Magnetophoretic focusing on submicron particles dispersed in a polymer-stabilized magnetic fluid

Abstract

Magnetophoresis is the migration of particles upon the application of an inhomogeneous magnetic field. The overall goal of this work was to investigate the magnetophoretic focusing of non-magnetic particles suspended in magnetic fluids, which are colloidal suspensions of nano-sized magnetic particles. With the magnetic fluid as the solvent, dispersed non-magnetic particles behave as if they were diamagnetic due to the difference in magnetic susceptibility between them and the surrounding magnetic continuum. When an inhomogeneous magnetic force is applied, a magnetic force acts on the colloidal particles, the magnitude of which is linearly proportional to the volume of the particles, the difference in the magnetic susceptibilities of the particles and the surrounding magnetic fluid, and the gradient of the square of the magnetic field. One potential application for this phenomenon is in the separation of submicron biological particles such as viruses, cell fragments, DNA and inclusion bodies. Magnetic fluids have several characteristics that make them attractive for use in separation. For example, they can be tailored to the separation needs at hand, manipulated using external magnetic fields, and completely removed through magnetic filtration. Since the scope of the work was to use physical forces for attaining the desired separations, the magnetic particles were designed and synthesized without any chemical affinity to the solute to be separated. They were prepared by coprecipitation of iron (II) and (III) ions to form magnetite, which is coated by a comb copolymer that serves two purposes: to limit growth of magnetite to about 10 nm and to stabilize the particles against aggregation.

(cont.) The polymer was prepared by a reaction between amine-terminated polyethylene oxide and polyacrylic acid. Characterization of the particles was done experimentally and theoretically. Dynamic light scattering was used to measure the diffusion coefficient and the hydrodynamic diameter of the particles, while transmission electron microscopy was used to measure the diameter of the magnetic core. Since the structure of the magnetic fluid is an important parameter in its application in any magnetophoretic separation, we characterized the aggregation behavior of the magnetic fluids using different theoretical techniques. Monte Carlo simulation was used to understand the clustering in sterically-stabilized magnetic fluids. Simulation results agree favorably with the scattering experiments with regards to the cluster sizes and fractal dimensions. The characterization of a closely related system, a charge stabilized magnetic fluid, was also performed to explain the finite cluster size observed experimentally. Next, we investigated magnetophoretic focusing in the synthesized magnetic fluid, as a means to separate submicron colloidal particles based on size. The magnetophoresis concepts were validated experimentally by monitoring the dynamic evolution of the concentration profile of fluorescently-tagged polymer beads of various sizes in a magnetic fluid upon the application of an inhomogeneous magnetic field. Polymer beads larger than 0.2 /um focused at the point of zero force, and the effect of the magnetic field on the particles was correlated with their size...