Chemistry & Biochemistry Doctoral Work

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Adsorption Kinetics and Dynamics of Small Molecules on Graphene and Graphene oxide
(North Dakota State University, 2018) Sivapragasam, Nilushni
Graphene is an allotrope of carbon composed of sp2 hybridized carbon and arranged into a honeycomb lattice. Graphene is a mechanically strong material (200 times stronger than steel) and has high carrier mobility, high thermal conductivity, and high optical transparency. Owing to these outstanding properties graphene is used in many applications; often graphene is used on a support instead as a free-standing graphene. When graphene is utilized it can adsorb many molecules and this adsorption could be influenced by the support. Furthermore, comparing the adsorption of such molecules on the support alone and on the supported graphene (graphene on the support) could provide details on the transparency of graphene; transparency can be defined as the identical interactions of a molecule on a supported graphene and the respective support. Therefore, this dissertation focused on studying the adsorption kinetics and dynamics of selected molecules (water, benzene, n-alkane, and carbon dioxide) on two different types of graphene: chemical vapor deposited (CVD) and physical vapor deposited (PVD) graphene. In addition, the chemically inert graphene was functionalized with oxygen to produce graphene oxide and the reactivity of graphene oxide on carbon dioxide adsorption was studied. All the experiments were carried at ultrahigh vacuum conditions to ensure an atomically clean environment. The PVD graphene was synthesized on Ru(0001) and was further functionalized with oxygen to produce graphene oxide. The surface characterizations were carried out by various surface analytical techniques: Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). The adsorption kinetics and dynamics were studied by thermal desorption spectroscopy (TDS) and molecular beam scattering techniques, respectively. Transparency of graphene, support effects, and the reactivity of graphene oxide were mapped. The studies clearly showed that the transparency of graphene depends on the polarizability of the molecule and the supports; the supports indeed influenced the adsorption of molecules on graphene. In addition, graphene oxide did not react with CO2 to produce any reaction products but it enhanced the CO2 adsorption.
Atropisomeric Chromophores as Catalysts and Substrates for Asymmetric Light Induced Transformations
(North Dakota State University, 2016) Vallavoju, Nandini
Photochemical transformations hold a unique place as they can provide access to molecules with unique stereochemical and structurally complex scaffolds, thus serving as a complementary approach to thermal transformations. However, asymmetric photoreactions have been under-explored due to the challenges in controlling the nature of the excited state(s). Various elegant strategies have been developed by chemists to address this bottleneck and achieved varying degrees of success. This dissertation describes a novel and unique strategy that employs atropisomeric thioureas as organo-photocatalysts to perform desired chemical transformations. The motivation of the thesis is to develop an alternative strategy that does not depend on energy/electron transfer processes to initiate the photoreactions. This dissertation describes another unique strategy that employs atropisomeric chromophores where axial chirality in the reactant is transferred to point chirality in the photoproduct(s). This research explains about rotamers control in the ground state that allows stereospecific phototransformations in the excited state(s) thus leading to enantioenriched product(s). The chapter 1 introduces the fundamental differences between asymmetric photochemical reactions and conventional thermal method. Further, an overview of various methodologies developed towards asymmetric photochemical transformations are detailed. In chapter 2 and chapter 3, various thiourea-based organo-photocatalysts were developed for enantioselective intramolecular [2+2] photocycloaddition of coumarin derivatives. The atropisomeric thioureas were found to be efficient in promoting the photocycloaddition leading to the corresponding products with high enantioselectivity (77-96% ee) at low catalyst loading (1-10 mol%). The photocatalytic cycle is proposed to proceed by the mechanism of ‘energy sharing’ via the formation of both static and dynamic complexes (exciplex formation), which is promoted by hydrogen bonding. Chapter 4 describes the intermolecular [2+2]-photocycloaddition of coumarin with tetramethylethylene promoted by thiourea catalysts. The photocatalytic cycle of coumarin mediated by thioureas is proposed to proceed via a combination of minimized aggregation, enhanced intersystem crossing and altered excited state lifetime(s), which is promoted by hydrogen bonding. 5 describes the enantiospecific hydrogen abstraction of atropisomeric enone carboxamides leading to spiro-β-lactam photoproduct(s). Divergent photoreactivity was observed based on restricted bond rotation(s) in atropisomeric substrates, when compared to their achiral analogue. The hydrogen abstraction also proceeded efficiently under visible light sensitized irradiation.
