Flexible Composite Fluorescent Optical Fiber Sensor Embedded with Li₂HfO₃:Er³⁺/Yb³⁺ for Physiological Temperature Monitoring

Abstract Flexible temperature sensors are crucial for monitoring physiological parameters in smart human wearable devices, however, there is a challenge to realize reproducible, sensitive, real‐time, and in‐situ temperature sensing simultaneously. Herein, Er 3+ /Yb 3+ co‐doped Li 2 HfO 3 phosphor thermometries have been designed and prepared, which demonstrate bright upconversion luminescence and precise temperature sensing capabilities. The temperature sensing characteristics of thermally coupled energy states in Li 2 HfO 3 :Er 3+ , Yb 3+ are explored based on the fluorescence intensity ratio technique, with a maximum relative sensitivity of 0.875% K −1 and a temperature uncertainty of 0.017 K at 300 K. In addition, flexible composite fluorescent optical fiber temperature sensors have been fabricated, and the fiber with a double‐cladding structure is employed to monitor human body temperature and airflow heat with satisfying performances. This work not only provides a promising rare earth phosphor thermometry but also opens a new avenue for the development of flexible optical fiber temperature sensors for health management.