Potential-modulation spectroscopy at solid/liquid and liquid/liquid interfaces.

Potential-modulation spectroelectrochemical methods at solid/liquid and liquid/liquid interfaces are reviewed. After a brief summary of the basic features and advantages of the methods, practical applications of potential-modulation spectroscopy are demonstrated using our recent studies of solid/liquid and liquid/liquid interfaces, including reflection measurements for a redox protein on a modified gold electrode and fluorescence measurements for various dyes at a polarized water/1,2-dichloroethane interface. For both interfaces, the use of linearly polarized incident light enabled an estimation of the molecular orientation. The use of a potential-modulated transmission-absorption measurement for an optically transparent electrode with immobilized metal nanoparticles is also described. The ability of potential-modulated fluorescence spectroscopy to clearly elucidate the charge transfer and adsorption mechanisms at liquid/liquid interfaces is highlighted.     

Femtosecond time-resolved electronic sum-frequency generation spectroscopy: a new method to investigate ultrafast dynamics at liquid interfaces

We developed a new surface-selective time-resolved nonlinear spectroscopy, femtosecond time-resolved electronic sum-frequency generation (TR-ESFG) spectroscopy, to investigate ultrafast dynamics of molecules at liquid interfaces. Its advantage over conventional time-resolved second harmonic generation spectroscopy is that it can provide spectral information, which is realized by the multiplex detection of the transient electronic sum-frequency signal using a broadband white light continuum and a multichannel detector. We studied the photochemical dynamics of rhodamine 800 (R800) at the air/water interface with the TR-ESFG spectroscopy, and discussed the ultrafast dynamics of the molecule as thoroughly as we do for the bulk molecules with conventional transient absorption spectroscopy. We found that the relaxation dynamics of photoexcited R800 at the air/water interface exhibited three characteristic time constants of 0.32 ps, 6.4 ps, and 0.85 ns. The 0.32 ps time constant was ascribed to the lifetime of dimeric R800 in the lowest excited singlet (S(1)) state (S(1) dimer) that is directly generated by photoexcitation. The S(1) dimer dissociates to a monomer in the S(1) state (S(1) monomer) and a monomer in the ground state with this time constant. This lifetime of the S(1) dimer was ten times shorter than the corresponding lifetime in a bulk aqueous solution. The 6.4 ps and 0.85 ns components were ascribed to the decay of the S(1) monomer (as well as the recovery of the dimer in the ground state). For the 6.4 ps time constant, there is no corresponding component in the dynamics in bulk water, and it is ascribed to an interface-specific deactivation process. The 0.85 ns time constant was ascribed to the intrinsic lifetime of the S(1) monomer at the air/water interface, which is almost the same as the lifetime in bulk water. The present study clearly shows the feasibility and high potential of the TR-ESFG spectroscopy to investigate ultrafast dynamics at the interface.     

Colloidal shape controlled by molecular adsorption at liquid crystal interfaces

The ability to control finely the structure of materials remains a central issue in colloidal science. Due to their elastic properties, liquid crystals (LC) are increasingly used to organize matter at the micrometer scale in soft composites. Textures and shapes of LC droplets are currently controlled by the competition between elasticity and anchoring, hydrodynamic flows, or external fields. Molecules adsorbed specifically at LC interfaces are known to orient LC molecules (anchoring effect), but other induced effects have been poorly explored. Using specifically designed amphitropic surfactants, we demonstrate that large-shape transformations can be achieved in direct LC/water emulsions. In particular, we focus on unusual nematic filaments formed from spherical droplets. These results suggest new approaches to design new soft LC composite materials through the adsorption of molecules at liquid crystal interfaces.     

Nanosecond rapid freezing of liquid benzene under shock compression studied by time-resolved coherent anti-Stokes Raman spectroscopy

Nanosecond time-resolved coherent anti-Stokes Raman spectroscopy is used to investigate the shock-induced liquid-solid phase transition and crystallization of liquid benzene. Temporal evolution of the Raman shift of the ring-breathing and C-H stretching modes is investigated. A metastable supercompressed state and a liquid-solid phase transition are observed under shock compression. Time-resolved Raman spectra reveal that the liquid state is initially a metastable state and rapidly transforms to the solid state within 25 ns under shock compression at 4.2 GPa.     

