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2006, 110, 5179-5181 Published on Web 02/25/2006
Synthesis of Ultrafine Gd2O3 Nanoparticles Inside Single-Wall Carbon Nanohorns Jin Miyawaki,*,† Masako Yudasaka,*,†,‡ Hideto Imai,‡ Hideki Yorimitsu,§,| Hiroyuki Isobe,§,⊥ Eiichi Nakamura,§ and Sumio Iijima†,‡,# JST/SORST, c/o NEC, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan, NEC, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan, Department of Chemistry and JST/ERATO, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, JST/ PRESTO, and Meijo UniVersity, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya 468-8502, Japan ReceiVed: February 6, 2006; In Final Form: February 10, 2006
The large diameter of single-wall carbon nanohorns (SWNHs) allows various molecules to be easily incorporated in hollow nanospaces. In this report, we prove that the nanospaces of SWNHs even work as the chemical reaction field at high temperature; that is, Gd-acetate clusters inside SWNHs were transformed into ultrafine Gd2O3 nanoparticles with their particle size retained even after heat-treatment at 700 °C. This indicates that the confinement of the Gd-acetate clusters in a deep potential well of the SWNH nanospaces prevented a migration to form larger particles, giving rise to ultrafine Gd2O3 nanoparticles of 2.3 nm in average diameter, which is much smaller than the case without SWNHs. The Gd2O3 nanoparticles thus obtained were demonstrated to be actually useful to the magnetic resonance imaging. We believe that the presented effectiveness of the inner hollow spaces of SWNHs, therefore, also those of the carbon nanotubes, for high-temperature chemical reactions should be highlighted, and that the thus produced novel nanomaterials are promising to expand the fields of nanoscience.
Single-wall carbon nanohorns (SWNHs)1 have a tubular structure like single-wall carbon nanotubes, but their larger diameters (2-5 nm) allow various kinds of materials to be easily incorporated inside SWNHs.2 Notably, dexamethasone, an antiinflammatory drug, or cisplatin, an anti-cancer drug, has been incorporated inside SWNHs and released with its original biological integrity maintained; this suggests SWNHs can be applied as drug carriers.3 Gadolinium-acetate clusters with diameters of about 2 nm have also been encapsulated in SWNHs.4 If we can transform the Gd-acetate to Gd-oxide, Gd2O3, while retaining the particle size through confinement effects in SWNH nanospaces, the resultant ultrafine Gd2O3 nanoparticles should improve the performance of various Gd2O3 applications, such as phosphor matrix and catalyst.5,6 In our current work, we heat-treated Gdacetate clusters incorporated inside SWNHs and successfully produced such ultrafine Gd2O3 nanoparticles inside inert SWNHs. In this transformation, the inner hollow nanospaces of SWNHs apparently suppressed the migration and coalescence of the particles. The efficacy of this nanometer-scale sizing of Gd2O3 was demonstrated through nuclear magnetic resonance (NMR) measurements and magnetic resonance (MR) imaging. We first describe the preparation and structural analysis of the Gd2O3-containing SWNHs. Gd-acetates were encapsulated * Corresponding authors. E-mail:
[email protected]; yudasaka@ frl.cl.nec.co.jp. Phone: +81-29-856-1940, Fax: +81-29-850-1366. † JST/SORST. ‡ NEC. § The University of Tokyo. ⊥ JST/ PRESTO. # Meijo University. | Current address: Kyoto University, Nishikyo, Kyoto 615-8510, Japan.
