Highly Sensitive Fluorescence Assay of Enterotoxin A in Milk Using Carbon Quantum Dots as a Fluorophore

Abstract

Staphylococcal enterotoxin A (SEA) is an exotoxin that releases from gram-positive bacteria, and it is a source of food poisoning. Here, the luminescence method is introduced to quantify and detect SEA using carbon quantum dots (CQDs) as fluorophore; SEA antibody loaded Fe3O4 nanoparticles (Fe3O4@Ab) are prepared for SEA extraction. SEA in standards and milk samples were conjugated with Fe3O4@Ab (Fe3O4@Ab@SEA), separated by a 1.4 T magnet, and dispersed in phosphate buffer saline. The fluorescence of CQDs is quenched when mixed with Fe3O4@Ab@SEA, without elution of SEAs from the Fe3O4@Ab@SEA. It is a fast and reliable method compared with the immunoassay counterpart. At optimum quantification conditions, a linear relationship between SEAs concentration and CQDs fluorescent quenching was observed from 4 to 2000 pg mL−1 with the detection limit 2 pg mL−1. Repeatability and reproducibility were calculated as a percentage of relative standard deviations and were from 1.56 to 11.76%. Finally, the proposed method was applied to measure the SEA contents of four sterilized and homogenized milk samples without fat precipitations and have shown significant reliability.