Diffuse reflection broad bandwidth sum frequency generation from particle surfaces

Windowless detection geometry for sum frequency scattering spectroscopy in the C-D and amide I regions

Understanding the structure and chemistry of nanoscopic surfaces is an important challenge for biointerface sciences. Sum frequency scattering (SFS) spectroscopy can specifically probe the surfaces of nanoparticles, vesicles, liposomes, and other materials relevant to biomaterial research, and, as a vibrational spectroscopy method, it can provide molecular level information about the surface chemistry. SFS is particularly promising to probe the structure of proteins, and other biological molecules, at nanoparticle surfaces. Here, amide I spectra can provide information about protein folding and orientation, while spectra in the C-D and C-H stretching regions allow experiments to determine the mode of interaction between particle surfaces and proteins. Methods used currently employ a closed liquid cell or cuvette, which works extremely well for C-H and phosphate regions but is often impeded in the amide I and C-D regions by a strong background signal that originates from the window material of the sample cells. Here, we discuss a windowless geometry for collecting background-free and high-fidelity SFS spectra in the amide I and C-D regions. We demonstrate the improvement in spectra quality by comparing SFS spectra of unextruded, multilamellar vesicles in a sample cuvette with those recorded using the windowless geometry. The sample geometry we propose will enable new experiments using SFS as a probe for protein-particle interactions.

Flexible approach to vibrational sum-frequency generation using shaped near-infrared light

We describe a new approach that expands the utility of vibrational sum-frequency generation (vSFG) spectroscopy using shaped near-infrared (NIR) laser pulses. We demonstrate that arbitrary pulse shapes can be specified to match experimental requirements without the need for changes to the optical alignment. In this way, narrowband NIR pulses as long as 5.75 ps are readily generated, with a spectral resolution of about 2.5  cm-1, an improvement of approximately a factor of 3 compared to a typical vSFG system. Moreover, the utility of having complete control over the NIR pulse characteristics is demonstrated through nonresonant background suppression from a metallic substrate by generating an etalon waveform in the pulse shaper. The flexibility afforded by switching between arbitrary NIR waveforms at the sample position with the same instrument geometry expands the type of samples that can be studied without extensive modifications to existing apparatuses or large investments in specialty optics.

Powder sum-frequency generation as a versatile method for infrared optical alignment

The sum-frequency generation (SFG) in potassium dihydrogen phosphate (KDP) powder with μm-grade particle size is successfully demonstrated under various experimental conditions. Two focused beams of 870 nm and 1369 nm are used for SFG excitation. SFG is observed under different excitation energies. The SFG intensity shows isotropy with different observation azimuths. The intersection angle between two excitation beams is not limited by conventional phase-matching conditions, and it owns the flexibility of a very large allowed range, e.g., it can be 0°∼100° in this work. The polarization combination of excitation beams is not limited either. Thanks to the non-toxicity, low price, and low SFG threshold properties of KDP material and the optical flexibility, this powder SFG technology is a versatile method and is expected to be applied to various situations of optical alignment, e.g., surface SFG, four-wave mixing, coherent anti-Stokes Raman spectroscopy, multi-color laser excitation, etc. The effect of potential powder SFG-assisted optical alignments is also discussed. Extension of this method to multi-beams, tight focusing beams, and plasmonic polariton devices is proposed.

Organic constituents on the surfaces of aerosol particles from Southern Finland, Amazonia, and California studied by vibrational sum frequency generation

This article summarizes and compares the analysis of the surfaces of natural aerosol particles from three different forest environments by vibrational sum frequency generation. The experiments were carried out directly on filter and impactor substrates, without the need for sample preconcentration, manipulation, or destruction. We discuss the important first steps leading to secondary organic aerosol (SOA) particle nucleation and growth from terpene oxidation by showing that, as viewed by coherent vibrational spectroscopy, the chemical composition of the surface region of aerosol particles having sizes of 1 μm and lower appears to be close to size-invariant. We also discuss the concept of molecular chirality as a chemical marker that could be useful for quantifying how chemical constituents in the SOA gas phase and the SOA particle phase are related in time. Finally, we describe how the combination of multiple disciplines, such as aerosol science, advanced vibrational spectroscopy, meteorology, and chemistry can be highly informative when studying particles collected during atmospheric chemistry field campaigns, such as those carried out during HUMPPA-COPEC-2010, AMAZE-08, or BEARPEX-2009, and when they are compared to results from synthetic model systems such as particles from the Harvard Environmental Chamber (HEC). Discussions regarding the future of SOA chemical analysis approaches are given in the context of providing a path toward detailed spectroscopic assignments of SOA particle precursors and constituents and to fast-forward, in terms of mechanistic studies, through the SOA particle formation process.

