Synthesis of Au-Based Porous Magnetic Spheres by Selective Laser

By combining the selective laser heating in liquid and acid treatment processes, .... Full melting and fast solidification of all particles in core–...
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Synthesis of Au-Based Porous Magnetic Spheres by Selective Laser Heating in Liquid Zaneta Swiatkowska-Warkocka,* Kenji Kawaguchi, Yoshiki Shimizu, Alexander Pyatenko, Hongqiang Wang, and Naoto Koshizaki* Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, 305-8565 Ibaraki, Japan S Supporting Information *

ABSTRACT: We report the synthesis of Au-based submicrometer-sized spherical particles with uniform morphology/ size and integrated porosity-magnetic property in a single particles. The particles are synthesized by a two-step process: (a) selective pulsed laser heating of colloidal nanoparticles to form particles with Au-rich core and Fe-rich shell and (b) acid treatment which leads to formation of porous architecture on particle surface. The simple, fast, inexpensive technique that is proposed demonstrates very promising perspectives for synthesis of composite particles.

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INTRODUCTIONS Functional hybrid particles composed of two or more components are expected to display a combination of the properties associated with each material. In addition, one may also expect to observe new properties and capabilities due to coupling between two different materials. Therefore, heterostructured particles have wider applications than singlecomponent particles.1,2 Magnetic particles offer wide-ranging applications in the fields of magnetic resonance imaging, cancer therapy, and drug delivery and separation.3,4 On the other side, gold particles have attracted extensive attention due to the biocompability and good affinity for binding to amine (−NH2) or thiol (−SH) terminal groups.5 The magnetic properties of such gold-based magnetic particles could provide many additional applications, e.g., biomedical uses such as cell separation, enzyme immobilization, diagnostic, molecular biology, MRI contrast, drug targeting, and hyperthermia.6−10 As a form of bifunctional nanomaterials, nanoparticles combining gold (Au) and iron (Fe) or iron oxides (FexOy) inherit from the two components excellent surface chemistry, special optical properties, and superparamagnetic properties, all of which would greatly enhance the potential and broaden the application of such composite bifunctional nanomaterials. As a result, successful strategies for the synthesis of bifunctional gold−iron oxide nanoparticles are recognized as one of the major advances in nanobiotechnology.11 Conventionally, gold/ iron oxide (Au/FexOy) nanocomposite particles are typically prepared by decomposing iron pentacarbonyl Fe(CO)5 or iron acetylacetonate Fe(acac)3 on the surface of gold nanoparticles12−14 or by reducing hydrogen tetrachloroaurate hydrate (HAuCl4·3H2O) or gold acetate Au(OOCCH3)3 in the presence of magnetite nanoparticles.15,16 However, the chemical agents involved in the synthesis often cannot be © 2012 American Chemical Society

removed and remain as residual molecules on the particle surfaces. Moreover, most of these approaches are limited to synthesizing particles with diameters smaller than 30 nm and relatively small surface area. At the same time, micrometer and submicrometer spheres have attracted much attention in many applications such as adsorbents for high-pressure liquid chromatography, calibration standards, and spacers for liquid crystals, inks, catalysis.17 Porosity dramatically increases surface of particles and therefore enhances the opportunities for their applications as absorbents, catalysis supports, and drug delivery vehicules.18−20 A laser ablation in liquid technique was considered a safe, simple, and versatile way to produce nanocrystals and nanosized oxides. As an extension of this technology, our group proposed an innovative method, pulsed laser selective heating of colloidal nanoparticles in solution, in order to prepare submicrometer-sized spheres, such as metal or metal oxides.21,22 However, it has never been used to synthesize the noble-metal/magnetic composites. In this communication, we adopt this method for producing submicrometer gold/ironoxide (Au/FexOy) nanocomposite spherical particles. An unfocused laser was used to irradiate the mixture of colloidal solutions of gold and magnetite, resulting in submicrometer sphere formation. Acid treatment has led to formation of porous architecture on particle surface. By combining the selective laser heating in liquid and acid treatment processes, we demonstrate a facile, but powerful strategy for the generation of Au-based composite particles with well-defined spherical Received: September 30, 2011 Revised: February 21, 2012 Published: February 24, 2012 4903

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Figure 1. (a) TEM image of a mixture of gold and magnetite nanoparticles before laser irradiation. The inset shows the red-brown color of a solution of Au+Fe3O4 before laser irradiation. (b) SEM and (c) TEM images of particles obtained after laser irradiation. The inset shows the gray color of solution after laser irradiation.

