Effect of Cobalt Doping Concentration on the Crystalline Structure and

Article pubs.acs.org/JPCC

Effect of Cobalt Doping Concentration on the Crystalline Structure and Magnetic Properties of Monodisperse CoxFe3−xO4 Nanoparticles within Nonpolar and Aqueous Solvents Ling Hu,† Caroline de Montferrand,‡ Yoann Lalatonne,‡,§ Laurence Motte,‡ and Arnaud Brioude*,† †

Laboratoire des Multimatériaux et Interfaces, Université Lyon 1, CNRS UMR 5615, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France ‡ Laboratoire CSPBAT, Université Paris 13, CNRS UMR 7244, 93017 Bobigny Cedex, France § Service de Médecine Nucléaire, Hôpital Avicenne, 125 route de Stalingrad, 93009 Bobigny Cedex, France S Supporting Information *

ABSTRACT: In this work, we investigate the effect of cobalt substitution on the size evolution, crystal structure, and magnetic properties of Fe3O4 nanoparticles. Monodisperse CoxFe3−xO4 nanoparticles were prepared, using a one-step method, by direct heating process of iron(III) and cobalt(II) acetylacetonates in high-boiling-point inert organic solvent. The quantities of precursors added were based on stoichiometric Fe/Co ratio of desired ferrite. Elemental analyses ICP-AES evidenced successful cobalt doping. The doped particles showed a cobalt-deficient composition. Transmission electron microscopy demonstrated the large changes of particle size as a function of cobalt doping. The magnetization measurements showed an unchanged saturation magnetization only up to x = 0.24, beyond which it significantly decreased. To make the as-synthesized nanoparticles suitable for biomedical applications, oleic acid ligands are exchanged with caffeic acid molecules leading to stable nanoparticles in physiological conditions. (Fe3+)A[Fe3+, Fe2+]BO4. The magnetic moment density of magnetic ferrites MxFe3−xO4 depends mainly on the nature of M2+, as Fe3+ are antiferromagnetically coupled.18 In the octahedral sites of cubic spinel structure, the Co2+ ions present a higher anisotropic character than both Fe2+ and Fe3+.19 The outlook for their potential biomedical applications remains promising. Various preparation procedures of cobaltdoped ferrite nanoparticles, including coprecipitation,20−22 sol− gel methods,23 combustion reaction,24 hydrothermal25,26 and thermal decomposition,24,27,28 have been studied. It has been found that preparation methods play an important role on the structural and magnetic properties of similar composition ferrite nanoparticles. Sorescu et al.26 reported the preparation of cobalt- and nickel-doped magnetite by the hydrothermal method. Cobalt substitution increased dramatically the hysteresis phenomenon and coercivity, while nickel addition caused a slight decrease in these parameters. Fatima Fontes Lelis et al.29 showed the obtention of cobalt-doped magnetite by a coprecipitation method. Mössbauer parameters suggested that the amounts of Fe2+ decreased continually with the increase in cobalt concentration. The cobalt substitution tended to grow particles in uniform agglomerates and increased the

1. INTRODUCTION Magnetite Fe3O4 and cation-substituted magnetite MxFe3−xO4 (M = Co, Mn, Fe, Ni, Mg, etc.) are some of the most commonly investigated magnetic inverse spinel ferrites. Magnetic spinel ferrite nanoparticles (NPs) present applications in many technological application areas, such as magnetic recording media,1−4 microwave devices,5 catalysis,6 and ferrofluid technology.7 Furthermore, nanotechnology-based devices and nanomaterials have been demonstrated to be greatly useful for improving the efficiency and efficacy of biomedical applications. Among different types of nanomaterials, magnetic ferrite nanoparticles have set the most successful example for medical applications of inorganic nanoparticles. Because of their ultrafine size, biocompatibility, and superparamagnetic properties, these nanoparticles are already approved for various applications such as cellular labeling or cell separation8,9 and enhanced resolution magnetic resonance imaging10,11 or undergoing preclinical and clinical evaluation for applications such as drug delivery,12 tissue repair,13 cell and tissue targeting,14 transfection,15 and more recently magnetic immunoassay.16 In the cubic structure of magnetite, 1/3 of the interstices are tetrahedrally coordinated with oxygen and 2/3 are octahedrally coordinated.17 The tetrahedral sites (A) are completely occupied by Fe3+ and the octahedral ones (B) by equal amounts of Fe3+ and Fe2+, as presented by the formula © 2012 American Chemical Society

