Anal. Chem. 2007, 79, 2179-2183
Correspondence
Effect of Polycyclic Aromatic Hydrocarbons on Detection Sensitivity of Ultratrace Nitroaromatic Compounds Hong-Xia Zhang,† Qing Chen,† Rui Wen,† Jin-Song Hu, and Li-Jun Wan*
Beijing National Laboratory for Molecular Science (BNLMS), Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100080, P. R. China
Polycyclic aromatic hydrocarbons (PAHs) with different numbers of π-electrons and geometric symmetry of π-systems, including anthracene, phenanthrene, pyrene, triphenylene, perylene, benzo[ghi]perylene, and coronene, were chosen to modify glassy carbon electrodes (GCEs) by self-assembling. The self-assembled monolayer of PAHs was investigated by STM and was used in the electrochemical detection of nitroaromatic compounds (NACs). The results indicate that PAH-modified GCE shows higher sensitivity to NACs than an unmodified one. Among the seven different PAHs, coronene-modified GCE exhibits the highest sensitivity to 2,4,6-trinitrotoluene and 1,3,5-trinitrobenzene.
gated.3 Recently, the study on SAMs has been shifting from a fundamental understanding of the formation and structure of SAMs toward their potential technological applications in the field of chemical sensors.4-9 For example, thioctic acid SAMs was used for pH sensing by an electrostatic interaction method.5 Tetradentate SAMs was used for selective binding of Cu2+ through highly selective ligands.6 Crown-annelated tetrathiafulvalene was employed for an Ag+ sensor by host-guest interactions.7 It is known that noncovalent interaction between donor-acceptor of π-π electrons plays an important role in stabilizing the double-helical structure of DNA10 and intercalating drugs into DNA.10,11 The π-π interaction widely exists between organic molecules and can be used for preparing SAMs.12-16
The research and development efforts to detect ultratrace amounts of nitroaromatic compounds (NACs) have been accelerated by the needs in public security and environmental protection. Compared with the conventional methods, the electrochemical method can reduce the cost and complexity of the instrument for NAC detection.1 Moreover, the surface of the electrode used in an electrochemical measurement can be chemically modified with designable molecules or materials, through which the prerequisites for possible applications would be established.2 Self-assembled monolayers (SAMs) are known to create ordered and oriented molecular layers and offer an opportunity to functionalize surfaces with controlled morphologies and reactivities.3 Because of their stability and the ease of preparation, surface modifications with SAMs have been extensively investi-
(3) (a) Huskens, J.; Deij, M. A.; Reinhoudt, D. N. Angew. Chem. Int. Ed. 2002, 41, 4467. (b) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (c) Thery-Merland, F.; Methivier, C.; Pasquinet, E.; Hairault, L.; Pradier, C. M. Sens. Actuators B 2006, 114, 223. (d) Li, S. S.; Xu, L. P.; Wan, L. J.; Wang, S. T.; Jiang, L. J. Phys. Chem. B 2006, 110, 1794. (e) Kondo, T.; Horiuchi, S.; Yagi, I.; Ye, S.; Uosaki, K. J. Am. Chem. Soc. 1999, 121, 391. (f) Yamada, R.; Uosaki, K. Langmuir 2001, 17, 4148. (4) (a) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Mater. 2000, 12, 1315. (b) Crooks, R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219. (c) Schierbaum, K. D.; Weiss, T.; van Velzen, E. U. T.; Engbersen, J. F. J.; Reinhoudt, D. N.; Gopel, W. Science 1994, 265, 1413. (5) (a) Cheng, Q.; Brajter-Toth, A. Anal. Chem. 1992, 64, 1998. (b) Cheng, Q.; Brajter-Toth, A. Anal. Chem. 1995, 67, 2767. (c) Cheng, Q.; BrajterToth, A. Anal. Chem. 1996, 68, 4180. (6) (a) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (b) Steinberg, S.; Tor, Y.; Sabatani, E.; Rubinstein, I. J. Am. Chem. Soc. 1991, 113, 5176. (c) Steinberg, S.; Rubinstein, I. Langmuir 1992, 8, 1183. (7) Moore, A. J.; Goldenberg, L. M.; Bryce, M. R.; Petty, M. C.; Monkman, A. P.; Marenco, C.; Yarwood, J.; Joyce, M. J.; Port, S. N. Adv. Mater. 