Methods for preparing uniformly sized microparticles, with an optional polymeric coating generally include: 1) providing nanoparticles, preferably having a size of between 1 nm and 100 nm; 2) adding a hydrophobic surface layer to the nanoparticles; 3) making a suspension of the hydrophobic nanoparticles and a polymerization initiator in an hydrophobic solvent; 4) dissolving a monomer in the hydrophobic solvent; 5) making an emulsion by dispersing droplets of the hydrophobic solvent in a continuous aqueous phase with an emulsifier; 6) sizing the first emulsion to provide a second emulsion of the same basic components but in which the droplets are substantially uniform and between 2 and 20 μm in size; 7) evaporating at least a substantial portion of the dispersed hydrophobic droplets to assemble nanoparticles to form microparticles suspended now in the aqueous phase; 8) optionally replacing the first surfactant with a second surfactant, which is preferably a polymerizable surfactant; 9) optionally adding a polymerizable monomer to the aqueous phase and allowing it to adsorb into the microparticle; 10) polymerizing the monomer(s) to provide a polymer layer on the microparticles; and 11) functionalizing the polymer surface layer of the microparticles with one or more polymer, nanoparticle or biological macromolecular layers. In a related method the nanoparticles are provided with a hydrophilic surface layer, and the emulsion is made by dispersing the aqueous nanoparticles in a continuous hydrophobic phase. The microparticles have a polymeric coating and a magnetic core that comprises in excess of 50% of the particle's mass. The polymeric shell may contain one or more functional groups capable of forming chemical bonds useful in managing a variety of separations.