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Alternate Layer-by-Layer Adsorption of Single- and Double-Walled Carbon Nanotubes Wrapped by Functionalized β-1,3-Glucan Polysaccharides Kouta Sugikawa,† Munenori Numata,‡ Kenji Kaneko,† Kazuki Sada,† and Seiji Shinkai*,†,§ Graduate School of Engineering, Kyushu UniVersity, Graduate School of Life and EnVironmental Science, Kyoto Prefectural UniVersity, and Institute of Systems, Information Technologies and Nanotechnology (ISIT), 203-1, Motooka, Nishi-ku, Fukuoka 819-0385, Japan ReceiVed July 11, 2008. ReVised Manuscript ReceiVed October 15, 2008 A great deal of attention has been focused on exploiting novel methods to fabricate thin carbonaceous capsules from multiple components for advanced materials. A layer-by-layer (LbL) method is therefore being introduced to synthesize thin and multi-carbon nanotube (CNT)-based hollow capsules from CNT complexes with cationic or anionic complementarily functionalized β-1,3-glucans as building-blocks. These ionic β-1,3-glucans wrap around single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs) to form water-soluble complexes with ionic groups on their exterior surface. Alternate self-assembly of these CNT complexes on the silica particles is demonstrated in solution by electrostatic interactions. The LbL adsorption processes were carefully monitored by ζ-potential measurements, frequency shifts of a quartz crystal microbalance (QCM), and electron micrographs. Silica particles were then dissolved away by HF acid to obtain CNT-based hollow capsules composed of SWNTs and DWNTs. We believe that these novel surface adsorption methods are useful for potential design of CNT-based advanced functional materials.
Introduction Hollow capsules, such as biomaterial or carbonaceous capsules, have attracted a great deal of attention because they have many unique properties in various research fields including nanoreaction templates, catalyst supports, adsorbents, biomedical devices, and lithium ion batteries.1-8 Carbon nanotubes (CNTs), such as single-walled (SWNTs) or multiwalled carbon nanotubes (MWNTs), have been chosen as components for synthesizing carbonaceous capsules, which might act as the new electron-conductive functional materials.9-28 In recent years, various techniques have been used to construct different CNT* Corresponding author.
[email protected]. † Graduate School of Engineering, Kyushu University. ‡ Graduate School of Life and Environmental Science, Kyoto Prefectural University. § Institute of Systems, Information Technologies and Nanotechnology (ISIT). (1) De La Escosura, A.; Verwegen, M.; Sikkema, F. D.; Comellas-Aragones, M.; Kirilyuk, A.; Rasing, T.; Nolte, R. J. M.; Cornelissen, J. L. M. C. Chem. Commun. 2003, 1542. (2) Douglas, T.; Young, M. Science 2006, 312, 873. (3) Tang, Z. Y.; Kotov, N. A.; Goersig, M. Science 2002, 297, 237. (4) Yehai, Y.; Mary, B. C.-P.; Qing, Z. Small 2007, 3, 24. (5) Sun, X. M.; Li, Y. D. J. Colloid Interface Sci. 2005, 291, 7. (6) Yu, J. C.; Hu, X. L.; Li, Q.; Zheng, Z.; Xu, Y. M. Chem. Eur. J. 2006, 12, 548. (7) Soppimath, K. S.; Tan, D. C. W.; Yang, Y. T. AdV. Mater. 2005, 17, 318. (8) Zhang, W.-M.; Hu, J.-S.; Guo, Y.-G.; Zheng, S.-F.; Zhong, L.-S.; Song, W.-G.; Wan, L.-J. AdV. Mater. 2008, 20, 1160. (9) Tans, S. J.; Devoret, M. H.; Dai, H.; Thess, A.; Smalley, R. E.; Geerligs, L. J.; Dekker, C. Nature 1997, 386, 474. (10) Tans, S. J.; Dekker, C. Nature 2000, 404, 834. (11) Fukushima, T.; Kosaka, A.; Ishimura, Y.; Yamamoto, T.; Takigawa, T.; Ishii, N.; Aida, T. Science 2003, 300, 2072. (12) Ikeda, A.; Hayashi, K.; Konishi, T.; Kikuchi, J. Chem. Commun. 2004, 1334. (13) Wei, J.; Zhu, H.; Jiang, B.; Ci, L.; Wu, D. Carbon 2003, 41, 2495. (14) Rouse, J. H.; Lillehei, P. T.; Sanderson, J.; Siochi, E. J. Chem. Mater. 2004, 16, 3904. (15) Sato, M.; Sano, M. Langmuir 2005, 21, 11490. (16) Moya, S. E.; Ilie, A.; Bendall, J. S.; Hernandez-Lopez, J. L.; Ruiz-Garcia, J.; Huck, W. T. S. Macromol. Chem. Phys. 2007, 208, 603. (17) Qin, S. H.; Qin, D. Q.; Ford, W. T.; Zhang, Y. J.; Kotov, N. A. Chem. Mater. 2005, 17, 2131.