Conformational dynamics of bistable RNAs studied by time-resolved NMR spectroscopy

The structural transition between two alternate conformations of bistable RNAs has been characterized by time-resolved NMR spectroscopy. The mechanism, kinetics, and thermodynamics underlying the global structural transition of bistable RNAs were delineated. Both bistable RNA conformations and a partial unstructured RNA of identical sequence could be trapped using photolabile protecting groups. This trapping allowed for an investigation of the initial folding from an unfolded RNA to one of the preferred conformations of the bistable RNA and of the structural transitions involved. Folding of the secondary structure elements occurs rapidly, while the global structural transition of the bistable RNA occurs on a time scale of minutes and shows marked temperature dependence. Comparison of these results with bistable systems previously investigated leads to the prediction of activation enthalpies (DeltaH++) associated with global structural transitions in RNA.     

Hydrogen-bonding molecular ruler surfactants as probes of specific solvation at liquid/liquid interfaces

Resonance-enhanced, second harmonic generation (SHG) is used to measure the electronic structure of solutes sensitive to specific solvation adsorbed to liquid/liquid and liquid/solid interfaces. Here, specific solvation refers to solvent–solute interactions that are directional and localized. N-methyl-p-methoxyaniline (NMMA) is a solute whose first allowed electronic transition wavelength remains almost constant (∼315 nm) in non-hydrogen-bonding solvents regardless of solvent polarity. However, in hydrogen-bond-accepting solvents such as dimethylsulfoxide, NMMA’s absorbance shifts to longer wavelengths (320 nm), whereas in hydrogen-bond-donating solvents (e.g., water), the absorbance shifts to shorter wavelengths (∼300 nm). SHG experiments show that at alkane/silica interfaces, surface silanol groups serve as moderately strong hydrogen-bond donors as evidenced by NMMA’s absorbance of 307 nm. At the carbon tetrachloride/water interface, NMMA absorbance also shifts to slightly shorter wavelengths (298 nm) implying that water molecules at this liquid/liquid interface are donating strong hydrogen bonds to the adsorbed NMMA solutes. In contrast, experiments using newly developed molecular ruler surfactants with NMMA as a model hydrophobic solute and a hydrophilic, cationic headgroup imply that, as NMMA migrates across an aqueous/alkane interface, it carries with it water that functions as a hydrogen-bond-accepting partner.     

Lactone cleavage reaction kinetics of rhodamine dye at liquid/liquid interfaces studied by micro-two-phase sheath flow/two-photon excitation fluorescence microscopy

Two-photon excitation fluorescence microscopy was combined with the two-phase microflow system in order to measure the fast interfacial reaction rate at liquid/liquid interfaces. The lactone cleavage kinetics of octadecylrhodamine B (C(18)RB) at the toluene/water and heptane/water interfaces was studied by this new method. The organic solution containing the nonfluorescent lactone of C(18)RB was made to flow as an inner flow with an aqueous outer sheath flow. The diameter of the inner flow was <20 microm. A focused fundamental beam of a Ti:sapphire pulse laser of 780 nm was irradiated to the interface, and emitted fluorescence from the fluorescent product was detected by a charge-coupled device (CCD) camera or a streakscope. The increase in the concentration of the fluorescent form of C(18)RB was measured along the interface of the inner flow of the toluene/water and heptane/water systems for 80 micros just after the contact of two phases. The analysis made by the time-dependent Langmuir adsorption model with the aid of the digital simulation method gave the cleavage reaction rate constants of the lactone form of C(18)RB at the liquid/liquid interfaces.     

