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Individual Single-Wall Carbon Nanohorns Separated from Aggregates Minfang Zhang,*,† Takashi Yamaguchi,‡ Sumio Iijima,†,‡ and Masako Yudasaka*,† National Institute of AdVanced Industrial Science and Technology, Higashi 1-1-1, Tsukuba 305-8565, Japan, and Nagoya UniVersity, Chiikusa-ku, Nagoya, 464-8601, Japan ReceiVed: April 23, 2009; ReVised Manuscript ReceiVed: May 30, 2009
The isolation of individual single-wall carbon nanohorns (SWNHs) from spherical dahlia-shaped aggregates of SWNHs has been successfully achieved for the first time by sucrose density gradient centrifugation. The shapes of the individual SWNHs took straight and two- and three-way branched forms. The lengths of the straight SWNHs or of each arm of the branched types were 10-70 nm, and their diameters were 2-10 nm. This new type of nanocarbon tubules will advance research of the applications of carbon nanostructures. Single-wall carbon nanohorns (SWNHs)1 are nanometer-sized single-graphene tubules having horn-shaped tips. Thousands of SWNHs assemble to form a spherical aggregate (SWNHag) with a diameter of 80-120 nm. A high purity SWNHag is produced abundantly by CO2 laser ablation of graphite without the use of metal catalysts and without auxiliary heating.1,2 The SWNHag has a large surface area and abundant hollow interior space,3 and it is easily chemically functionalized.4 Therefore, its potential applications to various fields have progressed, as exemplified by drug delivery systems (DDS).5 The anticancer effect of cisplatin was enhanced by incorporation inside SWNHag, as demonstrated by intratumoral injection into subcutaneously transplanted tumors in mice.5d Zinc phthalocyanine (ZnPc)-loaded SWNHag exhibited remarkable anticancer effects by phototherapy when injected intratumorally.5e ZnPc-SWNHag enabled a dual phototherapy approach: ZnPc was the photodynamic agent, and SWNHag, the photohyperthermia agent. While the application investigations have been developed, the fundamental structure of the individual SWNH (SWNHin) has not yet been clarified. On the basis of transmission electron microscopy (TEM) observations of SWNHag, SWNHin have been estimated to have diameters of 2-5 nm and lengths of 40-50 nm.1 In this study, for the first time, the structure of SWNHin was revealed by separating SWNHin from the asprepared SWNHag, and tubules with multiple branches as well as straight tubules were found. In addition, small-sized aggregates of SWNHs (SWNHsm) were available. To our best knowledge, SWNHin and SWNHsm are new categories of nanocarbon materials, and will be useful in various applications. For example, the use of SWNH-DDS is compromised by the trapping of SWNHag by the reticuloendotherial system (RES), which makes it difficult for the therapeutic agent to reach the tumors through intravenous injections.6 Because of their small * To whom correspondence should be addressed. E-mail: m-zhang@ aist.go.jp (M.Z.);
[email protected] (M.Y.). † National Institute of Advanced Industrial Science and Technology. ‡ Nagoya University.
10.1021/jp9037705 CCC: $40.75
Figure 1. (a) Centrifugation tube in which a sodium cholate dispersion of ox-SWNHag was placed on top of sucrose solutions made of three layers with different concentrations: 5, 10, and 30%. (b) The same tube after centrifugation for 4 h at 4600g. Four zones were designated as I-IV. (c) Particle-size distributions for layers I-IV measured by the dynamic light scattering method.
sizes, SWNHin or SWNHsm would not be as easily captured by the RES as single-wall carbon nanotubes (SWNTs)7 of ∼100 nm length. The separation process started from the oxidation of as-grown SWNHag (as-SWNHag): as-SWNHag was heated in air from about 20 to 450 °C at a temperature increase rate of 1 °C/min, followed by natural cooling (ox-SWNHag). This slow combustion is the best process because it produces less carbonaceous dusts.8 The influence of the end temperature on the SWNH separation is described in the Supporting Information (section 1). The ox-SWNHag (10 mg) obtained was mixed with 10 mL of aqueous solution of sodium cholate (SC, 2 mg/mL) by sonication (∼300 W, 1.5 h). The ox-SWNHag/SC dispersion was placed on top of layered sucrose solutions in a centrifuge tube (Figure 1a). Here, sucrose solutions of three different concentrations were layered (sucrose concentrations: top, 5%; middle, 10%; bottom, 30%), and each layer volume was about 2 mL (Figure 1a). After the centrifugation for 4 h at 4600g, the dispersions in the centrifuge tubes were isolated in batches from the four zones, numbered I to IV, from top to bottom (Figure 2009 American Chemical Society
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Figure 2. Typical TEM image of an ox-SWNH aggregate (a) and images of the particles in layer I (Figure 1b) (b and c), straight ox-SWNHs (d and e), branched ox-SWNHs (f and g), and small aggregates (h and i). Scale bars: 50 nm (a and b), 100 nm (c), and 10 nm (d-i).
