Ultrasensitive optical thermometer based on abnormal thermal quenching Stark transitions operating beyond 1500 nm
Guotao Xiang, Meng‐Lin Yang, Qing Xia, Sha Jiang, Yongjie Wang, Xianju Zhou, Li Li, Li Ma, Xiaojun Wang, Jiahua Zhang
Abstract
Abstract Light with wavelength longer than 1500 nm has great potential to afford deep bio‐tissue penetration due to its extremely weak photon scattering and undetectable autofluorescence in vivo . Here, in order to satisfy the requirements for thermometry during the tumor hyperthermia process, an ultrasensitive optical thermometer operating beyond 1500 nm is developed by employing the thermally coupled Stark sublevels of Er 3+ : 4 I 13/2 → 4 I 15/2 transition based on fluorescence intensity ratio ( FIR ) technology in Yb 3+ and Er 3+ codoped BaY 2 O 4 . Compared with the typical upconversion (UC) material β‐NaYF 4 : Yb 3+ /Er 3+ and Y 2 O 3 : Yb 3+ /Er 3+ , BaY 2 O 4 : Yb 3+ /Er 3+ shows more intense red Er 3+ : 4 F 9/2 → 4 I 15/2 transition and 1.5 μm near‐infrared (NIR) Er 3+ : 4 I 13/2 → 4 I 15/2 transition induced by its larger phonon energy and higher quenching concentration of Er 3+ . An equivalent four‐level model is proposed to investigate the temperature characteristics of the NIR emission, from which four Stark transitions are separated from the raw spectra, named α, β, γ, and δ respectively. Then, the NIR thermal sensing performance have been developed by utilizing the FIR of I β to I α and I γ to I α . More importantly, an ultra‐high sensitivity for optical thermometry has been obtained through the combination of transition β and γ, especially in the physiological temperature region. Furthermore, the detection depth of NIR light in bio‐tissues is assessed by an ex vivo test, demonstrating that the maximal detection depth of NIR emission can reach to 8 mm without any influence on optical thermometry. These findings indicate that Yb 3+ and Er 3+ codoped BaY 2 O 4 is a remarkable contender for optical thermometry in deep tissue with ultra‐high sensitivity.