Received: May 5, 2011 Revised: November 25, 2011 Published: January 13, 2012 4349

dx.doi.org/10.1021/jp205088x | J. Phys. Chem. C 2012, 116, 4349−4355

The Journal of Physical Chemistry C

Article

rapidly to 298 °C and kept at this temperature for 30 min before cooling down to room temperature. The nanoparticles were precipitated through the addition of 20 mL of ethanol and separated via centrifugation, the resulting product was then dispersed in cyclohexane. Centrifugation was applied once more to remove any undispersed residue. The final product was redispersed in cyclohexane. Table 1 shows the composition of four samples synthesized.

coercive field. Salazar-Alvarez et al. 19 investigated the production of near-stoichiometric CoxFe3−xO4+δ nanoparticles in large quantities by precipitation of metallic ions from an alkaline aqueous medium. The range of particle sizes determined from the XRD patterns showed that the particle size decreased as the [Fe(2,3)+]/[Co2+] and ([Fe2+] + [Co2+])/ [OH−] ratios increased. The coercivity increased with the particle size over a wide range of sizes. In the absence of surfactants, the nanoparticles prepared by these methods were strongly agglomerated and exhibited the hysteresis loops of ferrimagnetic nature at room temperature. The room temperature superparamagnetic behaviors of cobalt ferrite nanoparticles were observed by the research work of Gyergyek30 based on thermal decomposition of Co- and Fe-oleates complexes, at the end of which each particle is well isolated. The dispersed state of obtained nanoparticles is thus an important parameter which affects the magnetic properties of the samples. In the present paper, monodisperse cobalt-doped magnetite nanoparticles were synthesized using a modified synthesis based on the two-steps procedure first presented by Sun et al.27 Here, a “one-step” direct heating process of metal salts in highboiling-point inert organic solvent is described. The effects of cobalt substitution on the size evolution, crystal structure, and magnetic properties are investigated. To make as-synthesized nanoparticles suitable for biomedical applications, oleic acid ligands are exchanged with caffeic acid molecules. The caffeic acid presents two major properties as ligand for water-soluble nanoparticles. Firstly, the catechol function, as observed with dopamine molecules, is a well-known solid anchor on metal oxide surface which possesses irreversible binding affinity.31,43 Secondly, the large number of COOH functionalities on the magnetic core of the particle can be used as precursor groups for the covalent coupling of biomolecules and leads to electrostatic interaction between nanoparticles to facilitate their dispersion under physiologic conditions.32 These different points allowing a good stability over time and the control of the interfacial chemistry are crucial for the use of magnetic nanoparticles for biomedical applications. Finally, the magnetic properties of such nonpolar and water-dispersed ferrite nanoparticles are studied.

Table 1. Co/Fe Molar Ratio Used in the Synthesis of CoxFe3‑xO4 Nanoparticles sample pure magnetite sample I II III

Co(acac)2 (mmol)

Fe(acac)3 (mmol)

