spectrum was examined, and the corresponding emission at a relevant wavelength of excitation was also measured. After correction for the refractive index of the different solvents determined by Abbe’s refractometer, the quantum yields were calculated using the expression in eq 1.

standard methods before use. Silica gel column chromatography (CC) employed silica gel (200−300 mesh; Qingdao Makall Group Co., Ltd., Qingdao, China). Solvents were dried in a routine way and redistilled. 1H NMR and 13C NMR spectra were recorded in CDCl3, DMSO-d6, or CD3CN-d3 on a Varian Mercury 600 or 400 spectrometer, and resonances (δ) are given in parts per million relative to tetramethylsilane (TMS). The following abbreviations were used to designate chemical shift multiplicities: s = singlet, d = doublet, t = triplet, m = multiplet, and br = broad. High resolution mass spectra (HRMS) were acquired in positive mode on a WATERS MALDI SYNAPT G2 HDMS (MA, USA). Melting points were taken on a Buchi B545 melting point apparatus and were uncorrected. Compounds M1 and M2 were synthesized according to the reported method.26 Synthesis of Intermediates and Probe 1. 7-Diethylamino-3-nitro-chromene-2-one (M1). 4-Diethylamino-salicylaldehyde (3.86 g, 20 mmol), ethylnitroacetate (2.93 g, 22 mmol), 0.3 mL of piperidine, and 0.6 mL of glacial acetic acid were dissolved in 50 mL of n-BuOH, and the reaction mixture was refluxed for 24 h. Orange solids were formed during cooling. The resultant solids were washed with n-BuOH and petroleum ether (2 × 10 mL) and finally dried under vacuum conditions, which afforded an orange solid (3.18 g). Yield: 61%. m.p.: 198− 199 °C. 1H NMR (400 MHz, DMSO-d6): δ 9.03 (s, 1H), 7.74 (d, J = 9.2 Hz, 1H), 6.92 (d, J = 8.8 Hz, 1H), 6.64 (s, 1H), 3.54 (q, J = 6.8 Hz, 4H), 1.16 (t, J = 7.2 Hz, 6H). 13C NMR (100 MHz, DMSO-d6): δ 158.28, 156.98, 154.42, 143.86, 133.21, 125.90, 111.29, 106.03, 96.01, 44.66, 12.25. HRMS calcd for [M + Na+]: 285.0851. Found: 285.0836. 3-Amino-7-diethylamino-chromene-2-one (M2). SnCl2· 2H2O (19.4 g, 85.81 mmol) and 60 mL of 37% HCl were added into a 100 mL round-bottomed flask. Next, compound M1 (3.0 g, 11.45 mmol) was added portion-wise, and the resultant solution was further stirred at room temperature for 6 h. Then, a solution of 5 M NaOH was employed to neutralize the excessive acid, followed by extraction with ethyl acetate. The organic layer was dried with anhydrous Na2SO4 and evaporated to dryness. The crude product was obtained as a brown solid (1.57 g). Yield: 59%. m.p.: 81−82 °C. 1H NMR (600 MHz, DMSO-d6): δ 7.20 (d, J = 9.0 Hz, 1H), 6.70 (s, 1H), 6.61 (d, J1 = J2 = 2.4 Hz, 1H), 6.50 (s, 1H), 5.07 (s, 2H), 3.35 (q, J = 6.6 Hz, 4H), 1.09 (t, J = 7.2 Hz, 6H). 13C NMR (100 MHz, CD3CN-d3): δ 160.73, 152.11, 147.88, 129.33, 126.82, 113.39, 110.85, 110.18, 98.37, 45.07, 12.65. HRMS calcd for [M + Na+]: 255.1109. Found: 255.1095. N-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)-2,4 dinitrobenzenesulfon-amide (1). 2,4-Dinitrobenzenesulfenyl chloride (319 mg, 1.2 mmol) in 6 mL of CH2Cl2 was slowly added to the solution of 3-amino-7-diethylamino-chromene-2-one (232 mg, 1 mmol) and Et3N (101 mg, 1 mmol) dissolved in 10 mL

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(1) dx.doi.org/10.1021/ac403885n | Anal. Chem. 2014, 86, 3037−3042


