Deep-Tissue Temperature Sensing Realized in BaY₂O₄:Yb³⁺/Er³⁺ with Ultrahigh Sensitivity and Extremely Intense Red...

In this paper, BaY₂O₄:Yb³⁺/Er³⁺, a high efficient red upconversion (UC) material, is first utilized as an optical thermometer in the biological window, accomplished through the fluorescence intensity ratio (FIR) of thermally coupled Stark sublevels of ⁴F_9/2 (FIR_(654/663)). The maximum absolute sensitivity of FIR_(654/663)) is 0.19% K^-1 at 298 K, which is much higher than most previous reports about FIR-based temperature sensors located in the biological windows. More importantly, the groove of FIR_(654/663) for thermometry is nicely located in the physiological temperature range, indicating its potential thermometry application value in biomedicine. Furthermore, a simply ex vivo experiment is implemented to evaluate the penetration depth of the red emission in biological tissues, revealing that a detection depth of 6 mm can be achieved without any effect on the FIR values of I₆₅₄ to I₆₆₃. Beyond that, the temperature sensing behaviors of the thermally coupled levels ²H_11/2 and ⁴S_3/2 (FIR_(523/550)) are also investigated in detail. In the studied temperature range, the absolute sensitivity of FIR_(523/550) monotonously increases with the rising temperature and reaches its maximum value 0.31% K^-1 at 573 K. All the results imply that BaY₂O₄:Yb³⁺/Er³⁺ is a promising candidate for deep-tissue optical thermometry with high sensitivity.