Heterodyne-detected Sum Frequency Generation Research Articles

Depth-profiling alkyl chain order in unsaturated lipid monolayers on water.

Unsaturated lipids with C=C groups in their alkyl chains are widely present in the cell membrane and food. The C=C groups alter the lipid packing density, membrane stability, and persistence against lipid oxidation. Yet, molecular-level insights into the structure of the unsaturated lipids remain scarce. Here, we probe the molecular structure and organization of monolayers of unsaturated lipids on the water surface using heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy. We vary the location of the C=C in the alkyl chain and find that at high lipid density, the location of the C=C group affects neither the interfacial water organization nor the tail of the alkyl chain. Based on this observation, we use the C=C stretch HD-SFG response to depth-profile the alkyl chain conformation of the unsaturated lipid. We find that the first 1/3 of carbon atoms from the headgroup are relatively rigid, oriented perpendicular to the surface. In contrast, the remaining carbon atoms can be approximated as free rotators, introducing the disordering of the alkyl chains.

Heterodyne-Detected Sum-Frequency Generation Vibrational Spectroscopy Reveals Aqueous Molecular Structure at the Suspended Graphene/Water Interface.

Graphene, a transparent two-dimensional conductive material, has brought extensive new perspectives and prospects to various aqueous technological systems, such as desalination membranes, chemical sensors, energy storage, and energy conversion devices. Yet, the molecular-level details of graphene in contact with aqueous electrolytes, such as water orientation and hydrogen bond structure, remain elusive or controversial. Here, we employ surface-specific heterodyne-detection sum-frequency generation (HD-SFG) vibrational spectroscopy to re-examine the water molecular structure at a freely suspended graphene/water interface. We compare the response from the air/graphene/water system to that from the air/water interface. Our results indicate that theχ(2)yyz spectrum recorded from the air/graphene/water system arises from the topmost 1-2 water layers in contact with the graphene, with the graphene itself not generating a significant SFG response. Compared to the air/water interface response, the presence of monolayer graphene weakly affects the interfacial water. Graphene weakly affects the dangling O-H group, lowering its frequency through its interaction with the graphene sheet, and has a small effect on the hydrogen-bonded O-H group. Molecular dynamics simulations confirm our experimental observation. Our work provides molecular insight into the interfacial structure at a suspended graphene/water interface, relevant to various technological applications of graphene.

Substrate effect on charging of electrified graphene/water interfaces.

Graphene, a transparent two-dimensional (2D) conductive electrode, has brought extensive new perspectives and prospects to electrochemical systems, such as chemical sensors, energy storage, and energy conversion devices. In many of these applications, graphene, supported on a substrate, is in contact with an aqueous solution. An increasing number of studies indicate that the substrate, rather than graphene, determines the organization of water in contact with graphene, i.e., the electric double layer (EDL) structure near the electrified graphene, and the wetting behavior of the graphene: the graphene sheet is transparent in terms of its supporting substrate. By applying surface-specific heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy to the silicon dioxide (SiO2)-supported graphene electrode/aqueous electrolyte interface and comparing the data with those for the calcium fluoride (CaF2)-supported graphene [Y. Wang etal., Angew. Chem., Int. Ed., 2023, 62, e202216604], we discuss the impact of the different substrates on the charging of both the graphene and the substrate upon applying potentials. The SiO2-supported graphene shows pseudocapacitive behavior, consistent with the CaF2-supported graphene case, although the surface charges on SiO2 and CaF2 differ substantially. The SiO2 surface is already negatively charged at +0.57 V (vs. Pd/H2), and the negative surface charge is doubled when negative potentials are applied, in contrast with the CaF2 case, where the positive charge is reduced when negative potentials are applied. Interestingly, the charging of the graphene sheet is almost identical between the negatively charged SiO2 surface and positively charged CaF2 surface, demonstrating that the graphene charging is decoupled from the charging of the substrates.

