NIR‐II/III Luminescence Ratiometric Nanothermometry with Phonon‐Tuned Sensitivity

Abstract Luminescence nanothermometers are promising for noninvasive, high resolution thermographics ranging from aeronautics to biomedicine. Yet, limited success has been met in the NIR‐II/III biological windows, which allow temperature evaluation in deep tissues. Herein, a new type of phonon‐based ratiometric thermometry is described that utilizes the luminescence intensity ratio (LIR) between holmium (Ho³⁺) emission at ≈1190 nm (NIR‐II) and erbium (Er³⁺) emission at ≈1550 nm (NIR‐III) from a set of oxide nanoparticles of varying host lattices. It is shown that multi‐phonon relaxation in Er³⁺ ions and phonon‐assisted transfer process in Ho³⁺ ions play a significant role in LIR determination through channeling the harvested excitation energy to the corresponding emitting states. As a result, temperature sensitivity can be tuned by the dominant phonon energy of host lattice, thus endowing aqueous yttrium oxide (Y₂O_3, 376 cm⁻¹) nanoparticles to have a relative temperature sensitivity of 1.01% K⁻¹ and absolute temperature sensitivity of 0.0127 K⁻¹ at 65 °C in a physiological temperature range (25–65 °C). And their temperature sensing for biological tissues is further explored and the influence of water and chicken breast on thermometry is discussed. This work constitutes a solid step forward to build sensitive NIR‐II/III nanothermometers for biological applications.