Ionothermal synthesis of hexagonal-phase NaYF4:Yb ,Er/Tm upconversion nanophosphorsw

Rare-earth ions doped luminescence upconversion nanoparticles have attracted much attention in recent years owing to their superior spectroscopic properties, mainly arising from the existence of stable intermediate states, which may result in potential applications in many fields, especially in biology/ biomedicine. Among these materials, hexagonal-phase NaYF4 is reported as one of the most efficient hosts for performing infrared-to-visible photon conversion in the doped rare-earth ions. For their in vivo biological application, the prerequisites for an optimal universal bioprobe are watersolubility, small size and high luminescence efficiency. Up to now, the most general reported method to prepare pure hexagonal NaYF4:Yb ,Er/Tm upconversion nanocrystals (UP-NCs) with small size and good dispersibility has been the co-thermolysis of trifluoroacetate precursors at high temperature. However, these synthesized nanoparticles are hydrophobic and require further modification on their surfaces before biological applications. Although use of high temperature facilitates the formation of the thermally stable hexagonal phase, it requires rigorous experimental conditions and the decomposition of trifluoroacetate is highly toxic. Just a few papers have reported that water-soluble and biocompatible hexagonal NaYF4:Yb ,Er/Tm UP-NCs with size around 50 nm could be obtained either hydrothermally or solvothermally. It is, however, still difficult to control the size and phase purity. Hence, there is a challenge to develop a facile synthetic route to obtain small hydrophilic pure hexagonal NaYF4:Yb ,Er/Tm nanophosphors under relatively mild conditions. In this communication we report our recent successful attempt in synthesizing such nanocrystals in ionic liquid (IL) media, which appear as ‘‘green solvents’’ due to their chemical stability, very low vapor pressure and non-flammability, and have been applied in many fields, ranging from synthesis, catalysis, separation and electrochemistry. The syntheses hinge on the solvents being predominantly ionic. Thermal reactions using ionic liquids as reaction media are termed as ‘‘ionothermal’’ to distinguish from hydrothermal and solvothermal methods, which take place in a predominantly molecular solvent. Recently, the synthesis of inorganic nanostructures in IL media has attracted extensive attention, including metals, metal oxides, metal alloys, metal fluorides, and so on. Based on the requested components and hydrophilic property of the ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate, [Bmim][BF4], was chosen as the solvent for manufacturing nanostructured hexagonal-phase NaYF4:Yb ,Er/Tm UP-NCs. The imidazolium cations provided the in situ capping reagent to prevent the NaYF4 nucleation centers from growing up, while the tetrafluoroborate anions introduced a new fluorine source according to partial hydrolysis. Therefore, this IL could act as solvent and reaction agent, as well as template. Pure hexagonal-phase NaYF4:Yb ,Er/Tm nanoparticles with small size were obtained in this system at 160 1C. The as-prepared nanocrystals were water-soluble and displayed strong upconversion luminescence, suitable for biological applications. The ionothermal process reported here offers a new alternative in synthesizing NaYF4:Yb ,Er/Tm upconversion nanophosphors. In a typical synthetic route, a given amount of sodium chloride and rare-earth nitrate were added into a beaker containing 10 ml of BmimBF4 and stirred for 30 min at 80 1C; the above mixed solution was then transferred into a 23 mL Teflon-lined autoclave and kept at 160 1C for 18 h (see Fig. S1w), before being cooled to room temperature and diluted with absolute ethanol or acetone as appropriate. Finally, the precipitates were collected through centrifugation at a speed of 6000 rpm, washed with absolute ethanol, and dried in vacuum at 50 1C (see ESIw). The characterization of NaYF4:20%Yb,2%Er/Tm is summarized in Fig. 1. From the low-resolution micrograph, Fig. 1A, it is easily seen that the synthesized particles take on mixed morphologies (spherical, brick-like, hexagonal-shaped) and present some small holes. The corresponding HRTEM image, Fig. 1B, reveals the presence of clear crystal lattices from various directions, which means that the individual particles are polycrystalline. The fast Fourier transformation (FFT) pattern shown in Fig. 1C is composed of strong rings, pointing also to the polycrystalline nature of the particles. a Key Laboratory of Excited-State Processes, Changchun Institute of Optics Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, P. R. China. E-mail: [email protected] Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WS Amsterdam, The Netherlands. E-mail: [email protected] w Electronic supplementary information (ESI) available: Experimental and instrumental details, IR and SEM images. See DOI: 10.1039/b915517a