Facile transformation of FeO/Fe₃O₄ core-shell nanocubes to Fe₃O₄ via magnetic stimulation

Here, we propose the use of magnetic hyperthermia as a means to trigger the oxidation of Fe_1-xO/Fe_3-δO₄ core-shell nanocubes to Fe_3-δO₄ phase. As a first relevant consequence, the specific absorption rate (SAR) of the initial core-shell nanocubes doubles after exposure to 25 cycles of alternating magnetic field stimulation. The improved SAR value was attributed to a gradual transformation of the Fe_1-xO core to Fe_3-δO₄, as evidenced by structural analysis including high resolution electron microscopy and Rietveld analysis of X-ray diffraction patterns. The magnetically oxidized nanocubes, having large and coherent Fe_3-δO₄ domains, reveal high saturation magnetization and behave superparamagnetically at room temperature. In comparison, the treatment of the same starting core-shell nanocubes by commonly used thermal annealing process renders a transformation to γ-Fe₂O₃. In contrast to other thermal annealing processes, the method here presented has the advantage of promoting the oxidation at a macroscopic temperature below 37 °C. Using this soft oxidation process, we demonstrate that biotin-functionalized core-shell nanocubes can undergo a mild self-oxidation transformation without losing their functional molecular binding activity.