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.
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