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

Abstract

Achieving a bright red upconverting emission under a 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, for the first time, synergetic Yb³⁺/Tm³⁺ cascade-sensitized NaErF₄ with a high quantum yield under a low excitation power density was developed for the to address the above issues. In detail, high levels (e.g., 69%) of Yb³⁺ were used as the sensitizer to realize the efficient population of the ³H₅ state of co-doped (e.g., 1%) energy-bridging Tm³⁺ ions. Moderate Er³⁺ content (e.g., 30%) provided suitable Er³⁺–Er³⁺ cross-relaxation, sufficient bridging of energy utilizing centers (Tm³⁺(³H₅) → Er³⁺(⁴I_13/2)), and restrained coupled surface and concentration quenching effects for synergistic red emission and red-to-green ratio enhancement. Incredibly, an optimized nanostructure provides a single chromatic red emission with a red-to-green ratio as high as ∼25.0, and a 53.5-fold intensity enhancement for the red emission as compared with that of the well-known, highly efficient β-NaYF₄:Yb/Er. The first example of Yb³⁺ → Tm³⁺ → Er³⁺ cascade-sensitized ultrabright UCNPs exhibits an intense red emission even under an extremely low excitation power density of 0.2 W cm⁻², showing great attractiveness for biological applications.