Development of a detector-free semiquantitative immunochromatographic assay with major aflatoxins as target analytes

Recent trend toward semiquantitative strip tests has been driven by a strong demand for rapid screening of toxic or harmful substances in food. Here, a detector-free semiquantitative immunochromatographic assay (ICA) has been developed. In the assay, the test zone is formed by dispensing three test lines [TLs (TL-I, TL-II and TL-III)] with the same capture reagent onto a nitrocellulose membrane. The competition between the migrating analyte and the immobilized capture reagent on three TLs for the limited number of antibody binding sites causes less nanogold-Ab probe to accumulate on TLs, which provides an easily visible color owing to the red color itself. When analyte concentration is up to the threshold level, no color appears on the TL. That's to say, the appearing number of color bands after a reaction indicates the content (range) of analyte. Analyte concentration levels can thus be determined by observing the number of TLs developed in the test zone. Taking major aflatoxins as target analytes, the threshold levels for TL-I-III and the visual detection limit of the detector-free semiquantitative strip (DFQ-strip) assay were determined, respectively. Results show that the DFQ-strip assay is superior to traditional ICAs because it offers multiple dynamic detection ranges and semiquantitative analytical capability.