(Invited) Direct Probe of Electrochemical Pseudocapacitive pH Jump at a Graphene Electrode

Molecular-level insight into interfacial water at buried electrode interface is essential in electrochemistry, but spectroscopic probing of the buried interfaces remains challenging. Here, using surface-specific heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy, we directly access the interfacial water in contact with the graphene electrode supported on calcium fluoride (CaF2). The HD-SFG signal is obtained by overlapping the infrared and visible pulses at the sample position. An SFG signal is enhanced when the infrared frequency is resonant with the molecular vibration. The HD-SFG can provide the information on the orientation of the interfacial molecules. With the molecular specificity, orientational specificity, and surface specificity of the HD-SFG, we clarify the molecular level insight into the structure of the electrified graphene-water interface. At the negative voltage, we found the negative HD-SFG peak at 3630 cm-1, which manifests that the O-H group points to the bulk water, not to the CaF2 supported graphene. This O-H group is the Ca-O-H group, indicating that the chemical reaction occurs at the CaF2 surface. By monitoring the variation of the HD-SFG spectra upon the various applied potential, We find phase transition-like variations of the HD-SFG spectra. We attribute such a variation not to charging/discharging of graphene but to charging/discharging of the CaF2 substrate through the pseudocapacitive process. The potential-dependent spectra are nearly identical to the pH-dependent spectra, evidencing that the pseudocapacitive behavior is associated with a substantial local pH change induced by water dissociation between the CaF2 and graphene. Our work evidences the local molecular-level effects of pseudocapacitive charging at an electrode/aqueous electrolyte interface.

Molecular Orientations of Myristic Acid Derivatives with Different Perfluoroalkyl Chain Lengths at the Air/Water Interface Evaluated by Heterodyne-Detected Sum Frequency Generation Spectroscopy

Surface-related properties of perfluoroalkyl compounds such as hydrophobicity and adhesion force can have a close relationship with the orientational order of the molecules at the surface. Myristic acid (MA) derivatives with perfluoroalkyl (Rf) groups have been known to form a well-ordered monolayer at the air/gold substrate interface. In this study, molecular orientations of the MA derivative monolayers at the air/water interface were investigated by heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. We estimated the molecular orientation of MA derivative monolayers with a marker band observed at around 1370 cm–1. The Rf chain length and the surface pressure dependence of the molecular orientation of four MA derivatives were examined. The marker bands were observed except for the derivative with the shortest Rf chain at the higher surface pressure, whereas the marker bands were observed only for the two derivatives with relatively longer Rf chain lengths at the lower surface pressure. The observation indicates that the longer the Rf chain is and the higher the surface pressure is, MA derivatives tend to adopt a standing orientation closer to perpendicular to the interface.

Does the Sum-Frequency Generation Signal of Aromatic C-H Vibrations Reflect Molecular Orientation?

Organic molecules with aromatic groups at the aqueous interfaces play a central role in atmospheric chemistry, green chemistry, and on-water synthesis. Insights into the organization of interfacial organic molecules can be obtained using surface-specific vibrational sum-frequency generation (SFG) spectroscopy. However, the origin of the aromatic C-H stretching mode peak is unknown, prohibiting us from connecting the SFG signal to the interfacial molecular structure. Here, we explore the origin of the aromatic C-H stretching response by heterodyne-detected SFG (HD-SFG) at the liquid/vapor interface of benzene derivatives and find that, irrespective of the molecular orientation, the sign of the aromatic C-H stretching signals is negative for all the studied solvents. Together with density functional theory (DFT) calculations, we reveal that the interfacial quadrupole contribution dominates, even for the symmetry-broken benzene derivatives, although the dipole contribution is non-negligible. We propose a simple evaluation of the molecular orientation based on the aromatic C-H peak area.

Ions Speciation at the Water–Air Interface

In typical aqueous systems, including naturally occurring sweet and salt water and tap water, multiple ion species are co-solvated. At the water-air interface, these ions are known to affect the chemical reactivity, aerosol formation, climate, and water odor. Yet, the composition of ions at the water interface has remained enigmatic. Here, using surface-specific heterodyne-detected sum-frequency generation spectroscopy, we quantify the relative surface activity of two co-solvated ions in solution. We find that more hydrophobic ions are speciated to the interface due to the hydrophilic ions. Quantitative analysis shows that the interfacial hydrophobic ion population increases with decreasing interfacial hydrophilic ion population at the interface. Simulations show that the solvation energy difference between the ions and the intrinsic surface propensity of ions determine the extent of an ion's speciation by other ions. This mechanism provides a unified view of the speciation of monatomic and polyatomic ions at electrolyte solution interfaces.

