Wrapping of Self-Organized Fluorescent Nanofibers with a Silica Wall

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Langmuir 2009, 25, 776-780

Wrapping of Self-Organized Fluorescent Nanofibers with a Silica Wall Mutsumi Kimura,* Noritoshi Miki, Daisuke Suzuki, Naoya Adachi, Yoko Tatewaki, and Hirofusa Shirai Department of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu UniVersity, Ueda 386-8567, Japan, and CollaboratiVe InnoVation Center of Nanotech Fiber (nanoFIC), Shinshu UniVersity, Ueda 386-8567, Japan ReceiVed September 12, 2008. ReVised Manuscript ReceiVed NoVember 13, 2008 Amphiphilic compounds 1 and 2, composed of an aromatic pyrene core and an amphiphilic three-branched unit, were synthesized and investigated for their self-organizing process in solution by UV-vis, fluorescence spectra, Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and fluorescence microscopes. While 2 formed spherical objects in a mixed solvent of methanol and water, 1 assembled into long, flexible, and fluorescent fibers through π-π stacking of pyrene cores and hydrogen bonding among amide groups. The fluorescence spectra and morphologies strongly depended on the concentration and solution temperature. The fibrous assemblies were wrapped with an ultrathin silica wall by the acidic sol-gel polymerization of tetraethoxysilane (TEOS). A transmission electron microscopy (TEM) image after the sol-gel polymerization showed discrete fibrous structures with a uniform diameter of 3.5 nm and several micrometers in length. The thickness of the silica wall and the inner diameter of one fiber were estimated to be 0.5 nm and 2.5 nm, respectively. The observed inner diameter of the fiber was almost compatible with the width of the cylindrical assembly made of 1. The pyrene unit in 1 can interact with the sidewall of single-walled carbon nanotubes (SWNTs) through π-π interaction, and the adsorption of 1 onto the surface of SWNTs could disrupt the formation of bundles. The accumulation of oligomeric silica species at the hydrophilic surface created organic-inorganic nanoscopic fibers containing electronic conductive SWNTs.

Introduction Spontaneous organization processes of functional molecules into well-defined nanostructures, such as fibers, wires, channels, particles, two-dimensional sheets, and cages, provide a new approach for the fabrication of nanoscopic molecular-based devices.1 In the self-organization processes, small molecules are built-up into nanostructures with the help of intermolecular noncovalent forces such as hydrogen-bonding, van der Waals interactions, and π stacking. Organized states of functional molecules show fascinating electroconductive, photophysical, molecular-recognition, and transportation properties.2 The control of spatial arrangement and orientation of functional molecules within nanostructures promises the creation and enhancement of properties of organized states. A number of functional molecules have been designed as molecular components for the construction of functional nanostructures through self-organization processes using noncovalent intermolecular interactions. However, these self-organized nanostructures, constructed by using weak intermolecular interactions among low-molecular-weight functional molecules, dissociate into single molecular components by external stimuli such as heat, change of solvent polarity, and light irradiation. The preservation of these self-organized nanostructures is a novel way to obtain functional materials * Corresponding author. Mailing address: Department of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567, Japan. TEL & FAX: +81-268-21-5499. E-mail: [email protected]. (1) (a) Supramolecular Materials and Technologies, Reinhoudt, D. N. Ed.; Perspectives in Supramolecular Chemistry Series; John Wiley & Sons: Chichester, U.K., 1999; Vol. 4. (b) Supramolecular Chemistry; Steed, J. W., Atwood, J. L., Eds.; John Wiley & Sons: Chichester, U.K., 2002. (2) (a) ComprehensiVe Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., Macnicol, D. D., Vo¨gtle, F., Lehn, J. -M., Sauvage, J. -P., Hosseini, M. W., Eds.; Pergamon: New York, 1999. (b) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed. 2006, 45, 38. (c) Zhao, Y. S.; Fu, H.; Peng, A.; Ma, Y.; Xiao, D.; Yao, J. AdV. Mater. 2008, 20, 2859.

