Tutorials in vibrational sum frequency generation spectroscopy. III. Collecting, processing, and analyzing vibrational sum frequency generation spectra

"Nonlinear" pursuit of understanding pollutant accumulation and chemistry at environmental and biological interfaces

Over the past few decades, the public recognition of the prevalence of certain classes of pollutants, such as perfluoroalkyl substances and nanoplastics, within the environment, has sparked growing concerns over their potential impact on environmental and human health. Within both environmental and biological systems, the adsorption and structural organization of pollutants at aqueous interfaces can greatly impact the chemical reactivity and transformation. Experimentally probing chemical behavior at interfaces can often pose a problem due to bulk solvated molecules convoluting molecular signatures from interfacial molecules. To solve this problem, there exist interface-specific nonlinear spectroscopy techniques that can directly probe both macroscopic planar interfaces and nanoplastic interfaces in aqueous environments. These techniques can provide essential information such as chemical adsorption, structure, and reactivity at interfaces. In this perspective, these techniques are presented with obvious advantages for studying the chemical properties of pollutants adsorbed to environmental and biological interfaces.

Development of electronic sum frequency generation spectrophotometer to assess the buried interfaces

The interfacial region between two bulk media in organic semiconductor based devices, such as organic field-effect transistors (OFETs), organic light-emitting diodes, and organic photovoltaics, refers to the region where two different materials such as an organic material and an electrode come in contact with each other. Although the interfacial region contains a significantly smaller fraction of molecules compared to the bulk, it is the primary site where many photoinduced excited state processes occur, such as charge transfer, charge recombination, separation, energy transfer processes, etc. All such photoinduced processes have a dependence on molecular orientation and density of states at the interfaces, therefore having an understanding of the interfacial region is essential. However, conventional spectroscopic techniques, such as surface-enhanced Raman scattering, x-ray photoelectron spectroscopy, atomic force microscopy, etc., face limitations in probing the orientation and density of states of interfacial molecules. Therefore, there is a need for noninvasive techniques capable of efficiently investigating the interfaces. The electronic sum frequency generation (ESFG) technique offers an interface selectivity based on the principle that the second-order nonlinear susceptibility tensor, within the electric dipole approximation, is zero in the isotropic bulk but nonzero at interfaces. This selectivity makes ESFG a promising spectroscopy tool to probe the molecular orientation and density of states at the buried interface. For beginners interested in employing ESFG to study the density of states at the interface, a detailed description of the experimental setup is provided here.

SynDLP is a dynamin-like protein of Synechocystis sp. PCC 6803 with eukaryotic features

Dynamin-like proteins are membrane remodeling GTPases with well-understood functions in eukaryotic cells. However, bacterial dynamin-like proteins are still poorly investigated. SynDLP, the dynamin-like protein of the cyanobacterium Synechocystis sp. PCC 6803, forms ordered oligomers in solution. The 3.7 Å resolution cryo-EM structure of SynDLP oligomers reveals the presence of oligomeric stalk interfaces typical for eukaryotic dynamin-like proteins. The bundle signaling element domain shows distinct features, such as an intramolecular disulfide bridge that affects the GTPase activity, or an expanded intermolecular interface with the GTPase domain. In addition to typical GD-GD contacts, such atypical GTPase domain interfaces might be a GTPase activity regulating tool in oligomerized SynDLP. Furthermore, we show that SynDLP interacts with and intercalates into membranes containing negatively charged thylakoid membrane lipids independent of nucleotides. The structural characteristics of SynDLP oligomers suggest it to be the closest known bacterial ancestor of eukaryotic dynamin.

Vibrational sum frequency spectroscopy of thin film interfaces

We describe a basic theoretical treatment of how film-substrate and substrate-environment (air, water, and solution) interfaces can be selectively probed by controlling the film thickness and beam angles in a visible-infrared sum frequency generation experiment. In this model, we also account for the unique interfacial environment that may have optical properties that differ from the adjacent bulk phases. We see that this affects components of the electric field that are perpendicular to the surface such as when p-polarized light is used. We then provide an example using the glass-polydimethylsiloxane-air system and model the fields at both surfaces of the polymer. This is followed by some practical considerations for setting up such experiments and some typical experimental results.