Gold nanorods (GNRs) are synthesized in a micellar solution of hexadecyltrimethylammonium bromide (CTAB). The CTAB, a cationic surfactant, formed bilayers on GNRs that stabilized colloidal dispersion of the GNRs. Replacement of the CTAB with functional molecules is a key process to realize practical application of GNRs. The most important point of the replacement is how prevents the aggregation when the CTAB layers on GNRs are replaced with alternative surface-stabilizing molecules. In many cases, aggregated GNRs did not re-disperse in water even after sonication of those. If dried GNRs could be re-dispersed in water, it would be very advantageous for practical application.

Phosphatidylcholine-passivated GNRs (PC-GNRs) ware prepared by the addition of hydrochloric acid in the presence of 6-amino-1-hexanethiol hydrochloride (AHT). The aggregates were dried in vacuum, and then re-dispersed in water. Absorption spectra of the re-dispersed PC-GNRs indicated that the nanorods dispersed in water without forming aggregates. We found that the re-dispersion of the dried nanorod-aggregates depended on AHT in the nanorod solutions.

In this work, we formed aggregates of GNRs by using hydrochloric acid in the presence of AHT. The aggregates were dried in vacuum, and then re-dispersed in water. The dried GNRs showed metallic gold color, but dispersed in water again without forming aggregates. On the surface of the dried GNRs, there was no static and densely-packed surface stabilizing layer, such as alkanethiols or polymers. Our GNRs have soft-shells but stable even in the solid state. The GNRs could be kept for long terms in solid state, and when they were dispersed in water again, their surfaces were wrapped with the minimum surface stabilizing layers. Accordingly, additional surface modification on the GNRs will be easily realized by using thiol-derivatives.