In nature, aqueous solutions often move collectively along solid surfaces (for example, raindrops falling on the ground and rivers flowing through riverbeds). However, the influence of such motion on water-surface interfacial chemistry is unclear. In this work, we combine surface-specific sum frequency generation spectroscopy and microfluidics to show that at immersed calcium fluoride and fused silica surfaces, flow leads to a reversible modification of the surface charge and subsequent realignment of the interfacial water molecules. Obtaining equivalent effects under static conditions requires a substantial change in bulk solution pH (up to 2 pH units), demonstrating the coupling between flow and chemistry. These marked flow-induced variations in interfacial chemistry should substantially affect our understanding and modeling of chemical processes at immersed surfaces.

Inside a Superatom: The M 7 q (M = Cu, Ag, q = 1 + , 0, 1À) Case

No abstract

The enhancement of the vibrational sum-frequency generation (SFG) signal from molecules adsorbed on metallic nanopillars excited at a resonance frequency matching their localized surface plasmons is reported. The nanopillars stand vertically on a metal surface and possess two plasmon modes that can be selectively excited by either the incident visible laser beam, or the generated SFG beam itself. This nanostructured platform increases the molecular SFG signal of a monolayer by two orders of magnitude. The localization and the geometry of the two plasmon modes enables to probe the molecules adsorbed onto the vertical nanopillar wall, or on the top of it, or on the fl at surface between the pillars, selectively. In practice, this spatial selectivity is set by switching the polarization of the visible and SFG beams at resonance. Owing to the largely improved sensitivity combined with a specifi c spatial selectivity, plasmon-enhanced SFG boosts the versatility of second-order vibrational SFG spectroscopy or microscopy. This makes them promising platforms in the development of analytical molecular devices. 245SFG measurement alignment procedure and the data reproducibility are discussed. Complementary SFG spectra at different wavelengths are also provided.

The vibrational response of p-nitrothiophenol (p-NTP) self-assembled monolayers (SAMs), on Pt and on Au, has been investigated by combining theoretical methods with vibrational spectroscopies. Experimentally, the vibrational signatures have been measured using infrared spectroscopy (IR), polarization-modulation reflection absorption infrared spectroscopy (RAIRS), and with sum frequency generation spectroscopy (SFG). Theoretically, density functional theory calculations (DFT) have predicted the molecular vibrations and have estimated their IR vectors and Raman tensors, necessary to simulate the spectra and therefore to interpret the vibrational signatures as well as to retrieve the molecular orientation. So, a tilt angle (ϑ) of 60°for the molecular 1,4-axis of p-NTP has been directly estimated for both Pt and Au SAMs by the polarizationdependent SFG data. Then, combining these results with RAIRS measurements has allowed to determine rotation angles (ξ) of 56°and 66°for p-NTP SAMs on Pt and Au, respectively.