based superstructures to utilize their properties. The poor solubility/dispersion of pristine CNTs, however, has made it difficult to control their structures. Layer-by-Layer (LbL) assembly is one of the most well-established methods to create controlled two- or three-dimensional CNT architectures.14-28 In this way, alternating adsorption of CNTs and polymers attracted to each other through the use of donor-acceptor,14 van der Waals,15,16 hydrogen bond,17-19 or electrostatic interactions20-22 results in uniform growth of CNT-based films. The LbL method has also been utilized in the creation of CNT-based hollow capsules by using spherical cores as templates. Sano, Shinkai, and others first reported the creation of a CNT cage structure utilizing the amine-nanotube interaction and the nanotube-nanotube hydrophobic interaction on silica gel templates and the removal of templates via HF etching.23 Thereafter, other groups have reported the synthesize of CNTbased hollow capsules by LbL methods utilizing hydrophobic or electrostatic interactions.24-28 In these reports, however, only one component, such as SWNTs or MWNTs, except for the polyelectroyte supports, were present in the hollow capsules. To create more functional CNT-based hollow capsules, one must (18) Shi, J. H.; Qin, Y. J.; Luo, H. X.; Guo, Z. X.; Woo, H. S.; Park, D. K. Nanotechnology 2007, 18, 365704. (19) Ishibashi, A.; Yamaguchi, Y.; Murakami, H.; Nakashima, N. Chem. Phys. Lett. 2006, 419, 574. (20) Mamedov, A. A.; Kotov, N. A.; Prato, M.; Guldi, D. M.; Wicksted, J. P.; Hirsch, A. Nat. Mater. 2002, 1, 190. (21) Kim, B.; Park, H.; Sigmund, W. M. Langmuir 2003, 19, 2525. (22) Shim, S. S.; Podsiadlo, P.; Lilly, D. G.; Agarwal, A.; Lee, J.; Tang, Z.; Ho, S.; Ingle, P.; Paterson, D.; Lu, W.; Kotov, N. A. Nano Lett. 2007, 7, 3266. (23) Sano, M.; Kamino, A.; Okamura, J.; Shinkai, S. Nano Lett. 2002, 2, 531. (24) Shi, J.; Cheng, Z.; Qin, Y.; Guo, Z.-X. J. Phys. Chem. 2008, 112, 11617. (25) Correa-Duarte, M. A.; Kosiorek, A.; Kandulski, W.; Giersig, M.; Salgueirino-Maceira, V. Small 2006, 2, 220. (26) Huang, X. J.; Li, Y.; Im, H. S.; Yarimaga, O.; Kim, J. H.; Jang, D. Y.; Cho, S. O.; Cai, W. P.; Choi, Y. K. Nanotechnology 2006, 17, 2988. (27) Jin, H. J.; Choi, H. J.; Yoon, S. H.; Myung, S. J.; Shim, S. E. Chem. Mater. 2005, 17, 4034. (28) Ji, L. J.; Ma, J.; Zhao, C. G.; Wei, W.; Wang, X. C.; Yang, M. S.; Lu, Y. F.; Yang, Z. Z. Chem. Commun. 2006, 1206.
