Gold Nanoparticles-Based Colorimetric Immunoassay of Carcinoembryonic Antigen with Metal–Organic Framework to Load Quinones for Catalytic Oxidation of Cysteine

This work reported gold nanoparticles (AuNPs)-based colorimetric immunoassay with the Cu-based metal-organic framework (MOF) to load pyrroloquinoline quinone (PQQ) for the catalytic oxidation of cysteine. In this method, both Cu²⁺ and PQQ in the MOF could promote the oxidation of inducer cysteine by redox cycling, thus limiting the cysteine-induced aggregation of AuNPs and achieving dual signal amplification. Specifically, the recombinant carcinoembryonic antigen (CEA) targets were anchored on the MOF through the metal coordination interactions between the hexahistidine (His₆) tag in CEA and the unsaturated Cu²⁺ sites in MOF. The CEA/PQQ-loaded MOF could be captured by the antibody-coated ELISA plate to catalyze the oxidation of cysteine. However, once the target CEA in the samples bound to the antibody immobilized on the plate surface, the attachment of CEA/PQQ-loaded MOF would be limited. Cysteine remaining in the solution would trigger the aggregation of AuNPs and cause a color change from red to blue. The target concentration was positively related to the aggregation and color change of AuNPs. The signal-on competitive plasmonic immunoassay exhibited a low detection limit with a linear range of 0.01-1 ng/mL. Note that most of the proteins in commercial ELISA kits are recombinant with a His₆ tag in the N- or C-terminal, so the work could provide a sensitive plasmonic platform for the detection of biomarkers.