Direct Probe of Electrochemical Pseudocapacitive pH Jump at a Graphene Electrode.

Molecular-level insight into interfacial water at a buried electrode interface is essential in electrochemistry, but spectroscopic probing of the interface remains challenging. Here, using surface-specific heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy, we directly access the interfacial water in contact with the graphene electrode supported on calcium fluoride (CaF2 ). We find phase transition-like variations of the HD-SFG spectra vs. applied potentials, which arises not from the charging/discharging of graphene but from the charging/discharging of the CaF2 substrate through the pseudocapacitive process. The potential-dependent spectra are nearly identical to the pH-dependent spectra, evidencing that the pseudocapacitive behavior is associated with a substantial local pH change induced by water dissociation between the CaF2 and graphene. Our work evidences the local molecular-level effects of pseudocapacitive charging at an electrode/aqueous electrolyte interface.

On the Fresnel factor correction of sum-frequency generation spectra of interfacial water.

Insights into the microscopic structure of aqueous interfaces are essential for understanding the chemical and physical processes on the water surface, including chemical synthesis, atmospheric chemistry, and events in biomolecular systems. These aqueous interfaces have been probed by heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy. To obtain the molecular response from the measured HD-SFG spectra, one needs to correct the measured ssp spectra for local electromagnetic field effects at the interface due to a spatially varying dielectric function. This so-called Fresnel factor correction can change the inferred response substantially, and different ways of performing this correction lead to different conclusions about the interfacial water response. Here, we compare the simulated and experimental spectra at the air/water interface. We use three previously developed models to compare the experiment with theory: an advanced approach taking into account the detailed inhomogeneous interfacial dielectric profile and the Lorentz and slab models to approximate the interfacial dielectric function. Using the advanced model, we obtain an excellent quantitative agreement between theory and experiment, in both spectral shape and amplitude. Remarkably, we find that for the Fresnel factor correction of the ssp spectra, the Lorentz model for the interfacial dielectric function is equally accurate in the hydrogen (H)-bonded region of the response, while the slab model underestimates this response significantly. The Lorentz model, thus, provides a straightforward method to obtain the molecular response from the measured spectra of aqueous interfaces in the H-bonded region.

Experimental and Theoretical Heterodyne-Detected Sum Frequency Generation Spectroscopy of Isotopically Pure and Diluted Water Surfaces.

The χ(2) (second-order nonlinear optical susceptibility) spectrum of the water surface has been a matter of debate for a few decades. Here, we report that we experimentally measured the isotopic dilution dependence of the χ(2) spectrum and theoretically reproduced it by employing the quantum/classical mixed approach with a new idea to subtract an artifact. The present theoretical framework allows for clarifying the effects of the intramolecular, intermolecular, and Fermi resonance couplings on the OH-stretch vibrational spectra of water at the surface as well as in the bulk.

Accurate molecular orientation at interfaces determined by multimode polarization-dependent heterodyne-detected sum-frequency generation spectroscopy via multidimensional orientational distribution function.

Many essential processes occur at soft interfaces, from chemical reactions on aqueous aerosols in the atmosphere to biochemical recognition and binding at the surface of cell membranes. The spatial arrangement of molecules specifically at these interfaces is crucial for many of such processes. The accurate determination of the interfacial molecular orientation has been challenging due to the low number of molecules at interfaces and the ambiguity of their orientational distribution. Here, we combine phase- and polarization-resolved sum-frequency generation (SFG) spectroscopy to obtain the molecular orientation at the interface. We extend an exponentially decaying orientational distribution to multiple dimensions, which, in conjunction with multiple SFG datasets obtained from the different vibrational modes, allows us to determine the molecular orientation. We apply this new approach to formic acid molecules at the air-water interface. The inferred orientation of formic acid agrees very well with ab initio molecular dynamics data. The phase-resolved SFG multimode analysis scheme using the multidimensional orientational distribution thus provides a universal approach for obtaining the interfacial molecular orientation.

