Molecular-level interaction of dopamine with lipids studied by sum frequency generation spectroscopy.

Molecular interactions at interfaces have a significant effect on the wetting properties of surfaces on a macroscale. Sum frequency generation (SFG) spectroscopy, one of a few techniques capable of probing such interactions, generates a surface vibrational spectrum sensitive to molecular structures and has been used to determine the orientation of molecules at interfaces. The purpose of this review is to assess SFG spectroscopy's ability to determine the molecular orientations of interfaces composed of fluorinated organic molecules. We will explore three different types of fluorinated organic material-based interfaces, naming liquid-air, solid-air, and solid-liquid interfaces, to see how SFG spectroscopy can be used to gain valuable and unique information regarding the molecular orientation of each interface. We hope this review will help to broaden the understanding of how to employ SFG spectroscopy to obtain more complex structural information for various fluorinated organic material-based interfaces in the future.

Structure of adsorbed molecular layer on fused quartz surface determined sequentially in sodium stearate solution, dry Ar, pure water, and dry Ar by sum frequency generation spectroscopy

The responses of the N-alkyl diamine groups to variations in pH affect their conformations and surface activities, making them relevant to applications relying on interfacial interactions, such as controlled emulsification and mineral flotation. An in-depth understanding of interfacial self-assembly is crucial. Herein, a molecular-level study was performed to investigate the adsorption and self-assembly of N-dodecylpropane-1,3-diamine (DPDA) at the air-water (A/W) interface using sum frequency generation (SFG) spectroscopy and molecular dynamics (MD) simulations. The SFG spectra of DPDA, acquired under three pH conditions, suggest that the protonation of the DPDA diamine group influences the alkyl chain arrangement at a varying degree at the A/W interface. Analysis of the di-cationic DPDA SFG spectrum at a low pH showed fewer gauche defects at low concentration, as indicated by the relatively higher intensity ratio (ICH3SS/ICH2SS) of 18.1 ± 0.6. The density profiles from MD simulations at different surface areas per molecule and pH conditions, showing varying degrees of packing, support the observation of gauche defects in SFG. With MD simulation, the radial distribution factor for di-cationic species has the highest probability of forming hydrogen bonds compared to mono-cationic and non-ionic species. These g(r) probability results conform with observations obtained from SFG spectroscopy, where we observed a strong hydrogen bond interaction at low pH conditions with di-cationic species, forming tetrahedrally arranged water molecules at the A/W interface. Overall, comprehensive insights will facilitate the visualization of alkyl diamines and their potential derivatives at the A/W interface, enabling a better understanding of their behavior across various applications.

Carboxylic acid-terminated monolayers on crystalline silicon surfaces can be readily modified with biological macromolecules for the fabrication of semiconductor-based biosensing devices. They were prepared by acid-catalyzed hydrolysis of alkoxycarbonyl (ester)-terminated monolayers and studied by vibrational sum frequency generation (SFG) spectroscopy. The C-H vibration region of the SFG spectra consists of strong methyl bands with significant contributions from methylene stretching modes, indicating that these monolayers are generally ordered but with considerable gauche defects in the alkyl chains in comparison with n-alkyl monolayers. After hydrolysis, the methylene stretching modes prevail, with "residues" of the methyl bands, indicating incomplete hydrolysis and disruption of the monolayer structure. This work demonstrates that SFG is capable of providing quantitative information on structure-reactivity correlations in organic monolayers.

The vapor–liquid interface structures of two typical organic carbonates, propylene carbonate (PC) and dimethyl carbonate (DMC), are investigated in collaboration of sum frequency generation (SFG) spectroscopy and molecular dynamics (MD) simulation. The present general molecular model for organic carbonates well reproduces various liquid properties, including density, heat of vaporization, and infrared, Raman and SFG spectra. The MD simulation revealed contrasting interface structures between PC and DMC. The PC interface exhibits layered structure of oscillatory orientation, while the DMC interface is quite random. The structural feature of the PC interface is mainly attributed to dimer formation of PC molecules. We elucidated that the different surface structures are manifested in their Im[χ(2)] SFG spectra in the C═O stretching band, showing opposite signs of bipolar peaks between the two liquids.

