Oppositely charged, nanoionic nanoparticles can act as “universal surfactants” regulating the growth of ionic microcrystals. This phenomenon derives from a subtle interplay between crystal growth and cooperative electrostatic adsorption of the nanoparticles onto crystal faces. In addition to the electrostatic interactions acting in the system, the nature of salts is also important in the sense that for the same Debye screening length, different salts can mediate formation of markedly different assemblies including supraspheres, nanoneedles, or nanoplates. The method can be further extended to coat non-ionic crystals with appropriately functionalized nanoparticles.