Supramolecular structuring of nanoparticle ligand shells revealed by liquid-phase AFM

Polymer grafts are a ubiquitous component of nanocrystal (NC) ligand shells and impart critical functionalities, including biocompatibility and colloidal stability. Although many techniques have been used for elucidating the supramolecular structure of polymer chains once grafted to the NC surface, few of these are able to provide a complete picture of the ligand shell under experimentally relevant conditions ( e.g ., low polymer densities, various solvent environments). Here, we use high-resolution atomic force microscopy imaging along with coarse-grained molecular dynamics simulations to directly image polymers grafted onto single Ag NCs in both air and water to better understand the ligand-solvent interactions that dictate ligand shell structuring. For hydrophobic grafts under poor solvent conditions, the grafts self-associate to form a patchy shell, as previously reported. However, for hydrophilic grafts under good solvent conditions, this patchy morphology persists and is observed even for polyethylene glycol-functionalized NCs (i.e. PEGylated) in water. Polymer structuring is also observed amongst assembled NCs, with the formation of polymer bridges forming across patches. These unexpected results suggest that surface patchiness may be more widespread in colloidal NC systems than previously assumed, and this surface heterogeneity could play an important role in governing collective NC phenomena such as self-assembly.