Vibrational Sum Frequency Generation Research Articles

Selective Action of Antimalarial Hydroxychloroquine on the Packing of Phospholipids and Interfacial Water Associated with Lysosomal Model Membranes: A Vibrational Sum Frequency Generation Study.

Understanding the structural change of lysosomal membranes induced by hydroxychloroquine (HCQ) drug is essential as it has been considered as one of the probable mechanisms of its antimalarial action. In this context, vibrational sum frequency generation (VSFG) spectra of the O-H region of water and C-H of the hydrocarbon chain of negatively charged and zwitterionic phospholipids associated with the lysosomal membrane in the absence and presence of different concentrations of HCQ have been measured at the air/water interface. The interfacial water at the negatively charged and zwitterionic lipids gets restructured in the presence of HCQ; however, the mechanism of restructuring is different due to the charge of the head groups of lipids. Interestingly, the presence of HCQ leads to a disorder in the negatively charged lipids, irrespective of their chemical nature, mainly by creating the gauche defect in the hydrocarbon chain of the lipid. In contrast, the ordering of the zwitterionic lipid does not show any appreciable change with the addition of HCQ. The finding on the selectivity of HCQ in affecting the ordering of the lipid depending on its head group charge and restructuring of interfacial water may be useful in understanding the molecular level mechanism of the antimalarial action of HCQ.

Deciphering the Properties of Nanoconfined Aqueous Solutions by Vibrational Sum Frequency Generation Spectroscopy.

When confined between walls at nanometer distances, water exhibits surprisingly different properties with reference to bare interfacial water. Based on computer simulations, we demonstrate how vibrational sum frequency generation (VSFG) spectroscopy can be used-even with very mild symmetry breaking-to discriminate multilayer water in wide slit pores from both bilayer and monolayer water confined within molecularly narrow pores. Applying the technique, the VSFG lineshapes of monolayer, bilayer, and multilayer water are found to differ in characteristic ways, which is explained by their distinct density stratifications giving rise to different H-bonding patterns in the respective solvation layers.

The role of resonant coupling in vibrational sum-frequency-generation spectroscopy: Liquid acetonitrile at the silica interface

Vibrational sum-frequency-generation (VSFG) spectroscopy is a versatile technique for probing molecular organization at interfaces. There is a growing recognition that dynamic phenomena, such as reorientation and intermolecular vibrational coupling, can also influence VSFG spectra. The silica/liquid acetonitrile interface is a useful system for exploring these effects in more detail. The organization of acetonitrile at this interface has been well studied, and is known to resemble that of a supported lipid bilayer in many ways. Here isotopic dilution is used to explore the influence of resonant intermolecular coupling of methyl symmetric stretches on the VSFG spectroscopy of this system. VSFG spectra in the methyl stretching region at this interface show a blue shift, a decrease in linewidth, and a higher-than-expected intensity upon dilution in deuterated acetonitrile. We demonstrate that resonant coupling influences VSFG spectral shifts through the infrared transition. Using molecular simulations, we show that our experimental observations are consistent with resonant coupling between methyl transition dipoles being a significant, but not the dominant, contribution to the observed spectral shift upon isotopic dilution. Furthermore, our molecular simulations demonstrate that resonant coupling accounts partially for changes in linewidth and intensity. These classical molecular dynamics simulations also elucidate the behavior of the isotropic Raman spectrum in the bulk liquid upon isotopic dilution. We further simulate the resonant coupling among cyano stretches in this system. These simulations match the experimental shift due to the Raman non-coincidence effect shift of the CN stretch in the bulk liquid, but suggest that the corresponding resonant-coupling-induced shift in the VSFG spectrum at the silica interface is minimal. Our experimental and simulation results indicate that proximity to an interface can cause substantial changes in the resonant coupling of vibrations in a liquid.

Vibrational Sum Frequency Generation Spectra of Water-Vapor Interfaces Covered by Alcohols: Effects of Surface Coverage and Coupling between Oscillators.

The present study deals with the effects of varying coverage of water surface by alcohols on the vibrational sum frequency generation (VSFG) spectrum of interfacial water. We have considered two different alcohols: Tertiary butyl alcohol (TBA) whose alkyl part is fully branched and stearyl alcohol (STA) which has a long linear alkyl chain with larger hydrophobic surface area than that of TBA. With increase of the alcohol concentration, the hydrogen bonded OH stretch region of the VSFG spectrum is found to change following a regular trend for the STA-water system, whereas non-monotonic variation of the VSFG spectrum is observed for the TBA-water system which can be correlated with the presence of very different interactions of TBA molecules at different concentrations. On increasing the concentration of TBA, the hydrophobic groups get more tilted towards the water phase and significant hydrophobic interactions are introduced at higher concentrations. Whereas, for STA, there is a gradual increase in the hydrophilic interaction. Because of stacking interactions between the long chain alkyl groups, the hydrophobic parts stay outward from the water phase at higher concentrations and a regular change in the VSFG spectrum is observed. We have also presented a computationally efficient scheme to calculate the VSFG spectrum of interfacial systems for coupled oscillators which is expected to be beneficial for the treatment of coupling where the interfacial system size is inherently large.

