Doping Ag2S quantum dots with Pb yields significantly enhanced in vivo fluorescence imaging in the NIR-II window and comparable toxic effects

Silver chalcogenide (Ag2X; X = S, Se, Te) quantum dots (QDs) have facilitated the biomedical applications of QDs due to their low toxicity and tunable emission reaching the second near-infrared window (NIR-II window, 1000-1700 nm). The photoluminescence intensity of Ag2S QDs and Ag2Se QDs is considerably enhanced via Pb doping. However, the developed Pb-doped Ag2S QDs lacked in vivo verification, and the toxicity of the Pb-doped Ag2X QDs was only preliminarily assessed. Although QDs containing Pb have been widely introduced into the field of biomedicine, there are still concerns regarding toxicity associated with Pb. In the present study, we prepared polyethylene glycol (PEG)-modified Pb-doped Ag2S QDs with better biocompatibility and tested their in vivo bioimaging and toxicity in comparison with Ag2S QDs modified with the same PEG under identical concentrations. Results indicated that the PEG-modified Pb-doped Ag2S QDs significantly enhanced the brightness of in vivo bioimaging and spatial resolution for vessels inside the tumor without increasing toxicity, assessed based on cell viability post incubation for 96 h, intracellular ROS level, DNA damage, early apoptosis and mitochondrial injury. Our study demonstrated that the PEG-modified Pb-doped Ag2S QDs could serve as an advanced fluorescent probe for various biomedical applications.