Visual and fluorometric lateral flow immunoassay combined with a dual-functional test mode for rapid determination of tetracycline antibiotics.

A fluorometric immunochromatographic assay (FICA) is described where ZnCdSe/ZnS quantum dots (QDs) act as fluorescent label and gold nanoparticles (AuNPs) act as quencher. The assay works in the "turn-on" mode, i.e. the fluorescent signal (best measured at excitation/emission wavelengths of 302/525 nm) increases with the increase of analyte concentration. This assay can detect tetracycline antibiotics including tetracycline, oxytetracycline, chlortetracycline, and doxycycline. It is not interfered by other veterinary drugs. The visual limits of detection (LODs) for the tetracycline antibiotics are 2 μg·L-1 in buffer, 20 μg·L-1 in milk, and 40 μg·kg-1 in animal muscle tissue. The assay (including sample treatment) can be performed within 30 min. The FICA based on "turn on" mode is more sensitive than the colloidal gold-based immunochromatographic assay (CGICA) and quantum dot-based immunochromatographic assay (QDICA) based on "turn off" mode using either AuNPs or QDs as signal labels. One strip can simultaneously provide the fluorescent test results in the "turn on" mode on the basis of QD luminescence quenching under UV light. The colorimetric test is of the "turn off" mode based on the formation of a red coloration due to the use of AuNPs under natural light. The use of such a dual-functional test mode allows for rapid semi-quantitative determination of tetracycline antibiotics in milk and tissue samples. Graphical abstract Schematoc of a fluorometric immunochromatographic assay (FICA) based on fluorescence quenching of quantum dot (QD) by gold nanoparticle (AuNP) combined with a dual-functional test mode under UV light (turn on mode) and natural light (turn off mode) to visually detect tetracycline antibiotics.