Determining the time-dependent effective relaxation time of biofunctionalized magnetic nanoparticles conjugated with biotargets by using a high-T_c SQUID-based ac susceptometer for a magnetic immunoassay

In this study, we determined the time-dependent effective relaxation time (τ_eff) 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-T_c superconducting quantum interference device-based ac susceptometer. The detected θ(t) was then analyzed to derive τ_eff(t), the results of which showed that τ_eff(t) increased over time. Additionally, the change in τ_eff after the BMNs conjugated with the AFP (Δτ_eff) increased with the AFP concentration (Φ_AFP). 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 τ_eff. We established the relationship between Δτ_AFP and Φ_AFP and thus provide a foundation for assaying unknown quantities of AFP.