Controlled Shell Growth Enables High Quantum Yield Upconverting Nanoparticles for Thermal Sensing and Bioimaging

Abstract Lanthanide‐doped upconversion nanoparticles (UCNPs) are versatile materials for biomedical applications owing to their ability to convert low‐energy near‐infrared (NIR) light into higher‐energy emission. This nonlinear optical process allows for deep tissue penetration with minimal autofluorescence, scattering, and phototoxicity, making UCNPs ideal for dual‐function bioimaging and nanoscale temperature sensing. However, their use is often limited by low luminescence quantum yields ( QY ). Here, a novel synthesis strategy is reported to obtain NaYbF 4 :Tm 3+ @NaYF 4 core@shell UCNPs with controlled shell thickness to reduce surface quenching. The optimized nanoparticles exhibit strong upconversion emissions, high QYs up to 5.5%, and long luminescence lifetimes reaching 1.2 ms. Thicker shells significantly enhance performance, placing this material among the most efficient Tm‐based UCNP systems reported for biological applications. Dual ratiometric thermometric parameters enable accurate, contactless temperature readouts in the 30–60 °C range, with each parameter showing optimal thermal sensitivity at different subranges. Cytotoxicity assays confirm high biocompatibility (93% cell viability), and confocal super‐resolution (SR) imaging reveals strong, specific UC luminescence signals in NIH‐3T3 fibroblasts. These results demonstrate the potential of nanocrystal engineering in enhancing the optical performance of UCNPs and highlight their potential as a versatile material for dual‐function thermal sensing and imaging in biological environments.