Erbium Single‐Band Nanothermometry in the Third Biological Imaging Window: Potential and Limitations

Abstract 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 ⁴I_13/2 → ⁴I_15/2 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.