10.1021/jp0607622 CCC: $33.50
inside SWNHs with holes (NHox) according to a procedure similar to one used before.4 The structure of Gd-acetates incorporated inside NHox was confirmed by infrared absorption spectra and thermogravimetric analysis (TGA) accompanying mass spectrometric analysis (data not shown). The Gd-acetateencapsulated NHox are referred to as “Gd(OAc)3-NHox” in this report. For the thermal transformation of Gd-acetates to Gd2O3, Gd(OAc)3-NHox was heat-treated at 700 °C in an Ar flow (300 cm3 min-1) for 1 h. Hereafter, we abbreviate the heattreated Gd(OAc)3-NHox to “Gd2O3-NHox”. The structure of the Gd2O3 in Gd2O3-NHox was studied by X-ray diffraction (XRD) at room temperature. The location and the size of the Gd2O3 particles were examined by transmission electron microscopy (TEM) at 120 kV of high-acceleration voltage. The quantity of Gd2O3 was estimated from the amount of TGA residue at 1000 °C in O2. NMR spin-lattice relaxation time (T1) measurements and MR imaging were performed at 14.1 T (600 MHz) and 4.7 T (200 MHz), respectively, using MR imaging systems (Unity INOVA600 and Unity INOVA4.7T, Varian). The Gd2O3-NHox was homogeneously dispersed in phosphate buffered saline (PBS) to model living-body environments. Here, the Gd2O3-NHox was dispersed in 3% agarose gels to prevent it sinking to the bottom in the PBS.7 The control samples were PBS/agarose, NHox/PBS/agarose (10 mg cm-3 of PBS/agarose), and Gd2O3powder/PBS/agarose. T1 of the water proton was measured using a standard inversion recovery pulse sequence at 30 °C. The MR imaging experiments were done at room temperature using a 60 mm × 35 mm PBS/agarose plate, in which letter-molds “JST” of Gd2O3-NHox/PBS/agaraose were embedded. The T1© 2006 American Chemical Society
5180 J. Phys. Chem. B, Vol. 110, No. 11, 2006
Figure 1. Typical TEM images of Gd2O3-NHox. The lattice distance of black particles enclosed inside the NHox sheaths was 0.30-0.32 nm, which is consistent with the layer spacing of cubic Gd2O3(222) (d222 ) 0.312 nm).8
Letters
Figure 3. Relationship between Gd concentration and the T1 relaxation rate, T1-1, for Gd2O3-NHox (square), NHox (circle), and Gd2O3-powder (triangle) dispersed in PBS/agarose gel (cross). The average particle diameter of the Gd2O3 powder, determined from XRD, was about 44 nm.
Figure 4. A T1-weighted MR image of PBS/agarose gel plate. The molded “JST” regions contain Gd2O3-NHox of 0.5 mg cm-3 or 1 mg cm-3. Scale bar: 10 mm.
Figure 2. Diameter distributions of Gd2O3 particles encapsulated in sheaths of NHox for Gd2O3-NHox obtained from two hundreds TEM images. Heat-treatment at 700 °C did not change the average diameters of Gd-compound clusters encapsulated in SWNH sheaths.
weighted image was acquired using a spin-echo pulse sequence with repetition time/echo time ) 300 ms/16 ms. TEM observation of the Gd2O3-NHox revealed black particles enclosed inside the SWNH sheaths (Figure 1), which was similar to the case of previously reported results for Gd(OAc)3-NHox.4 It is known that Gd-acetate changes to Gd2O3 above 640 °C.5 The XRD profiles of Gd2O3-NHox exhibited a weak peak corresponding to cubic Gd2O3. Close-up observation of the particles inside the SWNH sheaths showed a lattice distance of 0.30-0.32 nm, which is consistent with the layer spacing of cubic Gd2O3(222) (d222 ) 0.312 nm).8 We confirmed that the black particles of Gd2O3-NHox were water-insoluble by TGA and TEM. These results indicate that the water-soluble Gd-acetate particles inside the SWNH sheaths were successfully changed to the water-insoluble Gd2O3 particles by the heattreatment. TGA revealed that the Gd quantity of Gd(OAc)3NHox (1.3 at. %) was maintained even after heat-treatment at 700 °C (1.6 at. %). The particle-size distributions were the same before and after the heat-treatment, indicating that the heattreatment produced ultrafine Gd2O3 nanoparticles of about 2.3 nm in average diameter (Figure 2). In the absence of NHox, however, heat-treatment in Ar at 700 °C changed Gd(OAc)3‚ 4H2O to Gd2O3 particles with larger diameters of about 19 nm. These results indicate that NHox prevented particles of the Gdacetate or its thermal derivatives from either subliming or migrating during the heat-treatment. We think that the deep potential fields of the SWNH nanospaces confined the Gdacetate and its thermal derivatives, thus preventing the sublimation and the migration to coalesce to larger particles. This unique ability of NHox to stabilize the adsorbed molecules has also been reported elsewhere.9 Therefore, the application of NHox is important for producing ultrafine Gd2O3 nanoparticles. We examined the effect of the small sizes of the Gd2O3 particles through the NMR measurements. Here, the paramagnetic Gd-moments are known to provide an effective relaxation path for nuclei of water protons, so the spin-lattice relaxation
