Intense green upconversion in core-shell structured NaYF4:Er,Yb@SiO2 microparticles for anti-counterfeiting printing

Rare-earth-doped NaYF4 nanoparticles are considered crucial upconversion luminescent materials with many applications, including anti-counterfeiting printing, biological imaging, and optical information storage. This work presents progress in obtaining hydrophilic pigments based on NaYF4:Er,Yb@SiO2 nanoparticles, and their utilization in water-based ink printing for anti-counterfeiting applications. Coating NaYF4:Yb/Er microparticles with SiO2 shells can provide several benefits for printing purposes, such as improved stability, dispersion, and ease of printing. The hydrophilic core-shell structured β-NaYF4:Er, Yb@SiO2 microparticles were successfully prepared using the hydrothermal synthesis and sol-gel Stober method. The obtained microparticles were characterized by Scanning Electron Microscopy, X-ray diffraction, and Energy-dispersive X-ray spectroscopy. Such characterizations confirm that the Er3+ and Yb3+ ions are incorporated in NaYF4 cores, the NaYF4:Er, Yb cores are in hundreds of nanometer sizes, and the NaYF4:Er, Yb microparticles are coated by a silica layer of 50 nm thick. Both NaYF4:Er, Yb and NaYF4:Er, Yb@SiO2 behave as up-conversion luminescent microparticles for both green and red emissions. The CIE chromaticity coordinates indicate that both the nanoparticles have their greenish color under excitation at 980 nm. Although decorating the NaYF4:Er, Yb nanoparticles with the SiO2 shell decreases luminescent intensity (by a factor of 2), this step is still necessary to achieve hydrophilic surfaces for preparing water-based printing inks. Using the synthesized NaYF4:Er, Yb@SiO2 as a pigment, a new water-based ink formula was created based on Polyamide as a polymer matrix. The obtained ink is printable on paper and on transparent and flexible Polyethylene substrate. The printed patterns with a height of 2 cm and a width of 2.5 cm were observed clearly under the irradiation of a 980 nm LED. Their durability in terms of sharpness and color was also assessed after six months.