Investigation of structural, chemical and magnetic modifications of nanoparticles surface during the synthesis of ferrite based electric double layered magnetic fluids

INVESTIGATION  OF  STRUCTURAL,  CHEMICAL  AND  MAGNETIC  MODIFICATIONS  OF  NANOPARTICLES  SURFACE  DURING  THE  SYNTHESIS 

OF  FERRITE  BASED  ELECTRIC  DOUBLE  LAYERED  MAGNETIC  FLUIDS.

 

M. H. Sousa 1 , F. A. Tourinho 1 , J. Depeyrot 2 , E. Dubois 3 , R. Perzynski 3 .

 

1.      Complex Fluids Group, Universidade de Brasilia, Instituto de Quimica, Caixa Postal 04478, CEF 70919-970, Brasilia, Brazil.

2.      Complex Fluids Group, Universidade de Brasilia, Instituto de Fisica, Caixa Postal 04455, CEF 70919-970, Brasilia, Brazil.

3.      Laboratoire des Milieux D’esordonnes et Heterogenes, associated, Universite Pierre et Marie Curie (Paris 6), case 78, 4 Place Jussieu, 75252, Paris Cedex 05, France.

 

Ferrite particles with grain sizes of the order of few nanometers are emerging as reliable miscible able to solve complex engineering problems and, very recently, as promising materials for biomedi­cal applications. At the nanometric scale, the size reduction leads to interesting magnetic properties such as superparamagnetism, enhanced anisotropy and surface effects, which are of great applica­tion in biomedicine [1]. Indeed, such nanoparticles, which have dimensions smaller or compatible a biological entities, can be coated with biological molecules and dispersed in a liquid carrier leading to colloidal solution called magnetic fluids or ferrofluids. Due to their liquid properties and sensi­tive to an applied magnetic field, these materials can be made to deliver packages any as anticancer drugs or radionuclide atoms to a targeted region of the body, such as a tumour. The elaboration of a conventional ferrofluid involves at least two fundamental requirements: the particle preparation, in which the method of synthesis determines the crystallographic structure the shape, the size and size distribution, the surface chemical state and consequently the magnetic properties of the grains; then peptization of the particles into a carrier liquid, in order a magnetic sol. In this case, the colloidal stability will depend first, on the dimension on the particles, sufficiently small to avoid precipitation due to gravitation, and second, on the scale

surface repulsion, provided by steric and/or coulombic interactions, to prevent agglomeration due to Van der Waals and magnetic dipole forces. Nanoferrites of MeFe2O4 are good candidates in future biomedical purposes. Indeed, the presence of  the divalent metallic ion Me (usually a d-block metal) in such nanoparticles would improve the determination of their in vivo bio-distribution by titration of sush ion in blood probes [2]. It is therefore of  great importance to investigate some basic char­acteristics such as structure, chemical composition and magnetization, before binding biomolecules on the particle surface and proceed to medical applications. 

In this work, we study ferrofluids based on ferrite nanoparticles (Me = Co, Ni, Zn) that are prepared in a three step procedure. The particles are coprecipitated by alkalinizing 1 : 2 muixtures of Me (II) and Fe(III) at 100°C, under vigorous stirring. Changing the velocity of mixing the reagents lead to nanoparticles with differente mean sizes ranging from 3 up to 12 nm. In order to prevent the dis­solition of the synthesized material in the acidic carrier (the dispersion medium), the particles are hydrotermally treated with a ferric nitrate solution at 100°C. Finally, the particles are electrostati­cally peptized in aqueous media by controlling the pH and the ionic strength of the colloidal solu­tion. This process yields the so-called electric double layered magnetic fluids.

Moreover, to reduce the polydispersion, the synthesized samples are centrifuged at 4000 RPM dur­ing 15 minutes. After centrifugation, the resulting precipitate and supernatant are redispeprsed, forming stable sols of high quality. The hydrothermal treatment changes the chemical composition of the nanoparticles with the formation of a protective shell on the particle surface with a high chemical stability [3]. Here our main goal is to experimentally investigate the formation of this shell, its composition, thickness, the changes induced or not in the global magnetic properties of  the particles using systems based on CoFe2O4, Cu Fe2O4, NiFe2O4 and ZnFe2O4 nanoparticles.

After the hydrothermal treatment, the molar ratio [Fe] / [Me] of our nanoparticles is determined by atomic absorption spectroscopy and found to increase from its stoichiometric value 2, up to 10, de­pending on the grain size: smaller the particle, larger the gain in iron. The pH and temperature con­ditions during the hydrothermal treatment are probably responsible for the aggregation of iron from solution onto the particles surface. Moreover the freshly-synthesized ferrite nanoparticles can also act as «nuclei», inducing the iron adsorption by diffusive mechanism. X- ray powder diffracto­grams, obtained after the hydrothermal treatment, do not present any significant modifications on both the crystalline structure and mean size of the particles. We therefore propose that the particles are made by a MeFe2O4 nucleus surrounded by a surface shell, iron-rich, with a mean chemical composition corresponding to Fe2O3. From such a core/ surface strategy combined with measure­ments of the density of colloidal solution, we estimate the shell thickness of the order of a spinel unit cell. Using a Squid device, a detailed room temperature magnetization measurement is per­formed in order to characterize the magnetic colloids. The values of magnetic dipolar interaction parameters, deduced from initial susceptibility measurements, are lower than 1, indicating a regime of non-interacting particles. Then, a Langevin analysis (independed particles) allows evaluating the magnetic moment distribution of the synthesized nanoparticles, the mean value varying from 600 to 3000 Bohr magnetons depending on the sample. Magnetization measurements performed on ferrite powders before and after the hydrothermal treatment show that the surface shell contributes to the total magnetization of the grains. Furthermore, Mossbauer spectroscopy measurements, performed in the presence of an applied magnetic field, indicate that the cation distribution of the ferrite core is different from that of an ideal spinel bulk [4]. The total magnetization of our nanoparticles is there­fore the result of cation redistribution in the core and a magnetic surface shell.

 

References:

 

1.      Q. A. Pankhurst, J. Connolly, S. K. Jones, J. Robson. // J. Phys. D.: Appl. Phys. 36 (2003) R167.

2.      E. Hasmonay, J. Depeyrot, M. H. Sousa, F. A. Tourinho, J.-C. Bacri, Yu. L. Raikher,               Y. Rosenman, R. Perzynski. // J. Appl. Phys. 88 (2000) 6628.

3.      M. H. Sousa, F. A. Tourinho, J. Depeyrot, G. J. Da Silva, M. C. F. L. Lara. // J. Phys. Chem. B 105 (2001) 1168.

4.      E. C. Sousa, M. H. Sousa, G. F. Goya, H. R. Rechenberg, M. C. F. L. Lara, F. A. Tourinho, J. Depeyrot. // J. Magn. Mat. Mat. (to appear in 2004). C. R. Alves, R. Aquino, M. H. Sousa, H. R. Rechenberg, G. Goya, F. A. Toutinho, J. Depeyrot. // J. Matastable Nanocryst. Mater. (to appear in 2004).