Lattice-Regulated CaNb₂O₆ Niobate for High-Performance NIR-II Luminescent Thermometry and Flexible Optical Sensing

Rare-earth-doped luminescent materials show great promise for noncontact temperature sensing. However, their practical implementation is often hindered by poor dopant compatibility and limited integration into flexible devices. Herein, we report a Li⁺-mediated structural engineering strategy that significantly enhances the dopant tolerance of a CaNb₂O₆ (CNO) niobate host. The incorporation of Li⁺ ions facilitates lattice contraction and charge compensation, enabling high-loading and homogeneous incorporation of multiple rare-earth ions (Er³⁺, Ho³⁺, Yb³⁺, and Nd³⁺) while effectively suppressing impurity formation. By strategically engineering multichannel energy transfer pathways, particularly a phonon-assisted Yb³⁺ → Er³⁺/Ho³⁺ sensitization network, the Yb³⁺-sensitized CNO: Er³⁺, Ho³⁺ system exhibits superior near-infrared II thermometric performance under 980 nm excitation, achieving a high relative sensitivity of 0.92% K^-1 and a temperature resolution of 0.66 K in the biologically relevant range (313-373 K). Furthermore, the CNO: Yb³⁺, Er³⁺, Ho³⁺ phosphor is successfully incorporated into a flexible polydimethylsiloxane film, demonstrating stable and reliable temperature sensing in aqueous environments. A prototype bendable optical fiber sensor is also fabricated, showcasing its great potential for wearable health monitoring and biomedical thermal imaging. This work establishes a systematic paradigm from host lattice structural regulation to flexible device integration, providing a novel and generalizable strategy for advancing intelligent thermal sensing technologies.