Magnetic nanoparticle-enhanced biosensor based on grating-coupled surface plasmon resonance.

A highly sensitive surface plasmon resonance (SPR) biosensor employing magnetic nanoparticle (MNP) assays is presented. In the reported approach, MNPs simultaneously served as "vehicles" for rapid delivery of target analyte from a sample to the sensor surface and as labels increasing the measured refractive index changes that are associated with the binding of target analyte. An optical setup based on grating-coupled surface plasmon resonance (GC-SPR) was used with a magnetic field gradient applied through the sensor chip for manipulating with MNPs on its surface. Iron oxide MNPs and a sensor surface with metallic diffraction grating were modified with antibodies that specifically recognize different epitopes of the analyte of interest. The sensitivity of the biosensor was investigated as a function of mass transport of the analyte to the sensor surface driven by diffusion (free analyte) or by the magnetic field gradient (analyte bound to MNPs). Immunoassay-based detection of β human chorionic gonadotropin (βhCG) was implemented to evaluate the sensitivity of the MNP-enhanced GC-SPR biosensor scheme. The results reveal that the sensitivity of βhCG detection was improved by 4 orders of magnitude compared with the regular SPR sensor with direct detection format, and a limit of detection below pM was achieved.