1998, 10, 395. (8) Flink, S.; Boukamp, B. A.; van den Berg, A.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 1998, 120, 4652. (9) Kitano, H.; Makino, Y.; Kawasaki, H.; Sumi, Y. Anal. Chem. 2005, 77, 1588. (10) Saenger, W. Principles of Nucleic Acid Structure; Springer-Verlag: New York, 1984; pp 132-140. (11) Wakelin, L. P. G. Med. Res. Reol. 1986, 6, 275. (12) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525. (13) Lewis, F. D.; Daublain, P.; Santos, G. B. D.; Liu, W.; Asatryan, A. M.; Markarian, S. A.; Fiebig, T.; Raytchev, M.; Wang, Q. J. Am. Chem. Soc. 2006, 128, 4792. (14) Okabe, A.; Fukushima, T.; Ariga, K.; Aida, T. Angew. Chem. Int. Ed. 2002, 41, 3414. (15) Zyss, J.; Ledoux-Rak, I.; Weiss, H.-C.; Blaser, D.; Boese, R.; Thallapally, P. K.; Thalladi, V. R.; Desiraju, G. R. Chem. Mater. 2003, 15, 3063.
* Corresponding author. E-mail:
[email protected]. Fax: (+86) 10-62558934. † Also in Graduate School of CAS, Beijing, P. R. China. (1) (a) Krausa, M.; Schorb, K. J. Electroanal. Chem. 1999, 461, 10. (b) Kalvoda, R. Electroanal. 2000, 12, 1207. (c) Zhu, X.; Han, K.; Li, G. Anal. Chem. 2006, 78, 2447. (2) (a) Wang, J.; Hocevar, S. B.; Ogorevc, B. Electrochem. Commun. 2004, 6, 176. (b) Walcarius, A. C. R. Chim. 2005, 8, 693. (c) Li, S.; Wang, X.; Beving, D.; Chen, Z.; Yan, Y. J. Am. Chem. Soc. 2004, 126, 4122. (d) Weng, J.; Xue, J.; Wang, J.; Ye, J.-S.; Cui, H.; Sheu, F.-S.; Zhang, Q. Adv. Funct. Mater. 2005, 15, 639. (e) Inoue, T.; Kirchhoff, J. R. Anal. Chem. 2000, 72, 5755. (f) Fireman-Shoresh, S.; Turyan, I.; Mandler, D.; Avnir, D.; Marx, S. Langmuir 2005, 21, 7842. (g) Song, Y.-Y.; Zhang, D.; Gao, W.; Xia, X.-H. Chem. Eur. J. 2005, 11, 2177. (h) Liu, G.; Lin, Y. Anal. Chem. 2005, 77, 5894. 10.1021/ac0618268 CCC: $37.00 Published on Web 02/01/2007
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NACs, with one or more nitro substitutes on the benzene ring, are strong π-electron acceptors. On the other hand, polycyclic aromatic hydrocarbons (PAHs) are π-electron donors with rich π-electrons and planar structures,17 which facilitates complexation with suitable acceptors. The fluorescence quenching of pyrene was proved to be an indirect detection method for NACs.18 The quenching mechanism is generally thought to involve the formation of a charge-transfer complex between pyrene and NACs, in which aromatic quenchers serve as electron acceptors with PAH fluorophores. NACs and PAHs have also been investigated in other donor-acceptor π-systems, showing the strong π-π interactions between them.13-15 An electrochemical sensor for ultratrace NACs using mesoporous silica (MCM-41) to modify the glassy carbon electrode (GCE) was recently reported.19a The high sensitivity is attributed to the strong adsorption of NACs by MCM-41 and large surface area of the working electrode resulting from MCM-41 modification. In a previous report, we demonstrates a novel electrochemical method for fast and sensitive detection of ultratrace 2,4,6trinitrotoluene (TNT) based on the triphenylene (TP) functionalized multiwalled carbon nanotubes (MWCNTs).19b Electrochemical experiment suggests, compared to an MWCNT-modified electrode, the TP-MWCNT-modified electrode results in both faster response and higher sensitivity to TNT detection. These results show that the attachment of TP on MWCNTs leads to more receptor sites to TNT, associated with the coordinative recognition of TP and MWCNTs to TNT, and results in improvement of response and sensitivity. In the present paper, a series of PAHs, including anthracene, phenanthrene, pyrene, triphenylene, perylene, benzo[ghi]perylene, and coronene, are used to modify GCEs. The effect of the PAHs-modified GCEs on the detecting sensitivity of NACs is intensively investigated. The results show that the sensitivity is generally enhanced by PAH modifications, with coronene-modified GCE exhibiting the highest sensitivity. The level of enhancements is influenced by the number of π-electrons and the geometric symmetry of the π-system of PAHs, resulting in different peak potentials and overall shapes of NACs in voltammograms. EXPERIMENTAL SECTION The chemical structures of all PAHs used in the study are illustrated in Figure 1. 