LAUSR

Near‐infrared (NIR) nanothermometers are sought after in biomedicine when it comes to measuring temperatures subcutaneously. Yet, temperature sensing within the third biological imaging window (BW‐III), where the highest contrast images can be obtained, remains relatively unexplored. Here, LiErF4/LiYF4 rare‐earth nanoparticles (RENPs) are studied as NIR nanothermometers in the BW‐III. Under 793 nm excitation, LiErF4/LiYF4 RENPs emit around 1540 nm, corresponding to the 4I13/2 → 4I15/2 radiative transition of Er3+. The fine Stark structure of this transition allows to delineate intensity regions within the emission band that can be used for single‐band ratiometric nanothermometry. These nanothermometers have a relative temperature sensitivity of ≈0.40% °C−1. The temperature‐dependent energy transfer to the surrounding solvent molecules plays a significant role in the thermometric properties of the RENPs. In addition, Ce3+ ions are doped in the core of the RENPs to examine whether it affects the NIR emission and temperature sensitivity. Ce3+ at 1 mol% marginally influences the downshifting emission intensity of the RENPs, yet increases the relative thermal sensitivity to ≈0.45% °C−1. Furthermore, Ce3+ quenches the visible upconversion emission of the RENPs. Together, LiErF4:Ce3+/LiYF4 RENPs enable single‐band photoluminescence nanothermometry in the BW‐III, with the future possibility of its integration within multifunctional decoupled theranostic nanostructures.