Abstract

The highly packed cetyltrimethylammonium bromide (CTAB) bilayer built up on the surface of gold nanorods
(AuNRs) when synthesized by the seed-mediated procedure hampers the complete ligand exchange
under experimental conditions that preserves the stability of the dispersions. In the present work, a ligand
exchange protocol by using carboxy-terminated alkanethiols of different chain lengths by means of a
green approach that uses only aqueous solutions is presented. The protocol is based on the knowledge
of the stability in the aqueous solution of both the starting CTAB-AuNRs and the final products that help
in the choice of the experimental conditions used for ligand exchange. The characterization of the CTAB
protective layer as well as the study of its colloidal stability in solution has helped us to design an appropriate
methodology. Cyclic voltammetry of CTAB-AuNRs demonstrates the high stability of the bilayer
showing the existence of a two-dimensional phase transition from a highly ordered to a less organized
phase. Other techniques such as XPS, FT-IR and Raman spectroscopy provide information about the
structure of the layer and UV-visible-NIR spectroscopy establishes the stability conditions in aqueous
solution. We have chosen an exchange procedure for 11-mercaptoundecanoic acid (MUA) and 16-mercaptohexadecanoic
acid (MHDA) based on a one-pot methodology under conditions where all the
species involved are stable. The protocol, however, can be extended to different chemical functionalities
that are considered useful to be applied in living systems. Under these conditions the complete exchange
of CTAB by the mercaptoderivatives was successful as demonstrated by the different characterization
techniques used: UV-visible-NIR, FT-IR, Raman, XPS spectroscopy, cyclic voltammetry and transmission
electron microscopy (TEM).