containing well-defined molecular assemblies.3 Beginn and coworkers succeeded in the preservation of self-organized ion channels by the polymerization of peripheral olefin groups, and the resulting material showed selective ion transportation along the stacking axis of crown ether moieties within self-organized cylindrical structures.4 Flexible organic nanostructures can also fix their morphologies permanently by the deposition of inorganic walls.5 We previously showed the creation of organic-inorganic composites containing one-dimensional ordered stacking of amphiphilic metallophthalocyanines by liquid crystal template sol-gel polymerization of silica.6 The sol-gel polymerization of tetraethoxysilane (TEOS) in the presence of amphiphilic metallophthalocyanines allowed for the deposition of silica walls around each one-dimensional columnar assembly. Ichinose and Kunitake reported on wrapping of functional molecules with an ultrathin wall of metal oxides.7 The advantage of this method (3) (a) Mueller, A.; O′Brien, D. F. Chem. ReV. 2002, 102, 727. (b) Ichimura, K. Chem. ReV. 2000, 100, 1847. (c) Kimura, M.; Wada, K.; Ohta, K.; Hanabusa, K.; Shirai, H.; Kobayashi, N. Macromolecules 2001, 34, 4706. (d) Jin, W.; Fukushima, T.; Kosaka, A.; Niki, M.; Ishii, N.; Aida, T. J. Am. Chem. Soc. 2005, 127, 8284. (e) Wakabayashi, R.; Kubo, Y.; Kaneko, K.; Takeuchi, M.; Shinkai, S. J. Am. Chem. Soc. 2006, 128, 8744. (f) Kang, S. H.; Jung, B. M.; Chang, J. Y. AdV. Mater. 2007, 19, 2780. (4) (a) Beginn, U.; Zipp, G.; Mo¨ller, M. AdV. Mater. 2000, 12, 510. (b) Beginn, U.; Zipp, G.; Mourran, A.; Walther, P.; Mo¨ller, M. AdV. Mater. 2000, 12, 513. (5) (a) Lu, Y.; Yang, Y.; Sellinger, A.; Lu, M.; Huang, J.; Fan, H.; Haddad, R.; Lopez, G.; Burns, A. R.; Sasaki, D. Y.; Shelnutt, J.; Brinker, C. J. Nature 2001, 410, 913. (b) Tamaru, S.; Takeuchi, M.; Sano, M.; Shinkai, S. Angew. Chem., Int. Ed. 2002, 41, 853. (c) Numata, M.; Sugiyasu, K.; Hasegawa, T.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43, 3279. (d) Kishida, T.; Fujita, N.; Sada, K.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 7298. (e) Aida, T.; Tajima, K. Angew. Chem., Int. Ed. 2001, 40, 3803. (f) Okabe, A.; Fukushima, T.; Ariga, K.; Aida, T. Angew. Chem., Int. Ed. 2002, 41, 3414. (g) Ikegame, M.; Tajima, K.; Aida, T. Angew. Chem., Int. Ed. 2003, 42, 2154. (h) Zhang, Q.; Ariga, K.; Okabe, A.; Aida, T. J. Am. Chem. Soc. 2004, 126, 988. (i) Kimura, M.; Iwashima, Y.; Ohta, K.; Hanabusa, K.; Shirai, H. Macromolecules 2005, 38, 5055. (6) Kimura, M.; Wada, K.; Ohta, K.; Hanabusa, K.; Shirai, H.; Kobayashi, N. J. Am. Chem. Soc. 2001, 123, 2438. (7) (a) Ichinose, I.; Kunitake, T. Chem. Lett. 2001, 626. (b) Ichinose, I.; Kunitake, T. AdV. Mater. 2002, 14, 344. (c) Ichinose, I.; Hashimoto, Y.; Kunitake, T. Chem. Lett. 2004, 33, 656.

10.1021/la802991g CCC: $40.75  2009 American Chemical Society Published on Web 12/15/2008

