Reagentless amperometric carbohydrate antigen 19-9 immunosensor based on direct electrochemistry of immobilized horseradish peroxidase

A reagentless immunosensor for rapid determination of carbohydrate antigen 19-9 (CA19-9) in human serum was proposed. This strategy was based on the immobilization of antibody in colloidal gold nanoparticle modified carbon paste electrode and the direct electrochemistry of horseradish peroxidase (HRP) that was labeled to a CA19-9 antibody. The nanoparticles were efficient for preserving the activity of immobilized biomolecules. Thus, the immobilized HRP displayed its direct electrochemistry with a rate constant of 1.02 s−1. The incubation of the immunosensor in phosphate buffer solution (PBS) including CA19-9 antigen leading to the formation of antigen–antibody complex, which made the block of electron transfer of HRP toward electrode and resulted in significant peak current decrease of HRP. Under the optimal conditions, the current decrease was proportional to CA19-9 concentrations ranging from 2 to 30 U/ml with a detection limit of 1.37 U/ml at a current decrease by 10%. The immunosensor showed an acceptable accuracy compared with those obtained from immunoradiometric assays, with intra-assay coefficient of 7.3 and 6.9% at CA19-9 concentrations of 5 and 15 U/ml, respectively, and inter-assay coefficient of 9.6% at a CA19-9 concentration of 20 U/ml. The storage stability was acceptable in a pH 7.0 PBS at 4 °C for 10 days. This method avoids the addition of electron transfer mediator, thus simplifies the immunoassay procedure and decreases the analytical time. It provides a new promising platform for clinical immunoassay.