10.1021/la802211q CCC: $40.75 2008 American Chemical Society Published on Web 10/31/2008
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Figure 1. (A) Structures of CUR, CUR-N+, and CUR-SO3-, and schematic diagrams of the CNT-based multilayer capsule formation. (B) Frequency shifts for the alternate adsorption of CUR-N+/SWNT complexes (closed triangle)-CUR-SO3-/SWNT complexes (closed circle), and the alternate assembling adsorption of CUR-N+/DWNT complexes (closed square)-CUR-SO3-/SWNT complexes (closed circle) on QCM. (C) Relationship between the ζ-potential of an outermost surface and the assembling step in SWNT films (solid line) and SWNT/DWNT films (dashed line) on silica particles.
develop some more sophisticated CNT adsorption methods with ease.8 Natural β-1,3-glucan polysaccharides can act as a onedimensional host including several guest compounds, such as SWNTs and conductive polymers, into the helical tubular hollow inherent to β-1,3-glucans, resulting in the creation of watersoluble one-dimensional nanocomplexes.29-32 Furthermore, there have been several reports of synthesizing functionalized curdlans33 (CUR, one kind of β-1,3-glucans) and of wrapping SWNTs within them.34,35 More recently, the creation of the highly ordered hierarchical architectures of SWNTs were fabricated by the electrostatic interactions between functionalized CUR/SWNT complexes from ammonium-modified cationic CUR (CUR-N+) and sulfonate-modified anionic CUR (CUR-SO3-) complexes.35 It is therefore determined that CUR-N+/CNT and CUR-SO3-/ CNT complexes are expected to self-assemble alternately on ionic surfaces such as silica particles and melamin resins in the (29) Numata, M.; Asai, M.; Kaneko, M.; Hasegawa, T.; Fujita, N.; Kitada, Y.; Sakurai, K.; Shinkai, S. Chem. Lett. 2004, 33, 232. (30) Numata, M.; Hasegawa, T.; Fujisawa, T.; Sakurai, S.; Shinkai, S. Org. Lett. 2004, 6, 4447. (31) Li, C.; Numata, M.; Bae, A.-H.; Sakurai, K.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 4548. (32) Numata, M.; Asai, M.; Kaneko, M.; Bae, A.-H.; Hasegawa, T.; Sakurai, K.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 5875. (33) Hasegawa, T.; Umeda, M.; Numata, M.; Li, C.; Bae, A.-H.; Fujisawa, T.; Haraguchi, S.; Sakurai, K.; Shinkai, S. Carbohydr. Res. 2006, 341, 35. (34) Ikeda, M.; Hasegawa, T.; Numata, M.; Sugikawa, K.; Sakurai, K.; Fujiki, M.; Shinkai, S. J. Am. Chem. Soc. 2007, 129, 3979. (35) Numata, M.; Sugikawa, K.; Kaneko, K.; Shinkai, S. Chem. Eur. J. 2008, 14, 2398.
LbL manner.23-28,36-39 According to this method, one can selfassemble different guest polymers alternately by including one in CUR-N+ and another in CUR-SO3-. To prove this, we chose SWNTs and DWNTs as the guest compounds of CUR-N+ and CUR-SO3-. In this paper, we first report the self-assembly of CUR-N+/ SWNT and CUR-SO3-/SWNT complexes constructed on the surface of the silica particles by the LbL method and the creation of SWNT-based hollow capsules (in this case, hollow assemblies) with the controlled thin layers after dissolution of silica particles by HF etching (Figure 1A). In addition, the syntheses of alternate self-assembled SWNT/DWNT multilayers on the surface of silica particles and of corresponding hollow capsules with the same methods are performed in order to demonstrate that two different complexes can be assembled alternately by LbL methods just by changing the guest compounds wrapped by functionalized β-1,3 glucans. The LbL processes and the resulting hollow capsules have been thoroughly characterized by ζ-potentials, frequency shifts of the QCM, scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), and so forth. (36) Ma, Y.; Dong, W.-F.; Hempenius, A. M.; Mohwald, H.; Vancso, L. G. Nat. Mater. 2006, 5, 724. (37) Kida, T.; Mouri, M.; Akashi, M. Angew. Chem., Int. Ed. 2006, 45, 7534. (38) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mohwald, H. Macromolecule 1999, 32, 2317. (39) Kato, N.; Schuetz, P.; Fery, A.; Caruso, F. Macromolecules 2002, 35, 9780.
