Water dispersible ligand-free rare earth fluoride nanoparticles: water transfer NaREF-to-REF phase transformation

The chemical stability of oleate-capped sub-10 nm α- and β-NaREF 4 NPs (RE = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu for α- and RE = Pr, Nd, Sm, Eu, Gd, Tb, Dy for β-phase NPs) was evaluated under the acidic conditions used for ligand removal towards water dispersibility. It was found that for such small NPs, a pH lower than 3 was necessary for the water transfer to be efficient and to yield well-dispersed ligand-free NPs. In stark contrast to the generally considered good chemical stability of NaREF 4 , these conditions were observed to pose a risk to phase transformation of the NaREF 4 NPs into much larger, hexagonal- or orthorhombic-phase REF 3 , depending on the NP composition. A correlation between the thermodynamic stability of the α/β-NaREF 4 and the hexagonal/orthorhombic REF 3 phases - dictated by the RE ion choice - and the chemical stability of the NPs was found. For instance, β-NaGdF 4 NPs remained stable, while α-NaGdF 4 NPs underwent phase transformation into hexagonal GdF 3 . More general, NaREF 4 NPs based on lighter RE ions were more prone towards phase transformation, while those based on heavier RE ions exhibited stability. Herein, within the RE series, the borderline for phase transformation was identified as Tb/Dy for α-NaREF 4 NPs and Sm/Eu for β-NaREF 4 NPs, respectively. Also, given the large interest in luminescent NPs for, e.g. biomedical applications, optically active Ln 3+ ions (Ln = Nd, Eu, Tb, Er/Yb) were doped into α/β-NaGdF 4 host NPs, and the dopant influence on the chemical stability was evaluated. Steady state and time-resolved spectroscopy unveiled spectral features characteristic for Ln 3+ f-f transitions, i.e. downshifting and upconversion, before and after ligand removal. Overall, the results herein described emphasise the importance of minding the chemical procedure used for ligand removal of NaREF 4 NPs of different crystalline phases and RE compositions. Crystalline phase and rare-earth (RE) ion choice were identified as key parameters for NaREF 4 -to-REF 3 phase transformation versus water transfer during ligand removal from small NaREF 4 nanoparticles at low pH.