Dual-mode excitation β-NaGdF4:Yb/Er@β-NaGdF4:Yb/Nd core–shell nanoparticles with NIR-II emission and 5 nm cores: controlled synthesis via NaF/RE regulation and the growth mechanism

Abstract

Rare-earth doped upconversion nanoparticles (UCNPs) have attracted wide-ranging interest due to their potential applications in bioimaging and biolabeling, where a high emission intensity and small particle size are needed. However, common synthesis processes can be somewhat complex or require additional reactants, and the use of small-size NPs is limited by their weak luminescence intensity. In this work, uniform, monodisperse ∼5 nm β-NaGdF₄:20% Yb/2% Er NPs were synthesized via a simple one-pot synthesis method in the high boiling-point solvents 1-octadecene (ODE) and oleic acid (OA) using a single dose of NaF at an ultralow NaF concentration of NaF/RE = 4. The effects of NaF/RE, the reaction temperature, and the reaction time on the size and phase of the particles were investigated. High-quality β-NaGdF₄:20% Yb/2% Er@β-NaGdF₄:20% Yb/10% Nd core–shell NPs with a size of ∼13 nm, enhanced UC luminescence, long UC lifetimes (τ) for green (τ ≈ 150 μs) and red (τ ≈ 105 μs) emission under 980 and 808 nm excitation, revealed via lifetime imaging, and obvious emission in the near-infrared (NIR) II region were synthesized. The synthesized core–shell NPs may result in a decrease in the overheating effect and allow for a deep penetration depth when further used for bioimaging applications. This work could inspire the synthesis of other high-quality RE-doped UCNPs via adding a single dose of a small amount of NaF and could lay the foundation for the use of Gd-based UCNPs in biomedical applications.