Controlled synthesis and panchromatic printing of highly luminescent NaYF4:Ln3+ upconversion hollow microtubes for information encryption on various packaging substrates

Abstract Lanthanide-ion doped β-NaYF4 crystals with bright and tunable upconversion luminescence are urgently demanded in anti-counterfeiting printing. Herein, novel highly luminescent hexagonal NaYF4:Ln3+ upconversion hollow microtubes (UCHMs) were controlled synthesized by a one-pot hydrothermal method employing poly(ethylene imine) (PEI) as ligand. The surface of the synthesized UCHMs was decorated by water-soluble polymer of PEI, rendering the hydrophilic nature of UCHMs. Besides, the synthesized UCHMs exhibited bright upconversion luminescence under the irradiation of a 980 nm laser due to the unique structure and morphology. Time-dependent morphology evolution showed that the formation of UCHMs experience a morphology change from nanoparticles to hexahedron to microprisms, and ultimately to microtubes via a dissolution-reconstruction formation mechanism. Furthermore, the resulting UCHMs were used as pigments for preparation of luminescent inks and their application in anti-counterfeiting printing was explored. To realize panchromatic printing, we prepared three-primary-color (RGB) UCHMs by rational tailoring the doping pairs and molar ratio of the lanthanide ions in UCHMs. Based on the superposition principle of the red-green-blue as three primary colors, a wide array of luminescent inks with panchromatic luminescent colors were prepared. The successful creation of securely light-responsive smart encryption patterns on various packaging substrates such as art paper, aluminum sheet, polyethylene terephthalate (PET) film and cardboard by screen printing was demonstrated. Our strategy provides a new route for controlled synthesis of hydrophilic UCHMs and the as-prepared UCHMs have great potential in application of packaging anti-counterfeiting.