Ionic effectiveness in the self-assembly and crystallization of polymer grafted Au nanoparticles at the gas-liquid interface*

Salts in solutions promote aqueous surface self-assembly of gold nanoparticles (AuNPs) that are grafted with non-ionic and water soluble polymers such as poly(ethylene glycol) (PEG) or poly(N-isopropylacrylamide) (PNIPAM). Using AuNPs functionalized with PEG (PEG-AuNPs) as a model system, previous studies have demonstrated that ions can be ranked in their effectiveness to promote the two-dimensional superlattice formation of PEG-AuNPs. Similar ranking for the aqueous biphasic system (ABS) of PEG in salt solutions. However, we show that ionic species that do not readily induce ABS at ambient temperatures can lead to surface crystallization of PEG-AuNPs. Synchrotron x-ray reflectivity and grazing incidence small angle x-ray scattering methods are used to determine the structures of the surface assembly for various ionic species. Here, we report on grazing incidence x-ray fluorescence spectroscopic measurements that in conjunction with Compton and Thomson scattering reveal ion-specific distributions at the surface and in the bulk in the presence of PEG-AuNPs suspensions and compare them to salt solutions without NPs. We find that the ion distributions of CsCl and NaI correlate with the quality of surface PEG-AUNPs crystallization.