Sensitive Sum Frequency Generation Research Articles

Noncollinear Optical Parametric Amplification of Broadband Infrared Sum Frequency Generation Vibrational Spectroscopy.

Sum-frequency generation (SFG) vibrational spectroscopy is an invaluable tool in surface science, known for its specificity to surfaces and interfaces. Despite its wide application, it is often hampered by weak signal detection. Here, we present an innovative enhancement technique of postsample amplification, using a picosecond noncollinear optical parametric amplifier (NOPA). We conducted a systematical investigation into the impact of different intensities of pump and SFG seed light, as the input signal in NOPA, and demonstrated this method on the octadecanethiol (ODT) molecules on gold films. The amplified SFG by NOPA reproduced the SFG vibrational spectra, enhanced by about 4 orders of magnitude but with broader spectral resolution due to the short pulse width of the pump light in NOPA. This study makes it possible to realize highly sensitive SFG measurements, marking a significant advancement in spectroscopic analysis techniques.

Sum frequency spectroscopy studies on cell membrane fusion induced by divalent cations

Cell membrane fusion is a fundamental biological process involved in a number of cellular living functions. Regarding this, divalent cations can induce fusion of the lipid bilayers through binding and bridging of divalent cations to the charged lipids, thus leading to the cell membrane fusion. How-ever, the elaborate mechanism of cell membrane fusion induced by divalent cations is still needed to be elucidated. Here, surface/interface sensitive sum frequency generation vibrational spectroscopy (SFG-VS) and dynamic light scattering (DLS) were applied in this research to study the responses of phospholipid monolayer to the exposure of divalent metal ions i.e. Ca2+ and Mg2+. According to the particle size distribution results measured by DLS experiments, it was found that Ca2+ could induce inter-vesicular fusion while Mg2+ could not. An octadecyltrichlorosilane self-assembled monolayer (OTS SAM)-lipid monolayer system was designed to model the cell membrane for the SFG-VS experiment. Ca2+ could interact with the lipid PO2− head groups more strongly, resulting in cell membrane fusion more easily, in comparison with Mg2+. No specific interaction between the two metal cations and the C=O groups was observed. However, the C=O orientations changed more after Ca2+-PO2− binding than Mg2+ mediation on lipid monolayer. Meanwhile, Ca2+ could induce dehydration of the lipids (which should be related to the strong Ca2+-PO2− interaction), leading to the reduced hindrance for cell membrane fusion.

Vibrational Ground-State depletion for enhanced resolution sum frequency generation microscopy

The study of surfaces, which provides unique information on chemical and physical properties, has led to the development of specialized tools including Sum Frequency Generation (SFG) spectroscopy. To map the chemical heterogeneity across the surface plane, the SFG spectroscopic imaging has been developed. SFG imaging has proven to be a useful tool for studying surface specific species through their vibrational signatures. However, the effective resolution of the imaging is limited to micrometer length scales often by the diffraction limit of the mid-IR wavelengths deployed in vibrationally sensitive SFG imaging. Therefore, many interesting sub-micron features are out of reach for such imaging techniques. This article introduces a technique to achieve super-resolution using ground state depletion from a structured area around the focal-spot of a raster-scan imaging scheme. The proof-of-concept results are presented using hydrogen terminated silicon (Si(111)) as sample surface with results indicating a 3-fold resolution improvement.

Observing a Chemical Reaction at a Buried Solid/Solid Interface in Situ.

Chemical reactions are the most important phenomena in chemistry. However, chemical reactions at buried solid/solid interfaces are very difficult to study in situ. In this research, the chemical reaction between two solid polymer materials, a nylon film and a maleic anhydride (MAH) grafted poly(ethylene-octene) (MAHgEO) sample, was directly analyzed at the buried nylon/MAHgEO interface at the molecular level in real time and in situ, using surface and interface sensitive sum-frequency generation (SFG) vibrational spectroscopy. Disappearance of nylon signals indicated a chemical reaction between amine and hydrolyzed amide groups of nylon and MAH groups on the MAHgEO at the buried interface. The appearance of SFG signals from reaction products was also observed at the buried nylon/MAHgEO interface. The mechanism of the observed interfacial reaction was further analyzed. Temperature-dependent SFG experiments were performed to measure the activation energy of the interfacial reaction, enabling a comparison with that reported for the bulk materials. The interfacial chemical reaction between nylon and MAHgEO greatly improved the adhesion of these dissimilar materials. The detailed analysis of a chemical reaction between two polymers at the polymer/polymer buried interface underscores the utility of SFG as a powerful analytical tool to build understanding of buried interfaces and to accelerate the design of interfacial structures with desired properties.

