Here lanthanide-doped tetragonal nanocrystals of BaYF5 with strong near infrared-to-visible multicolor upconversion emissions have been controllably synthesized by a hydrothermal method. Opposite to well-established NaYF4 nanocrystals, we found that the sizes of the BaYF5 nanocrystals increase in proportion to that of the molar ratios of EDTA to lanthanide nitrates and decrease as the pH value of the precursor solution increases in presence of EDTA. These findings reveal that the nucleation process of BaYF5 nanocrystals is governed by the pH-sensitive chelating ability of EDTA with both Ba2+ and Ln3+. Additionally, it was found that single wavelength laser excitation of 980 nm can induce strong red, green, and blue upconversion emissions in BaYF5 nanocrystals doped with 18%Yb3+/2%Er3+, 18%Yb3+/2%Ho3+, and 18%Yb3+/2%Tm3+, respectively. Furthermore, after a facile surface poly(acrylic acid) ligand exchange reaction, such multicolor nanoparticles can be transferred into water and multiple other solvents for applications in biology and materials science.
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