Vibrational Sum Frequency Generation Research Articles

Adsorption, Orientation, and Speciation of Amino Acids at Air-Aqueous Interfaces for the Direct Air Capture of CO2.

Amino acids make up a promising family of molecules capable of direct air capture (DAC) of CO2 from the atmosphere. Under alkaline conditions, CO2 reacts with the anionic form of an amino acid to produce carbamates and deactivated zwitterionic amino acids. The presence of the various species of amino acids and reactive intermediates can have a significant effect on DAC chemistry, the role of which is poorly understood. In this study, all-atom molecular dynamics (MD) based computational simulations and vibrational sum frequency generation (vSFG) spectroscopy studies were conducted to understand the role of competitive interactions at the air-aqueous interface in the context of DAC. We find that the presence of potassium bicarbonate ions, in combination with the anionic and zwitterionic forms of amino acids, induces concentration and charge gradients at the interface, generating a layered molecular arrangement that changes under pre- and post-DAC conditions. In parallel, an enhancement in the surface activity of both anionic and zwitterionic forms of amino acids is observed, which is attributed to enhanced interfacial stability and favorable intermolecular interactions between the adsorbed amino acids in their anionic and zwitterionic forms. The collective influence of these competitive interactions, along with the resulting interfacial heterogeneity, may in turn affect subsequent capture reactions and associated rates. These effects underscore the need to consider dynamic changes in interfacial chemical makeup to enhance DAC efficiency and to develop successful negative emission and storage technologies.

Mount for spectroscopic analysis of samples under sustained tensile stress.

Spectroscopic methods offer valuable insights into the molecular and structural changes induced by stress, but existing techniques are often unable to perform real-time measurements during deformation. A novel solid open mount design is presented that enables spectroscopic investigations of materials under sustained tensile stress while maintaining crucial alignment of the optical system. The mount design allows for sample movement in response to applied strain while maintaining the position of the sample plane, ensuring consistent and reliable spectroscopic measurements. The effectiveness of the mount design is demonstrated with vibrational sum-frequency generation measurements of an elastomer, cured hydroxyl-terminated polybutadiene, and a plastic, high-density polyethylene, taken before, during, and after tensile deformation. The application of this mount to other spectroscopic techniques is discussed. The ability to collect spectroscopic data during a stress event would provide valuable insights into the behavior of stressed materials.

Modeling Vibrational Sum Frequency Generation Spectra of Interfacial Water on a Gold Surface: The Role of the Fermi Resonance.

Studying the hydrogen bonding structure of H2O at the metal-water interface is a highly complex yet fascinating endeavor. The intricate interactions and diverse orientations of water molecules on metal surfaces with varying potentials pose a significant challenge in elucidating the coupling between O-H stretching and H-O-H bending modes. In this study, we employed DFT-MD simulation to explore how the orientation of interfacial water molecules changes with the applied potential on the Au(111) surface. Based on the surface-specific velocity-velocity correlation function (ssVVCF) formula, we calculated vibrational sum frequency generation (VSFG) spectra for the O-H stretches. We found that three assigned peaks (∼3300, ∼3450, and 3650 cm-1) shifted toward lower frequencies as the potential moved toward more negative values. Our results align remarkably well with experimental Raman spectroscopy data. Notably, our VSFG analysis revealed a significant change in the VSFG spectra of the hydrogen-bonded O-H groups (∼3300 cm-1), switching from a negative to a positive sign with decreasing potential. This alteration suggests a substantial change in the orientation of these low-frequency O-H groups owing to their increased interactions with the Au surface. In contrast, the orientations of both the high-frequency O-H groups (∼3450 cm-1) and the dangling O-H groups (∼3650 cm-1) remained unaffected by the applied potentials. Furthermore, our analysis of the decomposed vibrational density of states (VDOS) for the H-O-H bending mode uncovered the coupling between the H-O-H bending and O-H stretching vibrations, known as the Fermi resonance. Our work suggests that the H-O-H bending vibration becomes restricted when water molecules transition from the ″one-H-down″ to the ″two-H-down″ conformation, leading to a redshift in the O-H stretching vibration through the Fermi resonance. By constructing the VSFG and decomposed VDOS spectra, we gained valuable insights into the structural changes that Raman spectra alone cannot fully interpret. Specifically, our analysis revealed the critical role of the Fermi resonance effect in shaping the spectroscopic signature of interfacial water molecules on the Au(111) surface.

