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High-magnetic-moment multifunctional nanoparticles for nanomedicine applications
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 311 (2007) 131–134 www.elsevier.com/locate/jmmm
High-magnetic-moment multifunctional nanoparticles for nanomedicine applications Yun Hao Xu, Jianmin Bai, Jian-Ping Wang The Center for Micromagnetics and Information Technologies, Department of Electrical and Computer Engineering, University of Minnesota, 200 Union St. SE, Minneapolis, MN 55455, USA Available online 8 December 2006
Abstract Multifunctional FeCo nanoparticles with narrow size distribution (less than 8% standard deviation) were fabricated by a novel physical vapor nanoparticle-deposition technique. The size of magnetic nanoparticles was controlled in the range from 3 to 100 nm. The shape of nanoparticles was controlled to be either spherical or cubic. The particles had a high specific magnetization of 226 emu/g at low saturation field, which is much higher than the currently commercialized iron oxide nanoparticles. Core–shell-type Co(Fe)–Au nanoparticles were produced by the same technique. They combined the high moment of the Co(Fe) core with the plasmonic feature of a Au shell. r 2006 Elsevier B.V. All rights reserved. Keywords: Nanoparticle; Magnetic particle; High moment; FeCo; Co–Au particle; Core–shell nanoparticle; Multifunction; Nanomedicine; Hyperthermia; MRI imaging; Plasmonic; Nano-shell
1. Introduction Magnetic nanoparticles have been extensively studied for biomedical applications such as hyperthermia, magnetic resonance contrast enhancement and drug delivery [1]. One of the most widely used particles are Fe oxide nanoparticles [1–3]. However, its relatively low magnetization starts to become a concern when the size of the particles becomes very small, so that detection [4] of the particles will be difficult. The efficiency of heating will be low and the enhancement of MR contrast will be small. High-moment magnetic nanoparticles [5,6] are a better choice for these applications. There were also some attempts to combine a biocompatible surface, such as Au, with Fe2O3 nanoparticles as a core–shell-type structure [7]. Au nano-shells were reported to show an enhanced plasmonic feature, which can be used for bio and medical labeling and thermal therapy [8]. Multifunctional nanoparticles combining highmoment magnetic nanoparticles with an ultra-thin Au surface have been demonstrated by coating Au onto the magnetic nanoparticles [9]. However, a thick Au shell is Corresponding author. Tel.: +612 625 9509; fax: +612 625 4583.
E-mail address: [email protected] (J.-P. Wang). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.11.174
desirable for nanomedicine applications such as labeling and targeting. Here we report the multifunctional FeCo nanoparticles with controllable size and shape and Co–Au magnetic nanoparticles with a thick Au shell.
2. Experiments The high-moment multifunctional nanoparticles were synthesized directly from the gas phase by using a physical vapor nanoparticle-deposition technique. The details of the experimental setup can be found in Refs. [5,6]. The principle of the method is first to vaporize the target material by magnetron sputtering and then to cool down the vapor through the collisions among the vaporized atoms and inertia gas molecules. Ar gas is used as the working gas for sputtering and as the cooling media. As soon as a supersaturation condition is reached, the vaporized target atoms form a nucleus. Some of the nuclei keep growing into nanoparticles. By controlling the conditions such as target composition, target setup, plasma density, gas flow, etc., magnetic nanoparticles can be formed with controlled size, size distribution, composition, phase and structure. The base pressure of the system was
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Y.H. Xu et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 131–134
