No abstract

In IR and Raman spectral studies, the congestion of the vibrational modes in the C-H stretching region between 2800 and 3000 cm(-1) has complicated spectral assignment, conformational analysis, and structural and dynamics studies, even with quite a few of the simplest molecules. To resolve these issues, polarized spectra measurement on a well aligned sample is generally required. Because the liquid interface is generally ordered and molecularly thin, and sum frequency generation vibrational spectroscopy (SFG-VS) is an intrinsically coherent polarization spectroscopy, SFG-VS can be used for discerning details in vibrational spectra of the interfacial molecules. Here we show that, from systematic molecular symmetry and SFG-VS polarization analysis, a set of polarization selection rules could be developed for explicit assignment of the SFG vibrational spectra of the C-H stretching modes. These polarization selection rules helped assignment of the SFG-VS spectra of vapor/alcohol (n = 1-8) interfaces with unprecedented details. Previous approach on assignment of these spectra relied on IR and Raman spectral assignment, and they were not able to give such detailed assignment of the SFG vibrational spectra. Sometimes inappropriate assignment was made, and consequently misleading conclusions on interfacial structure, conformation and even dynamics were reached. With these polarization rules in addition to knowledge from IR and Raman studies, new structural information and understanding of the molecular interactions at these interfaces were obtained, and some new spectral features for the C-H stretching modes were also identified. Generally speaking, these new features can be applied to IR and Raman spectroscopic studies in the condensed phase. Therefore, the advancement on vibrational spectra assignment may find broad applications in the related fields using IR and Raman as vibrational spectroscopic tools.

In this paper we report detailed examples of the surface sum frequency generation vibrational spectroscopy (SFG-VS) as a polarization spectroscopic technique for vibrational spectral band assignment and orientational analysis for molecular groups at the interfaces. Surface sum frequency generation vibrational spectroscopy (SFG-VS) has been widely used as an important spectroscopy probe for chemical bonding, structural conformation and molecular interactions of both fundamentally and technologically important interfaces. However, the potential for SFG-VS as a polarization spectroscopic technique is yet to be fully explored. In IR and Raman studies, polarized spectroscopy (PS) can provide information on molecular symmetries, which is necessary for accurate vibrational band assignment in complex chemical environments. We shall show that SFG-vibrational polarization spectroscopy (VPS) is the polarization spectroscopic tool on this purpose for the interface, along with its advantage of submonolayer sensitivity. This ability of SFG-VPS comes from the fact that vibrational bands from different symmetry types do not have strongest peak intensities in the same polarization configuration of the SFG-VPS spectra, because molecular groups are aligned or partially aligned at the interface. We chose to study the SFG-VPS of three diols, namely, ethylene glycol, 1,3-propanediol, and 1,5-pentanediol, at the vapor/liquid interfaces as model systems for the methylene-only molecules. The polarization analysis of the SFG spectra resulted in few explicit polarization selection rules or guidelines for the assignment of the CH stretching modes of the methylene groups. These results could be used to provide clarifications for some of the disagreements and controversies still exist in the literatures. These results can also shed light on the IR and Raman studies on methylene group in the bulk condensed phases.

The orientation, structure, and energetics of the vapor/acetone-water interface are studied with sum frequency generation vibrational spectroscopy (SFG-VS). We used the polarization null angle (PNA) method in SFG-VS to accurately determine the interfacial acetone molecule orientation, and we found that the acetone molecule has its C=O group pointing into bulk phase, one CH3 group pointing up from the bulk, and the other CH3 group pointing into the bulk phase. This well-ordered interface layer induces an antiparallel structure in the second layer through dimer formation from either dipolar or hydrogen bond interactions. With a double-layer adsorption model (DAM) and Langmuir isotherm, the adsorption free energies for the first and second layer are determined as deltaG degrees (ads,1) = - 1.9 +/- 0.2 kcal /mol and deltaG degrees (ads,2) = - 0.9 +/- 0.2 kcal /mol, respectively. Since deltaG degrees (ads,1) is much larger than the thermal energy kT = 0.59 kcal /mol, and deltaG degrees (ads,2) is close to kT, the second layer has to be less ordered. Without either strong dipolar or hydrogen bonding interactions between the second and the third layer, the third layer should be randomly thermalized as in the bulk liquid. Therefore, the thickness of the interface is not more than two layers thick. These results are consistent with previous MD simulations for the vapor/pure acetone interface, and undoubtedly provide direct microscopic structural evidences and new insight for the understanding of liquid and liquid mixture interfaces. The experimental techniques and quantitative analysis methodology used for detailed measurement of the liquid mixture interfaces in this report can also be applied to liquid interfaces, as well as other molecular interfaces in general.