Article

Scheme 2. Synthetic Route for Probe 1a

Living Cell Imaging and Cytotoxicity Study. Human embryonic kidney (HEK293) cells were cultured in DMEM supplemented with 10% (V/V) fetal bovine serum and 1% (V/ V) 100 mM sodium pyruvate (Gibco). The cells were seeded on 24-well plates in the culture medium and incubated overnight at 37 °C and 5% CO2. Then, probe 1 (20 μM) dissolved in a culture medium containing 0.1% (V/V) DMSO was added into the cells, and the cells were incubated for 30 min at 37 °C. As control experiments, the cells were pretreated with PBS or N-methylmaleimide (NMM, 500 μM) dissolved in PBS for 1 h. Then, the cell samples were washed three times to remove the remaining NMM, followed by treatment with probe 1 (20 μM) in the culture media containing 0.1% (V/V) DMSO. After 30 min of incubation, the treated cells and controls were imaged by inverted fluorescence microscopy (Olympus IX71, Japan) with a 20× objective lens. To study the cytotoxicity, HEK293 cells were seeded at 1 × 105 cells per well in 24-well plates and incubated for 24 h before treatment, followed by exposure to different concentrations (2−50 μM) of probe 1 for an additional 24 h. To determine cell viability, the colorimetric metabolic activity assay with 3(4,5-dimethylthiazol-2-yl)-3,5-diphenytetrazolium bromide (MTT) was used. Cell samples then were incubated with MTT solution (containing 5 mg/mL MTT reagent in PBS) for 4 h, and after removal of the MTT solution, dimethyl sulfoxide was added to dissolve the formazan crystals. The absorbance was measured at 540 nm with a microplate reader (SpectraMax M5, Molecular Devices). Each experiment was performed at least three times. The cytotoxic effect of the probe was assessed through the ratio of the absorbance of the probe treated sample versus the control sample. Measurements of Thiophenol in Water Samples. Considering the possible interference that other components may cause in real samples, the level of thiophenols in water samples was determined using the standard addition method (often referred to as “spiking” samples) according to previous publications.12,16,27 Prior to the real sample detection, the fluorescence of probe 1 (10 μM) in the presence of various concentrations of thiophenol was quantified, and the corresponding plot was prepared as the standard curve (see Figure S1 in the Supporting Information). The crude water samples from the Changjiang River and East Lake in Wuhan city were passed through a microfiltration membrane before use. The pH values of the water samples (50 mL) were adjusted using a sodium phosphate buffer (0.01 M, pH 7.4), and aliquots of the water samples were then spiked with different concentrations of thiophenol (0.05, 0.5, 5 μM) that had been accurately prepared. The resulting samples were further treated with probe 1 in a phosphate buffer (0.01 M, pH 7.4) to give the final mixtures (2.0 mL) containing probe 1 (final concentration = 10 μM) and thiophenol (final concentration = 0.05, 0.5, or 5 μM). The solutions were incubated for 60 min at 30 °C, and the fluorescence was measured with a microplate reader (SpectraMax M5, Molecular Devices). The results (Table 2) were reported as the mean ± standard deviation of triplicate experiments.

a

Reagent and conditions: (a) NO2CH2COOC2H5, n-BuOH, reflux; (b) SnCl2·2H2O, 37% HCl, r.t.; (c) 2,4-dinitrobenzene-1-sulfonyl chloride, Et3N, CH2Cl2, r.t.

