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.

中文翻译：

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聚（N-异丙基丙烯酰胺）包覆的金纳米粒子的二维结晶

表面敏感的X射线反射率和掠入射小角度X射线散射揭示了聚合物包覆的金纳米颗粒（接枝有聚（N-异丙基丙烯酰胺）；PNIPAM–AuNPs）在水悬浮液/蒸气界面处自组装并结晶。柠檬酸盐稳定的AuNPs（标称直径为5和10 nm）被6 kDa PNIPAM-硫醇进行配体交换，从而在AuNPs周围形成了电晕刷，这些电刷非常稳定并分散在水性悬浮液中。令人惊讶的是，在10–35°C的温度范围内，没有观察到对PNIPAM–AuNPs的表面富集或自组装产生热敏效应的明确证据。但是，向悬浮液中添加简单的盐（在这种情况下为NaCl）会引起PNIPAM–AuNPs向水表面迁移，并且在阈值盐浓度以上会形成二维晶体。10 nm PNIPAM–AuNPs形成具有平面内三角结构的高度有序的单层，