Effect of Washing on the Surface Chemistry of Silica Nanoparticles

Abstract The surface chemistry of silica nanoparticles is central to their use in applications such as CO2 adsorption, environmental remediation, or drug delivery, yet the role of postsynthesis washing remains poorly understood. The surface of silica nanoparticles produced by the conventional Stöber process, in which tetraethoxysilane reacts with water in the presence of ammonia, is covered with silanol groups and residual species from the synthesis, such as ethoxy groups and ammonia. Here, we systematically investigate how washing with ethanol, water, or water/hydrochloric acid affects the concentration of silanols, ethoxy groups, and ammonia on the surface of silica nanoparticles. Using 29Si, 1H, and 13C solid-state nuclear magnetic resonance spectroscopy, complemented by infrared spectroscopy and elemental analysis, we demonstrate that washing with ethanol effectively removes adsorbed ammonia, but has little effect on the distribution of silanol groups or on the amount of ethoxy groups on the silica surface. In contrast, water and water/HCl effectively hydrolyze surface species, increasing the density of single silanols and removing residual surface-bound ethoxy groups and ammonia. Subsequent surface functionalization with (3-aminopropyl)triethoxysilane (APTES) showed that all particles retain a sufficient number of silanol groups to achieve maximum APTES grafting, suggesting that surface silanization processes are largely insensitive to variations in the washing history. At the same time, the washing protocol offers a means of modulating residual ethoxy groups and, to some extent, the binding mode of the grafted APTES. In particular, water-washed particles exhibit the least residual ethoxy groups after APTES grafting. These results show that careful washing can be used to control the surface chemistry of silica nanoparticles without compromising maximal APTES grafting, underscoring the robustness of the silica system for further functionalization.