Gold nanoparticle-based colorimetric ELISA for quantification of ractopamine

The work describes a gold nanoparticle-based colorimetric enzyme-linked immunosorbent assay (ELISA) for ractopamine. The ELISA is based on an indirect competitive approach. In the presence of ractopamine, gold(III) ions are oxidized by H₂O₂ to form red AuNPs. On the other hand, the AuNP in solution are purple-blue due to aggregation if the sample does not contain ractopamine. The absorption, best measured at 560 nm, increases linearly in the 2 to 512 ng·mL^-1 ractopamine concentration range, and the detection limit is as low as 0.35 ng·mL^-1 in urine. Ractopamine can also be detected visually, even in the presence of other β-agonists and antibiotics. The results obtained by this method are consistent with those obtained by LC-MS/MS as demonstrated by analysis of sheep urine. The ELISA method described here is inexpensive, easy-to-use, and suitable for rapid screening of ractopamine in animal samples. Graphical abstract Schematic presentation of a colorimetric indirect competitive immunoassay for ractopamine. It is based on the use of catalase labeled IgG and the measurement of the absorption of red gold nanoparticles (AuNPs) that are generated by the reaction of gold ions with H₂O₂. In the absence of ractopamine, the solution becomes blue.