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 CaSc₂O₄: Nd³⁺ 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 Nd³⁺: ⁴F_3/2 → ⁴I_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 Nd³⁺. These results indicate that Nd³⁺ single-doped CaSc₂O₄ 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.