Wrapping Nanofiber Assemblies with a Silica Wall

is that the functionalities of individual molecules or organizations within the inorganic framework can be evaluated without intermolecular interactions among the incorporated molecules. In this study, we report the self-organization properties of pyrenecored amphiphilic molecules 1 and 2 in polar solvents and the deposition of ultrathin silica walls around self-organized nanofibers. Single-walled carbon nanotubes (SWNTs) have been the most promising one-dimensional material for the construction of nanodevices because of their anisotropic electronic conductivity and mechanical strength.8 Much effort has been invested in recent years to achieve these devices by positioning individual SWNTs on a solid surface by using atomic force microscopy techniques.9 However, SWNTs aggregate to form bundles through their strong interaction among hydrophobic graphene sidewalls, and these bundles are heavily entangled with one another. Entangled bundles form insoluble three-dimensional networks. This uncontrollable aggregation of SWNTs is the main problem that occurs during the processing of individual SWNTs. Noncovalent functionalizations of SWNTs with polymers, proteins, DNA, polysaccharides, π-conjugated molecules, and amphiphilic molecules are of particular interest.10 Such functionalizations improve their processability, as well as modify their chemical and physical properties while still preserving nearly all of the intrinsic properties of SWNTs. The adsorption of these noncovalent modifiers onto the surface of SWNTs could disrupt the formation of bundles, and noncovalent functionalization could improve the solubility of SWNTs in water and organic solvents. In this context, we also expected that synthesized amphiphilic molecules 1 and 2 act as noncovalent modifiers of SWNTs since the pyrene units in 1 and 2 can interact strongly with the sidewalls of SWNTs through π-π stacking interaction.11 We also demonstrate the wrapping of SWNTs with silica walls by the noncovalent functionalization of amphiphilic molecules.

Experiment 1

General. H NMR spectra were measured on a Bruker AVANCE 400 FT-NMR spectrometer. Mass spectral data were obtained using a PerSeptive Biosystems Voyager DE-Pro spectrometer with dithranol as a matrix. Fourier transform infrared (FT-IR) spectra were obtained using a Shimazu FTIR-8400 spectrometer. UV-vis and fluorescence spectra were measured on a Shimazu MultiSpec-1500 spectrophotometer and a JASCO FP-750, respectively, by using a water-jacketed cuvette to control temperature. An X-ray diffraction (XRD) pattern was measured with CuKR radiation on a Rigaku XRD-DSC diffractometer. Raman spectra were obtained on a HaloLab 5000 spectrophotometer at an excitation wavelength of 532 nm. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2010 electron microscope at an acceleration voltage of 200 kV without any staining. The specimens for TEM were prepared by drop casting the solution onto amorphous carbon-coated copper (8) (a) Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, Ph. Appl. Phys. Lett. 1998, 73, 2447. (b) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (c) Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Nature 1996, 381, 678. (d) Baughman, R. H.; Chiu, C.; Zakhidov, A. A.; Iqbal, Z.; Barisci, J. N.; Spinks, G. M.; Wallace, G. G.; Mazzoldi, A.; De Rossi, D.; Rinzler, A. G.; Jaschinski, O.; Roth, S.; Kertesz, M. Science 1999, 284, 1340. (e) Gooding, J. J.; Wibowo, R.; Liu, J.; Yang, W.; Losic, D.; Orbons, S.; Mearns, F. J.; Shapter, J. G.; Hibbert, D. B. J. Am. Chem. Soc. 2003, 125, 9006. (9) (a) Avouris, P.; Hertel, T.; Martel, R.; Schmidt, T.; Shea, H. R.; Walkup, R. E. Appl. Surf. Sci. 1999, 141, 201. (b) Thelander, C.; Samuelson, L. Nanotechnology 2002, 13, 108. (c) Moriyama, S.; Fuse, T.; Suzuki, M.; Aoyagi, Y.; Ishibashi, K. Phys. ReV. Lett. 2005, 94, 186806. (10) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV 2006, 106, 1105. and related references therein. (11) (a) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838. (b) Nakashima, N.; Tomonari, Y.; Murakami, H. Chem. Lett. 2002, 638. (c) Tomonari, Y.; Murakami, H.; Nakashima, N. Chem.;Eur. J. 2006, 12, 4027.

