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

Abstract

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 × 10⁻⁹ M to 1.0 × 10⁻⁷ M, with a correlation coefficient of 0.9971 and a limit of detection of 1.8 × 10⁻⁹ M. The relative standard deviation for 6.0 × 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.