Tryptophan is one of the few amino acids that is intrinsically photoluminescent. This is because its side chain consists of indole. Indole’s photoluminescence has both fluorescence (emits for nanoseconds) and phosphorescence (emits for microseconds). Fluorescence emission comes from a singlet to singlet transition, while phosphorescence from a forbidden triplet to singlet transition. Taking advantage of tryptophan’s intrinsic emission, we can use it as a label-free probe for protein dynamics. For some of these dynamics, such as myosin binding to actin, the fluorescence lifetime of nanoseconds is too fast to monitor changes. The phosphorescence lifetime is much better suited to monitor these changes of large biomolecule interactions. Before any binding studies are developed, we have characterized the basic properties of indole’s phosphorescent properties. We began by embedding indole (as well as 5 – bromoindole) in a polymer matrix (PVA) to immobilize and thus increase the phosphorescence at room temperature. We discovered that using a longer wavelength of excitation (405 nm instead of 290 nm) we excite directly from the singlet state to the triplet state of indole, a typically forbidden process. This populates the triplet state without any transitions to the singlet state. This allows the polarization of phosphorescence emission to be preserved, and anisotropy measurements can be used to monitor biomolecular processes.

Driving through the disk of the Milky Way galaxy resides a gaseous stream that is associated with the Magellanic Clouds galaxies called the Leading Arm. The Milky Way will capture this stream of gas torn from the Magellanic Clouds to supply our galaxy with material to make future stars and planets. We study this gas cloud using Hubble Space Telescope observations to determine the complex's physical properties, such as the motion, temperature, ionization fraction, density, and total mass of the gas. With this observational data, we run computer simulations created with the Cloudy software to constrain these properties better. Measured ionization ratios and column densities from the Hubble observations act as inputs for our models. Studying these properties will better depict the processes that affect the stream of gas falling onto our galaxy's disk.

The first stars in the Universe, Pop III stars, formed out of the primordial hydrogen and helium sometime during the first billion years of cosmic time. Their formation ended the Cosmic Dark Ages. Despite their critical role in kick starting the formation of all “heavy” elements, including the carbon in our bodies and the oxygen we breathe, we do not know how massive these first stars were, and when and how the era of the first stars ended. While Pop III stars are too faint for a direct detection, their deaths are potentially visible by James Webb Space Telescope (JWST): a subset of Pop III stars end their lives as Pair Instability Supernova (PISN), explosions in which the entire star blows itself apart [and fling], flinging “heavy” elements into the Universe. However, what will the detection, or non-detection of a PISN tell us about the nature of the first stars? To answer this question, we need to fully explore the range of mass distribution of Population III stars to determine the physics which governed Cosmic Dawn. We present results from a new model which treats the distribution of Population III masses as free parameters. In this work, we attempt to determine whether the masses of the first stars can be constrained given various possible observational results from JWST.

The debate surrounding the fundamental mechanisms behind the antibacterial action of ZnO has led to increased interest in the impact of surface interactions on this behavior. In this regard, the impact of the different polar vs. non-polar surfaces of the anisotropic wurtzite ZnO crystal lattice are of particular interest. For this purpose, we developed a hydrothermal growth method that allows us to produce microscale ZnO crystals of tunable morphology with varying relative abundances of surfaces with desired polarities. The micron scale of the obtained crystals is critical to avoid internalization by bacteria as a means to isolate effects related to surface interactions. Simultaneously, at this scale, the high surface-to-volume ratio leads surface interactions to dominate, resulting in surface and near-surface defect states to become highly influential on this behavior. Photoluminescence is a powerful, non-destructive tool for characterizing the electronic structure of a material allowing us to observe the nature of the defect states present in our samples. Photoluminescence measurements were made over a range of temperatures for both predominantly polar and non-polar morphologies. Results of these investigations have allowed us to describe the electronic structure of these microcrystals. We show that both the nature and density of surface defects states are significantly impacted by the relative abundance of polar and non-polar surfaces.

Graphene quantum dots (GQDs) are unique derivatives of graphene that show promise in multiple biomedical applications as biosensors, bioimaging agents, and drug/gene delivery vehicles. Their ease in functionalization, biocompatibility, and intrinsic fluorescence enable those modalities. However, GQDs lack deep tissue magnetic resonance imaging (MRI) capabilities desirable for diagnostics. Considering that the drawbacks of MRI contrast agent toxicity are still poorly addressed, we develop novel Mn2+ or Gd3+ doped nitrogen-containing graphene quantum dots (NGQDs) to equip the GQDs with MRI capabilities and at the same time render contrast agents biocompatible. Water-soluble biocompatible Mn-NGQDs and Gd-NGQDs synthesized via single-step microwave-assisted scalable hydrothermal reaction enable dual MRI and fluorescence modalities. These quasi-spherical 3.9-6.6 nm average-sized structures possess highly crystalline graphitic lattice structure with 0.24 and 0.53 atomic % for Mn2+ and Gd3+ doping. This structure ensures high in vitro biocompatibility of up to 1.3 mg ml-1 and 1.5 mg ml-1 for Mn-NGQDs and Gd-NGQDs, respectively, and effective internalization in HEK-293 cells traced by intrinsic NGQD fluorescence. As MRI contrast agents with considerably low Gd and Mn content, Mn-NGQDs exhibit substantial transverse/longitudinal relaxivity (r 2/r 1) ratios of 11.190, showing potential as dual-mode longitudinal or transverse relaxation time (T 1 or T 2) contrast agents, while Gd-NGQDs possess r 2/r 1 of 1.148 with high r 1 of 9.546 mM-1 s-1 compared to commercial contrast agents, suggesting their potential as T1 contrast agents. Compared to other nanoplatforms, these novel Mn2+ and Gd3+ doped NGQDs not only provide scalable biocompatible alternatives as T1/T2 and T1 contrast agents but also enable in vitro intrinsic fluorescence imaging.

