Fluorescent imaging in the second near-infrared window (NIR-Ⅱ, 1.0~1.4 μm) is appealing due to minimal autofluorescence and negligible tissue scattering in this region, affording maximal penetration depth for deep tissue imaging with high feature fidelity.Simulations and modeling studies suggested that fluorophores with emission in the 1000-1320 nm NIR-Ⅱ region could significantly improve signal-to-noise ratio compared to those emitting at 650-950 nm (NIR-Ⅰ).Recent efforts have been devoted to identifying NIR-Ⅱ emitting agents for in vivo imaging applications.Quantum dots (QDs) such as PbSe, PbS, and CdHgTe with NIR emission have been successfully developed.However, the highly toxic nature of Pb, Cd and Hg is of concern for in vivo applications.Therefore, highly biocompatible NIR-Ⅱ fluorescent probes that do not contain Cd, Pd or Hg will facilitate biological imaging in this beneficial spectral region.Herein, we first reported a new type ofNIR QDs, Ag2S QDs, with emission in the NIR-Ⅱ region.