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.