Assembly of polyelectrolyte star block copolymers at the oil-water interface.

To understand and resolve adsorption, reconfiguration, and equilibrium conformations of charged star copolymers, we carried out an integrated experimental and coarse-grained molecular dynamics simulation study of the assembly process at the oil-water interface. This is important to guide development of novel surfactants or amphiphiles for chemical transformations and separations. The star block copolymer consisted of arms that are comprised of hydrophilic-hydrophobic block copolymers that are covalently tethered via the hydrophobic blocks to one point. The hydrophobic core represents polystyrene (PS) chains, while the hydrophilic corona represents quaternized poly(2-vinylpyridine) (P2VP) chains. The P2VP is modeled to become protonated when in contact with an acidic aqueous phase, thereby massively increasing the hydrophilicity of this block, and changing the nature of the star at the oil-water interface. This results in a configurational change whereby the chains comprising the hydrophilic corona are significantly stretched into the aqueous phase, while the hydrophobic core remains solubilized in the oil phase. In the simulations, we followed the kinetics of the anchoring and assembly of the star block copolymer at the interface, monitoring the lateral assembly, and the subsequent reconfiguration of the star via changes in the interfacial tension that varies as the degree-of-protonation increases. At low fractions of protonation, the arm cannot fully partition into the aqueous side of the interface and instead interacts with other arms in the oil phase forming a network near the interface. These insights were used to interpret the non-monotonic dependence of pH with the asymptotic interfacial tension from pendant drop tensiometry experiments and spectral signatures of aromatic stretches seen in vibrational sum frequency generation (SFG) spectroscopy. We describe the relationship of interfacial tension to the star assembly via the Frumkin isotherm, which phenomenologically describes anti-cooperativity in adsorbing stars to the interface due to crowding. Although our model explicitly considers long-range electrostatics, the contribution of electrostatics to interfacial tension is small and brought about by strong counterion condensation at the interface. These results provide key insights into resolving the adsorption, reconfiguration, and equilibrium conformations of charged star block copolymers as surfactants.

Direct observation of bicarbonate and water reduction on gold: understanding the potential dependent proton source during hydrogen evolution.

The electrochemical conversion of CO2 represents a promising way to simultaneously reduce CO2 emissions and store chemical energy. However, the competition between CO2 reduction (CO2R) and the H2 evolution reaction (HER) hinders the efficient conversion of CO2 in aqueous solution. In water, CO2 is in dynamic equilibrium with H2CO3, HCO3 -, and CO3 2-. While CO2 and its associated carbonate species represent carbon sources for CO2R, recent studies by Koper and co-workers indicate that H2CO3 and HCO3 - also act as proton sources during HER (J. Am. Chem. Soc. 2020, 142, 4154-4161, ACS Catal. 2021, 11, 4936-4945, J. Catal. 2022, 405, 346-354), which can favorably compete with water at certain potentials. However, accurately distinguishing between competing reaction mechanisms as a function of potential requires direct observation of the non-equilibrium product distribution present at the electrode/electrolyte interface. In this study, we employ vibrational sum frequency generation (VSFG) spectroscopy to directly probe the interfacial species produced during competing HER/CO2R on Au electrodes. The vibrational spectra at the Ar-purged Na2SO4 solution/Au interface, where only HER occurs, show a strong peak around 3650 cm-1, which appears at the HER onset potential and is assigned to OH-. Notably, this species is absent for the CO2-purged Na2SO4 solution/gold interface; instead, a peak around 3400 cm-1 appears at catalytic potential, which is assigned to CO3 2- in the electrochemical double layer. These spectral reporters allow us to differentiate between HER mechanisms based on water reduction (OH- product) and HCO3 - reduction (CO3 2- product). Monitoring the relative intensities of these features as a function of potential in NaHCO3 electrolyte reveals that the proton donor switches from HCO3 - at low overpotential to H2O at higher overpotential. This work represents the first direct detection of OH- on a metal electrode produced during HER and provides important insights into the surface reactions that mediate selectivity between HER and CO2R in aqueous solution.

