Controlling Transport and Chemical Functionality of Magnetic Nanoparticles

A wide range of metal, magnetic, semiconductor, and polymer nanoparticles with tunable sizes and properties can be synthesized by wet-chemical techniques. Magnetic nanoparticles are particularly attractive because their inherent superparamagnetic properties make them desirable for medical imaging, magnetic field assisted transport, and separations and analyses. With such applications on the horizon, synthetic routes for quickly and reliably rendering magnetic nanoparticle surfaces chemically functional have become an increasingly important focus. This Account describes common synthetic routes for making and functionalizing magnetic nanoparticles and discusses initial applications in magnetic field induced separations. The most widely studied magnetic nanoparticles are iron oxide (Fe2O3 and Fe3O4), cobalt ferrite (CoFe 2O4), iron platinum (FePt), and manganese ferrite (MnFe 2O4), although others have been investigated. Magnetic nanoparticles are typically prepared under either high-temperature organic phase or aqueous conditions, producing particles with surfaces that are stabilized by attached surfactants or associated ions. Although it requires more specialized glassware, high-temperature routes are generally preferred when a high degree of stability and low particle size dispersity is desired. Particles can be further modified with a secondary metal or polymer to create core-shell structures. The outer shells function as protective layers for the inner metal cores and alter the surface chemistry to enable postsynthetic modification of the surfactant chemistry. Efforts by our group as well as others have centered on pathways to yield nanoparticles with surfaces that are both easily functionalized and tunable in terms of the number and variety of attached species. Ligand place-exchange reactions have been shown quite successful for exchanging silanes, acids, thiols, and dopamine ligands onto the surfaces of some magnetic particles. Poly(ethylene oxide)-modified phospholipids can be inserted into nonpolar surface monolayers of as-prepared nanoparticles as a method for modifying the surface chemistry that induces water solubility. In general, reactive termini can subsequently be used to append a range of chemical groups. For example, surfactants with trifluoroethylester or azide termini have been used to attach a range of amine- or alkyne-containing species, respectively. Chemically functionalized magnetic nanoparticles are promising as advanced materials for analytical separations and analysis. Magnetic field flow fractionation leverages the size-dependent magnetic moments to purify and separate the components of a complex mixture of particles. Similarly, magnetic field gradients are useful for manipulating transport and separation in simple microfluidic devices. By either approach, magnet-induced transport of the particles is a simple method in which an attached reagent, catalyst, or bioanalytical tag can be moved between flow streams within a lab on a chip device.