(383t) Magnetophoresis and Field-Free Diffusion of Magnetic Nanoparticles in Liquid Media: Exploring the Effect of Nanoparticles Size and Concentration

Magnetic nanoparticles (MNPs) have been increasingly studied and successfully used as pivotal agents in chemical and environmental engineering problems due to physical traits that manifest at the nano-level and facilitates their precise manipulation through magnetophoresis. Magnetic separation is an essential step in many applications since it is often necessary to isolate a binding entity that is absorbed onto the nanoparticle’s surface or manipulate the particles’ trajectory toward a specific region [1]. Superparamagnetic iron oxide nanoparticles (SPIONs) are a class of MNPs that has been undergoing rapid advancement, finding place in many sorts of applications due to, among other properties, their superparamagnetic trait that translates into a zero net magnetization under field-free conditions. It is theorized that self-assembly may occur during magnetophoresis, making MNPs aggregate into larger superstructures and accelerate their separation due to larger magnetophoretic forces [2]. In contrast, their diffusion upon removal from a magnetic field causes particles to migrate back to the liquid medium in a dynamic fashion. While magnetophoresis is driven by a magnetic gradient, diffusion is driven by a concentration gradient within the liquid phase. This latter aspect is poorly explored in literature despite being present in systems that demand restoration and recycling of MNPs, for example. The goal of this study is two-fold: first, quantitively explore the effect of SPIONs initial concentration and size in a water dispersion while considering self-assembly during the separation process. Second, we intend to evaluate the extent to which formed aggregates affect diffusion of SPIONs back to the water medium. To study the behavior of SPIONs in these conditions, we will employ an image-based approach capable of estimating mass concentration profiles based on pixel intensity values of magnified pictures [3]. The commercial SPIONs employed in this study have a Fe₃O₄ magnetic core with a polymeric coat and will be purchased from Ocean Nanotech (San Diego, CA, USA) in the form of stable aqueous dispersions. To perform the batch magnetic separation of SPIONs, a quadrupole magnetic sorter (QMS) is used as the source of the magnetic gradient necessary to induce SPIONs’ magnetophoresis. Magnified pictures are then taken immediately after removal from the QMS using a portable microscope to characterize the timely extent of the separation. Following, diffusion is tracked through a series of pictures taken within different time intervals. By using this approach combined with a factorial experimental design, we expect to estimate the dependency of separation and diffusion of SPIONs to the dispersion’s initial concentration. While the Einstein-Stokes equation assumes an inverse relationship between diffusivity and particle size, the influence of particle concentration on this parameter remains to be further explored.

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2. Faraudo, J.; Camacho, J. Cooperative magnetophoresis of superparamagnetic colloids: theoretical aspects. Colloid and Polymer Science 2009, 288, 207-215, doi:10.1007/s00396-009-2107-z.
3. Wu, X.; Gomez-Pastora, J.; Zborowski, M.; Chalmers, J. SPIONs self-assembly and magnetic sedimentation in quadrupole magnets: Gaining insight into the separation mechanisms. Sep Purif Technol 2022, 280, doi:10.1016/j.seppur.2021.119786.