From bioactive saponins to carbon quantum dots: Upcycling Xanthoceras sorbifolium husk saponins for expanded theranostic applications

While the saponins from Xanthoceras sorbifolium husks (XSS) possess significant pharmacological activities, their application is limited by issues such as poor stability and singular therapeutic function. This study presents an innovative transformation route of XSS into nitrogen-doped carbon quantum dots (Saponin-CQDs) via a one-pot hydrothermal method. Comprehensive characterization confirmed the synthesis of Saponin-CQDs, which exhibited a nanoscale size (∼1.40 nm), strong blue fluorescence (quantum yield of 3.4%), and excellent water dispersibility. Crucially, this transformation facilitated a functional leap: while the carbonization process modulated the direct biological potency, the Saponin-CQDs acquired superior fluorescence imaging capability and markedly enhanced photostability. Although their direct antioxidant and cytotoxic effects were weaker than those of native XSS, Saponin-CQDs demonstrated a significantly improved biosafety profile, with reduced toxicity towards normal hepatocytes (HL-7702) and a higher selectivity index against cancer cells. They were successfully employed for real-time cytoplasm imaging in hepatocellular carcinoma (HepG2) cells. This work demonstrates that converting bioactive XSS into CQDs represents a novel strategy for engineering a therapeutic natural product into a multifunctional theranostic nanoplatform. This paradigm shift, which prioritizes functional expansion over mere bioactivity replication, paves the way for potential applications in bioimaging, drug delivery, and integrated nanomedicine.