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Assembly of β-Cyclodextrins Acting as Molecular Bricks onto Multiwall Carbon Nanotubes Kesong Liu,†,‡ Honggang Fu,*,†,§ Ying Xie,† Lili Zhang,† Kai Pan,† and Wei Zhou† Laboratory of Physical Chemistry, School of Chemistry and Materials Science, Heilongjiang UniVersity, 150080 Harbin, Center of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, 100080 Beijing, and State Key Laboratory of Theoretical and Computation Chemistry, Jilin UniVersity, 130023 Changchun, People’s Republic of China ReceiVed: July 19, 2007; In Final Form: October 3, 2007

Multiwall carbon nanotube (MWCNT)-β-cyclodextrin (β-CD) composites have been successfully synthesized through combining polymer wrapping and layer-by-layer self-assembly techniques. The obtained materials were characterized in detail by X-ray diffraction, thermogravimetric analysis-differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and nulcear magnetic resonance. The analytical results indicated that the introduction of β-CD debundled MWCNT bundles and endowed MWCNTs with uniform architecture. Furthermore, the obtained MWCNT-β-CD composites possessed good dispersibility in both ethanol and water media, and the solution was found to be very stable for several weeks. To further clarify the interaction between MWCNTs and β-CD, a molecular mechanics (MM) method was performed for the first time. Combining experimental results with MM simulations, a reasonable formation mechanism is also presented in this paper. The driving forces for the formation of well-defined MWCNT-β-CD composites originated from (1) van der Waals forces between MWCNTs and β-CD and (2) hydrogen-bonding interaction between adjacent β-CD molecules. Both forces induced the ordered assembly and arrangement of β-CD onto the surface of MWCNTs, where β-CD molecules acted as molecular bricks.

Introduction Since the discovery of carbon nanotubes (CNTs) by Iijima in 1991,1 there has been intense activity related to the synthesis, structure, properties, and applications of CNTs due to their unique electronic, mechanical, and chemical properties.2-4 High molecular weights and strong intertube forces cause CNTs to exist as bundles (or ropes) in their native state, which results in the insolubility nature in most common solvents and thus limits the manipulation, processing, characterization, and practical application of CNTs. Much work has been devoted to improving the solubility and processibility of single-wall carbon nanotubes (SWCNTs) through covalent or noncovalent modification methods.5-14 The conventional covalent surface chemical modification strategies with concentrated sulfuric acid, nitric acid, or other strong oxidizing agents tend to achieve the solubilization of CNTs.14 However, these aggressive approaches have been demonstrated to introduce a significant number of defects, deteriorate the intrinsic electronic and mechanical properties, and thus eliminate some potential advantages of CNTs.15 Furthermore, the traditional acid treatment of CNTs causes problems of corrosion and environmental pollution. For these purposes, to achieve the dissolubility and preserve the sp2 nanotube structures and thus the electronic characteristics, it is imperative to develop a novel and environmentally friendly method to functionalize CNTs. * To whom correspondence should be addressed. Phone: 86-045186608458. Fax: 86-0451-86673647. E-mail: [email protected]. † Heilongjiang University. ‡ Chinese Academy of Sciences. § Jilin University.

Cyclodextrins (CDs) are captivating molecules, appealing to investigators in both pure research and applied technologies due to unique physical and chemical merits, which have been investigated for more than 100 years. Generally, the most common members of CDs are R-CD, β-CD, and γ-CD, which comprise six, seven, and eight glucose units, respectively. Among them, β-CD is a particularly interesting candidate for a variety of applications, in the fields of catalysis, molecular recognition, increasing solubility, and environmental protection. Furthermore, β-CD molecule has two rims of hydroxyl groups, i.e., a primary (tail) and a secondary (head) hydroxyl group, which can either react with substrates themselves or be used to attach other functional groups. Although a series of SWCNTCD composites have been synthesized,16,17 less research has been conducted to prepare multiwall carbon nanotube-CD (MWCNTCD) composites, to clarify the interaction between CNTs and CD, and thus to explain their formation mechanism, although MWCNTs could have more interesting properties and a wider range of applications in comparison with SWCNTs.18,19 On the basis of the above consideration and in combination with the characteristics of MWCNTs and β-CD, in this paper, we synthesized MWCNT-β-CD composites through assembling the β-CD molecules on the surface of MWCNTs. The synthetic method combines the polymer wrapping technique and layerby-layer self-assembly approach, allowing the noncovalent attachment of β-CD on MWCNTs through van der Waals forces between CNTs and β-CD and hydrogen-bonding interaction between the adjacent β-CD molecules as the principal driving forces. Furthermore, there are significant differences in the morphology and solubility of MWCNTs before and after introducing β-CD. This novel synthetic strategy presents the

