Aqueous Solvation Dynamics at Metal Oxide Surfaces

Broadband transient absorption (TA) spectroscopy, three-pulse photon echo peak shift (3PEPS), and anisotropy decay measurements were used to study the solvation dynamics in bulk water and interfacial water at ZrO2 surfaces, using Eosin Y as a probe. The 3PEPS results show a multiexponential behavior with two subpicosecond components that are similar in bulk and interfacial water, while a third component of several picoseconds is significantly lengthened at the interface. The bandwidth correlation function from TA spectra exhibits the same behavior, and the TA spectra are well reproduced using the doorway-window picture with the time constants from PEPS. Our results suggest that interfacial water is restricted to a thickness of less than 5 angstrom. Also the high-frequency collective dynamics of water does not seem to be affected by the interface. On the other hand, the increase of the third component may point to a slowing down of diffusional motion at the interface, although other effects, may play a role, which are discussed

Solvation dynamics at liquid/metal-IV oxide interfaces

This thesis presents a comparative study of the ultrafast solvation dynamics of liquids in bulk conditions, and at the interfaces of metal-IV oxides (specifically TiO2 and ZrO2), using ultrafast spectroscopy techniques. In the first part of the thesis we report on results of photon-echo peak-shift and pump super-continuum probe spectroscopy, that provided complementary ways for characterizing the solvation function of a dye, Eosin-Y, in aqueous solution. The solvation dynamics of the dye was studied both, dissolved in an aqueous buffer, and adsorbed to the surface of ZrO2 nanoparticles that were in turn, suspended in an aqueous medium. The results from both techniques indicate that only minor changes between the bulk and interfacial environments, are manifested in the solvation dynamics of Eosin-Y on the fastest timescales of the process (sub-picosecond). On the other hand, on the longest timescales ( >1 ps), we obtained consistent evidences for a slower solvation dynamics at the interface than in bulk water. From these results we concluded, that the presence of the ZrO2 surface affects the dynamics of librational motions and intermolecular vibrations of the hydrogen-bond network of water, only in a very narrow region of no more than 0.5 nm around the metal oxide. The long time behavior, on the other hand, was explained as due to hindered translational diffusion dynamics of the solvent molecules in the proximity of the interface. In order to overcome the limitations inherent to using dyes as probe targets of the solvation dynamics at interfaces, in the second part of this thesis we performed time-resolved Optical Kerr Effect experiments on liquids in the pores of nano-structured films of ZrO2. Three liquids were investigated: acetonitrile (an aprotic polar solvent), cyclohexane (apolar) and water (polar, strongly H-bond networked solvent). In all the cases, significantly slower dynamics were detected in the pores, as compared to the bulk behavior of each solvent. The most significant case was that of water, where the characteristic time constants of the dynamics in the bulk, were increased by a factor of 3 in the film. The results are discussed considering the possible physical models that determine the dynamics of solvation at the interface. In addition to this, using the same experimental setup, we carried out a detailed characterization of the non-resonant nonlinear optical response of nano-structured films of TiO2, by means of Transient Lensing, Cross Phase Modulation measurements, and Optical Kerr Effect spectroscopy. This investigation led us to define future experimental developments, that will allow the extension of our present investigations of solvent dynamics at the surface of ZrO2, to the interfaces of TiO2, of special relevance in several applications

Raman-induced signals in optical Kerr effect measurements of water with elliptically polarized pulses

In 2-color optical Kerr effect measurements of water, addnl. signals can occur in the pulse overlap region if the energy difference between pump and probe pulses approaches the Raman resonance of the OH-stretch vibration. These features can be understood by taking into account the polarization of the pump beam and the chirp of the pulses. We present a simple model that describes well the exptl. results and shows the dependence of these features on the ellipticity of the pump beam polarization. [on SciFinder (R)

