Exploring Ho3+‐Yb3+ and Nd3+‐Yb3+‐Ho3+ Doped LiLuF4 Nanothermometers for NIR‐II Imaging: Elucidation of the LiLuF4 Structure via 3D Electron Diffraction

Temperature measurements in biological systems deliver important information about the occurrence and location of diseases such as cancer. Also, certain therapies rely on local heating of tumor tissue, and therefore, feedback on the heating of nearby healthy tissue is valuable. However, classical thermometers are not suitable for precise temperature detection in living systems. Because of this, alternative ways to measure temperature, such as luminescence‐based thermometry, have been gaining interest and relevance for medical diagnostics and treatment support. In this work, LiLuF4‐based nanocrystals are employed to develop novel ratiometric thermometers based on the rarely explored Ho3+‐Yb3+ and Nd3+‐Yb3+‐Ho3+ systems. After coating with a phospholipid bilayer, these thermometers show very good stability and thermometric performance in water and in skin‐mimicking tissue phantoms. The proposed Ho‐Yb‐Nd thermometry system allows constructing three separate thermometers within one material under one excitation wavelength. As there has been no scheelite single‐crystal structure of LiLuF4 reported to date, its single‐crystal structure is determined with the use of three‐dimensional Electron Diffraction analysis (3D ED) employing highly crystalline nanoparticles (<50 nm), which emphasizes the exciting potential of 3D ED for determining the crystal structure of inorganic nanoparticles.