Recent Advances in Analyzing Protein and Peptide Structures at Interfaces Using Vibrational Sum-Frequency Generation

Recent Advances in Analyzing Protein and Peptide Structures at Interfaces Using Vibrational Sum-Frequency Generation

Probing Silica–Kaolinite Interactions with Sum Frequency Generation Spectroscopy

Treating the oil sands tailings ponds is a major challenge because of the vast amounts of tailings and the need for a reliable treatment technique for releasing water and generating the highly consolidated material required for land reclamation. Treatment with chemicals such as lime (calcium (hydr)oxide) is a promising technology for tailings dewatering and consolidation, particularly at higher pH. Given that kaolinite and silica minerals are the main constituents of many oil sands, we have investigated the influence of lime and NaOH addition on the silica/aqueous kaolinite interface over the pH range 7.4-12.4 using vibrational sum frequency generation spectroscopy (SFG). With lime addition, at pH 12.0 and above we observe a complete disappearance of the vibrational features of the interfacial water molecules for planar silica in contact with an aqueous dispersion of kaolinite particles. A concurrent increase in the amount of adsorbed kaolinite on the silica surface at pH 12.0 and above is observed, shown in the increased intensity of the kaolinite SFG peak at 3694 cm-1. This suggests that the absence of water features in the SFG spectra is associated with conditions that facilitate dewatering. With NaOH addition, however, the interfacial water SF intensity is still significant even under highly alkaline conditions despite the increase in adsorbed kaolinite at high pH. To better understand the SFG observations and get a deeper insight into the chemistry of the silica/aqueous kaolinite interface, we measure the ζ-potential on the planar silica/aqueous interface and kaolinite aqueous dispersions under the same pH conditions with NaOH and lime addition.

Observation of Strong Synergy in the Interfacial Water Response of Binary Ionic and Nonionic Surfactant Mixtures.

Interfacial vibrational footprints of the binary mixture of sodium dodecyl sulfate (SDS) and hexaethylene glycol monododecyl ether (C12E6) were probed using heterodyne detected vibrational sum frequency generation (HDVSFG). Our results show that in the presence of C12E6 at CMC (70 μM) the effect of SDS on the orientation of interfacial water molecules is enhanced 10 times compared to just pure surfactants. The experimental results contest the traditional Langmuir adsorption model predictions. This is also evidenced by our molecular dynamics simulations that show a remarkable restructuring and enhanced orientation of the interfacial water molecules upon DS- adsorption to the C12E6 surface. The simulations show that the adsorption free energy of DS- ions to a water surface covered with C12E6 is an enthalpy-driven process and more attractive by ∼10 kBT compared to the adsorption energy of DS- to the surface of pure water.

Temporal Profile of Nonresonant Sum-Frequency Signal from Single-Crystal Silicon Depends on Crystal Orientation.

Proper analysis of vibrational sum-frequency generation (VSFG) spectra requires that the nonresonant contribution be dealt with correctly. This work shows that the temporal profile of the nonresonant SFG response varies with crystal facing and sample orientation for single-crystal Si and is significantly different than what is observed with polycrystalline Au. These considerations will affect the use of time-delay methods to experimentally suppress the nonresonant signal in broadband SFG measurements. Time-resolved or phase-sensitive SFG measurements will also need to properly account for these effects in post-processing.

Selective IR super-resolution imaging of β-keratins at the bulk or interface in feather detected by using a nonlinear optical process

We developed the new IR super-resolution microscope by using a 4-wave mixing (4-wave), which is a third-order nonlinear optical process, and carried out the IR super-resolution imaging of the cross section of the rachis of an avian feather. We clearly observed strong signals in the entire region of the rachis at the amide I vibration of β-keratin in both of the XXYY and YYXX polarization combination. These results are different from images detected by using the vibrational sum-frequency generation (VSFG) method. While the VSFG imaging detects molecules only from the interface, the 4-wave method enables us to observe the signal from the bulk area. We concluded that the four repeating units of β-keratins in the bulk area which are suggested by X-ray diffraction studies are visualized in the 4-wave detected method. We also applied two IR super-resolution microscopies for the barb and discuss the site dependence of the orientation, distribution and concentration of β-keratin.

The Unexpected Role of Cations in the Self-Assembly of Positively Charged Amphiphiles at Liquid/Liquid Interfaces.