Langmuir, Vol. 25, No. 2, 2009 777 grids (400 mesh). Molecular dimensions were estimated by SPARTAN version 4.0 (Wave Function, Inc., Irvine, CA). Materials. All chemicals were purchased from commercial suppliers and used without purification. All solvents were distilled before each procedure. Purified HiPco-SWNTs were purchased from Carbon Nanotechnologies, Inc. The purification of product was carried out by the combination of adsorption column chromatography (silica gel, Wakogel C-200) and recycling preparative HPLC (Japan Analytical Industry, model LC-908). Analytical thin-layer chromatography was performed on commercial Merck plates coated with silica gel 60 F254. Amphiphilic molecules 1 and 2 were synthesized from gallic acid methyl ester by three steps. The threebranched intermediated materials were synthesized according to a literature procedure reported by Percec et al.12 1: A mixture of benzoic acid having three 10-(2-(2-(2-methoxyethoxy)- ethoxy)ethoxy)decanoyl chains13 (0.9 g, 0.84 mmol), 1-pyrene methylamine hydrochloride (0.29 g, 1.1mmol), and triethylamine (0.17 mL, 1.12mmol) were dissolved in dry dichloromethane under a nitrogen atmosphere. 4-Dimethylaminopyridium toluene-p-sulfonate (DPTS) (0.074 g, 0.254mmol) and 1,3-dicyclohexylcarbodiimide (DCC) (0.52 g, 2.52mmol) were added to this solution. The reaction mixture was stirred for 2 days at room temperature. The reaction mixture was filtered and the filtrate was concentrated by dryness by a vacuum evapolator. The residue was purified by a silica gel column and by recycling preparative HPLC with CHCl3 to give the desired product in 81.4% yield as a colorless oily waxy material. 1H NMR (CDCl3, 400.13 MHz): δ (ppm) ) 8.34 (1H, d, J ) 9.1 Hz, Haromatic), 8.20 (2H, d, J ) 7.6 Hz, Haromatic), 8.16 (2H, q, J ) 5.3 Hz, Haromatic), 8.07 (2H, s, Haromatic), 8.04 (2H, m, Haromatic), 6.98 (2H, s, Haromatic), 6.57 (1H, br, -CONH-), 5.32 (2H, d, J ) 5.2 Hz, -ArCH2O-), 3.93 (6H, m, -OCH2-), 3.66-3.53 (36H, m, -OCH2CH2-), 3.44 (6H, t, J ) 6.8 Hz, -OCH2-), 3.37 (9H, s, -OCH3), 1.80-1.72 (6H, m, -CH2-), 1.58-1.51 (12H, m, -CH2-), 1.46-1.36 (6H, m, -CH2-), 1.30-1.26 (30H, m, -CH2-); 13C NMR (CDCl3, 100.61 MHz): δ (ppm) )167.0, 152.1, 141.3, 131.2, 130.8, 129.2, 128.4, 127.5, 126.2, 125.4, 122.9, 108.1, 105.9, 71.9, 71.5, 70.6, 70.5, 69.3, 58.9, 42.6, 29.6, 29.5, 26.2, 25.9; MALDI-TOF-Ms: 1291.66 ([M + H]+ calcd for C75H119NO16: 1290.74). 2 was synthesized from amphiphilic benzoic acid and 1-pyrene methanol by the same procedure used for 1. Yield 35% (colorless oily waxy material). 1H NMR (CDCl3, 400.13 MHz): δ (ppm) ) 8.40 (1H, d, J ) 9.0 Hz, Haromatic), 8.20 (2H, d, J ) 7.6 Hz, Haromatic), 8.12 (2H, q, J ) 5.4 Hz, Haromatic), 8.04 (6H, m, Haromatic), 7.2 (2H, s, Haromatic), 6.00 (2H, d, J ) 5.3 Hz, -ArCH2O-), 3.93 (6H, m, -OCH2-), 3.65-3.52 (36H, m, -OCH2CH2-), 3.48 (6H, t, J ) 6.9 Hz, -OCH2-), 3.32 (9H, s, -OCH3), 1.79-1.70 (6H, m, -CH2-), 1.58-1.50 (12H, m, -CH2-), 1.46-1.36 (6H, m, -CH2-), 1.29-1.23 (30H, m, -CH2-); 13C NMR (CDCl3, 100.61 MHz): δ (ppm) ) 166.4, 152.8, 138.6, 142.6, 131.7, 131.1, 129.2, 128.1, 127.8, 127.3, 126.1, 122.9, 108.1, 71.9, 71.5, 70.6, 70.5, 69.3, 65.3, 59.3, 29.6, 29.5, 26.2, 25.9; MALDI-TOF-Ms: 1314.6 ([M + Na]+ calcd for C75H119O17: 1291.73). Wrapping of Self-Organized Nanofibers with a Silica Wall. 1 (1.2 mg, 9.3 × 10-1µmol) was dissolved in a mixed solution of 0.5 mL of methanol, 0.5 mL of water, and TEOS (4.5 µL, 2.0 µmol). Then 100 µL of conc. HCl was added to the aqueous gel at 35 °C for 20 h. The solid product was collected, washed with water, and dried at 50 °C. Dispersion of SWNTs. 1 (5.0 mg) and SWNTs (1.0 mg) were added into 10 mL of CHCl3, and the mixed solution was sonicated in a bath-type sonicator for 1 h. After evaporation, 10 mL of methanol was added to the residue, and the mixed solution was sonicated again for 5 min. After sonication, the samples were centrifuged at (12) (a) Percec, V.; Johansson, G.; Unger, G.; Zhou, J. J. Am. Chem. Soc. 1996, 118, 9855. (b) Jahansson, G.; Percec, V.; Unger, G.; Zhou, J. Macromolecules 1996, 29, 646. (c) Percec, V.; Ahn, C.-H.; Unger, G.; Yeardley, D. J. P.; Mo¨ller, M.; Sheiko, S. S. Nature 1998, 391, 161. (13) (a) Beginn, U.; Zipp, G.; Mo¨ller, M. Chem.;Eur. J. 2000, 6, 2016. (b) Percec, V.; Cho, W.-D.; Ungar, G.; Yeardley, D. J. P. Chem.;Eur. J. 2002, 8, 2011.

