Programmable Photoresponsive Behavior of Gold Nanoparticles Enabled by Dynamic Covalent Chemistry

Light-triggered self-assembly of gold nanoparticles (AuNPs) has garnered significant attention due to its high spatiotemporal precision, reversible tunability, and potential applications in biosensing, drug delivery, and photothermal therapy. Nevertheless, conventional AuNPs functionalization is achieved by robust thiyl–gold bonds, which render subsequent thiol ligand exchange extremely slow and hinder precise control over ligand surface coverage. Consequently, the irradiation wavelength for the existing systems is confined to a single, fixed value. Herein, a modular platform was constructed by functionalizing AuNPs with hydrazone-terminated ligands (PHT AuNPs). The chemical structure of the surface ligands on the PHT AuNPs was further modified via hydrazone exchange with different azobenzene hydrazides (i.e., azobenzene-hydrazide BAB, fluorinated azobenzene-hydrazide FAB, and a nonphotoresponsive hydrazide NPH). Experimental results demonstrated that exchanging with 5 equiv of BAB or FAB hydrazides yields binary mixed-ligand monolayers (BAB0.60/PHT0.40 or FAB0.34/PHT0.66 AuNPs), which undergo reversible assembly under ultraviolet or green light irradiation, respectively. Furthermore, the photoresponsiveness of AuNPs can be erased by subsequent exchange with NPH. This work establishes a hydrazone exchange approach for postmodification of nanoparticle surfaces, enabling the rational design of nanomaterials with programmable, multilight-responsive behaviors for future photonic devices and intelligent nanotechnologies.