Co/Fe molar ratio

−

0.6

−

0.1 0.2 0.4

0.5 0.4 0.2

1:5 1:2 2:1

2.3. Surface Modification: Preparation of WaterSoluble Nanoparticles. To stabilize the particles in an aqueous solution, the organic nonpolar surfactant (oleic acid, OA) was exchanged with a catechol molecule (caffeic acid, CA). A 1 mL portion of an aqueous caffeic acid solution at pH 10 was added to 1 mL of the cyclohexane nanoparticles dispersion. The mixture was sonicated for 30 min and then stirred for 2 h. The organic nonpolar surfactant was diluted by adding 2 mL of cyclohexane, followed by centrifugation. The supernatant was then discarded. The particles in aqueous phase were washed with water (pH = 2) and finally dispersed in 1 mL of water (pH = 7). 2.4. Analytical and Characterization Methods. Transmission electron microscopy (TEM) was performed using a Topcon EM 002B microscope operating at 200 kV. Samples were prepared by air-drying drops of the as-prepared colloidal solutions on carbon-coated copper grids. Elemental analyses were carried out by an ICP-AES ≪Activa≫ Jobin Yvon. Nanoparticle surface charge was characterized by Zetasizer Nano ZS Malvern. Fourier transformed infrared (FT-IR) spectra were recorded on a Nicolet 380, ThermoScientific. The magnetic properties of nanoparticles were recorded at room temperature using a Superconducting Quantum Interference Device (SQUID).

2. EXPERIMENTAL SECTION 2.1. Chemicals. The products necessary for our experiments are as follows: iron(III) acetylacetonate Fe(acac)3 (Aldrich, ≥99.9% trace metals basis); cobalt(II) acetylacetonate Co(acac)2 (Acros Organics); 1,2-hexadecanediol (Aldrich, technical grade 90%); oleylamine (Fluka, tech.≥70%); oleic acid (Aldrich, tech. 90%); dibenzyl ether (Aldrich 99%); and caffeic acid (Sigma Aldrich, ≥98%). 2.2. Synthesis of Monodisperse CoxFe3−xO4 Nanoparticles. CoxFe3−xO4 nanoparticles were prepared, under inert atmosphere, according to a modified method first described by Sun.27 The strategy is using iron(III) and cobalt(II) acetylacetonates as precursors in the high-temperature organic solution containing the reducing reagent (1,2hexadecanediol) and surfactants (oleic acid and oleylamine). The quantities of precursors added were based on stoichiometric Fe/Co molar ratio of desired ferrite. In a typical synthesis, a total of 0.6 mmol of the metal precursors, 0.6 mmol of 1,2-hexadecanediol, 12 mmol of oleylamine, and 12 mmol of oleic acid were added to 20 mL of dibenzyl ether and stirred vigorously for 30 min. Under reflux, the mixture was heated

3. RESULTS AND DISCUSSION We have modified the Sun’s method27 by using a one-step heating process. The molar ratio of 1,2-hexadecanediol to metal precursors was decreased from 5 to 1, while the molar ratio of oleylamine and oleic acid to metal precursors was increased. The use of 1,2-hexadecanediol was believed to mediate the availability of the cations for deposition and therefore improve the nucleation of the cobalt ferrite nanoparticles.33 However, the high cost of 1,2-hexadecanediol made this process difficult to scale up for mass production. Xie et al.18 have tested different relatively cheap diols whose hydrocarbon chains are shorter than 1,2-hexadecandiol. It was found that these diols tended to yield irregularly shaped nanoparticles. In 2009, the strong reductive ability of oleylamine for the thermal decomposition of Fe(acac)3 was reported by Xu.34 The presence of an excess amount of oleylamine in our synthesis approach allows to provide a reductive environment and thus reduce the amount of 1,2-hexadecanediol used. The mixture 4350



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was heated directly from room temperature to reflux (298 °C). This rapid rise in temperature combined with highly reducing environment promoted the decoupling of nucleation and growth stages. The large-scale applications of ferrite nanoparticles require that the nanoparticles remain stable in carriers of different polarity. For stabilization in nonpolar solvents, the nanoparticles should have their surface modified by the adsorption of long-chain carboxylic acids. In our work, we increased the amount of oleic acid added to ensure nanoparticles’s colloidal stabilization. The oleic groups present on the nanoparticle surface could be replaced via ligand exchange reactions with various capping agents which can provide direct solubility in water or expose functional hydroxyl groups.35 Indeed, for biological applications, water solubility of NPs is indispensable. By the ligand exchange method, a catechol ligand (caffeic acid, CA) was used to replace the original surfactant OA on the surface of NPs. 3.1. Cobalt Deficient Growth. The results obtained for cobalt and iron determination by ICP-AES are presented in Table 2. Although doping was successful, the particles showed a

