A general and mild route to highly dispersible anisotropic magnetic colloids for sensing weak magnetic fields

There has been growing interest in the preparation of anisotropic magnetic nanoparticles with well-controlled shapes and excellent colloidal dispersity. Here we report a general and mild approach for the preparation of highly dispersible anisotropic magnetic colloids through the low-temperature conversion of nonmagnetic templates. The key of our strategy is to modify the template surface with a layer of porous silica to provide a spatially confined environment necessary for the preservation of the overall morphology and improve the shell permeability necessary for the conversion at lower temperatures. The conversion under milder conditions not only is of practical importance for improving energy and cost-effectiveness but also produces magnetic particles with excellent colloidal dispersity by alleviating aggregation. We demonstrate the universality of this approach in the porous-silica-protected conversion of different samples including -Fe2O3 and FeOOH into anisotropic magnetic particles with excellent colloidal dispersity. Based on highly dispersible anisotropic magnetic particles, we also demonstrate the fabrication of responsive photonic crystals for detecting very weak magnetic fields that are one order of magnitude higher than the earth magnetic field. Our study paves the way for new applications of anisotropic magnetic nanoparticles in self-assembly, catalysis, photonic crystals, liquid crystals, biomedicine and pharmaceutics.