Tandem solar cells, which are cells with two light-absorbing junctions instead of one, can produce more energy than traditional single-junction cells. However, there remain challenges related to efficiency, processing cost, and applicability in cloudy areas. Our goals were to model the performance of ideal tandem cells under cloudy skies, determine if different cell geometries improve this performance, and better understand the reaction that produces copper zinc tin sulfide (CZTS) nanocrystal inks, a low-cost but as-of-yet low-efficiency light absorber material. For modeling, we wrote a Mathematica script that performs detailed balance calculations for standard tandem cells and corrugated-rooftop-shaped tandem cells under various cloudy spectra, taking into account the incidence angle and reflectance. These calculations tell us the efficiency as a function of band gap for each spectrum, from which we extract the optimal band gap of the absorber. For the nanocrystal inks, we performed the solution-phase reaction of copper, zinc, and tin precursors with a hot injection of sulfur to form nanocrystals dispersed in the solvent oleylamine. We analyzed combinations of the precursors, the mixture over the course of the reaction, and the final product using spectroscopy, and we interpreted any change as the formation of complexes or new molecules as the nanocrystals formed. Our modeling results show a noticeable shift in the optimal band gap under cloudy conditions, and if we find higher efficiencies for the corrugated rooftop than for a flat plate under cloudy skies, then we conclude that appropriate changes in cell design should be implemented in cloudy areas. We expect our spectroscopic analysis of the nanocrystal reaction to give us a clearer understanding of the chemistry at the molecular scale, which may point to areas needing improvement that would lead to higher-efficiency CZTS cells.

 A Plasma Pencil Atmospheric Mass Spectrometer (PPAMS) is a new surface analysis device which allows mass spectrometry (MS) to be conducted on samples under normal atmospheric conditions. This enables surface analysis on samples that could previously not be analyzed under the current standard vacuum-based techniques, broadening the field of surface analysis. However, characterization of this method of surface analysis needs to be done in order to maximize the benefits of the PPAMS device and to determine the extent to which the traditional vacuum-based methods can be replaced. This process of characterization involves determining the ionization rate of the plasma pencil probe, performing various experiments while changing parameters (i.e. sample concentration, thickness, and gas type), and comparing results to established methods. Our current investigation is focused on the determination of the ionization rate of the PPAMS device. Initial experiments were performed on polyurethane-based samples dissolved in dimethyl acetamide and spun-coated onto a gold-coated silicon wafer. These samples were then probed by the PPAMS over controlled time intervals. A multivariate statistical method, Principal Components Analysis (PCA), was used to evaluate whether the MS signals obtained varied as a function of exposure time. We hypothesized that the signal would resemble our bare gold background once all of the polyurethane had been ionized. Instead, the MS signal was found to stay constant over a 15 minute exposure time implying a longer exposure time is needed. Current work is focused on evaluating the effect of PPAMS exposure to polymer thickness as a secondary means of evaluating ionization. Future work will center on investigating the changes to the ionization rate under different experimental conditions. The results of our device characterization will provide valuable insight into the limits of the PPAMS device that will enable improved experimental design.

This project’s goal is producing microporous scaffold for tissue growth. These scaffolds are made out of crosslinked 2-hydroxyethyl methacrylate (HEMA), a hydrogel. They are formed by polymerizing the HEMA around polymethyl methacrylate (PMMA) beads that are of a constrained and predetermined size. The PMMA is washed away and we are left with a porous scaffold of precise pore size. These scaffolds are used to encourage tissue growth and reduce inflammation and the foreign body reaction. The purpose of my research is to investigate blood vessel penetration into scaffolds by producing the largest sized scaffolds I can using novel techniques. We are currently limited to tissue scaffolds of a maximum thickness of 1 mm using our current techniques. With a larger thickness our scaffolds could be used for many more applications that are presently unavailable with a thin scaffold. Our methods are varied and include creating multiple mold designs, polymerizing techniques, and chemical washing techniques. Once I can create these larger scaffolds we will test them in vivo to evaluate the foreign body reaction as well as angiogenesis in the region. There is a large range of possible future applications for these scaffolds. They could be tuned to encourage many different types of tissue growth.

