Gold nanorods (Au NRs) are the archetype of a nanoantenna, enabling the directional capture, routing, and concentration of electromagnetic fields at the nanoscale. Solution-based synthesis methods afford advantages relative to top-down fabrication but are challenged by insufficient precision of structure, presence of byproducts, limited tunability of architecture, and device integration. This is due in part to an inadequate understanding of the early stages of Au NR growth. Here, using phase transfer via ligand exchange with monothiolated polystyrene, we experimentally demonstrate the complete evolution of seed-mediated Au NR growth in hexadecyltrimethylammonium bromide (CTAB) solution. Au NR size and shape progress from slender spherocylinders at short reaction times to rods with a dumbbell profile, flattened end facets, and octagonal prismatic structures at later stages. These evolve from a single mechanism and reflect the majority of reported Au NR morphologies, albeit reflecting different stages. Additionally, the fraction of nonrod impurities in a reaction is related to the initial distribution of the structure of the seed particles. Overall, the observations of early and intermediate stage growth are consistent with the formation of a surfactant bilayer on different crystal facets at different growth stages due to a fine balance between kinetic and thermodynamic factors.

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 fi nely 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 briefl y 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 briefl y examined, and explained why the scientifi c 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 fi eld.surfactants and reducing agents have been employed to produce gold NRs, but have been shown to not be able to yield the same fi nely tunable control over the morphology if used without CTAB. [ 14 ] In addition, it has been shown that the purity of CTAB highly infl uences the ability to synthesize gold NRs. [ 15 ] Recent work by Wadams et al. [ 16 ] has demonstrated that the NR growth is mostly susceptible to the action of CTAB during the early stages, dominated by epitaxial micellar adsorption of the surfactant and by adatom reorganization. The same authors have also shown that each stage is characterized by thermodynamically stable crystallographic facets at the tips and the sides of the rods that evolve during growth (see Figure 1 ). [ 17 ] Because of these discoveries, the above studies have opened a Pandora's box for the scientists interested in gold NRs. Because CTAB is so intimately connected to our synthetic ability to tune the nanoparticle's morphology, we cannot consider it only a mere surface stabilizer, and hence its replacement with other capping agents through ligand exchange should be considered a complex reaction, whose mechanism needs to be thoroughly understood via carefully devised thermodynamic studies. Ligand exchange is often employed as a tool to functionalize the NRs for a specifi c application and it is only rarely the primary subject of investigation. Many of the applications of gold NRs, whether in plasmonics or biological imaging, are closely dependent upon our ability to tailor their surface functionalization and tune their properties. However, the lack of understanding of ligand exchange will most likely negatively impact our ability to employ NRs in technological appl...

Gold nanorods have attracted more attention than other gold particle morphologies because of their tunable optical response, which is a product of simply changing the rod's aspect ratio. Advances in bottom-up synthetic methods of gold nanorods have greatly improved yields as well as allowed access to a range of aspect ratios. Despite great strides being made in synthetic methods, the growth mechanism of gold nanorods has been debated in recent years. A recent mechanism outlined by Park et al. shows that nanorods undergo five distinct stages of growth. Stages I and II of this growth mechanism are dominated by the epitaxial micellar adsorption of surfactant to the growing crystal and adatom migration during rod reconstruction, respectively. Both of these processes, occurring early in particle growth, clearly dominate the production of anisotropic species, as well as the resultant morphology. Therefore, our hypothesis is that nanorod growth should be most susceptible to crystal habit modification during early growth stages. In this work, we show that the addition of surfactants, having a structure similar to that of the primary morphology-guiding surfactant, cetyltrimethylammonium bromide, during stages I and II of nanorod growth drastically influences the final dimensionality and morphology of nanorods. In contrast, addition after longer periods of time has little or no influence on rod structure. Our results bolster the growth mechanism outlined by Park et al., proving nanorod growth is most susceptible to crystal habit modification during growth stages that are dominated by epitaxial micellar adsorption and adatom reorganization. Furthermore, this work emphasizes the sensitivity of nanorod growth to the existing micellar state of the growth solution.

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