10.1021/jp0756754 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/04/2008

952 J. Phys. Chem. C, Vol. 112, No. 4, 2008 following paramount advantages: (1) the introduction of β-CD has been proven to be capable of debundling MWCNT bundles, (2) the obtained MWCNT-β-CD composites possess good solubility/dispersibility in both ethanol and water. Owing to the high biocompatibility nature of β-CD, the obtained novel supramolecular self-assemblied composites were expected to have potential applications in the biology and medicine fields through embedding guest molecules (such as biological polymers, dyes, inorganic molecules, et al.) into the β-CD cavities utilizing hydrophobic interactions to form host-guest inclusion composites or supramolecular assemblies on the outer surface of MWCNTs. Furthermore, in this work, the mysterious interaction between CNTs and β-CD was revealed for the first time by using a molecular mechanics (MM) method. A reasonable formation mechanism based on the combination of MM calculation and experimental results is also presented in this paper. Further understanding of the formation mechanism is pivotal for a rational design and the reproducible construction of CNT-CD composites upon man’s volition. Experimental Section Preparation of MWCNT-β-CD Composites. Typically, in the current investigation, small amounts of raw MWCNTs and β-CD were ground in an agate mortar and pestle with the dropwise addition of ethanol (1 mL) over the first 10 min. After further grinding for 3 h, the resulting black powder was then heated at 75 °C for 24 h using a vacuum drying oven. Finally, the dried black product was gently ground into a fine homogeneous powder. The masses (g) of each component are (MWCNTs/β-CD/EtOH) 0.0108/0.32/0.79, 0.0108/0.16/0.79, 0.0108/0.08/0.79, 0.0108/0.04/0.79, 0.0108/0.02/0.79, 0.0108/ 0.01/0.79, 0.0108/0.005/0.79, and 0/0.32/0.79, designated as samples a, b, c, d, e, f, g, and h, respectively. Characterization. Powder X-ray diffraction (XRD) patterns were obtained using Cu KR radiation (λ ) 1.5406 Å) with a Rigaku D/max-IIIB diffractometer. Thermogravimetric analysisdifferential scanning calorimetry (TG-DSC) measurements were carried out using a Netzsch STA 449C thermoanalyzer under an argon flow of 20 mL min-1. Scanning electron microscopy (SEM) micrographs were obtained using a Hitachi S-4300 scanning electron microscope operating at 20 kV. The transmission electron microscopy (TEM) experiment was performed on a Philips CM 200 FEG electron microscope with an acceleration voltage of 200 kV. Carbon-coated copper grids were used as the sample holder. Atomic force microscopy (AFM) experiments were conducted on an SPI 3800N probe station (Seiko Instruments Inc., Japan). Solid-state 13C nuclear magnetic resonance (NMR) spectra were acquired on a Bruker DSX-300 NMR spectrometer. Computational Details. The relevant MM simulations were performed with the DISCOVER code, which is very suitable for modeling large systems and allows refinement of the geometry of a structure until it satisfies a certain criterion. The algorithm for geometry optimization is a cascade of the steepest descent, conjugate gradient, and Newton methods.20 To accurately describe the movements of the atomic nuclei on the potential energy surface, the COMPASS force field is selected and used in the calculations.21 The contributions of the nonbond interactions, including the hydrogen bond, electrostatic, and van der Waals terms, to the total potential energy are treated by an atom-based method embedded in the code. The (40, 0) CNT used in the calculations is 38 Å in length and saturated by hydrogen atoms at both ends to avoid the boundary effect. For all four considered models, e.g., the attachments of the head or

