Molecular recognition and size control of nanosized self-assembled polyoxometalate structures

Hydrophilic polyoxometalate (POM) macroanions are inorganic polyelectrolytes which offer a direct connection between simple ions and organic polyelectrolytes or colloids.;Many types of POM macroanions are highly soluble but undergo reversible self-assembly to form uniform, stable, soft, single-layer vesicle-like "blackberry" structures containing more than 1000 individual POMs in dilute solutions. A complete, continuous transition from discrete POM macroions to blackberry structures, and then back to discrete POM macroions, was reported for the first time in the system of {Mo132}/water/acetone, with the anionic {Mo132} polyoxomolybdate macroions. The complete transition revealed that electrostatic interactions are the dominant driving forces of POM blackberry formation. The charge density on the macroions is found to greatly affect the blackberry formation and dissociation, as the counter-ion association around the POM macroanions is important. Furthermore, the blackberry size can be accurately controlled by changing the charge density of the macroions by changing the solvent quality or pH and by adding cationic species to solution. Recent studies have shown that blackberry formation is a selective process by which the individual POM macroions exhibit a molecular recognition to self-assembly with only macroions like themselves. Many unexpected and intriguing phenomena have been observed in these novel systems.