A CdTe/CdS/ZnS core/shell/shell QDs-based OFF-ON fluorescent biosensor for sensitive and specific determination of L-ascorbic acid

Herein we report a quantum dots (QDs)-based OFF-ON fluorescent biosensor for the sensitive and specific determination of L-ascorbic acid. The proposed one-pot L-ascorbic acid detection method is quite simple, rapid and convenient because of the elimination of the modification and separation procedures. In this contribution, the N-acetyl-L-cysteine (NAC)-capped CdTe/CdS/ZnS core/shell/shell QDs were synthesized in aqueous phase. Subsequently, KMnO4 was added into solution and attached to the QDs surface to effectively quench the fluorescence of the QDs, which rendered the QDs into fluorescence OFF status. After the addition of L-ascorbic acid into the QDs-KMnO4 system, the fluorescence of the QDs was then turned ON because L-ascorbic acid could bind with KMnO4 and break KMnO4 away from the surface of the QDs. Under the optimized conditions, the relative restored fluorescence intensity was directly proportional to the concentration of L-ascorbic acid in the range of 8.0 x 10(-9) M to 1.0 x 10(-7) M, with a correlation coefficient of 0.9971 and a limit of detection of 1.8 x 10(-9) M. The relative standard deviation for 6.0 x 10(-8) M L-ascorbic acid was 2.1% (n - 5). There was almost no interference from few common ions, carbohydrates, nucleotides and amino acids. The proposed method was applied to the determination of L-ascorbic acid in three synthetic samples, human urine samples and vitamin C tablets with satisfactory results. The possible fluorescence quenching mechanism of this fluorescent sensor was further investigated by UV-vis spectroscopy.