Co-deficient composition. For samples I and II, the amount of cobalt ions inserted was only half of the theoretical stoichiometry. This content was slightly increased for sample III. These differences between experimental and theoretical reports are probably due to the difference in thermal stability of iron(III) and cobalt(II) acetylacetonates.36 In addition, the presence of a large excess in oleylamine reduced the decomposition temperature of metal complex and thus influenced the growth mechanism of the nanoparticles. Under reductive environment, iron(III) ions are partially reduced to iron(II) ions which may compete with Co(II) ions in the growth process. 3.2. Size Evolution. Pure and doped magnetite nanoparticles were also observed by means of transmission electron microscopy (TEM) (Figure 1). A characteristic diffraction pattern is given in Figure 1 showing the specific crystallographic planes of the two phases Fe3O4 (JCPDS 019-0629) and CoFe2O4 (JCPDS 022-1086). This diffraction pattern cannot allow us to distinguish between those two phases. From their corresponding size distribution histograms (Figure 2), it can be seen that in contrast to pure Fe3O4 nanoparticles (10.2 ± 2 nm), the average particle size of doped samples I and II (13.9 ± 1.6 nm for sample I and 14.8 ± 1.9 nm for sample II) is significantly increased. The particles can be formed if the nuclei are saturated in the reaction medium. The decomposition temperature of cobalt(II) acetylacetonate is lower than that of iron(III). The oxide-based nuclei appeared quickly. This quick nucleation led to the nuclei saturation at early stage, and the nuclei previously formed contributed spontaneously to the growth process, giving larger particles. When the amount of cobalt(II) precursor increased greatly

Table 2. ICP-AES Results Obtained for Cobalt and Iron Determination Co/Fe molar ratio

chemical formula

sample

theor.

exptl.

theor.

exptl.

I II III

1:5 1:2 2:1

1:11.6 1:5.2 1:0.9

Co0.5Fe2.5O4 CoFe2O4 Co2FeO4

Co0.24Fe2.76O4 Co0.48Fe2.52O4 Co1.58Fe1.42O4

Figure 1. TEM images and a characteristic diffraction pattern of Fe3O4 (A), sample I (B), sample II (C), and sample III (D). 4351



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Figure 2. Size distribution histograms of Fe3O4 (A), sample I (B), sample II (C), and sample III (D).

(samples III), the nucleation was spontaneous, resulting in very small ferrite nanoparticles and high polydispersity in particle size. 3.3. Surface Modification for Aqueous Dispersion. We now propose to exchange the ligands on the surface of the CoxFe3−xO4 particles, to generate water-soluble nanoparticles for biocompatible applications. Indeed, the magnetic properties of ferrite nanoparticles find a large domain of applications in biomedicine, point of care diagnostic, etc. TEM images (Figure 3) highlight the morphology conservation during the phase transfer. Moreover, ICP-AES data confirm that cobalt doping does not change before (Co:Fe = 1:5.2) and after (1:5.5) ligand exchange. Figure 3C displays infrared spectra of Co0.48Fe2.52O4 NPs coated with oleic (sample II) and caffeic acid. The IR spectra of pure oleic acid and caffeic acid are given for reference. The most relevant peak assignement of oleic acid is the carbonyl absorbance at 1711 cm−1. Several weak modes are also observed at 1465, 1413, and 1285 cm−1 corresponding to v(−COO−), v(−COH), and v(−C−O), respectively. In the oleylamine IR spectrum (not shown), the most characteristic peaks at 1647 and 787 cm−1 can be ascribed to NH2 scissoring mode and NH2 wagging vibration.37 The caffeic acid is characterized by different vibration bands: the aromatic ring CC stretching vibration bands at 1639, 1602, 1522, and 1376 cm−1, the vibration bands C−O of the carboxylic acid at 1293 cm−1, and C−O stretching vibration bands at 1280 cm−1.38 The spectrum of as-synthesized nanoparticles (coated with oleic acid and oleylamine) presents a strong peak at 587 cm−1 corresponding to Fe−O within magnetite structure.39,40 The weak peak at 716 cm−1 is due to CH−CH stretching mode. The two new peaks at 1534 and 1409 cm‑1 are assigned to

Figure 3. TEM images before (A) and after (B) transfer of sample II (scal bare represents 20 nm), infrared spectra (C) of oleic acid (black dotted line), oleic acid coated NPs (black line), caffeic acid (red dotted line), and caffeic acid coated NPs (red line).

