Dye-to-Er3+ Direct Energy Transfer for Enhancing Up- and Down-conversion Luminescence in Sub-10 nm NaErF4

Dye sensitization enhances the luminescence of lanthanide nanoparticles by improving light-harvesting. Typically, Yb³⁺ 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 Er³⁺ spans energy levels from UV to NIR, making it ideal for multicolor excitation. We developed a strategy to directly sensitize Er³⁺ 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 Er³⁺ emissions at ²H_11/2 (525 nm) and ²P_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.