A novel lateral flow immunoassay strip based on a label-free magnetic Fe3O4@UiO-66-NH2 nanocomposite for rapid detection of Listeria monocytogenes

Listeria monocytogenes (L. monocytogenes) is one of the most lethal pathogenic bacteria. Although the traditional microbial culture method has high sensitivity and selectivity for the diagnosis of L. monocytogenes, it is time-consuming and not suitable for on-site detection. A rapid, convenient and visualized on-site detection method is particularly needed. In this work, Fe3O4@UiO-66-NH2 was prepared for both magnetic separation and lateral flow immunoassay (LFIA) for the detection of L. monocytogenes by taking advantage of the easy separation of the magnetic core Fe3O4 and the high surface area of the outer layer UiO-66-NH2. Fe3O4@UiO-66-NH2 with a high surface area and good water-dispersibility and optical properties was synthesized by a simple hydrothermal process. It could directly adsorb on the surface of target bacteria and form Fe3O4@UiO-66-NH2-bacteria conjugates, without the labeling of an antibody. After magnetic separation and concentration, the Fe3O4@UiO-66-NH2-bacteria conjugates were detected by the antibody on the test line of the LFIA strip, resulting in a visible orange band. The capture efficiency and LFIA detection of Fe3O4@UiO-66-NH2 were optimized in this study. Under the optimal conditions, a good linear correlation between the test line intensity and the concentration of L. monocytogenes was obtained in the range of 10(5)-10(8) CFU mL(-1), and the limit of detection was 2.2 x 10(6) CFU mL(-1) by the naked eye. The Fe3O4@UiO-66-NH2-based LFIA strip showed strong specificity for L. monocytogenes, and the detection took 45 min without culture enrichment. Therefore, the proposed Fe3O4@UiO-66-NH2-based strip showed the advantages of simple synthesis, being label-free, low cost, good selectivity and convenience.