Transverse oxidation of gold nanorods assisted by selective end capping of silver oxide

An approach for the transverse oxidation of Au nanorods has been demonstrated by selectively capping the ends of the nanorods with Ag2O through a hydrothermal reaction. The tight Ag2O capping protects the ends of the nanorods and restricts oxidation to occurring only at the side surfaces. Such transverse oxidation leads to a gradual reduction in the effective diameter of the nanorods, an increase in the effective aspect ratio, and therefore a red shift in the longitudinal plasmon resonance. The red shift in the longitudinal plasmon resonance can reach up to 250 nm and be controlled by varying the oxidation time. In addition, the Ag2O capping and transverse oxidation process is also applicable for Au nanorods with different longitudinal plasmon resonance wavelengths. Furthermore, the transverse oxidation is nonuniform. It produces dimples on the side surfaces of the Au nanorods. Numerical electrodynamic calculations indicate that the oxidized nanorods exhibit stronger electric field intensity enhancements at the dimples for both the longitudinal and transverse plasmon resonances in comparison to the unoxidized nanorods. We believe that this transverse oxidation approach will be beneficial to the design and preparation of Au nanostructures for various biotechnological applications.