Bandgap and Interface Engineering of Ag–Au–S Quantum Dots toward Bright Near-Infrared Emission and Deep-Tissue Bioimaging

Abstract Low-toxicity colloidal quantum dots (QDs) with tunable near-infrared (NIR) emission are promising for bioimaging because biological tissues exhibit relatively low light scattering and absorption in the 700–1700 nm optical window. However, it remains challenging to achieve high photoluminescence quantum yields (PLQYs), aqueous stability, and tunable emission simultaneously. Here, we demonstrate efficient NIR emission from Ag–Au–S QDs by combining composition control with heterointerface engineering. Varying the Ag/Au ratio tuned the bandgap from 1.39 to 1.88 eV and shifted the PL peak from 1130 to 745 nm. QDs prepared with an Ag/(Ag+Au) precursor ratio of 0.75 had an average diameter of 2.5 nm and consisted of a polycrystalline Ag3AuS2 core surrounded by an amorphous Ag–S-rich surface layer. These QDs exhibited a PL peak at 800 nm with a PLQY of 3.4%. In–S shell coating increased the PLQY to 33.6%. Subsequent ZnS treatment produced Janus-like Ag–Au–S@In–Zn–S QDs exhibiting a PL peak at 840 nm with a PLQY of 38.9%, consistent with suppression of nonradiative recombination through surface passivation. After ligand exchange with 3-mercaptopropionic acid, the QDs were readily dispersed in water while retaining a PLQY of 27.0% and exhibited excellent PL stability in biologically relevant media. The QDs enabled in vivo deep-tissue imaging, with NIR PL detected from tissues at depths of up to 15 mm. These findings demonstrate that Ag–Au–S-based QDs are promising low-toxicity NIR probes for biomedical imaging.