Novel colorimetric enzyme immunoassay for the detection of carcinoembryonic antigen

We developed a highly sensitive colorimetric enzyme immunoassay for the detection of carcinoembryonic antigen (CEA). This method employed gold nanoparticles (AuNPs) as carriers of anti-CEA antibody labeled with biotin, which served as an affinity tag for streptavidin-horseradish peroxidase (streptavidin-HRP) binding. Using this strategy, about 12 HRP molecules were coated onto each AuNP. Through “sandwich-type” immunoreaction, the AuNP–anti-CEA–HRP complex was brought into the proximity of magnetic microparticle. As a result, HRP molecules confined at the surface of the “sandwich” immunocomplexes catalyzed the enzyme substrate and generated an optical signal. The spectrophotometric measurement confirmed effective signal amplification. The signals were linearly dependent on CEA concentrations from 0.05 to 50 ng mL−1 in a logarithmic plot, with a detection limit of 48 pg mL−1. Intra- and inter-assay coefficients of variation were <8.5%. The CEA concentrations of serum specimens assayed by the developed immunoassay showed consistent results in comparison with those obtained by a conventional enzyme-linked immunosorbent assay. The developed method thus proved its potential use in clinical immunoassay of CEA.