Nature-driven blue-emissive N, S-CDs: Harnessing sequential “switch-off-on” fluorescence signals for detection of chrysin and Al³⁺ along with cellular imaging versatility

• N, S-CDs were synthesized via in situ microwave-assisted synthesis from snake melon. • A sequential “Switch-off-on” fluorescence probe based on N, S-CDs was developed for chrysin and Al 3+ . • The exceptional characteristics of the N, S-CDs enabled their application in cell imaging. • The sensing mechanism for chrysin and Al 3+ was deeply investigated. • The method could efficiently determine chrysin and Al 3+ in real samples. . Novel biocompatible blue emissive nitrogen and sulfur co-doped carbon dots (N, S-CDs) were synthesized via in situ microwave-assisted synthesis from snake melon ( Cucumis melo var. flexuosus ) for the first time. The N, S-CDs have been utilized for fluorimetric determination of chrysin (CHR). The fluorescence emitted by the N, S-CDs (λ em = 460 nm) decreased upon the addition of CHR due to dynamic quenching and the inner filter effect (IFE). The reduction in emission intensity exhibits a linear relationship over a concentration range of 2.0-275.0 μM with a low limit of detection (LOD) of 0.0837 μM. By contrast, the quenched fluorescence of the N, S-CD/CHR system was restored upon the addition of Al³⁺, which efficiently displaced CHR from the surface of the N, S-CDs, prompting fluorescence "switch-on". The retrieved fluorescence intensity was proportional to Al 3+ concentration over a range of 50.0-4000.0 μM, with a LOD of 2.96 μM. This sequential approach was efficiently utilized to determine CHR and Al 3+ in real pharmaceutical, biological, and environmental samples with accurate and precise findings. Furthermore, due to their minimal cytotoxicity, the N, S-CDs were also successful for cellular imaging on HepG2 cells. The proposed sensing platform provides several merits, including simplicity, cost-efficiency, long-lasting stability, high selectivity, and exceptional sensitivity. These findings not only contribute to the growing field of carbon-based nanomaterials but also offer a sustainable and multifunctional platform for environmental and biomedical applications. .