In-ex site selectivity: a novel mechanism for the controlled fabrication of hollow multi-shelled nanostructures.

Site selectivity is fundamental to fabricating complex nanostructures. Herein, we introduce "In-Ex site selectivity," a novel mechanism for regulating deposition on a liquid template containing internal solid particles. The selectivity dictates whether deposition occurs on the interior (In) solid surface or the exterior (Ex) surface of the liquid template. It enables the fabrication of advanced nanomaterials including Hollow Multi-shelled Structures (HoMS)-nanoparticles featuring discrete layers with interstitial gaps. Selectivity is achieved by tuning the seed's surface affinity via incubating or doping with a treatment reagent. In a demonstration, treatment with Cationic Pink FG (CPFG) selectively deactivates the inner surface of the silica shell within the droplet template, rendering it inert to further silica growth. This promotes subsequent silica deposition exclusively on the external liquid surface, ultimately yielding uniform double-layer hollow shells. Without treatment, deposition occurs on the inner shell, forming a single thickened shell. The selectivity can be achieved using a diverse range of molecules, from cationic, neutral, and anionic molecules, to even DNA. The process is highly sensitive, with certain reagents effective at ppb levels. Triple-layer nanoshells were also synthesized. This work presents a robust, gentle method for fabricating HoMS and expands the concept of site selectivity in nanosynthesis.