Liu et al. tail of the β-CD molecule to the perfect CNT (p-CNT) or defected CNT (d-CNT) surface, the optimization procedure is repeated until the force on the atoms is less than 0.001 (kcal/ mol)/Å and the energy change less than 5.0 × 10-6 kcal/mol. Results The typical SEM images of raw MWCNTs and sample a and the representative TEM and AFM micrographs of sample a are presented in Figure 1. From the SEM image shown in Figure 1A, we can clearly see that the pristine MWCNTs aggregate in bundles with a disordered arrangement. The representative SEM micrograph of β-CD-modified raw MWCNTs (i.e., sample a) shown in Figure 1B reveals the resultant materials possess uniform coatings and present randomly oriented quasi-nanobeltlike structures. The obtained composites were mostly 1-2 µm in length and 0.1-0.2 µm in diameter, which is much thicker than the raw MWCNTs. Comparing parts A and B of Figure 1, significant differences can be clearly observed before and after introducing β-CD molecules. Furthermore, the surface of the obtained composites is smooth, and the edges of the belts are well-defined. A small amount of excess original β-CD was still present in sample a, which is consistent with the below TGDSC results. The TEM micrographs at different magnifications shown in Figure 1C reveal the final MWCNT-β-CD composites present quasi-nanobelt-like structures, which is in good agreement with the SEM observations. To further characterize the morphology and structure of sample a, AFM observations were also conducted. The representative two-dimensional AFM image and the corresponding section analysis are shown in Figure 1D. The AFM results further confirm the above SEM and TEM observations. Furthermore, the section analysis shown in Figure 1D indicates that the resultant MWCNT-β-CD composites possess a smooth surface and the height is about 60 nm. All the SEM, TEM, and AFM observations strongly demonstrate the introduction of β-CD debundled MWCNT bundles and stabilized individual nanotubes. The obtained MWCNT-β-CD composites are expected to be used as fundamental building blocks for fabricating multifunctional nanomaterials possessing specific physical and chemical properties through embedding functional polymers and other heterogeneous components. To compare with the resultant MWCNT-β-CD composites, the original β-CD after being ground with ethanol in the absence of MWCNTs (i.e., sample h) was characterized by SEM, and the typical SEM micrograph is shown in Figure 2. From Figure 2, it is clearly seen that β-CD presents a spherical structure with a uniform size, which is greatly different from the morphologies of MWCNT-β-CD composites shown in Figure 1. In comparing Figures 1 and 2, we can tentatively assume the existence of β-CD induced the self-assembly behavior along the MWCNT axes. This also indirectly demonstrates there must be special interactions between MWCNTs and β-CD. The solubility/dispersibility of MWCNT-β-CD composites was tested by ultrasonicating them in ethanol solution for 5 min and comparing their behavior to that of the raw MWCNTs. While the latter began to precipitate almost immediately at the bottom of the vial after sonication (see Figure 3A), presenting the insoluble nature of raw MWCNTs, however, MWCNTβ-CD exhibited good solubility/dispersibility in ethanol, forming a homogeneous solution, which can be very stable for more than four weeks. Evidently, the introduction of β-CD strongly increased the solubility of raw MWCNTs. Similar good solubility/dispersibility can also be found in a water medium (see Figure 3B). This can be attributed to the hydroxyl groups of

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Figure 1. Typical SEM images of raw MWCNTs (A) and sample a (B), TEM micrographs of sample a at different magnifications (C), and AFM images of sample a (D).

Figure 3. Optical micrographs of the dispersibility of raw MWCNTs and sample a dispersed in ethanol (A) and water (B) (0.1 mg mL-1). Figure 2. Representative SEM image of β-CD after being ground with ethanol.

β-CD interacting with the polar groups of the solvent.22-25,34-37 Because the obtained MWCNT-β-CD composites display homogeneous dispersion and simultaneously possess the inherent characteristics of β-CD and MWCNTs, the presented composite was expected to have potential biochemical and biomedical applications.26-28 Further investigation on the solubility/dispersibility of MWCNTs (raw and acid-treated)-β-CD in aqueous and/or various alcohol (methanol, ethanol, propanol, butanol, et al.) solutions is now in progress in our study group. Figure 4 shows the XRD patterns for β-CD, raw MWCNTs, and samples a-h. The powder XRD patterns for samples a-h are clearly different from the sum of the individual patterns of β-CD and raw MWCNTs. It is well-known that packing of the CD molecules in the crystal lattices occurs in one of two principal modes, described as cage-type and channel-type structures.29-32 From Figure 4, a diffraction peak centered around 2θ ) 17.9° can be clearly observed in samples a-h,

indicating a channel-type structure.30,33 Furthermore, the XRD pattern for samples a-h is partly similar to that of the CDbased inclusion complex, which has been reported to have the channel-type structure.31,33-35 Therefore, the obtained XRD results indicated that channel-type β-CD structures rather than cage-type β-CD structures were formed in the resultant MWCNT-β-CD composites, which can be further confirmed by the below 13C NMR observations. With the increase of the amount of β-CD, two observed trends can be easily found from Figure 4: (1) the intensity of the typical diffraction peaks of MWCNTs located at 2θ ) 26° and 43° was noticiceably decreased; (2) the full width at half-maximum of the XRD diffraction peak centered around 2θ ) 17.9° was increased gradually and then almost maintained unchangeable. This indirectly indicates that, upon an increase of the amount of β-CD, the self-assembly behavior of β-CD onto the MWCNTs becomes stronger and then trends to constant. Thermoanalytical techniques, especially TG-DSC, are widely applied to confirm the presence of interactions between CD and other heterogeneous components. By comparing the thermal

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Figure 4. XRD patterns of β-CD, raw MWCNTs, and samples a-h.

