Ligand Exchange on Gold Nanorods: Going Back to the Future

Ligand exchange on gold nanorods (NRs) is still too often dismissed or not given the importance it should deserve. The many applications of gold NRs, mainly in plasmonics, biological imaging, and sensing, are made possible by finely tuning not only the optical properties of the metallic core but also the tethered functional groups. Gold NRs are mainly synthesized by using CTAB as the morphology-guiding surfactant, and an intimate relationship between the crystallographic facets of the rod and the CTAB bilayer exists. Because of this, it is imperative to fully understand the ligand exchange mechanisms that allow replacing CTAB with functional ligands, including the energetic contributions. Here, the major applications of gold NRs are briefly overviewed, and what is known about ligand exchange mechanisms is summarized, as well as why it is important to achieve complete removal of CTAB, including the techniques that are used to characterize the exchange reaction products. The concept of interface in gold NRs is briefly examined, and explained why the scientific community should focus more on understanding and characterizing it. Starting from the published literature, the reader is guided through the reasons why it is thought that ligand exchange on gold NRs is perhaps the next grand challenge in the nanoparticle field.