Dictating nanorod assembly via the hydrophilic/hydrophobic transition of polymer ligands based on Hofmeister effect

Self-organization of anisotropic nanostructures into polymer/inorganic superstructures are of great research interest for various applications owing to their enhanced directional properties, such as plasmonics and catalysis. However, the controllable assembly and oriented growth of nanorods in solution without external template assistance remains a great challenge. Here, we report a facile Hofmeister-effect-based assembly strategy for fabricating ordered arrangements of polyethylene glycol-tethered gold nanorods (AuNRs@PEG) with controlled orientation and dimension. By adjusting the concentration of typical kosmotropic ions, we can precisely modulate the hydrophilic-to-hydrophobic transition of PEG ligands by the dehydration of PEG polymer ligands, enabling controlled nucleation and growth of nanorods in solution. The weak hydrophobicity of PEG ligands drives lying-down nanorods to form 2D smectic liquid crystalline phases, which serves as templates for hierarchical growth into 3D liquid crystalline phases. In contrast, the strong hydrophobicity induces standing nanorods to form hexagonally packed monolayer superlattices and face-centered cubic (FCC) superlattices. Additionally, we achieve in-situ transitions between solid-solid superstructures from smectic liquid crystalline to hexagonal packing by altering environmental conditions. This work provides a significant enhancement to the resources available for the fabrication of diverse anisotropic structures and reconfigurable materials.