Conventional wisdom suggests that cations play a minimal role in the assembly of cationic amphiphiles. Here, we show that at liquid/liquid (L/L) interfaces, specific cation effects can modulate the assemblies of hydrophobic tails in an oil phase despite being attached to cationic headgroups in the aqueous phase. We used oligo-dimethylsiloxane (ODMS) methyl imidazolium amphiphiles to identify these specific interactions at hexadecane/aqueous interfaces. Small cations, such as Li+, bind to the O atoms in the ODMS tail and pin it to the interface, thereby imposing a kinked conformation─as evidenced by vibrational sum frequency generation spectroscopy and molecular dynamics simulations. While larger Cs+ ions more readily partition to the interface, they do not form analogous complexes. Our data not only point to ways for controlling amphiphile structure at L/L interfaces but also suggest a means for the separation of Li+, or related applications, in soft-matter electronics.

Elucidating Interfacial Chain Conformation of Superhydrophilic Polymer Brushes by Vibrational Sum Frequency Generation Spectroscopy.

Surface-tethered macromolecules (polymer brushes) are a potent means to modify surfaces with stimuli-responsive properties while avoiding delamination problems. This vibrational sum frequency generation spectroscopy study describes how the conformation of hydrophilic polymer brushes changes in response to environmental conditions, that is, changes in humidity (in air) and upon exposure to liquid water. Three hydrophilic brushes were prepared on silicon oxide surfaces by surface-initiated reversible deactivation radical polymerization of cationic (quaternary ammonium), anionic (sulfonate), and zwitterionic (containing both) monomers. The average tilt angle of methyl groups was analyzed and used to deduce the chain conformations of the polymer brushes. In air, the brush films absorb water and swell with increasing humidity. This is accompanied by the rotation of interfacial polymer chains. The degree of water uptake and chain conformation vary with the nature of the charged hydrophilic moieties. The hydrophilic polymer brush surfaces appear to remain relatively dry except in near-condensation conditions. In water, the quaternary ammonium groups of cationic and zwitterionic brushes are aligned nearly parallel to the surface. The anionic brush chains appear to assume nearly random conformations in water.

Unanticipated Hydrophobicity Increases of Squalene and Human Skin Oil Films Upon Ozone Exposure.

The C-H and O-H oscillators on the surfaces of thin films of human-derived skin oil and squalene are probed under ambient conditions (300 K, 1 atm total pressure, 40% RH) using second-order vibrational spectroscopy and contact angle goniometry before and after exposure to ppb amounts of ozone. Skin oil and squalene are found to produce different vibrational sum frequency generation spectra in the C-H stretching region, while exposure to ozone results in surface spectra for both materials that is consistent with a loss of C-H oscillators. The measured contact angles show that the hydrophobicity of the films increases following exposure to ozone, consistent with the reduction in C═C···H2O ("πH") bonding interactions that is expected from C═C double bond loss due to ozonolysis and indicating that the polar functional groups formed point toward the films' interiors. Implications for heterogeneous indoor chemistry are discussed.

Surface Properties of Saponin—Chitosan Mixtures

The surface properties of saponin and saponin-chitosan mixtures were analysed as a function of their bulk mixing ratio using vibrational sum-frequency generation (SFG), surface tensiometry and dilational rheology measurements. Our experiments show that saponin-chitosan mixtures present some remarkable properties, such as a strong amphiphilicity of the saponin and high dilational viscoelasticity. We believe this points to the presence of chitosan in the adsorption layer, despite its complete lack of surface activity. We explain this phenomenon by electrostatic interactions between the saponin as an anionic surfactant and chitosan as a polycation, leading to surface-active saponin-chitosan complexes and aggregates. Analysing the SFG intensity of the O-H stretching bands from interfacial water molecules, we found that in the case of pH 3.4 for a mixture consisting of 0.1 g/L saponin and 0.001 g/L chitosan, the adsorption layer was electrically neutral. This conclusion from SFG spectra is corroborated by results from surface tensiometry showing a significant reduction in surface tension and effects on the dilational surface elasticity strictly at saponin/chitosan ratios, where SFG spectra indicate zero net charge at the air-water interface.

Ion Adsorption and Desorption at the CaF2 -Water Interface Probed by Flow Experiments and Vibrational Spectroscopy.

The dissolution of minerals in contact with water plays a crucial role in geochemistry. However, obtaining molecular insight into interfacial chemistry is challenging. Dissolution typically involves the release of ions from the surface, giving rise to a charged mineral surface. This charge affects the interfacial water arrangement, which can be investigated by surface-specific vibrational Sum Frequency Generation (v-SFG) spectroscopy. For the fluorite-water interface, recent spectroscopic studies concluded that fluoride adsorption/desorption determines the surface charge, which contrasts zeta potential measurements assigning this role to the calcium ion. By combining v-SFG spectroscopy and flow experiments with systematically suppressed dissolution, we uncover the interplay of dominant fluoride and weak calcium adsorption/desorption, resolving the controversy in the literature. We infer the calcium contribution to be orders of magnitude smaller, emphasizing the sensitivity of our approach.