Star clusters have been incredibly useful tools for studying the history of the Milky Way because they allow us to determine relative ages based on their chemical abundances. However, most stars are not in clusters, and current methods used to determine ages for individual stars produce substantial uncertainties. A new age method enabled by the precise photometry data of the NASA Kepler satellite is asteroseismology. Asteroseismology allows us to probe the internal structure of stars that are affected by age and composition. This research aims to calibrate the relationships between age, chemical abundances, and asteroseismology by analyzing data of stars in star clusters, which provide an independent measure of the stars' ages. This project aims to expand upon the currently used age and chemical abundance range and triple the number of open star clusters used to calibrate the asteroseismic age-mass-chemical abundance relation. We have combined asteroseismology data for stars in clusters within the Kepler 2 campaign fields with uniformly determined follow-up spectroscopic abundances from observations from the MMT.

Micro- and nano-scale ZnO particles are known to inhibit the growth of bacteria. Though this phenomenon has been vigorously studied, the fundamental mechanisms driving this action remain unknown. Mechanisms proposed by other studies include: the production of reactive oxide species, release of zinc ions, damage to the cell wall due to interactions with ZnO surfaces, and the inhibition of enzymes. ZnO surface defects serve as reaction sites for the processes driving these bactericidal interactions. Additionally, through MIC assays, we found antibacterial action of microparticles to be comparable to that of nanoscale particles. This confirms that antibacterial action of ZnO is rooted in surface-surface interactions between bacteria and ZnO. Therefore, our studies focus on ZnO surface charge dynamics and surface defects using surface photovoltage methods. Surface photovoltage experiments were performed on commercial grade ZnO nanoparticles and hydrothermally grown ZnO microcrystals in conjunction with antibacterial assays to elucidate the surface and near-surface charge dynamics associated with antibacterial processes of the ZnO surfaces.

With the advent of graphene, there has been an interest in utilizing this material and its derivative, graphene oxide (GO) for novel applications in nanodevices such as bio and gas sensors, solid-state supercapacitors and solar cells. Although GO exhibits lower conductivity and structural stability, it possesses an energy band gap that enables fluorescence emission in the visible/near infrared leading to a plethora of optoelectronic applications. In order to allow fine-tuning of its optical properties in the device geometry, new physical techniques are required that, unlike existing chemical approaches, yield substantial alteration of GO structure. Such a desired new technique is one that is electronically controlled and leads to reversible changes in GO optoelectronic properties. In this work, we for the first time investigate the methods to controllably alter the optical response of GO with the electric field and provide theoretical modeling of the electric field-induced changes. Field-dependent GO emission is studied in bulk GO/polyvinylpyrrolidone films with up to 6% reversible decrease under 1.6 V µm−1 electric fields. On an individual flake level, a more substantial over 50% quenching is achieved for select GO flakes in a polymeric matrix between interdigitated microelectrodes subject to two orders of magnitude higher fields. This effect is modeled on a single exciton level by utilizing Wentzel, Kremer, and Brillouin approximation for electron escape from the exciton potential well. In an aqueous suspension at low fields, GO flakes exhibit electrophoretic migration, indicating a degree of charge separation and a possibility of manipulating GO materials on a single-flake level to assemble electric field-controlled microelectronics. As a result of this work, we suggest the potential of varying the optical and electronic properties of GO via the electric field for the advancement and control over its optoelectronic device applications.

The Smith Cloud is a fast-travelling gas cloud that is currently hurtling towards the Milky Way galaxy at about 170,000 miles per hour. If the cloud is able to reach the Galactic plane, it has the potential to supply the Milky Way with at least 2 million suns worth of gas. This gas can be used to make new stars, planets, and even meatballs. In this project, we use observations taken with the Hubble Space Telescope and the Green Bank Telescope. We fit our spectroscopic observations with line profiles to quantify the amount of gas and its motions. We then take measurements of the low- and high-ionization species of two small cloud fragments that lie adjacent to the main body of this large gas cloud. This enables us to constrain the processes that impact the Smith Cloud as it traverses the Galactic halo. Our investigation could provide great insight on how galaxies capture the gas that they use to form stars and planets.

Intrinsic and extrinsic motivation satisfy biological needs or desires. Behavior that is intrinsically motivated is not followed by any apparent reward, except for the behavior itself. Behavior that is extrinsically motivated is followed a separate, observable reward. The overjustification hypothesis states that after engaging in behavior as a means to an extrinsic reward, there will be a reduction in one’s intrinsic motivation to engage the behavior. The current study observed whether the overjusitification effect occurs in rats when using lever pressing as a measure of intrinsic motivation. For all rats, intrinsic motivation was measured in Phase 1 by the number of lever presses made by each rat in the absence of any observable reward. In Phase 2, one group continued to lever press without reward (Control), while the other group received a sucrose pellet (extrinsic reward) for each lever press. Lever pressing in the absence of reward (intrinsically motivated) was again measured in Phase 3. The extrinsic reward group emitted more lever pressing in the sessions at the start of Phase 3. Lever pressing decreased thereafter, but stabilized at a higher rate than the control group. The groups were then switched before Phase 2 was repeated. The overjustification effect was not observed in our study, but rather, reinforcement protected the response from habituation.