Ratiometric nanothermometer CaSc2O4: Nd3+ operating in biological window for deep-tissue photothermal therapy

Accurate and real-time temperature detection is an urgent requirement in photothermal therapy (PTT). In this work, the CaSc 2 O 4 : Nd 3+ nanorod with an average length of 150 nm has been determined to be an outstanding optical thermometer based on the fluorescence intensity ratio ( FIR ) of thermally coupled Stark sublevels of Nd 3+ : 4 F 3/2  →  4 I 11/2 transition, of which the excitation and emission wavelength are located in the first and second biological window respectively, resulting in an 8 mm penetration depth in the biological tissues. The maximal absolute sensitivity and relative sensitivity as well as the minimal temperature resolution of the present sample is 0.0008 K -1 , 0.0018 K -1 and 0.19  K respectively, which is superior to the vast majority of the same type thermometers. Besides that, the nanorods show a 7  K temperature increment after 180 s’ 808 nm-laser radiation, revealing its photothermal conversion capacity thanks to the considerable nonradiative relaxation processes among the metastable energy levels of Nd 3+ . These results indicate that Nd 3+ single-doped CaSc 2 O 4 nanorods can be utilized as the optical thermometer in the deep-tissue PTT process for real-time thermometry along with the function of light-to-heat conversion.