Stark splitting of intense red emission for Er³⁺ in O_h 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 KZnF₃ is utilized as a host material for Er³⁺ 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 Er³⁺. Group theory is employed to decompose the spectral branching of Er³⁺ under the point group symmetry of KZnF₃, 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 KZnF₃: Yb³⁺, Er³⁺ 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 KZnF₃:Yb³⁺, Er³⁺ as a promising material for optical thermometry in various biological applications. Graphic abstract: (Figure presented.).