Broad Bandwidth Sum Frequency Generation Research Articles

Ionic Binding of Na+ versus K+ to the Carboxylic Acid Headgroup of Palmitic Acid Monolayers Studied by Vibrational Sum Frequency Generation Spectroscopy

Ionic binding of alkali ions Na(+) and K(+) to the carboxylic acid headgroups of fatty acid monolayers is studied as a proxy toward understanding the fundamental chemistry in cell biology. In this study, we used broad-bandwidth sum frequency generation (BBSFG) vibrational spectroscopy to investigate the ionic binding event that leads to deprotonation and complex formation of fatty acid headgroups. Palmitic acid (C(15)H(31)COOH) exists as a monolayer on aqueous surfaces. Surface vibrational stretch modes of palmitic acid from 1400 cm(-1) to 3700 cm(-1) were observed (nu(s)-COO(-), nu-C horizontal lineO, nu-C-H, nu-O-H of -COOH, free OH). Palmitic acid is mostly protonated at the aqueous surface at neutral pH (approximately 6). However, various degrees of deprotonation are initiated by the introduction of Na(+) and K(+) that results in the complexation of K(+):COO(-) and solvent separated Na(+):COO(-). Evidence in several spectral regions indicates that K(+) exhibits stronger ionic binding affinity to the carboxylate anion relative to Na(+).

Structure of Butanol and Hexanol at Aqueous, Ammonium Bisulfate, and Sulfuric Acid Solution Surfaces Investigated by Vibrational Sum Frequency Generation Spectroscopy

The organization of 1-butanol and 1-hexanol at the air-liquid interface of aqueous, aqueous ammonium bisulfate, and sulfuric acid solutions was investigated using vibrational broad bandwidth sum frequency generation spectroscopy. There is spectroscopic evidence supporting the formation of centrosymmetric structures at the surface of pure butanol and pure hexanol. At aqueous, ammonium bisulfate, and at most sulfuric acid solution surfaces, butanol molecules organize in all-trans conformations. This suggests that butanol self-aggregates. The spectrum for the 0.052 M butanol in 59.5 wt % sulfuric acid solution is different from the other butanol solution spectra, that is, the surface butanol molecules are observed to possess a significant number of gauche defects. Relative to surface butanol, surface hexanol chains are more disordered at the surface of their respective solutions. Statistically, an increase in the number of gauche defects is expected for hexanol relative to butanol, a six carbon chain vs a four carbon chain. Yet, self-aggregation of hexanol at its aqueous solution surfaces is not ruled out because the methylene spectral contribution is relatively small. The surface spectra for butanol and hexanol also show evidence for salting out from the ammonium bisulfate solutions.

Real-Time Investigation of Lung Surfactant Respreading with Surface Vibrational Spectroscopy

The respreading of a lung surfactant monolayer at the air-water interface is investigated with broad bandwidth sum frequency generation (BBSFG) spectroscopy. The lung surfactant mixture contains chain perdeuterated dipalmitoylphosphatidylcholine (DPPC-d62), palmitoyloleoylphosphatidylglycerol (POPG), palmitic acid (PA), and KL4 (a 21-residue polypeptide analogue to the surfactant protein SP-B). DPPC-d62 serves as a probe molecule for the spectroscopic investigation. The BBSFG spectra of DPPC-d62 in the lung surfactant mixture are obtained in the C-D stretching region in real-time during film compression and expansion in a Langmuir trough. The BBSFG intensity of the CD3 stretch peak from DPPC-d62 terminal methyl groups is used as a measure of the interfacial density of DPPC-d62 after careful consideration of orientation effects. For the first time, the interfacial loss of DPPC in a complex lung surfactant mixture is quantified. Spectroscopic results reveal that there is an 18% DPPC-d62 interfacial loss during film respreading. However, the surface pressure-area isotherm measurements demonstrate that there is a rather large trough area reduction (37%) during film expansion. The relatively small interfacial loss of DPPC-d62 and the rather large trough area reduction indicate that the respreading of DPPC and non-DPPC components in the lung surfactant is not uniform and a surface refinement process exists during film compression and expansion. This refinement process results in a DPPC-enriched monolayer with a significant depletion of non-DPPC components after film respreading. Implication for replacement surfactant design from this work is discussed.

