Producing ultra-thin silica coatings on iron-oxide nanoparticles to improve their surface reactivity

The reactivity of the relatively inert surfaces of iron-oxide magnetic nanoparticles can be significantly improved by coating the surfaces with silica. Unfortunately, however, this nonmagnetic silica layer tends to dilute the magnetic properties of the nanoparticles. Therefore, the silica layer should be as continuous, homogeneous, and as thin as possible.In this investigation we coated superparamagnetic maghemite nanoparticles by hydrolysis and the polycondensation of tetraethyl orthosilicate (TEOS), with the ethanol solution of TEOS being added to a stable suspension of citric acid-coated nanoparticles. The influences of the various parameters of the procedure on the quality of the coatings were systematically evaluated. The quality of the silica layer was characterized using electron microscopy and by performing leaching of the nanoparticles in HCl, while the surface reactivity was tested by grafting (3-aminopropyl) triethoxysilane (APS) onto the nanoparticles. We observed that the surface concentration of the grafted APS strongly increased when the nanoparticles were coated with a silica layer. The choice of experimental conditions for the coating procedure that favors the heterogeneous nucleation of silica on the surfaces of the nanoparticles enabled the preparation of very thin silica layers, less than 2 nm thick. By decreasing the amount of added TEOS to correspond to a monolayer of -Si-OH at the nanoparticles' surfaces, their surface reactivity could be very much improved, and with a reduction in their magnetization of only ∼10%.