In the theory of condensed-phase spectroscopy, local field effect is of general importance to account for intermolecular electrostatic interactions. The present paper extends the microscopic treatment of local field effects on the sum frequency generation (SFG) spectroscopy to incorporate quadrupole interactions, since their roles have been increasingly recognized in the SFG spectroscopy. The extended theory involves some corrections to the conventional formulas of the nonlinear susceptibilities in both the interface and bulk regions, including the χIQB term. Fresnel transformations for the interface and bulk susceptibilities are rigorously applied, which implies inseparability of the interface and bulk signals in PSS and PPP cases. We examined the influence of the corrections with quantitative calculations of the susceptibilities, including dipolar and quadrupolar interactions.

Two kinds of monolayered films of ion-paired macrocyclic amphiphiles formed from anionic surfactant 1,2-bis[(dodecyloxy)carbonyl]ethane-1-sulfonate and tetracationic macrocycles with different molecular sizes were prepared on hydrophilic silica plates by the Langmuir−Blodgett technique. Vibrational sum-frequency generation (SFG) spectroscopy was applied to investigate the packing structure of the alkyl chains of the films. The SF intensity ratio of the CH symmetric stretching mode of the methylene group to that of methyl group in the alkyl chains increased when the occupied area of the macrocycle was larger than the sum of the cross-sectional areas of the hydrophobic alkyl tails. The appearance of a methylene CH band in the SFG spectra implies the presence of gauche conformation in the alkyl chains. This indicates that the molecular size of the cyclic tetracation hydrophilic head exclusively governs the packing structure of the alkyl group hydrophobic tails in the monolayer state. The results show that mo...

Sum frequency generation (SFG), a second-order nonlinear optical process, is electric-dipole forbidden in systems with inversion symmetry. As a result, it has been used to study chiral media and interfaces, systems intrinsically lacking inversion symmetry. This thesis describes recent progresses in the applications of and new insights into SFG from chiral media and interfaces. SFG from solutions of chiral amino acids is investigated, and a theoretical model explaining the origin and the strength of the chiral signal in electronic-resonance SFG spectroscopy is discussed. An interference scheme that allows us to distinguish enantiomers by measuring both the magnitude and the phase of the chiral SFG response is described, as well as a chiral SFG microscope producing chirality-sensitive images with sub-micron resolution. Exploiting atomic and molecular parity nonconservation, the SFG process is also used to solve the Ozma problems. Sum frequency vibrational spectroscopy is used to obtain the adsorption behavior of leucine molecules at air-water interfaces. With poly(tetrafluoroethylene) as a model system, we extend the application of this surface-sensitive vibrational spectroscopy to fluorine-containing polymers.

We developed a multireflection data collection method in order to improve the signal-to-noise ratio (SNR) and sensitivity of sum frequency generation (SFG) spectroscopy, which we refer to as multireflection SFG, or MRSFG for short. To achieve MRSFG, a collinear laser beam propagation geometry was adopted and trapezoidal Dove prisms were used as sample substrates. An in-depth discussion on the signal and SNR in MRSFG was performed. We showed experimentally, with "m" total internal reflections in a Dove prism, MRSFG signal is ∼m times that of conventional SFG; SNR of the SFG signal-to-background is improved by a factor of >m(1/2) and <m. MRSFG also improved the SFG sensitivity to resolve weak vibrational signals. Surface molecular structures of adsorbed ethanol molecules, polymer films, and a lipid monolayer were characterized using both MRSFG and conventional SFG. Molecular orientation information on lipid molecules with a 9% composition in a mixed monolayer was measured using MRSFG, which showed a good agreement with that derived from 100% lipid surface coverage using conventional SFG. MRSFG can both improve the spectral quality and detection limit of SFG spectroscopy and is expected to have important applications in surface science for studying structures of molecules with a low surface coverage or less ordered molecular moieties.