Wedge-Based Design for Phase Stable and Phase Accurate Heterodyne-Detected Sum-Frequency Generation Spectroscopy.

Phase sensitive and heterodyne-detected (HD) sum-frequency generation (SFG) spectroscopy offers the ability to separate the nonlinear susceptibility into its real and imaginary components. This provides information about the absolute orientation of molecules at interfaces while also producing an absorptive spectrum that is linear in spectral composition and can easily be decomposed into different spectral components. However, simultaneously obtaining phase accuracy and phase stability remains a challenge in SFG. Here we present a new experimental design for HD-SFG spectroscopy that incorporates a wedge pair to accurately control the timing between the local oscillator and the sample signal. This experimental approach provides high phase accuracy and long-time phase stability in a compact and flexible configuration.

Deprotonation of phenol linked to a silicon dioxide surface using adaptive feedback laser control with a heterodyne detected sum frequency generation signal.

The development of laser-controlled surface reactions has been limited by the lack of decisive methods for detecting evolving changes in surface chemistry. In this work, we demonstrate successful laser control of a surface reaction by combining the adaptive feedback control (AFC) technique with surface sensitive spectroscopy to determine the optimally shaped laser pulse. Specifically, we demonstrate laser induced deprotonation of the hydroxyl group of phenol bound to a silicon dioxide substrate. The experiment utilized AFC with heterodyne detected vibrational sum frequency generation (HD-VSFG) as the surface sensitive feedback signal. The versatile combination of AFC with HD-VSFG provides a route to potentially control a wide range of surface reactions.

Heterodyne-Detected Sum Frequency Generation Spectroscopic Study of Weakly Hydrogen-Bonded Water at Charged Lipid Interfaces, Revisited

Heterodyne-Detected Sum Frequency Generation Spectroscopic Study of Weakly Hydrogen-Bonded Water at Charged Lipid Interfaces, Revisited

Effect of Head Group on Low-Level Ozone Oxidation of Unsaturated Phospholipids on a Water Surface

Abstract The effect of the head group on the oxidation of unsaturated phospholipids under low-level ozone was investigated. The phospholipids with head groups of glycerol and choline show different surface-pressure profiles but similar degradation kinetics of the C=C moieties was observed by heterodyne-detected sum frequency generation spectroscopy.

In Situ Monitoring of the Unsaturated Phospholipid Monolayer Oxidation in Ambient Air by HD-SFG Spectroscopy

The pulmonary surfactant monolayer is indispensable for the respiratory system. Recently, it was reported that some unsaturated lipids of the pulmonary surfactants are oxidized by low-level ozone in ambient air. However, the molecular-level understanding of the reaction mechanism is still limited due to technical difficulties. We applied heterodyne-detected sum frequency generation (HD-SFG) spectroscopy to probe the reaction process of an unsaturated phospholipid monolayer (1-palmitoyl-2-oleoyl-glycero-3-phosphocholine, POPC), which is one of the major lipids in the pulmonary surfactant, under low-level ozone (30 ± 5 ppb). The HD-SFG spectroscopy realized the accurate peak assignments of the spectra and the identification of molecular species with high sensitivity, which were impossible with previous measurements. The time-resolved spectra indicated that the C═C moiety in the unsaturated alkyl chain is selectively oxidized by ozone with a time constant of 22 ± 3 min by first-order reaction kinetics. Furthermore, it was revealed for the first time that the reaction product of the POPC monolayer under low-level ozone is not the carboxylic form but the aldehyde form based on the vibrational spectroscopy results. The present study has deepened our molecular-level understanding of the oxidation mechanism of unsaturated lipids that are widely found in many biological systems.

