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

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 Ag₂ 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 Ag₂ S to Yb³⁺ 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 Yb³⁺ 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.