Physicochemical control of solvation and molecular assembly of charged amphiphilic oligomers at air-aqueous interfaces

Hypothesis: Understanding the rules that control the assembly of nanostructured soft materials at interfaces is central to many applications. We hypothesize that electrolytes can be used to alter the hydration shell of amphiphilic oligomers at the air-aqueous interface of Langmuir films, thereby providing a means to control the formation of emergent nanostructures.Experiments: Three representative salts – (NaF, NaCl, NaSCN) were studied for mediating the self-assembly of oligodimethylsiloxane methylimidazolium (ODMS-MIM+) amphiphiles in Langmuir films. The effects of the different salts on the nanostructure assembly of these films were probed using vibrational sum frequency generation (SFG) spectroscopy and Langmuir trough techniques. Experimental data were supported by atomistic molecular dynamic simulations.Findings: Langmuir trough surface pressure – area isotherms suggested a surprising effect on oligomer assembly, whereby the presence of anions affects the stability of the interfacial layer irrespective of their surface propensities. In contrast, SFG results implied a strong anion effect that parallels the surface activity of anions. These seemingly contradictory trends are explained by anion driven tail dehydration resulting in increasingly heterogeneous systems with entangled ODMS tails and appreciable anion penetration into the complex interfacial layer comprised of headgroups, tails, and interfacial water molecules. These findings provide physical and chemical insight for tuning a wide range of interfacial assemblies.

Cholesterol Changes Interfacial Water Alignment in Model Cell Membranes.

The nanoscopic layer of water that directly hydrates biological membranes plays a critical role in maintaining the cell structure, regulating biochemical processes, and managing intermolecular interactions at the membrane interface. Therefore, comprehending the membrane structure, including its hydration, is essential for understanding the chemistry of life. While cholesterol is a fundamental lipid molecule in mammalian cells, influencing both the structure and dynamics of cell membranes, its impact on the structure of interfacial water has remained unknown. We used surface-specific vibrational sum-frequency generation spectroscopy to study the effect of cholesterol on the structure and hydration of monolayers of the lipids 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and egg sphingomyelin (SM). We found that for the unsaturated lipid DOPC, cholesterol intercalates in the membrane without significantly changing the orientation of the lipid tails and the orientation of the water molecules hydrating the headgroups of DOPC. In contrast, for the saturated lipids DPPC and SM, the addition of cholesterol leads to clearly enhanced packing and ordering of the hydrophobic tails. It is also observed that the orientation of the water hydrating the lipid headgroups is enhanced upon the addition of cholesterol. These results are important because the orientation of interfacial water molecules influences the cell membranes' dipole potential and the strength and specificity of interactions between cell membranes and peripheral proteins and other biomolecules. The lipid nature-dependent role of cholesterol in altering the arrangement of interfacial water molecules offers a fresh perspective on domain-selective cellular processes, such as protein binding.

Probing the Interfacial Molecular Structure of a Co‐Prussian Blue In Situ

AbstractMolecular‐level insight into the interfacial composition of electrodes at the solid‐electrolyte and the solid‐electrode interface is essential to understanding the charge transfer processes, which are vital for electrochemical (EC) and photoelectrochemical (PEC) applications. However, spectroscopic access to both interfaces, particularly upon application of an external bias, remains a challenge. Here, in situ surface sensitive vibrational sum‐frequency generation (VSFG) spectroscopy is used for the first time to directly access the interfacial structure of a cobalt‐containing Prussian blue analog (Co‐PBA) in contact with the electrolyte and TiO2/Au surface. Structural and compositional changes of the Prussian blue layer during electrochemical oxidation are studied by monitoring the stretching vibration of the CN group. At open circuit potential, VSFG reveals a non‐homogeneous distribution of oxidation states of metal sites: FeIII–CN–CoII and FeII–CN–CoIII coordination motifs are dominantly observed at the Co‐PBA|TiO2 interface, while it is only the FeII–CN–CoII unit at the electrolyte interface. Upon increasing the potential applied to the electrode, the partial oxidation of FeII–CN–CoII to FeIII–CN–CoII is observed followed by its transformation to FeII–CN–CoIII via charge transfer and, finally, the formation of FeIII–CN–CoIII species at the interface with TiO2 and the electrolyte.

