Continuous-wave near-infrared stimulated-emission depletion microscopy using downshifting lanthanide nanoparticles.

Stimulated-emission depletion (STED) microscopy has profoundly extended our horizons to the subcellular level. However, it remains challenging to perform hours-long, autofluorescence-free super-resolution imaging in near-infrared (NIR) optical windows under facile continuous-wave laser depletion at low power. Here we report downshifting lanthanide nanoparticles that enable background-suppressed STED imaging in all-NIR spectral bands (λ = 808 nm, λ = 1,064 nm and λ = 850-900 nm), with a lateral resolution of below 20 nm and zero photobleaching. With a quasi-four-level configuration and long-lived (τ > 100 μs) metastable states, these nanoparticles support near-unity (98.8%) luminescence suppression under 19 kW cm saturation intensity. The all-NIR regime enables high-contrast deep-tissue (~50 μm) imaging with approximately 70 nm spatial resolution. These lanthanide nanoprobes promise to expand the application realm of STED microscopy and pave the way towards high-resolution time-lapse investigations of cellular processes at superior spatial and temporal dimensions.