Colorimetric immunoassay for human chorionic gonadotropin by using peroxidase-mimicking MnO2 nanorods immobilized in microplate wells.

A colorimetric immunoassay is described for the pregnancy marker human chorionic gonadotropin (HCG). The assay is based on the use of MnO2 nanorods acting as a peroxidase mimic. The nanorods were prepared by a hydrothermal method using EDTA as a template. They exhibit excellent peroxidase-like activity and stability. The nanorods were immobilized in a chitosan matrix in the wells of a BSA-modified microplate which then were further modified with streptavidin and biotinylated capture antibodies. The specific recognition between HCG and the antibodies in wells inhibit the peroxidase-like activity of the nanorods. Hence, the substrate 3,3',5,5'-tetramethylbenzidine is less efficiently catalytically oxidized by H2O2 to form a blue colored product. This results in a decrease in the absorbance at 652 nm. Response is linear in the 0.5 to 400 mIU·mL-1 HCG concentration range, and the detection limit is 0.36 mIU·mL-1 under optimized conditions. The method is highly specific, acceptably reproducible and stable. It was applied to the determination of HCG concentration in serum samples, and results were in good agreement with data obtained by the reference method." Graphical abstract Schematic presentation of colorimetric immunoassay for human chorionic gonadotropin (HCG) by using MnO2 nanorods with peroxidase-like activity immobilized in microplate wells. The specific recognition between HCG and the antibodies in wells inhibits the peroxidase-like activity of the nanorods. TMB: 3,3',5,5'-tetramethylbenzidine.