Fe/FeOx NANOPARTICLES OBTAINED BY MECHANOSYNTHESIS OF FE NANOCORES PLUS THE ADDITION OF AN OXIDE SHELL. FABRICATION STRATEGIES AND EFFICIENCY AS TRANSDUCTORS OF RADIOFREQUENCY MAGNETIC-FIELD ENERGY INTO HEAT.

MUÑOZ MEDINA, GUILLERMO; JUNCAL, LUCIANA; DE SOUSA E.; MEYER, MARCOS; GUSTAVO A. PASQUEVICH; MENDOZA ZÉLIS P; FRANCISCO H SANCHEZ

Vitoria

Conferencia; XVII Latin American Conference On The Applications Of The Mössbauer Effect; 2022

Universidade Federal do Espírito Santo

Biomedical applications of magnetic nanoparticles (NPs) require biocompatibility and strong magnetic response. A strategy to optimize these features is the development of core/shell, Fe/FeOx (FeOx: iron oxide) NPs, to take advantage of the Fe bulk magnetization of 217 Am2/kg and of the FeOx biocompatibility. We prepared the Fe-cores by mechanosynthesis, through the solid reaction of FeCl3 and Mg[1],[2], followed by the addition of the FeOx shell, either by chemical coprecipitation2,[3] or by ambient oxidation. The mechanosynthesis step produces Fe cores of 35 nm mean size with high crystallinity and strong magnetic response. Their Mössbauer spectra display only a sextet which cannot be distinguished from the calibration α-Fe foil one. The mean value of their saturation magnetization at room temperatures (RT) is of 172 Am2/kg, consistent with the presence of an initial oxide mean layer of less than 2 nm.After performing the steps of chemical coprecipitation (using the initial cores as nucleation seeds) and of citrate stabilization, highly stable aqueous-colloids of Fe/FeOx@cit were obtained, with mean values of 9 nm and 1.5 nm for final core and shell thicknesses, respectively. Their RT saturation magnetization was above 130 Am2/kgFe. The RT Mössbauer spectrum of the dried colloid is well described by three components: A sextet from α-Fe (Bhf =33.0(1) T, d = 0.00(1) mm/s, e = 0.00(1) mm/s), a doublet with D = 0.65(2) mm/s and a very broad sextet. Both the broadened sextet and the doublet present an isomer shift d = 0.35(1) mm/s. Electron diffraction performed during TEM observations revealed reflections only from α-Fe and from a spinel structure consistent with magnetite and/or maghemite. Given the Mössbauer results, the oxide was identified as mostly maghemite. The z-potential of the colloid was almost constant at the pH interval 5 - 11, displaying a mean value of -42.2 mV, thus confirming the presence of the FeOx@cit shell.The magnetic heating efficiency of these colloids under radiofrequency (RF) magnetic fields (for concentrations in the range 0.08-2.50 g/L) was very promising, based on an exhaustive comparison with values reported in the literature for the relevant quantities SAR and ILP. obtained for Fe, Fe/FeOx and FeOx NPs of comparable sizes under accepted RF clinical conditions. Additionally, we have observed, through MTT tests, that B16-F0 murine melanoma cells internalized with these colloids present a viability of 80%.The ambient oxidation step of the initial cores was carried on at room temperature in open air. It gives rise to the gradual growth of an oxide-hydroxide phase. We have studied the as-synthesized samples (S1), which contain the Fe-cores, NaCl (inert dispersant medium employed during the mechanosysnthesis) and MgCl2 (reaction product) as well as samples containing just the Fe cores (S2), obtained after repeated washing of the former in MilliQ water. We have recorded the oxidation kinetics, in situ and ex situ, by gravimetric analysis, magnetic gravimetry, vibrating sample magnetometry (VSM) and Mössbauer effect spectrometry, at RT. It was found that the transformation kinetics is much faster for S1 samples, due to the salts’ hygroscopicity. The evolution of the total mass, the relative abundance of the Fe containing phases, and the magnetic properties of the core/shell NPs was studied. The Mössbauer spectra revealed the progressive intensity diminution of the α-Fe sextet subspectrum (Bhf = 33.0(1) T, δ = 0.00(1) mm/s), and the progressive intensity increase of a doublet subspectrum (Δ = 0.34-0.35 mm/s, δ = 0.67-0.70 mm/s), tentatively associated with ferrihydrite[4]. The growth and extinction of a transitory phase with hyperfine parameters consistent with Fe2+ was observed during the first 2 h in S1 samples. For these samples, were total transformation occurred within a week, the intensity of the VSM signal continuously decreased with time, to less than 1 % of the initial values, due to the paramagnetic-like response of the oxide-hydroxide phase. We have estimated the field dependence of the magnetic moment, low-field susceptibility, and coercivity, of the Fe-cores, as a function of oxidation time, by deconvoluting the isothermal magnetization cycles of the samples into those of the Fe cores, and those of the non-ferromagnetic part of the samples. It was found that the shape of the cycles evolves with time while the coercivity of the Fe-cores presents a maximum, a fact that was interpreted in terms of the continuous diminution of the Fe-cores and the expected dependence of the magnetization behavior with core size.