Characterization of magnetic relaxation when biofunctionalized magnetic nano-particles are associated with biomarkers in the liquid state in biomedical applications

This study determined the characteristics of the time-dependent effective relaxation time τ_eff and magnetization M when biofunctionalized magnetic nanoparticles (BMNPs) associated with biomarkers in a liquid immunoassay. Carcinoembryonic antigen (CEA) was used as the biomarker. BMNP is an anti-CEA that was coated onto dextran-coated Fe₃O₄ and labeled as Fe₃O₄-anti-CEA. The phase lag θ of M with respect to the applied field H was measured using a sensitive homemade alternative-current susceptometer, and θ was measured using lock-in detection. The results were used to estimate τ_eff using the relationship tanθ(t) = ωτ_eff(t), where 2πf = ω and f is the excitation frequency. Δτ_eff increased with Φ_CEA, where Δτ_eff was the increment of τ_eff after Fe₃O₄-anti-CEA associated with CEA, and Φ_CEA was the concentration of CEA. Additionally, M enhanced when Φ_CEA increased. We attributed these enhancements to magnetic dipole-dipole interactions among MNPs that contributed extra M and in turn enhanced τ_eff. Magnetic clusters after the association were verified using a transmission electron microscope. This study established the relationship between Δτ_eff/τ_eff,0 and Φ_CEA, where τ_eff,0 = τ_eff (Φ_CEA = 0), allowing the assay of an unknown amount of CEA molecules.