Low power density 980 nm-driven ultrabright red-emitting upconversion nanoparticles via synergetic Yb3+/Tm3+ cascade-sensitization

Achieving bright red upconverting emission under low excitation power density is the sole approach to address the issues of tissue overheating and limited penetration depth for the classic 980 nm driven green-emitting UCNPs. Herein, synergetic Yb3+/Tm3+ cascade-sensitized NaErF4 with high quantum yield under low excitation power density is developed for the first time to address the above issues. In details, high-level (e.g., 69%) Yb3+ is designed as sensitizer to realize efficient population of 3H5 state of co-doped (e.g., 1%) energy bridging Tm3+ ions. Moderate Er3+ content (e.g., 30%) provides suitable Er3+-Er3+ cross relaxation, sufficient bridging energy utilizing centers (Tm3+(3H5)→Er3+(4I13/2)), and restrained coupled surface and concentration quenching effects for synergistic red emission and red-to-green ratio enhancement. Incredibly, an optimized nanostructure provides single chromatic red emission with red-to-green ratio as high as ~25.0, and 53.5-fold intensity enhancement for the red emission compared with that of well-known highly efficient β-NaYF4:Yb/Er. The first example of Yb3+→Tm3+→Er3+ cascade-sensitized ultrabright UCNPs provide intense red emission even under an extremely low excitation power density of 0.2 W cm-2, showing great attractiveness for biological applications.