Designing complex functional nanostructures requires a high level of control over the individual system components, and this relies on a deep understanding of mechanisms and dynamic processes involved in their assembly. Amphiphilic polymers are an ideal system to achieve this goal, since they form a variety of hierarchical nanostructures and act as stimuli-responsive ligands in nanoparticle self-assembly. In this work, the addition of NaCl to an aqueous suspension of gold nanoparticles capped with amphiphilic polymer, poly(N-isopropylacrylamide) (pNIPAM-capped Au nanoparticles) promotes liquid/liquid phase separation by confining the polymer to a lower-density salt-deficient aqueous phase. The process is manifested by self-assembly of the aqueous polymer solution into globular structures, where the polymer-capped Au nanoparticles first reside at the interface between the NaCl-rich bulk solution and the pNIPAM-containing solution. As the globular structures assemble, their surface is decorated with polymer-grafted nanoparticles exhibiting hexagonal packing. Their interiors are filled with a lower density aqueous solution of pNIPAM. As a result, these buoyant decorated globes float to the liquid/air interface and then collapse, forming two-dimensional interfacial crystalline nanoparticle domains.
The evolution of this process was captured using liquid phase scanning transmission electron microscopy and conventional transmission electron microscopy in combination with surface-sensitive scattering techniques. Understanding how these initial structures form is enabling more precise controlling of the self-assembly of nanoparticles in aqueous solutions.
A. Londoño-Calderon, W. Wang, J. J. Lawrence, W. Bu, D. Vaknin , T. Prozorov, “Salt-induced Liquid-Liquid Phase Separation and Interfacial Crystal Formation in poly(N-isopropylacrylamide)-Capped Gold Nanoparticles” Journal of Physical Chemistry C, 2021, 125 (9), 5349–5362, https://doi.org/10.1021/acs.jpcc.0c11307.