Orientation and organization of gold nanorods on a substrate using a strong magnetic field: Effect of aspect ratio

The effects of magnetic processing on the orientation and the organization of three different types of gold nanorods (AuNRs) [l-AuNR (aspect ratio (AR) = 8.3), m-AuNR (AR = 5.0), and s-AuNR (AR = 2.5)] using a strong magnetic field were investigated. This investigation was performed using the extinction and the polarized extinction spectra that correspond to surface plasmons on a glass plate, and scanning electron microscopy (SEM). For m-AuNR, the results obtained from the extinction spectra and SEM images showed that a side-by-side aggregation of AuNRs formed in the presence of a magnetic field (10 T). In the absence of the magnetic field, side-by-side AuNRs aggregates were not observed. The polarized extinction spectra and the SEM images on the plates indicated that the long axes of AuNRs were oriented parallel to the magnetic field. Similar effects of magnetic processing on the orientation and the organization of AuNRs were observed in the l-AuNR. The magnitude of magnetic orientation in the l-AuNR was larger than that in the m-AuNR. No orientation or organization effects for the s-AuNRs on glass plates were observed. The effect of AR on the magnetic orientation of three AuNRs confirms that the magnetic orientation of AuNRs is because of the anisotropy in the magnetic susceptibilities of the adsorbed CTAB on AuNRs. When the magnetic processing was performed using the dilute concentration of aqueous solutions of m-AuNR, the opposite magnetic orientation, that the long axes of m-AuNRs were oriented perpendicular to the magnetic field, was obtained. The magnetic orientation is probably responsible for the magnetic property of the pristine m-AuNR.

► Magnetic orientation and magnetic organization depend on aspect ratio of gold nanorods.
► Increasing aspect ratio enhances magnitude of magnetic orientation of gold nanorods.
► Magnetic orientation do not achieve in gold nanorods with short aspect ratio.
► Origin of magnetic orientation of gold nanorods is magnetic susceptibility anisotropy of CTAB on gold nanorods.