Self-assembly of colloidal nanoparticles into encapsulated hollow superstructures

As a distinct type of nanocapsules, hollow superstructures of inorganic nanoparticles have attracted increasing attention due to their controllable permeability, convenient functionalization, and efficient surface utilization. Conventionally, they are produced by assembling nanoparticles against expensive sacrificial templates. Herein, a general emulsion-based method is reported to assemble colloidal nanoparticles into submicron hollow superstructures, involving first co-assembly of colloidal nanoparticles with organic additives to form clusters, then overcoating the clusters with a polymer shell, and finally removing the organic additives and re-dispersing nanoparticles by exposing to a good solvent. The key to the success of this process is the re-assembly of nanoparticles against the polymer shells as driven by the capillary force during solvent evaporation, producing hollow superstructures. Such a space-confined assembly process can be well controlled by choice of solvents and their evaporation rates. This general technique provides an open and low-cost platform for creating hollow superstructures of various inorganic nanoparticles, offering many opportunities for exploring unique applications that can take advantage of the collective properties of the constituent nanoparticles and the permeable nanoshell structures.