pioneering study, equally sized positively and negatively charged gold NPs were coassembled into colloidal crystals with a structure mimicking that of sphalerite (M 2+ S 2-, where M = Zn and Fe). However, it was subsequently demonstrated that the process, which occurred at 70 ºC, involved a substantial degree of thiolate-ligand exchange between the positively and negatively charged NPs, affording two populations of zwitterionic NPs: one having a small excess of positive charge and the other, a small excess of negative charge (in fact, thiolates on gold are labile and can readily desorb at temperatures exceeding 60 ºC ). An alternative approach to provide inorganic NPs with electric charge is to encapsulate them within charged protein cages. Although crystalline assemblies of protein-encapsulated NPs have been obtained, this method suffers from a relatively low charge density on the protein surface. Therefore, assemblies resulting from electrostatic interactions between heavily charged NPs-in particular, superionic NPs-have remained unknown.