Figure 3. (a) Histogram of ox-SWNHs with various forms in zone I. (b) Diameter distribution of ox-SWNHin. (c) Length distribution of straight ox-SWNHin. Distribution of (d) overall length and (e) each branch length of branched ox-SWNHin.
1b). Each layer volume was about 2 mL. The particle-size distributions estimated from the dynamic light scattering (DLS) (Figure 1c) show small particle sizes of about 30 nm in zone I, 70 nm in II, 100 nm in III, and 180 nm in IV. Since we were interested in the small-sized aggregates, we mainly analyzed the contents in zone I. TEM (Supporting Information, section 2) images confirmed that zone I contained the ox-SWNHin and ox-SWNHsm (Figure 2b and c). The ox-SWNHin took several shapes: almost straight and two- or three-way branched forms (Figure 2d-g). These configurations are indeed extremely different from the typical ox-SWNHag (Figure 2a). The numbers of ox-SWNHin and oxSWNHsm were counted (Figure 3a). The straight ox-SWNHin, including slightly curved structures, represented about 27% of the material, and the branched ox-SWNHin (two-, three-, or
higher-way branched), about 34%. Approximately 23% was the SWNHsm (Figure 2h and i), and the remaining ∼16% were agglomerates of tubules and/or graphenes. The diameter at maximum position (often from the middle of the tube) and length of ox-SWNHin or each branch of the ox-SWNHin were measured for the 480 objects in the TEM images. The diameters (Figure 3b) ranged from 2 to 10 nm, with most (about 70%) from 4 to 7 nm. The lengths of many straight SWNHin (Figure 3c) were 10-70 nm, and the overall lengths of branched SWNHin (Figure 3d) were 20-80 nm, where each arm was 10-60 nm (Figure 3e). These diameters and lengths largely agreed with those estimated from TEM observations in the first report of SWNHag in 1999.1 The ox-SWNHsm usually had sizes of 10-50 nm (Figure 2h and i), and often exhibited tubes aligning roughly in parallel (Figure 2i). TEM observation showed that there were ox-SWNHsm (50-80 nm) and a little of SWNHin in zone II, and SWNHag with sizes of 80-120 nm were in zone III. In zone IV, there were giant graphitic particles (GG-ball) besides large aggregates and agglomerates of SWNHs (not shown). We performed a sucrose gradient centrifugation (SGC) method on as-SWNHag (no oxidation). In the top layer, the number of SWNHin decreased, while those of SWNHsm and large SWNHag increased, and their ratio was approximately 1:50-100. Indeed, the DLS measurements showed the larger particle-size distribution (>50 nm) (Supporting Information, section 3). This is reasonable because the density of asSWNHag, 1.25 g/cm3, is less than that of ox-SWNHag, 2.05 g/cm3,3b meaning that ox-SWNHag settled out more quickly than did as-SWNHag. The existence of SWNHin and SWNHsm in the SGC top layer obtained from as-SWNHag suggests that both are contained in as-SWNHag. However, this result cannot eliminate the possibility that some of the ox-SWNHin and oxSWNHsm were generated by the oxidation from the large SWNHag.
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Letters incorporation of Gd2O3. The individual SWNHs are a new category of nanocarbon structures, having diameters larger than and lengths shorter than single-wall carbon nanotubes. The small SWNH aggregates are also a new type of carbon nanostructure. We believe that both will open up new fields of nanocarbon technology. Acknowledgment. We thank Dr. Azami and Dr. Kasuya for supplying as-grown single-wall carbon nanohorns. Supporting Information Available: Experimental details and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 4. TEM images of Gdox@SWNH after sucrose gradient centrifugation separation. Straight Gdox@SWNH (a), branched Gdox@SWNH (b and c), and small aggregates of Gdox@SWNH (d). Scale bars: 20 nm (a-c) and 50 nm (d).
