Dye-to-Er<sup>3+</sup> Direct Energy Transfer for Enhancing Up- and Down-conversion Luminescence in Sub-10 nm NaErF<sub>4</sub>
Fei Zhao, Huan Ling, Wenrui Zhang, Yunxiang Zhang, Qian Liu
Abstract
Dye sensitization enhances the luminescence of lanthanide nanoparticles by improving light-harvesting. Typically, Yb 3+ serves as an energy bridge but absorbs at a single transition, limiting dyes’ options (λ ex > 700 nm) due to the spectral overlap requirement. In contrast, the emitter Er 3+ spans energy levels from UV to NIR, making it ideal for multicolor excitation. We developed a strategy to directly sensitize Er 3+ upconversion (UCL) and downconversion luminescence (DCL) by using cyanine dyes. Cy5 demonstrated the greatest enhancement, achieving a 1942-fold UCL and 70-fold DCL increase compared to nanoparticles alone (Er-NPs) under 980 nm excitation. Smaller Er-NPs exhibited brighter dye-sensitized luminescence due to enhanced interfacial energy transfer. A 2 nm inert shell produced the brightest UCL, while thicker shells improved DCL. Dye-sensitized Er 3+ emissions at 2 H 11/2 (525 nm) and 2 P 3/2 (408 nm) enabled temperature monitoring with a maximum sensitivity ( S a ) of 3.69%/K. This approach holds significant potential for optical temperature sensing and medical imaging.