Exploring the Post-Separation Dynamic Behavior of Magnetic Nanoparticles through a Simple Image-Based Approach

Magnetic nanoparticles (MNPs), typically smaller than 100 nm, are usually synthesized from elements with magnetic properties like cobalt, nickel, iron, and their oxides. These particles may possess unique physical properties such as superparamagnetism, large surface area-to-volume ratios, and potential for surface functionalization, making them valuable for applications in targeted drug delivery, biosepararation, and environmental remediation. Magnetophoresis, which denotes the induced motion of MNPs through a viscous medium under a magnetic field, is one approach for manipulating and separating these particles.

Cooperative magnetophoresis can enhance the mobility of MNPs under certain conditions. Through magnetic dipole-dipole interactions, MNPs may aggregate into clusters that respond more effectively to external magnetic fields in terms of induced motion. This process is particularly useful for ultra-small particles (<50 nm), which typically exhibit low magnetophoretic velocities. Following, some applications require particle disaggregation accompanied by diffusion after magnetic separation to restore surface area and maintain their functional efficiency, especially in processes that involve pathogen capture and nanophotocatalysis, for instance. Despite its importance, this aspect of MNPs post-separation remains underexplored.

In this study, we examined the diffusion behavior of commercially available superparamagnetic iron oxide nanoparticles (SPIONs) of varying sizes (Ocean Nanotech, CA, USA) in the absence of a magnetic field (i.e. field free conditions). Pre-separation was conducted using a custom-built quadrupole magnetic sorter (QMS) that provides a constant magnetic gradient that induces MNPs magnetophoresis. To track SPIONs diffusion in an aqueous dispersion, we used a straightforward image-based approach capable of estimating mass concentration profiles based on image grayscale data, enabling real-time analysis of SPION dispersions at different initial concentrations.

This methodology can be applied as part of an analysis for recyclability of SPIONs in biocompatible fluids, with results relevant to biomedical and environmental applications. By fitting the collected concentration data into a diffusion model, we can estimate SPION diffusion coefficients based on particle size, according to the Stokes-Einstein equation. These findings contribute to a deeper understanding of MNP behavior in field-free conditions, aiding in the development of more efficient and sustainable magnetic nanoparticle technologies.