Synchronous Passivation Boosts the NIR-II Luminescence Efficiency of Ag2S Nanocrystals for Effective Deep Tissue Lymphatic Mapping

Silver sulfide nanocrystals (Ag2S NCs) with second near-infrared (NIR-II) emission and excellent biocompatibility hold great attraction for in vivo imaging. However, current Ag2S NCs suffer severely with low photoluminescence quantum yields (PLQYs), limiting the penetration depth. Herein, based on computational prediction, we proposed a facile synchronous passivation strategy employing non-toxic MgCl2 as a synchronous passivator for simultaneous Mg-doping and chlorination of Ag2S NCs (Syn-Ag2S). Spectroscopic studies including femtosecond transient absorption measurement (fs-TAS) and temperature-dependent PL spectra revealed that syn-passivation successfully enlarged nonradiative activation energy and reduced the defect states and exciton-photon interaction, thus resulting in a record high absolute PLQY of 46 +/- 2% at 1220 nm emission among toxic-element-free NIR-II probes. The boosted emission unlocks the potential of Ag2S NCs for deep lymph node imaging. The profile of deep lumbar lymph nodes was clearly distinguished by Syn-Ag2S, while clinical indocyanine green (ICG) failed to do so. This strategy also establishes an approach for developing other ultrabright and non-toxic NCs used not only in bioimaging but also in photovoltaics and LEDs.