Surfactant Layers on Gold Nanorods

Abstract

Conspectus Gold nanorods (Au NRs) are an exceptionally promising tool in nanotechnology due to three key factors: (i) their strong interaction with electromagnetic radiation, stemming from their plasmonic nature, (ii) the ease with which the resonance frequency of their longitudinal plasmon mode can be tuned from the visible to the near-infrared region of the electromagnetic spectrum based on their aspect ratio, and (iii) their simple and cost-effective preparation through seed-mediated chemical growth. In this synthetic method, surfactants play a critical role in controlling the size, shape, and colloidal stability of Au NRs. For example, surfactants can stabilize specific crystallographic facets during the formation of Au NRs, leading to the formation of NRs with specific morphologies. The process of surfactant adsorption onto the NR surface may result in various assemblies of surfactant molecules, such as spherical micelles, elongated micelles, or bilayers. Again, the assembly mode is critical toward determining the further availability of the Au NR surface to the surrounding medium. Despite its importance and a great deal of research effort, the interaction between Au NPs and surfactants remains insufficiently understood, because the assembly process is influenced by numerous factors, including the chemical nature of the surfactant, the surface morphology of Au NPs, and solution parameters. Therefore, gaining a more comprehensive understanding of these interactions is essential to unlock the full potential of the seed-mediated growth method and the applications of plasmonic NPs. A plethora of characterization techniques have been applied to reach such an understanding, but many open questions remain. In this Account, we review the current knowledge on the interactions between surfactants and Au NRs. We briefly introduce the state-of-the-art methods for synthesizing Au NRs and highlight the crucial role of cationic surfactants during this process. The self-assembly and organization of surfactants on the Au NR surface is then discussed to better understand their role in seed-mediated growth. Subsequently, we provide examples and elucidate how chemical additives can be used to modulate micellar assemblies, in turn allowing for a finer control over the growth of Au NRs, including chiral NRs. Next, we review the main experimental characterization and computational modeling techniques that have been applied to shed light on the arrangement of surfactants on Au NRs and summarize the advantages and disadvantages for each technique. The Account ends with a “Conclusions and Outlook” section, outlining promising future research directions and developments that we consider are still required, mostly related to the application of electron microscopy in liquid and in 3D. Finally, we remark on the potential of exploiting machine learning techniques to predict synthetic routes for NPs with predefined structures and properties.

Size, Diffusion, and Sedimentation of Gold Nanorods

“…These values are very consistent across the three methods shown and provide strong statistical evidence that CTAB forms a bilayer on the GNR surface. We note there is excellent agreement with the values obtained by Liz-Marzán and colleagues using SANS data . While the mean values obtained here are in strong agreement, the standard deviations increase from SAXS to DLS to AUC.…”

Fast and Facile Etching of Gold Nanorods by N-Halosuccinimides: Toward Multicolorimetric Identification and Quantification of 20 Natural Amino Acids

“…As it appears, even though the length of AuNRs is reduced as a result of etching by NBS and NCS, the width of AuNRs remains unchanged. It indicates that these reagents selectively etch AuNRs from the heads owing to [111] facets with the lowest degree of CTAB capping . Interestingly, in contrast to NBS, the etching of AuNRs using NCS at pH 5 produces dog-bone nanoparticles.…”

Gold nanohexagrams via active surface growth under sole CTAB control

“…For this reason, it is difficult to attract a positively charged tertiary amine group unless anions are involved. Despite extensive usage of CTAB, there is no clear assignment for the CTA + and Br − ions in CTAB binding, other than that the binding is cooperative 13,14 and that CTAB forms a bilayer structure. 15,16…”