Fabrication of One-Dimensional Iron Oxide/Silica Nanostructures with High Magnetic Sensitivity by Dipole-Directed Self-Assembly

Weakly ferromagnetic iron oxide nanoparticles with narrow size distribution were prepared by a high-temperature hydrolysis reaction. The magnetic nanoparticles can self-assemble into one-dimensional particle chains on substrates as well as in colloid dispersion through magnetic dipolar interaction without the help of an applied magnetic field. The 1D assemblies were further harnessed to prepare 1D nanostructures with aligned magnetic nanoparticle inclusions and a continuous silica shell. One-dimensional iron oxide/silica nanostructures were prepared through chainlike self-assembly of the magnetic nanoparticles in a mixture of ethanol, ammonia, and water followed by a sol−gel process of TEOS at the surface of the assemblies. The length of the nanostructures can be controlled by the amplitude of ultrasonication, and the thickness of the silica coating can be tuned by the dosage of TEOS. The 1D nanostructures exhibit highly magnetic sensitivity. In the presence of an applied magnetic field, the nanostructures can be oriented and align along the direction of the external magnetic field.