Enhanced upconversion luminescence and modulated paramagnetic performance in NaGdF₄:Yb, Er by Mg²⁺ tridoping

In this work, a new strategy to enhance upconversion emission has been realized for the first time, based on β-NaGdF₄:Yb³⁺, Er³⁺ nanocrystals (UCNC) using tridoping with magnesium (Mg²⁺) ions. X-ray diffraction (XRD) and inductively coupled plasma-atomic emission spectroscopy data indicated successful incorporation of the Mg²⁺ ions in β-NaGdF₄:Yb³⁺, Er³⁺. The effect of the Mg²⁺ tridopant on the crystal structure, upconversion luminescence, and paramagnetization in β-NaGdF₄:Yb³⁺, Er³⁺ was investigated in detail. The red, green and violet emission intensities were clearly enhanced and the maximum emission intensities were enhanced by about 9, 6, and 7 times, respectively, by tridoping with 13.5 mol% Mg²⁺ ions. The possible mechanism for enhancing upconversion emission was determined by the alteration of inter-ionic distance and the distortion of crystal structure based on the XRD data. The paramagnetic property can be modulated by the introduction of Mg²⁺ ions and the core-shell structure, and this can be ascribed to the change in the gadolinium molar percentage. Additionally, a synergistic effect on emission intensity enhancement was observed in a Mg²⁺-doped core-shell structure. At last, an improved ligand exchange method has been designed to render oleic acid-stabilized 13.5 mol% Mg²⁺-doped UCNCs dispersible in water and their three day cytotoxicity was assayed using stem cells. It is expected that this material could be applied in biomedical nanoprobes, bioseparation, magnetic resonance imaging, and so on.