Multimodal Multiphoton Imaging of the Lipid Bilayer by Dye-Based Sum-Frequency Generation and Coherent Anti-Stokes Raman Scattering.

Coherent anti-Stokes Raman scattering (CARS) imaging is widely used for imaging molecular vibrations inside cells and tissues. Lipid bilayers are potential analytes for CARS imaging due to their abundant CH2 vibrational bonds. However, identifying the plasma membrane is challenging since it possesses a thin structure and is closely apposed to lipid structures inside the cells. Since the plasma membrane provides the most prominent asymmetric location within cells, orientation sensitive sum-frequency generation (SFG) imaging is a promising technique for selective visualization of the plasma membrane labeled by a nonfluorescent and SFG-specific dye, Ap3, when using a CARS microscope system. In this study, we closely compare the characteristics of lipid bilayer imaging by dye-based SFG and CARS using giant vesicles (GVs) and N27 rat dopaminergic neural cells. As a result, we show that CARS imaging can be exploited for the visualization of whole lipid structures inside GVs and cells but is insufficient for identification of the plasma membrane, which instead can be achieved using dye-based SFG imaging. In addition, we demonstrate that these unique properties can be combined and applied to the live-cell tracking of intracellular lipid structures such as lipid droplets beneath the plasma membrane. Thus, multimodal multiphoton imaging through a combination of dye-based SFG and CARS can serve as a powerful chemical imaging tool to investigate lipid bilayers in GVs and living cells.

The Role of Specific Ion Effects in Ion Transport: The Case of Nitrate and Thiocyanate

The selective transport of trivalent rare earth metals from aqueous to organic environments with the help of amphiphilic "extractants" is an industrially important process. When the amphiphilic extractant is positively charged or neutral, the coextracted background anions are not only necessary for charge balance but also have a large impact on extraction efficiency and selectivity. In particular, the opposite selectivity trends observed throughout the lanthanide series in the presence of nitrate and thiocyanate ions have not been explained. To understand the role of background anions in the phase transfer of lanthanide cations, we use a positively charged long-chain aliphatic molecule, modeling a common extractant, and gain molecular level insight into interfacial headgroup-anion interactions. By combining surface sensitive sum frequency generation spectroscopy with X-ray reflectivity and grazing incidence X-ray diffraction, we observed qualitative differences in the orientational and overall interfacial structure of nitrate and thiocyanate solutions at a positively charged Langmuir monolayer. Though nitrate adsorbs without dramatic changes to the solvation structure at the interface or the monolayer ordering, thiocyanate significantly alters the water structure and reduces monolayer ordering. We suggest that these qualitatively different adsorption trends help explain a reversal in system selectivity toward lighter or heavier lanthanides in solvent extraction systems in the presence of nitrate or thiocyanate anions.

Quantitative determination of the nonlinear bulk and surface response from alpha-quartz using phase sensitive SFG spectroscopy

We investigate the nonlinear optical response of alpha-quartz with phase-sensitive vibrational sum frequency generation (SFG) spectroscopy. Phase and amplitude of the generated sum frequency signal are determined by a complex interplay between signal contributions from the surface and the bulk of the crystal with the experimental geometry, in particular, the quartz azimuth. By combining anisotropy measurements with spectral- and phase resolution in our SFG experiment, we study this interplay and show that we can quantitatively isolate and characterize the contributions from the surface and bulk of the crystal. The obtained optical constants in combination with the presented mathematical framework fully describe the nonlinear response and allow for a precise determination of the phase of the overall SFG signal of quartz for any experimental geometry. This knowledge is of particular relevance because quartz is commonly used as a reference in phase sensitive SFG experiments. In such studies, the surface contribution is typically neglected and a constant value for the phase of the sum frequency response is assumed. Our study shows that this assumption is not generally accurate by revealing the considerable impact that the surface contribution can have on the overall response of the crystal.

