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Raman mapping was used to determine the lipid distribution inside human mesenchymal stem cells during induced adipogenesis by monitoring C-H stretching bands of the fats inside the lipid droplets. By incorporating deuterated glucose into the cell culture medium during induction it was possible to distinguish whether or not downstream metabolites, either in lipid droplets or in the cytoplasm, had been formed before or after the adipogenic cascade, because C-D stretching bands are 1/√2 shifted compared to the C-H bands. Thus, metabolites formed after the initiation of the process displayed both C-H and C-D stretching bands and so were forming during induced adipogenesis rather than prior to it. With the ability to distinguish small putative lipid drops formed by the induction of adipogenesis from those pre-formed in the cell, it was possible to analyze spectral changes occurring in the droplets at the earliest stages of adipogenesis. There were two key findings. Firstly, Raman spectra of lipid droplets evolved over time, suggesting that their composition at the early stages was not the same as at the later stages. Secondly, it was apparent that the proportion of unsaturated fats in droplets was higher at early stages than it was at later stages, suggesting that unsaturated fats arrive in the droplets faster than saturated ones.

Electron transfer dynamics in the oxide layers of the working electrodes in both dye-sensitized solar cells and photocatalysts greatly influences their performance. A proper understanding of the distribution of surface and bulk energy states on/in these oxide layers can provide insights into the associated electron transfer processes. Metal ions like Iron (Fe), Copper (Cu) and Tin (Sn) doped onto TiO2 have shown enhanced photoactivity in these processes. In this work, the structural, optical and transient properties of Fe, Cu and Sn doped TiO2 nanocrystalline powders have been investigated and compared using EDX, Raman spectroscopy, X-ray Photoelectron spectroscopy (XPS), and Transient Absorption spectroscopy (TAS). Surface free energy states distributions were probed using Electrochemical Impedance spectroscopy (EIS) on Dye Sensitized Solar Cells (DSSC) based on the doped TiO2 photoanodes. Raman and XPS Ti2p3/2 peak shifts and broadening showed that the concentration of defects were in the order: Cu doped TiO2 > Fe doped TiO2 > Sn doped TiO2 > pure TiO2. Nanosecond laser flash photolysis of Fe and Cu doped TiO2 indicated slower transient decay kinetics than that of Sn doped TiO2 or pure TiO2. A broad absorption peak and fast transient decay at 430 nm for Fe doped TiO2 was ascribed to an increase in surface hole concentration resulting in poor current density in the Fe doped TiO2 photoanodes relative to pure TiO2, Sn or Cu doped anodes. The charge transfer capacitance and the calculated electron lifetimes correlated well with the trend in current density of the photoanodes (Sn > Cu > pure TiO2). The poor performance of Fe doped cells is due to faster recombination of injected electrons with surface holes while those of Sn and Cu were more influenced by the concentration of their bulk defects. These results demonstrate that the choice of selected metal ions doping onto TiO2 for a desired application should take into consideration the influence of bulk defect concentrations, the energy state distribution and the electron transfer properties in/on the oxide photoanodes.

We investigate the role of oxygen vacancies in photochromism of molybdenum oxide amorphous films. MoO3−x films with a wide range of initial stoichiometries are deposited using R.F. unbalanced magnetron sputtering. The evolution of visible light absorption in conjunction with Raman spectra for these films is studied in detail during the course of UV-irradiation to correlate the color change to the Mo6+ to Mo5+ conversion. For films which have an initially more complete stoichiometry, and hence lower oxygen vacancy concentration, the color change fully correlates with conversion of Mo6+ to Mo5+. This behavior is consistent with the group of models of photochromism of transition metal oxides which disregard the presence of oxygen vacancies in the films. However, coloration of the films with initially greater deviation from complete stoichiometry, and hence higher oxygen vacancy concentration, is not accompanied by any significant conversion of Mo6+ to Mo5+. This behavior is consistent with the group of models of photochromism of transition metal oxides which rely on the presence of oxygen vacancies. Overall, the reported results demonstrate the importance of the initial stoichiometry in the photochromism of MoO3−x amorphous films, particularly in the initial stages of UV-irradiation.