Protein-Mediated Changes in Membrane Fluidity and Ordering: Insights into the Molecular Mechanism and Implications for Cellular Function.

Probing protein-membrane interactions is vital for understanding biological functionality for various applications such as drug development, targeted drug delivery, and creation of functional biomaterials for medical and industrial purposes. In this study, we have investigated interaction of Human Serum Albumin (HSA) with two different lipids, dipalmitoylphosphatidylglycerol (dDPPG) and dipalmitoylphosphatidylcholine (dDPPC), using Vibrational Sum Frequency Generation spectroscopy at different membrane fluidity values. In the liquid-expanded (LE) state of the lipid, HSA (at pH 3.5) deeply intercalated lipid chains through a combination of electrostatic and hydrophobic interactions, which resulted in more ordering of the lipid chains. However, in the liquid-condensed (LC) state, protein intercalation is decreased due to tighter lipid packing. Moreover, our findings revealed distinct differences in HSA's interaction with dDPPG and dDPPC lipids. The interaction with dDPPC remained relatively weak compared to dDPPG. These results shed light on the significance of protein mediated changes in lipid characteristics, which hold considerable implications for understanding membrane protein behavior, lipid-mediated cellular processes, and lipid-based biomaterial design.

Ladungstransferdynamiken in organisch–anorganischen hybriden Heterostrukturen – Einblicke mithilfe von Summenfrequenzspektroskopie

AbstractOrganisch–anorganische Heterostrukturen spielen eine zentrale Rolle in modernen elektronischen und optoelektronischen Anwendungen. Dazu gehören Photodetektoren und Feldeffekttransistoren sowie die Umwandlung von Solarenergie mithilfe von Photoelektroden für Farbstoffsolarzellen, photoelektrochemischen Zellen und organischen Photovoltaiksystemen. Die Leistung solcher Systeme wird zu großen Teilen durch die Dynamik des Ladungstransfers an und zwischen (inneren) Grenzflächen, z. B. zwischen einem Halbleiter mit großer Bandlücke und molekularen Sensibilisatoren und/oder Katalysatoren, bestimmt. Daher ist ein detailliertes Verständnis der Beziehung zwischen Struktur, Dynamik und Funktion solcher funktionalen Grenzflächen notwendig, um mögliche Limitationen der Leistungsfähigkeit dieser Materialien und Systeme auf molekularer Ebene zu erklären. Die Summenfrequenzspektroskopie (SFS) als grenzflächenempfindliche Technik ermöglicht es, spezifische chemische Informationen über Grenzflächen zu erhalten. Wird diese in ein Anregungs‐Abfrage‐Schema integriert, liefert sie neben solchen chemischen Erkenntnissen eine ultraschnelle Zeitauflösung. Dieser Mini‐Review‐Artikel diskutiert die Vorteile und das Potenzial der SFS für die Untersuchung von Ladungstransferdynamiken an Grenzflächen und von strukturellen Veränderungen an inneren Grenzflächen. Es wird ein wichtiger Beitrag dieser einzigartigen spektroskopischen Einblicke in ansonsten unzugängliche Grenzflächen vorgestellt, der, so hoffen wir, neue Wege für ein besseres Verständnis der funktionsbestimmenden Prozesse in komplexen Materialien aufzeigt. Damit sollen Gemeinschaften, die sich dem Design von Materialien und Bauteilen widmen, mit Spektroskopikern zusammengebracht werden.

Charge Transfer Dynamics in Organic-Inorganic Hybrid Heterostructures-Insights by Vibrational-Sum Frequency Generation Spectroscopy.

