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ABSTRACT: The adsorption of water at solvent/silica interfaces was studied using IR-visible sum frequency generation (SFG) vibrational spectroscopy. We discovered a water layer between toluene and silica with no detectable free OHs. The water layer without free OHs showed resistance against further adsorption of water molecules. This "water-resistant" water surface was very stable at room temperature and did not become a regular water layer with free OH over a period of 12 h in water-saturated toluene. However, this special structure of water was not observed at heptane/silica interfaces, at which free OHs were observed. The study indicates that interactions between solvents and water molecules can significantly change the interfacial water properties.
Langmuir 11/2010; 26(21):16397-400. · 4.19 Impact Factor
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ABSTRACT: We report applications of two-photon excitation fluorescence (2PEF) microscopy with subdiffraction-limit resolution for green-fluorescent-protein-tagged cell imaging. The microscope integrates 2PEF microscopy and stimulated emission depletion microscopy in one microscope that has the benefits of both techniques: intrinsic three-dimensional resolution, confined photobleaching, and subdiffraction-limit resolution. The subdiffraction-limit resolution was demonstrated by resolving green-fluorescent-protein-tagged caveolar vesicles located within a distance shorter than the diffraction limit of a regular 2PEF microscope, which is approximately 250 nm even with the best optics. The full width at half-maximum of the effective point-spread function for the 2PEF microscope was estimated to be approximately 54 nm.
Biophysical Journal 12/2009; 97(12):3224-8. · 3.65 Impact Factor
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ABSTRACT: The competitive adsorption of toluene and n-alkanes at binary solution/silica interfaces was studied at room temperature using IR-visible sum frequency generation vibrational spectroscopy. The surface coverage of toluene for toluene−pentane, toluene−heptane, and toluene−tetradecane mixtures was measured over the complete mole fraction range from 0 to 1. The competitive adsorption process was reversible, and the toluene coverage only depended on the bulk mole fraction, not on the history of the system. The estimated molar adsorption free energy of toluene is 3.4 ± 0.3, 1.8 ± 0.3, and 0.84 ± 0.3 kJ/mol higher than pentane, heptane, and tetradecane, respectively. Overall, toluene competes favorably on silica, and the molar adsorption free energy of alkanes increases as the chain length increases. It is consistent with the observed SFG spectra, indicating that the alkanes lie flat on the silica surface.
11/2009;
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ABSTRACT: Surface-enhanced IR-visible sum frequency generation (SFG) was studied using Ag and Au films over nanospheres (AgFON and AuFON), which provided large area of reproducible nanoscale structures with well-defined morphology. SFG vibrational spectra for a self-assembled monolayer of octadecanethiol were investigated on spheres with diameters ranging from 300 nm to 620 nm. With an input wavelength of 532 nm, a sphere diameter near 360 nm produced the maximum enhancement, which was consistent with the localized surface-plasmon resonant wavelength identified by minimum reflectivity. The measured second-order susceptibility for the asymmetric CH3 stretch mode of octadecanethiol was enhanced by up to 27 times on AgFON, which corresponded to an SFG enhancement factor of approximately 730. The SFG enhancement factor for AuFON was a quarter of that from AgFON. The SFG enhancement factor for the symmetric CH3 mode was about 10 times smaller than that for the asymmetric CH3 mode. This difference can be explained by the highly directional electrical-field parallel to the surface, created at the junction between nanospheres. Polarization-dependent studies also indicated that excitations with the electrical field parallel to the surface was the main contribution to the observed surface enhancements, and the electrical field perpendicular to the surface offered little enhancement.
Physical Chemistry Chemical Physics 06/2009; 11(18):3436-42. · 3.57 Impact Factor
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ABSTRACT: Structures of water molecules at water/silica interfaces, in the presence of alkali chloride, were investigated using infrared−visible sum frequency vibrational spectroscopy. Significant perturbations of the interfacial water structure were observed on silica surfaces with the NaCl concentration as low as 1 × 10−4 M. The cations, which interact with the silica surface via electrostatic interaction, play key roles in perturbing the hydrogen-bond network of water molecules at the water/silica interface. This cation effect becomes saturated at concentrations around 10−2 to 10−1 M, where the sum frequency generation peaks at 3200 and 3400 cm−1 decrease by 75%. Different alkali cation species (Li+, Na+, and K+) produce different magnitudes of perturbation, with K+ > Li+ > Na+. This order can be explained by considering the effective ionic radii of the hydrated cations and the electrostatic interactions between the hydrated cations and silica surfaces. The interfacial water structure associated with the 3200 cm−1 band is more vulnerable to the cation perturbation, suggesting that the more ordered water structure on silica is likely associated with the vincinal silanol groups, which create a higher local surface electrical field on silica.
05/2009;
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Advanced Materials 05/2008; 20(12):2280 - 2284. · 13.88 Impact Factor
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ABSTRACT: We present the first measurement of the buried surface electronic states of the conjugated polymer poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) using two-dimensional (2D) IR-visible sum frequency generation (SFG). SFG electronic spectra were obtained by scanning the frequencies of both incident visible and IR beams and used to study the surface electronic transitions associated with the C-C stretching of benzene rings located at the backbone of MEH-PPV. Because of the surface confinement effects, the polymer conformation, and consequently the electronic states, at the film/solid interface are different from those of the bulk film. Theoretical analysis based on an oligomer model was employed to estimate the conjugation-length distributions of MEH-PPV at interfaces. Assuming a Gaussian conjugation-length distribution, it was found that the conjugation-length distribution at the MEH-PPV/solid interface was centered at 5.8 monomer units. Similar surface effects were also observed at the air/polymer interface, with a shorter average conjugation length of 5.1 monomer units.
The Journal of Physical Chemistry B 03/2008; 112(8):2315-8. · 3.70 Impact Factor
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ABSTRACT: Surface structure relaxations caused by temperature changes at the free surface of poly(methyl methacrylate) were studied using IR-visible sum-frequency generation (SFG). A polarization-rotating technique was introduced to enhance the sensitivity of SFG for monitoring the surface structure relaxations during a cooling process. A new surface structure relaxation was observed at 67 degrees C. This temperature does not match any known structure relaxation temperatures for the bulk and is 40 degrees C below the bulk glass transition temperature. As expected for a free-surface phenomenon, the surface relaxation temperature was found to be independent of film thickness in the range of 0.1-0.5 microm.
The Journal of Physical Chemistry B 02/2008; 112(3):694-7. · 3.70 Impact Factor
