A simple colorimetric and fluorescent “on-off-on” dual-mode sensor based on cyan fluorescent carbon dots/AuNPs for the detection of L-cysteine and Zinc thiazole

Cyan fluorescent carbon dots (G-CDs) were prepared from Glutathione (GSH) and m-phenylenediamine by one-step hydrothermal method. Interestingly, the synthesized G-CDs were modified with many functional groups, which can reduce Au3+ to Au-nanoparticles (AuNPs). The formed AuNPs led to fluorescence quenching of G-CDs because of internal filter effect (IFE). Furthermore, a fluorescent and colorimetric dual-mode sensor based on G-CDs/AuNPs was constructed and further applied for the analysis of L-cysteine (L-cys) and Zinc thiazole (ZTH). The fluorescence of G-CDs/AuNPs recovered from "turn-off" to "turn-on", which may be owing to the interaction of L-cys and ZTH with AuNPs, resulting in the aggregation of AuNPs and disappearance of IFE. At the same time, the surface plasmon resonance (SPR) of AuNPs was reduced. The dual-mode sensing platform not only had high sensitivity and selectivity for L-cys and ZTH, but also conducted visual detection. The fluorescence and absorbance detection limits for L-cys and ZTH were 0.01 µM, 0.1 µM, and 0.035 mg/L, 0.05 mg/L, respectively. It should be noted that G-CDs/AuNPs sensor has been successfully applied to monitor L-cys and ZTH in skin lotion, milk, lettuce, tea and other real samples, proving the potential application.