Synthesis of superparamagnetic nanoporous iron oxide particles with hollow interiors by using prussian blue coordination polymers

Our recent work has demonstrated that well-defined hollow interiors can be created inside Prussian Blue (PB) nanoparticles through controlled chemical etching in the presence of poly(vinylpyrrolidone) (Angew. Chem., Int. Ed.2012, 51, 984). By calcination of these PB nanoparticles as starting precursors, we can successfully synthesize nanoporous iron oxides with hollow interiors. From scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the original hollow cavities of PB nanocubes are shown to be retained after crystal transformation to iron oxides. Also, the obtained hollow iron oxides show a very high surface area because of their nanoporous shells, as illustrated by N₂ gas adsorption-desorption analysis. By tuning the applied calcination temperatures and selecting the PB nanoparticles with different hollow cavities, crystalline α-Fe₂O₃, and γ-Fe₂O₃ can be selectively formed in the products without formation of any impurity phases. Field-dependent magnetization measurements indicate that the nanoporous hollow iron oxides exhibit a very interesting superparamagnetic property at room temperature. Especially, nanoporous hollow γ-Fe₂O₃ particles with well-developed crystallinity possess sufficient saturation magnetization (M _s) value. Such reasonable M_s value, together with the superparamagnetic property, high specific surface area, and internal hollow cavity, makes our nanoporous iron oxides a very promising platform for future biomedical applications.