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Kimura et al.

Scheme 1a

Figure 1. (a) Fluorescence spectra of 1 in methanol and methanol-water of ratios 9:1, 3:2, and 1:1 v/v. [1] ) 19.3 µM, excitation wavelength ) 350 nm. (b) Concentration and (c) temperature-dependent fluorescence spectra of 1 in methanol-water (1:1 v/v), [1] ) 3.9-19.3 µM, and the temperature increased from 15 to 55 °C. (d) Fluorescence microscope image of the hydrogel made of 1 in methanol-water (1:1 v/v) at 25 °C, and the inset shows the image at 55 °C. [1] ) 0.25 g/L (0.19 mM).

a

a: Br(CH2)10(OCH2CH2)3OCH2, K2CO2; b: KOH; c: 1-pyrenemethylamine hydrochloride or 1-pyrenemethanol, DCC/DPTS.

2000g, and the supernatant was then carefully decanted. The suspension was filtered through a 0.1 µm filter (Millipore Co) and washed with methanol several times. The residue was collected and resuspended in 10 mL of methanol by sonication for 10 min at room temperature. Wrapping of SWNT-Containing Nanofibers with a Silica Wall. Water was added to the methanol solution of TEOS and 1 to form a gel. Hydrochloric acid was added as a catalyst for the sol-gel polymerization of TEOS, and the mixture was allowed to stand at room temperature (approximately 25 °C) for 7 days. One drop of the gel was placed onto carbon-coated copper grids (400 mesh), dried in vacuo, and then underwent TEM observation.

Results and Discussion Amphiphilic compounds 1 and 2, in which an aromatic pyrene core connected with three-branched amphiphilic segments through amide or ester groups, were synthesized according to Scheme 1. The synthesized amphiphilic compounds 1 and 2 exhibited excellent solubility in polar and nonpolar solvents except for hydrocarbons. Various low-molecular-weight compounds have recently been found to assemble into nanoscopic fibers in organic solvents and water.14 These compounds are assembled into flexible and long nanoscopic fibers through well-ordered intermolecular interactions such as hydrogen bonding, van der Waals packing, π-π stacking, and electrostatic interactions. When water was added to a homogeneous methanol solution of 1, a firm gel was formed when the concentration was greater than (14) (a) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133. (b) Hanabusa, K.; Yamada, M.; Kimura, M.; Shirai, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1949. (c) Menger, F. M.; Caran, K. L. J. Am. Chem. Soc. 2000, 122, 11679. (d) Estroff, L. A.; Hamilton, A. D. Angew. Chem., Int. Ed. 2000, 39, 3447. (e) Iwamura, R.; Yoshida, K.; Masuda, M.; Yase, K.; Shimizu, T. Langmuir 2002, 14, 3034. (f) Nakashima, T.; Kimizuka, N. AdV. Mater. 2002, 14, 1113. (g) Kiyonaka, S.; Sada, K.; Yoshimura, I.; Shinkai, S.; Kato, N.; Hamachi, I. Nat. Mater. 2004, 3, 58. (h) Shirakawa, M.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 4164. (i) Ajayaghosh, A.; Varghese, R.; Praveen, V. K.; Mahesh, S. Angew. Chem., Int. Ed. 2006, 45, 3261. (j) Ajayaghosh, A.; Varghese, R.; Mahesh, S.; Praveen, V. K. Angew. Chem., Int. Ed. 2006, 45, 7729. (k) Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644.

