A general strategy to fabricate ligand-free water-soluble up-conversion nanoparticles

It is a generally accepted method to synthesize the monodisperse NaYF4:Yb3+, Er3+ with uniform size and shape by using oleic acid (OA) as surfactant or solvent. However, the obtained oleate-capped up-conversion nanoparticles NaYF4:Yb3+, Er3+ (Ln-UCNPs, Ln = Yb3+, Er3+) have inherent hydrophobia properties, which should be processed by complicated post-treatments to render them water dispersible before used in biomedicine. Herein, we introduce a facile approach, ultrasonic separation, to obtain water-soluble and ligand-free Ln-UCNPs by analyzing the capping effect between Ln(3+) and the carboxy group of oleate anions. After ultrasonic separation, the ligand-free of Ln-UCNPs disperse in water and ethanol easily, which are characterized by transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FTIR), thermogravimetric analyses (TGA), nuclear magnetic resonance (NMR) and zeta potential. The experiments demonstrate that the present method is simple and effective to remove oleate layers from the surface of NaYF4:Yb3+, Er3+, and worthy of being generalized. (C) 2014 Elsevier B.V. All rights reserved.