Broadband Coherent Anti-Stokes Raman Spectroscopy Characterization of Polymer Thin Films

Broadband coherent anti-Stokes Raman spectroscopy (CARS) is demonstrated as an effective probe of polymer thin film materials. A simple modification to a 1 kHz broad bandwidth sum frequency generation (SFG) spectrometer permits acquisition of CARS spectra for polymer thin films less than 100 nm thick, a dimension relevant to organic electronic device applications. CARS spectra are compared to the conventional Raman spectra of polystyrene and the resonance-enhanced Raman spectra of poly(3-hexylthiophene). The CARS spectra obtained under these conditions consistently demonstrate enhanced signal-to-noise ratio compared to the spontaneous Raman scattering. The sensitivity of the CARS measurement is limited by the damage threshold of the samples. The dielectic properties of the substrate have a dramatic effect on the detected signal intensity. For ultrathin films, the strongest signals are obtained from fused silica surfaces. Similar to surface-enhanced Raman scattering (SERS), Au also gives a large signal, but contrary to SERS, no surface roughening is necessary.

DPPC Langmuir Monolayer at the Air−Water Interface: Probing the Tail and Head Groups by Vibrational Sum Frequency Generation Spectroscopy

Dipalmitoylphosphatidylcholine (DPPC) is the predominant lipid component in lung surfactant. In this study, the Langmuir monolayer of deuterated dipalmitoylphosphatidylcholine (DPPC-d62) in the liquid-expanded (LE) phase and the liquid-condensed (LC) phase has been investigated at the air-water interface with broad bandwidth sum frequency generation (BBSFG) spectroscopy combined with a Langmuir film balance. Four moieties of the DPPC molecule are probed by BBSFG: the terminal methyl (CD3) groups of the tails, the methylene (CD2) groups of the tails, the choline methyls (CH3) in the headgroup, and the phosphate in the headgroup. BBSFG spectra of the four DPPC moieties provide information about chain conformation, chain orientation, headgroup orientation, and headgroup hydration. These results provide a comprehensive picture of the DPPC phase behavior at the air-water interface. In the LE phase, the DPPC hydrocarbon chains are conformationally disordered with a significant number of gauche configurations. In the LC phase, the hydrocarbon chains are in an all-trans conformation and are tilted from the surface normal by 25 degrees. In addition, the orientations of the tail terminal methyl groups are found to remain nearly unchanged with the variation of surface area. Qualitative analysis of the BBSFG spectra of the choline methyl groups suggests that these methyl groups are tilted but lie somewhat parallel to the surface plane in both the LE and LC phases. The dehydration of the phosphate headgroup due to the LE-LC phase transition is observed through the frequency blue shift of the phosphate symmetric stretch in the fingerprint region. In addition, implications for lung surfactant function from this work are discussed.

Molecular Orientation of Self-assembled Monolayer of Octadecanethiol on Platinum Surface Studied by Femtosecond Broad-bandwidth Sum Frequency Generation Spectroscopy

Abstract The molecular orientation in self-assembled monolayer (SAM) of octadecanethiol (ODT) on platinum surface prepared in ethanol solution was examined by broad-bandwidth sum frequency generation (BB-SFG) spectroscopy. The bands attributed to the CH3 symmetric and asymmetric stretching vibrational modes were observed in the SFG spectra. From the analysis of SFG intensity ratio of these two modes, the alkyl chain on platinum was found to be more perpendicular to the substrate than those on gold surface.

Spectroscopic and Computational Studies of Aqueous Ethylene Glycol Solution Surfaces

The combination of Monte Carlo, ab initio, and DFT computational studies of ethylene glycol (EG) and EG-water hydrogen-bonding complexes indicate that experimental vibrational spectra of EG and EG-water solution surfaces have contributions from numerous conformations of both EG and EG-water. The computed spectra, derived from harmonic vibrational frequency calculations and a theoretical Boltzmann distribution, show similarity to the experimental surface vibrational spectra of EG taken by broad-bandwidth sum frequency generation (SFG) spectroscopy. This similarity suggests that, at the EG and aqueous EG surfaces, there are numerous coexisting conformations of stable EG and EG-water complexes. A blue shift of the CH2 symmetric stretch peak in the SFG spectra was observed with an increase in the water concentration. This change indicates that EG behaves as a hydrogen-bond acceptor when solvated by additional water molecules. This also suggests that, in aqueous solutions of EG, EG-EG aggregates are unlikely to exist. The experimental blue shift is consistent with the results from the computational studies.