Amorphous iron-(hydr) oxide networks at liquid/vapor interfaces: In situ X-ray scattering and spectroscopy studies

Surface sensitive X-ray reflectivity (XR), fluorescence (XF), and grazing incidence X-ray diffraction (GIXD) experiments were conducted to determine the accumulation of ferric iron Fe (III) or ferrous iron Fe (II) under dihexadecyl phosphate (DHDP) or arachidic acid (AA) Langmuir monolayers at liquid/vapor interfaces. Analysis of the X-ray reflectivity and fluorescence data of monolayers on the aqueous subphases containing FeCl(3) indicates remarkably high levels of surface-bound Fe (III) in number of Fe(3+) ions per molecule (DHDP or AA) that exceed the amount necessary to neutralize a hypothetically completely deprotonated monolayer (DHDP or AA). These results suggest that nano-scale iron (hydr) oxide complexes (oxides, hydroxides or oxyhydroxides) bind to the headgroups and effectively overcompensate the maximum possible charges at the interface. The lack of evidence of in-plane ordering in GIXD measurements and strong effects on the surface-pressure versus molecular area isotherms indicate that an amorphous network of iron (hydr) oxide complexes contiguous to the headgroups is formed. Similar experiments with FeCl(2) generally resulted with the oxidation of Fe (II)-Fe (III) which consequently leads to ferric Fe (III) complexes binding albeit with less iron at the interface. Controlling the oxidation of Fe (II) changes the nature and amount of binding significantly. The implications to biomineralization of iron (hydr) oxides are briefly discussed.     

Conformation changes of albumin in its interaction with physiologically active compounds as studied by quasi-elastic light scattering spectroscopy and ultrasonic method

The effect of pH and binding of ten physiologically active compounds (PAC) on conformational organization of human serum albumin (HSA) in aqueous solutions has been studied using two different methods. The hydrodynamic sizes of albumin globule and its subunits were obtained from diffusion coefficients measured by quasi-elastic light scattering. The adiabatic volume compressibility of albumin was evaluated from ultrasonic velocity and density measurements. It was found, that albumin globule has the most compact configuration (hydrodynamic diameter 59-62 A and molar compressibility 5.6 m(3) Pa(-1) mol(-1)) at physiological pH 7.4. The changes in pH, both increase to 8.0 and decrease to 5.4, result in the growth of globule size to 68-81 A. An additional peak corresponding to diffusion of the separate albumin subdomains (hydrodynamic diameter 32-40 A) is observed in the light scattering spectra and globule compressibility decrease to 4.5-2.8 m(3) Pa(-1) mol(-1) at the acidic shift of pH. The additional peak was not displayed and globule compressibility increased to 6.4 m(3) Pa(-1) mol(-1) at the basic shift of pH. The acidic changes were attributed to unfolded and elastic conformation of albumin with a high motility of separate subdomains, whilst the basic changes correspond to a closed compressible configuration of albumin molecule. The interaction with propranolol, clonidine, phenylephrine, carbachol and tripeptide fMLP, which hinder adenylate cyclase (AdC) and activate Ca-polyphosphoinisitide (Ca-PPI) signaling system of a cell, initiates structural rearrangements similar to acidic transitions of albumin. Isoproterenol, yohimbine, diphenhydramine, chlorpromazine and atropine, which activate AdC and hinder Ca-PPI, cause conformational changes of albumin similar to basic transitions. The results obtained are consistent with the idea of structural and pharmacological similarity among the drugs inside the marked groups.     

Ion-association aggregation of an anionic porphyrin at the liquid/liquid interface studied by second harmonic generation spectroscopy

Interfacial ion-association adsorption and aggregation of a water-soluble porphyrin, tetrakis(4-sulfonatephenyl)porphyrin (TPPS) diacid, which was promoted by a cationic cetyltrimethylammonium ion (CTA(+)), was studied by second harmonic generation (SHG) spectroscopy. Comparing the interfacial SH spectrum with the transmission absorption spectrum of TPPS in the aqueous solution elucidated the aggregation behavior of TPPS at the heptane/water interface. The time-dependent SHG spectra for TPPS aggregation and the interfacial tension lowering in the presence of CTA(+) were discussed on the basis of an electrostatic adsorption model. Then, it was suggested that TPPS diacid was highly concentrated by the ion-association with CTA(+) at the interface, which was the intermediate state before the final aggregated state.     

