Sodium Assists Controlled\nSynthesis of Cubic Rare-Earth\nOxyfluorides Nanocrystals for Information Encryption and Near-Infrared-IIb\nBioimaging

Rare-earth oxyfluoride (REOF) colloidal nanocrystals\n(NCs) suffer\nfrom a low photoluminescence efficiency due to their small size with\npoor crystallinity and a detrimental surface quenching effect. Herein,\nwe introduce an innovative approach that involves doping sodium ions\ninto REOF NCs to produce monodisperse, size-controllable, well-crystallized,\nand highly luminescent colloidal REOF core/shell NCs. The Na<sup>+</sup> doping allows for successfully synthesizing the cubic REOF NCs with\na tunable size from 6 to 30 nm. Further fabrication of the core/shell\nNCs doped with Na<sup>+</sup> results in enhancements up to 1062 (Ho<sup>3+</sup>), 1140 (Er<sup>3+</sup>), and 2212 (Tm<sup>3+</sup>) folds\nin upconversion luminescence and 17.7 folds (Er<sup>3+</sup>) in downconversion\nluminescence compared to that of core/shell NCs without doping Na<sup>+</sup> ions. These NCs were subsequently developed into multicolor\nluminescent inks, demonstrating significant potential application\nfor information security, and used for near-infrared-IIb (NIR-IIb)\n(1500–1700 nm) <i>in vivo</i> imaging, which exhibits\na high-resolution <i>in vivo</i> dynamic imaging capability\nwith a signal-to-noise ratio of 5.28. These results present the way\nto the controlled synthesis of efficient luminescent cubic LuOF: RE<sup>3+</sup>/LuOF core/shell NCs, expanding the toolkit of rare-earth\ndoped NCs in diverse applications such as advanced encoding encryption,\nvaried fluorescence imaging, and biomedicine.