On the excitation wavelength dependence of the fluorescence of bacteriorhodopsin

We present a systematic study of the time-integrated fluorescence of native bacteriorhodopsin, as a function of excitation wavelength across the visible absorption band. While the fluorescence max. is unaffected, the spectrum broadens on the high-energy side, with decreasing excitation wavelengths. In addn., there is no mirror image relation between emission and absorption, even for the longest excitation wavelengths. By comparison with the retinal cation in soln., we attribute these observations to vibrationally hot emission, and to the topol. of the excited state surface on the way to isomerization

Modelling of aqueous solvation of eosin Y at the rutile TiO2(110)/water interface

Mol. dynamics simulations at 298 K are used to study an aq. dissolved dye (eosin Y) adsorbed at the TiO2(1 1 0) surface to ext. static and dynamic information of solvation. Differences in the phys. behavior of the dye at the surface and in bulk H2O are compared with recent transient absorption and photon echo expts. within the limits of linear response. The calcd. solvent dynamics features fast contributions, which change very little at the surface as well as a slow component, which slows down by a factor of 2 at the interface, in good agreement with exptl. data. [on SciFinder (R)

Functional electric field changes in photoactivated proteins revealed by ultrafast Stark spectroscopy of the Trp residues

Ultrafast transient absorption spectroscopy of wild-type bacteriorhodopsin (WT bR) and 2 tryptophan mutants (W86F and W182F) is performed with visible light excitation (pump) and UV probe. The aim is to investigate the photoinduced change in the charge distribution with 50-fs time resolution by probing the effects on the tryptophan absorption bands. A systematic, quantitative comparison of the transient absorption of the 3 samples is carried out. The main result is the absence in the W86F mutant of a transient induced absorption band observed at ≈300–310 nm in WT bR and W182F. A simple model describing the dipolar interaction of the retinal moiety with the 2 tryptophan residues of interest allows us to reproduce the dominant features of the transient signals observed in the 3 samples at ultrashort pump-probe delays. In particular, we show that Trp86 undergoes a significant Stark shift induced by the transient retinal dipole moment. The corresponding transient signal can be isolated by direct subtraction of experimental data obtained for WT bR and W86F. It shows an instantaneous rise, followed by a decay over ≈500 fs corresponding to the isomerization time. Interestingly, it does not decay back to zero, thus revealing a change in the local electrostatic environment that remains long after isomerization, in the K intermediate state of the protein cycle. The comparison of WT bR and W86F also leads to a revised interpretation of the overall transient UV absorption of bR

Liquid as template for next generation micro devices

Liquids have fascinated generations of scientists and engineers. Since ancient Greece, the perfect natural shape of liquids has been used to create optical systems. Nowadays, the natural shape of liquid is used in the fabrication of microlens arrays that rely on the melting of glass or photoresist to generate high quality lenses. However shrinkage normally associated to the liquid to solid phase transition will affect the initial shape and quality of the liquid structure. In this contribution, a novel fabrication technique that enables the encapsulation and replication of liquid templates without affecting their natural shape is presented. The SOLID (SOlid on LIquid Deposition) process [1] allows for a transparent solid film to be deposited and grown onto a liquid template (droplet, film, line) in a way that the liquid shapes the overgrowing solid layer. The resulting configuration of the SOLID devices is chemically and mechanically stable and is the base of a huge variety of new micro-nano systems in the field of microfluidics, biomedical devices and micro-optics among others. The SOLID process enables in a one step process the encapsulation of liquid microlenses, fluidics channels, drug reservoir or any naturally driven liquid structure. The phenomenon and solid-liquid interface resulting from the SOLID process is new and still unexploited. The solid layer used for the SOLID process chosen in this paper is poly-para-xylylene called Parylene, a transparent biocompatible polymer with excellent mechanical and chemical properties. Moreover, as the solid layer is growing over a liquid template, atomically smooth surfaces channels can be obtained [2]. The polymerization of Parylene does not exert stress and does not change the shape of the liquid; this latter aspect is particularly interesting for manufacturing naturally driven liquid structures. In this paper the authors explore the limits of this new method by testing different designs of SOLID encapsulated structures and their potential to deliver next generation micro devices