In this study, we determined the time-dependent effective relaxation time (τeff) of biofunctionalized magnetic nanoparticles (BMNs) that were conjugated with biotargets for magnetic immunoassays. The BMNs were anti-alphafetoprotein (AFP)-coated onto dextran-coated magnetic nanoparticles to yield Fe3O4–anti-AFP. The biotarget was AFP. The time-dependent phase delay θ(t) of magnetization relative to the applied field strength was determined using a homemade high-Tc superconducting quantum interference device-based ac susceptometer. The detected θ(t) was then analyzed to derive τeff(t), the results of which showed that τeff(t) increased over time. Additionally, the change in τeff after the BMNs conjugated with the AFP (Δτeff) increased with the AFP concentration (ΦAFP). We attribute this to the additional magnetization generated when the BMNs conjugated with the AFP, which thereby reduced the Néel and Brownian relaxations and consequently improved the τeff. We established the relationship between ΔτAFP and ΦAFP and thus provide a foundation for assaying unknown quantities of AFP.
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