Stark splitting of intense red emission for Er 3+ in O h symmetry sites realizing optical temperature sensing in biological applications

Abstract Optical temperature sensing based on the fluorescence intensity ratio (FIR) of red emission for lanthanide ions holds significant relevance in non‐contact temperature measurement for biological application. In this study, the perovskite‐structured KZnF 3 is utilized as a host material for Er 3+ to achieve a high‐purity upconversion (UC) red emission. The observed Stark splitting of the red emission peak provides evidence of the energy level splitting of Er 3+ . Group theory is employed to decompose the spectral branching of Er 3+ under the point group symmetry of KZnF 3 , allowing for the derivation of Stark splitting energy levels induced by the crystal field effect. The optical temperature‐sensing behavior of the red UC luminescence was investigated, specifically examining the FIR of the splitting sub‐peaks, which exhibited an exponential relationship with temperature. The KZnF 3 : Yb 3+ , Er 3+ demonstrated a relative sensitivity ( S r ) of 0.00182%·K −1 at 298 K, highlighting its excellent response to temperature. Ex vivo bio‐thermometry experiments conducted on chicken breast validated the material's ability to penetrate biological tissues and showed its significant sensitivity of the FIR to temperature. These results establish KZnF 3 :Yb 3+ , Er 3+ as a promising material for optical thermometry in various biological applications.