Amine Grafting on Silica Nanoparticles in Water: A Systematic Study

The amine functionalization of silica nanoparticles (SiNPs) using aminotrialkoxysilanes is typically conducted in organic solvents and is widely applied across diverse fields. Recent studies have demonstrated that this modification can also be achieved in aqueous media at low temperatures; however, the impact of water as the primary solvent on the formation and morphology of alkoxysilane grafting remains unexplored. To address this gap, we systematically investigated the influence of key experimental parameters (including particle size and concentration, the type and concentration of aminoalkoxysilanes, and pH) on grafting efficiency. Our results reveal that the highest amine grafting density in water is achieved using a large excess of (3-aminopropyl)trimethoxysilane (APTMS) in dilute SiNP solutions with relatively large particle diameters (Dh ∼ 150 nm) under basic conditions. The so-modified SiNPs exhibit a smooth surface with a reproducible surface amine density of ∼12 NH2 nm-2 (among the highest recorded in the literature). Such density is attributed to the grafting of short, polymerized aminotrialkoxysilane chains on the silica surface. In addition, while this dense APTMS graft is sensitive to hydrolytic degradation, storage at low temperatures preserves a high grafting density (∼10 NH2 nm-2) over several weeks. This study provides a comprehensive understanding of the experimental conditions governing amine grafting on SiNPs in aqueous media and offers practical guidelines for the preparation of SiNPs with dense amine functionalization.