Carbon dots prepared from Litchi chinensis and modified with manganese dioxide nanosheets for use in a competitive fluorometric immunoassay for aflatoxin B.

An enzyme-linked immunoassay is described for the fluorometric determination of aflatoxin B1 (AFB). It is based on the use of carbon dots (CDs) synthesized by using Litchi chinensis as the carbon source via a hydrothermal method. The CDs were modified with MnO nanosheets upon which their blue fluorescence (with excitation/emission peaks at 340/425 nm) is quenched. In the presence of AFB, a competitive immunoreaction takes place between (1) AFB conjugated to bovine serum albumin and deposited in the wells of a microplate, and (2) antibody against AFB and labeled with alkaline phosphatase (ALP). On subsequent addition of ascorbic acid 2-phosphate, it will be hydrolyzed by ALP to form ascorbic acid and phosphate. The ascorbic acid produced reduces MnO nanosheets to Mn ions which then are relased from the CDs. This causes the recovery of fluorescence. Under optimum conditions, fluorescence decreases linearly with increasing AFB concentration in the range from 1.0 ng·kg to 10 μg·kg, and the limit of detection is 0.69 ng·kg. The precision of this method (expressed as RSD) is ±10.3%. The accuracy was tested by analyzing both naturally contaminated and spiked food samples, and the results were in good agreement with those obtained by the established ELISA. Graphical abstract Enzyme hydrolysate-triggered redox reaction of carbon dot-functionalized MnO nanosheets was developed for the fluorescence immunoassay of aflatoxin B in foodstuff.