acase@tulane.edu

acase@tulane.edu

Electron transfer is one of the fundamental process occurring in many chemical reactions. Electron transfer process has been under intensive study for many applications, for example artificial photosynthesis, where electrons from photo-excited chromophore molecules are harnessed to produce solar fuels in various forms. Transition metal complexes, such as ruthenium and rhenium complexes, play an important role in the continuing development of artificial photosynthetic devices. The electron transfer process in chromophores involving transition metal complexes often occurs on an ultrafast time scale from sub-ps to ns. To resolve such dynamics, ultrafast spectroscopic techniques are required. A variety of ultrafast techniques, such as time-resolved infrared spectroscopy and multi-pulse transient absorption spectroscopy, were used in this study to unravel the excited state electron transfer dynamics in a series of Re(I) complexes. Transition metal complexes often feature excited states that involve only partial electron transfer between the electron donating and accepting ligands, even for ligands with strong electron donating and accepting properties. It is often difficult to design a compact complex feature a full electron transfer excited state. Therefore, part of the work presented in this thesis was dedicated to the study of the electron transfer extent in the excited states of a series of [Re(N,N)(CO)3L]+ compounds, where N,N stands for electron accepting and L stands for electron donating ligands. By carefully designing the structure and redox properties of both the electron donor and acceptor, we demonstrated that essentially a full-electron charge transfer excited state can be prepared, while the designed Re(I) complex is still compact. To further extend the understanding of the electron transfer in transition metal complexes, modulation of the electron transfer rate in a compact Re(I) complex was studied. By perturbing the electron transfer process with a femtosecond mid-IR pulse, we showed that a 28% increase of the electron transfer rate was achieved. This study demonstrated the possibility of using a small energy mid-IR quanta to change the energy conversion process in a chromophore. Vibrational energy transfer in molecules is another important process in nature. Detailed understanding of the vibrational energy transfer on a molecule level is fundamentally important and essential for the development of molecular optical devices. It was recently discovered that the transport of vibrational energy in molecules can be fast and efficient due to its ballistic character. To understand the mechanism of the ballistic energy transport, experiments with several series of oligomers were performed using a relaxation-assisted two-dimensional infrared method. The energy transport speed was found to be dependent on transport initiation method and the transport pathways for different cases of initiation were identified. Detailed analysis on the chain band structure, group velocity and vibrational relaxation dynamics is presented. / acase@tulane.edu

The goal of this research is to study the neutralization of sulfuric acid by engine oils, and more specifically study how the presence of different oil additives affects the acid-neutralizing performance of engine oils by using capillary videomicroscopy. Nowadays the formulation of engine oils has been changing due to a trend of different regulations around the world that seek to diminish the emission of atmospheric pollution from all types of vehicles driven by internal combustion engines. In the particular case of large marine ships powered by low-speed two-stroke diesel engines, pollutant emissions are high given that the marine fuel they use can contain up to 4.5 wt. % of sulfur. But this sulfur content cap in marine fuel is bound to diminish dramatically during the ongoing decades due to regulations and therefore, the industry is coming up with new engine oil formulations accordingly as to comply with these changes. Here a technique called capillary videomicroscopy was used to study new changes to engine oil formulations. The reaction and dispersion of a sulfuric acid micro-droplet into formulations of marine cylinder lubricants (MCL) was studied by microscopically observing and measuring the shrinking of a micropipette-produced droplet in real time. It was found that MCL formulations having a base number (BN) of 40 had an acid-neutralizing performance comparable to those of having BN 70. On the other hand, the addition of fatty alcohols as final additives to MCL formulations so as to boost the MCL’s acid neutralization performance was found to be slightly effective although phase separation due to alcohol insolubility in MCL at room temperatures and other resilient phases formed upon reaction can be detrimental, hence the use of fatty alcohols for boosting any MCL formulation cannot be generalized and should be studied for each formulation. In the case of passenger car motor oils (PCMO), substitution of traditional oil additives by new sulfur-free additive species is driven by the need to prevent the catalytic converter's poisoning by eliminating any sulfur present in the exhaust gas. The effect of the polymeric dispersant on the acid neutralization performance was also studied. The formation of clear, thin and resilient shells surrounding sulfuric acid droplets upon reaction with some MCLs was noticed to be a detrimental aspect towards their acid neutralization performance and more importantly, due to the formation of potential precursors for cylinder liner engine deposits. Finally it is shown a modification of the capillary videomicroscopy technique that allowed long-term monitoring of the fate of microscopic particles while reacting or dissolving under flow, by suspending them using a balance between buoyancy and drag force from creep flow. / acase@tulane.edu

