Synthesis of Uniform Ferrimagnetic Magnetite Nanocubes
Dokyoon Kim, Nohyun Lee, Mihyun Park, Byung Hyo Kim, Kwangjin An, and Taeghwan Hyeon* National CreatiVe Research InitiatiVe Center for Oxide Nanocrystalline Materials and School of Chemical and Biological Engineering, Seoul National UniVersity, Seoul 151-744, Korea Received November 5, 2008; E-mail: thyeon@snu.ac.kr
Nanoparticles have been intensively studied due to their potential
applications as well as their fundamental size-dependent properties.1 For many of these applications, the synthesis of uniform nanopar- ticles is of key importance because the electrical, optical, and magnetic properties of the nanoparticles are strongly dependent on their dimensions.1 Over the past decade, uniform nanoparticles with various compositions, sizes, and shapes have been synthesized using several different methods including hot-injection and heat-up processes.1,2 Among various nanoparticles, magnetic nanoparticles have attracted significant attention for many technological applica- tions including magnetic resonance imaging contrast agents, magnetic carriers for drug delivery systems, biosensors, and bioseparation.3 Although ferri- or ferromagnetic nanoparticles are desirable for many of these applications, superparamagnetic nano- particles of <30 nm have generally been used because nearly no preparation method is available for the synthesis of uniform ferrimagnetic nanoparticles larger than 30 nm. Interestingly, mag- netosomes in magnetotactic bacteria are composed of single-domain magnetite nanocrystals in the 30-100 nm range and often with cube or cuboctahedron shapes.4 These magnetosomes have attracted a lot of attention because of their biotechnological applications derived from their narrow size and shape distribution and inherent biocompatibility. In our continuous effort to improve the synthesis of magnetic nanoparticles,5 we herein report on the synthesis of uniform magnetite (Fe3O4) nanocubes ranging in size from 20 to 160 nm. In a typical synthesis, iron(III) acetylacetonate (0.71 g) was added to a mixture of oleic acid (1.13 g) and benzyl ether (10.4 g). The mixture solution was degassed at room temperature for 1 h and then heated to 290 °C at the rate 20 °C/min under vigorous magnetic stirring. The reaction mixture was maintained at this temperature for 30 min. After cooling to room temperature, a mixture of toluene and hexane was added to the solution. The solution was then centrifuged to precipitate the Fe3O4 nanocubes. The separated precipitate was washed using chloroform. The TEM image (Figure Figure 1. TEM images of (a) 79-nm-sized Fe3O4 nanocubes (inset: HRTEM 1a) revealed that the synthesized nanocubes have a uniform edge image); (b) mixture of truncated cubic and truncated octahedral nanoparticles with an average dimension of 110 nm; (c) 150-nm-sized truncated length of 79 nm (78.5 nm ( 6.9 nm). The high-resolution TEM nanocubes; (d) 160-nm-sized nanocubes; (e) 22-nm-sized nanocubes. (f) (HRTEM) image showed the highly crystalline nature of the Schematics showing the overall shape evolution of the Fe3O4 nanoparticles. nanocubes (inset of Figure 1a), and the measured d-spacing value (4.18 Å) was twice the value of the magnetite (400) plane. The X-ray diffraction pattern clearly indicated that the synthesized the HRTEM study, we concluded that the nanocubes are formed nanocubes are magnetite (Fe3O4) rather than similarly structured as a result of fast growth along 〈111〉 directions, and the surfaces maghemite (γ-Fe2O3) (Figure S1).5a of the final nanocubes correspond to {100} planes, which is similar The dimensions of the nanocubes could be controlled by varying to the magnetite nanocrystals found in magnetotactic bacteria.4 The the experimental conditions. For example, when the amount of overall shape evolution is illustrated in Figure 1f. Kinetically benzyl ether was reduced to 5.2 g and the reaction time was controlled growth under high monomer concentration seems to be increased to 1 h, ∼110-nm-sized particles composed of truncated responsible for this anisotropic growth.2a When 0.40 g of 4-biphe- cubes and truncated octahedra were obtained (Figure 1b). By nylcarboxylic acid and 1.27 g of oleic acid were used, while keeping increasing the reaction time to 1.5 and 2 h, the particles grew to a all the other reaction conditions the same for the synthesis of 79- larger and more perfect cubic shape with edge dimensions of 150 nm-sized nanocubes, smaller 22-nm-sized nanocubes were synthe- nm (Figure 1c) and 160 nm (Figure 1d), respectively. Based on sized (Figure 1e and Figure S2). 