Freezing of Binary Colloidal Systems for the Formation of Hierarchy Assemblies

Cryo-etch scanning electron microscopy (cryo-etch SEM) of aqueous gels composed of colloidal silica nanoparticles in the 1−40 nm range and liposomes of ∼200 nm gave unique morphologies. The aqueous gels are frozen at subcooled liquid nitrogen and fractured to obtain a fresh surface. High-vacuum sublimation of ice from the freshly exposed surface (etching) results in the formation of a hierarchy assembly, characterized by granular fences composed of colloidal silica and liposomes surrounded by empty areas in which amorphous ice originally resided. The biocompatible character of this ice segregation induced self-assembly (ISISA) process that allows for the preservation of the structural integrity of liposomes within the assembly is demonstrated by fluorescence anisotropy performed at the binary colloidal aqueous gels and differential scanning calorimetry and electron microscopy at the hierarchy assembly. The resulting assembly shows an interesting dual character, with one colloidal entity supporting the structure (e.g., silica) and the other providing functionality (e.g., liposomes).