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Experimental Section Materials. CUR used here was purchased from Tokyo Kasei Ind. Co. SWNTs (lot no: po304) and DWNTs (lot no: dw0923) were obtained from Carbon Nanotechnologies, Inc. Template silica particles with 5 µm in average size were purchased from Fuji Silicia Chemical Ltd. The synthesis of CUR-N+ and CUR-SO3- was carried out according to the method previously reported by us.35 Preparation of CUR-N+/CNT and CUR-SO3-/CNT Complexes. CUR-N+/SWNT and CUR-SO3-/SWNT complexes were prepared and purified by gel column chromatography (Sephadex, G-100, eluted with water) as we reported in our previous paper.23 We also prepared CUR-N+/DWNT complexes by the same procedure. Synthesis of CUR/CNT-Coated Silica Particles. A 0.6 mL aqueous suspension of silica particles (50 mg/mL) was incubated in 0.8 mL of CUR-N+/CNT and CUR-SO3-/CNT complex solutions (2 µM NaCl) alternately at room temperature for 20 min under gentle vibration. The CUR/CNT-coated silica particles were then rinsed three times in distilled water with a centrifuge (1000 g, 10 min) and dried in an oven (60 °C, 6 h). The immersion and drying processes were continued for 9 cycles to create the CNT layers on the surface of silica particles. CUR-N+/SWNT and CUR-N+/DWNT complexes were chosen as the source of CUR-N+/CNT, and CURSO3-/SWNT complexes as that of CUR-SO3-/CNT. Each concentration was determined by UV-vis absorption spectroscopy.12 As a result, the powders of silica particles turned color from white to black after the adsorption processes, which implies the formation of CUR/CNT-coated silica particles. Removal of Core Silica Particles. 2.3% aqueous HF solution (2 mL) was added into the black CUR/CNT-coated silica particles (ca. 6 mg), and the mixture was incubated for 12 h at 4 °C to etch away the silica cores.25 The cloudy mixture became transparent, which means dissolution of the silica cores. After 12 h of incubation, the black products accumulated at the bottom, and then the upper 90% of clear supernatant was carefully decanted. With addition of water to the bottom residue, the decantation procedure was repeated three times to remove dissolved silica thoroughly. Quartz Crystal Microbalance (QCM) Measurements. A QCM setup with a frequency counter (Hewlett Packard, Universal Counter 53131A) and an oscillator (U.S.I, UQ-100) for 9 MHz QCM electrodes was used for QCM measurements. The cleaned QCM was incubated in 1 M of sodium 2-mercaptoethanesulfonate solution for 24 h to obtain the anionic surfaces, and then washed with pure water. The anionic modified QCM was immersed into an aqueous CUR-N+/CNT complex solution (4.0 µM NaCl) for 20 min at room temperature, rinsed thoroughly with pure water, and then dried under N2 gas. After the measurement of the frequency shift, the QCM was immersed into an aqueous CUR-SO3-/CNT complex solution (4.0 µM NaCl) for 20 min at room temperature and dried under N2 gas. The same procedure was repeated 6 times to form CNT-layers on the surface of QCM by the LbL method. Microelectrophoresis and Dynamic Laser Light Scattering (DLS). ζ-Potentials and DLS measurements were performed by a Malvern Zetasizer apparatus in water. The particles and capsules dispersed in water were diluted for measurements. The suspension was injected into the measurement cell with a time gap between dilution and measurement shorter than 2 min. Raman Spectra. Raman spectra of CUR/CNT-coated silica particles were obtained using a JASCO NRS-3100KK laser Raman spectrometer (Ar laser, 532 nm). Transmission Electronic Microscopy (TEM) Observation. TEM images and energy dispersive X-ray spectroscopy (EDX) spectra were acquired using a JEOL TEM-2010 (accelerate voltage 120 kV) and a TECNAI-20, FEI (accelerate voltage 200 kV), respectively. Each solution was placed on a copper TEM grid, and then the TEM grid was dried under reduced pressure for more than 6 h before TEM observation. Scanning Electronic Microscopy (SEM) Observation. A HITACHI S-5000 was used for SEM studies under high vacuum. The diluted suspensions were dried under reduced pressure on mica.
