Using SAXS to analyze the magnetic nanoparticles structuring induced by external magneticfields

J. M. POLLI; P. MENDOZA ZÉLIS; D. CORAL; M. B. FERNÁNDEZ VAN RAAP

CABA

Encuentro; Symposium ''Frontiers in Physical Sciences'' 2016; 2016

International Center for Advanced Studies of Argentina and the Max Planck Liason Office for Latin America

Iron oxide magnetic nanoparticles (MNP) are being widely used for several applications, frommagnetic recording to drug delivery and magnetic hyperthermia (MH) for cancer treatment [1].Many of their properties strongly depend on the interactions between particles e.g. coercive fieldand magnetic susceptibility [2,3], and affect their performance for each application. For MNP incolloidal suspensions the interactions are usually magnetic and electrostatic, and may lead to MNPaggregation [1]. Especially, in intracellular MH applications the MNP are highly aggregated invesicles called endosomes. When MNP are submitted to radiofrequency field, with field amplitudearound 10-50 kA/m and frequency in the kHz range, the delay in the magnetic response yield to adissipative process. That process results in a heat transfer to the cells, an increases of itstemperature, and its subsequently death. The efficiency of the energy transduction fromelectromagnetic to thermic depends on the MNP properties, which are affected by the magneticinteractions between particles. This interaction is determined, among others, for the spatialdistribution of the nanoparticles. In colloids, this distribution strongly depends on the strength ofthe magnetic field applied. Small Angle X-Ray Scattering (SAXS) is a sensitive technique to analyzechanges in the structuring of MNP in colloidal suspensions.In this work, colloidal suspensions of magnetite nanoparticles used for MH applications, withparticle size of 10, 11 and 17 nm, with different aggregation degree in field absence are used toanalyze the MNP structuring under external magnetic field. This study was performed analyzing theanisotropic scattering patterns from SAXS measurements under static magnetic field with strengthin the same range of RF field amplitudes used for MH. Results show that when an external staticmagnetic field is applied, alignment is induced in the colloid. This alignment could be from individualMNP or magnetic aggregates, in both scenarios an anisotropic structuring is induced in the colloidalsuspension. In SAXS, the scattering patterns from anisotropic structures are also anisotropic, andthe typical models used to retrieve particle and structural information from isotropic patterns arenot adequate. An anisotropic model, based in the second order Legendre polynomes, is applied toanalyze the azimuthal scattering SAXS pattern, and used to determine the anisotropy degree (S) asfunction of field strength. This model allows to differentiate between individual or collectivestructuring of MNP in magnetic fields, in this way the scattering q-value vector at which S = 0corresponds to the size d = 2π/q of the smallest anisotropic structure.References[1]. D.F. Coral et al. Langmuir, 2016, 32 (5), pp 1201?1213.[2] P. Alliaet al. PhyscalReview B, 2001, 64, 144420.[3]. J.L. Dormannet al. Journal of Physics C: Solid States Physics, 1988, 21, 2015-2034.