Two-dimensional assembly of gold nanoparticles grafted with charged-end-group polymers

Hypothesis: Introducing charged terminal groups to polymers that graft nanoparticles enable Coulombic control over their assembly by tuning the pH and salinity of their aqueous suspensions. Experiments: Gold nanoparticles (AuNPs) are grafted with poly (ethylene glycol) (PEG) terminated with CH3 (charge-neutral), COOH (negatively charged) or NH2 groups (positively charged), and characterized with dynamic light scattering, -potential, and thermal gravimetric analysis. Liquid surface X-ray reflectivity (XR) and grazing incidence small-angle X-ray scattering (GISAXS) are used to determine the density profile and in-plane structure of the AuNPs assembly at the aqueous surface.Findings: Assembly of PEG-AuNPs at the liquid/vapor interface is tunable by adjusting pH or salinity for COOH but less for NH2 terminals. The distinct assembly behaviors are attributed to the overall charge of PEG-AuNPs as well as PEG conformation. COOH-PEG corona is more compact than those of the other terminal groups, leading to a crystalline structure with a smaller superlattice. The net charge per particle depends not only on the PEG terminal groups but also on the cation sequestration of PEG and the intrinsic negative charge of the AuNP surface. [1] The closeness to overall charge neutrality, and hydrogen bonding in play, brought by NH2-PEG, drive NH2-PEG-AuNPs to assembly and crystallinity without additives to the suspensions.