Optical depletion mechanism of upconverting luminescence and its potential for multi-photon STED-like microscopy

Simulated emission depletion (STED) microscopy is very powerful, but still suffers from small tissue penetration depth, photobleaching of fluorescent probes and complicated imaging systems. Here, we propose an optical luminescence depletion mechanism employing upconverting nanoparticles (UCNPs) and explore its potential for multi-photon STED-like microscopy. With the addition of Yb³⁺ ions in NaYF₄:Er³⁺ UCNPs, the two-photon green emission of Er³⁺ under 795-nm excitation was successfully depleted by 1140-nm laser through the synergetic effect of the excited state absorption and the interionic energy transfer. This STED-like depletion mechanism was systematically investigated using steady-state rate equations, evidenced by the surprising emerging of 478-nm emission. The green emission depletion efficiency was about 30%, limited by the current laser source. Our work indicates that NaYF₄:Yb³⁺/Er³⁺ UCNPs will be potential probes for multi-photon super-resolution microscopy with many advantages, including long-wavelength-induced large penetration, non-photobleaching and non-photoblinking properties, cost-effective and simplified imaging systems.