We describe the development of iron oxide-core, gold-shell nanoparticle probes for eventual use in cell or single molecule sorting technologies and as intracellular probes. These nanoparticles combine the potential for magnetic manipulation and plasmonic sensing in a single entity. The surface plasmon resonance of gold makes it possible to track the positions of individual nanoparticles with darkfield microscopy. In addition, the gold shell is more easily biofunctionalized than the surface of a bare iron oxide nanoparticle. This presentation will emphasize the synthesis and physical characterization of the hybrid nanoparticles. Eighteen nanometer magnetic cores are first synthesized in organic solvents, then transferred into water and coated with gold nanoparticle seeds that are further grown to a complete shell. Transmission electron microscope images obtained at various stages of the process reveal the development of the core-shell morphology. The completion of the gold shell produces an intensification and pronounced shift of the surface plasmon resonance peak in the optical absorbance spectrum. These particles are superparamagnetic at room temperature. Darkfield optical imaging shows the feasibility of detecting single nanoparticles undergoing Brownian motion. Magnetophoretic and drag force calculations will be described that indicate the potential utility of these nanoparticles for microfluidic single molecule and cell sorting applications. The calculations are tested by measuring magnetophoretic velocities using darkfield microscopy and a micropatterned magnetic array.