5 10 8 Torr. The pressure was increased to several hundreds of mTorr in order to generate nanoparticles efficiently. FeCo alloy and Co(Fe)–Au composite sputtering targets of 2 in in diameter were used to produce the FeCo and Co(Fe)–Au nanoparticles, respectively. The transmission electron microscopy (TEM) samples were collected on carbon film on a copper grid. A TECNAI G2 F30 with working voltage of 300 kV and a TECNAI T12 working at 120 kV were used to acquire TEM images, scanning TEM (STEM) images and energy dispersion Xray (EDX) spectra. A SQUID magnetometer was used to measure the magnetic properties of these particles. The mass of the nanoparticles was measured in an inductively coupled plasma optical emission spectrometer (ICP-OES). The specific saturation magnetization of the nanoparticles was obtained by dividing the saturation magnetic moments of a sample by its mass. 3. Results and discussion 3.1. FeCo high-magnetic-moment nanoparticles Two TEM bright field (BF) images of high-moment FeCo nanoparticles, with different shapes of spheres and cubes are shown in Fig. 1A and B, respectively. Other shapes such as octahedron and truncated octahedron (triangle/hexagonal projections) can also be fabricated [10]. The shape of the magnetic nanoparticles can greatly affect the local magnetic field. This feature can be used to enhance the contrast in MRI and to tag different species. FeCo nanoparticles have a natural oxide shell of 1–2 nm thick at the surface, which can be coated with biocompatible layers with similar techniques used in ferrite particles [11,12]. Size of these FeCo nanoparticles can be controlled to range from 3 to 100 nm with uniform size. Fig. 2A shows the hysteresis loops of FeCo nanoparticles with average size of 3.5, 20 and 35 nm at room temperature. The specific
magnetization of FeCo nanoparticles was measured to be 226 emu/g at room temperature, much higher than the 78.8 emu/g of g-Fe2O3. The moment difference between FeCo and FeO nanoparticles with the same size at a low field (not saturated) will be even larger. Superparamagnetic behavior was found in all three sizes, but with different susceptibility, which can be easily detected in magnetic measurements. This difference in susceptibility can be used to identify different species attached to nanoparticles with different sizes. Meanwhile, the size distribution of these nanoparticles can be controlled to be as small as 5%, which is shown in Fig. 2B. For this particular sample, a total number of 433 particles was measured. The distribution is well fitted by a normal distribution function, which suggests that the growth of these nanoparticles is by adding atoms to the existing nucleuses or particles instead of agglomerating nucleuses and particles. The mean size is 24.4 nm and the standard deviation is 1.04 nm from fitting. Magnetic nanoparticles with high moments are very promising in enhancing the heating efficiency of hyperthermia applications. Since the specific loss power due to Ne´el relaxation is proportional to the square of the moment of the particle [13], FeCo magnetic nanoparticles will generate about nine times the loss power of that of a magnetite nanoparticle with the same mass if the magnetic moment of particle is saturated in the magnetic field. In this case, the FeCo nanoparticle will have smaller size than that of magnetite due to its higher density. However, it requires a rather large field to saturate the nanoparticles. In an approachable field of 100 Oe, an FeCo nanoparticle has a magnetic moment of 1.17 10 17 emu. Compared to 1.43 10 18 emu of a 5 nm g-Fe2O3 nanoparticle in the same field, the increase of effective moment is more than eight times and the enhancement of the loss power is more than 64 times. Loss power due to Ne´el relaxation has a narrow window for the size of nanoparticles to have optimal efficiency. On the other hand, loss power due to
Fig. 1. TEM BF images of FeCo nanoparticles: (A) spherical shape; (B) cubic shape.
ARTICLE IN PRESS Y.H. Xu et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 131–134
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Fig. 2. (A) Hysteresis loops of FeCo nanoparticles with mean size of 3.5, 20 and 35 nm; (B) size distribution of FeCo nanoparticles with mean size of 24.4 nm and a 5% distribution. Line is fitting by using a normal distribution.
Fig. 3. STEM images of Co–Au nanoparticles: (A) lower magnification; (B) higher magnification.
