Interfacial Self‐Assembly of Gold Nanoarrows into Macroscopic Monolayer Films with Tunable Orientations and Reversible Color Switching

Liquid–liquid interfacial self‐assembly represents a cost‐effective, adaptable, and efficient strategy to construct 2D nanoparticle assemblies, especially in generating large‐area, monolayer nanoparticle films with tunable architectures. However, controllable self‐assembly of nanoparticles with complex morphologies into macroscopic, functional monolayer films remains to be explored. This study demonstrates ligand‐mediated self‐assembly of gold nanoarrows (GNAs) into centimeter‐scale, homogeneous monolayer films with tunable orientations at the oil‐water interface. The particle orientation of the macroscopic monolayer films can be readily modulated by adjusting the relative adsorbed amounts of the hydrophilic and hydrophobic ligands, yielding monolayer GNA assemblies with two distinct orientations (namely, types I and II). Interestingly, after transferred onto a transparent polydimethylsiloxane substrate, the type II monolayer films exhibit a switchable color response toward the ethanol solvent due to reversible modulation of the interparticle spacing arising from switchable shrinkage and swelling of the polymeric ligand. Additionally, solvent‐responsive pattern encryption with rapid color switching and excellent cycle stability is achieved using the type II monolayer films as the active components, indicating their promising potential in information encryption and anti‐counterfeiting applications.