0.25 g/L (0.19 mM), in which one-dimensional self-assembled aggregates composed of 1 are formed through intermolecular interactions, and then the formed aggregates are entangled into three-dimensional network structures. On the other hand, a gel did not form in the same concentration and medium for 2, and this suggests different aggregation behaviors between 1 and 2. The aggregation behaviors of 1 and 2 in a mixture of water and methanol were studied by UV-vis, fluorescence, and 1H NMR spectra. The UV-vis spectrum of a dilute solution of 1 (19.3 µM) in methanol displayed sharp absorption peaks in the range of 200-400 nm. All the absorption peaks of 1 in methanol/ water (1:1 v/v) were broad, and the absorption maxima were slightly red-shifted. It is well-known that the fluorescence spectra of pyrene-containing molecules show significant dependence on microenvironmental polarity and intermolecular interaction among pyrene units.20 The steady-state fluorescence spectra for 1 were obtained by varying the mixed ratio of methanol and water at the same concentration (19.3 µM) under deareated conditions. Compound 1 in methanol exhibited sharp emission peaks at 376 and 395 nm, which is typical of a nonaggregated pyrene unit.15 With an increase in the water ratio, the emission intensities increased and a new strong broad emission band with a maximum at 430 nm appeared above a ratio of 1:1 v/v (Figure 1a). This emission can be ascribed to the formation of excimer species with partial overlap of the pyrene units.15 In contrast, 2, with an ester linkage between the pyrene unit and the threebranched amphiphilic segment, exhibited a broad emission centered at 480 nm for the same media and concentration at 25 °C; this suggests different aggregation modes of pyrene units between 1 and 2. The aggregation behavior of 1 also depended on concentration and temperature. Increasing the concentration of 1 in the mixed solvent resulted in the appearance of an intense emission band at 430 nm above 19.3 µM (Figure 1b). Furthermore, the fluorescence intensity at 430 nm was gradually decreased by heating from 6 to 45 °C, and a significant red shift in the emission maximum was observed from 430 to 480 nm above 50 °C. This (15) (a) Winnik, F. M. Chem. ReV. 1993, 93, 587. (b) Ihara, H.; Yamada, T.; Nishihara, M.; Sakurai, T.; Takafuji, M.; Hachisako, H.; Sagawa, T. J. Mol. Liq. 2004, 111, 73.

Wrapping Nanofiber Assemblies with a Silica Wall

Figure 2. TEM image of silica-coated nanoscopic fibrous assemblies of 1 and a molecular model of the cylindrical assembly of 1.

spectral change revealed the temperature-dependent formation of a sandwich-type excimer between two pyrene units (Figure 1c). When the sample was cooled to below 15 °C, the emission spectrum returned to that of a freshly prepared sample, indicating reversibility between a partial overlap excimer and a sandwichtype excimer between pyrenyl units in the aggregates.15 While the 1H NMR spectra of 1 in deuterated methanol (CD3OD) at room temperature indicated clearly resolved spectra due to the molecularly dissolved nature of 1 in a CD3OD, 1 in a mixed solvent of CD3OD and D2O (8:2 v/v) gave broad signals in the aromatic region. The broadening of the proton signals is also indicative of the aggregation of pyrene units through π-π stacking. Fluorescence microscopy images of the gel containing 1 at 25 °C clearly showed fibrous structures with lengths of several micrometers in the wet gel state (Figure 1d). The observed fibrous morphology was gradually lost with rising temperature and transferred to the spherical structures above 55 °C, as shown in the inset of Figure 1d. Powder XRD of the gel state provided a sharp diffraction peak at 0.35 nm, corresponding to the stacking distance between the pyrene units within the fibrous aggregates. FT-IR spectra of the gel state displayed a broadband assigned to hydrogen-bonding amide carbonyl stretching at 1620 cm-1. On the other hand, fluorescence microscope images of 2 ([2] ) 0.25 g/L) in methanol-water showed only spherical morphologies at 25 and 55 °C, indicating that the linkage between the pyrene core and the three-branched amphiphilic segment strongly affects the morphologies of supramolecular assemblies. By combining these data, we propose that the pyrene-containing amphiphilic compound 1 spontaneously assembles into fibrous aggregates that contain the partial overlap stacking of pyrene units and an intermolecular hydrogen-bonding network among amides. The organized structure of 1 in water-methanol transferred from one-dimensional fibers to the spherical assemblies as a result of the cleavage of the hydrogen-bond network among amides. Sol-gel polymerization of inorganic monomer around supramolecular assemblies of organic molecules can create various unique inorganic nanostructures such as fibers, hollow fibers, vesicles, and lamellar structures.16 Oligomeric silica species interact with the hydrophilic triethylene glycol segments located at the surface of fibrous aggregates, and the fibrous morphology can be fixed by wrapping with a silica wall. Figure 2 displays (16) Van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980. and related references therein.