Rechargeable Li batteries provide a high energy density to satisfy our daily life. However, a deep understanding of electrochemical electrode/electrolyte interfaces is of crucial importance for designing electrolytes or electrode materials to achieve high-performance Li batteries. In this work, the advantages of sum frequency generation (SFG) spectroscopy are outlined in studying the solvent adsorption manners, the static electric double layer (EDL) structures, the initialization and evolutions of the solid electrolyte interphase (SEI), and the catalytic regulation of interfacial Li+ desolvation dynamics. The fundamental logic to correlate the interfacial molecular information to the transformation, transfer, and diffusion of the ionic charge carriers, and thus to the battery performance, is also highlighted. The technical challenges and solutions for conducting operando SFG on battery systems are discussed, and the physical models beyond the traditional EDL theory for SFG data interpretation are prospected. Finally, efforts to push in situ SFG techniques forward in high time resolution and real-time fine-spectral feature resolution, as well as the applicability in practical rough interface morphologies and chemical heterogeneity, are further prospected.

An improved time correlation function (TCF) description of sum frequency generation (SFG) spectroscopy was developed and applied to theoretically describing the spectroscopy of the ambient water/vapor interface. A more general TCF expression than was published previously is presented-it is valid over the entire vibrational spectrum for both the real and imaginary parts of the signal. Computationally, earlier time correlation function approaches were limited to short correlation times that made signal processing challenging. Here, this limitation is overcome, and well-averaged spectra are presented for the three independent polarization conditions that are possible for electronically nonresonant SFG. The theoretical spectra compare quite favorably in shape and relative magnitude to extant experimental results in the O-H stretching region of water for all polarization geometries. The methodological improvements also allow the calculation of intermolecular SFG spectra. While the intermolecular spectrum of bulk water shows relatively little structure, the interfacial spectra (for polarizations that are sensitive to dipole derivatives normal to the interface--SSP and PPP) show a well-defined intermolecular mode at 875 cm(-1) that is comparable in intensity to the rest of the intermolecular structure, and has an intensity that is approximately one-sixth of the magnitude of the intense free O-H stretching peak. Using instantaneous normal mode methods, the resonance is shown to be due to a wagging mode localized on a single water molecule, almost parallel to the interface, with two hydrogens displaced normal to the interface, and the oxygen anchored in the interface. We have also uncovered the origin of another intermolecular mode at 95 cm(-1) for the SSP and PPP spectra, and at 220 cm(-1) for the SPS spectra. These resonances are due to hindered translations perpendicular to the interface for the SSP and PPP spectra, and translations parallel to the interface for the SPS spectra. Further, by examining the real and imaginary parts of the SFG signal, several resonances are shown to be due to a single spectroscopic species while the "donor" O-H region is shown to consist of three distinct species-consistent with an earlier experimental analysis.

The kinetics of proteins at interfaces plays an important role in biological functions and inspires solutions to fundamental problems in biomedical sciences and engineering. Nonetheless, due to the lack of surface-specific and structural-sensitive biophysical techniques, it still remains challenging to probe protein kinetics in situ and in real time without the use of spectroscopic labels at interfaces. Broad-bandwidth chiral sum frequency generation (SFG) spectroscopy has been recently developed for protein kinetic studies at interfaces by tracking the chiral vibrational signals of proteins. In this article, we review our recent progress in kinetic studies of proteins at interfaces using broad-bandwidth chiral SFG spectroscopy. We illustrate the use of chiral SFG signals of protein side chains in the C–H stretch region to monitor self-assembly processes of proteins at interfaces. We also present the use of chiral SFG signals from the protein backbone in the N–H stretch region to probe the real-time kinetics of proton exchange between protein and water at interfaces. In addition, we demonstrate the applications of spectral features of chiral SFG that are typical of protein secondary structures in both the amide I and the N–H stretch regions for monitoring the kinetics of aggregation of amyloid proteins at membrane surfaces. These studies exhibit the power of broad-bandwidth chiral SFG to study protein kinetics at interfaces and the promise of this technique in research areas of surface science to address fundamental problems in biomedical and material sciences.