Methanol Reaction with Sulfuric Acid: A Vibrational Spectroscopic Study

The reaction between methanol and sulfuric acid (SA) was investigated using Raman and vibrational broad bandwidth sum frequency generation spectroscopies. Evidence for the formation of methyl hydrogen sulfate (MHS) was obtained by the presence of a new peak in the 800 cm-1 region, not present in either the neat methanol or concentrated sulfuric acid spectra. This peak is attributed to the singly bonded OSO symmetric stretch of MHS. The maximum yield of MHS with a large SA excess (7 SA/1 methanol) is shown to be (95 ± 5)% at −(15 ± 2) °C. No evidence was found to suggest formation of dimethyl sulfate.

1-Methyl Naphthalene Reorientation at the Air-Liquid Interface upon Water Saturation Studied by Vibrational Broad Bandwidth Sum Frequency Generation Spectroscopy.

Vibrational broad bandwidth sum frequency generation spectroscopy was employed to investigate the surface structure of neat 1-methyl naphthalene (1-MN) and the reorientation of the 1-MN molecules upon saturation of the 1-MN liquid with water. The neat 1-MN liquid molecules have their aromatic rings aligned antiparallel to one another with their methyl groups alternating out of the surface and into the subsurface region from molecule to molecule. With the introduction of relatively few water molecules into the 1-MN liquid (1:336 water/1-MN) a rearrangement of the surface molecules is induced, leading to an increased number density of the methyl groups arranged such that more methyl groups are oriented in the same direction into the air phase at the air-liquid 1-MN interface. Surface tension measurements reveal an increase in the surface tension upon water saturation of the 1-MN liquid, indicating surface activity of the water in the 1-MN solution. It is also clear that the reorientation of the surface 1-MN molecules is reversible.

Orientation of o-, m-, and p-Methylbenzylmercaptans Adsorbed on Au(111) Probed by Broad-Bandwidth Sum Frequency Generation Spectroscopy

The molecular orientation in self-assembled monolayers of ortho, meta, and para isomers of methylbenzylmercaptans (OMBM, MMBM, and PMBM, respectively) on Au(111) has been studied using broad-bandwidth sum frequency generation spectroscopy and cyclic voltammetry. Cyclic voltammograms for the reductive desorption suggest that all methylbenzylmercaptans form closely packed monolayers. Sum frequency vibrational spectra for OMBM, MMBM, and PMBM show the CH3 symmetric stretching vibration mode. The vibrational intensities are greatest in PMBM and smallest in OMBM, implying the difference in the orientation of the methyl group. The vibrational intensities are compared with the hyperpolarizabilities of methylbenzylmercaptans from a calculation based on the density functional theory. OMBM molecules adsorb on Au(111) tilting their benzyl moieties so that the direction of the methyl group is parallel with the surface, whereas MMBM and PMBM molecules adsorb on Au(111) with their benzyl moieties being perpendicular to the surface.

Surface Studies of Aqueous Methanol Solutions by Vibrational Broad Bandwidth Sum Frequency Generation Spectroscopy

Neat methanol (CH3OH) and aqueous methanol solutions were investigated using broad bandwidth sum frequency generation (BBSFG), Raman spectroscopy, and FTIR spectroscopy. BBSFG results indicate a net orientation of methanol molecules at the air−liquid interfaces of neat methanol and aqueous methanol solutions. However, as the methanol concentration increases above 0.57 methanol mole fraction the methanol molecules become less ordered, i.e., a larger distribution of orientation angles of the CH3 transition moment about the surface normal exists. This was further verified by the BBSFG results from partially deuterated methanol (CD3OH) aqueous solutions. A red-shift of the CH3 symmetric stretch (CH3−SS) frequency with the increase of methanol concentration observed by BBSFG, Raman, and FTIR spectroscopy suggests a changing hydrogen-bonding configuration between the methanol and the water molecules at the surface and in the bulk. Moreover, the surface studies strongly suggest that methanol is a more efficient hydrogen-bonding acceptor when the methanol molecule resides in the interfacial region.

Total-internal-reflection broad-bandwidth sum frequency generation spectroscopy of hexadecanethiol adsorbed on thin gold film deposited on CaF2.

Sum frequency vibrational spectra for hexadecanethiol (HDT) adsorbed on thin gold film deposited on the surface of a CaF2 prism have been measured using total-internal reflection broad-bandwidth sum frequency generation (TIR-BBSFG) spectroscopy. The bands attributed to the CH3 symmetric and asymmetric stretching vibrational modes were observed in the sum frequency vibrational spectra. The orientation of the methyl groups was analyzed using the ratio of the intensities of the two modes. The methyl groups of HDT on the thin gold film were much more randomly orientated than those on Au( 111).

