A versatile and scalable route for tailoring silica shells on hydrophobic nanoparticles

Abstract

Encapsulating hydrophobic nanoparticles (NPs) with a hydrophilic silica shell has proven critical for expanding their applicability to aqueous media, favoring their incorporation into composite materials, and tunning their surface properties (specific surface, thickness and spacing) during the design of nano/micro systems. However, a generalized, scalable method that achieves simultaneous control over both shell thickness and applicability across chemically diverse core materials remains a significant synthetic challenge. Here, we present a versatile and scalable methodology for robustly coating various hydrophobic core materials with a silica shell of precisely tunable thickness. Our approach integrates an efficient ligand-exchange procedure, utilizing nitrosonium tetrafluoroborate (NOBF4) to substitute surface oleate ligands with weakly coordinating anions, thereby enabling phase transfer to N,N-Dimethylformamide (DMF). This hydrophilic intermediate is then directly coated using a modified Stöber process. By systematically tuning the NH3/TEOS ratio and the total precursor concentration, we demonstrate precise control over the shell thickness, successfully synthesizing uniform silica coatings ranging from thin (~6 nm) to ultra-thick shells (over 500 nm, yielding total diameters >1 µm) on individual nanoparticles. We validate the versatility of this integrated protocol by applying it to both rare-earth Upconversion Nanoparticles (UCNPs) of varying shapes, crystal phases and sizes, and magnetic Fe3O4 nanoparticles. This robust and reproducible method offers a generalized platform for the high-throughput production of core-shell NP@SiO2 structures with tailored dimensions, which is essential for engineering advanced materials for diverse applications in sensing, catalysis, and composite systems.