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 τeff 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 τeff using the relationship tan θ(t) = ωτeff(t), where 2πf = ω and f is the excitation frequency. Δτeff increased with ΦCEA, where Δτeff was the increment of τeff after Fe3O4-anti-CEA associated with CEA, and ΦCEA was the concentration of CEA. Additionally, M enhanced when ΦCEA increased. We attributed these enhancements to magnetic dipole–dipole interactions among MNPs that contributed extra M and in turn enhanced τeff. Magnetic clusters after the association were verified using a transmission electron microscope. This study established the relationship between Δτeff/τeff,0 and ΦCEA, where τeff,0 = τeff (ΦCEA = 0), allowing the assay of an unknown amount of CEA molecules.