Sub-3 nm Lanthanide-Doped Double Perovskite Quantum Dots: A Multifunctional Platform for Thermal-Enhanced Upconversion Nanothermometry and Magnetic Resonance Imaging.

Conventional perovskite microcrystals commonly exhibit luminescence thermal quenching and functional singularity, severely limiting their practical utility. The inherent toxicity and instability of lead-based variants further restrict their biomedical applicability. To address these challenges, we synthesized ultrasmall (∼2.5 nm) lead-free Cs2NaGdCl6:Yb3+,Er3+ double perovskite quantum dots (QDs) through an optimized variable-temperature hot-injection approach. These QDs display an anomalous thermal enhancement in upconversion luminescence, attributed to temperature-dependent desorption of surface -OH groups, which enables highly sensitive optical nanothermometry. Simultaneously, they exhibit efficient broadband self-trapped exciton emission under UV excitation. Following surface modification with 2-aminoethylphosphonic acid (AEP), the QDs acquire good hydrophilicity and biocompatibility. Moreover, the Gd3+-rich composition confers outstanding T1-weighted magnetic resonance imaging (MRI) capability with a high relaxivity of 8.23 mM-1s-1. The successful demonstration of in vivo tumor imaging confirms their potential as effective MRI contrast agents. This study establishes ultrasmall lead-free double perovskite QDs as a versatile multifunctional platform integrating nanothermometry and bioimaging functionalities.