(Invited) In Situ Time-Resolved Vibrational Spectroscopic Probe of Catalyst Structures, Dynamics and Reaction Mechanisms on Semiconductor Photoelectrodes

Attachment of molecular catalysts onto semiconductor or metal electrodes is a promising approach for rational design of efficient photoelectrochemical cells for solar water splitting and CO2 reduction. The understanding and improvement of such devices requires fundamental understanding of the structure and dynamics of catalysts as well as the interfacial electrolyte environment at electrode-liquid interfaces. Vibrational sum frequency generation (VSFG) spectroscopy is an in situ interface specific technique for probing interfacial properties at the semiconductor electrode/electrolyte interface at the molecular level. In this talk, we discuss our recent efforts in developing and applying in situ time-resolved VSFG for studying structural and dynamics of rhenium bipyridyl and other molecular CO2 reduction catalysts on semiconductor and metal electrodes. We have shown that by combining VSFG experiment with advanced computational modeling, catalyst adsorption geometry can be determined; through electrochemical Stark effect, local electric field and the potential profile in the electric double layer under light and dark conditions can be directly measured; using VSFG as a time-dependent probe, the interfacial charge transfer dynamics can be directly measured and key reactive intermediates can be identified.

Theoretical Sum Frequency Generation Spectra of Protein Amide with Surface-Specific Velocity-Velocity Correlation Functions.

Vibrational sum frequency generation (vSFG) spectroscopy is widely used to probe the protein structure at interfaces. Because protein vSFG spectra are complex, they can only provide detailed structural information if combined with computer simulations of protein molecular dynamics and spectra calculations. We show how vSFG spectra can be accurately modeled using a surface-specific velocity-velocity scheme based on ab initio normal modes. Our calculated vSFG spectra show excellent agreement with the experimental sum frequency spectrum of LTα14 peptide and provide insight into the origin of the characteristic α-helical amide I peak. Analysis indicates that the peak shape can be explained largely by two effects: (1) the uncoupled response of amide groups located on opposite sides of the α-helix will have different orientations with respect to the interface and therefore different local environments affecting the local mode vibrations and (2) vibrational splitting from nearest neighbor coupling evaluated as inter-residue vibrational correlation. The conclusion is consistent with frequency mapping techniques with an empirically based ensemble of peptide structures, thus showing how time correlation approaches and frequency mapping techniques can give independent yet complementary molecular descriptions of protein vSFG. These models reveal the sensitive relationship between protein structure and their amide I response, allowing exploitation of the complicated molecular vibrations and their interference to derive the structures of proteins under native conditions at interfaces.

Frontispiece: A Happy Get‐Together – Probing Electrochemical Interfaces by Non‐Linear Vibrational Spectroscopy

Electrochemical interfaces are key structures in energy storage and catalysis. Hence, a molecular understanding of the active sites at these interfaces, their solvation, the structure of adsorbates, and the formation of solid-electrolyte interfaces are crucial for an in-depth mechanistic understanding of their function. Vibrational sum-frequency generation (VSFG) spectroscopy has emerged as an operando spectroscopic technique to monitor complex electrochemical interfaces due to its intrinsic interface sensitivity and chemical specificity. Thus, this Review discusses the happy get-together between VSFG spectroscopy and electrochemical interfaces. For more information, see the full Review by R. De and B. Dietzek-Ivansic (DOI: 10.1002/chem.202200407).

Effect of deuteration on a phosphatidylcholine lipid monolayer structure: New insights from vibrational sum-frequency generation spectroscopy

We used vibrational sum-frequency generation (VSFG) spectroscopy to elucidate the possible effect of various levels of isotopic substitution (H/D) on the properties of the DPPC monolayer by probing DPPC/D2O interface. We found that deuteration of the choline group has a great impact on monolayer properties, while monolayers with deuterated alkyl chains do not exhibit any differences under our experimental conditions. In addition, deuteration of the choline group strongly affected the hydration of the phosphate group. We showed by probing symmetric stretching vibration of phosphate group that denser packing only slightly reduced the hydration of DPPC-d13 and DPPC-d75 monolayers. Moreover, addition of calcium ions, which generally cause a marked dehydration of the lipid monolayer, had no effect on lipid monolayers with deuterated choline group. We proposed that one way to explain this experimental finding could be deuteration induced changes in the structure of lipid’s choline group, resulting in a well-hydrated but Ca2+ ion blocking structure. These results have important implications for various spectroscopic techniques, which commonly use deuteration of phospholipids to circumvent overlapping between vibrational bands.