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

Hydrophobic, oleylamine-stabilized magnetite nanoparticles (Fe₃O₄ NPs) dispersed in hexanes can assemble into dense coatings on the surface of silica-overcoated gold nanorods (SiO₂-GNRs) dispersed in ethanol by mixing. In this nonaqueous heteroaggregation process, Fe₃O₄ NPs are destabilized when ethanol is added, resulting in core/satellite Fe₃O₄-SiO₂-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 Fe₃O₄-SiO₂-GNRs occurs quickly, while avoiding homoaggregation of Fe₃O₄ NPs or SiO₂-GNRs. Fe₃O₄-SiO₂-GNRs retain the longitudinal surface plasmon resonance of the gold nanorod cores and are magnetically responsive and separable. The Fe₃O₄ NPs remain bound on the surface of the Fe₃O₄-SiO₂-GNRs during multiple cycles of magnetic extraction and redispersion. Oleylamine ligands on the Fe₃O₄ NPs render the Fe₃O₄-SiO₂-GNRs dispersible in nonpolar solvents. Functionalization of the outer Fe₃O₄ surface with poly­(ethylene glycol) catechol (PEG-catechol) for PEGylation results in PEG-Fe₃O₄-SiO₂-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.