Label-Free Quantitative Immunoassay of Fibrinogen in Alzheimer Disease Patient Plasma Using Fiber Optical Surface Plasmon Resonance

We present a real-time quantitative immunoassay to detect fibrinogen in the blood plasma of Alzheimer’s disease patients using multimode fiber optical sensors in which surface plasmon resonance (SPR) was employed. Nanometer-thick bimetals including silver and aluminum were coated onto the core surface of the clad-free part (5 cm long) of the fiber for SPR excitation at the He-Ne laser wavelength of 632.8 nm. The histidine-tagged peptide was then coated on the metal surface to immobilize the fibrinogen antibody for the selective capture of fibrinogen among the proteins in the patient blood plasma. The SPR fiber optical sensor enabled quantitative detection of concentrations of fibrinogen from the different human patient blood at a detection limit of ∼20 ng/ml. We also observed a correlation in the fibrinogen concentration measurement between enzyme-linked immunosorbent assay and our SPR fiber-based sensors. This suggests that the presented SPR fiber-based sensors that do not rely on the use of labels such as fluorophores can be used for a real-time quantitative assay of a specific protein such as fibrinogen in a human blood that is known to contain many other kinds of proteins together.