hysteresis is at a similar level than the Ne´el losses, while it only decreases slowly with particle diameter until the superparamagnetic critical size is reached. The tolerance of the size distribution for high heating efficiency is greatly increased. This feature is highly desirable for scale up manufacturing. The upper limit of hysteresis losses is 4pMsHc if the easy axis of particles is aligned parallel with external field, where Ms is saturation magnetization and Hc is the coercivity at a certain testing time. The coercivity of magnetic nanoparticles increases with the increase in frequency. Since the magnitude of the AC field for magnetic hyperthermia application is limited at a fixed working frequency from a safety viewpoint, the highest useful coercivity of the nanoparticles is thus limited. High Ms materials thus increase the hysteresis losses. It needs to be pointed out that Hc for small particles expressed in Refs. [14,15] will increase by a factor of about two for highfrequency applications [16]. Magnetic hyperthermia is such a high-frequency application. The high magnetization of
FeCo nanoparticles gives them significant advantages over iron oxide particles or gadolinium chelates as an MRI image contrast agent in applications where a strong T2effect is desired. The transverse relaxation rate, DR2, is proportional to M2s [17], where Ms is the saturation magnetization of the nanoparticles. In addition, the local magnetic field distribution is strongly affected by the shape of the particles. For example, cubic particles will give much sharper field gradients than spherical particles. The shape anisotropy thus also serves as a mechanism of enhancing the MRI contrast since the local field gradients tend to accelerate the loss of phase coherence of the spins. 3.2. Co–Au multifunctional nanoparticles Fig. 3A is a STEM BF image of a collection of Co–Au nanoparticles with an average diameter of about 14 nm. The contrast inside a particle reflects the amount of scattered electrons by specific location. Because Au has
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easily replaced by FeCo and other materials for future applications. 4. Conclusions
Fig. 4. Hysteresis loops of Co–Au core–shell-type nanoparticles measured at 5 and 300 K.
higher z number than Co, Au scatters more electrons than Co does. It makes the parts containing more Au appear darker in the image than the parts containing more Co. This feature is more clearly seen in the higher resolution STEM BF image (Fig. 3B). It should be noticed here that the light part around the particle in the image is not part of the particle but the amorphous carbon support film affected by the deposition of particles. The distribution of Co and Au is not uniform throughout the particle. By using EDX spectroscopy with electron beam size smaller than 1 nm, the distribution of the Co and Au in a single particle was resolved [18]. Au was found more profound at the edge/surface of the particle, while the intensity of Co had a peak at the center of the particle. These results suggest a core–shell-type structure for the Co–Au nanoparticles. XPS data confirmed the existence of Au at the surface of these nanoparticles. Fig. 4 shows the hysteresis loops of Co–Au nanoparticles at 300 and 5 K, respectively. These nanoparticles are superparamagnetic at 300 K and ferromagnetic at 5 K. A coercivity of about 350 Oe is found at 5 K. Diffusion of atoms happens at an elevated temperature during the particle growth and also thereafter. Au atoms segregate at the surface of the nanoparticles to lower the total energy of the nanoparticles. Au has a substantially lower surface energy of its (1 1 1), (1 0 0) and (1 1 0) planes, 1.61, 1.71 and 1.79 J/m2, respectively, than the 2.74 J/m2 of the (0 0 1) plane of Co, which is the plane of lowest surface energy of Co [18]. Besides, Au atoms are larger than Co atoms. By segregating Au atoms to the surface, both the surface energy and strain energy [19] will be lowered due to the surface energy difference between Co and Au crystal planes and the difference of the sizes of their atoms. This new technique will allow us to prepare thick Au shell with high-moment magnetic core for the first time. Co can be
