Branched NaYF4:Yb, Er Up-Conversion Phosphors with Luminescent Properties for Anti-Counterfeiting Application

Rational design and fabrication of multimodal imaging nanoprobes are of great significance for many imaging applications. Here, we report the systhesis of water-soluble hexagonal NaYF4:Yb, Er up-conversion phosphors (UCPs) with branched structure by hexadecylpyridinium chloride (CPC) and tri-sodium citrate (Na3Cit) binary cationic/anionic capping agents assisted hydrothermal reaction. The crystal phase, morphology, and size of NaYF4:Yb, Er UCP were controlled by the concentration of Na3Cit and CPC, molar ratio of NaF:RE3+, hydrothermal temperature, hydrothermal time, as well as analyzed by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD). By tuning and optimizing hydrothermal conditions, NaYF4 UCPs experienced the phase transition from pure cubic phase to mixed cubic/hexagonal phases to pure hexagonal phase, as well as morphology changes from spherical nanoparticles to short pillar structures, and ultimately to branched structures. Due to the special geometrical shape, the branched NaYF4:Yb, Er UCPs could be blended with host materials without further surface modification. As a proof-of-concept, branched NaYF4:Yb, Er phosphors were then incorporated into polyvinyl alcohol (PVA) matrix to develop a novel luminescence composite film. The resulting PVA/NaYF4:Yb, Er composite film possessed optical transparence at daylight and emitted green fluorescence at the irradiation of a 980 nm laser. NaYF4:Yb, Er phosphors and hybrid PVA/NaYF4:Yb, Er demonstrate high potential of fluorescent materials used for anti-counterfeiting and other optical applications.