Thermal Reshaping of Gold Microplates: Three Possible Routes and Their Transformation Mechanisms

Unexpectedly high thermal stability of Au nanotriangle@mSiO2 yolk-shell nanoparticles

The shape of Au nanoparticles (NPs) plays a crucial role for applications in, amongst others, catalysis, electronic devices, biomedicine, and sensing. Typically, the deformation of the morphology of Au NPs is the most significant cause of loss of functionality. Here, we systematically investigate the thermal stability of Au nanotriangles (NTs) coated with (mesoporous) silica shells with different morphologies (core-shell (CS): Au NT@mSiO2/yolk-shell (YS): Au NT@mSiO2) and compare these to 'bare' nanoparticles (Au NTs), by a combination of in situ and/or ex situ TEM techniques and spectroscopy methods. Au NTs with a mesoporous silica (mSiO2) coating were found to show much higher thermal stability than those without a mSiO2 coating, as the mSiO2 shell restricts the (self-)diffusion of surface atoms. For the Au NT@mSiO2 CS and YS NPs, a thicker mSiO2 shell provides better protection than uncoated Au NTs. Surprisingly, the Au NT@mSiO2 YS NPs were found to be as stable as Au NT@mSiO2 CS NPs with a core-shell morphology. We hypothesize that the only explanation for this unexpected finding was the thicker and higher density SiO2 shell of YS NPs that prevents diffusion of Au surface atoms to more thermodynamically favorable positions.

Laser-Induced Ion Formation and Electron Emission from a Nanostructured Gold Surface at Laser Fluence below the Threshold for Plasma Formation

The laser formation of positive and negative ions on a nanostructured metal surface is observed at laser fluence below the plasma formation threshold. The laser radiation energy dependences of the yield of positive and negative Au ions and charged clusters as well as electrons from the laser-induced nanostructures on the surface of gold are obtained at laser fluence below the plasma formation threshold using a pulsed laser with a wavelength of 355 nm and a pulse duration of 0.37 ns. It is shown that the ratio of the signals of positive and negative ions is constant over the entire range of the laser radiation energies, while the ion signal dependence on the laser radiation energy is described by a power function with an exponent of 9. The role of gold nanoparticles with a size of less than 5 nm in the formation of Au ions and charged Au clusters is discussed.

Characterization of Substrates and Surface-Enhancement in Atomic Force Microscopy Infrared Analysis of Amyloid Aggregates

Atomic force microscopy infrared (AFM-IR) spectroscopy is an emerging analytical technique that can be used to probe the structural organization of specimens with nanometer spatial resolution. A growing body of evidence suggests that nanoscale structural analysis of very small (<10 nm) biological objects, such as viruses and amyloid aggregates, requires substrates that must fit strict criteria of low surface roughness and low IR background, simultaneously. In this study, we examine the suitability of a broad range of substrates commonly used in AFM and IR fields, and we determined that silicon, zinc sulfide, and calcium fluoride are the most ideal substrates for nanoscale imaging of amyloid oligomers, protein aggregates that are directly linked to the onset and progression of neurodegenerative diseases. Our data show that these substrates provide the lowest roughness and the lowest background in the 800-1800 cm^-1 spectral window from all examined AFM and IR substrates. We also investigate a contribution of surface enhancement in AFM-IR by the direct comparison of signal intensities from oligomers located on silicon and gold-coated silicon surfaces. We found that metallization of such substrates provides a factor of ~7 enhancements to the IR signal and induces an equivalent enhancement of the sample background in the 950-1250 cm^-1 spectral region.

Plasmon-Driven Chemistry on Mono- and Bimetallic Nanostructures

ConspectusHot carriers are highly energetic species that can perform a large spectrum of chemical reactions. They are generated on the surfaces of nanostructures via direct interband, phonon-assisted intraband, and geometry-assisted decay of localized surface plasmon resonances (LSPRs), which are coherent oscillations of conductive electrons. LSPRs can be induced on the surface of noble metal (Ag or Au) nanostructures by illuminating the surfaces with electromagnetic irradiation. These noble metals can be coupled with catalytic metals, such as Pt, Pd, and Ru, to develop bimetallic nanostructures with unique catalytic activities. The plasmon-driven catalysis on bimetallic nanostructures is light-driven, which essentially enables green chemistry in organic synthesis. During the past decade, surface-enhanced Raman spectroscopy (SERS) has been actively utilized to study the mechanisms of plasmon-driven reactions on mono- and bimetallic nanostructures. SERS has provided a wealth of knowledge about the mechanisms of numerous plasmon-driven redox, coupling, and scissoring reactions. However, the nanoscale catalytic properties of both mono- and bimetallic nanostructures as well as the underlying physical cause of their catalytic reactivity and selectivity remained unclear for decades.In this Account, we focus on the most recent findings reported by our and other research groups that shed light on the nanoscale properties of mono- and bimetallic nanostructures. This information was revealed by tip-enhanced Raman spectroscopy (TERS), a modern analytical technique that has single-molecule sensitivity and subnanometer spatial resolution. TERS findings have shown that plasmonic reactivity and the selectivity of bimetallic nanostructures are governed by the nature of the catalytic metal and the strength of the rectified electric field on their surfaces. TERS has also revealed that the catalytic properties of bimetallic nanostructures directly depend on the interplay between the catalytic and plasmonic metals. We anticipate that these findings will be used to tailor synthetic approaches that are used to fabricate novel nanostructures with desired catalytic properties. The experimental and theoretical results discussed in this Account will facilitate a better understanding of TERS and explain artifacts that could be encountered upon TERS imaging of a large variety of samples. Consequently, plasmon-driven chemistry should be considered as an essential part of near-field microscopy.

Tip-enhanced Raman imaging of photocatalytic reactions on thermally-reshaped gold and gold–palladium microplates

The plasmonic/photocatalytic properties of thermally-reshaped walled gold–palladium microplates (WAu@PdMPs) have been explored by TERS.

Nanoscale Structural Characterization of Individual Viral Particles Using Atomic Force Microscopy Infrared Spectroscopy (AFM-IR) and Tip-Enhanced Raman Spectroscopy (TERS)

Viruses are infections species that infect a large spectrum of living systems. Although displaying a wide variety of shapes and sizes, they are all composed of nucleic acid encapsulated into a protein capsid. After virions enter the host cell, they replicate to produce multiple copies of themselves. They then lyse the host, releasing virions to infect new cells. The high proliferation rate of viruses is the underlying cause of their fast transmission among living species. Although many viruses are harmless, some of them are responsible for severe diseases such as AIDS, viral hepatitis, and flu. Traditionally, electron microscopy is used to identify and characterize viruses. This approach is time- and labor-consuming, which is problematic upon pandemic proliferation of previously unknown viruses, such as H1N1 and COVID-19. Herein, we demonstrate a novel diagnosis approach for label-free identification and structural characterization of individual viruses that is based on a combination of nanoscale Raman and infrared spectroscopy. Using atomic force microscopy-infrared (AFM-IR) spectroscopy, we were able to probe structural organization of the virions of Herpes Simplex Type 1 viruses and bacteriophage MS2. We also showed that tip-enhanced Raman spectroscopy (TERS) could be used to reveal protein secondary structure and amino acid composition of the virus surface. Our results show that AFM-IR and TERS provide different but complementary information about the structure of complex biological specimens. This structural information can be used for fast and reliable identification of viruses. This nanoscale bimodal imaging approach can be also used to investigate the origin of viral polymorphism and study mechanisms of virion assembly.