By using a gas-phase-condensation technique, we have successfully fabricated high-moment FeCo magnetic nanoparticles with narrow size distribution and different size and controllable shape. The specific magnetization of FeCo nanoparticles was determined to be 226 emu/g. With the controllable size, shape and high magnetic moment, these FeCo nanoparticles can be used to enhance MR-imaging contrast, target and label different medical species, and improve heating efficiency in hyperthermia. Co–Au core–shell-like structure nanoparticles were produced by the same technique. Combining the Au nano-shell and magnetic properties of magnetic core, these nanoparticles can be used to attach to organic molecules with thiolterminations by taking advantage of the well-established gold-thiol chemistry. The plasmonic feature of the thick Au shell can be used for medical labeling together with the high-magnetic-moment function. Acknowledgments Authors acknowledge support from NSF NFC at University of Minnesota, Seagate Technology, and INSIC Tape program. The authors would also like to thank Dr. Stuart McKernan at the Characterization facility of University of Minnesota. References [1] U. Ha¨feli, W. Schu¨tt, J. Teller, et al. (Eds.), Scientific and Clinical Applications of Magnetic Carriers, Plenum Press, New York, 1997. [2] A. Halbreich, J. Roger, J.N. Pons, et al., Biochimie 80 (1998) 379. [3] A. Jordan, R. Scholz, W. Peter, et al., J. Magn. Magn. Mater. 201 (1999) 413. [4] P. Alivisatos, Nat. Biotech. 22 (2004) 47. [5] J.P. Wang, in: PMRC Proceeding, Sendai, Japan, 2004. [6] Y.H. Xu, J.M. Qiu, J.M. Bai, et al., J. Appl. Phys. 97 (2005) 10J305. [7] J.L. Lyon, D.A. Fleming, M.B. Stone, et al., Nano-Lett. 4 (2004) 719. [8] J.B. Jackson, N.J. Halas, Proc. Natl. Acad. Sci. 101 (2004) 17930. [9] J. Bai, J.P. Wang, Appl. Phys. Lett. 87 (2005) 152502. [10] J.M. Qiu, J.P. Wang, Appl. Phys. Lett. 88 (2006) 192505. [11] D.K. Kim, M. Mikhaylova, Y. Zhang, et al., Chem. Mater. 15 (2003) 1617. [12] M. Chen, S. Yamamuro, D. Farrell, et al., J. Appl. Phys. 93 (2003) 7551. [13] R. Hergt, W. Andra, C.G. d’Ambly, et al., IEEE Trans. Magn. 34 (1998) 3745. [14] E. Kneller, Encyclopedia of Physics, in: H.P.J. Wijn (Ed.), Ferromagnetism, XVIII/2, Springer, New York, 1966, pp. 438–544. [15] R. Hergt, W. Andra, C.G. d’Ambly, et al., IEEE Trans. Magn. 34 (1998) 3745. [16] M.P. Sharrock, IEEE Trans. Magn. 26 (1990) 193. [17] A. Bjornerud, L. Johansson, NMR Biomed. 17 (2004) 464. [18] H.L. Skriver, N.M. Rosengaard, Phys. Rev. B 46 (1992) 7157. [19] G. Wang, M.A. Van Hove, P.N. Ross, et al., Prog. Surf. Sci. 79 (2005) 28.
Journal of Magnetism and Magnetic Materials 311 (2007) 131–134 www.elsevier.com/locate/jmmm
High-magnetic-moment multifunctional nanoparticles for nanomedicine applications Yun Hao Xu, Jianmin Bai, Jian-Ping Wang The Center for Micromagnetics and Information Technologies, Department of Electrical and Computer Engineering, University of Minnesota, 200 Union St. SE, Minneapolis, MN 55455, USA Available online 8 December 2006
Abstract Multifunctional FeCo nanoparticles with narrow size distribution (less than 8% standard deviation) were fabricated by a novel physical vapor nanoparticle-deposition technique. The size of magnetic nanoparticles was controlled in the range from 3 to 100 nm. The shape of nanoparticles was controlled to be either spherical or cubic. The particles had a high specific magnetization of 226 emu/g at low saturation field, which is much higher than the currently commercialized iron oxide nanoparticles. Core–shell-type Co(Fe)–Au nanoparticles were produced by the same technique. They combined the high moment of the Co(Fe) core with the plasmonic feature of a Au shell. r 2006 Elsevier B.V. All rights reserved. Keywords: Nanoparticle; Magnetic particle; High moment; FeCo; Co–Au particle; Core–shell nanoparticle; Multifunction; Nanomedicine; Hyperthermia; MRI imaging; Plasmonic; Nano-shell
1. Introduction Magnetic nanoparticles have been extensively studied for biomedical applications such as hyperthermia, magnetic resonance contrast enhancement and drug delivery [1]. One of the most widely used particles are Fe oxide nanoparticles [1–3]. However, its relatively low magnetization starts to become a concern when the size of the particles becomes very small, so that detection [4] of the particles will be difficult. The efficiency of heating will be low and the enhancement of MR contrast will be small. High-moment magnetic nanoparticles [5,6] are a better choice for these applications. There were also some attempts to combine a biocompatible surface, such as Au, with Fe2O3 nanoparticles as a core–shell-type structure [7]. Au nano-shells were reported to show an enhanced plasmonic feature, which can be used for bio and medical labeling and thermal therapy [8]. Multifunctional nanoparticles combining highmoment magnetic nanoparticles with an ultra-thin Au surface have been demonstrated by coating Au onto the magnetic nanoparticles [9]. However, a thick Au shell is Corresponding author. Tel.: +612 625 9509; fax: +612 625 4583.
