Confining Excitation Energy in Er³⁺-Sensitized Upconversion Nanocrystals through Tm³⁺-Mediated Transient Energy Trapping

KGaA, Weinheim A new class of lanthanide-doped upconversion nanoparticles are presented that are without Yb3+or Nd3+sensitizers in the host lattice. In erbium-enriched core–shell NaErF4:Tm (0.5 mol %)@NaYF4nanoparticles, a high degree of energy migration between Er3+ions occurs to suppress the effect of concentration quenching upon surface coating. Unlike the conventional Yb3+-Er3+system, the Er3+ion can serve as both the sensitizer and activator to enable an effective upconversion process. Importantly, an appropriate doping of Tm3+has been demonstrated to further enhance upconversion luminescence through energy trapping. This endows the resultant nanoparticles with bright red (about 700-fold enhancement) and near-infrared luminescence that is achievable under multiple excitation wavelengths. This is a fundamental new pathway to mitigate the concentration quenching effect, thus offering a convenient method for red-emitting upconversion nanoprobes for biological applications.