454 9 J. AM. CHEM. SOC. 2009, 131, 454–455 10.1021/ja8086906 CCC: $40.75 2009 American Chemical Society COMMUNICATIONS
Figure 2. Magnetic behavior of the Fe3O4 nanocubes measured at 300 K:
(a) M-H curves for 22-nm- (red), 80-nm- (blue), and 160-nm- (black) sized nanocubes; (b) size-dependent coercivity (error bar: size distribution). Figure 3. Fluorescence spectra showing the change of emission intensity before and after treating the solutions of histidine-tagged green fluorescent 5a protein (green) or nonhistidine-tagged Cy5-labeled normal mouse IgG (red) The current synthetic process is relatively easy to scale up. with Fe3O4 nanocubes. For example, when the reaction size for the synthesis of 79-nm- sized nanocubes was increased by a factor of 10, 1.6 g of the observed for red-emitting Cy5-labeled normal mouse IgG without nanocubes with an edge length of 49 nm were obtained (Figure histidine-tag, indicating the limited extent of the binding (Figure 3 S3). Recently, Yang and co-workers synthesized uniform magnetite right). nanocubes with sizes ranging from 6.5 to 30 nm from thermal In summary, we synthesized uniform ferrimagnetic magnetite reaction of Fe(acac)3 in a mixture of 1,2-hexadecandiol, oleic acid, nanocubes in the size range from 20 to 160 nm. The magnetic oleylamine, and benzyl ether.6a Chen and co-workers synthesized behavior of the nanocubes was characterized, and magnetic relatively uniform 50-nm-sized magnetite nanocubes from a sol- separation of the histidine-tagged protein was demonstrated. These vothermal reaction of ferrocene with H2O2 in a mixture containing synthesized nanocubes are expected to find many applications where polyvinylpyrrolidone, water, and alcohol.6b soft-magnet properties are useful. The synthesized Fe3O4 nanocubes are a soft ferrimagnetic Acknowledgment. The present work was supported by the material with high saturation magnetization and low coercivity. KOSEF through the National Creative Research Initiative Program. Figure 2 shows the magnetic behavior of the nanocubes measured at 300 K. All the samples showed hysteresis loops in the M-H Supporting Information Available: Experimental procedures. curves, indicating the ferrimagnetic nature of the Fe3O4 nanocubes TEM, XRD, and SQUID data. This material is available free of charge (Figure 2a). The coercivities of various nanocube sizes are plotted via the Internet at http://pubs.acs.org. in Figure 2b. In general, the coercivity of the superparamagnetic nanoparticles is zero above the blocking temperature. As the particle References size increases, the coercivity at a given temperature increases rapidly (1) (a) The Chemistry of Nanostructured Materials; Yang, P., Ed.; World upon the size exceeding the superparamagnetic limit, saturates when Scientific Publishing: Singapore, 2003. (b) Nanoscale Materials in Chem- istry; Klabunde, K. J., Ed.; Wiley-Interscience: New York, 2001. (c) the size approaches the single-domain limit, and gradually decreases Nanoparticles; Schmid, G., Ed.; Wiley-VCH: Weinheim, 2004. (d) Semi- to the bulk value upon passing the multidomain region (Figure conductor and Metal Nanocrystals; Klimov, V. I., Ed.; Marcel Dekker, Inc.: New York, 2004. S4).1b,7 By fitting the measured coercivity to the single-domain (2) (a) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Angew. Chem., Int. particle model1b (trend line in Figure 2b), the superparamagnetic Ed. 2007, 46, 4630–4660. (b) Donega, C. D.; Liljeroth, P.; Vanmaekelbergh, D. Small 2005, 1, 1152–1162. limit of ∼20 nm was estimated, which is in reasonable accord with (3) (a) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, the previous reports.8 Although the single-domain limit of 128 nm 287, 1989–1992. (b) Hyeon, T. Chem. Commun. 