End-to-end assembly of CTAB-stabilized gold nanorods by citrate anions

We report studies on the end-to-end assembly of CTAB-stabilized gold nanorods (GNRs) dispersing in an aqueous solution. The addition of sodium citrate to the solution of GNRs with an aspect ratio (AR) of 3 causes the end-to-end assembly accompanied with a red-shift of longitudinal surface plasmon resonance (LSPR) band of the GNRs in the optical extinction spectrum. The red-shift in the LSPR band continues until the solution becomes colorless and transparent, whereas adding CTAB brings the color change to a stop, resulting in the long term-stable LSPR of the end-to-end connected GNRs. Added citrate anions are found to be adsorbed on only the end faces of GNRs so that they are considered to neutralize the surface charge of the GNR ends, resulting in the end-to-end assembly of the GNRs. On the other hand, GNRs with an AR of 16 are end-to-end assembled by decreasing CTAB concentration in the solution. The assembled GNRs are welded at their connecting points by the further decrease of CTAB concentration in the solution. Such end-to-end aggregated GNRs presumably have an advantage of electron conduction, whereas the end-to-end connected GNRs with a slight space between two GNRs could be ideal substrate for surface enhanced Raman spectroscopy.