Bio‐Temperature Sensing via Thermal Crossover Between <i> 4f ⁵ 5d ¹ </i> and <i> ⁵ D _J </i> States of Sm ²⁺ in Nanocrystals

ABSTRACT The unique luminescent properties of lanthanides have driven significant advances in remote temperature sensing. However, conventional intensity ratio thermometers governed by Boltzmann equilibrium require high temperatures, often exceeding safe restrictions for biological applications. Meanwhile, luminescence intensity ratio thermometers based on multiple emission centers also suffer from poor stability and batch‐to‐batch variation, undermining measurement reliability. Here, we introduce a novel single‐emission‐center BaFCl:Sm 2+ thermometer specifically for applying in biological sensing. Kinetic rate equations and experimental results indicate that the thermal crossover between the 4f 5 5d 1 and 5 D J states of Sm 2+ is mainly responsible for the thermal luminescence performance. More importantly, our thermometer shows high relative sensitivity (4.57% K −1 , 293 K) and excellent temperature resolution (0.13, 293 K) in the physiological range of 293–333 K. Furthermore, laser spot heating and intracellular experiments confirm this reliable temperature‐dependent response at the luminescence wavelengths of 643 and 688 nm. These results highlight the promise for precise temperature sensing in biological systems.