Ultrathin MgO Coating of Superparamagnetic Magnetite Nanoparticles

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Ultrathin MgO Coating of Superparamagnetic Magnetite Nanoparticles by Combined Coprecipitation and Sol−Gel Synthesis Laura De Matteis,⊗,† Laura Custardoy,⊗,† Rodrigo Fernández-Pacheco,‡ César Magén,‡,§,⊥ Jesús M. de la Fuente,†,⊥ Clara Marquina,*,§,∥ and M. Ricardo Ibarra†,‡,§ †

Instituto de Nanociencia de Aragón (INA), Universidad de Zaragoza, Mariano Esquillor s/n, 50018 Zaragoza, Spain Laboratorio de Microscopías Avanzadas (LMA)-Instituto de Nanociencia de Aragón (INA), Universidad de Zaragoza, 50018-Zaragoza, Spain § Departamento de Física de la Materia Condensada, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain ⊥ Fundación ARAID, 50004-Zaragoza, Spain ∥ Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC-Universidad de Zaragoza, Pedro Cerbuna 12, 50009-Zaragoza, Spain ‡

S Supporting Information *

ABSTRACT: Superparamagnetic magnetite nanoparticles coated with an ultrathin (∼1 nm) magnesium oxide (MgO) layer have been synthesized by combining coprecipitation and sol−gel methods. A thorough chemical and structural characterization has been carried out by means of high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (EDS), dynamic light scattering (DLS), and thermogravimetric analysis (TGA). Aberration corrected HRTEM experiments with subangstrom spatial resolution have allowed us to distinguish the ultrathin MgO shell that grows epitaxially on the magnetic cores. The capability of the MgO shell to protect the magnetic nuclei from oxidation up to 600 °C has been demonstrated. The magnetic properties of the material have been studied before and after the coating procedure. The superparamagnetism of the magnetite nuclei at room temperature is preserved even after calcination. The possibility of obtaining particles of controlled size coated with an isolating layer of nanometric thickness and high thermal stability makes the combination of the two synthesis methods used in this work a starting procedure to obtain nanometric powders suitable for technological applications, such as high-frequency electronics. These particles, after the appropriated functionalization, are also potential candidates to be used in biomedical applications. KEYWORDS: magnetite nanoparticles, magnesium oxide coating, sol−gel, coprecipitation, superparamagnetic particles

1. INTRODUCTION Magnetic nanoparticles are very suitable for a broad range of applications, like the synthesis of ferrofluids, data storage, catalysis, as solid supports for biomolecule immobilization, and in bioapplications in general. In many cases they have been already integrated in commercial applications.1 In particular, magnetic iron oxide nanoparticles are among the most commonly used. They are biocompatible and by tailoring their size their magnetic behavior can be varied from ferrimagnetic to superparamagnetic, which makes them suitable for a wide range of biomedical and biotechnological applications, like magnetic separation of biomolecules, magnetic fluid hyperthermia, magnetic resonance imaging, and target delivery of bioactive molecules .2−5 To obtain the desired magnetic properties, the knowledge and the control of chemico-physical characteristics of the nanoparticles are necessary, along with the stability of the material, its biocompatibility, and the possibilities of its surface to be functionalized with the biomolecules of interest.1 Magnetite © 2012 American Chemical Society

nanoparticles can be produced by different methods, using both physical and chemical techniques. Among them, the coprecipitation of ferric and ferrous cations in aqueous solution is an easy, reproducible, and cheap route of synthesis, and it offers the possibility of tuning the reaction conditions to control the chemical and magnetic properties of the nanoparticles.2,6 However, further oxidation of the magnetite can result in a degradation of the magnetic properties, so the magnetic core has to be coated with an organic or inorganic material to form a protecting shell. This is also advantageous to avoid the aggregation of the particles because of the large surface-tovolume ratio and therefore to their high surface energies, or because of dipolar magnetic interactions. Many different coatings have been already developed, consisting of polymers, silica, or metal oxides.1,7 Ideally, the coating material should Received: August 5, 2011 Revised: December 26, 2011 Published: January 4, 2012 451

