This study designed a "turn-off-on" fluorescence analysis method based on carbon quantum dots (CQDs) to detect metal ions and amino acids in real sample systems. CQDs were derived from green pomelo peel via a one-step hydrothermal process. The co-doped CQDs with N and S atoms imparted excellent optical properties (quantum yield = 17.31%). The prepared CQDs could be used as fluorescent "turn-off" probes to detect Fe3+ with a limit of detection of 0.086 µM, a linear detection range of 0.1-160 µM, and recovery of 83.47-106.53% in water samples. The quenched CQD fluorescence could be turned on after adding L-cysteine (L-Cys), which allowed detection of L-Cys with a detection limit of 0.34 µM and linear range of 0.4-85 µM. Recovery of L-Cys in amino acid beverage was 87.08-122.74%. Visual paper-based testing strips and cellulose/CQDs composite hydrogels could be also used to detect Fe3+ and L-Cys.
Keywords: Fe3+; L-cysteine; carbon quantum dots; fluorescence probe; pomelo peel; turn-off–on.