Figure 2. (a) TEM image and (b,c) SEM images and particle size distribution histogram of Au/FexOy particles after acid treatment.

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RESULTS AND DISCUSSION The size and shape of the particles before and after laser irradiation were examined by field emission scanning electron microscope (FE-SEM). The average diameter of raw magnetite nanoparticles was 5 nm, and that of gold nanoparticles was 20 nm. Both particles were rather spherical. Initial particles were strongly agglomerated (Figure 1a). When irradiation started, large particles were observed in greater and greater amounts. After laser irradiation for 1 h, only large particles with diameter 450 nm were observed (Figure 1b and c). Figure 1b and c indicates that spherical particles with smooth surfaces were formed after laser irradiation. Their spherical shape clearly indicates melt formation during the process, suggesting that the temperature of the particles transiently increases above the melting points of iron oxide (magnetite: 1600 °C) and gold (1064 °C). The transmission electron microscope (TEM) image in Figure 1c indicates that particles prepared by pulsed laser irradiation have a core/shell structure. The average core diameter and shell thickness were 300 and 75 nm. These morphological changes are reflected in the optical properties of the particles, whose solutions change color from red-brown to gray (Figure 1a and b). The absorption spectrum of Au+Fe3O4 suspension before laser irradiation shows a peak of surface plasmon resonance (SPR) at 528 nm, which cannot be found in the absorbance spectra of suspension after laser irradiation (Figure S1 in the Supporting Information and related analysis). The spectrum resulting from energy dispersive X-ray spectroscopy (EDS) of a single submicrometer spherical particle indicates the formation of a multicomponent particle, since the particle is composed of iron (Fe), gold (Au), and oxygen (O). EDS data clearly demonstrate that the particles have Au-rich core and Fe-rich shell. To remove the Fe-rich shell, the Au/FexOy composite particles were treated with hydrochloric acid (HCl), which can dissolve Fe and Fe oxides but not Au. A TEM image revealed particles 230 nm in average size after the acid treatment (Figure 2a−c). While the particles had a smooth and nonporous morphology before acid treatment (Figure 1b and c), they had rough surfaces and highly porous structures after acid treatment (Figure 2a−c). The atomic ratios of Au:Fe determined by EDS spectra at the edge and center of single isolated particles were

morphology, uniform submicrometer-sized of about 230 nm, and integrated porosity-magnetic properties.

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EXPERIMENTAL PROCEDURES

The raw magnetite (Fe3O4) nanoparticles were prepared by conventional coprecipitation from FeCl2·4H2O and FeCl3·6H2O at high values of pH. Iron salts were dissolved in water with magnetic stirrer for 1 h. The pH value was increased by adding NaOH. The color of the solution turned to black immediately, inducting magnetite formation. Magnetite particles were removed from the solution by using a permanent magnet, and were washed several times with deionized water. Finally, the Fe3O4 nanoparticles were dispersed in ethanol (C2H5OH). Source gold nanoparticles were prepared by pulsed laser ablation in liquid (PLAL). A gold plate target placed in ethanol was ablated for 30 min by a Nd:YAG (yttrium aluminum garnet) laser with 1064 nm wavelength, 30 Hz repetition rate, and 80 J/pulse·cm2 fluence. Suspensions of Au nanoparticles in C2H5OH (7.5 mL; 0.5 mM) and magnetite nanoparticles in C2H5OH (7.5 mL; 0.5 mM) were mixed and transferred to a sealed cell with quartz window to introduce laser light. The mixture was irradiated by unfocused laser light for 1 h using the second harmonic (532 nm) of an Nd:YAG laser operated at 30 Hz with a fluence of 100 mJ/pulse·cm2. During irradiation, ultrasonic stirring was used to prevent sedimentation and gravitational settling of the suspension. Then, Au/FexOy composite particles were treated with hydrochloric acid (HCl), which can dissolve Fe and Fe oxides but not Au, and we obtained Au/AuFe porous particles. The mixture was further centrifuged and washed with distilled water and ethanol several times. The morphology of the obtained gold/iron oxide particles was observed by a field emission scanning electron microscope (SEM; Hitachi S4800) and a transmission electron microscope (TEM; JEOL JEM 2010). The average particle size was determined by measuring the diameters of 200 particles from SEM images. The formed Au/ FexOy phases and composition were determined by a powder X-ray diffractometer (XRD; Rigaku Ultima IV) with Cu Kα radiation. The optical properties of suspensions with dispersed Au, Fe3O4, and Au/ FexOy particles were characterized by a UV−vis spectrophotometer (Shimadzu UV-2100PC). A highly sensitive superconducting quantum interference device (SQUID; Quantum Design, MPMS) magnetometer was employed to measure the magnetic properties of the nanocomposite particles. 4904