In our previous studies,9 we used general centrifugation but not the SGC method to separate as-SWNHag with small sizes of about 50-80 nm from the larger-sized as-SWNHag and graphitic particles. The ox-SWNHag settled down to the bottom quickly; therefore, the separation of ox-SWNHag was difficult.9a Using the SGC method in this study, we were able to separate ox-SWNHin from ox-SWNHag because the particles with different sizes had different settling rates in the solutions.10 It is worth noting that the size and shape distributions of SWNHs would change by choosing different thicknesses of the zones. The more defined process such as preparing a uniform gradient of sucrose gradient solution by using a gradient maker or repeating SGC several times will improve the separation efficiencies. It has been reported that SWNTs could be sorted by length and even types of metallic and semiconducting tubes.11 We believe that the yield and structure uniformity of SWNHin would be improved by the more defined SGC. To prove that the ox-SWNHin and ox-SWNHsm had hollow inner spaces, we incorporated Gd2O3 into them. The incorporation method is described in detail in the Supporting Information (section 4). Briefly, the Gd acetate was incorporated inside oxSWNHs in methanol, followed by heat treatment at 1200 °C to change the Gd acetate to Gd2O3 and to close the holes of ox-SWNH.12,13 The obtained material is designated as Gdox@ SWNH. The obtained Gdox@SWNH was similarly dispersed in SC solution and followed by SGC. The dispersion at the SGC top layer was filtered by using an ultrafiltration membrane (Millipore, MWCO 30000) and washed four times with hydrochloric acid solution (30%, 5 mL) and two times with deionized water (5 mL). TEM images of Gdox@SWNH showed (Figure 4) that the Gd2O3 was located inside of the SWNHin and SWNHsm. In summary, we have successfully separated the individual SWNHs and small SWNH aggregates from the oxidized SWNH aggregates by sucrose density gradient centrifugation. The individual SWNHs had several different shapes such as straight and two-way or three-way branched, which were identified for the first time in this study. The diameters and lengths of the individual SWNHs or of each branch of the branched SWNHs ranged from 2 to 10 nm and 10 to 70 nm, respectively. The small SWNH aggregates had diameters of 10-50 nm. These new SWNH tubules had hollow inner spaces as proved by the
References and Notes (1) Iijima, S.; Yudasaka, M.; Yamada, R.; Bandow, S.; Suenaga, K.; Kokai, F.; Takahashi, K. Chem. Phys. Lett. 1999, 309, 165. (2) (a) Kasuya, D.; Yudasaka, M.; Takahashi, K.; Kokai, F.; Iijima, S. J. Phys. Chem. B 2002, 106, 4947. (b) Azami, T.; Kasuya, D.; Yuge, R.; Yudasaka, M.; Iijima, S.; Yoshitake, T.; Kubo, Y. J. Phys. Chem. C 2008, 112, 1330. (3) (a) Murata, K.; Kaneko, K.; Kokai, F.; Takahashi, K.; Yudasaka, M.; Iijima, S. Chem. Phys. Lett. 2000, 331, 14. (b) Murata, K.; Kaneko, K.; Steele, W. A.; Kokai, F.; Takahashi, K.; Kasuya, D.; Yudasaka, M.; Iijima, S. Nano Lett. 2001, 1, 197. (c) Yang, C. M.; Kasuya, D.; Yudasaka, M.; Iijima, S.; Kaneko, K. J. Phys. Chem. B 2004, 108, 17775. (d) Bekyarova, E.; Kaneko, K.; Kasuya, D.; Murada, K.; Yudasaka, M.; Iijima, S. Langmuir 2002, 18, 4138. (4) (a) Tagmatarchis, N.; Maigne, A.; Yudasaka, M.; Iijima, S. Small 2006, 2, 490. (b) Cioffi, C.; Campidelli, S.; Brynetti, F.; Meneghetti, M.; Prato, M. Chem. Commun. 