The thermal reorganization of DNA immobilized at the silica/buffer interface: a vibrational sum frequency generation investigation.

We have monitored the interactions of DNA strands immobilized on silica at the buried solid/liquid interface using vibrational sum frequency generation. We find that the nucleobases exhibit net order even prior to hybridization for immobilized single strands. Moreover, varying the temperature of the hybridized samples leads to spectral changes from the thymine nucleobases that are consistent with duplex dissociation.

Following the azide-alkyne cycloaddition at the silica/solvent interface with sum frequency generation.

The Cu(I) -catalyzed 1,3-dipolar azide-alkyne cycloaddition (CuAAC) has arisen as one of the most useful chemical transformations for introducing complexity onto surfaces and materials owing to its functional-group tolerance and high yield. However, methods for monitoring such reactions in situ at the widely used silica/solvent interface are hampered by challenges associated with probing such buried interfaces. Using the surface-specific technique broadband sum frequency generation (SFG), we monitored the reaction of a benzyl azide monolayer in real time at the silica/methanol interface. A strong peak at 2096 cm(-1) assigned to the azides was observed for the first time by SFG. Using a cyano-substituted alkyne, the decrease of the azide peak and the increase of the cyano peak (2234 cm(-1) ) were probed simultaneously. From the kinetic analysis, the reaction order with respect to copper was determined to be 2.1, suggesting that CuAAC on the surface follows a similar mechanism as in solution.

A combined vibrational sum frequency generation spectroscopy and atomic force microscopy study of sphingomyelin-cholesterol monolayers.

A combination of vibrational sum frequency generation spectroscopy and atomic force microscopy is used to study the changes in morphology and conformational order in monolayers prepared from three natural sphingomyelin (SM) mixtures as a function of surface pressure and cholesterol concentration. The most homogeneous SM gave monolayers with well-ordered acyl chains and few gauche defects with relatively small effects of either increasing surface pressure or cholesterol addition. Heterogeneous SM mixtures with a mixture of acyl chain lengths or with significant fractions of unsaturated acyl chains had much larger contributions from gauche defects at low surface pressure and gave increasingly well-ordered monolayers as the surface pressure increased. They also showed substantial increases in lipid chain order after cholesterol addition. Overall, these results are consistent with the strong hydrogen bonding capacity of SM leading to well-ordered monolayers over a range of surface pressures. The changes in acyl chain order for natural SMs as a function of cholesterol are relevant to formation of sphingolipid-cholesterol enriched domains in cell membranes.

Specific Cation Effects on the Bimodal Acid-Base Behavior of the Silica/Water Interface.

Using nonresonant second harmonic generation spectroscopy, we have monitored the change in surface charge density of the silica/water interface over a broad pH range in the presence of different alkali chlorides. Planar silica is known to possess two types of surface sites with pKa values of ∼4 and ∼9, which are attributed to different solvation environments of the silanols. We report that varying the alkali chloride electrolyte significantly changes the effective acid dissociation constant (pKa(eff)) for the less acidic silanol groups, with the silica/NaClaq and silica/CsClaq interfaces exhibiting the lowest and highest pKa(eff) values of 8.3(1) and 10.8(1), respectively. Additionally, the relative populations of the two silanol groups are also very sensitive to the electrolyte identity. The greatest percentage of acidic silanol groups was 60(2)% for the silica/LiClaq interface in contrast to the lowest value of 20(2)% for the silica/NaClaq interface. We attribute these changes in the bimodal behavior to the influence of alkali ions on the interfacial water structure and its corresponding effect on surface acidity.

Hydrogen bonding at the water surface revealed by isotopic dilution spectroscopy.

