High-fluoride-induced rapid synthesis and universal modulation of hexagonal phase NaYF4

Abstract

Hexagonal phase NaYF₄ (β-NaYF₄) is extensively utilized in bioimaging and fluorescent labeling due to its excellent biocompatibility and optimal up-conversion luminescence in the visible light region. However, conventional hydrothermal methods for producing β-NaYF₄ typically necessitate prolonged reaction times and are prone to impure phase mixtures, thus lacking efficiency in meeting increasing demand. In our study, we have developed a rapid and straightforward hydrothermal method for synthesizing β-NaYF₄ by controlling the nucleation rate through adjusting the concentration of F⁻ in the reaction solution, effectively managing the phase transition process. The use of high fluoride concentrations allows multiple NaYF₄ molecules in the precursor to rapidly reach the critical state required for transitioning from cubic phase to hexagonal phase, resulting in swift formation of hexagonal phase crystals. Furthermore, we have successfully synthesized NaGdF₄, NaYbF₄, and NaErF₄ using this method, demonstrating its universality. Additionally, thorough investigation into the luminescence properties of Yb³⁺ and Er³⁺ co-doped β-NaYF₄ has been conducted. When encapsulated as a polymer film, variations in emitted light color with temperature indicate potential applications in temperature sensing. The rapid and efficient synthesis developed in this study effectively addresses the supply and demand contradiction for high-quality β-NaYF₄, providing significant support for subsequent research in biological and related fields.