Silica and core-shell nanospheres by supercritical fluid assisted sol-gel synthesis

Fabrication of monodisperse pure silica and core-shell nanospheres (Fe2O3@SiO2 and Fe2O3/QD@SiO2) was done in straight forward one-pot method combining sol-gel chemistry and supercritical fluids. For the core-shell materials, preformed nanoparticles stabilized in hexane were dispersed in a sol containing a silicon alkoxide, water and acetone as the solvent. The precursor solution was introduced in an autoclave and pressurized with compressed CO2. Then, pressure and temperature were raised over the supercritical conditions of the CO2/acetone mixture allowing the silicon alkoxide to hydrolyze and condensate forming the silica shell. The composite gel nanospheres got dried as the solvent was extracted at supercritical conditions. They present a narrow particle size distribution (with less than 30% polidispersity). Each nanosphere consists of a functional core of several non-contacting γFe2O3 nanoparticles and/or quantum dots surrounded by a microporous silica shell. Their size can be tuned by controlling the reaction conditions. The same synthetic approach is followed to fabricate microporous pure silica nanoparticles. Advantages of the method are short reaction times compared to non-supercritical sol-gel procedures, high reaction yield and purity of the product. In addition, the method has potentiality to be scaled up.