Lateral flow immunoassay for 5-hydroxyflunixin based on near-infrared fluorescence molecule as an alternative label to gold nanoparticles.

A high-affinity monoclonal antibody (mAb) has been prepared and separately a gold nanoparticle (AuNP)-based and a near-infrared (NIR) fluorescence-based lateral flow immunoassay (LFA) developed for determination of 5-hydroxyflunixin residue in raw milk. The AuNP and IRDye® 800CW were used to label anti-5-hydroxyflunixin mAb to form the AuNP-mAb and NIR dye-mAb conjugates, respectively. Quantitative determination of 5-hydroxyflunixin was achieved by imaging the optical or fluorescence intensity of the AuNP-mAb and NIR dye-mAb captured on the test line. As a result, the detection limits of the AuNP-based LFA and NIR dye-based LFA were 0.82 and 0.073 ng/mL in raw milk, respectively. The considerable improvement on assay sensitivity of the NIR-based LFA can be attributed to the lower background and less antibody consumption per test than that of the AuNP-based LFA. The spiking experiment by the NIR-based LFA yielded 85.7-112.6% recovery with a relative standard deviation below 14%, indicating that it has satisfactory assay accuracy and precision. Furthermore, the analytical results of actual samples by the NIR dye-based LFA were consistent with that by instrumental analysis. Therefore, these results demonstrated that the NIR dye is an ideal alternative label to the conventional AuNP for the development of LFA for veterinary drugs in animal-origin food. Graphical abstract.