2003, 927–934. (c) Gu, H.; Xu, K.; Xu, C.; Xu, B. Chem. Commun. 2006, 941–949. (d) Bulte, was estimated for Fe3O4 nanospheres,1b our nanocubes did not show J. W. M.; Kraitchman, D. L. NMR Biomed. 2004, 17, 484–499. (e) Xie, J.; any sign of coercivity decrease up to 160 nm. The shape anisotropy Chen, K.; Lee, H.-Y.; Xu, C.; Hsu, A. R.; Peng, S.; Chen, X.; Sun, S. J. Am. Chem. Soc. 2008, 130, 7542–7543. (f) Dumestre, F.; Chaudret, B.; and the interaction between closely spaced nanocubes might be Amiens, C.; Renaud, P.; Fejes, P. Science 2004, 303, 821–823. (g) Zeng, responsible for this increased critical size of the single domain H.; Rice, P. M.; Wang, S. X.; Sun, S. J. Am. Chem. Soc. 2004, 126, 11458– particles,4a and more rigorous investigation should be conducted 11459. (h) Desvaux, C.; Amiens, C.; Fejes, P.; Renaud, P.; Respaud, M.; Lecante, P.; Snoeck, E.; Chaudret, B. Nat. Mater. 2005, 4, 750–753. to achieve a comprehensive understanding of the results. (4) (a) Dunin-Borkowski, R. E.; McCartney, M. R.; Frankel, R. B.; Bazylinski, To explore the potential application of the nanocubes, we applied D. A.; Pósfai, M.; Buseck, P. R. Science 1998, 282, 1868–1870. (b) Mann, S.; Frankel, R. B.; Blakemore, R. P. Nature 1984, 310, 405–407. them to the separation of histidine-tagged proteins. Fractionation (5) (a) Park, J.; An, K.; Hwang, Y.; Park, J.-G.; Noh, H.-J.; Kim, J.-Y.; Park, of biomolecules by exploiting their different affinities to metal ions J.-H.; Hwang, N.-M.; Hyeon, T. Nat. Mater. 2004, 3, 891–895. (b) Kim, D.; Park, J.; An, K.; Yang, N.-K.; Park, J.-G.; Hyeon, T. J. Am. Chem. was first introduced as the concept of immobilized metal ion affinity Soc. 2007, 129, 5812–5813. chromatography.9 Intermediate transition metal ions such as Ni(II), (6) (a) Yang, H.; Ogawa, T.; Hasegawa, D.; Takahashi, M. J. Appl. Phys. 2008, 103, 07D526-1–07D526-3. (b) Xiong, Y.; Ye, J.; Gu, X.; Chen, Q.-w. J. Cu(II), and Zn(II) based on Pearson’s hard and soft acid classifica- Phys. Chem. C 2007, 111, 6998–7003. tion are often used for these applications.9 The use of nickel (7) (a) Modern Magnetic Materials: Principles and Applications; O’Handley, R. C., Ed.; Wiley-Interscience: New York, 1999. (b) Introduction to the nanoparticles or nickel-containing magnetic nanostructures was Magnetic Materials; Cullity, B. D., Ed.; Addition-Wesley: London, 1972. recently demonstrated.10 In our experiments, we employed the as- (8) (a) Kovalenko, M. V.; Bodnarchuk, M. I.; Lechner, R. T.; Hesser, G.; Schäffler, F.; Heiss, W. J. Am. Chem. Soc. 2007, 129, 6352–6353. (b) synthesized nanocubes without any nickel incorporation. Following Klokkenburg, M.; Vonk, C.; Claesson, E. M.; Meeldijk, J. D.; Erné, B. H.; our previously reported procedures,10a 59% loading and 50% Philipse, A. P. J. Am. Chem. Soc. 2004, 126, 16706–16707. (9) (a) Porath, J.; Carlsson, J.; Olsson, I.; Belfrage, G. Nature 1975, 258, 598– retrieval (84% release of the load) of histidine-tagged green 599. (b) Porath, J. Trends Anal. Chem. 1988, 7, 254–259. fluorescent protein were observed (Figure 3 left). We believe that (10) (a) Lee, I. S.; Lee, N.; Park, J.; Kim, B. H.; Yi, Y.-W.; Kim, T.; Kim, the loading capacity can be increased further after optimization T. K.; Lee, I. H.; Paik, S. R.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 10658–10659. (b) Xu, C.; Xu, K.; Gu, H.; Zheng, R.; Liu, H.; Zhang, X.; because the highest protein adsorption on Fe(III) is reached at low Guo, Z.; Xu, B. J. Am. Chem. Soc. 2004, 126, 9938–9939. (c) Lee, K.-B.; pH (<6) and low ionic strength condition.9b In a control experiment, Park, S.; Mirkin, C. A. Angew. Chem., Int. Ed. 2004, 43, 3048–3050. only a 13% decrease in the fluorescent emission intensity was JA8086906