Studies on the self-assembly of metal nanoparticles (NPs) in the presence of ions are motivated by the biosensing applications of NP clusters and the capability to control the morphology of clusters of oppositely charged NPs. The effect of ions has been explored for the self-assembly of metal NPs capped solely with ionic ligands, whereas, in general, the surface of NPs can be coated with a mixture of ligands interacting with each other by non-electrostatic forces. In the present work, we examined the kinetics of self-assembly of gold nanorods capped with a mixture of low-molecular weight ionic molecules and nonpolar polymer ligands. We show that in contrast with earlier reports on the effect of electrolytes on NP self-assembly, the driving force for the accelerated self-assembly of nanorods is the reduction in polymer solubility in the presence of ions, rather than the screening of the electric double layer of the charged ligands. The reported results are important for NP self-assembly occurring in mixed solvents, in which attraction forces between nonpolar ligands are governed by the balance between solvent-solvent and solvent-salt interactions. Furthermore, the addition of salts can be used to increase the rate of nanorod self-assembly, which, otherwise, is an intrinsically slow process.
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