Label-free immunosensing for α-fetoprotein in human plasma using surface plasmon resonance

In this study, we attempted to develop a surface plasmon resonance (SPR)-based immunoassay sensor to detect α-fetoprotein (AFP) in human plasma at the nanogram level, as is required for clinical diagnosis of hepatocellular tumors. A self-assembled monolayer (SAM) surface of tri(ethylene glycol) (TEG) and carboxyl group-terminated hexa(ethylene glycol) (HEG) was employed to suppress the nonspecific adsorption of plasma components onto the sensor surface. AFP was detected by a sandwich-type immunoassay using two kinds of antibodies, primary and secondary, in this system. The SPR signal shift was further enhanced by applying an antibody (polyclonal) against the second antibody. With this method, the SPR signals were highly intensified, and so nanogram levels (ng/ml) of AFP could be easily detected with a high signal/noise ratio, as is necessary for clinical diagnosis. It is expected that our SPR-based immunoassay method can also be applicable to the detection of several other tumor markers that are present in low concentrations in human blood.