Influence of Viscosity on Linear Dichroism Change in Magnetic Nanoparticles Under Damped Oscillating Magnetic Field

Abstract

Magnetically induced linear dichroism in a suspension of magnetic nanoparticle (MNP) reflects the rotational motion of the MNP. This phenomenon should be useful to measure the local viscosity around the MNP. In this study, we investigated the influence of the viscosity of an MNP suspension on the linear dichroism, and then a couple of robust parameters for viscosity measurements were found. We measured the polarizing angle change in an MNP suspension in a damped oscillating magnetic field (DOMF). Just after the application of DOMF, the polarizing angle change increased. The oscillation of the polarizing angle, whose frequency was twice that of DOMF, was observed. We focused on the amplitude and the phase of this oscillation. The amplitude of the polarizing angle decreased with an increase in the viscosity, and the variation can be fitted with a linear fractional function. The amplitude also depends on the MNP concentration. This fact is undesirable for local viscosity measurement because it is difficult to estimate the precise concentration of MNP in a sample, such as a biological cell. We found the proportionality between the amplitude and the peak value of the polarizing angle change. The proportional coefficient had a similar dependence on the viscosity to the amplitude, but it did not depend on the MNP concentration. Therefore, this proportional coefficient seems to be useful for viscosity measurement. The phase of the polarizing angle change increases with the viscosity and does not depend on the MNP concentration. Since the phase varied with the field amplitude, it is expected that the use of an ac magnetic field with constant amplitude and a lock-in amplifier makes the viscosity measurement sensitive and precise.