Facile fabrication of water-soluble Ln3+-doped β-NaGdF4 nanocrystals (Ln = Ce, Tb, Eu, Dy) with multicolor luminescence and magnetic properties

Water-soluble Ln3+-doped β-NaGdF4 (Ln = Ce, Tb, Eu, Dy) nanocrystals were synthesized via a hydrothermal method followed by phase transferring. The nanocrystals crystallize in the hexagonal phase as β-NaGdF4. Tunable multicolor down-conversion emissions can be achieved from Ln3+-doped β-NaGdF4 nanocrystals under a single ultraviolet excitation. During the down-conversion emission, energy transfer process like Ce3+ → Gd3+ → (Gd3+)n → Ln3+ occurred, in which Gd3+ ions play an important intermediate role. The magnetic properties of the obtained nanocrystals were investigated in detail. The nanocrystals display paramagnetic and superparamagnetic properties with mass magnetic susceptibility values of 6.77 × 10−5 emu/g · Oe and 1.92 × 10−3 emu/g · Oe at 300 K and 2 K, respectively. The T1-weighted magnetic resonance image that measured in aqueous solutions with different Gd3+ concentrations reveals the contrast brightening increases with the concentration of Gd3+ ions. Such magnetic/luminescent dual-function material may have promising applications in multiple biolabels and magnetic resonance imaging.