1532 nm-driven multimode optical thermometry within biological window enabled by Er3+ self-sensitized upconversion luminescence

Abstract

The development of optical thermometers operating within the biological windows is crucial for safe and precise deep-tissue temperature sensing. This work reports a self-sensitized Er³⁺-doped LaScO₃ phosphor for upconversion (UC) luminescence and thermometry under 1532 nm excitation which lies within the longer wavelength region of the second biological window. The UC mechanisms of the samples involving multi-photon processes are systematically investigated. Multiple optical thermometry is developed through the fluorescence intensity ratio (FIR) technique using transitions from Er³⁺:⁴F_9/2,⁴I_9/2 and ⁴I_11/2 levels. While the thermometers based on thermally coupled Stark sublevels show reliable performance, a significantly improved sensitivity of 5.22% K⁻¹ is achieved by employing the non-thermally coupled energy pair ⁴F_9/2 and ⁴I_9/2. The accuracy of the thermometric readings is validated against those of a standard infrared thermometer. Furthermore, ex vivo penetration experiments demonstrate the superior potential of the ⁴I_11/2 → ⁴I_15/2 transition for temperature probing in deep tissues. These findings demonstrate that LaScO₃:Er³⁺ is a high-performance candidate for deep-tissue optical thermometry.