ConspectusSurface ligands are vital to the colloidal synthesis
of noble-metal
nanocrystals with well-controlled sizes and shapes for various applications.
The surface ligands not only dictate the formation of nanocrystals
with diverse shapes but also serve as a colloidal stabilizer to prevent
the suspended nanocrystals from aggregation during their synthesis
or storage. By leveraging the facet selectivity of some surface ligands,
one can further control the sites for growth or galvanic replacement
to transform presynthesized nanocrystals into complex structures that
are otherwise difficult to fabricate using conventional methods. Furthermore,
the presence of surface ligands on nanocrystals also facilitates their
applications in areas such as sensing, imaging, nanomedicine, and
self-assembly. Despite their popular use in enhancing the properties
of nanocrystals and thus optimizing their performance in a wide variety
of applications, it remains a major challenge to quantitatively determine
the coverage density of ligand molecules, not to mention the difficulty
of substituting or removing them without compromising the surface
structure and aggregation state of the nanocrystals.In this
Account, we recapitulate our efforts in developing methods
capable of qualitatively or quantitatively measuring, exchanging,
and removing the surface ligands adsorbed on noble-metal nanocrystals.
We begin with an introduction to the typical interactions between
ligand molecules and surface atoms, followed by a discussion of the
Langmuir model that can be used to describe the adsorption of surface
ligands. It is also emphasized that the adsorption process may become
very complex in the case of a polymeric ligand due to the variations
in binding configuration and chain conformation. We then highlight
the capabilities of various spectroscopy methods to analyze the adsorbed
ligands qualitatively or quantitatively. Specifically, surface-enhanced
Raman scattering, Fourier transform infrared, and X-ray photoelectron
spectroscopy are three examples of qualitative methods that can be
used to confirm the absence or presence of a surface ligand. On the
other hand, ultraviolet–visible spectroscopy and inductively
coupled plasma mass spectrometry can be used for quantitative measurements.
Additionally, the coverage density of a ligand can be derived by analyzing
the morphological changes during nanocrystal growth. We then discuss
how the ligands present on the surface of metal nanocrystals can be
exchanged directly or indirectly to meet the requirements of different
applications. The former can be done using a ligand with stronger
binding, whereas the latter is achieved by introducing a sacrificial
shell to the surface of the nanocrystals. Furthermore, we highlight
three additional strategies besides simple washing to remove the surface
ligands, including calcination, heating in a solution, and UV-ozone
treatment. Finally, we showcase three applications of metal nanocrystals
in nanomedicine, tumor targeting, and self-assembly by taking advantage
of the diversity of surface ligands bearing different functional groups.
We also offer perspectives on the challenges and opportunities in
realizing the full potential of surface ligands.