dx.doi.org/10.1021/cm202306z | Chem. Mater. 2012, 24, 451−456

Chemistry of Materials

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2. EXPERIMENTAL SECTION

have a good affinity for the iron oxide core, and it should not modify its magnetic properties.8 Although organic coatings are often used to prevent particle aggregation by passivating the iron oxide surface, and therefore enhancing the particle stability, the use of inorganic materials offers a better protection of the magnetic core from its oxidation.1,7 For instance, the physical properties of magnesium oxide (MgO), that is, low specific weight, high temperature stability because of its high melting point,9 and negligible lattice mismatch with the magnetite,10 make it a very promising coating material to achieve a good protection of the magnetic core.11 Inorganic coatings as silica and magnesium oxide are also used as dielectric layer to isolate magnetic nanoparticles in composites for high-frequency electronic applications. It is well established that, to meet the high permeability requirements, the coating has to be thin compared with the particle size.12,13 In this respect, research focuses nowadays not only on a good control of the magnetic nanoparticle size (what influences the magnetic coercivity and therefore the magnetic losses) but also on the good control of the coating at nanometric scale. As it has been stated by Suetsuna et al.13,14 the shell should have high thermal stability and high oxidation resistance, to preserve the magnetic nuclei from oxidation (and therefore preserve their magnetic properties) during the process of the composite formation. Ironbased soft magnetic composites with MgO coating have been reported, but the final product consists of aggregates of coated micrometric metallic powders.15,16 FePt nanoparticles have been embedded in a MgO matrix by thermal annealing, to develop ultrahigh density magnetic recording media as well as highenergy product permanent magnet composites. The MgO coating has been proven to prevent the magnetic cores from sintering at high temperatures (750 °C), providing the magnetic isolation, and preserving the magnetic properties necessary for magnetic recording applications.17 A comparative study of the magnetic properties has shown that MgO matrix leads to a better performance than a SiO2 coating.18 On the other hand, aiming at biomedical applications, MgO biocompatibility has been proved in some medical uses of bioactive materials for tissue regeneration,19−21 magnetic resonance imaging, and magnetic hyperthermia.22 Among the chemical methods, the sol−gel process represents a simple process that permits to obtain high purity and chemically homogeneous materials. Therefore, it can be used for coating magnetic nanoparticles obtained, for example, by coprecipitation.23 The combination of these two methods will be the way to obtain MgO-coated magnetic particles of a desired size and with a narrow size distribution, which is difficult to achieve by other methods reported in the literature.22 It will be possible, for example, to synthesize monodomain superparamagnetic MgO-coated magnetic nanoparticles, with better prospects as MRI contrast agent than ferromagnetic nanoparticles, as for relaxivity and stability of the suspension.24 In this work, magnetite superparamagnetic nanoparticles coated with an ultrathin MgO shell have been synthesized. A sol−gel process has been developed for this purpose by adjusting and modifying already known procedures to synthesize the inorganic MgO shell.25,26 The obtained particles have been characterized from a structural and magnetic point of view, as preliminary study of possible technological and biomedical applications.

2.1. Materials. Iron salts FeCl3·6H2O and FeCl2·4H20 were purchased from Sigma. Magnesium methoxide, 7−8% in methanol, was obtained from Alfa-Aesar; ethanol absolute and ammonia 30% were purchased from Panreac. H2O Milli-Q has been used in all the reactions. 2.2. Synthetic Procedure. Magnetic nanoparticles were prepared following two subsequent synthetic steps for the magnetite magnetic core and the magnesium oxide coating (see Scheme 1).

Scheme 1. Synthesis of the MgO-Coated Fe3O4 Nanoparticles

The magnetite particles were prepared by coprecipitation method27,28 with a slightly modified procedure. FeCl3·6H2O and FeCl2·4H2O in the ratio 3:1 were dissolved in water, and a proper amount of 30% NH3·H2O was then added quickly into the solution under gentle stirring. The color of the solution turned from orange to black immediately, indicating that the formation of the particles had started. The suspension was sonicated for 10 min and then left to rest for another 10 min. The magnetite was first separated magnetically to remove the reaction mixture and then was washed three times with water. Then the particles were washed once with ethanol to remove the residual water, and they were finally resuspended in ethanol. The MgO coating was obtained through a proper modification of sol−gel procedures already reported in the literature for the synthesis of MgO particles.25,26 The ethanol suspension of magnetite particles (6 mg/mL) was kept under sonication, and 1,5 mmol of magnesium dimethoxide (Mg(OCH3)2), together with an excess of water (1:20), was added. This mixture was kept under sonication for 1 h and subsequently was put under stirring for 12 h. Afterward it was heated under stirring at 70 °C for 5 h. The reaction mixture removal was obtained by centrifugation; then the particles were washed once with ethanol, and they were recovered by centrifugation. The remaining solvent was evaporated in ambient conditions and, finally, the obtained powder was calcined in air atmosphere at 600 °C for 30 min. Finally the particles were resuspended in ethanol under sonication. 2.3. Magnetic Particle Characterization. 2.3.1. Thermogravimetric Analysis (TGA). The TGA was carried out by using a SDT2960 instrument by heating from 25 to 800 °C and 10 °C/min in air. 2.3.2. X-ray Diffraction (XRD). The XRD spectra were obtained by using a “D-Max Rigaku” instrument equipped with a rotating 452



