Triple-Mode Protection with Ln³⁺ Ion-Doped Core-Heptad-Shell Single Nanocrystals for High-Level Security Applications

In this work, oleic acid (OA)-capped core-heptad-shell (CHS) nanocrystals (NCs) that exhibit multiple emissions achieved through downshifting and orthogonal upconversion are synthesized via layer-by-layer thermal decomposition. This method enables the downshifting process to be accommodated by doping ions in the inert space between two upconversion patterns (the core and fourth shell) and doping Ce/Tb or Ce/Eu ions in the NaGdF₄ layer for the first time. These developed CHS NCs exhibit different emission colors via 980 and 800 nm orthogonal upconversion and downshifting emissions under 256 nm UV excitation in hexane solvent. Furthermore, surface-functionalized OA is removed using mild acid treatment. The resulting bare CHS NCs disperse well in water and exhibit 21.60-fold and 43.59-fold higher Ce/Tb and Ce/Eu luminescence intensities, respectively, than the OA-capped CHS NCs. These NCs are mixed with a carboxymethylcellulose (CMC) polymer in an aqueous medium to form a CMC-CHS NC gel. Invisible patterns and QR codes are printed on nonfluorescent paper using gels and screen-printing techniques. These patterns and QR codes exhibit three different emission colors under three different excitations. This method can be used for high-level anticounterfeiting applications.