We report the effect of deposition conditions on the intrinsic color and photochromic properties of amorphous MoO3 thin films (a-films) deposited by R.F. unbalanced magnetron sputtering. Optical transmission spectroscopy was used to measure optical properties of the films. The conversion between Mo6+ and Mo5+ for as-deposited and UV irradiated films was characterized using XPS. Raman spectroscopy was used to confirm that the results of XPS were consistent with the bulk of the films. It is shown that absorption coefficient of as-deposited films increases with Mo5+ content. The temporal evolution of absorption coefficients for all films under UV light irradiation is measured using optical transmission spectroscopy. The largest change in absorption was observed for the film with the highest initial Mo5+ content. The temporal evolution of absorption coefficients for all the films shows initial fast rise within first minute of irradiation. XPS and Raman results show that for all films Mo5+ content increases as a result of UV irradiation except for the film with the highest initial Mo5+ content, for which the Mo5+ content decreases relative to Mo6+ despite the fact that the absorption of the film continues to rise. Further understanding of this mechanism is important since it will lead to enhanced photochromism and extend the photo-colorability of the films beyond the point at which the conversion of Mo6+ to Mo5+ is saturated.

Transient events in thin films and interfaces have been studied using the technique of time resolved pump-probe nanosecond Brewster angle microscopy. For p-polarized light there is a minimum reflectivity at the Brewster angle. When the interface is viewed with light that is both incident and reflected at the Brewster angle the resulting image is dark. Subsequent small changes is refractive index will then cause an increase in the reflectivity in affected regions providing high contrast images of an altered interface with a dark background level. This is the basis of Brewster angle microscopy. In the present work two synchronized nanosecond pulsed lasers were used in the pump-probe configuration in order to induce changes at an air-liquid interface and to monitor the resulting morphology changes over a range of time delays from nanosecond to milliseconds after laser-excitation. This method can be used to observe morphological changes in phase altering thin-films and molecular monolayers. Further it can be used to obtain information about transient photochemistry even in optically thin materials and nano-films. In the current work the method is used to monitor laser induced processes in phase separating binary liquid mixtures as well as in monolayers of photo-responsive amphiphilic molecules derived from spiropyran on water. The two systems are quite different but provide valuable comparisons.

A technique is described to observe transient events in thin interfacial films and monolayers. p-polarized light has minimum reflectivity at the Brewster angle. When an interface is viewed with light that is both incident and reflected at the Brewster angle the resulting image is dark. However, small refractive index changes can increase the reflectivity producing a high-contrast image of an altered interface with a dark background level. Using this phenomenon, with imaging optics, photo-induced phase change in Langmuir films was monitored. Two synchronized 5-ns pulsed lasers were used in the pump–probe configuration to induce changes at an air–liquid interface and to monitor the resulting morphology changes at selected time delays after photo-excitation. The photo-responsive layers were made from photochromic spiropyrans, having long aliphatic chain substituents. When irradiated with UV light the closed form of the molecule converts to a more planar ring open merocyanine form. This results in the layers changing their morphology in order to accommodate the new molecular form.

Self-assembled monolayers of 11-(3',3'-dimethyl-6,8-dinitrospiro[chromene-2,2'-indoline]-1'-yl) undecanoic acid (amphiphilic spiropyran) at the air-water interface are studied using Brewster angle reflectometry. Transient kinetics of the spiropyran to merocyanine conversion are recorded in a UV-pump, VIS-probe configuration. By varying the probe wavelength using an optical parametric oscillator, we are able to reconstruct absorption spectra of intermediate states with a time-resolution of 10 nanoseconds, limited by the temporal convolution of the two laser pulses. After UV irradiation, spiropyran converts to merocyanine in two stages. The first occurs within a timescale of several tens of nanoseconds and is heavily convoluted with the system response time, whereas the second stage occurs over a few hundred nanoseconds. During the rise time there is a small red shift in the transient absorption spectrum of ~20 nm. We assign the red shift and the slower kinetics to the isomerization of a merocyanine isomer cis about the central methine bond to those that are trans about the same bond.

Inspired by the amphiphilicity of graphene oxide (GO), the surface of water is used as a template for the assembly of a GO film. Methacrylate-functionalized GO sheets can be cross-linked instantaneously at the water-air interface to form a highly wrinkled membrane spreading over an extended area. The multiple covalent linkages amongst the GO sheets enhances the in-plane stiffness of the film compared to noncovalently bonded GO films. The highly convoluted GO membrane can be used in two applications: the promoting of spontaneous stem-cell differentiation towards bone cell lineage without any chemical inducers, and for supercapacitor electrodes. Due to reduced van der Waals restacking, capacitance values up to 211 F g(-1) can be obtained. The scalable and inexpensive nature of this assembly route enables the engineering of membranes for applications in regenerative medicine and energy-storage devices where secondary structures like nanotopography and porosity are important performance enhancers.

