Stable Monodisperse Pb_1-x Cd_x S Quantum Dots for NIR-II Bioimaging by Aqueous Coprecipitation of Bimetallic Clusters

Aqueous lead sulfide (PbS) quantum dots (QDs) synthesized by traditional methods are unstable, so that they are usually coated with cadmium sulfide (CdS) to prevent oxidation, which are complicated and not satisfactory for mass production. Here, stable ternary Pb_1-x Cd_x S QDs were synthesized by in situ coprecipitation of Pb_4-n Cd_n O₄ bimetallic clusters in an aqueous solution, which possess a uniform size of 4.0±0.2 nm and the second near-infrared (NIR-II) emission at 1100 nm with photoluminescence quantum yield (PLQY) as high as 17.72 %. Stored at 4 °C and in colloidal form, the PLQY of Pb_1-x Cd_x S QDs remained at 90.9 % of the initial value after 15 days, while stored as powder, the spectra did not change after 5 months. The high PLQY and good water compatibility of Pb_1-x Cd_x S QDs provide a good performance in vivo vasculature imaging and lymphatic system imaging at a very low power density (10 mW cm^-2 ) in the NIR-II window.