Environmentally friendly carbon quantum dots: A turn-off/turn-on probe for metal ions and norfloxacin

A straightforward and environmentally sustainable hydrothermal synthesis approach is introduced for creating carbon quantum dots (CQDs) from the Chinese herbal medicine (CHM) residue, with thorough studies on their structural and optical characteristics. A considerable quantum yield (QY) of 62.0 % and a blue-violet fluorescence were observed in the prepared CQDs. They possess excellent photoluminescence, high solubility in water, and good dispersibility, averaging 3.01 nm in size. Interestingly, the obtained CQDs can be served as a turn-off probe for Fe³⁺/Hg²⁺ as well as a turn-on probe for norfloxacin (NOR). Additionally, the limits of detection (LODs) for Fe³⁺, Hg²⁺, and NOR were measured at 0.48 μM, 1.67 μM, and 0.80 μM, respectively, which were markedly superior to those previously reported for CQDs, demonstrating their effectiveness in detecting Fe³⁺, Hg²⁺ and NOR in aqueous solutions. The fluorescence quenching of Fe³⁺ and Hg²⁺ was primarily attributed to the IFE and PET, respectively. In contrast, the fluorescence enhancement associated with NOR was predominantly due to hydrogen bonding interactions between CQDs and NOR, as well as the formation of complexes. The cytotoxicity assay revealed a cell viability rate exceeding 85 % at a concentration of 200 μg/mL, suggesting that the prepared CQDs have acceptable biocompatibility. This method proved successful in determining Fe³⁺, Hg²⁺ and NOR in tap and river water samples, suggesting good application prosperity of the CQDs-based probe.