High Permeability Silicone-Based Magnetic Microspheres for Microscale Force Spectroscopy

Abstract

A wide variety of magnetic microspheres are currently available for applications such as cell sorting, chemical separations, or microscale force spectroscopy. The smallest spheres (10 nm - 500 nm) generally consist of solid iron-oxide nanoparticles synthesized via the coprecipitation of iron salts, while many larger products (2 μm - 100 μm) consist of polymer microspheres which are subsequently saturated with magnetic nanoparticles. However, the latter process may result in incomplete saturation and therefore diminishing magnetic concentration as sphere diameter increases. In addition, a lack of confluence between these two methods of production has resulted in a dearth of products in the 0.5 - 2 μm range. We present here a bottom-up approach to magnetic microsphere fabrication which allows us to scale the diameter of our microspheres between 0.2 - 100 microns while maintaining a constant magnetic content throughout. In this approach, we use a novel magnetic silicone consisting of magnetite nanoparticles complexed with a monolayer of poly(dimethyl siloxane-co-aminopropylmethyl siloxane). The magnetic content of the composite may be tuned from 0 - 50% wt. without nanoparticle aggregation, resulting in a highly magnetic silicone fluid which is homogenous at scales below 100 nm. We demonstrate the production of microspheres of this material by an emulsion process in which microsphere diameter may be tuned by careful selection of surfactant type and concentration. Finally, we show that the resulting spheres compare favorably with leading commercial competitors in terms of magnetic force application.