Electrostatic Driven Self-Assembly of Polyoxometalate Macroions in Divalent Salt Solution

Supramolecular assembly of subnanometer-size macroions, including peptides, proteins, and lipids, in aqueous solutions has gained much interest for their wide potential applications ranging from nanomedicine to energy storage. The electrostatic attraction and self-assembly of multivalent macroions remain inadequately understood, where Debye−Huckel theory for small ions or the DLVO theory for charged colloids become inapplicable. Recently, we have found that inorganic polytungstate (Li₆H₂W₁₂O_40, {W₁₂}) polyoxometalate (POM), which is about 0.8 nm in diameter and bears 8 negative charges upon fully dissolved in water, can spontaneously assemble into well-organized structures of 5-10 mm in narrow size distribution in divalent salt solution. It is in sharp contrast to the thermodynamically stable {W₁₂} aqueous solution added with monovalent salt such as LiCl. As increasing the concentrations of {W₁₂} and CaCl₂, We have observed the transformation of supramolecular assemblies into various shapes, including rods, dumbbell shapes, and spheres, using both SEM and confocal microscopy. Such multivalent counterion-mediated electrostatic control of macroion assembly could be general and extended to other POMs and even hybrid inorganic-organic macroion mixtures, opening new approaches to develop nanoscale functional materials.