Lanthanide Tri-Doping Engineered Upconversion Emissions under Dual-Wavelength Excitation toward High-Resolution Imaging

Lanthanide-doped upconversion nanoparticles (UCNPs) have emerged as compelling optical probes due to their photostable narrowband emissions. Dual-wavelength excitation provides a powerful approach to tailoring upconversion emissions as it selectively targets specific 4f transitions. Nonetheless, achieving upconversion emission depletion, which quenches the emission with auxiliary irradiation, remains challenging because of competitive dual-wavelength cooperative excitation. Herein, we delineate a lanthanide tridoping strategy that exploits a re-excitation enabled emission depletion (REED) process to overcome this limitation. In REED, an additional laser re-excites already excited lanthanide ions, promoting their transitions from the emissive level to higher-lying ones. Meanwhile, the introduced codopants create interionic cross-relaxation (CR) and energy transfer (ET) pathways to depopulate these higher-lying levels. This synergistically combined CR/ET and REED depletes the population of the original emissive level, leading to remarkable emission reduction. As a proof-of-concept, the CR/ET-REED approach was deployed to deplete the green upconversion emissions of Er3+ in Er3+–Yb3+–Ln3+ (e.g., Tb3+, Eu3+, Dy3+) tridoped nanoparticles under 795 nm excitation and 1140 nm re-excitation. The depletion efficiency rose from 19.4% in Er3+ singly doped nanoparticles to 48.5% in tridoped ones. With these nanoparticles, high-resolution microscopic imaging was achieved under dual-wavelength excitation with a resolution of sub-250 nm. This work provides a generalizable strategy for developing high-resolution imaging probes via emission depletion and represents a paradigm shift for UCNPs toward tridoped systems.