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measured at 5 K (Figure S4 in the Supporting Information and related analysis) show the exchange bias and coercive field enhanced for the field cooling results before acid treatment and no exchange bias after acid treatment. It is well-known that exchange bias is an interfacial effect of the exchange coupling between antiferromagnetic (in this case FeO) and ferromagnetic (Fe) phases. Our results indicate that the significant FeO−Fe interface connection exists before acid treatment and it disappears after acid treatment. To determine the growth mechanism of Au/FexOy by pulse laser irradiation, we performed a series of experiments in which we varied the laser irradiation time from 5 min to 1 h (Figure S5 in the Supporting Information). After just 5 min, the irradiated particles became larger and started fusing together. The number of small particles decreased and the particles became larger with increasing irradiation time. After 60 min of irradiation, only submicrometer particles were observed. The XRD results (Figure S6 in the Supporting Information) indicate that AuFe alloy phase had already started to form after 5 min of irradiation. Increasing irradiation time increased the amount of AuFe phase in the particles. Scheme 1 briefly illustrated the formation of multicomponent structures of Au/AuFe hybrid particles with rough surfaces. The proposed formation mechanism consists of the following processes: (1) Fe3O4 (5 nm) nanoparticles are mixed with Au (20 nm) nanoparticles and dispersed in ethanol. (2) Because both particles are very small, it is possible to expect strong agglomeration. There are possibilities as follows: Fe3O4 nanoparticles form the agglomerates by themselves or/and agglomerate with Au. In this case much smaller Fe3O4 particles probably attach onto larger Au particles and agglomerate with quasi core−shell structure. (3) In one individual pulse, Au absorbs much more energy then Fe3O4 and transfers this energy to surrounding Fe3O4 nanoparticles. Full melting and fast solidification of all particles in core−shell agglomeration results in formation of the real core−shell particle. In this process of core−shell particle formation, Fe3O4 decomposes to FeO and outer layer decomposes to Fe. (4) By the repeated agglomeration, absorption and melting-solidification of particles, the core−shell particles become larger. (5) During the period of time between two consecutive pulses, some core− shell particles can approach very close to each other due to diffusion process. After melting, such particles can be merged to form larger particles. During each short period when the particle is in melting condition, diffusion of FeO and Fe from periphery to core region occurs. During this process, new portions of outer FeO layer can be reduced to Fe by ethanol pyrolysis products. Also, during this diffusion process AuFe alloy can be formed in core region.26,27 (6) The merging of cores of multicore−shell particles could occur not in one laser pulse, but during the several pulses. (7) Acid treatment

the same at 90:10 (Figure S2 in the Supporting Information), suggesting a homogeneous composition of the obtained particles. The structural changes in Au/FexOy following acid treatment were analyzed using X-ray diffraction (XRD) (Figure S3 in the Supporting Information). X-ray diffraction patterns of the particles taken before acid treatment were confirmed to be a composite of wustite (FeO), gold (Au), and gold−iron alloy (Au−Fe) phases. The AuFe alloy phase can be easily differentiated from the physical mixture of the individual metals by X-ray diffraction analysis. The (111) reflection of the fcc AuFe alloy phase (40.17°) is well separated from the proximate reflections of fcc Au (38.2°) and fcc Fe (44.5°). The weight ratio of Au/AuFe/FeO before acid treatment calculated from the ratio of the highest peaks is 45:36:19. The acid treatment product is predominantly Au, although few small peaks (2θ111 = 40.17° and 2θ200 = 46.72°) clearly indicated fcc AuFe alloy phase. The weight ratio of Au/AuFe after acid treatment is 80:20. We measured the magnetic properties of obtained particles to demonstrate their possibilities. The magnetic hysteresis loop measured in 300 K in Figure 3 indicates that Au/FexOy particles

Figure 3. Magnetic hysteresis loop pattern of Au/FexOy particles before (black line) and after acid treatment (green line) at 300 K.

are ferromagnetic and magnetically soft. Particle before and after acid treatment have coercivities of 55 and 64 Oe, with magnetization (Ms) of 37 and 9 emu/g, respectively. Obviously, the value of magnetization decreased while the value of coercivity increased with increasing Au content. The Ms bulk value for iron is 222 emu/g.23 The lower Ms value, compared to the bulk value of the Au/FexOy particles, arises from the nonmagnetic Au and FeO content. It should be noted that the magnetization of particles after acid treatment with a Au/Fe molar ratio of 5 is the same or higher than that of particles with Au/Fe molar ratios below 3.24,25 Magnetic properties not only show possibilities of obtained particles, but also manifest interactions between components. The hysteresis loops