Organic-inorganic heterostructures play a pivotal role in modern electronic and optoelectronic applications including photodetectors and field effect transistors, as well as in solar energy conversion such as photoelectrodes of dye-sensitized solar cells, photoelectrochemical cells, and in organic photovoltaics. To a large extent, performance of such devices is controlled by charge transfer dynamics at and across (inner) interfaces, e.g., between a wide band gap semiconductor and molecular sensitizers and/or catalysts. Hence, a detailed understanding of the structure-dynamics-function relationship of such functional interfaces is necessary to rationalize possible performance limitations of these materials and devices on a molecular level. Vibrational sum-frequency generation (VSFG) spectroscopy, as an interface-sensitive spectroscopic technique, allows to obtain chemically specific information from interfaces and combines such chemical insights with ultrafast time resolution, when integrated as a spectroscopic probe into a pump-probe scheme. Thus, this minireview discusses the advantages and potential of VSFG spectroscopy for investigating interfacial charge transfer dynamics and structural changes at inner interfaces. A critical perspective of the unique spectroscopic view of otherwise inaccessible interfaces is presented, which we hope opens new opportunities for an improved understanding of function-determining processes in complex materials, and brings together communities who are devoted to designing materials and devices with spectroscopists.

Working on a dream: bringing up the level of interface spectroscopy to the bulk level

Abstract Liquid interfaces are unique environments in which a variety of fundamental phenomena occur. Therefore, it is important to obtain a molecular-level understanding of liquid interfaces for both basic science and industrial applications. However, it is not an easy task to investigate molecules in the interface region that only has nanometer thicknesses. Second-order nonlinear spectroscopy, or even-order nonlinear spectroscopy more generally, is intrinsically interface-selective because the relevant nonlinear signal is generated only in the region in which the inversion symmetry is broken under the dipole approximation. In the past 2 decades, we have been developing and applying new interface nonlinear spectroscopic methods, aiming to bring up the level of knowledge on interfacial molecules to that on molecules in solution. During this attempt, we developed electronic sum-frequency generation spectroscopy, heterodyne-detected electronic sum-frequency generation spectroscopy, and heterodyne-detected vibrational sum-frequency generation spectroscopy, as well as fourth-order Raman spectroscopy. We also extended the methods to femtosecond time-resolved measurements. Using these methods, we are now able to study the structure and dynamics at liquid interfaces, in particular exposed interfaces such as air/liquid interfaces, at a similar level to the study for solution. I overview our interface research while describing thoughts we had at each turning point.

Potential Dependent Reorientation Controlling Activity of a Molecular Electrocatalyst.

The activity of molecular electrocatalysts depends on the interplay of electrolyte composition near the electrode surface, the composition and morphology of the electrode surface, and the electric field at the electrode-electrolyte interface. This interplay is challenging to study and often overlooked when assessing molecular catalyst activity. Here, we use surface specific vibrational sum frequency generation (VSFG) spectroscopy to study the solvent and potential dependent activation of Mo(bpy)(CO)4, a CO2 reduction catalyst, at a polycrystalline Au electrode. We find that the parent complex undergoes potential dependent reorientation at the electrode surface when a small amount of N-methyl-2-pyrrolidone (NMP) is present. This preactivates the complex, resulting in greater yields at less negative potentials, of the active electrocatalyst for CO2 reduction.

Monovalent ion-graphene oxide interactions are controlled by carboxylic acid groups: Sum frequency generation spectroscopy studies.

Graphene oxide (GO) is a two-dimensional, mechanically strong, and chemically tunable material for separations. Elucidating GO-ion-water interactions at the molecular scale is highly important for predictive understanding of separation systems. However, direct observations of the nanometer region by GO surfaces under operando conditions are not trivial. Therefore, thin films of GO at the air/water interface can be used as model systems. With this approach, we study the effects of alkali metal ions on water organization near graphene oxide films at the air/water interface using vibrational sum frequency generation (SFG) spectroscopy. We also use an arachidic acid Langmuir monolayer as a benchmark for a pure carboxylic acid surface. Theoretical modeling of the concentration-dependent sum frequency signal from graphene oxide and arachidic acid surfaces reveals that the adsorption of monovalent ions is mainly controlled by the carboxylic acid groups on graphene oxide. An in-depth analysis of sum frequency spectra reveals at least three distinct water populations with different hydrogen bonding strengths. The origin of each population can be identified from concentration dependent variations of their SFG signal. Interestingly, an interfacial water structure seemed mostly insensitive to the character of the alkali cation, in contrast to similar studies conducted at the silica/water interface. However, we observed an ion-specific effect with lithium, whose strong hydration prevented direct interactions with the graphene oxide film.

