It has been long recognized that the surface chemistry of silica, and in particular the type and relative amount of surface bound silanol groups, plays a critical role in many of the properties associated with the material, where a typical example is the discrepant adsorption behavior observed depending on the pretreatment history of the surface. However, in spite of its importance, the direct probing of specific surface silanol groups under water has been hampered by instrumental limitations. Here we make use of vibrational sum frequency spectroscopy (VSFS) to first, identify under water the OH stretch of isolated surface silanols, and second, explore its acid/base behavior and dependence on the surface pretreatment method. The properties of other types of silanol groups (i.e. hydrogen bonded/geminal) are also inferred from the data. The ability to directly probe these functional groups under water represents a crucial step to further improving our understanding of this widely used mineral oxide.

We report on the recent scientific research contribution of non-linear optics based on Sum-Frequency Generation (SFG) spectroscopy as a surface probe of the plasmonic properties of materials. In this review, we present a general introduction to the fundamentals of SFG spectroscopy, a well-established optical surface probe used in various domains of physical chemistry, when applied to plasmonic materials. The interest of using SFG spectroscopy as a complementary tool to surface-enhanced Raman spectroscopy in order to probe the surface chemistry of metallic nanoparticles is illustrated by taking advantage of the optical amplification induced by the coupling to the localized surface plasmon resonance. A short review of the first developments of SFG applications in nanomaterials is presented to span the previous emergent literature on the subject. Afterwards, the emphasis is put on the recent developments and applications of the technique over the five last years in order to illustrate that SFG spectroscopy coupled to plasmonic nanomaterials is now mature enough to be considered a promising research field of non-linear plasmonics.

We experimentally determine the effective nonlinear second-order susceptibility of gold over the visible spectral range. To reach that goal, we probe by vibrational two-color sum-frequency generation spectroscopy the methyl stretching region of a dodecanethiol self-assembled monolayer adsorbed on a gold film. The sum-frequency generation spectra show a remarkable shape reversal when the visible probe wavelength is tuned from 435 to 705 nm. After correcting from Fresnel effects, the methyl stretching vibrations serve as an internal reference, allowing to extract the dispersion of the absolute phase and relative amplitude of the effective nonlinear optical response of gold in the visible range.