Modulation of Exciton Dynamics in AgInS 2 and AgInS 2 /ZnS Quantum Dots by Baicalein for Fluorescence Lifetime Imaging Microscopy of Cancer Cells

Nanoscale ternary chalcogenide quantum dots are promising optical probes for chemical sensing and bioimaging owing to their tunable emission and low toxicity. Flavonoids, a class of natural antioxidants and biological hydroxyphenols, exhibit environmentally sensitive redox behavior that can be exploited for fluorescence-based sensing. In this work, biocompatible silver indium sulfide quantum dots (AIS QDs) and core–shell silver indium sulfide/zinc sulfide quantum dots (AIS/ZnS QDs) were synthesized using an eco-friendly aqueous route and capped with 3-mercaptopropionic acid ligands. The sensing of the polyphenolic flavonoid baicalein by these quantum dots was systematically investigated with emphasis on exciton dynamics and optical properties. Steady-state photoluminescence (PL), time-resolved fluorescence spectroscopy, and ultrafast transient absorption spectroscopy revealed significant PL quenching without changing fluorescence lifetimes, resulting in ultrafast electron transfer. ZnS shell passivation reduced trap states, enhancing carrier lifetimes and suppressing nonradiative recombination. In vitro studies showed low cytotoxicity, and fluorescence lifetime imaging microscopy (FLIM) confirmed localization in MDA-MB-231 breast cancer cells in strong contrast against cellular autofluorescence. These findings highlight the potential of flavonoid-sensing AIS QDs and AIS/ZnS QDs as biocompatible nanoscale probes for optical biosensing and bioimaging.