Determining nonlinear optical coefficients of metals by multiple angle of incidence heterodyne-detected sum-frequency generation spectroscopy

We illustrate a technique by which heterodyne-detected sum-frequency generation spectroscopy is performed at multiple angles of incidence in order to decompose components of the second-order susceptibility tensor when all beams are polarized parallel to the plane of incidence. As an illustration, we study the non-vibrationally resonant gold response. We benchmark our results by comparing with measurements obtained in a polarization scheme that isolates a single element of the susceptibility tensor. Our technique is particularly valuable in the case of metal substrates, where the surface selection rule often prevents spectra from being acquired in multiple beam polarizations.

Chirality discrimination at the carvone air/liquid interfaces detected by heterodyne-detected sum frequency generation

The chiral signal of the carvone air/liquid interface is probed by heterodyne-detected sum frequency generation (HD-SFG) without the electronic resonance. The chiral SFG spectra exhibit two distinguishable spectral signatures. Four chiral vibrational peaks of the R- and S-carvone molecules are with opposite signs, which can directly determine the surface molecular chirality. Two achiral vibrational peaks are also observed with the same sign. The different spectral signatures can provide a detailed chirality characterization at the molecular interface.

Heterodyne-detected sum frequency generation of water at surfaces with varying hydrophobicity.

Aqueous surfaces mediate many atmospheric, biological, and technological processes. At the interface, the bulk hydrogen-bonding network is terminated and the interfacial water molecules restructure according to the surface chemistry of the interface. Given the complexity of both natural and technical aqueous interfaces, self-assembled monolayers provide a platform for controllably tuning the chemical composition of the surface and thus the water restructuring. Here, we study a hydrophobic monolayer, a hydrophilic monolayer, and a mixed hydrophobic/hydrophilic monolayer in contact with water. Monolayers composed of both hydrophilic and hydrophobic chains mimic the complex and heterogeneous chemical composition of natural and technological surfaces. By employing heterodyne-detected sum frequency generation, the purely absorptive vibrational line shape of interfacial water is measured experimentally. We examined the structure of the interfacial water in contact with each of the monolayers by analyzing the relative dipole moment orientations and fitting the imaginary component of χ(2) with a combination of Lorentzian and Gaussian line shapes. For all of the monolayers, the hydrogen-bonded water points toward the monolayer, which is opposite of the orientation of the hydrogen-bonded water at the air-water interface. Additionally, a strongly hydrogen-bonded water species exists for the monolayers containing hydrophilic chains. The spectroscopic results suggest that the microscopic water structure in contact with the mixed monolayer is dominated by the hydrophilic parts of the monolayer, while the contact angle shows that at the macroscopic level the surface properties lie closer to the pure hydrophobic monolayer.

Enhanced structural disorder at a nanocrystalline ice surface.

Detailed knowledge of the structure and dynamics of the surface of ice particles is of considerable importance for understanding catalytic reactions in the upper atmosphere. Here we report the enhanced structural disorder specific at a nanoscale ice island studied by using heterodyne-detected vibrational sum-frequency generation spectroscopy under ultrahigh vacuum. Ultrathin films of isotopically diluted HOD crystalline ice are grown on Rh(111), whose average height (≥1.4 nm) is controlled by varying the nominal film thickness. The Im χ(2) spectrum of the hydrogen-bonded O-H stretching band shows a bipolar line shape reflecting the orientation-dependent hydrogen bond length alternation in the subsurface of the ice island. The peak splitting and the bandwidth of the bipolar spectrum increase with a decrease in the nominal film thickness. This is ascribed to the significant enhancement of structural disorder at the surface of the ice island as the terrace size is decreased. Temperature dependence of the Im χ(2) spectra of the hydrogen bonded O-H stretching band indicates that the thermal expansivity of the top layer increases upon decreasing the island size. In addition, the stretching frequency of the dangling OD band at the island surface with the average height less than 18 nm shows a systematic blue shift with increasing temperature from 100 to 145 K: this is in stark contrast to thick ice films and bulk ice showing a negligible peak shift at a temperature lower than 180 K. These findings indicate that the anharmonicity of the intermolecular potential at the top layer of the ice island is strongly enhanced upon decreasing its terrace size, providing valuable insights for understanding the properties of ice particles in the outer atmosphere including polar mesospheric clouds.