Ultrasensitive NIR‐II Ratiometric Nanothermometers for 3D In Vivo Thermal Imaging

Abstract Luminescent nanothermometry, particularly the one based on ratiometric, has sparked intense research for non‐invasive in vivo or intracellular temperature mapping, empowering their uses as diagnosis tools in biomedicine. However, ratiometric detection still suffers from biased sensing induced by wavelength‐dependent tissue absorption and scattering, low thermal sensitivity ( S r ), and lack of imaging depth information. Herein, this work constructs an ultrasensitive NIR‐II ratiometric nanothermometer with self‐calibrating ability for 3D in vivo thermographic imaging, in which temperature‐insensitive lanthanide nanocrystals and strongly temperature‐quenched Ag 2 S quantum dots are co‐assembled to form a hybrid nanocomposite material. Precise control over the amount ratio between two sub‐materials enables the manipulation of heat‐activated back energy transfer from Ag 2 S to Yb 3+ in lanthanide nanoparticles, thereby rendering S r up to 7.8% °C −1 at 43.5 °C, and higher than 6.5% °C −1 over the entire physiological temperature range. Moreover, the luminescence intensity ratio between two separated spectral regions within the narrow Yb 3+ emission peak is used to determine the depth information of nanothermometers in living mice and correct the effect of tissue depth on 2D thermographic imaging, and therefore allows a proof‐of‐concept demonstration of accurate 3D in vivo thermographic imaging, constituting a solid step toward the development of advanced ratiometric nanothermometry for biological applications.