Gold icosahedral nanocages: Facile synthesis, optical properties, and fragmentation under ultrasonication

Versatile template-directed synthesis of gold nanocages with a predefined number of windows

Highly symmetrical gold nanocages can be produced with a controllable number of circular windows of either 2, 3, 4, 6 or 12 via an original fabrication route. The synthetic pathway includes three main stages: the synthesis of silica/polystyrene multipod templates, the regioselective seeded growth of a gold shell on the unmasked part of the silica surface and the development of gold nanocages by dissolving/etching the templates. Electron microscopy and tomography provide evidence of the symmetrical features of the as-obtained nanostructures. The optical properties of nanocages with 4 and 12 windows were measured at the single particle level by spatial modulation spectroscopy and correlated with numerical simulations based on finite-element modeling. The new multi-step synthesis approach reported here also allows the synthesis of rattle-like nanostructures through filling of the nanocages with a guest nano-object. With the potential to adjust the chemical composition, size and geometry of both the guest particle and the host cage, it opens new routes towards the fabrication of hollow nanostructures of high interest for a variety of applications including sensing devices, catalytic reactors and biomedicine.

Physical Transformations of Noble-Metal Nanocrystals upon Thermal Activation

ConspectusThe last two decades have witnessed the successful development of noble-metal nanocrystals with well-controlled properties for a variety of applications in catalysis, plasmonics, electronics, and biomedicine. Most of these nanocrystals are kinetically controlled products greatly deviated from the equilibrium state defined by thermodynamics. When subjected to elevated temperatures, their arrangements of atoms are expected to undergo various physical transformations, inducing changes to the shape, morphology (hollow vs solid), spatial distribution of elements (segregated vs alloyed/intermetallic), internal structure (twinned vs single-crystal), and crystal phase. In order to optimize the performance of these nanocrystals in various applications, there is a pressing need to understand and improve their thermal stability.By integrating in situ heating with transmission electron microscopy or X-ray diffraction, we have investigated the physical transformations of various types of noble-metal nanocrystals in real time. We have also explored the atomistic detail responsible for a physical transformation using first-principles calculations, providing insightful guidance for the development of noble-metal nanocrystals with augmented thermal stability. Specifically, solid nanocrystals were observed to transform into pseudospherical particles favored by thermodynamics by reducing the surface area while eliminating the facets high in surface energy. For nanocrystals of relatively large in size, a single-crystal lattice was more favorable than a twinned structure. When switching to core-shell nanocrystals, the elevation in temperature caused changes to the elemental distribution in addition to shape transformation. The compositional stability of a core-shell nanocrystal was found to be strongly dependent on the shape and thus the type of facet expressed on the surface. For hollow nanocrystals such as nanocages and nanoframes, their thermal stabilities were typically inferior to the solid counterparts, albeit their unique structure and large specific surface area are highly desired in applications such as catalysis. When a metastable crystal structure was involved, phase transition was also observed at a temperature close to that responsible for shape or compositional change. We hope the principles, methodologies, and mechanistic insights presented in this Account will help the readers achieve a good understanding of the physical transformations that are expected to take place in noble-metal nanocrystals when they are subjected to thermal activation. Such an understanding may eventually lead to the development of effective methods for retarding or even preventing some of the transformations.

Gold nanocages for effective photothermal conversion and related applications

Gold nanocages are highly effective in converting light to heat, making them versatile for an array of photothermal applications.

The synthesis of Ag-coated tetrapod gold nanostars and the improvement of surface-enhanced Raman scattering

In this work, the novel tetrapod gold nanostars with four specific numbers of sharp tips were synthesized with good uniformity, high yield and chemical stability. The size of tetrapod gold nanostars has been increased effectively by increasing the amount of HAuCl₄ or DEA added to the reaction solution, and the corresponding in-plane localized surface plasmon resonance (LSPR) band can shift from 700 to 1100 nm. In order to combine the advantages of different metals, the Ag-coated tetrapod gold nanostars were prepared with different Ag coating thicknesses by increasing the volume of AgNO₃. With the increase of Ag coating thickness, the morphology of the nanoparticles has changed from gold nanostars with four sharp tips to bimetallic cuboids with four sharp tips and finally to crore-shell cuboids completely. The surface-enhanced Raman scattering (SERS) activity of Ag-coated tetrapod gold nanostars has also been improved with the different Ag coating thicknesses. Combined the tip effect and the enhancement effect of Ag, the SERS activity has the strongest enhancement when the four sharp tips of tetrapod gold nanostars are not completely coated by the Ag layer but still exposed to the outsides. For tetrapod gold nanostars with larger size to grow Ag-coated tetrapod gold nanostars, the bimetallic cuboids with four sharp tips nanostructure have the biggest SERS enhancement factor (EF) of 1.73 × 10⁶ for Rhodamine 6G (R6G), making it an ideal candidate in SERS-based sensing application.

Photothermal transformation of Au-Ag nanocages under pulsed laser irradiation

Pulsed laser irradiation has emerged as an effective means to photothermally transform plasmonic nanostructures after their use in different biomedical applications. However, the ability to predict the products after photothermal transformation requires extensive ex situ studies. Here, we report a systematic study of the photothermal transformation of Au-Ag nanocages with a localized surface plasmon resonance at ca. 750 nm under pulsed laser irradiation at different fluences and a pulse duration of 5 ns. At biologically relevant laser energies, the pulsed laser transforms Au-Ag nanocages into pseudo-spherical, solid nanoparticles. The solid nanoparticles contained similar numbers of Au and Ag atoms to the parent Au-Ag nanocages. At increased laser fluences (>16 mJ cm-2) and number of pulses (>150), the average diameter of the resulting pseudo-spherical particles increased due to the involvement of Ostwald ripening and/or attachment-based growth. The changes in optical properties as a result of the transformation were validated using simulations based on the discrete dipole approximation method, where the spectral profiles and peak positions of the initial and final states matched well with the experimentally derived data. The results may have implications for the future use of Au-Ag nanocages in biomedicine, catalysis, and sensing.

Dual-template cascade synthesis of highly multi-branched Au nanoshells with ultrastrong NIR absorption and efficient photothermal therapeutic intervention

A universal dual-template cascade strategy for the synthesis of multi-branched gold nanoshells with ultrastrong NIR absorption for tumor photothermal therapy.

Shape-Controlled Synthesis of Colloidal Metal Nanocrystals by Replicating the Surface Atomic Structure on the Seed

Controlling the surface structure of metal nanocrystals while maximizing the utilization efficiency of the atoms is a subject of great importance. An emerging strategy that has captured the attention of many research groups involves the conformal deposition of one metal as an ultrathin shell (typically 1-6 atomic layers) onto the surface of a seed made of another metal and covered by a set of well-defined facets. This approach forces the deposited metal to faithfully replicate the surface atomic structure of the seed while at the same time serving to minimize the usage of the deposited metal. Here, the recent progress in this area is discussed and analyzed by focusing on the synthetic and mechanistic requisites necessary for achieving surface atomic replication of precious metals. Other related methods are discussed, including the one-pot synthesis, electrochemical deposition, and skin-layer formation through thermal annealing. To close, some of the synergies that arise when the thickness of the deposited shell is decreased controllably down to a few atomic layers are highlighted, along with how the control of thickness can be used to uncover the optimal physicochemical properties necessary for boosting the performance toward a range of catalytic reactions.