Scheme 1. Schematic Illustration of the Formation of Au/AuFe Particles

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(5) Daniel, M.-C.; Astruc, D. Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. Chem. Rev. 2004, 104, 293−346. (6) Bao, J.; Chen, W.; Liu, T. T.; Zhu, Y. L.; Jin, P. Y.; Wang, L. Y.; Liu, J. F.; Wei, Y. G.; Li, Y. D. Bifunctional Au-Fe3O4 Nanoparticles for Protein Separation. ACS Nano 2007, 1, 293−298. (7) Gu, H. W.; Xu, K. M.; Xu, C. J.; Xu, B. Biofunctional Magnetic Nanoparticles for Protein Separation and Pathogen Detection. Chem. Commun. 2006, 941−946. (8) Gupta, A. K.; Gupta, M. Synthesis and surface engineering of iron oxide nanoparticles for biomedical applications. Biomaterials 2005, 26, 3995−4021. (9) Zhai, Y.; Zhai, J.; Wang, Y.; Guo, S.; Ren, W.; Dong, S. Fabrication of Iron Oxide Core/Gold Shell Submicrometer Spheres with Nanoscale Surface Roughness for Efficient Surface-Enhanced Raman Scattering. J. Phys. Chem. C 2009, 113, 7009−7014. (10) Park, H. Y.; Schadt, M. J.; Wang, L. Y.; Lim, I.-I. S.; Njoki, P. N.; Kim, S. H.; Jang, M. Y.; Luo, J.; Zhong, C. J. Fabrication of Magnetic Core@Shell Fe-Oxide@Au Nanoparticles for Interfacial Bioactivity and Bio-separation. Langmuir 2007, 23, 9050−9056. (11) Stoeva, S. I.; Huo, F.; Lee, J. S.; Mirkin, C. A. Three-layer Composite Magnetic Nanoparticle Probes for DNA. J. Am. Chem. Soc. 2005, 127, 15362−15363. (12) Yu, H.; Chen, M.; Rice, P. M.; Wang, S. X.; White, R. L.; Sun, S. H. Dumbbell-like Bifunctional Au−Fe3O4 Nanoparticles. Nano Lett. 2005, 5, 379−382. (13) Shi, W. L.; Zeng, H.; Sahoo, Y.; Ohulchanskyy, T. Y.; Ding, Y.; Wang, Z. L.; Swihart, M.; Prasad, P. N. A general approach to binary and ternary hybrid nanocrystals. Nano Lett. 2006, 6, 875−881. (14) Shevchenko, E. V.; Bodnarchuk, M. I.; Kovalenko, M. V.; Talapin, D. V.; Smith, R. K.; Aloni, S.; Heiss, W.; Alivisatos, A. P. Gold/Iron Oxide Core/Hollow-Shell Nanoparticles. Adv. Mater. 2008, 20, 4323−4329. (15) Xu, Z. C.; Hou, Y. L.; Sun, S. H. Magnetic core/shell Fe3O4/Au and Fe3O4/Au/Ag nanoparticles with tunable plasmonic properties. J. Am. Chem. Soc. 2007, 129, 8698−8699. (16) Wang, L.; Luo, J.; Maye, M. M.; Fan, Q.; Rendeng, Q.; Engelhard, M. H.; Wang, Ch.; Lin, Y.; Zhong, Ch.-J. Iron oxide−gold core−shell nanoparticles and thin film assembly. J. Mater. Chem. 2005, 15, 1821−1832. (17) Arshady, R. Microspheres, Microcapsules & Liposomes; Citus: London, 1999. (18) Colmenares, L.; Jusys, Z.; Behm, R. J. Electrochemical Surface Characterization and O2 Reduction Kinetics of Se Surface-Modified Ru Nanoparticle-Based RuSey/C Catalysts. Langmuir 2006, 22, 10437−45. (19) Shim, M.; Kam, N. W. S.; Chen, R. J.; Li, Y. M.; Dai, H. J. Functionalization of carbon nanotubes for biocompatibility and biomolecular recognition. Nano Lett. 2002, 2, 285−288. (20) Luo, B.; Xu, S.; Luo, A.; Wang, W. R.; Wang, S. L.; Guo, J.; Lin, Y.; Zhao, D. Y.; Wang, C. C. Mesoporous Biocompatible and aciddegradable magnetic colloidal nanocrystal clusters with sustainable stability and high hydrophobic drug loading capacity. ACS Nano 2011, 5, 1428−1435. (21) Ishikawa, Y.; Shimizu, Y.; Sasaki, T.; Koshizaki, N. Boron carbide spherical particles encapsulated in graphite prepared by pulsed laser irradiation of boron in liquid medium. Appl. Phys. Lett. 2007, 91, 161110. (22) Wang, H. Q.; Pyatenko, A.; Kawaguchi, K.; Li, X. Y.; Swiatkowska-Warkocka, Z.; Koshizaki, N. Selective pulsed heating for the synthesis of semiconductor and metal submicrometer spheres. Angew. Chem., Int. Ed. 2010, 49, 6361−6364. (23) Dale, L. H. Synthesis, properties, and applications of iron nanoparticles. Small 2005, 1, 482−501. (24) Chiang, I.-C.; Chen, D.-H. Synthesis of Monodisperse FeAu Nanoparticles with Tunable Magnetic and Optical Properties. Adv. Funct. Mater. 2007, 17, 1311−1316.

