Colloidal Stability and Magnetophoresis of Gold-Coated Iron Oxide Nanorods in Biological Media

Magnetic iron oxide nanorods are coated with a gold colloid (Fe/Au NRs) to form core–shell particles that combine magnetic and plasmonic properties in a single nanostructure. Three different macromolecules are employed to surface functionalize the nanorod in order to promote colloidal stability of these particles in elevated ionic strength media (∼154 mM NaCl equivalent) that are appropriated for biomedical applications. With a 10 000 molecular weight poly­(ethylene glycol) (PEG), the NRs flocculated and sedimented within a few minutes. However, Pluronic F127 or poly­(diallyldimethylamonium chloride) (PDDA) coatings yielded stable dispersions for up to 20 h. These NRs exhibit two absorbance peaks at 530 nm and ∼740 nm corresponding to the transverse and longitudinal surface plasmon resonances (SPR). In addition to dynamic light scattering (DLS), spectrophotometry can also be used to monitor dispersion stability because the 530 nm SPR peak shape changes when agglomerates form. The magnetophoretic migration time of these particles, monitored by suspension opacity measurement by light dependent resistor (LDR) under low gradient magnetic separation (LGMS), was prolonged from 1.5 min to ∼8 min after surface functionalization.