Self‐Reduced Eu ²⁺ /Eu ³⁺ Co‐Doped Phosphors for High‐Sensitivity Flexible Optical Fiber Sensing

ABSTRACT Conventional optical thermometry based on dual‐valent Eu 2+ /Eu 3+ often necessitates complex synthesis under reducing atmospheres and suffers from limited sensitivity. Additionally, typical optical thermometric materials are predominantly in powder form and lack suitable application carriers. Herein, a highly sensitive optical thermometry is developed based on dual‐valent Eu 2+ /Eu 3+ emissions in a self‐reduced Ba 2 ZnSi 2 O 7 (BZSO) host, synthesized conveniently in air without a reducing atmosphere. The fluorescence intensity ratio of the thermally coupled Eu 2+ broadband and Eu 3+ sharp‐line emissions enables temperature sensing, achieving a maximum relative sensitivity ( S r ) of 2.92% K −1 at 396 K. Then, strategic Sr 2+ substitution for Ba 2+ induces lattice contraction, strengthening the crystal field around Eu 2+ and simultaneously enhancing the thermal stability of Eu 3+ emission and the temperature sensitivity. The optimized B 1.95 S 0.05 ZSO:0.07Eu 2+ ,Eu 3+ achieves a superior S r of 3.93% K −1 at 389 K. Furthermore, the optimized phosphor is incorporated into a polydimethylsiloxane to fabricate a flexible and stretchable optical fiber. The resulting fiber conforms closely to human skin, exhibiting great potential for physiological temperature monitoring, and also enables real‐time, in situ temperature sensing in aquatic environments. This work presents a facile and effective strategy for designing high‐performance, flexible optical thermometers for environmental and biomedical applications.