Second harmonic generation phase measurements of Cr(VI) at a buried interface.

Surface second harmonic generation (SHG) phase measurements are carried out on methyl ester-functionalized fused quartz/water interfaces in the presence and absence of Cr(VI). The experiments are performed at pH 7, room temperature, and a chromate concentration of 10(-4) M, which corresponds to monolayer Cr(VI) coverage. The liquid/solid interface is probed from the fused quartz side by directing the probe light field at 580 nm onto the interface together with an SHG reference signal at 290 nm that is collinear with the fundamental. The phase difference of the SHG signals generated at the interface in the presence and absence of Cr(VI) is 85 degrees, which is consistent with SHG resonance enhancement observed for the surface-bound Cr(VI) near 290 nm. The optical arrangement discussed here does not require vacuum technology or optics that compensate for the dispersion of the fundamental and the second harmonic E-fields in the two condensed-phase media. This approach is general and can be applied for analyzing thermodynamic and kinetic data derived from SHG measurements of physical and chemical processes occurring at any buried interface.

DNA single strands tethered to fused quartz/water interfaces studied by second harmonic generation.

Second harmonic generation (SHG) is used to study oligonucleotides at aqueous/solid interfaces for the first time. Detailed thermodynamic state information for interfacial DNA single strands, namely, the interfacial charge density, the interfacial potential, and the change in the interfacial energy density, is obtained. The phosphate groups on the DNA backbone serve as intrinsic labels that do not require DNA modification other than surface attachment. This approach is broadly applicable for the investigation of DNA during its interaction with biological targets, as well as charged biopolymers in general, and has important implications for predicting and controlling macromolecular interactions, improving biodiagnostics, and understanding life processes.

Nonlinear optical studies of the agricultural antibiotic morantel interacting with silica/water interfaces.

It is now known that the untreated discharge of pharmaceuticals into the environment can impact human health and development and lead to increased drug resistance in biota. Here, we present the first direct interface-specific studies that address the mobility of the widely used agricultural antibiotic morantel, which is commonly present in farm runoff. Surface-bound morantel was spectroscopically identified using second harmonic generation (SHG) via a two-photon resonance of its n-pi* transition and in the C-H stretching region by vibrational sum frequency generation (VSFG). Resonantly enhanced SHG adsorption isotherm measurements carried out at the silica/water interface between 6 x 10(-7) and 5 x 10(-5) M morantel concentration result in a free energy of adsorption of 42(2) kJ/mol at pH 7. Finally, real-time tracking of morantel interaction with the silica/water interface shows that the binding events are fully reversible, consistent with its high mobility in silica-rich soil environments. This work thus indicates that pharmaceuticals discharged into the environment can enter the groundwater supply of municipal water systems, at which point their removal is challenging. In addition, the high mobility of morantel in silica-rich soil environments could lead to developing increased interaction of this antibiotic with target organisms, which could respond by increased drug resistance.

Kinetic studies of chromium (VI) binding to carboxylic acid- and methyl ester-functionalized silica/water interfaces important in geochemistry.

Real-time kinetic measurements of hexavalent chromium binding to fused silica surfaces functionalized with carboxylic acid and methyl ester terminal groups are performed in situ using resonantly enhanced surface second harmonic generation (SHG) at pH 7 and 300 K. These functional groups were chosen because of their high abundance in humic acids and related biopolymers. Kinetic measurements are conducted in the submonolayer regime using chromate solution concentrations ranging from 1 x 10(-6) to 2 x 10(-5) M. The adsorption rates were analyzed using the standard Langmuir model and the Frumkin-Fowler-Guggenheim model. The desorption kinetics are consistent with a first-order process. These results indicate that hexavalent chromium mobility in carboxylic acid- and ester-rich soil environments increases with decreasing chromate concentrations. Based on the measured half-lives of the adsorbed Cr(VI) species, remobilization of bound hexavalent chromium due to natural or anthropogenic events that lower the chromate concentration in the aqueous phase can occur within minutes.

Interfacial acidities, charge densities, potentials, and energies of carboxylic acid-functionalized silica/water interfaces determined by second harmonic generation.

Second-harmonic studies were carried out to determine the interfacial acidity, the potential, and the interfacial energy density of an acid-functionalized silica/water interface between pH 2 and 12. The interfacial potential changes over 3 orders of magnitude, from 10-2 mV to several tens of millivolts, and the interfacial energy density changes by 7 orders of magnitude, from less than 10-7 mJ/m2 to several millijoules per square meter. The methodology presented in this study provides quantitative thermodynamic information necessary for understanding and predicting how solvated species interact with functionalized organic adlayers at liquid/solid interfaces over a wide pH range.