Interface bond structure, in addition to the well-known size and shape quantum confinement effects, is another factor that affects the properties of nanomaterials that is less known and studied. Inspired by the thiol-bridging "staple" motif (RS-Au-SR, Jadzinsky; et al. Science 2007, 318, 430.) discovered from monothiol-stabilized gold nanoclusters, dithiol ligand 2,3-dimercaptopropanesulfonic (DMPS) acid has been employed to synthesize dithiol-protected Au clusters (DTCs). The structure and property of the Au DTCs are studied to probe two effects: the entropy gain of dithiol over monothiol ligand protection and the constraint to the formation of the thiol bridging surface bonding. The hydrodynamic sizes of Au DTCs were estimated by diffusion nuclear magnetic resonance (NMR). The size distribution, Au core plus ligands on solid support, was confirmed by atomic force microscope (AFM) imaging. Size-dependent optical properties were observed. Au(4) clusters at high purity, characterized by mass spectrometry and organic-metal ratio confirmed by thermogravimetric analysis (TGA), display a characteristic absorbance band at 282 nm. The proton chemical environments as well as Au-S bond information of the Au(4) cluster were fully elucidated by (13)C-(1)H heteronuclear single-quantum coherence (HSQC) in conjunction with other two-dimensional (2D) NMR techniques. The Au-S bonding was further studied in thiol stretching by infrared and Au(4f) and S(2p) electrons by X-ray photoelectron spectroscopy (XPS). One possible structure of the Au(4) cluster has been proposed that needs further theoretical studies or single-crystal confirmation.

Current rectification is well known in ion transport through nanoscale pores and channel devices. The measured current is affected by both the geometry and fixed interfacial charges of the nanodevices. In this article, an interesting trend is observed in steady-state current-potential measurements using single conical nanopores. A threshold low-conductivity state is observed upon the dilution of electrolyte concentration. Correspondingly, the normalized current at positive bias potentials drastically increases and contributes to different degrees of rectification. This novel trend at opposite bias polarities is employed to differentiate the ion flux affected by the fixed charges at the substrate-solution interface (surface effect), with respect to the constant asymmetric geometry (volume effect). The surface charge density (SCD) of individual nanopores, an important physical parameter that is challenging to measure experimentally and is known to vary from one nanopore to another, is directly quantified by solving Poisson and Nernst-Planck equations in the simulation of the experimental results. The flux distribution inside the nanopore and the SCD of individual nanopores are reported. The respective diffusion and migration translocations are found to vary at different positions inside the nanopore. This knowledge is believed to be important for resistive pulse sensing applications because the detection signal is determined by the perturbation of the ion current by the analytes.

Recent developments in sum frequency generation-vibrational spectroscopy (SFG-VS) have shown that SFG-VS is not only a spectroscopic probe of the molecular interfaces but also an important tool for studying molecular spectroscopy in general. It has been demonstrated that through polarization and symmetry analysis a few sets of polarization selection rules can explicitly help the vibrational spectral assignment. This worked because of the coherent nature and the strong orientational dependence of the SFG process from the molecular interfaces or films. In this work, we further discuss the dependence on the experimental configurations in the SFG-VS polarization analysis. Such experiment and analysis can further increase the ability of the SFG-VS as the tool for discerning spectral details and overlapping spectral features, as well as the ability to obtain detailed molecular orientational information. The experimental configuration dependence of the SFG spectra is most significant in the ppp polarization combination, less significant in the sps and ssp polarization combinations, due to the fact that ppp is the result of the combination of four different susceptibility tensor elements, while both ssp and sps are of a single tensor element. Such complexity of the ppp spectrum, which is very useful as demonstrated in this work, used to be avoided in the SFG-VS studies. The spectral details of the SFG-VS from the vapor/methanol, vapor/ethanol, and vapor/ethylene glycol interfaces are studied in different experimental configurations. The strong experimental configuration dependence observed for these liquid interfaces also indicated that they possess well-ordered interfacial structures. The concepts and conclusions in this report may find future applications in studying more complex molecular interfaces and films.

The role of oxidative stress in skeletal health is unclear. The present study investigated whether a high dietary intake of antioxidant nutrients (vitamins C and E, b-carotene, animal-derived vitamin A, retinol equivalents, Zn and Se) is associated with a reduced risk of hip fracture in elderly Chinese. This 1:1 matched case -control study involved 726 elderly Chinese with hip fracture and 726 control subjects, recruited between June 2009 and May 2013. Face-to-face interviews were conducted to determine habitual dietary intakes of the above-mentioned seven nutrients based on a seventy-nine-item FFQ and information on various covariates, and an antioxidant score was calculated. After adjustment for potential covariates, dose-dependent inverse associations were observed between the dietary intake of vitamin C, vitamin E, b-carotene, and Se and antioxidant score and the risk of hip fracture (P for trend #0·005). The OR of hip fracture for the highest (v. lowest) quartile of intake were 0·39 (95 % CI 0·28, 0·56) for vitamin C, 0·23 (95 % CI 0·16, 0·33) for vitamin E, 0·51 (95 % CI 0·36, 0·73) for b-carotene, 0·43 (95 % CI 0·26, 0·70) for Se and 0·24 (95 % CI 0·17, 0·36) for the antioxidant score. A moderate-to-high dietary intake of retinol equivalents in quartiles 2 -4 (v. 1) was found to be associated with a lower risk of hip fracture (OR range: 0·51 -0·63, P, 0·05). No significant association was observed between dietary Zn or animal-derived vitamin A intake and hip fracture risk (P for trend .0·20). In conclusion, a higher dietary intake of vitamins C and E, b-carotene, and Se and a moderateto-high dietary intake of retinol equivalents are associated with a lower risk of hip fracture in elderly Chinese.