E-mail address: [email protected] (J.-P. Wang). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.11.174
desirable for nanomedicine applications such as labeling and targeting. Here we report the multifunctional FeCo nanoparticles with controllable size and shape and Co–Au magnetic nanoparticles with a thick Au shell.
2. Experiments The high-moment multifunctional nanoparticles were synthesized directly from the gas phase by using a physical vapor nanoparticle-deposition technique. The details of the experimental setup can be found in Refs. [5,6]. The principle of the method is first to vaporize the target material by magnetron sputtering and then to cool down the vapor through the collisions among the vaporized atoms and inertia gas molecules. Ar gas is used as the working gas for sputtering and as the cooling media. As soon as a supersaturation condition is reached, the vaporized target atoms form a nucleus. Some of the nuclei keep growing into nanoparticles. By controlling the conditions such as target composition, target setup, plasma density, gas flow, etc., magnetic nanoparticles can be formed with controlled size, size distribution, composition, phase and structure. The base pressure of the system was
ARTICLE IN PRESS 132
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5 10 8 Torr. The pressure was increased to several hundreds of mTorr in order to generate nanoparticles efficiently. FeCo alloy and Co(Fe)–Au composite sputtering targets of 2 in in diameter were used to produce the FeCo and Co(Fe)–Au nanoparticles, respectively. The transmission electron microscopy (TEM) samples were collected on carbon film on a copper grid. A TECNAI G2 F30 with working voltage of 300 kV and a TECNAI T12 working at 120 kV were used to acquire TEM images, scanning TEM (STEM) images and energy dispersion Xray (EDX) spectra. A SQUID magnetometer was used to measure the magnetic properties of these particles. The mass of the nanoparticles was measured in an inductively coupled plasma optical emission spectrometer (ICP-OES). The specific saturation magnetization of the nanoparticles was obtained by dividing the saturation magnetic moments of a sample by its mass. 3. Results and discussion 3.1. FeCo high-magnetic-moment nanoparticles Two TEM bright field (BF) images of high-moment FeCo nanoparticles, with different shapes of spheres and cubes are shown in Fig. 1A and B, respectively. Other shapes such as octahedron and truncated octahedron (triangle/hexagonal projections) can also be fabricated [10]. The shape of the magnetic nanoparticles can greatly affect the local magnetic field. This feature can be used to enhance the contrast in MRI and to tag different species. FeCo nanoparticles have a natural oxide shell of 1–2 nm thick at the surface, which can be coated with biocompatible layers with similar techniques used in ferrite particles [11,12]. Size of these FeCo nanoparticles can be controlled to range from 3 to 100 nm with uniform size. Fig. 2A shows the hysteresis loops of FeCo nanoparticles with average size of 3.5, 20 and 35 nm at room temperature. The specific
magnetization of FeCo nanoparticles was measured to be 226 emu/g at room temperature, much higher than the 78.8 emu/g of g-Fe2O3. The moment difference between FeCo and FeO nanoparticles with the same size at a low field (not saturated) will be even larger. Superparamagnetic behavior was found in all three sizes, but with different susceptibility, which can be easily detected in magnetic measurements. This difference in susceptibility can be used to identify different species attached to nanoparticles with different sizes. Meanwhile, the size distribution of these nanoparticles can be controlled to be as small as 5%, which is shown in Fig. 2B. For this particular sample, a total number of 433 particles was measured. The distribution is well fitted by a normal distribution function, which suggests that the growth of these nanoparticles is by adding atoms to the existing nucleuses or particles instead of agglomerating nucleuses and particles. The mean size is 24.4 nm and the standard deviation is 1.04 nm from fitting. Magnetic nanoparticles with high moments are very promising in enhancing the heating efficiency of hyperthermia applications. Since the specific loss power due to Ne´el relaxation is proportional to the square of the moment of the particle [13], FeCo magnetic nanoparticles will generate about nine times the loss power of that of a magnetite nanoparticle with the same mass if the magnetic moment of particle is saturated in the magnetic field. In this case, the FeCo nanoparticle will have smaller size than that of magnetite due to its higher density. However, it requires a rather large field to saturate the nanoparticles. In an approachable field of 100 Oe, an FeCo nanoparticle has a magnetic moment of 1.17 10 17 emu. Compared to 1.43 10 18 emu of a 5 nm g-Fe2O3 nanoparticle in the same field, the increase of effective moment is more than eight times and the enhancement of the loss power is more than 64 times. Loss power due to Ne´el relaxation has a narrow window for the size of nanoparticles to have optimal efficiency. On the other hand, loss power due to
Fig. 1. TEM BF images of FeCo nanoparticles: (A) spherical shape; (B) cubic shape.