Enhanced Sensitivity of Lateral Flow Strip Biosensors Based on Enyzmatic Reaction and Nanomaterials
(North Dakota State University, 2014) Xu, Hui
Ultrasensitive detection for trace amount of proteins plays pivotal role in the diagnosis of specific diseases in clinical application, basic discovery research and the improvement of proteomics. Recently, lateral flow strip biosensor (LFSB) has gained considerable attention for protein analysis. Compared with the traditional immunoassays, LFSB has several advantages: user-friendly format, short assay time (generally several minutes), less interference due to chromatographic separation, a relatively low cost, and no requirements for skilled technicians. This ideal technique is suitable for on-site testing by people who are untrained. Traditional gold nanoparticles (GNPs) based LFSB have been used for qualitative and semiquantitative analysis, the application of GNP-based LFSB is limited by its low sensitivity. In this dissertation, different nanomaterials and advanced detection technologies have been used to enhance the LFSB sensitivities. An ultrasensitive LFSB based on horseradish peroxidase (HRP)/GNP dual labels was developed for qualitative (Yes/No) and quantitative detection of protein. The LFSB signal was enhanced dramatically by introducing the second tracer (enzyme) on the GNP surface. The detection limit of LFSB was 100 times lower than that of GNP-based LFSB. A fluorescent LFSB based on enzyme tracers was developed for sensitive detection of proteins. Alkaline Phosphatase (ALP) was selected as a label to prepare the LFSB. The signal was from the fluorescent emission of the ELF-97 alcohol precipitate which was the product of ALP catalyzed dephosphorylation of ELF-97 phosphate. ALP-conjugated antibody (ALP-Ab) functionalized gold nanoparticles (GNPs) were used as labels for the development of a chemiluminescence-based quantitative LFSB. The use of detection and GNPs as enzyme carriers allowed accurate and sensitive analyte detection. GNP-decorated silica nanorods (GNP-SiNRs) were synthesized and employed as the labels for ultrasensitive detection of proteins on the LFSB. Owing to its biocompatibility and convenient surface modification, SiNRs were used as carriers to load numerous GNPs. The signal of the GNP-SiNR based LFSB was enhanced significantly compared to the GNP-based LFSB since more GNPs were captured through the sandwich-type immunoreactions.
From Nanocontainer to Nanocatalyst: Mechanistic Studies of [2+2] Photodimerization of Coumarin Derivatives within Cucubit[8]URIL
(North Dakota State University, 2013) Pemberton, Barry Charles
Controlling photoreactions remains a formidable challenge to chemists who have developed several approaches with varying degrees of success to achieve high reactivity/selectivity. Following nature's footprints, chemists have explored the use of confined media for controlling photoreactions. This thesis explores catalytic aspects of a water-soluble supramolecule known as a cucurbituril. Cucurbituril is a macrocyclic oligomer with a large enough cavity to sequester two guest molecules of appropriate size. The guest molecules explored in this thesis is coumarins. The model investigation involves host-guest complexes between cucurbit[8]uril (CB[8]) and coumarin to study the [2+2] photodimerization in water through various spectroscopic techniques. Our initial investigations explored the formation of host-guest complexes with coumarin guests that interacted with CB[8] host. This host guest complexation was used to explore and control photochemical reaction and photophysical properties of encapsulated coumarin guest molecules. The host-guest complexation was found to be dependent on the polarity of the coumarin and the volume constraints imparted by the CB[8] cavity. Observational insights from various coumarins provided insights into formation of host-guest complexes with CB[8]. Some coumarins do not form complexes but if they do they can form 1:1 and 1:2 host guest complexes as well as dynamic host-guest complexes (mixture of 1:1 and 1:2 host-guest complexes). Using dynamic host-guest complexes, we explored the use of CB[8] as a photocatalysts. Photodimerization of 6-methylcoumarin was explored as a model system to understand the supramolecular aspects of photocatalysis. The mechanism for photocatalysis was elucidated using various spectroscopic techniques. Both steady state and time-resolved experiments were carried to ascertain the thermodynamic and kinetic aspects of the supramolecular catalytic process. Spectroscopic investigations provided insights into vital role of dynamic complexes in the catalytic cycle as well as the extrusion of photoproduct from the cavity to enable turnover in the system. Thus this investigation provided an opportunity to build an overall picture of a novel supramolecular photocatalytic process in water. This will undoubtedly foster further development in the area of supramolecular photocatalysis.
