Probing Orientational Dynamics of Magnetic Nanoparticles in Opaque Samples Using Near-Infrared Magnetic Linear Birefringence

We demonstrated the advantage of magnetic linear birefringence (MLB) measurement for studying the orientation dynamics of magnetic nanoparticles (MNPs) in various environments. It is expected that MNPs will be utilized as active probes of nanoscale measurements, such as nanorheology and homogeneous bioassay. The optical anisotropy induced in MNP suspensions under an AC magnetic field, including MLB, reflects the physical rotation of the MNP itself. MLB is measurable with near-infrared light, which can reduce undesirable extinction due to the scattering or absorption by the sample suspension. In this study, we built an apparatus for precise MLB measurement by refurbishing the previous one to measure magnetic linear dichroism under an AC magnetic field and confirmed the relationship between the MLB signal and the orientation of MNP. Then, two systems that were opaque for visible light were examined to show the unique advantage of the MLB measurement of MNPs: (1) MLB measurement applied to the MNP suspension with a wide concentration range, and the effect of the interparticle interaction on the orientation dynamics could be detected from MLB frequency spectra. (2) Magneto-liposomes (ML), small vesicles containing MNPs, could be measured, and the frequency spectra could provide information on the condition of MNPs in ML. Furthermore, it was possible to detect the phase transition of the lipid bilayer from the gel to the liquid crystal phase.