Figure 5. TG-DSC curves of β-CD (A) and sample a (B).

Figure 6. 13C NMR spectra of β-CD and sample a (the inset is the atom numbering in the glucopyranosic unit).

behavior of β-CD with its corresponding composites, the TGDSC results can provide some useful information about what kind of interaction occurs between β-CD and the raw MWCNTs. Figure 5A shows the TG-DSC curves of β-CD. A first endothermic effect occurs at 98 °C and corresponds to the dehydration process of β-CD. The water loss is confirmed by

the TG curve, indicating that β-CD contains 10 mol of water. A second thermal effect located at 217 °C represents a process corresponding probably to a molecular reorganization of β-CD, where the TG curve is flat and no mass loss is detected. The last endothermic peak at 301 °C is associated with the decomposition of β-CD, which also can be confirmed by the TG curve.36 The TG-DSC curves of sample a are shown in Figure 5B. Comparing the TG-DSC results shown in parts A and B of Figure 5, a distinct difference can be found. Three endothermic features are clearly presented in Figure 5B. The first endothermic peak positioned at 80 °C is associated with the removal of volatile components. Noteworthy, in Figure 5B, a new endothermic peak located at 321 °C is found. It is clear evidence of an interaction between the original β-CD and raw MWCNTs, where the β-CD self-assembles around the surface of the MWCNTs. The retention of the β-CD endothermic peak at 296 °C is due to the presence of a small amount of excess β-CD, besides the β-CD packed onto the MWCNTs, which can be further confirmed by the above SEM observation. NMR spectroscopy is another technique used to analyze the structure of CD-based composites either in solution or in the solid state. Solid-state 13C NMR spectra of β-CD and sample a are shown in Figure 6. Significant differences between β-CD and sample a can be clearly found from Figure 6. It is wellknown that β-CD molecules have a less symmetrical conforma-

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Figure 7. Schematic representation of the reasonable formation mechanism of MWCNT-β-CD composites presented in this work.

tion in the crystalline state.30,33,37-40 The NMR spectrum of β-CD shown in Figure 6 is similar to that previously reported and exhibits resolved carbon resonances from each of the glucose units, which is reflected by strong splitting for all C1C6 resonances.30 However, in this case, for sample a, the resolved resonances disappear and each carbon of the glucose unit is observed as a single peak. These unique phenomena can also be observed in other CD-based composites.30,41-43 The obtained NMR results indicate that the β-CD molecules in MWCNT-β-CD composites adopt a more symmetrical conformation and each glucose unit of β-CD is in a similar environment.30,40-45 Furthermore, all of these 13C NMR results suggest that β-CD molecules in the resultant MWCNT-β-CD composites are packed in a channel-type structure rather than in a cage-type structure,30,43 which is very consistent with the above XRD observations. Discussion The above detailed characterization indicates MWCNT-βCD composites have been successfully synthesized. Moreover, the presented materials possess good solubility. Because the interaction between heterogeneous components can strongly affect the conformational preferences and functions of supramolecules, thorough understanding of the intermolecular interactions between CNTs and CD is very necessary, which is essential to design and construct novel CNT-CD composites upon man’s volition. On the basis of the above considerations, in this paper, utilizing MM simulations and in combination with our experimental results, a possible formation mechanism is