removes the Fe-rich shell containing FeO, Au-rich Au−Fe alloy phase, and Fe dissolved from the AuFe particle surface, which leads to the porous surface of Au/AuFe particles.

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CONCLUSIONS In conclusion, we demonstrated a novel methodology to prepare Au/AuFe submicrometer spheres by combining selective laser heating of nanoparticles dispersed in liquid and acid treatment processes. To the best of the authors’ knowledge, this is the first time synthesis of Au-based particles has been demonstrated with well-defined spherical morphology, uniform submicrometer size and integrated porositymagnetic properties. The porous structure significantly increases the surface area of the particles. As demonstrated above, after acid treatment the particles did indeed possess the magnetic properties of AuFe alloy. Additionally, our method gives us the possibility to easily change the magnetic properties of the particles by changing some experimental conditions, Au:Fe ratio and/or laser fluence.28 We believe that Au/AuFe particles may provide great promise for bioseparation and catalysis because of their magnetic properties and porous structure.

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ASSOCIATED CONTENT

S Supporting Information *

UV−vis spectra of Au nanoparticles, Fe3O4 nanoparticles, mixture of Au and Fe3O4 particles before laser irradiation and Au/FexOy particles after laser irradiation. EDS results for particles after acid treatment. XRD spectra for particles before and after acid treatment. Magnetization hysteresis loops for particles before and after acid treatment and detailed analysis on magnetic properties at 5 K. TEM and XRD analysis of particles obtained by pulsed laser irradiation with different irradiation time: 5, 10, 15, 30, and 60 min. The material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; koshizaki.naoto@aist. go.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by KAKENHI 2008734, and the magnetization measurements were conducted at the NanoProcessing Facility, supported by IBEC Innovation Platform, AIST.

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REFERENCES

(1) Cozzoli, P. D.; Pellegrino, T.; Manna, L. Synthesis, properties and perspectives of hybrid nanocrystal structures. Chem. Soc. Rev. 2006, 35, 1195−1208. (2) Zeng, H.; Sun, S. H. Syntheses, properties and potential applications of multicomponent magnetic nanoparticles. Adv. Funct. Mater. 2008, 18, 391−400. (3) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. Applications of magnetic nanoparticles in biomedicine. J. Phys. D: Appl. Phys. 2003, 36, R167−R181. (4) Lu, A.-H.; Salabas, E.; Schüth, F. Magnetic Nanoparticles: Synthesis, Protection, Functionalization, and Application. Angew. Chem., Int. Ed. 2007, 46, 1222−1244. 4906

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(25) Liu, H. L.; Wu, J. H.; Min, J. H.; Kim, Y. K. Synthesis of monosized magnetic-optical AuFe alloy nanoparticles. J. Appl. Phys. 2008, 103, 07D529. (26) Teng, X.; Yang, H. Synthesis of face-centered tetragonal phase FePt nanoparticles from Pt@Fe2O3 core-shell nanoparticles. J. Am. Chem. Soc. 2003, 125, 14559−14563. (27) Wang, C.; Peng, S.; Lacroix, L. M.; Sun, S. Synthesis of high magnetic moment CoFe nanoparticles via interfacial diffusion in core/ shell structured Co/Fe nanoparticles. Nano Res. 2009, 2, 380−385. (28) Swiatkowska-Warkocka, Z.; Kawaguchi, K.; Wang, H.; Katou, Y.; Koshizaki, N. Controlling exchange bias in Fe3O4/FeO composite particles prepared by pulsed laser irradiation. Nanoscale Res. Lett. 2011, 6, 226−232.

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