ARTICLE IN PRESS Y.H. Xu et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 131–134
133
Fig. 2. (A) Hysteresis loops of FeCo nanoparticles with mean size of 3.5, 20 and 35 nm; (B) size distribution of FeCo nanoparticles with mean size of 24.4 nm and a 5% distribution. Line is fitting by using a normal distribution.
Fig. 3. STEM images of Co–Au nanoparticles: (A) lower magnification; (B) higher magnification.
hysteresis is at a similar level than the Ne´el losses, while it only decreases slowly with particle diameter until the superparamagnetic critical size is reached. The tolerance of the size distribution for high heating efficiency is greatly increased. This feature is highly desirable for scale up manufacturing. The upper limit of hysteresis losses is 4pMsHc if the easy axis of particles is aligned parallel with external field, where Ms is saturation magnetization and Hc is the coercivity at a certain testing time. The coercivity of magnetic nanoparticles increases with the increase in frequency. Since the magnitude of the AC field for magnetic hyperthermia application is limited at a fixed working frequency from a safety viewpoint, the highest useful coercivity of the nanoparticles is thus limited. High Ms materials thus increase the hysteresis losses. It needs to be pointed out that Hc for small particles expressed in Refs. [14,15] will increase by a factor of about two for highfrequency applications [16]. Magnetic hyperthermia is such a high-frequency application. The high magnetization of
FeCo nanoparticles gives them significant advantages over iron oxide particles or gadolinium chelates as an MRI image contrast agent in applications where a strong T2effect is desired. The transverse relaxation rate, DR2, is proportional to M2s [17], where Ms is the saturation magnetization of the nanoparticles. In addition, the local magnetic field distribution is strongly affected by the shape of the particles. For example, cubic particles will give much sharper field gradients than spherical particles. The shape anisotropy thus also serves as a mechanism of enhancing the MRI contrast since the local field gradients tend to accelerate the loss of phase coherence of the spins. 3.2. Co–Au multifunctional nanoparticles Fig. 3A is a STEM BF image of a collection of Co–Au nanoparticles with an average diameter of about 14 nm. The contrast inside a particle reflects the amount of scattered electrons by specific location. Because Au has
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Y.H. Xu et al. / Journal of Magnetism and Magnetic Materials 311 (2007) 131–134
easily replaced by FeCo and other materials for future applications. 4. Conclusions
Fig. 4. Hysteresis loops of Co–Au core–shell-type nanoparticles measured at 5 and 300 K.