In recent years, the unique optical properties supported by noble metal nanostructures have fostered great interest in the field of biosensing. Studies have concluded that incident light can excite collective electron oscillations, called surface plasmon polaritons (SPP), which propagate as waves along planar metal film surfaces. When confined to sub-wavelength physical features, these waves resonate and produce tremendous, localized electric fields that significantly enhance optical phenomenon. Surface-enhanced Raman spectroscopy (SERS) arises from the concentrated electric fields supported by irradiated nanostructured noble metal films. Due to its chemical specificity, and extraordinary Raman signal intensification, SERS has emerged as powerful tool for chemical and biological sensing applications. Our group utilizes lithographic techniques to manufacture arrays of quasi three-dimensional (Q3D) nanostructured patterns with precise geometric dimensions. The physical dimensions of Q3D nanostructures may be tuned such that incident light selectively induces electric field enhancement on the top, continuous gold surface that contains nanoholes, or in the bottom, discontinuous phase comprised of gold nanodisks. As a consequence, Q3D nanostructured arrays may be specifically designed to detect larger analytes (e.g. microorganisms) that contact the top continuous phase, or molecular species capable of passing through nanoholes to contact the nanodisks. In this study, Q3D nanostructures designed for nanodisk enhancement are utilized to detect pesticide molecules at low concentrations in aqueous solution. Results from this research may aid in the development of a microfluidic device for on-site environmental monitoring of river systems. 

The glass transition temperature is a feature of polymer systems that dictates the transition between glassy and molten-like states in amorphous solids. Glassy polymers have many advantageous properties and are highly desirable for commercial and industrial uses. However, the conditions that lead to glass formation are usually well above room temperature, making implementation of most polymer glasses unfeasible. In a recent discovery, ultra-stable poly(methyl methacrylate) (PMMA) glasses were formed using a technique known as matrix assisted pulsed laser evaporation (MAPLE). These results showed a significant decrease in the polymer’s density, while maintaining chemical stability under normal conditions. The exciting experimental discoveries hold great promise for future technological developments. However, there is much to understand about the underling atomic scale features that arise from MAPLE deposited glassy polymer films. To help answer some of these questions, molecular dynamics (MD) computer simulations were used to understand the mechanisms behind the ultra-stable polymer formation. Previous studies include observing the collapse of the polymer PMMA from an extended to a globular-like state under various solvent and temperature conditions. This poster will show how we track formation of the globular state using different order parameters (e.g., radius of gyration) and also show our generalized strategy for quickly building new polymer simulations. Understanding the fundamentals behind ultra-stable glass formation can unlock discoveries in the development and implementation of new materials.

A growing number of applications seek to use electrostatic effects in non-polar media. However, the science behind particle charging in such systems is not fully understood. Previous work has suggested that the presence of surface hydroxyl groups, when combined with certain surfactants, can induce charging on particles. These surfactants also serve to prevent particle aggregation through steric stabilization. However, dispersing these particles typically requires extended sonication or some other dispersive technique. One possible solution for decreasing the dispersion time is to first hydrophobically surface-treat the particles. Unfortunately, this process has the potential to remove the hydroxyl groups necessary for particle charging. Therefore, our group has investigated the effects of surface modification on particle hydrophobicity and chargeability. Silica particles on the micron and submicron scale were surface treated with tri-methoxy(octyl)silane and methoxy(dimethyl)octylsilane in order to create hydrophobic silica particles with varying amounts of hydroxide groups on their surfaces. The resulting surface energy and acid/base characteristics of these particles were then tested using Inverse Gas Chromatography, with the intent of comparing these properties to the particles' chargeability. This was assessed by measuring their electrophoretic mobilities using a Zeta Potential Analyzer. Our results indicated that particles could both be made hydrophobic and chargeable after surface treatments. However, further investigation is required in order to better understand the mechanism behind this charging. If the surface modification of these particles can preserve their chargeability while adding a steric barrier, this process could be used to reduce dispersion time for charged particles in non-polar media.

Methylobacterium extorquens AM1 is a methylotroph that metabolizes reduced C₁ compounds. In particular, methanol metabolism by M. extorquens AM1 requires two cofactors: tetrahydromethanopterin (H₄MPT) and tetrahydrofolate (H₄F). MeOH is first oxidized to formaldehyde, a highly toxic compound. MtdB is involved in formaldehyde oxidation to formate using H₄MPT and both NAD+ and NADP+ as co-substrates. Formate is the branching point for either further oxidation to CO₂ or assimilation. MtdA is a NADP+-specific enzyme that uses H₄F to generate methylene-H₄F, the intermediate that enters the assimilatory serine cycle. Previous in vitro studies have shown that MtdA is capable of using both cofactors and more efficient using H₄MPT. This suggests that it may have a dual role in both the oxidation and assimilation steps of methanol metabolism. However, in vivo, MtdA is unable to fully substitute MtdB as an mtdB mutant is auxotrophic on methanol. Interestingly, when MtdA was over-expressed the same mutant was no longer methanol sensitive. This observation poses two questions: 1) What is the role of MtdA in the H₄MPT pathway? And 2) Why is MtdA activity unable to substitute for MtdB activity in vivo? To answer these questions, we identified that an mtdB mutant is able to grow on methylamine and that this growth is H₄MPT dependent. This suggests that MtdA under this condition is able to substitute for MtdB. Further, we identified that a higher defect on growth on methylamine correlated with increased MtdA expression. This strongly suggests the accumulation of a toxic compound. We have also shown that the different phenotypes of mtdB on methanol and methylamine are due to an alternative pathway for methylamine oxidation. Together, our results suggest that MtdA is a key factor for regulation of carbon flux from the oxidative pathway and the assimilation pathway on one carbon compound metabolism.

