Heteroaggregation Approach for Depositing Magnetite Nanoparticles onto Silica-Overcoated Gold Nanorods

Hydrophobic, oleylamine-stabilized magnetite nanoparticles (Fe<sub>3</sub>O<sub>4</sub> NPs) dispersed in hexanes can assemble into dense coatings on the surface of silica-overcoated gold nanorods (SiO<sub>2</sub>-GNRs) dispersed in ethanol by mixing. In this nonaqueous heteroaggregation process, Fe<sub>3</sub>O<sub>4</sub> NPs are destabilized when ethanol is added, resulting in core/satellite Fe<sub>3</sub>O<sub>4</sub>-SiO<sub>2</sub>-GNRs within a few minutes. The composition of the solvent mixture allows tuning of the polarity and driving forces toward aggregation. At the optimal 2:1 volume ratio of hexanes:ethanol, heteroaggregation to form Fe<sub>3</sub>O<sub>4</sub>-SiO<sub>2</sub>-GNRs occurs quickly, while avoiding homoaggregation of Fe<sub>3</sub>O<sub>4</sub> NPs or SiO<sub>2</sub>-GNRs. Fe<sub>3</sub>O<sub>4</sub>-SiO<sub>2</sub>-GNRs retain the longitudinal surface plasmon resonance of the gold nanorod cores and are magnetically responsive and separable. The Fe<sub>3</sub>O<sub>4</sub> NPs remain bound on the surface of the Fe<sub>3</sub>O<sub>4</sub>-SiO<sub>2</sub>-GNRs during multiple cycles of magnetic extraction and redispersion. Oleylamine ligands on the Fe<sub>3</sub>O<sub>4</sub> NPs render the Fe<sub>3</sub>O<sub>4</sub>-SiO<sub>2</sub>-GNRs dispersible in nonpolar solvents. Functionalization of the outer Fe<sub>3</sub>O<sub>4</sub> surface with poly­(ethylene glycol) catechol (PEG-catechol) for PEGylation results in PEG-Fe<sub>3</sub>O<sub>4</sub>-SiO<sub>2</sub>-GNRs that disperse in water. In comparison with seeded growth or use of molecular cross-linkers to form multifunctional nanoparticles, heteroaggregation approaches are potentially quite general, simple, and efficient. The ability to continuously adjust the solvent polarity is expected to allow tuning of the heteroaggregation process for many different types and sizes of NPs.