Synthesis of highly monodisperse superparamagnetic iron oxide core@mesoporous silica shell particles with independently tunable size, and porosity

Superparamagnetic iron oxide nanoparticles, SPIONs, especially in the form of nanoscopic aggregates thereof, find broad applications as magnetic resonance imaging contrast agents, as biological tools for straightforward separation of biomolecules and cells, as magnetically tunable colorimetric sensors exhibiting structural colors, etc. Furthermore, if the SPIONs are covered with another oxide, typically silica, the size and refractive index of the particles can be modified. In addition, if the silica layer is mesoporous additional functionalities can be introduced. Such particles could serve as, for example, theranostic nanoparticles capable of serving both as drug carriers and contrast agents, as easily separable high-capacity adsorbers, and if the particles are monodisperse, colorimetric sensors. Many reports cover the synthesis of such SPION core-silica shell particles, but to date the structural tunability and reproducibility aspect has not received deserved attention. Here we report highly reproducible syntheses leading to SPION cores carrying silica shells with independently tunable shell thicknesses and porosities. The products are highly monodisperse in all cases, as evidenced by the fact that pellets of the particles show structural colors. The shell formation process is followed in detail, and is related to the applied synthesis parameters, which allows for rational further fine-tuning of the structural characteristics of the core-shell particles for a given application. • We report results related to the synthesis of highly monodisperse, superparamgnetic aggregates of iron oxide nanoparticles of various sizes with different silica shells of various thicknesses, and porosities. • The silica shell porosity can be adjusted independently of the total particle size and the silica shell thickness. • The possibility for independent tuning of total particle size, shell thickness, and shell porosity makes it possible to rationally optimize these core-shell particles for a given application.