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Cu anode and a graphite monocromator (therefore working with Kα1= 1.5405 Å). The diffractometer worked at 40 kV and 80 mA. Data were recorded in the 2θ range between 10° and 80° by using a step of 0.03° and 1s/step. 2.3.3. Dynamic Light Scattering (DLS) Analysis. The hydrodynamic size of particles in liquid suspension was measured in a Brookhaven 90Plus DLS instrument, by means of the PhotoCorrelation Spectroscopy (PCS) technique. 2.3.4. Transmission Electron Microscopy (TEM). Preliminary observation of the synthesized particles was carried out by bright field (BF) imaging in a FEI Tecnai T20 operated at 200 kV. The samples were prepared by resuspending the powder in ethanol under sonication, putting a drop of the suspension directly on a TEM carbon grid, and letting it to dry in air atmosphere before putting it inside the microscope. High Resolution TEM (HRTEM) images were obtained using an aberration corrected FEI Titan 60−300 operated at 300 kV. Correction of the spherical aberration of the objective lens leads to a spatial resolution below 0.1 nm and the minimization of delocalization effects,29 which have made possible the imaging of the ultrathin MgO shell. 2.3.5. Magnetic Characterization. Magnetization measurements were performed in a high sensitivity magnetometer MPMS-5S (Quantum Design Inc., U.S.A.) with superconducting quantum interference detection (SQUID). The temperature dependence of the magnetization from 5 to 320 K of dried powders and aqueous samples was measured, as well as magnetization hysteresis loops up to 5 T at different temperatures.

(just before the coating) and that of the MgO produced by sol−gel method are also shown for comparison. Because of the close lattice parameters of magnetite and MgO, the technique does not allow to distinguish the presence of MgO in the XRD spectrum shown in Figure 1 a because all the MgO reflections (in good agreement with the JCPDS file no. 45-0946) match the magnetite ones (JCPDS file no. 75-1610). Moreover, the sol−gel reaction parameters were chosen with the aim of forming a very thin MgO shell. Consequently, the MgO content could be less than 5% and therefore below the experimental resolution of the XRD technique. In spite of this, the X-ray analysis has been an important tool to demonstrate the presence of the MgO protecting shell around the nucleus. The XRD pattern displayed in Figure 1 a shows the typical reflections of the magnetite. In absence of this very good protecting layer, the calcination of the particles at 600 °C leads to the complete oxidation of the magnetite nucleus to hematite, as it can be seen in the spectrum displayed in Figure 1 c. Therefore, even if the MgO layer is very thin, it is able to perfectly protect the core from the oxidation. This hypothesis was confirmed by the HRTEM results discussed in the paragraphs below. The average size of the crystallites before and after the coating reaction has been estimated by means of the Scherrer formula d = kλ/β cos(θ), where λ is the Cu−Kα1 X-ray wavelength, β is the full width at half-maximum, and k is the shape factor. The results are 7.6 and 21.7 nm, respectively, analyzing the highest intensity peaks in the corresponding spectra in Figure 1. The small reflection appearing at 33 ± 0.03 degrees in the XRD spectrum of the MgO-coated magnetite could be associated to a small quantity of hematite ( JCPDS file no. 33-0664) formed during calcination, because of some residual magnetite nanoparticles that remained uncoated after the sol−gel process. The hydrodynamic diameter distribution of the as-synthesized magnetite particles coated by MgO was determined by DLS analysis. The particles resulted to be almost monodispersed, with a polydispersity index of 0.109 and a mean diameter of 30 nm. Unfortunately, it was not possible to measure the magnetite sample before the coating process since magnetic particles, before the passivation of the surface with the suitable coating, possess a tendency to aggregate and therefore, to quickly precipitate. To investigate the morphology, crystallographic structure, and chemical composition of the individual MgO-coated magnetite particles a thorough characterization by TEM techniques was carried out. BF-TEM images of both magnetite and MgO-coated magnetite particles are shown in Figure 2.

