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

Abstract

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 <i>T</i>₁-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.