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 4 f 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 Er 3+ in Er 3+ –Yb 3+ –Ln 3+ (e.g., Tb 3+, Eu 3+, Dy 3+ ) tridoped nanoparticles under 795 nm excitation and 1140 nm re-excitation. The depletion efficiency rose from 19.4% in Er 3+ 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.