Charged nanoparticles as supramolecular surfactants for controlling the growth and stability of microcrystals

Microcrystals of desired sizes are important in a range of processes and materials, including controlled drug release^{1, 2}, production of pharmaceutics and food^{3, 4}, bio-⁵ and photocatalysis⁶, thin-film solar cells⁷ and antibacterial fabrics⁸. The growth of microcrystals can be controlled by a variety of agents, such as multivalent ions⁹, charged small molecules¹⁰, mixed cationic–anionic surfactants^{11, 12}, polyelectrolytes^{13, 14} and other polymers¹⁵, micropatterned self-assembled monolayers^{16, 17}, proteins¹⁸ and also biological organisms during biomineralization^{19, 20}. However, the chief limitation of current approaches is that the growth-modifying agents are typically specific to the crystalizing material. Here, we show that oppositely charged nanoparticles can function as universal surfactants that control the growth and stability of microcrystals of monovalent or multivalent inorganic salts, and of charged organic molecules. We also show that the solubility of the microcrystals can be further tuned by varying the thickness of the nanoparticle surfactant layers and by reinforcing these layers with dithiol crosslinks.