Letters Wet-SEM Observation. A JEOL SS-550 was used for WetSEM studies. The obtained SWNT-based hollow capsule suspensions having CUR-N+/SWNT complexes on their outermost surfaces were glued up in QX-102 capsules (QUANTAMIX) whose membrane was treated with poly(sodium-4-styrenesulfonate) solution (30% w/v) to carry out Wet-SEM observation in order to examine the structure of SWNT-based hollow capsules in water. Atomic Force Microscopy (AFM). AFM images were acquired in air using a NanoScope IIIa (tapping mode). The sample was cast on mica and dried for more than 6 h under reduced pressure before AFM observation. Phenol/Sulfuric Acid Reaction System. Phenol/sulfuric acid reaction system is useful for determining the concentration of sugars. A calibration curve was made by mannose as a standard saccharide for the phenol/sulfuric-acid reaction system. An aqueous solution containing 5% (w/v) phenol was added to 200 µL of the mannose solution. The resultant solution became an intense yellow color after the addition of 1.0 mL of sulfuric acid to the mixture immediately. The solution was kept for 40 min at room temperature to complete the coloration reaction. The absorption maximum at 490 nm was plotted as a function of several mannose concentrations. The CUR/ CNT-coated silica particles and CNT-based hollow capsules containing an unknown amount of CUR were treated with phenol followed by sulfuric acid according to the same procedure. From the calibration chart, the amounts of CUR contained in CUR/CNTcoated silica particles or CNT-based hollow capsules were estimated.
Results and Discussion First, water-soluble SWNT complexes utilizing CUR-N+ or CUR-SO3- as one-dimensional hosts according to our previous report.34,35 Water-soluble DWNT complexes with CUR-N+ were also prepared and purified in a similar way as SWNT complexes. The obtained black solutions were apparently transparent, which implies that DWNTs are solubilized in aqueous solution. Dispersed CUR-N+/DWNT complexes with 0.5-1.2 µm length and 3.1-3.6 nm height are clearly observed after casting them on mica by AFM (Supporting Information Figure S1). It is clear from this result that DWNTs are finely solubilized and dispersed with CUR-N+ in water. The ζ-potentials of CUR-N+/SWNT, CUR-N+/DWNT, and CUR-SO3-/SWNT complexes in solutions were estimated to be +48.9 mV, +49.2 mV, and -49.5 mV, respectively, which indicated that they were stably dispersed in water and their ionic moieties were present on the exterior surface of CNT complexes. Alternate thin CNT layers composed of CUR-N+/CNT and CUR-SO3-/CNT complexes were grown on a QCM in order to monitor their growth behavior from the frequency shifts of the QCM.39 An aqueous CUR-N+/SWNT complex solution (4.0 µM NaCl) and an aqueous CUR-SO3-/SWNT complex solution (4.0 µM NaCl) were used. Three CUR-N+/SWNT and three CURSO3-/SWNT complex layers were assembled alternately on the anionic QCM. The frequency shift observed for each step was almost constant: the adsorbed amounts of CUR-N+/SWNT complexes and CUR-SO3-/SWNT complexes in each step were estimated to be 277 ( 23 ng/cm2 and 178 ( 19 ng/cm2, respectively, supporting the adsorption behavior in which CURN+/SWNT and CUR-SO3-/SWNT complexes are assembled on an ionic surface by the LbL method as shown in Figure 1B. The alternate self-assembly of CUR-N+/DWNT complexes and CUR-SO3-/SWNT complexes on the surface of QCM by the LbL method was also examined. The frequency shifts were induced as shown in Figure 1B, which was basically the same as that observed for the alternate adsorption of CUR-N+/SWNT complexes and CUR-SO3-/SWNT complexes. These results suggest that the alternate self-assembly of CUR-N+/DWNT and CUR-SO3-/SWNT complexes could also occur on the surface of silica particles by the use of the electrostatic interaction. As
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Figure 2. SEM images of (A) CUR-SWNT-coated silica particles having CUR-N+/SWNT complexes as an outermost surface and (B) CUR/ SWNT/DWNT-coated silica particles having CUR-N+/DWNT complexes as an outermost surface. TEM images of CUR/SWNT/DWNT-coated silica particles having nine layers in total (C) and five layers in total (D).