Fundamental Surface Properties and Gas-Surface Interactions of Two-Dimensional Materials
(North Dakota State University, 2019) Nayakasinghe, Mindika Tilan Abeyrathna
Heterogeneous model catalysis with supported nanomaterials on ultra-thin two-dimensional films has contributed significantly to improve the existing industrial catalytic processes, as well as to discover novel ways to enhance selectivity, specificity, and stability of the catalysts. Silica and zeolites are of particular interest, which has been widely utilized as catalysts and catalytic supports in several industrial processes. However, there are a limited number of surface science studies with zeolites due to the lack of surface analogs. Understanding the fundamental surface properties of silica and zeolites, involving the synthesis of surface analogs of silica and zeolites, characterization, surface modification, and screening for chemical and physical properties connected to the heterogeneous catalysis related applications utilizing advanced ultra-high vacuum-based surface science techniques is the main focus of this dissertation. Catalyst particles should be finely distributed on high surface area supports, in order to have high selectivity and specificity. Particle agglomeration during extreme catalyst operation (reaction) conditions decreases the efficiency of the catalysts over time. One common strategy to address the issue of particle agglomeration is to promote strong catalyst-support interactions. In this study, chemical reactivity of the inert silica was improved by doping with aluminum, which enhanced the polarity of silica (2D-zeolites) and hence the catalyst-support interactions compared to inert silica. Organohalide perovskite thin films are a fascinating class of material, which attract much attention in the recent past as the light harvesting materials in solar cells due to excellent power conversion efficiencies. However, poor thermal, chemical, and long-term stability limit the industrial applications of these organohalide perovskites. Gas-surface interactions on methylammonium lead iodide perovskite thin films were investigated in order to understand the thermal and the chemical degradation mechanisms utilizing UHV-based surface analytical techniques combined with computational calculations. Thermal stability improvement of the perovskite thin films by surface passivation using a protective chemical inhibition layer was successfully investigated experimentally.
Photons in Action: Asymmetric Synthesis and Polymer Degradation
(North Dakota State University, 2016) Raghunathan, Ramya
Video summarizing Ph.D. dissertation for a non-specialist audience.
Synthesis, Photophysics, and Nonlinear Absorption of Platinum (II) and Iridium (III) Complexes
(North Dakota State University, 2013) Li, Zhongjing
Square planar d8 platinum(II) complexes and octahedral d6 iridium(III) complexes were synthesized. Their photophysics were studied in detail. Structure-property relationship was studied by varying the substitution on the ligands or the π-conjugation extent of the ligands. In Chapter 2, bipyridyl platinum(II) bisstilbenylacetylide complexes (2-1 – 2-6) with different auxiliary substituents on the stilbenylacetylide ligands were synthesized. While the substitution of H on the 4'-position of stilbene by Br and OMe groups does not alter the photophysical properties of the complexes eminently, the photophysical properties are significantly tuned by the CHO, NO2 and NPh2 substituents. In Chapter 3, platinum(II) complexes (3-1 – 3-6) containing 6-[7-R-9,9-di(2-ethylhexyl)-9H-fluoren-2-yl]-2,2'-bipyridine (R = NO2, CHO, benzothiazol-2-yl, n-Bu,carbazol-9-yl, NPh2) ligands were synthesized. It is found that electron-withdrawing substituents (NO2, CHO, BTZ) and electron-donating substituents (n-Bu, CBZ, NPh2) exert distinct effects on the photophysics of the complexes. In chapter 4, platinum(II) complexes (4-1 – 4-6) containing 6-[9,9-di(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2'-bipyridine ligands (R = 4-R'-phenylethynyl with R'= NO2, BTZ, H and OCH3 or R = 4'-BTZ-phen-1-yl or BTZ) were synthesized. The effects of terminal substituents and the different π-conjugated linkages between the BTZ component and the C^N^N core on the photophysics of these ligands and complexes were systematically investigated. In Chapter 5, iridium(III) complexes (5-1 – 5-5) featuring 7-(benzothiazol-2-yl)-9,9-di(2-ethylhexyl)-9H-fluoren-2-yl attachment to the 2-phenylpyridine was synthesized and studied. The effects of the extent of the π conjugation was studied in by the comparison between 5-1 and 5-2, and the effect of the number of the 7-(benzothiazol-2-yl)-9,9-di(2-ethylhexyl)-9H-fluoren-2-yl unit was compared in 5-3 – 5-5. In Chapter 6, bypyridyl iridium(III) complexes (6-1 – 6-7) with different cylometallated arylpyridyl ligands were synthesized. The effects of π-conjugation extension and direction were systematically investigated. Most complexes showed moderate to strong ns transient absorption from visible to near-IR region, indicating stronger excited-state absorption than ground-state absorption in the corresponding region and potential application as reverse saturable absorption materials. Thus, their application as nonlinear absorption materials was demonstrated by reverse saturable absorption (RSA) upon 532 nm ns laser. The RSA trend can be deciphered by the absorption cross section ratio between the excited states and ground states (σex/σ0).
