Magnetically Responsive Silica Hollow Spheres: Straightforward Synthesis of Accessible Micro-Sized Containers

Silica hollow spheres are obtained as micro-sized containers with magnetic-field-induced mobility. The completely empty hollow spheres are proved accessible with features of confined environment, where an unexpected structured fluorescence emission with large Stokes' shift is observed. This multifunctional system is achieved through a controllable etching of magnetite from magnetite@silica core-shell with hydrochloric acid solution, resulting in silica hollow spheres coupled with remaining magnetite@silica. The increase in the etching time leads to an increase in the proportion of hollow to magnetite-occupied spheres, and the obtained hollow spheres are completely empty. The remaining magnetite particles have their crystalline structure preserved and even the material with higher proportion of hollow spheres presents high saturation magnetization, which grants response from the whole system induced by a magnetic field. The accessibility of the hollow structure is successfully observed by steady-state fluorescence spectroscopy using toluene as optical probe.