The air-water interface is perhaps the most common liquid interface. It covers more than 70 per cent of the Earth's surface and strongly affects atmospheric, aerosol and environmental chemistry. The air-water interface has also attracted much interest as a model system that allows rigorous tests of theory, with one fundamental question being just how thin it is. Theoretical studies have suggested a surprisingly short 'healing length' of about 3 ångströms (1 Å = 0.1 nm), with the bulk-phase properties of water recovered within the top few monolayers. However, direct experimental evidence has been elusive owing to the difficulty of depth-profiling the liquid surface on the ångström scale. Most physical, chemical and biological properties of water, such as viscosity, solvation, wetting and the hydrophobic effect, are determined by its hydrogen-bond network. This can be probed by observing the lineshape of the OH-stretch mode, the frequency shift of which is related to the hydrogen-bond strength. Here we report a combined experimental and theoretical study of the air-water interface using surface-selective heterodyne-detected vibrational sum frequency spectroscopy to focus on the 'free OD' transition found only in the topmost water layer. By using deuterated water and isotopic dilution to reveal the vibrational coupling mechanism, we find that the free OD stretch is affected only by intramolecular coupling to the stretching of the other OD group on the same molecule. The other OD stretch frequency indicates the strength of one of the first hydrogen bonds encountered at the surface; this is the donor hydrogen bond of the water molecule straddling the interface, which we find to be only slightly weaker than bulk-phase water hydrogen bonds. We infer from this observation a remarkably fast onset of bulk-phase behaviour on crossing from the air into the water phase.

Vibrational sum frequency generation spectroscopy using inverted visible pulses.

We present a broadband vibrational sum frequency generation (BB-VSFG) scheme using a novel ps visible pulse shape. We generate the fs IR pulse via standard procedures and simultaneously generate an 'inverted' time-asymmetric narrowband ps visible pulse via second harmonic generation in the pump depletion regime using a very long nonlinear crystal which has high group velocity mismatch (LiNbO3). The 'inverted' ps pulse shape minimally samples the instantaneous nonresonant response but maximally samples the resonant response, maintaining high spectral resolution. We experimentally demonstrate this scheme, presenting SFG spectra of canonical organic monolayer systems in the C-H stretch region (2800-3000 cm(-1)).

Temporal effects on spectroscopic line shapes, resolution, and sensitivity of the broad-band sum frequency generation.

Sum frequency generation (SFG) is a surface-selective spectroscopy that provides a wealth of molecular-level information on the structure and dynamics at surfaces and interfaces. This paper addresses the general issue of spectral resolution and sensitivity of the broad-band (BB) SFG that involves a spectrally narrow nonresonant (usually visible) and a BB resonant (usually infrared) laser pulses. We examine how the spectral width and temporal shape of the two pulses, and the time delay between them, relate to the spectroscopic line shape and signal level in the BB-SFG measurement. By combining experimental and model calculations, we show that the best spectral resolution and highest signal level are simultaneously achieved when the nonresonant narrow-band upconversion pulse arrives with a nonzero time delay after the resonant BB pulse. The nonzero time delay partially avoids the linear trade-off of improving spectral resolution at the expense of decreasing signal intensity, which is common in BB-SFG schemes utilizing spectral filtering to produce narrow-band visible pulses.

Effect of nanoscale geometry on molecular conformation: vibrational sum-frequency generation of alkanethiols on gold nanoparticles.

Vibrational sum frequency generation (VSFG) spectroscopy was used to study the nanoscale geometric effects on molecular conformation of dodecanethiol ligand on gold nanoparticles of varying size between 1.8 and 23 nm. By analyzing the CH3 and CH2 stretch transitions of dodecanethiol using the spectroscopic propensity rules for the SFG process, we observe the increase of the gauche defects in the alkyl chain of the ligand on the nanoparticle surface when the curvature approaches the size of the molecule ( approximately 1.6 nm). In contrast, linear infrared absorption and Raman spectra, governed by different selection rules, do not allow observation of the size-dependent conformational changes. The results are understood in terms of the geometric packing effect, where the curvature of the nanoparticle surface results in the increased conical volume available for the alkyl chain.

Rubbing-induced anisotropy of long alkyl side chains at polyimide surfaces.

Molecular organization at polyimide surfaces used as alignment layers in liquid crystal displays was investigated using vibrational sum frequency generation (SFG) spectroscopy. We focus on the orientation of the long alkyl side groups at the polymer surface using polarization-selected SFG spectra of the CH(3)- and CH(2)-stretch modes of the side chain. Mechanical rubbing and baking, an accepted industrial procedure used to produce pretilt of the liquid crystal, was found to induce pronounced azimuthal anisotropy in the orientational distribution of the alkyl side chains. Orientational analysis of the SFG vibrational spectra in terms of the azimuthal and tilt angles (in and out of plane, respectively) of the alkyl side chains shows their preferential tilt along the rubbing direction, with the azimuthal distribution narrower for stronger rubbed polymer samples.