Zwitterion Additives in the Stöber Process for Particle Size Tuning and Silica Surface Decoration

Zwitterionic colloidal materials, such as antifouling and biocompatible silica nanoparticles, are of potential utility for a variety of applications. This work investigates the use of three small-molecule zwitterions (ZIs): trimethylglycine (TMG), l-carnitine (CAR), and l-α-glycerophosphorylcholine (GPC), as additives in a traditional Stöber process that employs tetraethylorthosilicate (TEOS) as the silica precursor. Including a ZI additive in the reaction mixture was observed to reduce the average silica particle size and modify the particle surface with the zwitterionic moiety, to a varying extent based on the ZI selected. Silica particle size was characterized by dynamic light scattering (DLS) and confirmed using scanning electron microscopy (SEM). Zeta potential values were measured using phase analysis light scattering (PALS), and the particle surface was characterized by X-ray photoelectron spectroscopy (XPS). At the highest ZI concentration examined (0.213 molal, corresponding to a 1:1 ZI:TEOS molar ratio), GPC caused a 46% reduction in the average particle size compared to the control (no ZI). The persistent presence of each ZI additive on the particle surface following two deionized water washes and redispersion steps was evidenced by zeta potential values that approached 0 mV for samples prepared with the highest ZI concentration. Furthermore, high-resolution XPS N 1s spectra confirmed the presence of quaternary ammonium (N-Q) groups in all samples prepared using the zwitterionic additives at a concentration of 0.213 molal in the reaction mixture. For these samples, the N-Q/Si 2p peak area ratio of the TMG sample was observed to be approximately four times larger than in the cases of GPC and CAR. This straightforward method for utilizing small-molecule ZIs in the Stöber process highlights their promise as additives in silica preparation methods and for zwitterionic surface modification.