Gold nanoparticles based lateral flow immunoassay with largely amplified sensitivity for rapid melamine screening

The sensitivity of current lateral flow strip (LFS) based assays is a technical bottleneck. The authors describe a method for signal amplification to give a 10- to 25-fold improvement of the detection limit without compromising the assay time and operational mode. The assays includes the following steps: (a) immobilization of melamine antibody on gold nanoparticles (AuNPs); (b) immobilization of BSA antibody on other AuNPs; (c) coating a nitrocellulose (NC) membrane with coating antigen and secondary antibody against the melamine antibody as the test line and control line, respectively; (d) introducing an enhancement pad for signal amplification by immuno-recognition of the blocking protein on the first conjugation pad by the antibody on the enhancement pad. This results in dual labeling of AuNPs on the test line and enhances the optical intensity of test line at the same time of detection. The signal amplification makes the optical intensity change of the test line distinguishable even at low concentrations. The assay has a detection limit of 1.4 ppb (at an S/N ratio of 3). It was applied to the determination of melamine in milk, and the results obtained with this LFS are in good agreement with those obtained with instrumental methods. This demonstrates the practical applicability of the amplified LFS to real food samples. In our perception, this approach has a wide scope because it paves the way to the systematic amplification of the sensitivity of lateral flow strips.