Orientational Behavior and Vibrational Response of Glycine at Aqueous Interfaces.

Aqueous glycine plays many different roles in living systems, from being a building block for proteins to being a neurotransmitter. To better understand its fundamental behavior, we study glycine's orientational behavior near model aqueous interfaces, in the absence and presence of electric fields and biorelevant ions. To this purpose, we use a surface-specific technique called heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSFG). Using HD-VSFG, we directly probe the symmetric and antisymmetric stretching vibrations of the carboxylate group of zwitterionic glycine. From their relative amplitudes, we infer the zwitterion's orientation near surfactant-covered interfaces and find that it is governed by both electrostatic and surfactant-specific interactions. By introducing additional ions, we observe that the net orientation is altered by the enhanced ionic strength, indicating a change in the balance of the electrostatic and surfactant-specific interactions.

Unified picture of vibrational relaxation of OH stretch at the air/water interface

The elucidation of the energy dissipation process is crucial for understanding various phenomena occurring in nature. Yet, the vibrational relaxation and its timescale at the water interface, where the hydrogen-bonding network is truncated, are not well understood and are still under debate. In the present study, we focus on the OH stretch of interfacial water at the air/water interface and investigate its vibrational relaxation by femtosecond time-resolved, heterodyne-detected vibrational sum-frequency generation (TR-HD-VSFG) spectroscopy. The temporal change of the vibrationally excited hydrogen-bonded (HB) OH stretch band (ν=1→2 transition) is measured, enabling us to determine reliable vibrational relaxation (T1) time. The T1 times obtained with direct excitations of HB OH stretch are 0.2-0.4 ps, which are similar to the T1 time in bulk water and do not noticeably change with the excitation frequency. It suggests that vibrational relaxation of the interfacial HB OH proceeds predominantly with the intramolecular relaxation mechanism as in the case of bulk water. The delayed rise and following decay of the excited-state HB OH band are observed with excitation of free OH stretch, indicating conversion from excited free OH to excited HB OH (~0.9 ps) followed by relaxation to low-frequency vibrations (~0.3 ps). This study provides a complete set of the T1 time of the interfacial OH stretch and presents a unified picture of its vibrational relaxation at the air/water interface.

Exploring the influence of interfacial solvation on electrochemical CO2 reduction using plasmon‐enhanced vibrational sum frequency generation spectroscopy

AbstractAlthough interfacial solvation plays an important role in determining carbon dioxide reduction (CO2R) kinetics, present understanding of the potential dependent properties of the electrochemical double layer under conditions relevant for CO2R remains limited. This article summarizes the development and recent applications of plasmon‐enhanced vibrational sum frequency generation (VSFG) spectroscopy to study the effects of cation hydration and interfacial solvation on CO2R using CO as a vibrational Stark reporter. Results show that electrolyte cations retain their entire solvation shell upon adsorption to inactive sites, while active sites retain only a single water layer between the gold surface and the cation. Measurements also show that the total interfacial electric field can be separated into two contributions: one from the electrochemical double layer (Stern field) and another from the polar solvation environment (Onsager field). Surprisingly, correlating VSFG spectra with reaction kinetics reveals that it is the solvation‐mediated Onsager field that governs the chemical reactivity at the electrode/electrolyte interface. Measuring the interfacial water spectra during electrocatalysis also provides evidence for the proton source during H2 evolution, which competes with CO2R in aqueous electrolyte. These findings highlight the importance of directly probing cation hydration and interfacial solvation, which mediates reaction kinetics at electrochemical interfaces.

Observation of Electrostatically Driven Surface Adsorption in Mixed Surfactant Systems.

