Sculpting Silica Colloids by Etching Particles with Nonuniform Compositions

ABSTRACT: We present the synthesis of new shapes of colloidal
silica particles by manipulating their chemical composition and
subsequent etching. Segments of silica rods, prepared by the ammonia
catalyzed hydrolysis and condensation of tetraethylorthosilicate
(TEOS) from polyvinylpyrrolidone loaded water droplets, were
grown under different conditions. Upon decreasing temperature,
delaying ethanol addition, or increasing monomer concentration, the
rate of dissolution of the silica segment subsequently formed
decreased. A watery solution of NaOH (∼mM) selectively etched
these segments. Further tuning the conditions resulted in rod−cone
or cone−cone shapes. Deliberately modulating the composition along the particle’s length by delayed addition of (3-aminopropyl)-triethoxysilane (APTES) also allowed us to change the composition stepwise. The faster etching of this coupling agent in neutral conditions or HF afforded an even larger variety of particle morphologies while in addition changing the chemical functionality. A comparable step in composition was applied to silica spheres. Biamine functional groups used in a similar way as APTES caused a charge inversion during the growth, causing dumbbells and higher order aggregates to form.
These particles etched more slowly at the neck, resulting in a biconcave silica ring sandwiched between two silica spheres, which could be separated by specifically etching the functionalized layer using HF.