(20b) Refinement of the Relationship between Brownian Force and Magnetic Force on Superparamagnetic Iron Oxide Nanoparticles

In the past two decades, magnetic nanoparticles have gained great attention due to their unique chemical and physical properties. MNPs have been widely used within medical fields. In this regard, magnetic field-assisted bio-separation can be performed by the binding of the particle to the target molecule using affinity ligands, which creates a complex particle-biomolecule that can be later separated using a magnetophoretic device. However, even the most advanced magnetic separation systems present some issues. Current devices based on permanent magnets are not optimized, using relatively low field and gradients, making the MNPs recovery a challenging process. Also, magnetophoresis is a complex process and the physics behind it are not completely understood.

Recently, we experimentally demonstrated the fast manipulation/separation of 5-15 nm MNPs. We employed a novel, permanent magnet-based system, that integrate multiple permanent magnets in specific arrangements, allowing the generation of fields and gradients higher than the ones provided by current permanent magnet systems. Commercial superparamagnetic iron oxide nanoparticles, SPIONs, are the target MNPs to be separated, and state-of-the-art analytical tools Small Angle X-ray Scattering (SAXS) are employed to study the SPION behavior inside the system. In-situ SAXS study indicated that no particle aggregation/agglomeration occurred during the separation, implying no dipole-dipole interactions are involved. Moreover, the Comsol simulation are carried out to validate the magnetic fields generated in the system, as well as the particle tracing modeling.

To summarize, it is theoretically challenging to isolate MNPs that are smaller than 50 nm from the media due to the hindrance of the particle's magnetic motion by Brownian motion and viscous drag. Our modeling and experimental findings indicate that there is a need to improve the understanding of the interplay between magnetic force and Brownian force to accurately predict the separation ability or stability of MNPs.