Temperature Dependence of the Air/Water Interface Revealed by Polarization Sensitive Sum-Frequency Generation Spectroscopy.

The temperature dependence of the vibrational sum-frequency generation (vSFG) spectra of the air/water interface is investigated using many-body molecular dynamics (MB-MD) simulations performed with the MB-pol potential energy function. The vSFG spectra calculated for different polarization combinations are then analyzed in terms of molecular autocorrelation and cross-correlation contributions. To provide molecular-level insights into interfacial hydrogen-bonding topologies, which give rise to specific spectroscopic features, the vSFG spectra are further investigated by separating contributions associated with water molecules donating zero, one, or two hydrogen bonds to neighboring water molecules. This analysis suggests that the low frequency shoulder of the free OH peak which appears at ∼3600 cm-1 is primarily due to intermolecular couplings between both singly and doubly hydrogen-bonded molecules.

High-resolution broadband sum frequency generation vibrational spectroscopy using intrapulse interference.

We theoretically propose a new approach to obtain high-spectral-resolution sum frequency generation (SFG) vibrational spectra using intrapulse interference. By introducing a π-step phase modulation to the broadband 800 nm pulse, the broadband 800 nm laser pulse splits into two distinguishable pulses in the time domain with a fixed time delay. The resolution of the intrapulse interference SFG can be better than 1 cm-1 and is limited only by the spectral resolution of the spectrometer. This approach can accurately retrieve the amplitude and the relative phase of vibrational peaks. Additionally, the sensitivity of SFG is enhanced by adopting femtosecond IR and femtosecond visible pulses.

Hyperspectral imaging with laser-scanning sum-frequency generation microscopy

Vibrationally sensitive sum-frequency generation (SFG) microscopy is a chemically selective imaging technique sensitive to non-centrosymmetric molecular arrangements in biological samples. The routine use of SFG microscopy has been hampered by the difficulty of integrating the required mid-infrared excitation light into a conventional, laser-scanning nonlinear optical (NLO) microscope. In this work, we describe minor modifications to a regular laser-scanning microscope to accommodate SFG microscopy as an imaging modality. We achieve vibrationally sensitive SFG imaging of biological samples with sub-μm resolution at image acquisition rates of 1 frame/s, almost two orders of magnitude faster than attained with previous point-scanning SFG microscopes. Using the fast scanning capability, we demonstrate hyperspectral SFG imaging in the CH-stretching vibrational range and point out its use in the study of molecular orientation and arrangement in biologically relevant samples. We also show multimodal imaging by combining SFG microscopy with second-harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS) on the same imaging platfrom. This development underlines that SFG microscopy is a unique modality with a spatial resolution and image acquisition time comparable to that of other NLO imaging techniques, making point-scanning SFG microscopy a valuable member of the NLO imaging family.

Structure of the Fundamental Lipopeptide Surfactin at the Air/Water Interface Investigated by Sum Frequency Generation Spectroscopy.

The lipopeptide surfactin produced by certain strains of Bacillus subtilis is a powerful biosurfactant possessing potentially useful antimicrobial properties. In order to better understand its surface behavior, we have used surface sensitive sum frequency generation (SFG) vibrational spectroscopy in the C-H and C═O stretching regions to determine its structure at the air/water interface. Using surfactin with the leucine groups of the peptide ring perdeuterated, we have shown that a majority of the SFG signals arise from the 4 leucine residues. We find that surfactin forms a robust film, and that its structure is not affected by the number density at the interface or by pH variation of the subphase. The spectra show that the ring of the molecule lies in the plane of the surface rather than perpendicular to it, with the tail lying above this, also in the plane of the interface.

Probing Site-Specific Structural Information of Peptides at Model Membrane Interface In Situ.