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a TEM image after the sol-gel polymerization of TEOS in the presence of 1 without staining. Whereas the characteristic superstructure could not be obtained before the sol-gel polymerization using TEM, the image after the sol-gel polymerization showed discrete fibrous structures with a uniform diameter of 3.5 nm and several micrometers in length. The thickness of the silica wall and the inner diameter of one fiber were estimated to be 0.5 nm and 2.5 nm, respectively. The observed inner diameter of the fiber was almost compatible with the width of the cylindrical assembly made of 1, as estimated from the computer-generated molecular model as shown in Figure 2. The incorporated compound 1 within the silica fiber emitted a broad fluorescence peak at 430 nm, suggesting that the partial overlap of pyrene units as observed in the gel phase was maintained after wrapping with the silica wall. Furthermore, the fluorescence spectrum of 1 remained unchanged when immersed in methanol and heating above 60 °C, indicating the stabilization of molecular stacks by the deposition of silica wall on the surface of self-organized fibers. These results clearly support that the thin silica wall was deposited onto the surface of individual molecular assemblies and the cylindrical assembly of 1 was immobilized without a morphological change. SWNTs are insoluble in most solvents, but have been reported to form stable suspensions by treatment with noncovalent modifiers.10 The noncovalent modification of SWNTs with 1 was carried out according to the reported method.11 A black supernatant containing 1-SWNTs composite was obtained with no detectable solid precipitation for over 10 days. The adsorption of 1 on the surface of SWNTs prevented the SWNTs from aggregating and induced a stable suspension in methanol. In contrast, a solution of 2 and SWNTs after sonication produced an unstable suspension of visible insoluble solids, indicating a poor ability to form complexes of 2 and SWNTs. We compared the fluorescence and 1H NMR spectra of the 1-SWNTs solution with that of 1 alone. The fluorescence of 1 was almost completely quenched in 1-SWNTs solution, which is likely due to the energy transfer between pyrene units and SWNTs. 1H NMR signals for the pyrene-protons in CD3OD showed significant broadening and upfield shifts upon the attachment of the SWNTs. FT-IR spectra of the 1-SWNTs solution revealed an intermolecular hydrogen bond formation among 1. These spectra support the fact that the pyrene unit in 1 is adsorbed onto the SWNT surface as a result of strong π-π interactions, leaving peripheral amphiphilic chains to move freely in solution.17 This may lead to the functionalization of SWNTs with hydrophilic triethylene glycol chains that can accumulate oligomeric silica species around the SWNTs. We demonstrated wrapping noncovalent functionalized SWNTs with a silica wall by the sol-gel polymerization of TEOS. Shinkai and co-workers previously reported the preparation of silica rods under the template of SWNTs-poly(N-vinylpyrrolidone) composites.18 However, the resulting composites contained bundles of SWNTs. A TEM image of the sample after the sol-gel polymerization of TEOS showed a double-walled tubular structure as shown in Figure 3.19 The sol-gel polymerization of TEOS (17) (a) Srinivasan, S.; Babu, S. S.; Praveen, V. K.; Ajayaghosh, A. Angew. Chem., Int. Ed. 2008, 47, 5746. (b) Srinivasan, S.; Oraveen, V. K.; Philip, R.; Ajyayaghosh, A. Angew. Chem., Int. Ed. 2008, 47, 5750. (18) Asai, M.; Fujita, N.; Sano, M.; Shinkai, S. J. Mater. Chem. 2003, 13, 2145. (19) Endo, M.; Muramatsu, H.; Hayashi, T.; Kim, Y. A.; Terrones, M.; Dresselhaus, M. S. Nature 2005, 433, 476. (20) (a) Rao, A. M.; Richter, E.; Bandow, S.; Chase, B.; Eklund, P. C.; Williams, K. A.; Fang, S.; Subbaswamy, K. R.; Menon, M.; Thess, A.; Smalley, R. E.; Dresselhaus, G.; Dresselhaus, M. S. Science 1997, 275, 187. (b) Rao, A. M.; Chen, J.; Richter, E.; Eklund, P. C.; Haddon, R. C.; Venkateswaran, U. D.; Kwon, Y.-K.; Toma´nek, D. Phys. ReV. Lett. 2001, 86, 3895.