the amounts of CNT complexes in each layer were almost constant and the process was reproducible, one may expect that the control of the CNT layer thickness would be possible even on the silica particles. Subsequently, LbL assembly on the surface of the silica particles was also performed. In this study, silica particles with 5.0 ( 0.4 µm diameter were selected as cores in order to prepare the CNT-based hollow capsules by HF etching (Figure 1A).23,27,28 The ζ-potentials of the particles at each adsorption step were measured by microelectrophoresis to monitor the LbL growth of CUR-N+/CNT complexes and CUR-SO3-/CNT complexes on the silica particles.38 Prior to the microelectrophoresis measurements, the obtained black particles were redispersed in pure water. The ζ-potentials of the CUR/SWNT-coated silica particles shifted to +23.6 ( 3.4 mV when CUR-N+/SWNT complexes were deposited and to -24.0 ( 3.2 mV when CURSO3-/SWNT complexes were deposited (Figure 1C, solid line). These alternate changes in the ζ-potential are characteristic of the LbL formation of SWNT multilayers on silica particles, which is strong evidence for the stepwise alternate layer growth of CUR-N+/SWNT complexes and CUR-SO3-/SWNT complexes on the surface of silica particles. CUR-N+/DWNT and CURSO3-/SWNT multilayers were also prepared by the use of CURN+/DWNT and CUR-SO3-/SWNT complex solutions. The alternating ζ-potentials of +22.2 ( 2.3 mV (CUR-N+/DWNT-
outermost surface) and -27.4 ( 2.5 mV (CUR-SO3-/SWNToutermost surface) indicate again that the formation of hetero multilayers composed of DWNTs and SWNTs are formed, as shown in Figure 1C (dashed line). Raman spectra measurements were carried out to confirm the presence of both SWNTs and DWNTs in the obtained black particles. Two characteristic peaks (230 cm-1 and 157 cm-1) were present and attributed to SWNTs and DWNTs, respectively, with almost similar relative intensities versus the G-band (1590 cm-1), almost the same intensities as those obtained from raw SWNTs and DWNTs, respectively. These results suggests that two different complexes wrapped by functional β-1,3-glucans possessing complementary moieties can be assembled alternately on the surface of the silica particles. SEM images in Figure 2A,B show the presence of CUR-N+/ CNT and CUR-SO3-/CNT complex layers maintaining the original morphologies of CNTs with knitting patterns on the surface of silica particles. TEM observations were performed for CUR/SWNT- or CUR/SWNT/DWNT-coated silica particles having nine CNT layers in total in order to evaluate the thickness of CNT layers, in which CNT layers attached on the silica particles have 20-40 nm thickness are seen from Figure 2C and Supporting Information Figure S2. The thickness is in agreement with that predicted from the height of CUR/CNT complexes obtained from AFM V(Supporting Information Figure S1). Furthermore, we carried out TEM observations for CUR/SWNT/DWNT-coated
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Figure 3. TEM images of (A) SWNT-based hollow capsules having a total of nine layers and (B) SWNT/DWNT-based hollow capsules having a total of nine layers obtained after HF etching. (C) SEM images of SWNT/DWNT-based hollow capsules on mica. (D) Results of DLS measurements for CUR/SWNT-coated silica particles in water before (closed circle) and after (closed square) HF etching.
