Red emission nitrogen, boron, sulfur co-doped carbon dots for “on-off-on” fluorescent mode detection of Ag+ ions and l-cysteine in complex biological fluids and living cells

Herein, a simple and efficient fluorescent assay for Ag+ ions and l-cysteine (L-Cys) in complex biological fluids and living cells was first developed based on the fluorescent "on-off-on" mode of red emission nitrogen, boron, sulfur co-doped carbon dots (NBS-CDs). Red emission NBS-CDs were prepared via one-step hydrothermal synthesis by using 3-aminobenzeneboronic acid and 2,5-diaminobenzenesulfonic acid as precursors. Such NBS-CDs exhibited excellent optical properties and relatively high absolute fluorescent quantum yield compared with some reported NBS-CDs. Due to the strong quenching ability of Ag+ ions on the fluorescence of NBS-CDs, red emission NBS-CDs were used for the determination of Ag+ ions with high sensitivity and excellent selectivity. The fluorescence of NBS-CDs was recovered after the interaction between Ag+ ions and L-Cys, which realized the specific determination of L-Cys in human urine samples and human plasma samples. The established NBS-CDs-based fluorescent "on-off-on" sensor offered a relatively low detection limits of 0.35 μM for Ag+ ions and 0.045 μM for L-Cys based on three times signal-to-noise criteria. Notably, this strategy was applied for the visual detections of Ag+ ions and L-Cys in living human cancer cells (HeLa cells and MCF-7 cells). This method is simple, high sensitive, and excellent selectivity, which provided a new insight on the potential applications of NBS-CDs to develop the biosensor in clinical diagnosis and other biologically related areas.