Triple-Mode\nProtection with Ln<sup>3+</sup> Ion-Doped\nCore–Heptad-Shell Single Nanocrystals for High-Level Security\nApplications

In\nthis work, oleic acid (OA)-capped core–heptad-shell (CHS)\nnanocrystals (NCs) that exhibit multiple emissions achieved through\ndownshifting and orthogonal upconversion are synthesized via layer-by-layer\nthermal decomposition. This method enables the downshifting process\nto be accommodated by doping ions in the inert space between two upconversion\npatterns (the core and fourth shell) and doping Ce/Tb or Ce/Eu ions\nin the NaGdF<sub>4</sub> layer for the first time. These developed\nCHS NCs exhibit different emission colors via 980 and 800 nm orthogonal\nupconversion and downshifting emissions under 256 nm UV excitation\nin hexane solvent. Furthermore, surface-functionalized OA is removed\nusing mild acid treatment. The resulting bare CHS NCs disperse well\nin water and exhibit 21.60-fold and 43.59-fold higher Ce/Tb and Ce/Eu\nluminescence intensities, respectively, than the OA-capped CHS NCs.\nThese NCs are mixed with a carboxymethylcellulose (CMC) polymer in\nan aqueous medium to form a CMC–CHS NC gel. Invisible patterns\nand QR codes are printed on nonfluorescent paper using gels and screen-printing\ntechniques. These patterns and QR codes exhibit three different emission\ncolors under three different excitations. This method can be used\nfor high-level anticounterfeiting applications.