Silicon nanoparticles are attractive candidates for biological, photovoltaic and energy storage applications due to their size dependent optoelectronic properties. These include tunable light emission, high brightness, and stability against photo-bleaching relative to organic dyes (see Chapter 1). The preparation and characterization of silicon nanoparticle based hybrid nanomaterials and their relevance to photovoltaic and biological applications are described. The surface-passivated silicon nanoparticles were produced in one step from the reactive high-energy ball milling (RHEBM) of silicon wafers with various organic ligands. The surface structure and optical properties of the passivated silicon nanoparticles were systematically characterized. Fast approaches for purifying and at the same time size separating the silicon nanoparticles using a gravity GPC column were developed. The hydrodynamic diameter and size distribution of these size-separated silicon nanoparticles were determined using GPC and Diffusion Ordered NMR Spectroscopy (DOSY) as fast, reliable alternative approaches to TEM. Water soluble silicon nanoparticles were synthesized by grafting PEG polymers onto functionalized silicon nanoparticles with distal alkyne or azide moieties. The surface-functionalized silicon nanoparticles were produced from the reactive high-energy ball milling (RHEBM) of silicon wafers with a mixture of either 5-chloro-1-pentyne in 1-pentyne or 1,7 octadiyne in 1-hexyne to afford air and water stable chloroalkyl or alkynyl terminated nanoparticles, respectively. Nanoparticles with the ω-chloroalkyl substituents were easily converted to ω-azidoalkyl groups through the reaction of the silicon nanoparticles with sodium azide in DMF. The azido terminated nanoparticles were then grafted with monoalkynyl-PEG polymers using a copper catalyzed alkyne-azide cycloaddition (CuAAC) reaction to afford core-shell silicon nanoparticles with a covalently attached PEG shell. Covalently linked silicon nanoparticle clusters were synthesized via the CuAAC “click” reaction of functional silicon nanoparticles with α,ω-functional PEG polymers of various lengths. Dynamic light scattering studies show that the flexible globular nanoparticle arrays undergo a solvent dependent change in volume (ethanol> dichloromethane> toluene) similar in behavior to hydrogel nanocomposites. A novel light-harvesting complex and artificial photosynthetic material based on silicon nanoparticles was designed and synthesized. Silicon nanoparticles were used as nanoscaffolds for organizing the porphyrins to form light-harvesting complexes thereby enhancing the light absorption of the system. The energy transfer from silicon nanoparticles to porphyrin acceptors was investigated by both steady-state and time-resolved fluorescence spectroscopy. The energy transfer efficiency depended on the donor-acceptor ratio and the distance between the nanoparticle and the porphyrin ring. The addition of C60 resulted in the formation of silicon nanoparticle-porphyrin-fullerene nanoclusters which led to charge separation upon irradiation of the porphyrin ring. The electron-transfer process between the porphyrin and fullerene was investigated by femto-second transient absorption spectroscopy. Finally, the water soluble silicon nanoparticles were used as nanocarriers in photodynamic therapeutic application, in which can selectively deliver porphyrins into human embryonic kidney 293T (HEK293T) cells. In particular, the PEGylated alkynyl-porphyrins were conjugated onto the azido-terminated silicon nanoparticles via a CuAAC “click” reaction. The resultant PEGylated porphyrin grafted silicon nanoparticles have diameters around 13.5 ± 3.8 nm. The cryo-TEM and conventional TEM analysis proved that the PEGylated porphyrin grafted silicon nanoparticle could form the micelle-like structures at higher concentration in water via self-assembly. The UV-Vis absorption analysis demonstrated that the silicon nanoparticle could reduce the porphyrin aggregation in water which can reduce the photophysical activity of porphyrin. In addition, the nanoparticle complex was capable of producing singlet oxygen when the porphyrin units were excited by light. The cell studies demonstrated that the silicon nanoparticle could deliver the porphyrin drugs into HEK293T cells and accumulate in the mitochondria where the porphyrin could serve as an efficient photosensitizer to kill the cells via mitochondrial apoptotic pathway. / acase@tulane.edu

