Controlling the growth of "ionic" nanoparticle supracrystals

Dimensions and quality of supracrystals self-assembling from oppositely charged nanoparticles (NPs) can be controlled by changing the relative nanoparticle concentrations, NP polydispersity, and pH. In particular, excess nanoparticles of either polarity terminate the self-assembly process at desired stages by forming charged, stabilizing shells around the growing aggregates. In this way, average supracrystal sizes can be varied from several micrometers down to tens of nanometers. While larger crystals precipitate from the growing solution, those smaller than ca. 400 nm are soluble. The experimentally observed threshold size for solubility agrees with arguments based on the DLVO theory.