silica particles having five CNT layers in total. 15-20 nm thickness is seen from Figure 2D, which is almost one-half that of CNT-coated silica particles having nine CNT layers. These results suggest that each CNT layer is very thin and their total thickness can be precisely controlled by the number of LbL steps. The microelectronphoresis, TEM, and SEM characterizations consistently support the presence and formation of CNT-based multilayers on the silica particles. The synthesis of CNT-based hollow capsules was therefore expected by removing the silica particles by HF etching. Several minutes after the addition of aqueous HF solution to dissolve the silica cores, the cloudy mixture became transparent. As shown in Figure 3A,B, the presence of SWNT- and SWNT/DWNT-based hollow capsules with 4-6 µm average diameters was observed in the resultant black solution by TEM, which was in good agreement with the average diameters of silica particles used as the cores. The EDX analysis provides direct evidence that the capsule structure was composed of CUR-N+/CNT complexes and CUR-SO3-/CNT complexes, in which two characteristic peaks were present at 0.39 eV (N-K) and 2.30 eV (S-K) arising from CUR-N+ and CUR-SO3-, respectively (Supporting Information Figure S4). SEM images also showed collapsed capsule structures having a network structure even after HF etching (Figure 3C). We can also confirm the creation of the similarly collapsed SWNT-based hollow capsules created from the nine-layer CUR/SWNT-coated silica particles by AFM (Supporting Information Figure S6). The line profile for the collapsed capsule structures also revealed the 30 to 60 nm height, which was in agreement with the layer thickness obtained from TEM observation (Figure 2C,D). These
findings indicate that the structure of CNT layers is almost the same even after HF etching. A solution containing the SWNT-based hollow capsules was glued up in QX-102 capsules to be examined by Wet-SEM in order to achieve the direct image in water. The presence of spherical structures with 4-6 µm in average diameter was also confirmed, which supported the formation of SWNT-based hollow capsules within water as shown in Supporting Information Figure S5C. In addition, almost the same size of SWNT-based hollow capsules was confirmed by DLS measurements for both the CUR/ SWNT-coated silica particles before HF etching and the SWNTbased hollow capsules after HF etching (Figure 3D). The ζ-potentials of SWNT-based hollow capsules having CUR-N+/ SWNT complexes and SWNT/DWNT-based hollow capsules having CUR-N+/DWNT complexes were estimated to be +22.9 mV and +19.1 mV, respectively, which suggested the survival of CUR-N+ on the surface of the capsules even after HF etching. Furthermore, the amount of surviving CUR contained in CNTbased hollow capsules was estimated at 70-80% of CUR by a phenol/sulfuric acid reaction system after HF etching. These results suggest that the spherical shape and the structure are well-maintained in water even after HF etching.
Conclusion In conclusion, we demonstrated a novel method for the preparation of CNT-based hollow capsules fabricated by very thin CNT layers. Importantly, the shell thickness of these capsules is controllable by changing the LbL assembly number. As the ability to include guest compounds and the electrostatic moieties
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of functionalized CUR are used, one can layer the various ionic-CUR-wrapping guest polymers “alternately” with the same methods. A successfully demonstrated example is the preparation of SWNT/DWNT-based hollow hetero capsules in which the thickness of each layer was controlled. In our system, therefore, there is a potential advantage that different functional polymers can be included in each layer through the use of the electrostatic interaction of functionalized CUR. These CNTbased hollow hetero capsules may find new applications in many advanced research fields such as catalyst supports, adsorbents, biomedical devices, and lithium ion batteries.
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Acknowledgment. We thank Ms. M. Fujita for AFM measurements. This work was partially supported by a grant in aid for the Global COE Program. Supporting Information Available: Additional AFM images of CUR-N+/DWNT complexes, Raman spectra of CNT coated silica particles, TEM-EDS spectra. and SEM images of CNT-based hollow capsules. This material is available free of charge via the Internet at http://pubs.acs.org. LA802211Q
