Universal upconversion amplification via 4d–4f hybridization in Ln3+/MoO42− co-doped fluorapatite: mechanism and applications

Abstract

Lanthanide-doped fluorapatite (FAp:Ln³⁺) upconversion luminescence (UCL) probes exhibit good biocompatibility but suffer from low quantum yields under near-infrared (NIR) excitation. Here, we propose a universal MoO₄²⁻ doping strategy that achieves 74-fold green UCL enhancement in FAp:Yb/Er/Mo (FYEM) under 980 nm excitation (quantum yield: 0.004% at 50 W cm⁻²). Remarkably, stable Ln³⁺/MoO₄²⁻ dimers induce 4d–4f orbital hybridization, as validated by density functional theory (DFT) and X-ray photoelectron spectroscopy (XPS). This mechanism extends to self-sensitized systems: FAp:Er/Mo (FEM) achieves 156-fold UCL enhancement under 980 nm excitation, while FAp:Nd/Mo (FNM) integrates 84-fold UCL amplification with photothermal synergy (ΔT = 53.5 °C). Critically, the optimized FYEM material enables dual-mode imaging, with 808/980 nm excitation UCL and NIR-II downconversion luminescence for deep-tissue imaging, besides outstanding thermal sensitivity (S_A = 1.04% K⁻¹@323 K). This work establishes a biocompatible, multi-wavelength platform for deep-tissue diagnostics, photothermal therapy, and multimodal sensing and imaging.