High-Yb3+-Doped NaYbF4:10% Er@NaYF4 Alloyed Nanoparticles for Brightening Upconversion and Multi-Colorizing Information Encryption

Alloyed upconversion nanoparticles (aUCNPs, NaYbF4:10% Er@NaYF4) with high Yb3+ doping were designed to brighten luminescence, exploring multicolor patterns of information encryption. The luminescence intensity of aUCNPs was 2.3 times higher than that of common doped UCNPs reported (dUCNPs, NaYF4:20% Yb, 2% Er@NaYF4). Furthermore, because of the high Yb3+ doping (90%), both green and red emissions of aUCNPs were enhanced, similar to 4.2 and similar to 17.2 times stronger than that of NaYF4:20% Yb, 10% Er@NaYF4, respectively. The reasons for the aUCNPs having a brightened luminescence were explored by synthesizing four UCNPs with different structures and comparing their luminescence. These are attributed to their unique features: (1) a larger absorption cross section for accumulating energy, (2) more efficient energy transfer between Yb3+ and Er3+, and (3) their inert shell inhibiting surface quenching. Eventually, three colors including yellow, orange, and red were obtained by combining aUCNPs and neutral red (NR) taking advantage of the inner filter effect (IFE), and aUCNPs were thus successfully applied in information encryption with multicolor patterns.