Synthesis of Superparamagnetic Particles with Tunable Morphologies: The Role of Nanoparticle–Nanoparticle Interactions

Superparamagnetic microparticles are extensively used in the purification of biomolecules due to the speed and ease of magnetic separation. It is desirable that the microparticles used in biological affinity separations have both high surface area and high magnetic mobility to facilitate a high binding capacity of target biomolecules and their rapid removal from solution, respectively. Scaling laws for conventional spherical superparamagnetic microparticles are such that increasing the microparticle specific surface area results in a significant decrease in the magnetic mobility. More favorable combinations of these key parameters can be found if alternative microparticle morphologies are developed for use in affinity separations. Emulsion-templated self-assembly of iron oxide nanoparticles into microparticles using oil-in-water emulsions was carried out using a modified Couette shear mixer with separate inlet ports for the oil and aqueous phases, enabling high throughput microparticle synthesis. By controlling the dissolved nanoparticle concentration and nanoparticle surface activity at the droplet interfaces, the resulting microparticles were tuned to spherical, dimpled, or crumpled morphologies. The specific binding capacity and magnetic mobility of each type of microparticle were measured by a peroxidase-based colorimetric assay and by their magnetic field-induced motion in a viscous fluid, respectively. Superparamagnetic microparticles with dimpled and crumpled morphologies were found to have higher specific binding capacities compared to spherical microparticles, while maintaining high magnetic field velocities due to their high iron oxide content. Superparamagnetic microparticles with these novel morphologies would make excellent tools for affinity-based bioseparations where binding capacity and magnetic mobility are key factors.