Breaking the Sensitization-Passivation Trade-Off in Dye-Sensitized Upconversion Nanoparticles through LiLuF4-Enabled Directional Energy Transfer.

Dye-sensitized upconversion nanoparticles (UCNPs) are promising for biological and photonic applications, but their performance is limited by Yb3+-Er3+ back-energy transfer and excitation migration to surface quenchers. In conventional hexagonal NaY(Gd)F4 hosts, these coupled losses restrict the optimal Yb3+ content to approximately 20-30%, preventing the use of densely doped Yb3+-Er3+ cores for dye sensitization. Here, we overcome this limitation through host-topology codesign in LiYbF4:Er@LiLu1-xYbxF4@LiLu1-yNdyF4 nanoparticles. The tetragonal LiLuF4-type lattice enables a core containing 98% Yb3+ and 2% Er3+ with reduced concentration quenching and a more favorable net energy-transfer balance than hexagonal NaYF4. An Nd3+-rich outer shell captures dye-derived excitation, while a Yb3+-rich intermediate shell directs energy inward and suppresses surface dissipation. These elements are mutually enabling: the host supports the high-Yb3+ core, and the shell topology converts it into an efficient dye-sensitized emitter. The resulting architecture produces a >150-fold enhancement in 808-nm-excited green upconversion luminescence relative to dye-sensitized low-Yb3+ core nanoparticles. As a proof of concept, reaction-product-dependent turn-on signals enable discrimination of ortho-, meta-, and para-mercaptobenzoic acid at 1 μg mL-1. This work establishes host-topology codesign as a general strategy for controlling directional energy transfer in lanthanide nanostructures.