Versatile Nanoparticle Capsule Formation With Enhanced Encapsulation Efficiency via Solute‐Induced Liquid–Liquid Phase Separation

Nanoparticle capsules (NCs), capsule-like structures composed of inorganic nanoparticles (NPs), hold great potential for diverse applications due to their structure-driven functionality and the unique properties of the NPs. Despite advancements in NC formation methods, achieving both stable formation and efficient material encapsulation remains challenging. In this study, a simple and versatile method is developed for the formation of inorganic NCs. Inorganic NCs are formed from oligo(ethylene glycol) (OEG)-modified inorganic NPs via self-assembly at solute-induced liquid-liquid interfaces. First, Au NCs are formed from OEG-Au NPs. Their formation is confirmed by scanning transmission electron microscopic (STEM) and cryo-electron tomographic (ET) observations. The resulting Au NCs exhibited high stability, uniform size, and precise tunability under various conditions. The method is further extended to magnetic Fe₃O₄ NPs, affording size-controlled magnetic NCs, demonstrating broad applicability. Moreover, this approach enables the efficient encapsulation of various materials, including citrate-capped Au NPs and DNA molecules, into inorganic NCs, affording >2000-fold enrichment. This method paves the way for innovative application of inorganic NCs in drug delivery, nanoreactors, and beyond.