Domoic acid (DA) is a toxin that accumulates in edible shellfish through filter feeding and is the causing agent of a serious neurological condition known as amnesic shellfish poisoning. The increasing intensity and frequency of DA-producing algal blooms has drawn more attention to develop fast and effective ways to detect and quantify DA. At Yu Lab, we developed an effective method for the fast detection and quantification of DA using a surface plasmon resonance coupler and disperser (SPRCD) sensor. The SPRCD sensor is an optical instrument based on the coupling of light to electronic oscillations at the interface between a gold surface and a dielectric material. The sensor measures changes in the optical properties of the interface caused by the binding of a specific molecule to the gold surface. By modifying the gold surface such that only the molecule of interest adsorbs, we are able to detect and quantify low concentrations of the toxin in a sample. Using our method, we can detect concentrations of DA as low as 0.1ng/ml and quantify DA in concentrations ranging from 5ng/ml to 10ug/ml.

Organic photovoltaics (OPVs) are a promising low-cost option in developing renewable energies. OPVs are solar cells made from organic semiconductors instead of the more traditionally used inorganic semiconductors such as silicon. OPVs have several advantages including: low cost, ease of processing, physical flexibility, and highly tunable properties. Therefore, if the efficiency of OPVs can be increased, they could become a major, inexpensive renewable energy source. Electron-accepting materials used in OPVs have traditionally been fullerene-based due to their high electron mobility. However, fullerenes contribute little to light harvesting in an OPV as they have negligible absorption in the solar spectrum. Thus, our goal was to develop new electron-accepting materials for OPVs that absorb significantly in the solar spectrum as a means to increase light harvesting and therefore OPV efficiency. A series of novel oligomers with an oligothiophene-naphthalene diimide (NDI) structure was synthesized. All oligomers exhibited absorption in the visible region with band gaps ranging from 1.39 eV to 2.06 eV, and could therefore contribute to light harvesting in an OPV. Solution-processed solar cells using the new oligomers as the electron-acceptors showed power conversion efficiencies up to 1.5%. Overall, these new materials exhibited that non-fullerenes can function as the electron-accepting material in OPVs while contributing to light harvesting. This approach therefore shows promise for increasing OPV efficiency to make OPVs a viable, cost-effective renewable energy source.

The ability of the semiconductor FeS₂, or iron pyrite, to absorb sunlight makes it a cheap and abundant potential material for use in solar cells. Current photovoltaic studies using nanocrystalline pyrite rely on the synthesis of small, uniform crystals. However, pyrite naturally takes different crystal shapes depending on the conditions of its formation. Even the same shapes can have different surface chemistry, which directly impacts crystal growth direction. The mechanisms behind pyrite synthesis are not fully understood and are difficult to discern out of the complexity of real reaction mixtures. Density functional theory (DFT), a method for simplifying and calculating the quantum mechanics behind a physical system, allows for the numerical approximation of pyrite’s material properties. Our group performed DFT calculations of two common pyrite surfaces using the supercell approach. Slabs of pyrite separated by artificial vacuums were allowed to come to energy minimums. We hypothesized that the energetic and electronic properties of real pyrite surfaces would match those of the DFT-optimized supercells. Preliminary results agreed with published experimental data. After ensuring that our computations could reproduce physical properties, we put small molecules that could be found in reaction mixtures into contact with the lowest-energy pyrite surface and optimized using the same methods. The most-favorable interaction sites between pyrite and the reactants needed to make it are key components to the mechanisms of pyrite growth. Once these mechanisms are known, synthesis conditions can be controlled to produce only crystals fit for use in photovoltaic cells.

Self-assembling silver nanoparticles have been of great interest to the scientific community due to their unique optical properties and potential use in areas such as cancer therapy and semiconductors. This research explores using a simple one-pot technique to produce self-assembling silver nanoparticles with controllable geometries based on steric interactions. The core advantage of this method is that it employs a bottom-up approach using commonly available and relatively inexpensive materials. Successive doping of long hydrophilic polyethylene glycol (PEG) chains followed by a short hydrophobic alkane-thiol renders the nanoparticles amphiphilic. Clusters of different sizes self-assemble depending on the concentration of PEG added to the silver dispersion. As PEG concentration increases, the extent of clustering decreases as the long chained PEG molecules create a corona around the silver particles and inhibit functionalization with alkane-thiol. This synthesis method is analyzed with Dynamic Light Scattering (DLS), Small Angle X-ray Scattering (SAXS), and Transmission Electron Microscopy (TEM). While the samples follow the expected clustering size trends, data indicates that monodisperse silver is critical to the reproducibility of the technique.

