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 (<i>Syn</i>-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 <i>Syn</i>-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.