Washing-Free Displacement Immunosensor for Cortisol in Human Serum Containing Numerous Interfering Species

Simple and sensitive competitive immunosensors for small molecules are difficult to obtain, especially in serum containing numerous interfering species (ISs) with different concentrations. Herein, we report a washing-free and sensitive (competitive) displacement immunosensor for cortisol in human serum, based on electron mediation of Os(bpy)₂Cl₂ between an electrode and a redox label [oxygen-insensitive diaphorase (DI)] (i.e., electrochemical-enzymatic redox cycling). The anticortisol IgG-DI conjugate bound to a cortisol-immobilized electrode is displaced by competitive binding of cortisol in serum and diffuses away from the electrode during incubation; therefore, the concentration of the displaced conjugate near the electrode becomes very low, even without washing. Electrochemically interfering ascorbic acid is converted to a redox-inactive species by ascorbate oxidase during incubation. The remaining bound conjugate mainly contributes to electrochemical currents. Compared with ferrocene methanol, Fe(CN)₆^4-, and Ru(NH₃)₆³⁺, the electrochemical and redox cycling behaviors of Os(bpy)₂Cl₂ are influenced significantly less by ISs in serum. Comparative studies reveal that washing-free displacement assay shows better cortisol-induced signal change than three other assays. The surface concentration of cortisol immobilized on the electrode is optimized, because the electrochemical signal is highly dependent on the surface concentration. When the washing-free displacement immunosensor is applied for the detection of cortisol in artificial serum, cortisol is measured with a detection limit of ∼30 pM within 12 min. The cortisol concentrations measured in clinical serum samples agree well with those obtained using a commercial instrument. The new immunosensor is highly promising for the simple, sensitive, and rapid point-of-care detection of small molecules.