probe 1 enabled an investigation of its properties in an aqueous phosphate buffer (0.01 M, pH 7.4). We first examined its fluorescence in the absence of thiophenol. As expected, probe 1 alone exhibited almost no fluorescence and an extremely low quantum yield (Φ = 0.0024), when excited at the maximum absorption wavelength (λex = 370 nm). Since the pH value may affect the fluorescence behavior of the probe (1) and the expected fluorophore (2), we investigated the influence of pH on their fluorescence properties (Figure 1a). The resulting fluorescence change of probe 1 alone was independent of pH, which means both the chemical structure and fluorescence properties of this probe were relatively stable over a broad range from pH 5.0 to pH 10.0. In contrast to probe 1, the fluorescence of 2 that was isolated by column chromatography after chemical preparation, increased with the increment of pH and achieved the maximum value at pH 6.0. In addition, the distribution of fluorophore 2 and its protonated form from pH = 0 to 14 was calculated.28 As depicted in Figure 1b, the diethylamine group of fluorophore 2 remains protonated at low pH (pH ≤ 3.0) and also remains unchanged at neutral to high pH (pH ≥ 6.0). It is noteworthy that the pH-dependent fluorescence response of 2 is consistent with its corresponding distribution. The above study showed that physiological pH (pH 7.4) could be selected as an appropriate working pH for the sensing of thiophenols with the designed probe. As anticipated, upon the addition of thiophenol (4 equiv), a dramatic fluorescence intensity enhancement was observed after a few minutes at 515 nm (Figure 2). Thus, with probe 1, we had established an off−on fluorogenic system with a very large Stokes shift (145 nm) and a remarkable quantum yield improvement (Φ = 0.14). Next, the off−on fluorescence sensing mechanism of probe 1 was validated through the chemical reaction of the designed probe with thiophenol in phosphate buffered saline (PBS, pH = 7.4) solution. The products of the reaction, (2,4-dinitrophenyl)(phenyl)sulfane (3) and 3-amino-7-diethylamino-chromene-2-one (2), were successfully isolated by column chromatography and confirmed by 1H NMR and 13C NMR. Analysis of the reaction mixture with LC-HRMS confirmed that the molecular weight of [2 + H]+ was accurate (Calculated: 233.12845. Found: 233.12822; see Figure S15 in the Supporting Information). Sensitivity and Selectivity of Probe 1 to Thiophenols. To shed light on the sensitivity of probe 1 in pure aqueous solutions, the change in the fluorescence intensity of probe 1 was investigated by adding various concentrations of thiophenols. Considering the moderate range of environmental

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RESULTS AND DISCUSSION Probe Design. Starting with 4-(diethylamino)-2-hydroxybenzaldehyde and 2,4-dinitrobenzene-1-sulfonyl chloride, the new probe 1 was synthesized in three steps (Scheme 2). The chemical structures were well characterized (see Figures S2− S14 in the Supporting Information). The water solubility of 3039



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Figure 1. (a) The experimental study (λex = 370 nm, λem = 515 nm) of pH dependence of 10 μM probe 1 (black) and fluorophore 2 (red) in 0.01 M phosphate buffer (pH 7.4). (b) The calculated species distribution of fluorophore 2 (black) and its protonated form (red) at different pH values ranging from 0 to 14.

thiophenol. Notably, the maximum fluorescence intensity was about 280-fold enhanced in the presence of thiophenol (Figure 3a) and about 290-fold enhanced in the presence of p-methoxythiophenol (Figure 3b). Accordingly, for probe 1, thiophenol and p-methoxy-thiophenol had limits of detection (LODs) of ∼30 nM and ∼18 nM, respectively. The above kinetic study indicated that probe 1 could detect thiophenols with rapid response and high sensitivity. As indicated in Table 1, probe 1 showed its excellent analytical performance compared to other recently developed fluorescent probes for thiophenols. Moreover, the probe described in this article displayed the largest Stokes shift, ∼145 nm, indicating its promising selectivity in practical analysis. To explore the selectivity of the new probe, we investigated the interference from other analytes, such as relevant aliphatic thiols and other common nucleophiles (Figure 4). The aliphatic thiols tested included 2-methyl-2-propanethiol, t-butylmercaptan, cysteine, and glutathione, which are abundant in the biosphere. None of these agents triggered any fluorescence enhancement, even at high concentrations. In addition, no significant change in fluorescence emission intensity was observed upon the addition of some thiophenol analogues, like p-aminophenol or phenol. Only four selected thiophenols (thiophenol, p-amino-thiophenol, p-nitro-thiophenol, and p-

Figure 2. Time-dependent fluorescence spectra (λex = 370 nm) of probe 1 (10 μM) in the presence of thiophenol (40 μM) in 0.01 M phosphate buffer (pH 7.4). Inset: photos of samples illuminated with 365 nm UV light in the absence and presence of thiophenol (40 μM).

temperatures, we chose 30 °C as the working temperature without further optimization, although a higher temperature may be optimal for rapid thiophenol detection. We found that within 10 min the fluorescence intensity increased significantly, depending on the concentration of thiophenol or p-methoxy-

Figure 3. Fluorescence spectra (λex = 370 nm) of probe 1 (10 μM) in the presence of various concentrations of thiophenol (a) and p-methoxythiophenol (b) in a phosphate buffer (0.01 M, pH 7.4). Insets: dependence of relative fluorescence intensity (F/F0) on (a) thiophenol and (b) pmethoxy-thiophenol concentrations. 3040