2,4,6-Trinitrotoluene (TNT, 1 g/L in acetonitrile), 1,3,5-trinitrobenzene (TNB) (Sigma-Aldrich), 2,4dinitrotoluene (2,4-DNT), 1,3-dinitrobenzene (1,3-DNB) (TCI), anthracene (Merck-Schuchardt), pyrene (Alfa Aesar), triphenylene (Kanto Chemical), perylene, coronene (Aldrich), benzo[ghi]perylene (Accustandard), phenanthrene, N,N-dimethylformamide (DMF), toluene, and sodium chloride (Beijing Chemical Reagent Ltd.) were used as received. Milli-Q water was used in all experiments. The structure of coronene SAM was observed with a Nanoscope IIIa STM (Digital Instrument Co.) under ambient conditions (16) Tang, T.; Qu, J.; Mullen, K.; Webber, S. E. Langmuir 2006, 22, 26. (17) (a) Wang, Z.; Dotz, F.; Enkelmann, V.; Mullen, K. Angew. Chem. Int. Ed. 2005, 44, 1247. (b) Samori, P.; Severin, N.; Simpson, C. D.; Mullen, K.; Rabe, J. P. J. Am. Chem. Soc. 2002, 124, 9454. (c) Watson, M. D.; Fechtenkotter, A.; Mullen, K. Chem. Rev. 2001, 101, 1267. (18) Goodpaster J. V.; McGuffin, V. L. Anal. Chem. 2001, 73, 2004. (19) (a) Zhang, H.-X.; Cao, A.-M.; Hu, J.-S.; Wan, L.-J.; Lee, S.-T. Anal. Chem. 2006, 78, 1967. (b) Zhang, H.-X.; Hu, J.-S.; Yan, C.-J.; Jiang, L.; Wan, L.-J. Phys. Chem. Chem. Phys. 2006, 8, 3567.
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Figure 1. Chemical structures of PAHs used to modify GCE.
with mechanically cutting Pt/Ir (90/10%) tips.20 The sample was prepared by depositing DMF solution containing coronene onto freshly cleaved highly oriented pyrolytic graphite (HOPG) surface. Coronene molecules self-assembled onto the HOPG surface for 10 min, and the residual solution was rinsed with Milli-Q water. Before electrochemical experiments, the GCE was well polished with 0.3 µm alumina slurry, sonicated in Milli-Q water, and finally dried in an oven at 318 K. The cleaned electrode was immersed in DMF solution containing PAHs for 10 min. PAH molecules would self-assemble on the electrode surface to yield functionalized surfaces. The effect of immersing time and solution concentrations of PAHs on the detection is shown in Figures S1 and S2 of Supporting Information. After modification, the electrode was rinsed with Milli-Q water and used to detect the NACs by voltammetry. The supporting electrolyte solution used in the experiments was 0.5 M NaCl solution. High-purity nitrogen gas was used to bubble the solution to remove oxygen from the solution in the electrochemical cell; then a constant flow of high-purity nitrogen was maintained over the solution during the measurements. Voltammograms were performed with a Princeton Applied Research (PAR) model 2273 electrochemical system at room temperature. A conventional three-electrode electrochemical cell was employed in the experiment, which consists of the modified GC disk (3 mm in diameter) as the working electrode, a saturated calomel electrode as the reference electrode, and a platinum wire as the counter electrode. During the electrochemical detection, the potential was scanned linearly at 20 mV/s.1b,19 (Due to the carcinogen properties of some PAHs used in this method, the experiments must be carried out with through planning and adequate safety measures!) RESULTS AND DISCUSSION STM Investigation of Coronene SAMs on HOPG. Figure 2A and B show large-scale and high-resolution STM images of coronene on the HOPG surface. It can be seen that coronene molecules adsorb on HOPG and self-organize into a well-defined structure. The self-assembly is considered to be originated from the π-π interaction between coronene and HOPG surface, suggesting that PAHs would also form adlayers on GCE surface. As shown in Figure 2B, the coronene molecules are arranged in (20) (a) Wan, L. -J. Acc. Chem. Res. 2006, 39, 334. (b) Noda, H.; Wan, L. -J.; Osawa, M. Phys. Chem. Chem. Phys. 2001, 3, 3336.
