Electrochemical sensor for immunoassay of carcinoembryonic antigen based on thionine monolayer modified gold electrode

A sensor based on thionine monolayer modified gold electrode for determination of carcinoembryonic antigen (CEA) in human serum is proposed. The sensor is prepared by covalently binding thionine to a cysteamine self-assembled monolayer with p-phthaloyl chloride as a linkage, which gives a surface coverage of 8.97 ± 3.28 × l0−12 mol/cm2 for thionine. The electrochemistry of the immobilized thionine displays a surface-controlled electrode process with an average electron transfer rate constant of 1.47 ± 0.84 s−1. Based on an electrochemical enzyme-linked immunoassay by using the immobilized thionine as an electron transfer mediator between the electrode and the horseradish peroxidase (HRP) labeled anti-CEA antibody, a calibration curve with two linear ranges from 0.6 to 17 and 17 to 200 ng/mL and a detection limit of 0.2 ng/mL for CEA determination is obtained in pH 4.2 PBS containing 2.0 mmol/L H2O2 and 0.5 mol/L NaCl. The sensor shows a good accuracy. The precision and reproducibility are acceptable with the intra-assay CV of 4.9% and 5.9% at 10 and 100 ng/mL CEA concentrations, respectively, and the inter-assays CV of 7.8% at 100 ng/mL CEA. The response of thionine modified electrode shows only 1.6% decrease after 100 replicate measurements and the storage stability is acceptable in a pH 7.0 PBS at 4 °C for 1 week. The method avoids the addition of electron transfer mediator to the solution, thus is much simpler. The proposed method would be valuable for the diagnosis and monitoring of carcinoma and its metastasis.