Invisible to visible: Novel hydrophilic rare earth fluorescent composite inks for security applications

Rare earth nanomaterials exhibit remarkable characteristics, including real-time responsiveness, luminescence stability, and multicolor emission capabilities. Herein, NaYbF₄: <i>x</i>%Tb,<i>y</i>%Eu@NaYF₄ core–shell structured nanoparticles (CSNP) were synthesized with distinct fluorescence under both ultraviolet (UV) and near-infrared (NIR) excitation. It can be uniformly mixed with a transparent ink solution and loaded into ink cartridges to print customized graphics on copy papers. The graphics cannot be recognized under normal visible light, and the concealed information with high resolution can only be exposed under specific excitation light. Combining with cryptography, it facilitates the implementation of advanced information encryption techniques. Consequently, the innovative fluorescent ink materials hold significant promise for enhancing anti-counterfeiting and information encryption.