Isotope labeling is a powerful technique to probe detailed structures of biological molecules with a variety of analytical methods such as NMR and vibrational spectroscopies. It is important to obtain molecular structural information on biological molecules at interfaces such as cell membranes, but it is challenging to use the isotope labeling method to study interfacial biomolecules. Here, by individually (13)C═(16)O labeling ten residues of a peptide, Ovispirin-1, we have demonstrated for the first time that a site-specific environment of membrane associated peptide can be probed by the submonolayer surface sensitive sum frequency generation (SFG) vibrational spectroscopy in situ. With the peptide associated with a single lipid bilayer, the sinusoidal trend of the SFG line width and peak-center frequency suggests that the peptide is located at the interface beneath the lipid headgroup region. The constructive interferences between the isotope labeled peaks and the main peptide amide I peak contributed by the unlabeled components were used to determine the membrane orientation of the peptide. From the SFG spectral peak-center frequency, line width, and polarization dependence of the isotope labeled units, we deduced structural information on individual units of the peptide associated with a model cell membrane. We also performed molecular dynamics (MD) simulations to understand peptide-membrane interactions. The physical pictures described by simulation agree well with the SFG experimental result. This research demonstrates the feasibility and power of using isotope labeling SFG to probe molecular structures of interfacial biological molecules in situ in real time.

Multireflection sum frequency generation vibrational spectroscopy.

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.

Consequences of water between two hydrophobic surfaces on adhesion and wetting.

The contact of two hydrophobic surfaces in water is of importance in biology, catalysis, material science, and geology. A tenet of hydrophobic attraction is the release of an ordered water layer, leading to a dry contact between two hydrophobic surfaces. Although the water-free contact has been inferred from numerous experimental and theoretical studies, this has not been directly measured. Here, we use surface sensitive sum frequency generation spectroscopy to directly probe the contact interface between hydrophobic poly(dimethylsiloxane) (PDMS) and two hydrophobic surfaces (a self-assembled monolayer, OTS, and a polymer coating, PVNODC). We show that the interfacial structures for OTS and PVNODC are identical in dry contact but that they differ dramatically in wet contact. In water, the PVNODC surface partially rearranges at grain boundaries, trapping water at the contact interface leading to a 50% reduction in adhesion energy compared to OTS-PDMS contact. The Young-Dupré equation, used extensively to calculate the thermodynamic work of adhesion, predicts no differences between the adhesion energy for these two hydrophobic surfaces, indicating a failure of this well-known equation when there is a heterogeneous contact. This study exemplifies the importance of interstitial water in controlling adhesion and wetting.

Sodium Dodecyl Sulfate Adsorption onto Positively Charged Surfaces: Monolayer Formation With Opposing Headgroup Orientations

The adsorption and structure of sodium dodecyl sulfate (SDS) layers onto positively charged films have been monitored in situ with vibrational sum-frequency-generation (SFG) spectroscopy and surface plasmon resonance (SPR) sensing. Substrates with different charge densities and polarities used in these studies include CaF2 at different pH values as well as allylamine and heptylamine films deposited onto CaF2 and Au substrates by radio frequency glow discharge deposition. The SDS films were adsorbed from aqueous solutions ranging in concentration from 0.067 to 20 mM. In general the SFG spectra exhibited well resolved CH and OH peaks. However, at SDS concentrations between 1 and 8 mM the SFG CH and OH intensities decreased close to background levels. Combined data sets from molecular conformation, orientation, and order sensitive SFG with mass sensitive SPR suggest that the observed changes in SFG intensities above 0.2 mM are related to structural arrangements in the SDS layer. A model is proposed where the SFG intensity minimum between 1 and 8 mM is associated with a monolayer containing two headgroup orientations, one pointing toward the substrate and one pointing toward the solution phase. The SFG peaks observed at concentrations below 0.2 mM are dominated by the presence of adsorbed contaminants such as fatty alcohols (e.g., dodecanol), which are more surface active than SDS. As SDS solution concentration is increased above 1 mM SDS molecules are incorporated in the surface layer, with dodecanol continuing to be present in the surface layer for solution concentrations up to at least the critical micelle concentration.

Direct comparison of phase-sensitive vibrational sum frequency generation with maximum entropy method: Case study of water

We present a direct comparison of phase sensitive sum-frequency generation experiments with phase reconstruction obtained by the maximum entropy method. We show that both methods lead to the same complex spectrum. Furthermore, we discuss the strengths and weaknesses of each of these methods, analyzing possible sources of experimental and analytical errors. A simulation program for maximum entropy phase reconstruction is available at: http://lbp.epfl.ch/.

