Single-crystalline MFe₂O₄ nanotubes/nanorings synthesized by thermal transformation process for biological applications

We report a general thermal transformation approach to synthesize single-crystalline magnetic transition metal oxides nanotubes/nanorings including magnetite Fe₃O₄, maghematite γ-Fe ₂O₃, and ferrites MFe₂O₄ (M = Co, Mn, Ni, Cu) using hematite α-Fe₂O₃ nanotubes/nanorings template. While the straightforward reduction or reduction-oxides process was employed to produce Fe₃O₄ and γ-Fe2O3, the α-Fe₂O₃/M(OH)₂ core/shell nanostructure was used as precursor to prepare MFe₂O ₄ nanotubes via MFe₂O_4-x (0 < x < 1) intermediate. The transformed ferrites nanocrystals retain the hollow structure and single-crystalline nature of the original templates. However, the crystallographic orientation-relationships of cubic spinel ferrites and trigonal hematite show strong correlation with their morpologies. The hollow-structured MFe₂O₄ nanocrystals with tunable size, shape, and composition have exhibited unique magnetic properties. Moreover, they have been demonstrated as a highly effective peroxidase mimic catalysts for laboratory immunoassays or as a universal nanocapsules hybridized with luminescent QDs for magnetic separation and optical probe of lung cancer cells, suggesting that these biocompatible magnetic nanotubes/nanorings have great potential in biomedicine and biomagnetic applications.