Brewster angle reflectometry has been developed as a tool for determining the absorbance and refractive index changes in molecular monolayers containing spiropyran. The method is sensitive to changes in both the real and imaginary parts of the refractive index in the monolayers. It was used to monitor the conversion of spiropyran to merocyanine and the reversal of this reaction when the molecules were immobilised on quartz using silane coupling. An analytical solution of Fresnel formula allowed the transient reflectometry data to be converted into transient absorption information. Absorbances of transients as low as approximately 10(-6) were possible using the current apparatus with a single laser pulse transient measurement. It was found that spiropyran photoconverted to merocyanine with an efficiency of approximately 0.1. The photochemical reversion of converted merocyanine to spiropyran occurred with efficiencies of 0.03-0.2 and this was probably site dependent. It was found that the thermal conversion from merocyanine to spiropyran was slow and even after 10 min there was no significant thermal reversion. This measurement was possible because the real part of the refractive index of the monolayer could be monitored with time using an off-resonance probe at a wavelength where the merocyanine did not absorb light meaning that the probe did not photobleach the sample. Thus our method also provides a non-intrusive method for probing changes in molecules in thin films.

Raman mapping was used to determine the distribution of fats inside of adipocytes and human mesenchymal stem cells during adipogenesis. This proved to be a successful way of mapping the lipid distribution within the cells because of the strong C-H stretching bands of the fats inside the lipid droplets. Furthermore using deuterated glucose in the cell culture medium it was possible to distinguish any fats in droplets or in the cytoplasm that were formed before and after adipogenesis was initiated, because those formed after displayed both C-H and C-D stretching bands. The distributions of the initial metabolites of glucose and the resulting fats in the lipid drops could also be mapped in this way.

Bovine serum albumin (BSA) was space selectively immobilized onto a quartz substrate using photo-reactions initiated by pulsed laser irradiation of benzophenone derivatives tethered by silane coupling to the quartz. The excitation of the benzophenone with 266 nm light yields the pi-pi* triplet state with a quantum yield of 1. This triplet state then reacts with BSA in buffer solution in contact with the modified quartz. The triplet state may form a ketyl radical by reaction with the solution or it may abstract a proton directly from the BSA becoming covalently tethered to the surface. The entire deposition process was monitored in real time, in-situ using a Brewster angle microscope (BAM).

A nanosecond pulsed IR (1.9 microm) laser rapidly heated water, in an open vessel, to temperatures well below the boiling point. The subsequent dynamics of volume expansion were monitored using time-resolved interferometry in order to measure the increase in the water level in the heated area. The water expanded at less than the speed of sound, taking just less than 100 ns to increase its height by approximately 500 nm at surface temperature jumps of 20 K. The initial expansion was followed by an apparent contraction and then a re-expansion. The first expansion phase occurred more slowly than the timescale for bulk H-bond re-structuring of the water, as determined from vibrational bands in the Raman spectra, and represents the limit to the rate at which the overpressure caused by sudden heating can be released. The second phase of the expansion was caused by hydrodynamic effects and is accompanied by morphological changes resulting in light scattering as well as droplet spallation.

A new apparatus for nanosecond-time-resolved Brewster angle reflectometry is described that can be used to measure transient angle-resolved reflectivity changes in thin films and monolayers in a single pulsed laser shot. In order to achieve this, a cylindrical lens is placed in the probe beam path replacing the goniometer that is usually used for angular scanning in other systems. Using two synchronized nanosecond pulsed lasers in pump-probe configuration it is possible to measure the kinetics of photoinduced conformational changes by altering the delay between pump and probe pulses. The system was used to observe nanosecond time-resolved photodynamics in a spiropyran monolayer at the air-water interface. After UV excitation the spiropyran converted to its merocyanine form in two stages. The first stage occurred with a timescale close to the instrument time resolution (tens of nanoseconds) whereas the second stage occurred over a few hundred nanoseconds.

Two-dimensional condensation was initiated in a self-assembled mixed monolayer of spiropyran and octadecanol by a nanosecond laser pulse. The dynamics of the process were monitored using nanosecond pump-probe Brewster angle microscopy. Domain growth followed a power law with a growth exponent of 0.47 at a velocity approaching 20 ms(-1). This represents a limit for the rate of longitudinal signaling of pressure waves through a self-assembled amphiphilic layer at an interface and adds to our understanding of signal transmission rates in biomimetic membranes where morphological change in one region can be signaled to a more remote region.

A method is presented that enables the determination of transient absorption in Langmuir films made from a monolayer of spiropyran on water. This is achievable even though the optical pathlength of such a monolayer is < 10-9 m. The approach is to monitor reflectivity changes close to the Brewster angle, where the background of reflected light is minimized. This is the key to the sensitivity of the method. Relatively speaking the small changes in reflectivity due to changes in both real and imaginary parts of the refractive index are easier to observe with the intrinsically low backgrounds at the Brewster angle. Notably using Fresnel equations we can show that the real and imaginary parts of the refractive index can be independently assessed for ultrathin films and monolayers using the approach presented.