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bidentate (−COO−Fe) mode of oleic acid binding,41 whereas the band at 1598 cm‑1 is assigned to N−H bending, indicating the presence of oleylamine in the sample. Moreover, the fact that the peak at 1711 cm−1 corresponding to the carboxylic function of free oleic acid disappears, indicates that no unbounded oleic acid remains within sample II and strong interactions between the carboxylate and Co0.48Fe2.52O4 NPs occur.42 Hence oleic acid and oleylamine molecules prevent nanoparticles interactions through steric repulsion leading to a stable nonpolar ferrofluid. Catechol-derivative anchor groups possess irreversible binding affinity to iron oxide.43 Hence we choose the caffeic acid molecules to replace oleic acid coating and thus can optimally disperse ferrite nanoparticles under physiologic conditions (Figure 3B). Compared Co0.48Fe2.52O4 nanoparticles coated with oleic and caffeic acid, a number of changes are observed in the related infrared spectra confirming ligand exchange of oleic acid with caffeic acid (Figure 3C). The most notable is the appearance of additional bands relative to catechol species such as the aryl C− O stretch (∼1267 cm−1) and the strong characteristic band of a conjugated aromatic ketone (∼1609 cm−1),44 thus confirming the bidentate bonding of the catechol to the surface of the nanoparticles. The aqueous suspension of caffeic acid coated nanoparticles remains stable after several months’ storage at room temperature (see the Supporting Information). This high stability could be explained by electrostatic repulsions between particles which are quantified by the high negative zeta potential (∼−40 mV). 3.4. Magnetic Properties. Magnetic measurements were carried out at room temperature on lyophilized samples, except sample III coated with caffeic acid which was measured in solution. In agreement with differences observed in composition and structural properties, the nanoparticles show markedly different magnetic properties for various doping concentrations (Figure 4).

M2+ and their distribution between A and B sites are thus the fundamental aspects on the understanding of magnetic properties of metal-doped magnetite nanoparticles.29 In cobalt ferrite, Co2+ tends to replace Fe2+ preferentially in B sites. This preference can be explained by the fact that B sites can hold two times more metal ions than A sites.45 It is also important to note that in our case, oleic acid molecules were coated on the particle surface with a single layer structure proved by the absence of the characteristic IR band of the secondary layer in bilayer oleic acid coated magnetite nanoparticles at 1710 cm−1 (Figure 3C).46 This nonmagnetic surface layer, which can comprise a important fraction of the total mass, could have a significant effect on the magnetic properties of small nanoparticles. Zhang et al.47 reported that the total adsorption amount and cover density of oleic acid molecules on the nanoparticle surface decreased sharply with the increase of particle size, thus leading to a decrease of nonmagnetic species on the particle surface. Consequently the nonmagnetic layer has been subtracted from the magnetization value by determining the coating rate with TGA (see the Supporting Information). The saturation value of 10 nm undoped magnetite nanoparticles is 69 emu/g, close to the values found in the literature.48,49 The difference with the bulk value (92 emu/g) is attributed to a spin canting due to subcoordinated surface atoms.50,51 With cobalt content x from 0 to 0.24 (sample I), the saturate magnetization is unchanged (70 emu/g). This behavior is due to two opposite contributions. The cobalt substitution decreases the magnetization,52 and this effect is partially counteracted by the significant increase in particle size from 10 to 14 nm.53 When we doubled the cobalt doping concentration up to x = 0.48 (sample II), a rapid decrease of magnetization value was observed (40 emu/g) as expected for an increasing cobalt substitution.52 We can note that for sample III, even at relatively high fields, the saturation magnetization was not reached with a very low magnetization of 4 emu/g at 3000 kA/m (Figure 4D). This behavior is probably related to the presence of very small nanoparticles (Figure 1D) which could contribute only a negligible magnetic signal. The room temperature magnetization curves of the samples suggest superparamagnetic behavior, showing zero coercivity except for sample I. This room temperature superparamagnetic behavior is probably related to the dispersed state of particles.47 Concerning sample I, a low room temperature coercivity of 10kA/m is observed (Figure 4B). This coercive field is about five times lower than a same-sized CoFe2O4 particle54 and could be attributed to the presence of large particles with triangular and hexagonal shape (Figure 1B). Indeed, it has been demonstrated that the parameter which has the greatest impact onto the coercivity is the particle size,33,55 and there are no such large particles in sample II (Figure 1C). Consequently, this last sample presents no magnetic coercitivity. The enlarged views of magnetization curves compare the magnetization behaviors of MxFe3−xO4 nanoparticles before and after ligand exchange. The magnetic behavior has not been changed during the transfer. This confirms the crystal morphology conservation observed on TEM images as well as the cobalt doping conservation. This validates the softness of our ligand exchange procedure.

