Hydrothermal synthesis and inkjet printing of hexagonal-phase NaYF4: Ln3+ upconversion hollow microtubes for smart anti-counterfeiting encryption
Rational design and fabrication of well-defined upconversion phosphors are of great significance for anti-counterfeiting and security applications. Here, we present hydrothermal synthesis of novel upconversion hollow microtubes (UCHMs) composed of poly(acrylic acid) (PAA) functionalized hexagonal-phase NaYF4: Ln3+. The resulting UCHMs possess not only well-defined structures and uniform sizes, but also strong upconversion luminescence emission at room temperature. The structure–property relationship of the NaYF4: Ln3+ crystal between the phase transition from cubic to hexagonal and morphological changes from nanospheres to hollow microtubes is obtained by detailed time-dependent experiments, which help to derive a possible growth mechanism of the UCHMs. Furthermore, by doping different lanthanide ions and tailoring the molar ratio of the doped lanthanide ions in the UCHMs, three-primary-color (RGB) UCHMs are successfully produced. The RGB UCHMs are fabricated into fluorescent inks by dispersal in a solvent mixture of ethanol, water, and glycerol. The resulting RGB UCHMs inks are inkjet printed on common paper to produce hidden images under ambient conditions. However, the hidden images can be read out with red-green-blue luminescence under 980 nm light excitation. The UCHMs-based inks enable the creation of light-responsive smart encryption patterns in a secure way, and such image encryption can be spatially and selectively printed on documents for anti-counterfeiting applications.