3. RESULTS AND DISCUSSION At the end of the synthesis described in paragraph 2.2, calcination of the Mg(OH)2−coated particles was necessary to ensure the complete transformation of the Mg(OH)2 into a crystalline MgO shell (see Scheme 1). TGA of the Mg(OH)2coated particles performed in air atmosphere, showed that below 400 °C no appreciable loss of weight could be observed. Therefore particle calcination was performed in air at 600 °C. XRD analysis was carried out to investigate the role of the MgO coating in protecting the magnetic cores from oxidation. The XRD pattern of the MgO-coated magnetite nanoparticles after calcination is reported in Figure 1, together with the

Figure 1. XRD spectra of (a) MgO-coated magnetite calcinated at 600 °C, (b) as-synthesized magnetite (noncoated), (c) calcinated (at 600 °C) noncoated magnetite, and (d) MgO produced by sol−gel method. Full squares correspond to the diffraction peaks of the Fe3O4 structure; empty squares correspond to the diffraction peaks of the γ-Fe2O3 structure; X corresponds to the diffraction peaks of periclase.

Figure 2. BF-TEM image of magnetite (left) and MgO-coated magnetite (right) particles.

spectra of noncoated magnetite powder calcinated at 600 °C. The XRD pattern of the magnetite obtained by coprecipitation

The size of the uncoated magnetite ranges from 5 to 10 nm. A clear change in the morphology can be appreciated after the 453



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sol−gel reaction, which yields large particles, because of the formation of the MgO coating. Nevertheless, from these images it is not possible to distinguish the MgO shell from the magnetite core. Moreover, it is not clear if the coating involves one or more magnetic particles. Aberration corrected HRTEM revealed the core−shell structure of individual particles. Calculating the Fast Fourier Transform (FFT) of selected regions of an HRTEM image, a digital diffractogram of different regions of the particles is obtained. If the particle is oriented in a high symmetry crystallographic direction, that is, with a zone axis parallel to the optic axis of the microscope, the FFT reveals the periodicities associated to the lattice fringes of the particle crystal structure. Figure 3 is an example of a magnetite particle oriented in the

Figure 4. ZFC-FC temperature dependence of the magnetization of the Fe3O4 (a) and of the MgO-coated Fe3O4 (b). Measurements have been performed applying a magnetic field of 100 Oe.

the system behaves as superparamagnetic. The shape of the maximum in the ZFC curve shown in the figure evidences the existence of a distribution of blocking temperatures. Moreover, the rounded shape of the maximum makes it difficult to determine the value of Tmax, which is commonly accepted in the literature as an average blocking temperature in the case of a nonideal superparamagnetic system. In our particular case Tmax is approximately Tirr. For Stoner−Wohlfart particles, their volume is related to TB by means of the expression KV = 25kBTB, where K is the nanoparticle anisotropy constant and kB is the Boltzmann constant.30 Assuming that our nanoparticles behave following the Stoner−Wohlfart model, taking 100 K as TB, and using the value of K corresponding to bulk magnetite at room temperature31 the obtained particle diameter is 17 nm. Under the above-mentioned hypotheses and according to the literature30 this value has to be considered as an upper limit of the nanoparticle volume for which the superparamagnetic behavior would appear at a given temperature. The particle sizes estimated from the TEM images are below the diameter derived from the ZFC-FC measurements, and therefore both results are consistent. One reason for the discrepancy between the real nanoparticle dimensions and the diameter obtained from the ZFC-FC measurements is that the synthesized nanoparticles are neither spherical nor monodisperse, as assumed for using the equation above for the volume determination. Moreover, the discrepancy between the size measured on the TEM images and that derived from ZFC-FC magnetization measurements can also be because noncoated magnetite nanoparticles aggregate because of dipolar magnetic interactions, behaving as magnetic particles larger than what the morphology of the particles observed by TEM indicates. The existence of dipolar magnetic interactions can be inferred from the slope of the magnetization vs temperature curve well above Tirr (which does not obey the Curie law, as expected in absence of dipolar magnetic interactions). It should be also taken into

Figure 3. Aberration corrected HRTEM image of a magnetite nanoparticle epitaxially coated by a 1-nm-thick MgO layer. The insets show the FFT calculated from the areas marked with white squares.

