SPIONs self-assembly and magnetic sedimentation in quadrupole magnets: Gaining insight into the separation mechanisms.

Superparamagnetic iron oxide nanoparticles (SPIONs) are currently popular materials experiencing rapid development with potential application value, especially in biomedical and chemical engineering fields. Examples include wastewater management, bio-detection, biological imaging, targeted drug delivery and biosensing. While not exclusive, magnetically driven isolation methods are typically required to separate the desired entity from the media in specific applications and in their manufacture and/or quality control. However, due to the nano-size of SPIONs, their magnetic manipulation is affected by Brownian motion, adding considerable complexities. The two most common methods for SPION magnetic separation are high and low gradient magnetic separation (HGMS and LGMS, respectively). Nevertheless, the effect of specific magnetic energy fields on SPIONs, such as horizontal (perpendicular to gravity), high fields and gradients (higher than LGMS) on the horizontal magnetophoresis and vertical sedimentation of SPIONs has only recently been suggested as a way to separate very small particles (5 nm). In this work, we continue those studies on the magnetic separation of 5-30 nm SPIONs by applying fields and gradients perpendicular to gravity. The magnetic field was generated by permanent magnets arranged in quadrupolar configurations (QMS). Different conditions were studied, and multiple variables were evaluated, including the particle size, the initial SPIONs concentration, the temperature, the magnetic field gradient and the magnetic exposure time. Our experimental data show that particles are subjected to horizontal magnetic forces, to particle agglomeration due to dipole-dipole interactions, and to vertical sedimentation due to gravity. The particle size and the type of separator employed (i.e. different gradient and field distribution acting on the particle suspension) have significant effects on the phenomena involved in the separation, whereas the temperature and particle concentration affect the separation to a lesser extent. Finally, the separation process was observed to occur in less than 3 mins for our experimental conditions, which is encouraging considering the long operation time (up to days) necessary to separate particles of similar sizes in LGMS columns that also employ permanent magnets.