time of the protons neighboring Gd becomes shorter. Figure 3 shows the relationship between the T1 relaxation rate (T1-1) of water protons in the agarose gel and the Gd concentration for sample materials with different Gd2O3 particle sizes. Adding Gd2O3-NHox to the agarose gel significantly enhanced T1-1 of water protons, and T1-1 increased with the Gd concentration. At a Gd concentration of 25 mM, T1-1 was more than 10 times higher than that for the water protons in the NHox/PBS/agarose system, suggesting that ultrafine Gd2O3 nanoparticles on NHox work as a positive MR contrast agent. Note that for the larger particles of Gd2O3-powder (average particle diameter: about 44 nm), T1-1 was much lower than for Gd2O3-NHox. We attribute the larger T1-1 value of the smaller Gd2O3 particles to the greater surface area with smaller particles, which allows a larger number of water molecules to interact with the Gd2O3 particles at their surface. Finally, we show a T1-weighted MR image of a PBS/agarose gel plate, where the letters “JST” containing Gd2O3-NHox were molded. Even at Gd2O3-NHox of 0.5 mg cm-3, corresponding to a Gd concentration of 0.6 mM, a sharp bright image was observed, making it easy to identify the NHox locations in the T1-weighted MR image (Figure 4). To apply SWNHs as a drug delivery carrier, the potential of which was demonstrated by Murakami et al. or Ajima et al.,3 knowledge where drug-carrying SWNHs accumulate in a living body is indispensable to maximize the efficacy of chemotherapeutic treatments. Thus, we believe that presented results suggest the feasibility of applying Gd2O3-NHox as a labeled drug delivery carrier. In summary, we successfully produced ultrafine Gd2O3 nanoparticles with diameters of about 2.3 nm using the holeopened SWNHs. We believe that the confinement effect of the deep potential well in nanospaces of SWNHs on the encapsulated Gd2O3 nanoparticles prevented an increase in their particle size. The effectiveness of small-diameter Gd2O3 nanoparticles was revealed through the proton NMR measurements, and the potential of Gd2O3-deposited NHox as a positive MR contrast agent was demonstrated. We also expect to find that inert NHox can be used as a “nanometer-scale reaction chamber” with confined nanospaces not only for thermal reactions but also for any other chemical reactions to produce novel nanostructured materials.
Letters Acknowledgment. We thank Dr. T. Azami and Dr. D. Kasuya for preparing the SWNHs, and Dr. T. Ichihashi and Dr. K. Hata for their help in the TEM observations. In addition, we thank Varian Technologies Japan Ltd. and BioView Co. for the NMR measurements and the MR imaging. Supporting Information Available: Details of sample preparation and water-solubility examinations, XRD profiles of Gd2O3-NHox, TEM and TGA results after water-solubility examinations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Iijima, S.; Yudasaka, M.; Yamada, R.; Bandow, S.; Suenaga, K.; Kokai, F.; Takanashi, K. Chem. Phys. Lett. 1999, 309, 165. (2) (a) Murata, K.; Kaneko, K.; Steele, W. A.; Kokai, F.; Takahashi, K.; Kasuya, D.; Hirahara, K.; Yudasaka, M.; and Iijima, S. J. Phys. Chem. B 2001, 105, 10210. (b) Bekyarova, E.; Murata, K.; Yudasaka, M.; Kasuya, D.; Iijima, S.; Tanaka, H.; Kanoh, H.; Kaneko, K.; J. Phys. Chem. B 2003, 107, 4681. (c) Ajima, K.; Yudasaka, M.; Suenaga, K.; Kasuya, D.; Azami,
J. Phys. Chem. B, Vol. 110, No. 11, 2006 5181 T.; Iijima, S. AdV. Mater. 2004, 16, 397. (d) Yuge, R.; Ichihashi, T.; Shimakawa, Y.; Kubo, Y.; Yudasaka, M.; Iijima, S. AdV. Mater. 2004, 16, 1420. (3) (a) Murakami, T.; Ajima, K.; Miyawaki, J.; Yudasaka, M.; Iijima, S.; Shiba, K. Mol. Pharm. 2004, 1, 399. (b) Ajima, K.; Yudasaka, M.; Murakami, T.; Maigne´, A.; Shiba, K.; Iijima, S. Mol. Pharm. 2005, 2, 475. (4) Hashimoto, A.; Yorimitsu, 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. (5) Hussein, G. A. M., J. Phys. Chem. 1994, 98, 9657. (6) (a) Bhattacharyya, S.; Agrawal, D. C. J. Mater. Sci. 1995, 30, 1495. (b) Gunduz, G.; Uslu, I. J. Nucl. Mater. 1996, 231, 113. (c) Takahashi, K.; Tazaki, S.; Miyahara, J.; Karasawa, Y.; Niimura, N. Nucl. Instrum. Methods Phys. Res. 1996, A377, 119. (d) Caravan, P.; Ellison, J. J.; McMurry, T. J.; Lauffer, R. B. Chem. ReV. 1999, 99, 2293. (e) Park, J.-C.; Moon, H.-K.; Kim, D.-K.; Byeon, S.-H.; Kim, B.-C.; Suh, K.-S. Appl. Phys. Lett. 2000, 77, 2162. (7) We confirmed that the gelling with agarose did not induce any difference in the relaxation times of protons for a commercial contrast agent, Magnevist (data not shown). (8) Powder Diffraction File No. 65-3181. (9) Yudasaka, M.; Fan, J.; Miyawaki, J.; Iijima, S. J. Phys. Chem. B 2005, 109, 8909.