Examination of glass transitions in CO(2)-processed, peracetylated sugars using sum frequency generation spectroscopy

The present study utilizes vibrational sum frequency generation (SFG) spectroscopy to study changes in the surface crystallinity of various peracetylated sugars, a class of materials that have a high affinity for carbon dioxide (CO(2)). Studies of the solid-air interface of acetylated beta-cyclodextrin (Ac-beta-CD) and sucrose octaacetate (SOA) show that diffuse reflectance SFG spectroscopy is sensitive to changes in crystallinity from processing with either heat or solvation in CO(2), due to the loss of signal after glassification occurs. beta-d-Glucose pentaacetate (Ac-beta-GLC) was used as a control for this experiment due to the fact that it does not undergo a crystalline phase transition, regardless of processing conditions. The crystalline to amorpohous transitions of these bulk materials were verified using differential scanning calorimetry (DSC) as a function of thermal and CO(2) processing. In addition, preliminary results suggest that the SFG technique is sensitive in detecting the degree of crystallinity at the interface as a result of incomplete processing and presents new opportunities for the examination and detection of surface crystallinity changes.

Ethylenediamine at air/liquid and air/silica interfaces: protonation versus hydrogen bonding investigated by sum frequency generation spectroscopy

The adsorption process on mineral oxide surfaces is one of the most important factors influencing the migration and distribution of contaminants in the environment. Although there have been numerous studies carried out at the macroscopic scale, there is a lack of molecular-scale interfacial information. The molecular-scale information is often crucial for the determination of sorption mechanisms. In this study, sum frequency generation spectroscopy (SFG), a surface specific vibrational spectroscopy, has been employed to investigate the adsorption of ethylenediamine (H2N(CH2)2NH2, EDA) onto the amorphous SiO2 surface and EDA protonated products at air/liquid interfaces. The SFG spectra of EDA adsorbed on the silica surface and the singly protonated EDA solution are similar in both CH and NH stretching regions. These spectral similarities indicate that EDA molecules are strongly chemisorbed to the silica surface through the protonation of one EDA amine group by surface silanol OH groups, thereby forming a (H2N(CH2)2NH3)+(O-Si[triple bond])- surface complex. The SFG results also indicate that the surface acidity of the silanol OH groups (pKa (HOSi[triple bond])) is between the two pKa values of EDA (in the range of 7.56-10.71) at the air/silica interface.

Sum frequency generation studies of surfaces of high-surface-area powdered materials

We report the first observation of sum frequency generation (SFG) photons on high-surface-area powders, critically important materials in heterogeneous catalysis. We utilize SFG in total internal reflection (TIR) geometry and show that the TIR-SFG approach markedly reduces the destructive interference associated with nonlinear optical spectroscopy of small particle surfaces, making SFG studies of high-surface-area powders possible. The index of refraction of materials and their distance from the TIR-SFG prism are key parameters in generating and detecting the sum frequency signal. We find that TIR-SFG is highly sensitive to capillary condensation. To demonstrate the capability of the TIR-SFG technique, we measure the thermodynamics of methanol adsorption and desorption on high-surface-area SiO2.

Adsorption of 4-picoline and piperidine to the hydrated SiO2 surface: probing the surface acidity with vibrational sum frequency generation spectroscopy

Vapor adsorption is an important process influencing the migration and the fate of many organic pollutants in the environment. In this study, vibrational sum frequency generation (SFG) spectroscopy was used to study the adsorption of two surface acidity probe molecules, 4-picoline (pKa = 5.94) and piperidine (pKa = 11.24), onto the amorphous SiO2 surface. The adsorption of 4-picoline onto the silica surface occurs by forming weak hydrogen bonds between the nitrogen atoms of 4-picoline molecules and the hydrogen atoms of surface silanol OH groups. Piperidine molecules are strongly chemisorbed onto the SiO2 surface through the protonation of piperidine molecules by surface silanol OH groups. The SFG results indicate that the surface acidity constant of silanol OH groups (pKa-(HOSi triple bond)) is in the range of 5.94-11.24 at the air/solid interface. Although this range of surface acidity constants is quite wide, it is possible to narrow it by choosing probe molecules with a smaller pKa range. Together with theoretical prediction methods, adsorption studies using vibrational SFG spectroscopy are capable of quantifying the surface acidity of mineral oxides by carefully choosing the acidity probe molecules.

Vibrational sum frequency scattering from a submicron suspension

A novel application of vibrational sum frequency generation (VSFG) is developed to study the molecular properties of the surface of submicron particles in suspension. The Rayleigh-Gans-Debye scattering theory is extended to extract the local molecular response from the macroscopic nonlinearly scattered spectral intensity. These results demonstrate the use of VSFG to investigate quantitatively the surface molecular properties of submicron particles, dispersed in solution. It provides information on the order and density of alkane chains and allows us to determine the elements of the local second-order surface susceptibility.

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.