Stark splitting of intense red emission for Er3+ in Oh symmetry sites realizing optical temperature sensing in biological applications

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. Graphic abstract