Developing Upconversion (Yb3+–Er3+) and Downshifting (Yb3+–Ho3+–Er3+) PEG-Ylated Na3ZrF7 Degradable Nanoparticles as Ratiometric Luminescent Nanothermometers for Theranostic Applications

In this work, we report the synthesis of Na₃ZrF₇ nanoparticles as an appealing host matrix for lanthanide ions for both upconversion (bidoped, sensitizer-activator Yb³⁺–Er³⁺ in the vis and NIR-I regions) and downshifting (tridoped, sensitizer-activator-activator Yb³⁺–Ho³⁺–Er³⁺ in the NIR-II region) ratiometric luminescence nanothermometry in the physiological temperature range (293.15–323.15 K). We explored three different routes to reveal the most favorable synthetic approach for bioapplication purposes as well as investigated and characterized the temperature-dependent photoluminescence (PL) properties of the obtained nanoparticles. Furthermore, a biocompatible DSPE-PEG₂₀₀₀-based layer was incorporated to render the nanoparticles water-dispersible and to subsequently evaluate their degradation-induced in vitro toxicity toward HeLa and Normal Human Dermal Fibroblast (NHDF) cells. Additional in vitro tests were conducted to preliminarily evaluate the cellular uptake of the nanoparticles for future biological applications. We show that the synthetic conditions of the Na₃ZrF₇ nanoparticles and the solvent environment influence their morphology, crystalline phase, and PL emission profiles. To the best of our knowledge, this is the first effort at an extensive systematic synthesis approach for nanosized core-only and core–shell(s) Na₃ZrF₇ materials as ratiometric luminescent thermometers and the first reported temperature-sensing performance via Yb³⁺–Er³⁺ (NIR-I-to-vis) upconversion and Yb³⁺–Ho³⁺–Er³⁺ (NIR-I-to-NIR-II) downshifting mechanisms for this host material, both in cyclohexane and in an aqueous environment.