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.