proposed and summarized as follows (see Figure 7). To the best our knowledge, this is the first time the interactions between CNTs and β-CD molecules have been systematically and thoroughly revealed using MM simulations. Interactions between MWCNTs and β-CD Molecules. The high-resolution transmission electron microscopy (HRTEM) images presented in the Supporting Information (Figure S1) clearly show the pore is about 0.61 nm in diameter, which may be assigned to the cavity diameter of β-CD molecules (0.600.65 nm).22 Therefore, the HRTEM results imply that the β-CD cavities are perpendicular to rather than parallel to the MWCNT surface. Since a β-CD molecule has two different hydroxyl groups on the two ends of its cavity, i.e., a primary (tail) and a secondary (head) hydroxyl group (see Figure 7B), it is possible that either end (head or tail) of the β-CD molecule can interact with MWCNTs (see Figure 7C, mechanisms I and II). This is further confirmed by our MM theoretical calculation results. The calculated total potential energies of the individual β-CD, CNT, and β-CD-CNT composites are listed in the Supporting Information. Compared with the summation of the total potential energies of the individual β-CD molecule and CNTs, the total potential energies of the CNT-β-CD composites are smaller, which indicates the occurrence of an additional interaction between the β-CD molecule and the CNTs. The binding energy, defined as ECNTs+β-CD - (ECNTs + Eβ-CD), is -40.426 kcal/ mol when the head of the CD molecule approaches the p-CNTs, while it is -29.593 kcal/mol when the tail of the CD molecule attaches to the p-CNTs, which suggests that the attachment of the head of the β-CD molecule to the surface of p-CNTs is

956 J. Phys. Chem. C, Vol. 112, No. 4, 2008 more favorable. To further illustrate the interaction between CNTs and the β-CD molecule, the contributions of the internal and nonbond energies are taken into consideration. According to the breakdown of the total potential energy, one can easily see that the binding energy is mainly from the contribution of the nonbond energy, in more detail not from hydrogen bond or electrostatic interaction but from the van der Waals forces. This indicates that the interaction between CNTs and β-CD is not a chemical bond but nonbond van der Waals forces. To further confirm the above conclusion, the CNT surface possessing Stone-Wales defects was also subjected to the same simulations. An obvious change is found in that the van der Waals interaction between the tail of the β-CD molecule and the d-CNTs is enhanced. However, the difference in binding energy, -39.330 kcal/mol vs -41.650 kcal/mol, almost disappears when different ends of the β-CD molecule attach to the d-CNT surface, and the tail of the β-CD molecule can thus be attached to the d-CNT surface as easy as the head of the β-CD molecule. Therefore, MM theoretical calculation results indicate that the van der Waals forces truly exist in the CNT-β-CD composites and the specific interaction plays an essential driving force for the construction of the resultant composites, which is in good agreement with the above experimental results, especially the TG-DSC observations. It was expected that the van der Waals forces between CNTs and β-CD might be stronger than intertube forces of raw MWCNTs, which resulted in the debundling of raw MWCNTs. On the other hand, β-CD was not expected to form hydrophobic interaction with the raw MWCNTs as a result of the hydrophilic surface of β-CD. Intermolecular Interactions between β-CD Molecules. On the basis of the above experimental results, it is reasonable to assume that, besides the above-mentioned van der Waals forces between CNTs and β-CD, there must exist strong interaction between the adjacent β-CD molecules which also plays an important role in driving the formation of CNT-β-CD composites. In our case, during the formation of uniform CNT-βCD composites, β-CDs were expected to act as molecular bricks and pack onto the surface of MWCNTs through layer-by-layer assembly. Because a β-CD molecule has two different hydroxyl groups, it can have three different arrangement modes, namely, head-to-head, tail-to-tail, and head-to-tail (see Figure 7E). This also can be demonstrated by Shigekawa’s observations, where Shigekawa and his co-workers succeeded in the quantitative analysis of the intramolecular conformation of CD molecules.46 Their results indicated that CD molecules are arranged in headto-head or tail-to-tail conformation, and about 20% head-totail conformation was present in the molecule. Furthermore, the hydrogen-bonding interaction between two CD molecules is the dominant driving force that determines the conformation of the CD molecules. Therefore, the intermolecular hydrogen bonds between hydroxyl groups of the adjacent β-CD molecules were partly responsible for the formation of CNT-β-CD composites. From the above discussion, it is reasonable to assume the van der Waals forces between CNTs and β-CD and the hydrogen-bonding interaction between the adjacent β-CD molecules are the dominant driving forces during the β-CD wrapping and layer-by-layer self-assembly process. Both interactions drive β-CD molecules to rigidly arrange onto the surface of MWCNTs. Conclusions In summary, we have successfully designed and synthesized MWCNT-β-CD composites. The introduction of β-CD debundled raw MWCNTs bundles and stabilized individual

