Synergistic Ligand Engineering Suppresses Dye Aggregation Quenching and Photobleaching in Lanthanide Upconversion Nanoparticles

ABSTRACT Near‐infrared cyanine dyes are widely employed for sensitizing lanthanide upconversion luminescence (UCL), but generally suffer from aggregation‐caused quenching (ACQ) and photobleaching. Herein, we report a ligand engineering strategy utilizing pyridine‐2‐carboxylic acid (2PA) to competitively modify with cyanine dyes (e.g., IR808) on the lanthanide‐doped nanoparticles (e.g., Cs 2 NaYbF 6 :Er, Nd). Specifically, 2PA suppresses ACQ of dye via physical isolation, passivates surface defects to reduce lanthanide dopants quenching, and actively quenches singlet oxygen to enhance the photostability of the sensitized system. This synergy ultimately enhances the dye‐sensitized lanthanide UCL by over one order of magnitude and shows superior photostability under continuous stimulation. Remarkably, this strategy shows universality across multiple dye‐sensitized systems and demonstrates UCL enhancement and photostability improvement at the single‐particle level upon high‐power excitation. This work overcomes the fundamental bottlenecks in dye‐sensitized lanthanide systems, offering a facile strategy for designing high‐performance UCL nanoplatforms for versatile applications.