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 noticed that the refractive index of the crystalline quartz was not used properly. Correction of this erratum is made in the revised manuscript, which causes changes in values of and in text (as depicted in the resubmitted manuscript with changes remarked) and the scale of y axes in Fig. 2A, 2B, 4B, and Fig. S6, but does not affect any discussion nor conclusion of the manuscript. 2 ABSTRACTWe develop and verify a phase-sensitive second harmonic generation spectroscopic scheme that allows for direct determination of the absolute surface charge density and surface potential of a water interface without need of prior interfacial information. The method relies on selective probing of surface-field-induced reorientation order of water molecules in the electrical double layer and is, hence, independent of the interfacial molecular bonding structure. Application of this technique to a mixed surfactant monolayer on water suggests the manifest effect of the chain-chain interactions among the monolayer on adsorption of soluble ionic surfactants. We also deduce the third-order nonlinear susceptibility of bulk water and prove its applicability to analysis of charges of various water interfaces. In addition, we show that the Debye-Hückle theory should be avoided in the spectroscopic analysis for its potential significant error, as evidenced experimentally and theoretically. TOC GRAPHIC
Gold nanoparticles (AuNPs) chemically grafted on substrates are widely used as sensors due to their plasmonic properties. The efficiency and robustness of such sensors strongly depend on the molecular sublayer structure, which influences the distribution of AuNPs, and therefore the plasmonic properties of the layer. Few spectroscopic tools are able to sense the grafting layer both before and after particle deposition. Here, we use sumfrequency generation (SFG) spectroscopy to deeply investigate both the grafting layer and the immobilized AuNPs. We combine SFG with reflectance UV−visible spectroscopy and scanning electron microscopy (SEM) for 14 nm diameter AuNPs, dispersed on modified silicon surfaces with either amine or mixed amine/thiol terminated layers. SFG spectra show the specific vibrational fingerprint of each supporting layer through the amplitudes of methylene and methyl vibration modes and prove the presence of unreacted ethoxy groups from (3-aminopropyl) triethoxysilane. We establish a linear evolution of the absorbance amplitudes with AuNP surface coverage, a relationship valid up to the aggregation limit of 10 11 AuNPs·cm −2 . In the same way, SFG amplitudes follow a quadratic dependence with the UV−vis absorbance amplitudes, showing the close correlation between nonlinear and linear optical properties. In addition, the optical properties of the AuNP layers are stable for several months (plasmon position and damping) despite their storage in ambient air and long exposure to visible laser light.
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