Binary Ligand-Mediated Photochromic Tuning of Lanthanide-Doped Upconversion Nanoparticle Conjugates

Abstract

The ability to tune photochromic features of nanoparticle (NP)-ligand conjugates can vastly extend the utility of dynamic materials in the fields of drug delivery, bioimaging, and data storage. Herein, the photochromic tuning of lanthanide-doped upconversion nanoparticle (UCNP)-spiropyran (SP) conjugates through a binary ligand strategy is reported. It is found that the content of SP ligand on the surface of UCNPs can be rationally controlled by an SP-to-oleate ligand exchange reaction, allowing for regulating the steric congestion in the ligand layer. A highly congested SP layer not only leads to an ultralong lifetime for the purple open-ring state but also results in improved red fluorescence under UV excitation. The enhancement in steric congestion further enables increased activation energy for the ring-opening reaction for the bound SP ligand, permitting the generation of high-resolution self-erasing patterns in a UCNP-SP conjugate-doped polydimethylsiloxane film by local irradiation with either UV or near-infrared light. The findings provide new insights into the optical tuning of nanoconjugates by controlling the conformational dynamics of their mixed ligand layers.