Windowless detection geometry for sum frequency scattering spectroscopy in the C-D and amide I regions

The Surface of Colloidal Metal-Organic Framework Nanoparticles Revealed by Vibrational Sum Frequency Scattering Spectroscopy

Solvation shells strongly influence the interfacial chemistry of colloidal systems, from the activity of proteins to the colloidal stability and catalysis of nanoparticles. Despite their fundamental and practical importance, solvation shells have remained largely undetected by spectroscopy. Furthermore, their ability to assemble at complex but realistic interfaces with heterogeneous and rough surfaces remains an open question. Here, we apply vibrational sum frequency scattering spectroscopy (VSFSS), an interface-specific technique, to colloidal nanocrystals with porous metal-organic frameworks (MOFs) as a case study. Due to the porous nature of the solvent-particle boundary, MOF particles challenge conventional models of colloidal and interfacial chemistry. Their multiweek colloidal stability in the absence of conventional surface ligands suggests that stability may arise in part from solvation forces. Spectra of colloidally stable Zn(2-methylimidazolate)2 (ZIF-8) in polar solvents indicate the presence of ordered solvation shells, solvent-metal binding, and spontaneous ordering of organic bridging linkers within the MOF. These findings help explain the unexpected colloidal stability of MOF colloids, while providing a roadmap for applying VSFSS to wide-ranging colloidal nanocrystals in general.

"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.

Lysozyme Interaction with Phospholipid Nanodroplets Probed by Sum Frequency Scattering Vibrational Spectroscopy

When a nanoparticle (NP) is introduced into a biological environment, its identity and interactions are immediately attributed to the dense layer of proteins that quickly covers the particle. The formation of this layer, dubbed the protein corona, is in general a combination of proteins interacting with the surface of the NP and a contest between other proteins for binding sites either at the surface of the NP or upon the dense layer. Despite the importance for surface engineering and drug development, the molecular mechanisms and structure behind interfacial biomolecule action have largely remained elusive. We use ultrafast sum frequency scattering (SFS) spectroscopy to determine the structure and the mode of action by which these biomolecules interact with and manipulate interfaces. The majority of work in the field of sum frequency generation has been done on flat model interfaces. This limits some important membrane properties such as membrane fluidity and dimensionality─important factors in biomolecule-membrane interactions. To move toward three-dimensional (3D) nanoscopic interfaces, we utilize SFS spectroscopy to interrogate the surface of 3D lipid monolayers, which can be used as a model lipid-based nanocarrier system. In this study, we have utilized SFS spectroscopy to follow the action of lysozyme. SFS spectra in the amide I region suggest that there is lysozyme at the interface and that the lysozyme induces an increased lipid monolayer order. The binding of lysozyme with the NP is demonstrated by an increase in acyl chain order determined by the ratio of the CH₃ symmetric and CH₂ symmetric peak amplitudes. Furthermore, the lipid headgroup orientation s-PO₂^- change strongly supports lysozyme insertion into the lipid layer causing lipid disruption and reorientation. Altogether, with SFS, we have made a huge stride toward understanding the binding and structure change of proteins within the protein corona.

Orientation of the Dysferlin C2A Domain is Responsive to the Composition of Lipid Membranes

Dysferlin is a 230 kD protein that plays a critical function in the active resealing of micron-sized injuries to the muscle sarcolemma by recruiting vesicles to patch the injured site via vesicle fusion. Muscular dystrophy is observed in humans when mutations disrupt this repair process or dysferlin is absent. While lipid binding by dysferlin's C2A domain (dysC2A) is considered fundamental to the membrane resealing process, the molecular mechanism of this interaction is not fully understood. By applying nonlinear surface-specific vibrational spectroscopy, we have successfully demonstrated that dysferlin's N-terminal C2A domain (dysC2A) alters its binding orientation in response to a membrane's lipid composition. These experiments reveal that dysC2A utilizes a generic electrostatic binding interaction to bind to most anionic lipid surfaces, inserting its calcium binding loops into the lipid surface while orienting its β-sheets 30-40° from surface normal. However, at lipid surfaces, where PI(4,5)P2 is present, dysC2A tilts its β-sheets more than 60° from surface normal to expose a polybasic face, while it binds to the PI(4,5)P2 surface. Both lipid binding mechanisms are shown to occur alongside dysC2A-induced lipid clustering. These different binding mechanisms suggest that dysC2A could provide a molecular cue to the larger dysferlin protein as to signal whether it is bound to the sarcolemma or another lipid surface.