Sum frequency generation (SFG) spectroscopy was applied to investigate D2O adsorption on atomic layer deposition (ALD)-grown Al2O3, ZrO2, and TiO2 films at 94 ± 1 K. Film composition and thickness were characterized by ellipsometry and X-ray photoelectron spectroscopy (XPS). Additional SFG measurements were conducted on the SiO2/Si wafer and on a CoO film prepared by oxidizing Co foil. At D2O exposure below 3000 L, the spectra were dominated by interfacial features originating from the ice-oxide interface. These spectra exhibited a weak, broad O-D stretching band (OD3) centered at 2650 cm-1, tentatively attributed to (dissociated) water molecules hydrogen-bonded to the oxide surface; this assignment was supported by the absence of the OD3 feature on the SiO2/Si substrate. A sharp peak at 2730 cm-1 was also observed and assigned to the "free" O-D stretch (non-hydrogen-bonded with any neighboring molecule) of surface D2O molecules pointing into the vapor phase. Upon increasing D2O exposure, both the OD3 and "free" OD bands decreased in intensity and were replaced by weakly hydrogen-bonded OD2 and strongly hydrogen-bonded OD1 modes associated with the ice-vapor interface. As the exposure increased further, the OD2 and OD1 bands shifted to lower wavenumbers (2310 to 2284 cm-1) and became stronger, with the OD1 mode exhibiting a larger red shift and more pronounced intensity enhancement. No significant differences in water structure were observed on the Al2O3, ZrO2, and CoO films at the ice-vapor interfaces, apart from an approximately fivefold reduction in intensity on CoO, which is attributed to signal scattering from the rough CoO film/Co foil surface. However, when D2O exposure reached ≥30 000 L, the OD1 band on the TiO2 surfaces decreased substantially in intensity and shifted to much lower wavenumbers (2065 cm-1 at 30 000 L; 2030 cm-1 at 102 000 L) than on Al2O3 (2283 cm-1 at 90 000 L), ZrO2 (2293 cm-1 at 30 000 L), and CoO (2284 cm-1 at 900 000 L), indicating specific hydrogen-bonding interactions on the TiO2 surface.

CO dissociation and CO hydrogenation on smooth and ion-bombarded Pd(1 1 1): SFG and XPS spectroscopy at mbar pressures

Methanol decomposition on Pd(111) at 300 and 400 K was studied in situ from 5 _ 10-7 to 0.1 mbar by combining vibrational sum frequency generation (SFG) and X-ray photoelectron spectroscopy (XPS). Two competing decomposition pathways, i.e., dehydrogenation of CH3OH to CO and H2 and methanolic C-O bond scission, were observed by monitoring the time-dependent evolution of CO/CHxO and of carbonaceous deposits CHx (x = 0-3) via their vibrational and photoemission characteristics. Quantification of carbon-containing species was performed by XPS, while the preferred binding site of CHx was determined by SFG using CO as probe molecule for postreaction adsorption. In contrast to previous reports, Pd(111) was found to be quite active for methanolic C-O bond scission. The CHx formation rate strongly increased with pressure and temperature, leading to immediate catalyst deactivation at 0.1 mbar and 400 K. The combined SFG/XPS data suggest that the carbonaceous residues are highly dehydrogenated, such as CH or carbon atoms bonded to hollow sites. Complete dehydrogenation of CHx species and partial dissolution of atomic carbon in the Pd bulk most likely occurred even at 300 K. On the other hand, the CHx species was found to be unexpectedly thermally stable (up to ~600 K), until carbon dissolution and formation of carbon clusters take place. Regeneration with oxygen above 400 K was able to remove CHx deposits and to partially restore the initial adsorption properties. Corresponding experiments with CO did not produce any carbon signals, indicating that the cleavage of the C-O bond must occur via CHxO intermediates (and not within CO). Methanol decomposition at pressures up to 15 mbar and temperatures up to 550 K, followed by gas chromatography, did not produce measurable decomposition products, due to fast carbon poisoning under catalytic reaction conditions.