Improvement of Water Dispersion of Magnetic Nanoparticles and Their Effect on Magnetic Relaxation

Magnetic relaxation is a critical phenomenon in the application of magnetic nanoparticles (MNPs) in magnetic hyperthermia therapy and imaging. Effective magnetic relaxation time is an essential factor for understanding the relaxation process. Because the hydrodynamic volume is proportional to the Brownian relaxation time, it represents a significant parameter in this context. Because MNPs tend to aggregate, aggregates must be considered when determining hydrodynamic volumes, and several studies have adopted single-core models. However, in this study, we distinguished between the primary particle size (PPS), which refers to the diameter of individual MNPs, and secondary particle size (SPS), which represents the diameter of aggregates. We investigated whether conventional magnetic relaxation models could be applied under these considerations. The magnetic susceptibility calculated from the distributions of PPS and SPS was compared with that estimated from heat generation under an alternating magnetic field, revealing the importance of SPS in the discussion of Brownian relaxation. Furthermore, we conducted experiments on silica coating and ultrasonic treatment as strategies to reduce the average and variance of the SPS of MNPs. The experiments identified the optimal conditions for the ratio of MNPs to tetraethyl orthosilicate in the silica coating and exposure time of the ultrasonic treatment. The findings of this study present effective strategies for designing SPS of MNPs and provide evidence that aggregates are essential for understanding the magnetic properties of MNPs.