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

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 Al³⁺ 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 Al³⁺ 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.