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

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 approximate to 1190 nm (NIR-II) and erbium (Er3+) emission at approximate to 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 (Y2O3, 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 degrees C in a physiological temperature range (25-65 degrees 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.