Ultraviolet- and Near-Infrared-Excitable LaPO4:Yb3+/Tm3+/Ln3+ (Ln = Eu, Tb) Nanoparticles for Luminescent Fibers and Optical Thermometers

The widespread demand for multifunctional materials that can be used for anticounterfeiting purposes, as dual-mode phosphors, or as optical nanothermometers has inspired us to synthesize Yb3+/Tm3+/Ln3+ (Ln = Eu, Tb)-doped LaPO4 nanoparticles (NPs) and, based on these, luminescent fibers that utilize and extend the properties of the NPs. They show intense dual-mode color-tunable emission and temperature-dependent up-conversion (UC) luminescence, which makes them multifunctional and of a high potential applicability. The nanomaterials were obtained by a simple and fast coprecipitation method, yielding pure-phase monoclinic products. The products were used as luminescence activators in cellulose fibers, showing their potential applications for security purposes. They can emit strong UC and down-conversion luminescence within one particle, under excitation of commercially available near-infrared (NIR) and ultraviolet (UV) excitation sources. The pure red and green emissions under UV irradiation resulted from the presence of Eu3+ or Tb3+ ions in the structure of the products, whereas violet-blue emission was obtained under NIR excitation because of Yb3+ and Tm3+ codopants. After simultaneous UV/NIR double-laser excitation, they obtained NPs that show a complex luminescence resulting from Tm3+ (after energy transfer from Yb3+), Tb3+, or Eu3+. The obtained UC emission can be tuned, giving a huge color shift (from orange or green to blue). What is more, thanks to the presence of thermalized levels of Tm3+ ions, these materials can act as promising temperature nanosensors in a wide range of temperatures from 293 to 679 K. Using the fluorescence intensity ratio technique, relatively high thermal sensitivity (S r) was obtained, 0.024 and 0.022 K–1 for 293 K for the Yb3+/Tm3+/Eu3+ and Yb3+/Tm3+/Tb3+ samples, respectively.