Figure 2. (A) Large-scale and (B) high-resolution STM images of coronene adlayer on HOPG. Tunneling conditions: Ebias ) 430 mV and Itip ) 350 pA for (A) and Ebias ) 515 mV and Itip ) 412 pA for (B). (C) Molecular model of coronene adlayer.
Figure 3. Voltammograms for anthracene-modified GCEs in 0.5 M NaCl solution containing TNT (A), TNB (B), 2,4-DNT (C), and 1,3DNB (D).
a 6-fold symmetry. The distance between the adjacent coronene molecules is a ) b ) 11.2 ( 0.2 Å, and the angle R ) 60 ( 2°. Figure 2C is a schematic illustration of the structure. Anthracene- and Phenanthrene-Modified Electrodes. GCE was modified in 6 mM anthracene in DMF solution. The corresponding voltammograms were recorded in 0.5 M NaCl solution containing different concentrations of TNT. Figure 3 shows the voltammograms of anthracene-modified GCE in NAC-containing
solution. Compared with bare GCE, the anthracene-modified electrode shows higher sensitivity to TNT. The 1.1 µM TNT solution generates three redox signals at -0.50, -0.65, and -0.81 V, which are attributed to the stepwise reduction of three nitro groups.19,21 With decreasing TNT concentration, the peak current reduces. However, when TNT concentration was reduced to 132.1 nM, the first reduction peak is still visible and can be identified by comparing it with the voltammogram of the anthracenemodified electrode in an analyte free solution of 0.5 M NaCl. Unmodified GCEs were used as a parallel experiment to anthracene-modified GCEs to compare the detection sensitivity. The corresponding voltammograms (shown in Supporting Information Figure S3) indicate ∼4 times less sensitivity than that of anthracene-modified GCEs, to electrochemical detection of TNT. In addition, the overall voltammogram shape of TNT by anthracene-modified GCE is also different from that by unmodified GCE. Experiments were also carried out to detect other NACs with anthracene-modified electrodes in 0.5 M NaCl solution. As shown in Figure 3B, 703.9 nM TNB solution generates three well-defined redox signals at -0.46, -0.61, and -0.72 V, corresponding to the stepwise reduction of three Ar-NO2 groups.19,21 There were still weak peaks when TNB concentration was reduced to 140.8 nM, while no signal was found with the same concentration by unmodified GCE (shown in Supporting Information Figure S3). Figure 3C shows the voltammograms of trace 2,4-DNT. There are two redox signals at -0.66 and -0.82 V from the solution containing 329.4 nM 2,4-DNT.19,21 When 2,4-DNT concentration was reduced to 82.4 nM, the two redox signals were still visible, and this is ∼6 times more sensitive than that by unmodified GC electrode (see Figure S3 in Supporting Information). Figure 3D shows the voltammograms of trace 1,3-DNB. There are also two redox signals at -0.6 and -0.74 V in the solution containing 535.4 nM 1,3-DNB.19,21 There are still two weak peaks when 1,3-DNB concentration was reduced to 89.2 nM. As to these four NACs, the peak currents are all linear with the scan rates, showing the reduction signals originate from NACs adsorbed onto the anthracene-modified surface, the same as that by unmodified GCE. To compare the influence of the geometric symmetry of the π-system to NAC detection, phenanthrene, which has the same number of benzene units as anthracene, was also used to modify (21) (a) Wang, J.; Bhada, R. K.; Lu, J.; MacDonald, D. Anal. Chim. Acta 1998, 361, 85. (b) Haderlein, S. B.; Schwarzenbach, R. P. Environ. Sci. Technol. 1993, 21, 316. (c) Haderlein, S. B.; Weissmahr, K. W.; Schwarzenbach, R. P. Environ. Sci. Technol. 1996, 30, 612. (d) Weissmahr, K. W.; Haderlein, S. B.; Schwarzenbach, R. P.; Hany, R.; Nuesch, R. Environ. Sci. Technol. 1997, 31, 240. (e) Boyd, S. A.; Sheng, G. Y.; Teppen, B. J.; Johnston, C. J. Environ. Sci. Technol. 2001, 35, 4227.