Liu et al. nanotubes. The interaction between CNTs and β-CD was first revealed through MM calculations. The dominant driving forces coming from van der Waals forces between CNTs and β-CD and hydrogen-bonding interaction between the adjacent β-CD molecules were responsible for the formation of MWCNT-βCD composites. Furthermore, the obtained composites presented good solubility in both ethanol and water media. Such novel materials provide a possibility to further extend the applications of MWCNTs to biology, medicine, and other fields. Acknowledgment. This work was supported by the Key Program Projects of the National Natural Science Foundation of China (Grant No. 20431030), the National Natural Science Foundation of China (Grant No. 20671032), the program for new century excellent talents in university (Grant No. NCET04-0341), the China Postdoctoral Science Foundation (Grant No. 20060400102), and the Chinese Academy of Science K.C.Wong Post-doctoral Research Award Fund (Hong Kong). Supporting Information Available: Additional HRTEM images and a table of theoretical calculation results. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Iijima, S. Nature 1991, 354, 56-58. (2) Ajayan, P. M. Chem. ReV. 1999, 99, 1787-1800. (3) Piscanec, S.; Lazzeri, M.; Robertson, J.; Ferrari, A. C.; Mauri, F. Phys. ReV. B 2007, 75, 035427. (4) Yin, Y.; Vamivakas, A. N.; Walsh, A. G.; Cronin, S. B.; Unlu, M. S.; Goldberg, B. B.; Swan, A. K. Phys. ReV. Lett. 2007, 98, 037404. (5) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y. -S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253-1256. (6) Saini, R. K.; Chiang, I. W.; Peng, H. Q.; Smalley, R. E.; Billups, W. E.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. Soc. 2003, 125, 36173621. (7) Tasis, D.; Tagmatarchis, N.; Georgakilas, V.; Prato, M. Chem.s Eur. J. 2003, 9, 4001-4008. (8) Aitchison, T. J.; Ginic-Markovic, M.; Matisons, J. G.; Simon, G. P.; Fredericks, P. M. J. Phys. Chem. C 2007, 111, 2440-2446. (9) Liu, Y. Q.; Gao, L.; Sun, J. J. Phys. Chem. C 2007, 111, 12231229. (10) Ajayan, P. M.; Tour, J. M. Nature 2007, 447, 1066-1068. (11) Du, J. M.; Fu, L.; Liu, Z. M.; Han, B. X.; Li, Z. H.; Liu, Y. Q.; Sun, Z. Y.; Zhu, D. B. J. Phys. Chem. B 2005, 109, 12772-12776. (12) Ge, J. J.; Zhang, D.; Li, Q.; Hou, H. Q.; Graham, M. J.; Dai, L. M.; Harris, F. W.; Cheng, S. Z. D. J. Am. Chem. Soc. 2005, 127, 99849985. (13) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853-1859. (14) Banerjee, S.; Hemraj-Benny, T.; Wong, S. S. AdV. Mater. 2005, 17, 17-29. (15) Bahr, J. L.; Yang, J. P.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536-6542. (16) Chen, J.; Dyer, M. J.; Yu, M. F. J. Am. Chem. Soc. 2001, 123, 6201-6202. (17) Chambers, G.; Carroll, C.; Farrell, G. F.; Dalton, A. B.; McNamara, M.; Panhuis, M. I. H.; Byrne, H. J. Nano Lett. 2003, 3, 843-846. (18) Na, N.; Hu, Y. P.; Jin, O. Y.; Baeyens, W. R. G.; Delanghe, J. R.; Taes, Y. E. C.; Xie, M. X.; Chen, H. Y.; Yang, Y. P. Talanta 2006, 69, 866-872. (19) Kang, S. Z.; Cui, Z. Y.; Liu, L. Y.; Mu, J. Fullerenes, Nanotubes, Carbon Nanostruct. 2005, 13, 353-362. (20) Ermer, O. Struct. Bonding 1976, 27, 161-211. (21) Sun, H. J. Phys. Chem. B 1998, 102, 7338-7364. (22) Li, S.; Purdy, W. C. Chem. ReV. 1992, 92, 1457-1470. (23) Rekharsky, M. V.; Inoue, Y. Chem. ReV. 1998, 98, 1875-1917. (24) Connors, K. A. Chem. ReV. 1997, 97, 1325-1357. (25) Rouse, J. H. Langmuir 2005, 21, 1055-1061. (26) Uekama, K.; Hirayama, F.; Irie, T. Chem. ReV. 1998, 98, 20452076. (27) Hedges, A. R. Chem. ReV. 1998, 98, 2035-2044. (28) Liu, Y.; Liang, P.; Chen, Y.; Zhao, Y. L.; Ding, F.; Yu, A. J. Phys. Chem. B 2005, 109, 23739-23744.

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