Chemical Transformation of Lead Halide Perovskite into Insoluble, Less Cytotoxic, and Brightly Luminescent CsPbBr₃/CsPb₂Br₅ Composite Nanocrystals for Cell Imaging

Lead halide perovskite nanocrystals (NCs) have been widely investigated owing to their potential applications as optoelectronic devices. However, these materials suffer from poor water stability, which make them impossible to be applied in biomedicine. Here, insoluble CsPbBr₃/CsPb₂Br₅ composite NCs were successfully synthesized via simple water-assisted chemical transformation of perovskite NCs. Water plays two key roles in this synthesis: (i) stripping CsBr from CsPbBr₃/Cs₄PbBr₆ and (ii) modifying the coordination number of Pb²⁺ (six in CsPbBr₃ and Cs₄PbBr₆ vs eight in CsPb₂Br₅). The as-prepared CsPbBr₃/CsPb₂Br₅ composite NCs not only retain the photoluminescence quantum yield (up to 80%) and a narrow full width to half-maximum of 16 nm, but also present excellent water stability and low cytotoxicity. With these properties, the CsPbBr₃/CsPb₂Br₅ composite NCs were demonstrated as efficient fluorescent probes in live HeLa cells. We believe that our finding not only provides a new method to prepare insoluble, narrow-band, and brightly luminescent CsPbBr₃/CsPb₂Br₅ composite NCs, but also extend the potential applications of lead halides in biomedicine.