Mixtures of oppositely charged nanoparticles can control the size and stability of microcrystals of inorganic salts or charged organic molecules, Northwestern University’s Bartosz A. Grzybowski and coworkers report (Nat. Mater., DOI: 10.1038/nmat3202). The researchers crystallized inorganic salts, such as K2SO4, Na2SO4, and CaCO3, and organic molecules, such as l-lysine and vitamin B-5, in a 50:50 solution of positively and negatively charged thiol-coated gold or silver nanoparticles. The ratio of nanoparticle concentration to salt or molecule concentration controls the crystallization outcome: As the ratio increases, the crystal size decreases. During crystal growth the nanoparticles adsorb cooperatively to the crystal surfaces where they facilitate the adsorption of oppositely charged nanoparticles. When these nanoparticles adsorb, crystal growth slows down, stopping when the entire surface is coated by a mosaic of alternating positively and negatively charged nanoparticles. The researchers can also control the size via pH. As the pH decreases from 11 to 6.5, the nanoparticle layer becomes thicker. After the microcrystals are formed, they can be stabilized by cross-linking the nanoparticles with alkane dithiols.