Interpreting intensities in vibrational sum frequency generation (SFG) spectroscopy: CO adsorption on Pd surfaces

The lineshape and intensity of SFG signals of CO adsorbed on supported Pd nanoparticles and Pd(1 1 1) are analyzed. For CO/Pd(1 1 1) nearly symmetric lorentzian lineshapes were observed. Applying two different visible wavelengths for excitation, asymmetric lineshapes observed for the CO/Pd/Al 2O 3/NiAl(1 1 0) system are explained by a lower resonant and a higher non-resonant SFG signal and a change in the phase between resonant and non-resonant signals, most likely originating from an interband transition in the NiAl substrate. The relative intensity of different CO species (hollow, bridge, on-top) was modeled by DFT calculations of IR transition moments and Raman activities. While the (experimental) sensitivity of SFG towards different CO species strongly varies, the calculated IR and Raman activities are rather similar. The inability to exactly reproduce experimental SFG intensities suggests a strong coverage dependence of Raman activities or that non-linear effects occur that can currently not be properly accounted for.

Polymer-Silane Interactions Probed by Sum Frequency Generation Vibrational Spectroscopy

ABSTRACT Gaining an understanding of the molecular mechanisms of adhesion to polymeric materials is crucial for the design of better adhesives and adhesion promoters. Silane coupling agents are widely used as polymer-polymer adhesion promoters. However, a molecular level understanding of how silanes enhance adhesion between solid polymeric surfaces is largely unknown. Here we exploit the extreme surface sensitivity of sum frequency generation (SFG) vibrational spectroscopy to probe various interactions at buried interfaces between polymers and silanes in situ. It has been elucidated that silanes can adopt various conformations at the interfaces with different polymers depending on the chemical groups that comprise the silane and the surface-presenting groups on the polymer. Some silanes have been found to diffuse into certain polymers, and SFG has been used to monitor the moving polymer/silane interface and deduce the diffusion coefficient. Hydrogen bonding between polymer surface carbonyl groups and silane amino groups has also been detected. Finally we demonstrate that SFG can probe the buried interface between a polymer and a cured silicone elastomer, and the segregation of silane adhesion promoting molecules to the polymer/elastomer interface can be detected.

Order To Disorder Transition at The Polymer-Air Interface for Hydrophobic Octadecyl Side Chain Copolymers

ABSTRACTMolecular orientation of octadecyl alkyl side chains at the poly (vinyl octadecyl carbamates-co-vinyl acetate) polymer-air interface has been studied using surface sensitive sum frequency generation (SFG) spectroscopy. At 280C, below the side chain crystalline transition temperature, the SFG spectra show strong methyl vibrations indicating ordered alkyl side chains at the polymer-air interface. In the liquid state (980C), the SFG spectra show higher contributions from the methylene vibrations indicating higher gauche defects at the interface. This surface order to disorder transition is gradual and spans over a broad temperature range (50-900C), where the bulk is in the smectic liquid crystalline state.

Sum Frequency Spectra in the C-H Stretch Region of Adsorbates on Iron

Infrared-visible sum frequency generation (SFG) was used to record vibrational spectra in the C-H stretch region of single monolayer Langmuir-Blodgett films of stearic acid, octadecylamine, and p-octadecylphenyl acetic acid on iron oxide, demonstrating the sensitivity of SFG to about 0.1 monolayer. Within our present signal-to-noise limitations, it was not possible to obtain SFG spectra of the aromatic C-H stretch in p-octadecylphenyl acetic acid or the N-H stretch in octadecylamine. Vibrational spectra in the C-H stretch region have also been obtained for chemically adsorbed monolayers of stearic acid and oleic imidazoline on iron oxide; however an undetermined fraction of these spectra result from hydrocarbon contamination. Finally, an SFG spectrum was obtained at a buried interface, that of p-octadecylphenyl acetic acid on iron oxide under water. The potential of SFG as a surface analytical tool is discussed.