Dye-to-Er<sup>3+</sup> Direct Energy Transfer for\nEnhancing Up- and Down-conversion Luminescence in Sub-10 nm NaErF<sub>4</sub>

Dye sensitization enhances the luminescence\nof lanthanide\nnanoparticles\nby improving light-harvesting. Typically, Yb<sup>3+</sup> serves as\nan energy bridge but absorbs at a single transition, limiting dyes’\noptions (λ<sub>ex</sub> > 700 nm) due to the spectral overlap\nrequirement. In contrast, the emitter Er<sup>3+</sup> spans energy\nlevels from UV to NIR, making it ideal for multicolor excitation.\nWe developed a strategy to directly sensitize Er<sup>3+</sup> upconversion\n(UCL) and downconversion luminescence (DCL) by using cyanine dyes.\nCy5 demonstrated the greatest enhancement, achieving a 1942-fold UCL\nand 70-fold DCL increase compared to nanoparticles alone (Er-NPs)\nunder 980 nm excitation. Smaller Er-NPs exhibited brighter dye-sensitized\nluminescence due to enhanced interfacial energy transfer. A 2 nm inert\nshell produced the brightest UCL, while thicker shells improved DCL.\nDye-sensitized Er<sup>3+</sup> emissions at <sup>2</sup>H<sub>11/2</sub> (525 nm) and <sup>2</sup>P<sub>3/2</sub> (408 nm) enabled temperature\nmonitoring with a maximum sensitivity (<i>S</i><sub>a</sub>) of 3.69%/K. This approach holds significant potential for optical\ntemperature sensing and medical imaging.