2006, 2129. (c) Isobe, H.; Tanaka, T.; Maeda, R.; Noiri, E.; Solin, N.; Yudasaka, M.; Iijima, S.; Nakamura, E. Angew. Chem., Int. Ed. 2006, 45, 6676. (d) Zhang, M.; Yudasaka, M.; Ajima, K.; Miyawaki, J.; Iijima, S. ACS Nano 2007, 1, 265. (5) (a) Murakami, T.; Ajima, K.; Miyawaki, J.; Yudasaka, M.; Iijima, S.; Shiba, K. Mol. Pharmaceutics 2004, 1, 399. (b) Ajima, K.; Yudasaka, M.; Murakami, T.; Maigne, A.; Shiba, K.; Iijima, S. Mol. Pharmaceutics 2005, 2, 475. (c) Murakami, T.; Fan, J.; Yudasaka, M.; Iijima, S.; Shiba, K. Mol. Pharmaceutics 2006, 3, 407. (d) Ajima, K.; Murakami, T.; Mizoguchi, Y.; Tsuchida, K.; Ichihashi, T.; Iijima, S. ACS Nano 2008, 2, 2057. (e) Zhang, M.; Murakami, T.; Ajima, K.; Tsuchida, K.; Sandanayaka, A.; Ito, O.; Iijima, S.; Yudasaka, M. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 14773. (6) Miyawaki, J.; Zhang, M.; Iijima, S.; Yudasaka, M. In preparation, 2009. (7) Liu, Z.; Cai, W.; He, L.; Nakayama, N.; Chen, K.; Sun, X.; Chen, X.; Dai, H. Nat. Nanotechnol. 2007, 2, 47. (b) Liu, Z.; Davis, C.; Cai, E.; He, L.; Chen, X.; Dai, H. Proc. Natl. Acad. Soc. 2008, 105, 1419. (c) Liu, Z.; Chen, K.; Davis, C.; Sherlock, S.; Cao, Q.; Chen, X.; Dai, H. Cancer Res. 2008, 68, 6652. (8) Fan, J.; Yudasaka, M.; Miyawaki, J.; Ajima, K.; Murata, K.; Iijima, S. J. Phys. Chem. B 2006, 110, 1587. (9) (a) Zhang, M.; Yudasaka, M.; Miyawaki, J.; Fan, J.; Iijima, S. J. Phys. Chem. B 2005, 109, 22201. (b) Tamura, G.; Yudasaka, M.; Iijima, S. Jpn. J. Appl. Phys. 2009, 48, 015003. (10) Alberts, B.; Johnson, A.; Lewis J.; Raff, M.; Roberts, K.; Walter, P. Molecular Biology of The Cell, 4th ed.; Garland Science: New York, 2002; pp 478-480. (11) (a) Arnold, M.; Stupp, S.; Hersam, M. Nano Lett. 2005, 5, 713. (b) Arnold, M.; Green, A; Hulvat, J.; Stupp, S.; Hersam, M. Nat. Nanotechnol. 2006, 1, 60. (c) Sun, X.; Zaric, S.; Daranciang, D.; Welsher, K.; Lu, Y.; Li, X.; Dai, H. J. Am. Chem. Soc. 2008, 130, 6551. (d) Fagan, J. A.; Becker, M. L.; Chun, J.; Hobbie, E. K. AdV. Mater. 2008, 20, 1609. (e) Green, A.; Duch, M.; Hersam, M. Nano Res. 2009, 2, 69. (12) (a) Hashimoto, A.; Yomitsu, H.; Ajima, K.; Suenaga, K.; Isobe, H.; Miyawaki, J.; Yudasaka, M.; Iijima, S.; Nakamura, E. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 8527. (b) Yuge, R.; Ichihashi, T.; Miyawaki, J.; Yoshitake, T.; Iijima, S.; Yudasaka, M. J. Phys. Chem. C 2009, 113, 2741. (13) (a) Miyawaki, J.; Yuge, R.; Kawai, T.; Yudasaka, M.; Iijima, S. J. Phys. Chem. C 2007, 111, 1553. (b) Fan, J.; Yuge, R.; Miyawaki, J.; Kawai, T.; Iijima, S.; Yudasaka, M. J. Phys. Chem. C 2008, 112, 8600.
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