A strong understanding of the intricacies of blood coagulation is essential for proper treatment of coagulopathies such as hemophilia, hemorrhage, Von Willebrand’s disease, and numerous thrombosis related conditions. The coagulation behavior of blood is directly related to the structure of the supporting protein networks. Fibrinogen is a protein found in human blood that, upon polymerization into cross-linked fibers, forms the scaffold-like support structure of a blood clot. Based on the observation that emergency room patients with traumatic injuries experience a greater extent of hemorrhage and abnormal hemostasis, our group has investigated the effect of acidic oxidation on the structure and mechanical properties of fibrin gels. Purified fibrinogen was oxidized using hypochlorous acid to simulate the acidification of blood during trauma and shock. Rheological, x-ray scattering, and turbidity studies were performed to characterize the structural and mechanical differences between oxidized and normal fibrin gels. Preliminary results from these studies indicate the oxidation causes a delay in fibrin gelation as well as a decrease in the elastic modulus of the fiber network. Additionally, evidence from turbidity studies suggests that oxidation prevents lateral aggregation between protofibrils leading to an overall reduction in fiber diameter.

 The ability to resist nonspecific protein adsorption, low fouling, is an essential surface property necessary for medical implants and biosensors. Mixed-charge surfaces with net neutral uniformly distributed charged amino acid residues have been shown experimentally to highly resistant to nonspecific protein adsorption, and thus have become a new field for biomaterial design. These mixed-charge surfaces have analogues in nature: The interior surfaces, or cavities, of molecular chaperone protein complexes. In addition to assisting hundreds of proteins folding through nonspecific interaction, the cavities usually show transition between a more charged residues state when assisting protein folding and a less charged residues state when actively unfolding proteins. Thus if these interior cavities are well studied, they can be mimicked to create new surface designs. In this study, GroEL-GroES, one of the most well characterized chaperon complexes, is used along with 528 substrate proteins found in Escherichia coli (E.coli). We hypothesized that the interior surface of GroEL-GroES will interact nonspecifically and assist the substrate proteins in folding by lowering their folding free energy. In order to quantitatively calculate these values, we have extended entropic models by introducing a new enthalpy term derived from the knowledge of the residues on the cavity surface. The model not only includes entropic confinement effects but also accounts for energetic interactions between residues found on GroEL-GroES and the substrate protein. Applied to the substrate proteins, our model presents results that match the experimentally observed changes in folding free energies. Furthermore, the optimization of the surface residues distribution of the cavity suggested by this model shows which residues contribute most to stabilized folding. This study provides a better understanding of GroEL-GroES’s ability to nonspecifically assist protein folding. Finally, it also gives insight into the mixed-charge surface design projects where low fouling materials are used as medical devices or sensors.

In recent years, nanoscience has become a major focus: from technologies in solar cell coatings, to developments in biomolecular detection methods. Utilization of unique optical absorption properties of noble metals, such as gold or silver, opens the door for harnessing photons in new and innovative ways. For these reasons, great emphasis has been placed on the synthesis and characterization of nanostructures. The Pozzo Group has recently shown that structured gold clusters can be formed in solution by exploiting the hydrophobic and steric interactions in self-assembling nanoparticles. Hydrophilic polyethylene-glycol (PEG) chains control long-range steric interactions, while hydrophobic alkane-thiol chains induce clustering through short-range attraction. This simple method has the potential to be a cheap and viable option for nanoscale fabrication; starting from the molecular level and building up. This method of nanoclustering yields multi-particle aggregates in solution. Clusters of different geometries and sizes are formed based on the number of consistuent gold particles in each aggregate. It has been shown that the total amount of PEG adhered to the surface of the gold particle impacts the extent of clustering. At lower PEG concentrations, more diverse populations of nanoclusters are observed. In this project, size-exclusion chromatography (SEC) is being utilized for separating cluster populations. A cluster dispersion is pumped through a column that is packed with a porous stationary phase. By size-exclusion, the largest clusters exit the bottom of the column first, while the smaller clusters have a higher retention time as they navigate the pores of the matrix. The viability of SEC for controlled separation of cluster populations is being investigated on different systems, with the goal of isolating populations of similar aggregation types. Population analysis with SEC is simple and can be used to identify the fundamental parameters that lead to polydispersity in cluster size and shape.