  Magnetic nanoparticles (MNPs) have been extensively used as contrasts and
tracers for bioimaging, heating sources for tumor therapy, carriers for
controlled drug delivery, and labels for magnetic immunoassays. Here, we
describe a MNP relaxation dynamics-based magnetic particle spectroscopy (MPS)
method for the quantitative detection of molecular biomarkers. In MPS
measurements, the harmonics of oscillating MNPs are recorded and used as a
metric for the freedom of rotational motion, which indicates the bound states
of the MNPs. These harmonics can be collected from microgram quantities of iron
oxide nanoparticles within 10 seconds. Using a streptavidin-biotin binding
system, we demonstrate the feasibility of using MPS to sense these molecular
interactions, showing this method is able to achieve rapid, wash-free
bioassays, and is suitable for future point-of-care (POC), sensitive, and
versatile diagnosis.