Peptide Orientation at Emulsion Nanointerfaces Dramatically Different from Flat Surfaces

The adsorption of protein to nanoparticles plays an important role in toxicity, food science, pharmaceutics, and biomaterial science. Understanding how proteins bind to nanophase surfaces is instrumental for understanding and, ultimately, controlling nanoparticle (NP) biochemistry. Techniques probing the adsorption of proteins at NP interfaces exist; however, these methods have been unable to determine the orientation and folding of proteins at these interfaces. For the first time, we probe in situ with sum frequency scattering vibrational spectroscopy the orientation of model leucine-lysine (LK) peptides adsorbed to NPs. The results show that both α-helical and β-strand LK peptides bind the particles in an upright orientation, in contrast to the flat orientation of LKs binding to planar surfaces. The different binding geometry is explained by Coulombic forces between peptides across the particle volume.

Choose your own adventure: Picosecond or broadband vibrational sum-frequency generation spectroscopy

Vibrational sum-frequency generation (VSFG) spectroscopy is a method capable of measuring chemical structure and dynamics within the interfacial region between two bulk phases. At the core of every experimental system is a laser source that influences the experimental capabilities of the VSFG spectrometer. In this article, we discuss the differences between VSFG spectrometers built with picosecond and broadband laser sources as it will impact everything from material costs, experimental build time, experimental capabilities, and more. A focus is placed on the accessibility of the two different SFG systems to newcomers in the SFG field and provides a resource for laboratories considering incorporating VSFG spectroscopy into their research programs. This Tutorial provides a model decision tree to aid newcomers when determining whether the picosecond or femtosecond laser system is sufficient for their research program and navigates through it for a few specific scenarios.

Interfacial Steric and Molecular Bonding Effects Contributing to the Stability of Neutrally Charged Nanoemulsions

In cosmetic, pharmaceutical, and food applications, many active ingredients have limited bioavailability in an aqueous environment, and in that context, nanoemulsions provide a mechanism for encapsulation, protection, and transport. These dispersed oil droplets are on the order of 100s of nanometers in diameter and owe their long-term stability to emulsifiers that are commonly charged. More recently, applications have been utilizing nonionic species as stabilizing agents due to their enhanced biosafety. DLVO (named after Derjaguin, Landau, Verwey, and Overbeek) theory has been central in the description of colloid stability, which emphasizes repulsive electrostatic forces, while extended DLVO theory also accounts for steric effects. Past studies of nanoemulsions have largely employed charged surfactants and polyelectrolytes, making it difficult to decouple electrostatic and steric effects as they relate to droplet stability. To better understand steric and molecular factors contributing to the stability of "uncharged" droplets, we have created nanoemulsions with sodium dodecyl sulfate (SDS) and poly(N-vinylacetamide) (PNVA). Though SDS is anionic, with PNVA coating the droplet surfaces, the ζ-potentials of these nanoemulsions are ∼0 mV. Despite minimizing electrostatic contributions, these nanoemulsions are stable for upward of a month with interesting dynamics. By employing dynamic light scattering, vibrational sum frequency scattering spectroscopy, and calculating interaction pair potentials using extended DLVO theory, we learn that the thickness of the PNVA layer plays a critical role in stabilizing these "uncharged" nanoemulsions. Beyond the sterics, the molecular conformation of the PNVA strands also contributes to the droplet stability. The adsorbed PNVA strands are shown to form stratified, rigid polymer networks that prevent the nanoemulsions from rapid destabilization.

Dynamic Duo: Vibrational Sum Frequency Scattering Investigation of pH-Switchable Carboxylic Acid/Carboxylate Surfactants on Nanodroplet Surfaces

Surfactants containing pH-switchable, carboxylic acid moieties are utilized in a variety of environmental, industrial, and biological applications that require controlled stability of hydrophobic droplets in water. For nanoemulsions, kinetically stable oil droplets in water, surface adsorption of the anionic form of the carboxylic acid surfactant stabilizes the droplet, whereas a dominant surface presence of the neutral form leads to destabilization. Through the use of dynamic light scattering, ζ-potential, and vibrational sum frequency scattering spectroscopy (VSFSS), we investigate this mechanism and the relative surface population of the neutral and charged species as pH is adjusted. We find that the relative population of the two surfactant species at the droplet surface is distinctly different than their bulk equilibrium concentrations. The ζ-potential measurements show that the surface concentration of the charged surfactant stays nearly constant throughout the stabilizing pH range. In contrast, VSFSS shows that the neutral carboxylic acid form increasingly adsorbs to the surface with increased acidity. The spectral features of the headgroup vibrational modes confirm this behavior and go further to reveal additional molecular details of their adsorption. A significant hydrogen-bonding interaction occurs between the headgroups that, along with hydrophobic chain-chain interactions, assists in drawing more carboxylic acid surfactant to the interface. The charged surfactant provides the stabilizing force for these droplets, while the neutral surfactant introduces complexity to the interfacial structure as the pH is lowered. The results are significantly different than what has been found for the planar oil/water studies where stabilization of the interface is not a factor.