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Detection of Thiophenols in Living Cells. To study the ability of probe 1 to permeate cells and detect thiophenol in vivo, we performed a preliminary study in HEK293 cells and observed the results with inverted fluorescence microscopy. The direct addition of probe 1 (20 μM) to the cells resulted in bright green fluorescence compared to controls treated with either PBS or pretreated with N-methylmaleimide (NMM, 500 μM; Figure 5). Thus, probe 1 showed excellent cell permeation

Table 1. Comparison of Fluorescent Probes for Thiophenols

Figure 5. Fluorescence microscopy of probe 1 applied to HEK293 cells. (a) Brightfield image of HEK293 cells. (b) Fluorescence image of HEK293 cells treated with probe 1 (20 μM) for 30 min at 37 °C. (c) Fluorescence image of HEK293 cells pretreated with N-methylmaleimide (500 μM) for 30 min at 37 °C and then incubated with probe 1 (20 μM) for 30 min. (d) Merged images in a and b.

capability and could efficiently sense thiophenol in living cells or biological samples. In addition, we investigated the cytotoxicity of the designed probe in HEK293 cells. MTT assays showed that the cells remained in good condition when treated with probe 1 (50 μM) for as long as 4 h. Detection of Thiophenols in Water Samples. To validate its practicality in environmental science, we employed probe 1 in a standard addition method12,27 to determine thiophenol concentrations in water samples from both the Changjiang River and East Lake in Wuhan city. When probe 1 was added directly to the water samples, no significant fluorescence enhancement was observed. When the water samples were spiked with different thiophenol concentrations (0.05 μM, 0.5 μM, and 5 μM) and measured with the current methods, thiophenol recoveries were not less than 92% (Table 2). These results showed that the thiophenol in the water samples could be accurately quantified with good recovery. Thus, probe 1 was highly capable of thiophenol detection in water samples.

Figure 4. The relative fluorescence responses (F/F0) of probe 1 in the absence and presence of different analytes. The probe concentration was 10 μM, and other analytes were added at 40 μM.

methoxy-thiophenol) induced remarkable fluorescence enhancement when added to probe 1. According to the reaction principle of the thiolate-mediated SNAr reaction, a substitution that places a strong electron-withdrawing group on the benzene ring of thiophenol should seriously affect the off−on fluorescence response. As anticipated, p-nitro-thiophenol indeed produced a much lower fluorescence response in probe 1 than the other three thiophenols under the same conditions; this result was consistent with reports in the literature.11,16 Among all the tested analytes, probe 1 displayed the best sensitivity to thiophenols. It displayed a 280-fold fluorescence enhancement upon treatment with thiophenol, which indicated that probe 1 was particularly selective toward thiophenols, and it was not perturbed by any obvious interference.

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CONCLUSION In summary, we successfully designed an ICT-based, off−on fluorescent probe (1) by combining 3-amino-7-diethylaminocoumarin as the fluorophore and 2,4-dinitrobenzene-1sulfonamide as the recognition unit. This probe displayed a 3041



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Table 2. Determination of Thiophenol Concentrations in Water Samples sample

thiophenol spiked (μM)

Changjiang River water

0

East Lake water

0.05 0.5 5 0 0.05 0.5 5

thiophenol recovered (μM)

recovery (%)

not detected 0.052 ± 0.012 0.48 ± 0.023 4.74 ± 0.18 not detected 0.054 ± 0.015 0.47 ± 0.10 4.59 ± 0.22

104 96 95 108 94 92

large Stokes shift, rapid response, good sensitivity, and satisfactory specificity for thiophenol detection in a pure aqueous solution. In addition, probe 1 showed excellent cell permeation and low toxicity. Thus, probe 1 has great potential for the efficient quantification of thiophenols in biological systems. Furthermore, we successfully applied probe 1 to thiophenol determinations in water samples with quantitative recovery. Taken together, our results indicated that probe 1 holds promise for applications involving thiophenol sensing in environmental samples and biological samples.

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ASSOCIATED CONTENT

S Supporting Information *

Materials and chemicals used, a plot of the standard addition method used for the determination of total thiophenols, and NMR and HRMS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: 86-27-67867706. Fax: 86-27-67867141. E-mail: [email protected] *Tel.: 86-27-67867800. Fax: 86-27-67867141. E-mail: gfyang@ mail.ccnu.edu.cn. Author Contributions §

These two authors made equal contributions.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Natural Science Foundation of China (NSFC; No. 21102052 and 21372094) and the National Basic Research Program of China (No. 2010CB126103).

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REFERENCES

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