Chain-length-dependent change in the structure of self-assembled monolayers of n-alkanethiols on Au(111) probed by broad-bandwidth sum frequency generation spectroscopy

The structure of the self-assembled monolayers (SAMs) of n-alkanethiols [CH3(CH2)nSH, n=3–11, 13–15, 17] on Au(111) has been studied using broad-bandwidth sum frequency generation spectroscopy. Sum-frequency vibrational spectra show three pronounced CH3 vibrational modes for all alkanethiol investigated, indicating that the commonly accepted picture that the alkyl chain for the long-chain alkanethiol SAMs has the all-trans conformation applies even to the short chain SAMs. The chain-length dependence of the ratio of the intensity for the CH3 symmetric vibrational mode to that for the CH3 asymmetric mode clearly shows the odd–even effect due to the difference in the direction of methyl group for SAMs with odd and even n, also supporting that the alkyl chain of SAMs has the all-trans conformation. An analysis of the vibrational intensities with respect to the angle between the main axis of the methyl group and the surface normal reveals that the structure of the alkanethiol SAMs gradually changes with n.

The air-liquid interface of benzene, toluene, m-xylene, and mesitylene: a sum frequency, Raman, and infrared spectroscopic study.

The air-liquid interface and the liquid-phase of benzene, toluene, 1,3-dimethylbenzene, and 1,3,5-trimethylbenzene are studied using broad bandwidth sum frequency generation spectroscopy, Raman and infrared spectroscopy. A vibrationally resonant sum frequency response is observed from these surfaces in spite of the small hyperpolarizabilities, in particular, the zero and near-zero hyperpolarizabilities of benzene and 1,3,5-trimethylbenzene. The orientation of the aromatic rings of these compounds at their air-liquid interfaces is tilted relative to the surface plane. Thus, on average, the plane of the aromatic ring does not lie in the interfacial plane. Comparison of the square root of the sum frequency intensity to that of the Raman multiplied bythe infrared intensity provides additional information about the molecular environment at their respective air-liquid interface.

Diffuse reflection broad bandwidth sum frequency generation from particle surfaces.

We report the first vibrational sum frequency generation (VSFG) spectroscopic study from particle surfaces of powdered solids using a modified SFG approach, diffuse reflection broad bandwidth sum frequency generation (DR-BBSFG). The DR-BBSFG spectrum of sodium dodecyl sulfate (SDS, C(12)H(25)SO(4)Na) powdered solids was obtained. Five peaks were resolved by calculated fits. Possible origins of the SFG response from SDS particle surfaces are discussed. Potential applications of DR-BBSFG spectroscopy are addressed.

Broadband vibrational sum frequency generation spectroscopy of a liquid surface.

An important advance in surface science has been the evolution of sum frequency generation to the application of studying surface structure and chemistry of liquid surfaces at the molecular-level by probing the vibrational signatures of surface molecules. Recently, broad-bandwidth sum frequency generation (BBSFG) spectroscopy has become an important tool for investigating gas-solid interfaces. BBSFG spectroscopy allows, theoretically, a surface sum frequency spectrum to be acquired within one pulse of the laser. In this paper, the viability of BBSFG to study inherently small nonlinear response interfaces and the time-resolving capability of this surface-selective technology are demonstrated. Presented here are the first published accounts of spectra from a liquid surface utilizing the broad-bandwidth sum frequency technology with acquisition times as low as 500 milliseconds.

Broadband sum frequency generation with two regenerative amplifiers: temporal overlap of femtosecond and picosecond light pulses.

Sum frequency generation (SFG) spectroscopy is a valuable tool for studying interfaces such that the boundary between two adjoining phases can be probed with minimal interference from the adjacent bulk material. More recently, broad-bandwidth sum frequency generation (BBSFG) techniques are being explored. This technique using IR broad-bandwidth fs pulses overlapped with narrow-bandwidth ps pulses to obtain BBSFG spectra is described. In the BBSFG system design presented here, the fs pulse and the ps pulse that are generated in separate regenerative amplifiers are overlapped temporally. This temporal overlap process is discussed. In addition, images of the sum frequency response demonstrate its viability. The new approach in experimental design described here for this emerging technology, BBSFG, has application for studying time-dependent processes at interfaces that inherently produce low SFG signal levels such as air-aqueous interfaces.