The migration of living cells plays an important role in immune response, hemostasis, cancer progression, delivery of nutrients, and microfluidic technologies such cell separation/enrichment and flow cytometry. Using three-dimensional computational algorithm for multiphase viscoelastic flow and mass transport, this study is focused on the investigation of the effects of cell size, viscoelasticity, cortical tension, fluid inertia and cell-cell interaction on passive migration and deformation of leukocytes, and active deformation of circulating cells during chemotactic migration in a rectangular microchannel. The results of the passive migration modeling show that there is an almost linear increase in the distance between the wall and the lateral equilibrium position of liquid drops or leukocytes with the particle diameter-to-channel height ratio increased from 0.1 to 0.5. Drops with different bulk viscosities can be efficiently separated if their interfacial tension is low or the flow rate is sufficiently high. The microfluidic technology is well suited for the separation of leukocytes with different cytoplasmic viscosities and relaxation times, but it is much less sensitive to cortical tension. When a series of closely spaced cells with same size are considered, they generally undergo damped oscillation in both lateral and translational directions until they reach equilibrium positions where they become evenly distributed in the flow direction (self-assembly phenomenon). For a series of cells with different sizes, bigger cells could collide repeatedly with smaller ones and enter the other side of the channel (above or below the centerline). For a series of cells with different deformability, more deformable cells upon impact with less deformable cells move to an equilibrium position closer to the centerline. The results of our study provide better understanding of cell margination in bloodstream and cell separation/enrichment in microfluidic devices. The simulation data on active migration of cells show the formation of a finger- or lamellipodium-like projection of the cell membrane towards the chemoattractant source and indicate that lowering the cortical tension facilitates cell protrusion. / acase@tulane.edu

Although it is suggested that a dysfunctional stress response system may be associated with posttraumatic stress disorder (PTSD) the neurobiological underpinnings are not well established, especially in children. There is also limited research on how treatment for PTSD may impact associated physiology. Respiratory sinus arrhythmia (RSA) is a reliable measure of parasympathetic stress reactivity, and both resting RSA and RSA reactivity are physiological indicators related to children’s emotion functioning and regulation. The present study examined if pretreatment resting RSA levels predicted RSA reactivity at pretreatment and the trajectory of resting RSA, RSA reactivity, resting heart period (HP) and HP reactivity after Cognitive Behavioral Therapy (CBT). Forty-nine children who experienced at least 1 traumatic event and presented with PTSD symptoms were assessed for psychological measures, RSA and HP at pretreatment, post treatment and a 3-month follow up. At pretreatment, lower resting RSA was associated with increased RSA withdrawal. Analysis with repeated measures mixed models indicated that lower resting pretreatment RSA and lower RSA withdrawal increased during CBT, and individuals with higher resting RSA and RSA withdrawal decreased during CBT, so that those at the extreme ends of higher and lower indices converged in the middle by the end of treatment. These data suggest an optimal moderate range for resting RSA and RSA reactivity. There were also significant gender differences on RSA reactivity after CBT. Lower pretreatment resting RSA predicted lower resting heart period (higher heart rate) across all time points but did not change with CBT. Pretreatment resting RSA did not predict HP reactivity. Post hoc analysis also revealed that PTSD symptoms were significantly reduced after CBT but this change was not associated with pretreatment resting RSA levels. Overall, these results suggest that children may change physiologically after CBT and the direction of the changes may depend on initial resting RSA levels. / acase@tulane.edu