We employed heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy to obtain a molecular-level understanding of the interaction between the anionic surfactant sodium dodecyl ammonium sulfate (SDS) and the cationic surfactant dodecyltrimethylammonium bromide (DTAB). We observed that these surfactants show a strong cooperative effect on their adsorption to the water-air interface. Even at bulk concentrations 1000 times lower than the critical micelle concentrations of SDS and DTAB, a nearly complete surface surfactant layer is observed when both surfactants are present. This strong enhancement of the surface concentrations of DS- and DTA+ can be quantitatively explained from the favorable Coulomb interaction of the oppositely charged headgroups of DS- and DTA+ and the electrostatic interactions with their counterions. The HD-VSFG results are complemented by a modified Langmuir adsorption model in which we include the free energy associated with the electrostatic interactions of the surfactant ions and their counterions.

Tailored Monolayers of N-Heterocyclic Carbenes by Kinetic Control.

Despite the substantial success of N-heterocyclic carbenes (NHCs) as stable and versatile surface modification ligands, their use in nanoscale applications beyond chemistry is still hampered by the failure to control the carbene binding mode, which complicates the fabrication of monolayers with the desired physicochemical properties. Here, we applied vibrational sum-frequency generation spectroscopy to conduct a pseudokinetic surface analysis of NHC monolayers on Au thin films under ambient conditions. We observe for two frequently used carbene structures that their binding mode is highly dynamic and changes with the adsorption time. In addition, we demonstrate that this transition can be accelerated or decelerated to adjust the binding mode of NHCs, which allows fabrication of tailored monolayers of NHCs simply by kinetic control.

Spectral narrowing of a phonon resonance in time-domain sum-frequency spectroscopy

Sum-frequency generation (SFG) spectroscopy provides a versatile method for the investigation of noncentrosymmetric media and interfaces. Here, using tunable picosecond infrared (IR) pulses from a free-electron laser, the nonlinear optical response of 4H-SiC, a common polytype of silicon carbide, has been probed in the frequency and time domain by infrared-visible vibrational SFG spectroscopy. In the SFG spectra we observe a sharp resonance near the longitudinal optical phonon frequency, arising from linear optical effects due to the epsilon-near-zero regime of the IR permittivity. In the time domain, the buildup of the SFG intensity is linked to the free-induction decay of the induced coherent IR polarization. When approaching the frequency of the phonon resonance, a slower polarization dephasing is observed as compared to off-resonant IR excitation. Thus, by introducing a temporal delay between the IR and the visible up-conversion pulse we are able to demonstrate spectral narrowing of the phonon SFG resonance, as corroborated by model calculations. Published by the American Physical Society 2024

Surface stratification determines the interfacial water structure of simple electrolyte solutions

The distribution of ions at the air/water interface plays a decisive role in many natural processes. Several studies have reported that larger ions tend to be surface-active, implying ions are located on top of the water surface, thereby inducing electric fields that determine the interfacial water structure. Here we challenge this view by combining surface-specific heterodyne-detected vibrational sum-frequency generation with neural network-assisted ab initio molecular dynamics simulations. Our results show that ions in typical electrolyte solutions are, in fact, located in a subsurface region, leading to a stratification of such interfaces into two distinctive water layers. The outermost surface is ion-depleted, and the subsurface layer is ion-enriched. This surface stratification is a key element in explaining the ion-induced water reorganization at the outermost air/water interface.

Ultrafast Surface-Specific Spectroscopy of Water at a Photoexcited TiO2 Model Water-Splitting Photocatalyst.

A critical step in photocatalytic water dissociation is the hole-mediated oxidation reaction. Molecular-level insights into the mechanism of this complex reaction under realistic conditions with high temporal resolution are highly desirable. Here, we use femtosecond time-resolved, surface-specific vibrational sum frequency generation spectroscopy to study the photo-induced reaction directly at the interface of the photocatalyst TiO2 in contact with liquid water at room temperature. Thanks to the inherent surface specificity of the spectroscopic method, we can follow the reaction of solely the interfacial water molecules directly at the interface at timescales on which the reaction takes place. Following the generation of holes at the surface immediately after photoexcitation of the catalyst with UV light, water dissociation occurs on a sub-20 ps timescale. The reaction mechanism is similar at pH 3 and 11. In both cases, we observe the conversion of H2 O into Ti-OH groups and the deprotonation of pre-existing Ti-OH groups. This study provides unique experimental insights into the early steps of the photo-induced dissociation processes at the photocatalyst-water interface, relevant to the design of improved photocatalysts.