Non-labeled immunoassay based on zeta-potential analysis

Immunoassay has been widely used for the detection of target analytes in complex mixtures such as blood by using highly specific antigen-antibody interactions. In this work, antibodies against a target analyte was immobilized to magnetic beads and the change of surface charge of a magnetic bead was measured after the binding of a target protein by using a zeta potential analyzer. For the feasibility test of the zeta potential analysis for immunoassays, horseradish peroxidase (HRP) was used as a model target analyte. The limit of detection (LOD) of HRP from zeta potential analysis was estimated to be less than 1 ng/mL and the dynamic range was from less than 1 ng/mL to more than 1 μg/mL. The application to medical diagnosis was demonstrated by detection of C-reactive protein (CRP) which is known to be a biomarker for inflammatory diseases. In the case of CRP samples in human serum, the limit of detection was estimated to be around 10 ng/mL and the response curve shows also a linear correlation at the concentration range from 10 ng/mL to 1 μg/mL. From these results, the the immunoassay based on zeta potential analysis was considered to be feasible for medical diagnosis of CRP in serum.