Two-Dimensional Crystallization of Poly( N-isopropylacrylamide)-Capped Gold Nanoparticles

Surface-sensitive X-ray reflectivity and grazing incidence small-angle X-ray scattering reveal the structure of polymer-capped-gold nanoparticles (AuNPs that are grafted with poly( N-isopropylacrylamide); PNIPAM-AuNPs) as they self-assemble and crystallize at the aqueous suspension/vapor interface. Citrate-stabilized AuNPs (5 and 10 nm in nominal diameter) are ligand-exchanged by 6 kDa PNIPAM-thiol to form corona brushes around the AuNPs that are highly stable and dispersed in aqueous suspensions. Surprisingly, no clear evidence of thermosensitive effect on surface enrichment or self-assembly of the PNIPAM-AuNPs is observed in the 10-35 °C temperature range. However, addition of simple salts (in this case, NaCl) to the suspension induces migration of the PNIPAM-AuNPs to the aqueous surface, and above a threshold salt concentration, two-dimensional crystals are formed. The 10 nm PNIPAM-AuNPs form a highly ordered single layer with in-plane triangular structure, whereas the 5 nm capped NPs form short-range triangular structure that gradually becomes denser as salt concentration increases.