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Figure 3. (a) TEM image of SWNTs after coating with a silica wall, and the schematics of the assembly of amphiphilic modifier 1 and coating with a silica wall on individual SWNTs. First, SWNTs are functionalized with 1 through π-π interaction between the pyrene unit and the sidewall of SWNTs, followed by the formation of a silica wall around noncovalent functionalized SWNTs by the sol-gel polymerization of TEOS. (b) Photograph of a supramolecular gel of 1 and SWNTs.

produced a smooth coating on SWNTs over a large distance, with the outer tubular structure having a diameter of 4.6 nm. The observed diameter was about 1.3 times greater than that of the silica-coated fiber diameter composed of aggregated 1 and a silica wall. The inner diameter of the observed double-walled tubular structure was 1.2 nm, which corresponded to the diameter of a typical, individual SWNT produced using arc-discharge.20 This clearly shows the existence of unaggregated SWNTs within the nanoscopic fibers. Energy-dispersive X-ray analysis (EDXA) indicated the presence of Si and C within the sample. The FT-IR spectra showed the sum of the spectra of 1 and silica, supporting the inclusion of 1 within the sample and the formation of SiO-Si by the hydrolysis of TEOS. A Raman spectrum of the sample after the sol-gel polymerization indicated two peaks at 1338 and 1590 cm-1, which were ascribable to the D- and G-bands for SWNTs, that remained unaltered in comparison with those

Kimura et al.

for only SWNTs.19 In contrast, the frequencies of the radial breathing mode (RBM) for the silica-coated fiber were found to be 10 cm-1 higher than those observed for only SWNTs. This frequency shift in the RBM provides evidence of the dissociation of bundle structures caused by the functionalization of SWNTs with 1.21 These results provide clear evidence for the inclusion of SWNTs within organic-inorganic nanoscopic fibers using the noncovalent functionalization of individual SWNTs with 1 and for the deposition of silica walls. In conclusion, the present paper demonstrated the creation of organic-inorganic nanoscopic fibers containing fluorescent pyrene stacks by the deposition of a peripheral tri(ethylene glycol) layer of self-organized fibrous assemblies with a silica wall. The synthesized amphiphilic compound 1 assembled into the cylindrical aggregates through partial overlapping of pyrene units and through intermolecular hydrogen bonding among amide groups. The pyrene unit in 1 can interact with the sidewall of SWNTs through π-π interaction, and the adsorption of 1 onto the surface of individual SWNTs could disrupt the formation of bundles. The accumulation of oligomeric silica species at the hydrophilic surface created the organic-inorganic nanoscopic fibers that contained SWNTs. The resultant fibers can be considered as electronic cables that are composed of a conductive SWNT and an insulating silica wall. The wrapping of individual SWNTs with a silica wall will open a new opportunity for evaluating electronic and optical properties of SWNTs. We believe that the wrapping of SWNTs with insulated inorganic walls can be used to align and orientate SWNTs on solid substrates for the fabrication of nanodevices. Acknowledgment. This work was supported by projects for the “Innovation Creative Center for Advanced Interdisciplinary Research Areas” in Special Coordination Funds for Promoting Science and Technology from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Supporting Information Available: 1H NMR spectra of 1 in CD3OD and CD3OD-D2O, XRD pattern of supramolecular gel, UV-vis and fluorescence spectra of 1-SWNTs solution, and Raman spectra of 1-SWNTs wrapped with a silica wall (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. LA802991G (21) (a) Jishi, R. A.; Venkataraman, L.; Dresselhaus, M. S.; Dresselhaus, G. Chem. Phys. Lett. 1993, 209, 77. (b) Saito, R.; Tanaka, T.; Kimura, T.; Dresselhaus, G.; Dresselhaus, M. S. Phys. ReV. B 1998, 57, 4145. (c) Dalton, A. B.; Stephan, C.; Coleman, J. N.; McCarthy, B.; Ajayan, P. M.; Lefrant, S.; Bernier, P.; Blau, W. J.; Byrne, H. J. J. Phys. Chem. B 2000, 104, 10012.