Facile synthesize of upconversion β-NaYF4 capped with waterborne polyurethane prepolymer for packaging anti-counterfeiting

Due to unique optical properties and high information concealment, upconversion materials, with excellent luminescent properties and good water dispersibility, have received significant attention in many fields recently, particularly for packaging anti-counterfeiting. In a vast majority of parameters, chelating ligands played an essential role in controlling the phase structure, morphology and surface characteristics of the upconversion materials, thus produced an effect on photoluminescent properties. Herein, we demonstrated a facile hydrothermal synthetic strategy of β-NaYF4 upconversion microcrystals using waterborne polyurethane (WPU) prepolymer as the ligand. In order to achieve high intensity, the WPU polymer was prepared through molecular design and optimized preparation process, and the influence of WPU prepolymer with different ammonia contents on the luminescence property of the crystals was investigated thoroughly. The results of powder X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) indicated that the as-prepared NaYF4 crystal possessed hexagonal phase structure, a forked rod shape, high crystallinity and excellent crystal quality. Interestingly, the crystal capped with WPU prepolymer possessed good water dispersibility and its fluorescence intensity at 540 nm was 10∼15 times as higher as that of the crystal chelated with trisodium citrate (TSC) or ligand-free. The possible reasons have been proposed. It was demonstrated that the β-NaYF4 (Er3+/Tm3+, Yb3+/Er3+ or Yb3+/Tm3+ co-doped) crystals capped with WPU prepolymer would be promising candidates to prepare waterborne RGB anti-counterfeit inks which were eco-friendly and suitable for stamping designable patterns and handwriting on flexible packaging substrates. The patterns and handwritings were invisible under ambient conditions, while displayed bright multicolor fluorescence upon 980 nm irradiation, which confirmed that the as-prepared crystals exhibited high potential in packaging anti-counterfeiting.