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GCE. A 6 mM phenanthrene solution in DMF was used. The corresponding voltammograms to NAC detection (shown in Supporting Information Figure S4) indicates that phenanthrenemodified GCE has almost the same sensitivity as that by anthracene-modified GCE. Pyrene-, triphenylene-, and perylene-Modified Electrodes. Pyrene, triphenylene, and perylene were used to modify GCE in the detection of NACs. The corresponding voltammograms are shown in Supporting Information Figure S5, S6, and S7, respectively). The results show that the geometric symmetry of the π-system also influences detection sensitivity. Pyrene- or perylene-modified GCEs have almost the same sensitivity to NACs as that of anthracene-modified GCE. The sensitivities of triphenylene-modified electrode to TNT, 2,4-DNT, and 1,3-DNB change a little. However, triphenylene-modified GCE displays higher sensitivity to TNB, almost 5 times that of anthracene-modified GCE. When TNB concentration was reduced to 28.2 nM, three redox peaks were still visible. Both triphenylene and TNB molecules present 3-fold symmetry, and their corresponding parts of electrostatic potential have complementary charges,22 which may result in good recognition between triphenylene and TNB molecules. The shapes of the voltammograms from these three PAHmodified GCEs are different. For example, the first reduction peak of TNT is relatively enhanced by triphenylene- or perylenemodified GCE; as to TNB, the three reduction peaks by triphenylene-modified GCE have almost the same intensity, while the third reduction peak by other PAHs-modified GCEs can not been discerned at low TNB concentration. Benzo[ghi]perylene- and Coronene-Modified Electrodes. To further explore the π-π interactions between a PAH molecule and NAC, and to improve NAC detection sensitivity, benzo[ghi]perylene and coronene, which have more dense π-electrons, were chosen to modify GCEs. Since the solubility of benzo[ghi]perylene in DMF is very low, its saturated solution of ∼0.7 mM was used to modify GCE. The corresponding voltammograms to NACs are illustrated in Supporting Information Figure S8. Compared with voltammograms in TNT-containing solution by a 0.7 mM perylene-modified electrode (shown in Supporting Information Figure S2B), benzo[ghi]perylene is more sensitive than perylene. A 1.3 µM TNT solution generates three well-defined redox signals at -0.56, -0.71, and -0.86 V. In contrast with those of other PAHs-modified electrodes, all these three redox potentials shift to negative. Similarly, the redox potentials in the solutions containing TNB, 2,4-DNT, and 1,3-DNB also shift to negative. The saturated solution of coronene in DMF (∼1 mM) was used to modify GCE. Voltammograms of NACs by coronene-modified electrode are illustrated in Figure 4. Compared with TNT voltammograms by 1 mM perylene-modified electrode (shown in Supporting Information Figure S2C), the sensitivity to TNT is greatly improved. The 132.1 nM TNT generates three well-defined redox signals at -0.40, -0.50, and -0.66 V. There are still two strong peaks when TNT concentration was reduced to 66.0 nM, and this sensitivity is ∼8 times that from unmodified GCE. Moreover, compared with that of other PAHs-modified electrodes, all the three redox potentials shift to positive, and the peak shape changes also, which may be attributed to the highest symmetry 2182 Analytical Chemistry, Vol. 79, No. 5, March 1, 2007
Figure 4. Voltammograms for coronene-modified GCEs in 0.5 M NaCl solution containing TNT (A), TNB (B), 2,4-DNT (C), and 1,3DNB (D).