Interlayer water molecules in vanadium pentoxide hydrate. 8. Dynamic properties by quasi-elastic neutron scattering

The dynamics of water molecules in the layered vanadium pentoxide hydrate, V(2)O(5).nH(2)O, were studied by quasi-elastic neutron scattering (QENS) measurements. Heterogeneity of the dynamic properties was confirmed by alpha-relaxation model analysis. Translational diffusion of monolayer and double-layer water molecules is by site-to-site diffusion and is reduced relative to that of bulk water. Water molecules lose their mobility markedly and solidify with decreasing temperature. However, mobile water remains at 253 K. Rotational diffusion coefficients are unaffected by confinement and are very similar to the bulk values determined at temperatures in the range 253-298 K. The dynamic speed characterized by QENS is much faster than that expected from the data determined by deuterium NMR (DNMR) measurements at low temperatures.     

Reaction of diphenyldiazomethane with singlet oxygen studied by time-resolved IR spectroscopy

The mechanism of the reaction of diphenyldiazomethane 4a with singlet oxygen has been investigated by nanosecond time-resolved UV-vis (LFP) and IR (step-scan) spectroscopy. The experiments were performed with fullerene (C60) as photosensitizer for the generation of (1)O2 in nonpolar solvents (toluene and CCl4). The UV-vis experiments allowed us to monitor the formation of benzophenone O-oxide 1a, while in the IR experiments the bleaching of 4a and the formation of benzophenone 7a and N2O was observed. The kinetic data were evaluated using Monte Carlo simulation and DFT calculations. These methods allow us to present a consistent mechanistic scheme for the reaction of 4a with (1)O2 and to explain why the elusive dioxadiazole 5a as key intermediate is not directly observed.     

Evaporation of ethanol and ethanol-water mixtures studied by time-resolved infrared spectroscopy

The knowledge of the physics and the chemistry behind the evaporation of solvents is very important for the development of several technologies, especially in the fabrication of thin films from liquid phase and the organization of nanostructures by evaporation-induced self-assembly. Ethanol, in particular, is one of the most common solvents in sol-gel and evaporation-induced self-assembly processing of thin films, and a detailed understanding of its role during these processes is of fundamental importance. Rapid scan time-resolved infrared spectroscopy has been applied to study in situ the evaporation of ethanol and ethanol-water droplets on a ZnSe substrate. Whereas the evaporation rate of ethanol remains constant during the process, water is adsorbed by the ethanol droplet from the external environment and evaporates in three stages that are characterized by different evaporation rates. The adsorption and evaporation process of water in an ethanol droplet has been observed to follow a complex behavior: due to this reason, it has been analyzed by two-dimensional infrared correlation. Three different components in the water bending band have been resolved.     

Photodissociation of thioglycolic acid studied by femtosecond time-resolved transient absorption spectroscopy

Steady-state and time-resolved spectroscopies were employed to study the photodissociation of both the neutral (HS-CH(2)-COOH) and doubly deprotonated ((-)S-CH(2)-COO(-)) forms of thioglycolic acid (TGA), a common surface-passivating ligand used in the aqueous synthesis and organization of semiconducting nanostructures. Room temperature UV-Vis absorption spectroscopy indicated strong absorption by the S(1) and S(2) excited states at 250 nm and 185 nm, respectively. The spectrum also contained a weaker absorption band that extended to approximately 550 nm, which was assigned to the π(CO) (*)←n(O) transition. Femtosecond time-resolved transient absorption spectroscopy was performed on TGA using 400 nm excitation and a white-light continuum probe to provide the temporally and spectrally resolved data. Both forms of TGA underwent a photoinduced dissociation from the excited state to form an α-thiol-substituted acyl radical (α-TAR, S-CH(2)-CO(●)). For the acidic form of TGA, radical formation occurred with an apparent time constant of 60 ± 5 fs; subsequent unimolecular decay took 400 ± 60 fs. Similar kinetics were observed for the deprotonated form of TGA (70 ± 10 fs radical formation; 420 ± 40 fs decay). The production of the α-TAR was corroborated by the observation of its characteristic optical absorption. Time-resolved data indicated that the photoinduced dissociation of TGA via cleavage of the C-OH bond occurred rapidly (≤100 fs). The prevalence of TGA in aqueous semiconducting nanoparticles makes its absorption in the visible spectral region and subsequent dissociation key to understanding the behavior of nanoscale systems.     