(001) direction, in which a very thin shell of MgO of approximately 1 nm encapsulates the magnetite core. The local FFTs of the particle surface reveals the lattice fringes of MgO (001), whereas the FFT of the core shows the typical periodicities and crystal symmetry of the magnetite. Furthermore, the ultrathin MgO shell, coating completely the particle, grows epitaxially on magnetite. Proof of that is the continuity of the lattice fringes across the interface and the coincident orientation of the FFT reflections with very close spacings, such as magnetite (400) and MgO (200) with 2.1 Å. To study the magnetic properties of the magnetite and MgOcoated magnetite nanoparticles, magnetization measurements were performed. Prior to the coating procedure, the temperature dependence of a sample of water-dispersed magnetite nanoparticles was measured from 5 K up to 250 K. Zero-field cooled (ZFC) and field cooled (FC) magnetization curves (measured applying a magnetic field of 100 Oe) are displayed in Figure 4 a. As it can be seen in the figure, the ZFC and FC curves overlap from the highest measured temperature down to the so-called temperature of irreversibility, Tirr, which in our case is 100 K. In an ideal superparamagnetic system Tirr coincides with the blocking temperature (TB) above which 454



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magnetic field. As in the case of the noncoated samples, the high-field susceptibility observed in the isotherm can be ascribed to the highly anisotropic disordered spins in the surface of the magnetite cores.34 Despite the observed reduction of the particle magnetization after the MgO coating reaction and despite the lack of saturation, the achieved magnetization value (48 emu/g at 5 T, 41 emu/g at 1.5 T) is similar to that of iron oxide suspensions reported in the literature and used, for example, as superparamagnetic MRI contrast agents.24,35−37 Moreover, the superparamagnetic character of our particles ensures the reproducibility of their magnetic behavior, regardless of the value of the applied magnetic field. This is an advantage with respect to the use of ferromagnetic nanoparticles in all those applications that reproducibility would require to work at the saturation point, and therefore, in many cases, the use of very high magnetic fields.

account that the anisotropy constant value may be underestimated; first of all because the anisotropy energy increases with temperature (being at 250 K higher than at room temperature); second, because other sources of anisotropy different than the magnetocrystalline cannot be discarded, as it will be discussed later. The fact that no anomaly that could be ascribed to the Verwey transition (characteristic of multidomain magnetite particles) is observed in the ZFC-FC magnetization curve confirms the single domain nature of the synthesized particles.32,33 The superparamagnetic behavior was confirmed by the absence of remanence and coercivity in the magnetization isotherm measured at 250 K. The magnetization reached Ms = 77.5 emu/g at 1.5 T. This is a value 15% lower than the bulk magnetite magnetization value, which suggests that the magnetite nanoparticles consist of a crystalline magnetite core surrounded by a thin shell of disordered magnetic moments or by a very thin layer of a nonmagnetic iron oxide (whose thickness and content are below the experimental resolution of the HRTEM and XRD techniques). The existence of this shell would be the origin of surface anisotropy, the extra contribution to the anisotropy energy mentioned above, that adds to the uniaxial one.34 The hysteresis observed in the magnetization isotherm measured at 5 K confirmed the behavior of the nanoparticles system as a ferromagnetically blocked system, in good agreement with the observed irreversibility of the ZFC and FC processes below 100 K. The ZFC-FC magnetization measurements corresponding to the MgO-coated magnetite nanoparticles displayed in Figure 4 b were performed on the powder samples obtained after the calcination process. The temperature at which the ZFC and the FC curve merge is found to be 275 K, pointing to a superparamagnetic behavior of the nanoparticles above this temperature. A maximum in the ZFC curve is observed at Tmax = 230 K. The progressive separation of the ZFC and the FC curves below 275 K and the broadness of the ZFC maximum indicate that the sample is not monodisperse. These results suggest that some of the individually coated magnetite nanoparticles aggregate because of dipolar interactions. The formation during the sol−gel reaction of magnetite aggregates of different sizes coated by a layer of magnesium oxide can not be discarded. From 275 K down to 230 K the particles progressively block according to their size (starting by the largest ones), leading to a distribution of blocking temperatures. Assuming 230 K as TB, in the hypothesis that each aggregate can be considered also as a Stoner−Wohlfart magnetite nanoparticle, yields an average magnetic diameter around 22 nm. As in the case of the noncoated nanoparticles, besides the uniaxial anisotropy assumed in the Stoner−Wohlfart model, the existence of additional contributions to the anisotropy energy can not be discarded. They would lead to an increase of the anisotropy constant value, which in our calculations has been assumed as that of bulk magnetite at room temperature. The slope of the ZFC-FC magnetization curves points to the existence of dipolar interactions, as in the case of the noncoated sample. The fit of the inverse of the susceptibility above the Tirr of the MgO-coated and noncoated nanoparticles to a Curie− Weiss law, suggests that the MgO coating decreases the extent of the dipolar interactions between particles. As in the case of the magnetite particles, the magnetization isotherms of the MgO-coated nanoparticles at 300 and 5 K display the typical superparamagnetic and blocked behaviors respectively. At 300 K the magnetization did not saturate even at the largest applied

