We describe the development of iron oxide-core, gold-shell nanoparticle probes for eventual use as magnetically guided biosensor particles. The particles combine magnetization and plasmonics in a single nanoparticle entity. Monodisperse, eighteen nanometer diameter 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. A weak reduction method is employed to make iron oxide core, gold shell nanoparticles. Here a large particle size mismatch between the iron oxide core and the deposited gold seeds is crucial. Transmission electron microscopy (TEM) 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, as expected from Mie scattering theory. Theoretical analysis of the absorbance spectrum confirms the presence of 18 nm iron oxide particles coated with a 5 nm thick gold shell. Dynamic light scattering (DLS) analysis shows that at least 85% of these particles are singlet core-shell, with the remainder mostly doublets or triplets. Superconducting Quantum Interference Device (SQUID) measurements of magnetization curves indicate that these particles are superparamagnetic at room temperature. The magnetic moment of these particles at 10 K is approximately 54 emu/g. Suspensions of magnetic particles are readily and reversibly collected by a permanent magnet. Darkfield optical microscopy, with supporting random walk statistical analysis, demonstrates the feasibility of detecting single nanoparticles undergoing Brownian motion.

These nanoparticles can be sterically stabilized by physisorption of common macromolecules, and they remain stably dispersed in high ionic strength phosphate buffer saline (PBS, 154 mM NaCl), an environment suitable for the eventual application in biological fluids. Several different commonly available polyethers, dextran, and bovine serum albumin were tested in as nanoparticle stabilizers. Of the polymers considered, Pluronic F127 poly(ethylene oxide)-block¬-(propylene oxide)-block-(ethylene oxide) triblock copolymers were identified as the best stabilization agent for these particles, stabilizing them against flocculation in PBS for more than one month.