Carbon dot-mediated synthesis of NaYF4:Yb3+,Er3+@carbon dot composites with enhanced upconversion luminescence for temperature sensing†

Upconversion (UC) nanocrystals and carbon dots (CDs) have emerged as significant subjects of research interest across various fields, including biomedicine, fluorescence sensing, and anti-counterfeiting. This study presents a novel method for preparing NaYF₄:Yb³⁺,Er³⁺@CDs composites through the cubic-to-hexagonal phase transformation of NaYF₄ mediated by CDs. The formation of these composites was successfully confirmed through morphological and structural analyses. Notably, the composites were found to enhance UC emission and prolong luminescence lifetime, with these effects being dependent on the quantity of CDs used. The experimental results indicate that the enhanced UC emission in the composites is primarily due to the interception of quenching centers, such as hydroxyl (OH⁻) groups. Furthermore, these composites with improved UC emission have the potential to serve as highly sensitive optical thermometers based on the fluorescence intensity ratio technique, with optimal green emissions at 298 K and a maximum relative sensitivity of 1.08 K⁻¹. This work paves the way for advancements in UC luminescence and establishes a foundation for the design and fabrication of high-efficiency UC materials with potential applications in optical temperature sensing.