Facile transformation of FeO/Fe(3)O(4) core-shell nanocubes to Fe(3)O(4) via magnetic stimulation

Here, we propose the use of magnetic hyperthermia as a means to trigger the oxidation of Fe(1−x)O/Fe(3−δ)O(4) core-shell nanocubes to Fe(3−δ)O(4) 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−x)O core to Fe(3−δ)O(4), 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−δ)O(4) 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-Fe(2)O(3). 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.