Several additives, including polystyrene, cyclen, and polyamines, to the Stöber process allow the creation of shell-core silica particles. Two configurations of the shell-core morphology have been described. One is a silica core with an outer shell of an alternative material such as polystyrene. The second is a silica shell with a morphologically different core such as a polyamine.
The creation of the polystrene/silica core composite particles begins with creation of the silica cores via the one-step Stöber process. Once formed, the particles are treated with oleic acid, which is proposed to react with the surface silanol groups. Styrene is polymerized around the fatty-acid-modified silica cores. By virtue of size distribution of the silica cores, the styrene polymerizes around them evenly resulting composite particles are similarly sized.
The silica shell particles created with cyclen and other polyamine ligands are created in a much different fashion. The polyamines are added to the Stöber reaction in the initial steps along with the TEOS precursor. These ligands interact with the TEOS precursor, resulting in an increase in the speed of hydrolysis; however, as a result they get incorporated into the resulting silica colloids. The ligands have several nitrogen sites that contain lone pairs of electrons that interact with the hydrolyzed end groups of TEOS. Consequently, the silica condense around the ligands encapsulating them. Subsequently, the silica/ligand capsules stick together to create larger particles. Once all of the ligand has been consumed by the reaction the remaining TEOS aggregates around the outside of the silica/ligand nanoparticles, creating a solid silica outer shell. The resultant particle has a solid silica shell and an internal core of silica-wrapped ligands. The sizes of the particles cores and shells can be controlled through selection of the shape of the ligands along with the initial concentrations added to the reaction.