Synthesis, Photophysics, Nonlinear Absorption, and Photodynamic Therapy Study of Iridium(III) Complexes
(North Dakota State University, 2018) Liu, Bingqing
Video summarizing a Ph.D. dissertation for a non-specialist audience.
Ultrasensitive Lateral Flow Nucleic Acid Biosensors Based on Novel Macro-/Nano-Materials
(North Dakota State University, 2017) Takalkar, Sunitha
Video summarizing a Ph.D. dissertation for a non-specialist audience.
Understanding the Interaction Between Enzymes and Nanomaterials
(North Dakota State University, 2019) Neupane, Sunanda
The rapid development of nanoparticles (NPs) has impacted many fields including energy efficiency, material science, biosensing, and medical therapeutics. Recently, NPs have been utilized to immobilize enzymes. The so-formed enzyme- NP complex show great potential to increase the reusability of enzymes and catalytic efficiencies. Enzyme-NP complex can also advance enzyme delivery for therapeutics where NPs serve as the enzyme carrier. In all applications, the contact of NPs with biomacromolecules, especially proteins, is either necessary or inevitable, which can lead to alterations in adsorbed enzyme structure and function. In biocatalysis, such changes often reduce the desired catalytic activity; in living organisms these changes can even cause protein malfunction, raising concerns about public health and nanotoxicity. Therefore, understanding the correlation of enzyme structure and activity upon contact with NPs is essential. While enzyme activities can often be determined, the details of enzyme structural changes caused by NPs are underexplored for most enzyme-NP complexes. Obtaining the structural information is challenging due to the relatively large size of the complexes, high heterogeneity in enzyme binding, and complexities caused by the presence of NPs which limit most structure determination approaches. These challenges were overcome using a set of biophysical techniques especially site-directed spin labeling (SDSL) with Electron Paramagnetic Resonance (EPR). SDSL-EPR can measure site-specific structural information in the native state of enzyme/NP systems, regardless of the complexity, primarily due to its “penetrating” power which is only sensitive to the motion of the spin label. The focus of this dissertation was on T4 lysozyme (T4L), a representing model enzyme proven useful in many works. Gold Nanoparticles (AuNPs), Gold Nanorods (AuNRs), Silica Nanoparticles (SiNPs), and Carbon Nanotubes (CNTs) were the studied NPs. The interaction of T4L with each NPs was unique. The local structural information and the orientation of T4L in each NP was revealed based on which the possible docking mechanism for each case was proposed. The ultimate goals to reveal the structure-function relationship of enzymes on NPs and utilize this information to fine-tune enzyme adsorption on various NPs to 1) avoid NPs aggregation and 2) optimize NPs as enzyme carriers were met.
Visible Light Photocatalysis of N-N Bond Based Compounds
(North Dakota State University, 2016) Iyer, Akila
The well-established principles of organic photochemistry, offer chemists the fundamental understanding and tools for studying light induced chemical transformations. Employing visible light for photocatalysis, one can design and develop benign routes for the synthesis of new organic materials. In our present investigation, we have developed novel N-N bond based compounds for visible light mediated phototransformations. We have presented synthesis for targeting achiral/chiral N-N bond based compounds and their study for various light driven applications. To name a few applications, these compounds have shown to react smoothly under visible light, metal-free conditions for classical photoreactions, chloromethylation, asymmetric photocyclization and photopolymerization. A diverse range of compounds has shown to react smoothly to afford products in high yields. The scope of this methodology has been evaluated for both intermolecular and intramolecular reactions. Our work benefits from the ability of these compounds to undergo desired phototransformation in both solution and in crystalline media. We have provided photochemical and photophysical details that corroborates our experimental findings and highlights the role of excited state reactivity of the novel N-N bond based compounds. This thesis will be an effort to make chemists familiarize with potential of these compounds in light induced reactions.

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