Figure 4. Room temperature magnetizations of Fe3O4 (A), sample I (B), sample II (C), and sample III (D); insets: enlarged views of magnetization curves for oleic acid capped (black curves) and caffeic acid functionalized (red curves) for each sample.

The saturation magnetization of MxFe3−xO4 nanoparticles largely depends on their composition and particle size. Fe3+ ions are distributed in A and B sites within the spinel structure and are antiferromagnetically coupled. The chemical nature of

4. CONCLUSIONS Cobalt ferrite (CoxFe3−xO4), one of the most investigated ferrites, has successfully been synthesized by direct heating 4353



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(18) Xie, J.; Peng, S.; Brower, N.; Pourmand, N.; Wang, S. X.; Sun, S. H. Pure Appl. Chem. 2006, 78, 1003−1014. (19) Salazar-Alvarez, G.; Olsson, R. T.; Sort, J.; Macedo, W. A. A.; Ardisson, J. D.; Dolors Baro, M.; Gedde, U. W.; Nogues, J. Chem. Mater. 2007, 19, 4957−4963. (20) Olsson, R. T.; Salazar-Alvarez, G.; Hedenqvist, M. S.; Gedde, U. W.; Lindberg, F.; Savage, S. J. Chem. Mater. 2005, 17, 5109−5118. (21) Jacintho, G. V. M; Brolo, A. G.; Corio, P.; Suarez, P. A. Z.; Rubim, J. C. J. Phys. Chem. C 2009, 113, 7684−7691. (22) Melo, T. F. O.; da Silva, S. W.; Soler, M. A. G.; Lima, E. C. D.; Morais, P. C. Surf. Sci. 2006, 600, 3642−3645. (23) Niu, Z. P.; Wang, Y.; Li, F. S. J. Mater. Sci. 2006, 41, 5726−5730. (24) Nakagomi, F.; da Silva, S. W.; Garg, V. K.; Oliveira, A. C.; Morais, P. C.; Franco Junior, A.; Lima, E. C. D. J. Appl. Phys. 2007, 101, 09M514. (25) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Nature 2005, 437, 121− 124. (26) Sorescu, M.; Grabias, A.; Tarabasanu-Mihaila, D.; Diamandescu, L. J. Appl. Phys. 2002, 91, 8135−8137. (27) Sun, S. H.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. X. J. Am. Chem. Soc. 2004, 126, 273−279. (28) Jana, N. R.; Chen, Y. F.; Peng, X. G. Chem. Mater. 2004, 16, 3931−3935. (29) Fatima Fontes Lelis, M.; Fabris, J. D.; Nova Mussel, W.; Yoshihaki Takeuchi, A. Mater. Res. 2003, 6, 145−150. (30) Gyergyek, S.; Makovec, D.; Kodre, A.; Arcon, I.; Jagodic, M.; Drofenik, M. J. Nanopart. Res. 2010, 12, 1263−1273. (31) Xu, C. J.; Xu, K. M.; Gu, H. W.; Zheng, R. K.; Liu, H.; Zhang, X. X.; Guo, Z. H.; Xu, B. J. Am. Chem. Soc. 2004, 126, 9938−9939. (32) Lalatonne, Y.; Paris, C.; Serfaty, J. M.; Weinmann, P.; Lecouvery, M.; Motte, L. Chem. Commun. 2008, 2553−2555. (33) Song, Q.; Zhang, Z. J. J. Am. Chem. Soc. 2004, 126, 6164−6168. (34) Xu, Z. C.; Shen, C. M.; Hou, Y. L.; Gao, H. J.; Sun, S. H. Chem. Mater. 