Flow-injection immuno-bioassay for interleukin-6 in humans based on gold nanoparticles modified screen-printed graphite electrodes

A flow-injection electrochemical immunoassay system based on a disposable immunosensor for the determination of interleukin-6 (IL-6) was proposed. The immunosensor was prepared by entrapping horseradish peroxidase (HRP)-labeled IL-6 antibody into gold nanoparticles-modified composite membrane at a screen-printed graphite electrode. With a non-competitive immunoassay format, the immunosensor was inserted in the flow system with an injection of sample, and the injected sample containing IL-6 antigen was produced transparent immunoaffinity reaction with the immobilized HRP-labeled IL-6 antibody. The formed antigen–antibody complex inhibited partly the active center of HRP, and decreased the immobilized HRP to H₂O₂ reduction. The performance and factors influencing the performance of the immunosensor were investigated. Under optimal conditions, the current change obtained from the labeled HRP relative to thionine–H₂O₂ system was proportional to the IL-6 concentration in the range of 5–100 ng L⁻¹ with a detection limit of 1.0 ng L⁻¹ (at 3δ). The flow-injection immunoassay system could automatically control the incubation, washing and measurement steps with acceptable reproducibility and good stability. Moreover, the proposed immunosensors were used to analyze IL-6 in human serum specimens. Analytical results of clinical samples show the developed immunoassay has a promising alternative approach for detecting IL-6 in the clinical diagnosis.