Magnetic field induces nanoparticles structuring. A SAXS application: instrumental development at LNLS and data analysis

Full text: Small Angle X-ray scattering is a useful technique to characterize magnetic nanoparticles (MNP) structuring in suspension. Usually, the inter-particle interactions as steric, electrostatic or dipolar magnetic, induce the structuring of the even coated MNP resulting in fractal or more compact structures. The MNP response to external magnetic field is an important parameter for several MNP applications, this response can be affected by the structuring of the single particles in the suspension, and most important, the applied magnetic field can modify the MNP structuring. One example is the magnetic hyperthermia therapy, in this application, magnetic nanoparticles absorb energy from an alternating magnetic field when they are subjected to it, and release this energy as heat resulting in an increase of the MNP-suspension temperature. This temperature elevation is highly influenced by the dipolar interactions among particles, in this way, a modification of the MNP structuring results in a modification of the dipolar interactions and consequently in a modification of the temperature rise. For these reasons, the analysis of the structuring evolution under applied field is of importance and can be achieved measuring SAXS under applied static fields. In this work, a modification of the liquid sample holder for constant magnetic field application in the SAXS beamline is presented. The magnetic field is applied perpendicular to X-ray beam using constant NdFeB ring shapes magnets. The sample holder allows magnetic field strength, between 0 and 90 kA/m varying the sample-magnet distance. This field strength range is the same used in magnetic hyperthermia experiments. Scattering patterns obtained from this set-up are anisotropic. An anisotropic model, based on the second order Legendre polynomials, is applied to analyze the azimuthal scattering SAXS pattern, and used to determine the anisotropy degree (A) as function of field strength. This model allows to differentiate between individual or collective structuring of MNP in magnetic fields, in this way the scattering q-value vector at which A = 0 corresponds to the size d = 2π/q of the smallest anisotropic structure. Information about the structure factor is derived from the azimuthal integration of the 2D field-applied-pattern, keeping constant the form factor deduced from non-field scattering patterns. Results show that the structure factor change from fractal to hard sticky when magnetic field is applied, and that the anisotropy degree is reduced when the interparticle distance is increased by diluting the suspension. (author)