2009, 21, 1778−1780. (35) Lattuada, M.; Hatton, T. A. Langmuir 2007, 23, 2158−2168. (36) Hoene, J. V.; Charles, R. G.; Hickam, W. M. J. Phys. Chem. 1958, 62, 1098−1101. (37) Klokkenburg, M.; Hilhorst, J.; Erné, B. H. Vib. Spectrosc. 2007, 43, 243−248. (38) Xu, P.; Uyama, H.; Whitten, J. E.; Kobayashi, S.; Kaplan, D. L. J. Am. Chem. Soc. 2005, 127, 11745−11753. (39) Benyettou, F.; Lalatonne, Y.; Motte, L. Nanotechnology 2011, 22, 055102. (40) Hu, L.; Hach, D.; Chaumont, D.; Brachais, C. H.; Couvercelle, J. P.; Percheron, A. J. Sol−Gel Sci. Technol. 2009, 49, 277−284. (41) Wu, N. Q.; Fu, L.; Su, M.; Aslam, M.; Wong, K. C.; Dravid, V. P. Nano Lett. 2004, 4, 383−386. (42) Zhao, S. Y.; Lee, D. K.; Kim, C. W.; Cha, H. Q.; Kim, Y. H.; Kang, Y. S. Bull. Korean Chem. Soc. 2006, 27, 237−242. (43) Amstad, E.; Gillich, T.; Bilecka, I.; Textor, M.; Reimhult, E. Nano Lett. 2009, 9, 4042−4048. (44) Shultz, M. D.; Ulises Reveles, J.; Khanna, S. N.; Carpenter, E. E. J. Am. Chem. Soc. 2007, 129, 2482−2487. (45) Fan, X. A.; Guan, J. G.; Cao, X. F.; Wang, W.; Mou, F. Z. Eur. J. Inorg. Chem. 2010, 419−426. (46) Yang, K.; Peng, H. B.; Wen, Y. H.; Li, N. Appl. Surf. Sci. 2010, 256, 3093−3097. (47) Zhang, L.; He, R.; Gu, H. C. Appl. Surf. Sci. 2006, 253, 2611− 2617. (48) Hong, R. Y.; Li, J. H.; Cao, X.; Zhang, S. Z.; Di, G. Q.; Li, H. Z.; Wei, D. G. J. Alloys Compd. 2009, 480, 947−953. (49) Li, Z.; Chen, H.; Bao, H.; Gao, M. Chem. Mater. 2004, 16, 1391−1393. (50) Berkowitz, A. E.; Schuele, W. J.; Flanders, P. J. J. Appl. Phys. 1968, 39, 1261−1263. (51) Morales, M. P.; Veintemillas-Verdaguer, S.; Montera, M. I.; Serna, C. J. Chem. Mater. 1999, 11, 3058−3064. (52) Zheng, Y. H.; Xia, J. B. Phys. Rev. B 2005, 72, 195204.

process of metal salts in high-boiling-point inert organic solvent using Fe(acac)3 and Co(acac)2 as reactants. We demonstrated the subsequent effects on the size evolution, crystal structure, and magnetic properties with the cobalt doping concentration variation. The doped particles showed a cobalt-deficient composition. Large changes of particle size as a function of cobalt doping were demonstrated by TEM analysis. The magnetization measurements showed an unchanged saturation magnetization only up to x = 0.24, beyond which it significantly decreased. A rapid and easy ligand exchange route was then investigated to make these nanoparticles hydrophilic, without modifying their morphology, cobalt doping concentration, and magnetic behavior. This final step opens opportunities for biomedical applications.