There are few teacher trainings that focus on the development of social and emotional competencies for teachers (SECT) despite a large research base showing its relevance to teaching, learning, and academic success. Research suggests a major reason many current professional development (PD) models are not effective nor show utility to the immediate context of teachers is that they are externally developed and driven without consideration of the teachers’ voice. Further, they are not created to be context-specific. This study employs a qualitative, participatory action research design (PAR) to use the teachers’ perspectives at an urban elementary charter school to create a local model of professional development with the goal of enhancing social and emotional competencies of teachers. Findings reveal a local definition of PD, indicate the importance of considering macrosystemic policies, systems level policies and practices, and interpersonal supports as potential influences on SECT. / acase@tulane.edu

Synthesis and characterization of anisotropic micro- and nanoparticles, either in suspension or localized on a surface, are current areas of intense scientific interest because of their shape-tunable material properties with potential applications in catalysis, microelectronics, data storage and pharmaceutics. Electrochemical deposition represents a facile and versatile route to fabricate anisotropic particles since it offers a high degree of freedom in monitoring and manipulating particle growth processes. The first part of my dissertation presents an additive-mediated electrochemical approach to fabricate anisotropic copper micro- and nanoparticles. This work explores the possibility of using anisotropic copper particles as novel non-noble metal alternatives to expensive anode electrocatalysts (platinum and palladium) used in direct methanol fuel cells (DMFCs). Characterization using SEM, EDS, XRD and TEM confirms the anisotropic morphology and crystal structure of synthesized copper particles. A possible mechanism for anisotropic crystal growth is proposed based on preferential adsorption of additive ions onto selective crystal faces. Methanol oxidation is chosen as model experiment to test the electrocatalytic property of copper particles towards DMFC applications. Characterization using cyclic voltammetry demonstrates shape dependent enhancement in electrocatalytic activity of anisotropic copper particles for methanol oxidation. Chronoamperometry and thermal stability measurements indicate good catalyst stability and durability under steady-state conditions. The second part of my dissertation presents a novel electrochemical fabrication route to generate randomly rough surfaces over large areas. Surface roughness directly affects a material's performance at its functional interface. This work shows that by simple tuning of electrochemical deposition potential for a metal onto an electrode, island nucleation density can be systematically varied. Changes in nucleation density results in generation of thin films with different nanoscale surface roughness. Characterization using AFM illustrates the change in surface topography with applied potential. The fabricated roughness is successfully replicated onto other moldable soft materials (polystyrene and polyurethane) through an embossing and curing step. Roughness gradients were also generated by introducing a controlled mechanical retraction step to the process. Gradient surfaces serve as an effective probing tool for investigating a range of surface parameters in quick time using single experiment, enabling a cost-effective and high-throughput screening method. / acase@tulane.edu

In the United States, children that require mental health supports will most likely receive them in their school. However, there is a serious discrepancy between youth that require and those that receive psychological supports in public schools. This reality is even more pronounced in urban settings and with youth of color. Untreated mental health issues impact a multitude of life domains—only one of which is school achievement. For decades, the public school system at federal and state levels has focused reform efforts on a single developmental competency (i.e., academic skill) to the detriment and underdevelopment of other interdependent competencies (e.g., social-emotional skills [self-awareness, self-management and regulation, social awareness, relationship skills, and responsible decision-making]). Social and emotional skills are considered both a universal prevention against future psychological disturbances as well as core components in the treatment of many psychological disorders. Promoting social-emotional skills, however, requires the purposive establishment of a school climate that fosters such skills. The present study aimed to impact social-emotional factors and school climate by encouraging the participation of local stakeholders (i.e., teachers, parents, administrators, and students themselves) in problem solving and decision-making about how best to ensure school conditions foster the development of social and emotional competence. Analyses focused on both the creation of a plan for school climate reform as well as local perceptions of the facilitating and inhibiting factors of engaging in this process in an urban public school. / acase@tulane.edu