(D6h) of the π-system of coronene among these PAHs. Detection of TNB by coronene-modified GCE also shows much higher sensitivity than that of other PAHs-modified GCEs. There are still two peaks in the low concentration of 14.1 nM TNB, 10 times that from perylene-modified GCE, and 20 times that of unmodified GCE. As to the voltammograms of 2,4-DNT and 1,3-DNB by coronene-modified electrodes, the sensitivity is comparable to that from other PAHs-modified electrodes. The detection limits to NACs by different PAH-modified GCEs are listed in Table 1, and voltammograms of TNT and TNB by unmodified GCE and different PAHs-modified GCEs are compared in Figure 5. The detection limit is defined herein as the lowest concentration with identifiable response compared with the blank solution.23 Data in Table 1 and curves in Figure 5 indicate that the detection sensitivity is influenced by both the number of π-electrons and the geometric symmetry of the π-system of PAHs. (22) Gardner, J. W.; Yinon, J. Electronic noses & sensors for the detection of explosives; Kluwer Academic Publishers: London, 2003. (23) Hrapovic, S.; Majid, E.; Liu, Y.; Male, K.; Luong, J. H. T. Anal. Chem. 2006, 78, 5504.
Table 1. Comparison of Detection Sensitivity to NACs by Different PAH-Modified GCEs PAH detection limit, nM NACs
bare GC
anthracene
phenanthrene
pyrene
triphenylene
perylene
benzo[ghi]perylene
coronene
TNT TNB DNT DNB
528.3 281.6 494.2 89.2
132.1 140.8 82.4 89.2
132.1 140.8 82.4 89.2
132.1 140.8 82.4 89.2
132.1 28.2 82.4 89.2
132.1 140.8 82.4 89.2
252.4 140.8 82.4 178.5
66.0 14.1 82.4 178.5
Figure 5. Voltammograms for unmodified GCE and different PAHmodified GCEs in 0.5 M NaCl solution containing 254.2 nM TNT (A) and 281.6 nM TNB (B). Lines a-h are for unmodified GCE, and anthracene-, phenanthrene-, pyrene-, triphenylene-, perylene-, benzo[ghi]perylene-, and coronene-modified GCEs, respectively.
The existence of PAH adlayers on GCEs would not hinder NAC detection. Coronene, with most π-electrons and highest geometric symmetry of the π-system, has highest sensitivity to TNT and TNB detection. Triphenylene, because of the same π-system symmetry as that of TNB, shows high sensitivity to TNB. The results indicate that the modified molecules on GCEs will change the electrochemical responses of NAC molecules. The response is related to molecular chemical structures including π-electron and π-system symmetry. The intermolecular reaction between PAH molecules and NAC molecules is responsible for the sensitivity difference. By choosing a surface modifier, the detection sensitivity could be increased. The detailed mechanism of the PAH-modified GCEs on the enhanced sensitivity is in progress. CONCLUSION Seven PAHs, with different numbers of π-electrons and different geometric symmetry of the π-system, including an-
thracene, phenanthrene, pyrene, triphenylene, perylene, benzo[ghi]perylene, and coronene, have been used to modify GCEs to detect NACs. Corresponding voltammograms of NACs by modified and bare GCEs are compared. The results show improved sensitivity by PAH modification. The sensitivity is related to both the number and the geometric symmetry of the π-system. Among the seven different PAHs, coronene-modified GCEs exhibit highest sensitivity to TNT and TNB, with ∼8 times and ∼20 times that by unmodified GCEs, respectively, due to the most π-electrons and highest symmetry of π-system of coronene molecule. The results reported in this study will be important in both sensor fabrication and analytical chemistry.
ACKNOWLEDGMENT The authors thank the National Natural Science Foundation of China (Grants 20575070, 20673121, 20603041 and 20121301), National Key Project on Basic Research (Grant 2006CB806100), and the Chinese Academy of Sciences for financial support.
SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
Received for review September 28, 2006. Accepted January 15, 2007.
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