Selective etching of bifunctional core–shell organosilica micro/nanospheres to hollow structures

In this study, highly monodisperse cyanoethyl-/cyanopropyl-functionalized silica micro/nanospheres (CN-E-SiO₂/CN-P-SiO₂) have first been synthesized via one-pot sol–gel reaction at room temperature in the microemulsion system. Effects of the surfactant concentrations and the reaction time on the formation of CN-E-SiO₂/CN-P-SiO₂ have been discussed. Furthermore, bifunctional core–shell structures prepared using CN-E-SiO₂ and CN-P-SiO₂ as core and ureidopropyl-functionalized silica spheres (UD-SiO₂) as shell, denoted as CN-E-SiO₂@UD-SiO₂ and CN-P-SiO₂@UD-SiO₂, have also been etched separately in alkaline conditions. Compared with the original CN-E-SiO₂ and CN-P-SiO₂, transmission electron microscopy images showed that the diameter of CN-E-SiO₂@UD-SiO₂ and CN-P-SiO₂@UD-SiO₂ core–shell structures was all reduced during etching in alkaline condition, finally resulting in a hollow structure with a similar sphere size. In addition, for the sake of comparison, pure silica as core and UD-SiO₂ as shell, denoted as SiO₂@UD-SiO₂ core–shell structure, has also been prepared. However, transmission electron microscopy image indicated that the size and shape of SiO₂@UD-SiO₂ was almost unchanged in the whole etching process. It can be proved that selective etching of bifunctional core–shell organosilica is an optimal strategy for preparation of hollow materials.