Dynamic adsorption and characterization of phospholipid and mixed phospholipid/protein layers at liquid/liquid interfaces

Drop profile analysis tensiometry is applied to study the adsorption dynamics of phospholipids, proteins and phospholipid/protein mixtures at liquid/liquid interfaces. Measurements of the dynamic interfacial tension of phospholipid layers give information on the adsorption mechanism and the structure of the adsorption layer. The equilibrium and dynamic adsorption of pure protein solutions, i.e. human serum album (HSA), beta-lactoglobulin (beta-LG), beta-casein (beta-CA), can be explained well by the thermodynamic model of Frumkin and the diffusion-controlled adsorption theory. The adsorption behavior from mixed phospholipid/protein solutions was also investigated in terms of dynamic interfacial tensions. Interestingly, a "skin-like" folded film of pure protein or phospholipid/protein complex layers can be observed at curved surfaces at the water/oil interfaces. The addition of phospholipids accelerates the formation of the folded structure at the drop surface through co-adsorption of proteins.     

Molecular interactions of proteins and peptides at interfaces studied by sum frequency generation vibrational spectroscopy

Interfacial peptides and proteins are critical in many biological processes and thus are of interest to various research fields. To study these processes, surface sensitive techniques are required to completely describe different interfacial interactions intrinsic to many complicated processes. Sum frequency generation (SFG) spectroscopy has been developed into a powerful tool to investigate these interactions and mechanisms of a variety of interfacial peptides and proteins. It has been shown that SFG has intrinsic surface sensitivity and the ability to acquire conformation, orientation, and ordering information about these systems. This paper reviews recent studies on peptide/protein-substrate interactions, peptide/protein-membrane interactions, and protein complexes at interfaces and demonstrates the ability of SFG on unveiling the molecular pictures of complicated interfacial biological processes.     

Analysis of a quasi-elastic laser scattering spectrum using the maximum entropy method.

We have applied the maximum entropy method (MEM) to the analysis of quasi-elastic laser scattering (QELS) spectra and have established a technique for determining capillary wave frequencies with a higher time resolution than that of the conventional procedure. Although the QELS method has an advantage in time resolution over mechanical methods, it requires the averaging of at least 20-100 power spectra for determining capillary wave frequencies. We find that the MEM analysis markedly improves the S/N ratio of the power spectra, and that averaging the spectra is not necessary for determining the capillary wave frequency, i.e., it can be estimated from one power spectrum. The time resolution of the QELS attains the theoretical limit by using MEM analysis.     

Dynamic evolution of light-induced orientation of dye-doped liquid crystals in liquid phase studied by time-resolved optically heterodyned optical Kerr effect technique

Transient evolution of light-induced molecular reorientation both in 1-amino-anthraquinone (1AAQ) dye and azobenzene doped isotropic liquid crystals (LCs) were studied by time-resolved optically heterodyned optical Kerr effect method. The results give clear direct experimental proof that under short pulse (30 ps) excitation, LC molecules orientate toward the excitation light polarization direction in the 1AAQ/LC system. However, LC molecular orientation becomes orthogonal to the light polarization in azobenzene/LC system. Time-resolved excited-state absorption of 1AAQ and wavelength dependent excited-state absorption of azobenzene were also observed and their contributions to the early dynamics of the third order optical responses of the two systems were confirmed. A simplified two-level mean-field theory was derived to reveal the intensity dependence of orientation enhancement factor in azobenzene/LC system considering the photoisomerization process.     

Nonradiative deactivation of excited hemicyanines studied with submolecular spatial resolution by time-resolved surface second harmonic generation at liquid-liquid interfaces

The excited-state dynamics of aminostilbazolium dyes is known to be dominated by nonradiative deactivation through large-amplitude motion. In order to identify the coordinate(s) responsible for this process, the excited-state lifetimes of two dialkylaminostyryl-methylpyridinium iodides have been measured at liquid-liquid interfaces using time-resolved surface second harmonic generation. We found that the decay time of the excited-states of both compounds was increasing with the viscosity of the apolar phase, consisting of n-alkanes of varying length, but was unaffected by that of the polar phase, made of water/glycerol mixtures. This indicates that the nonradiative deactivation is associated with the twist of the dialkylaniline group, which is located in the apolar part of the molecule.