Bridging interactions and selective nanoparticle aggregation mediated by monovalent cations.

Selective aggregation and precipitation of like-charged nanoparticles (NPs) covered with carboxylate ligands can be induced by different monovalent cations. The ordering of critical concentrations required for NP precipitation is Cs(+) ≫ K(+) > Li(+) > Na(+) > Rb(+) and does not correlate with the size of hydrated cations M(+), nor can it be predicted by the Hofmeister series. On the other hand, different anions have no effect on the precipitation trends. These observations are rationalized by a theoretical model combining the elements of the DLVO theory with molecular-level calculations. The key component of the model is the cation-specific binding of various metal cations to the carboxylate ligands.