Role of the alkali-metal cation size in the self-assembly of polyoxometalate-monolayer shells on gold nanoparticles.

Polyoxometalate (POM)-monolayer stability constants, K, for three POM anions vary with the cation size, in the same order as that for increasing ion-pair formation with α-SiW(11)O(39)(8-) (1) in the early nucleation phase of monolayer self-assembly: Li(+) < Na(+) < K(+) < Cs(+). Cryo-TEM images demonstrating the use of the cation size to rationally control monolayer formation provide definitive evidence that the POM monolayers are electrostatically stabilized (ionic) shells, analogous in that respect to the monolayer walls of "hollow" POM-macroanion vesicles.