Synthesis and Luminescent Properties of Multi-Mode Core-Shell-Shell LaPO4:RE (RE³⁺ = Er³⁺, Eu³⁺) Microspheres for Anti-Counterfeiting Ink Applications

To meet the demands for product and consumer protection, we developed biocompatible multi-mode emitting core-shell-shell rare earth phosphates for anti-counterfeiting inks. These core-shell-shell structures feature SiO2 microspheres as the core, with LaPO4 nanocrystals doped with erbium (Er) and europium (Eu) as the interlayer, and an outer SiO2 shell. The SiO2@LaPO4:Er0.1,Eux@SiO2 (x = 0.01, 0.02, 0.03, 0.04, 0.05) microspheres display dual emissions when excited by 254 nm and 365 nm UV light, offering a novel anti-counterfeiting approach. Another core-shell-shell structure was created with LaPO4:Er3+ and LaPO4:Eu3+ separately coated on SiO2 cores, mixed, and then covered with a SiO2 shell. Under 980 nm near-infrared laser excitation, these structures emit bright upconverted green light, while 254/365 nm UV light induces red/yellow down-conversion luminescence. This design minimizes up-conversion luminescence quenching of Er3+ ions from cross-relaxation with Eu3+ ions. The SiO2 core minimizes energy loss, regulates particle size, and enhances luminescence. A certain thickness of the silicon shell layer also protects against luminescence quenching. The inks exhibit high dispersibility, non-reproducibility, and excellent luminescence, making them effective for anti-counterfeiting.