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ASSOCIATED CONTENT

S Supporting Information *

TGA experiements and DLS measurements. This 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].

■

ACKNOWLEDGMENTS We gratefully acknowledge the Service Analyse Texture of IRCE for ICP-AES analysis and the Laboratory of Condensed Matter Physics and Nanostructures (LPMCN) for access to TEM analysis. We are also grateful to Hicham Jouni for experiment assistance and Dominique Bonnin and Brigitte Leridon from LPEM, ESPCI ParisTech, for access to SQUID. This work was supported by grants from Region Ile-de-France.

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REFERENCES

(1) Woo, K.; Lee, H.; Ahn, J. P.; Park, Y. S. Adv. Mater. 2003, 15, 1761−1764. (2) Ngo, A. T.; Bonville, P.; Pileni, M. P. J. Appl. Phys. 2001, 89, 3370−3376. (3) Yi, D. K.; Lee, S. S.; Ying, J. Y. Chem. Mater. 2006, 18, 2459− 2461. (4) Zhao, W.; Gu, J.; Zhang, L.; Chen, H.; Shi, J. J. Am. Chem. Soc. 2005, 127, 8916−8917. (5) Pardavi-Horvath, M. J. Magn. Magn. Mater. 2000, 215, 171−183. (6) Gill, C. S.; Price, B. A.; Jones, C. W. J. Catal. 2007, 251, 145−152. (7) Pu, H. T.; Jiang, F. J. Nanotechnology 2005, 16, 1486. (8) Olsvik, O.; Popovic, T.; Skjerve, E.; Cudjoe, K. S.; Hornes, E.; Ugelstad, J.; Uhlen, M. Clin. Microbiol. Rev. 1994, 7, 43−54. (9) Gupta, A. K.; Curtis, A. S. G. Biomaterials 2004, 25, 3029−3040. (10) Amstad, E.; Zurcher, S.; Mashaghi, A.; Wong, J. Y.; Textor, M.; Reimhult, E. Small 2009, 5, 1334−1342. (11) Qiao, R.; Yang, C.; Gao, M. J. Mater. Chem. 2009, 19, 6274− 6293. (12) Gai, S.; Yang, P.; Li, C.; Wang, W.; Dai, Y.; Niu, N.; Lin, J. Adv. Funct. Mater. 2010, 20, 1166−1172. (13) Gupta, A. K.; Gupta, M. Biomaterials 2005, 26, 3995−4021. (14) Tchikov, V.; Winoto-Morbach, S.; Kronke, M.; Kabelitz, D.; Schutze, S. J. Magn. Magn. Mater. 2001, 225, 285−293. (15) Krotz, F.; de Wit, C.; Sohn, H. Y.; Zahler, S.; Gloe, T.; Pohl, U.; Plank, C. Mol. Ther. 2003, 7, 700−710. (16) Motte, L.; Benyettou, F.; de Beaucorps, C.; Lecouvey, M.; Milesovic, I.; Lalatonne, Y. Faraday Discuss. 2010, 149, 211−225. (17) Schwertmann, U.; Cornell, R. M. Iron oxides in the laboratory: Preparation and characterization; Wiley-VCH: New York, 2000. 4354



Article

(53) Demortière, A.; Panissod, P.; Pichon, B. P.; Pourroy, G.; Guillon, D.; Donnio, B.; Bégin-Colin, S. Nanoscale 2011, 3, 225−232. (54) Robinson, D. B.; Persson, H. H. J.; Zeng, H.; Li, G.; Pourmand, N.; Sun, S.; Wang, S. X. Langmuir 2005, 21, 3096−3103. (55) Maaz, K.; Mumtaz, A.; Hasanain, S. K.; Ceylan, A. J. Magn. Magn. Mater. 2007, 308, 289−295.

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