Robust immunosensing system based on biotin-streptavidin coupling for spatially localized femtogram mL⁻¹ level detection of interleukin-6

Detection of a very low amount of cytokines such as interleukin-6 (IL-6) in clinical fluids is important in biomedical research and clinical applications. Here, we demonstrate spatially-localized ultrasensitive (femtogram mL⁻¹) level detection of IL-6 in serum and in cell culture media. Our approach is based on a sandwich immunosensor fabricated on the surface of an optical fibre. Firstly, the biotinylated IL-6 capture antibody was immobilized on the fibre surface by biotin-streptavidin coupling. Then the fabricated fibre was used for capturing IL-6 followed by exposure to detection antibody which was labeled with the fluorescent magnetic nanoparticles to report the signal. A linear relationship between IL-6 concentration and the fluorescence signal was obtained in the range from 0.4 pg mL⁻¹ to 400 pg mL⁻¹ of IL-6, with the limit of detection down to 0.1 pg mL⁻¹. In addition, this optical fibre sensor was successfully applied for the localized detection of IL-6 with the spatial resolution of 200 μm and a sample volume of 1 μL. Finally, the performance of the fibre sensor was demonstrated by detection of IL-6 secreted by BV-2 cells with comparable performance of the conventional enzyme-linked immunosorbent assay (ELISA).