Magnetic properties of γ-FeOnanoparticles in a porous SiOshell for drug delivery.

A method is presented for synthesizing core-shell nanoparticles with a magnetic core and a porous shell suitable for drug delivery and other medical applications. The core contains multiple $\gamma$-Fe$_2$O$_3$ nanoparticles ($\sim$15~nm) enclosed in a SiO$_2$ ($\sim$100-200~nm) matrix using either methyl (denoted TMOS-$\gamma$-Fe$_2$O$_3$) or ethyl (TEOS-$\gamma$-Fe$_2$O$_3$) template groups. Low-temperature M{\"o}ssbauer spectroscopy showed that the magnetic nanoparticles have the maghemite structure, $\gamma$-Fe$_2$O$_3$, with all the vacancies in the octahedral sites. Saturation magnetization measurements revealed that the density of $\gamma$-Fe$_2$O$_3$ was greater in the TMOS-$\gamma$-Fe$_2$O$_3$ nanoparticles than TEOS-$\gamma$-Fe$_2$O$_3$ nanoparticles, presumably because of the smaller methyl group. Magnetization measurements showed that the blocking temperature is around room temperature for the TMOS-$\gamma$-Fe$_2$O$_3$ and around 250~K for the TEOS-$\gamma$-Fe$_2$O$_3$. Three dimensional topography analysis shows clearly that the magnetic nanoparticles are not only at the surface but have penetrated deep in the silica to form the core-shell structure.