higher z number than Co, Au scatters more electrons than Co does. It makes the parts containing more Au appear darker in the image than the parts containing more Co. This feature is more clearly seen in the higher resolution STEM BF image (Fig. 3B). It should be noticed here that the light part around the particle in the image is not part of the particle but the amorphous carbon support film affected by the deposition of particles. The distribution of Co and Au is not uniform throughout the particle. By using EDX spectroscopy with electron beam size smaller than 1 nm, the distribution of the Co and Au in a single particle was resolved [18]. Au was found more profound at the edge/surface of the particle, while the intensity of Co had a peak at the center of the particle. These results suggest a core–shell-type structure for the Co–Au nanoparticles. XPS data confirmed the existence of Au at the surface of these nanoparticles. Fig. 4 shows the hysteresis loops of Co–Au nanoparticles at 300 and 5 K, respectively. These nanoparticles are superparamagnetic at 300 K and ferromagnetic at 5 K. A coercivity of about 350 Oe is found at 5 K. Diffusion of atoms happens at an elevated temperature during the particle growth and also thereafter. Au atoms segregate at the surface of the nanoparticles to lower the total energy of the nanoparticles. Au has a substantially lower surface energy of its (1 1 1), (1 0 0) and (1 1 0) planes, 1.61, 1.71 and 1.79 J/m2, respectively, than the 2.74 J/m2 of the (0 0 1) plane of Co, which is the plane of lowest surface energy of Co [18]. Besides, Au atoms are larger than Co atoms. By segregating Au atoms to the surface, both the surface energy and strain energy [19] will be lowered due to the surface energy difference between Co and Au crystal planes and the difference of the sizes of their atoms. This new technique will allow us to prepare thick Au shell with high-moment magnetic core for the first time. Co can be
By using a gas-phase-condensation technique, we have successfully fabricated high-moment FeCo magnetic nanoparticles with narrow size distribution and different size and controllable shape. The specific magnetization of FeCo nanoparticles was determined to be 226 emu/g. With the controllable size, shape and high magnetic moment, these FeCo nanoparticles can be used to enhance MR-imaging contrast, target and label different medical species, and improve heating efficiency in hyperthermia. Co–Au core–shell-like structure nanoparticles were produced by the same technique. Combining the Au nano-shell and magnetic properties of magnetic core, these nanoparticles can be used to attach to organic molecules with thiolterminations by taking advantage of the well-established gold-thiol chemistry. The plasmonic feature of the thick Au shell can be used for medical labeling together with the high-magnetic-moment function. Acknowledgments Authors acknowledge support from NSF NFC at University of Minnesota, Seagate Technology, and INSIC Tape program. The authors would also like to thank Dr. Stuart McKernan at the Characterization facility of University of Minnesota. References [1] U. Ha¨feli, W. Schu¨tt, J. Teller, et al. (Eds.), Scientific and Clinical Applications of Magnetic Carriers, Plenum Press, New York, 1997. [2] A. Halbreich, J. Roger, J.N. Pons, et al., Biochimie 80 (1998) 379. [3] A. Jordan, R. Scholz, W. Peter, et al., J. Magn. Magn. Mater. 201 (1999) 413. [4] P. Alivisatos, Nat. Biotech. 22 (2004) 47. [5] J.P. Wang, in: PMRC Proceeding, Sendai, Japan, 2004. [6] Y.H. Xu, J.M. Qiu, J.M. Bai, et al., J. Appl. Phys. 97 (2005) 10J305. [7] J.L. Lyon, D.A. Fleming, M.B. Stone, et al., Nano-Lett. 4 (2004) 719. [8] J.B. Jackson, N.J. Halas, Proc. Natl. Acad. Sci. 101 (2004) 17930. [9] J. Bai, J.P. Wang, Appl. Phys. Lett. 87 (2005) 152502. [10] J.M. Qiu, J.P. Wang, Appl. Phys. Lett. 88 (2006) 192505. [11] D.K. Kim, M. Mikhaylova, Y. Zhang, et al., Chem. Mater. 15 (2003) 1617. [12] M. Chen, S. Yamamuro, D. Farrell, et al., J. Appl. Phys. 93 (2003) 7551. [13] R. Hergt, W. Andra, C.G. d’Ambly, et al., IEEE Trans. Magn. 34 (1998) 3745. [14] E. Kneller, Encyclopedia of Physics, in: H.P.J. Wijn (Ed.), Ferromagnetism, XVIII/2, Springer, New York, 1966, pp. 438–544. [15] R. Hergt, W. Andra, C.G. d’Ambly, et al., IEEE Trans. Magn. 34 (1998) 3745. [16] M.P. Sharrock, IEEE Trans. Magn. 26 (1990) 193. [17] A. Bjornerud, L. Johansson, NMR Biomed. 17 (2004) 464. [18] H.L. Skriver, N.M. Rosengaard, Phys. Rev. B 46 (1992) 7157. [19] G. Wang, M.A. Van Hove, P.N. Ross, et al., Prog. Surf. Sci. 79 (2005) 28.