4. CONCLUSIONS The sol−gel method has been used together with coprecipitation to coat magnetite nanoparticles with a layer of magnesium oxide. In the present case, magnetite nanoparticles below the monodomain size have been synthesized as starting material. This two-step procedure offers the possibility to coat particles of any desired size (that can be controlled by the coprecipitation reaction conditions) which is not possible with other methods reported in the literature for MgO coatings. The exhaustive characterization using the most advanced microscopy techniques has demonstrated that a 1 nm layer of MgO grows epitaxially on the magnetite cores. Moreover, such a thin layer has been proven to efficiently protect the magnetic core from oxidation, even at high temperatures well above room temperature. Having in mind the possible technological applications, the synthesis procedure presented here could be a starting point for the fabrication of soft-magnetic composites with MgO insulation, as no such powder size reduction and thinner MgO coatings have been reported up to date. Also the magnetization values of the synthesized nanoparticles, as well as their superparamagnetic behavior at room temperature, make them potential candidates for biomedical applications. The small hydrodynamic diameter of the particles obtained after the coating reaction opens good perspectives concerning the nanoparticles' stabilization in biocompatible media, reducing the probability of aggregation.

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ASSOCIATED CONTENT

S Supporting Information *

Thermogravimetry analysis, size histograms, DLS measurements, X-ray energy dispersive spectroscopy, magnetization isotherms and magnetic susceptibility vs temperature analysis are reported. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +34 976 76 1213. Fax: +34 976 76 1229. E-mail: [email protected]. Author Contributions ⊗

These authors contributed equally.

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ACKNOWLEDGMENTS This work was supported by the Spanish Ministry of Science and Innovation (MICINN) trough projects NANOBIOMED 455



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(30) Cullity, B. D. Introduction to Magnetic Materials; Addison-Wesley Publishing Company, Inc.: Reading, MA, 1972. (31) du Trémolet de Lacheisserie, É., Gignoux, D., Schlenker, M., Eds.; Magnetism, Materials and Applications; Springer: New York, 2005. (32) Goya, G. F.; Berquo, T. S.; Fonseca, F. C.; Morales, M. P. J. Appl. Phys. 2003, 94, 3520−3528. (33) Verges, M. A.; Costo, R.; Roca, A. G.; Marco, J. F.; Goya, G. F.; Serna, C. J.; Morales, M. P. J. Phys. D: Appl. Phys. 2008, 41, 134003− 134010. (34) Knobel, M.; Nunes, W.; Socolovsky, L.; Biasi, E. D.; Vargas, J.; Denardin, J. J. Nanosci. Nanotechnol. 2008, 8, 2836−2857. (35) Hu, F.; Wei, L.; Zhou, Z.; Ran, Y.; Li, Z.; Gao, M. Adv. Mater. 2006, 18, 2553−2556. (36) Villanueva, A.; Cañete, M.; Roca, A. G.; Calero, M.; VeintemillasVerdaguer, S.; Serna, J. C.; Morales, M. P.; Miranda, R. Nanotechnology 2009, 20, 115103−115109. (37) Kluchova, K.; Zboril, R.; Tucek, J.; Pecova, M.; Zajoncova, L.; Safarik, I.; Mashlan, M.; Markova, I.; Jancik, D.; Sebela, M.; Bartonkova, H.; Bellesi, V.; Novak, P.; Petridis, D. Biomaterials 2009, 30, 2855−2863.

(CONSOLIDER-INGENIO 2010 Program), CTQ200803739/PPQ, and EUI2008-00157. Financial support from the Autonomic Government of Aragon (DGA) is also acknowledged through ARAID Foundation and Grupos de Excelencia de la Diputación General de Aragón.

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Ultrathin MgO Coating of Superparamagnetic Magnetite Nanoparticles

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