Browsing by Author "Marti, Angel"

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    2D Materials in Lego Style: Synthesis, Characterizations and Applications
    (2015-12-04) Gong, Yongji; Ajayan, Pulickel M.; Jun, Lou; Marti, Angel
    Recently, the emergence and development of 2D materials with various optical and electrical properties has opened up new routes for electronic and optoelectronic device fabrication based on atomically thin layers. For example, graphene behaves as a semi-metal with extremely high mobility, hexagonal boron nitride (h-BN) is a good insulator and monolayer TMDs such as MoS2, MoSe2 and WSe2 are semiconductors with direct band gap. This diversity offers the opportunity to construct atomically thin electronics based entirely on 2D materials. One of the most promising applications is to get 2D integrated circuits to replace the traditional silicon based ones, which will be much thinner and faster. 2D materials can be considered to be analogous to Lego blocks. The Lego game is to use different Lego blocks to get a complicated Lego building. Similarly, we can use different 2D materials to get the corresponding integrated circuits or devices for energy related applications. Based on this purpose, we need different 2D blocks, which are the most fundamental parts in the 2D world, 2D materials with tunable properties, and different strategies to combine the 2D materials together. Chapter 1 focuses on synthesis, characterization and applications of pristine 2D materials, which are the fundamental blocks for the 2D world. In this part, we synthesized different 2D materials such as insulator (h-BN), metal (graphene) and semiconductors (MX2, M = metal and X = chalcogen) for different applications. There are two directions in this part: one is to explore new 2D materials and the other one is to improve the growth of 2D materials to push them closer to their real applications. Moreover, semiconductors with different band gap (from 1.1 eV to 2.8 eV) and different type (p type and n type) have been developed. Furthermore, we improved the growth of different 2D materials to get their millimeter-scale single crystals or even continuous film. In the coming Chapter 2, we focused on the 2D alloys. The purpose of alloying 2D materials is to engineer the phase and band gap by changing the composition in the alloys. By this, we can tune the optical and electrical properties in 2D materials very easily. The first project in this part is about h-BNC system, which can open a band gap in graphene system, resulting in both high mobilities and high ON-OFF ratio in their transistors. Then we developed the MoS2-xSex (x, 0-2) alloys, in which the band gap can be continuously tuned from 1.50 eV to 1.84 eV. At last, RexMo1-xS2 (x, 0-1) system is developed to study the phase transition with different x. In Chapter 3, heterostructures based on different 2D materials are developed by different strategies. For example, we can get h-BN/h-BNC/graphene lateral heterostructure by combing a conversion method and lithography. We also developed the heterostructures based on MoS2/WS2 and MoSe2/WSe2 by a one-step growth method and two-step growth method, respectively. In both of them, we can get the in-plane and vertical heterostructures. The interface of the in-plane interface is atomically seamless and sharp and the bilayer heterostructures have fixed stacking orientations, which are more advantageous than other methods. At last, we developed more complicated heterostructures, which can be composed by 3 or 4 different 2D materials. In Chapter 4, we further developed several different 3D structures constructed by 2D materials for energy storage and conversion. Basically, this part is inspired by graphene aerogel with porous 3D structure. The porous structure enables the access of electrolyte very easily and the graphene network has very good electrical conductivity, advantageous to work as electrochemical applications. In this part, we developed several different structures for different applications, including MoS2/GO as the anode for lithium ion battery, VO2/GO as the cathode for lithium ion battery and h-BNC as ORR catalyst. For the lithium ion battery, the structures developed here have better performance than the commercial ones with higher capacity, better stability and much higher charge and discharge rate. H-BNC aerogel can even beat the performance of commercial Pt/C as the ORR catalyst. In summary, the research based on 2D materials is like the Lego game, including exploring the Lego blocks (pristine 2D materials and their alloys) and combining them together to form the functional devices (2D heterostructures and 3D porous structure from 2D materials).
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    A Design Approach to the Synthesis and Characterization of Metal Phosphonate MOFs
    (2019-03-13) Barbee, Derek Blaine; Barron, Andrew; Adams, Wade; Marti, Angel
    Metal Phosphonate Metal-Organic Frameworks (MOFs) have recently garnered interest as a possible solution to many issues plaguing modern society including but not limited to sustainable energy, catalysis, climate change, biomedical applications, etc. While previous attempts at synthesizing metal phosphonate MOF species have been successful, the products are microcrystalline and generally exhibit small pore sizes. Additionally, due to the nature of synthesis, metal phosphonate MOFs exhibit significant defect loading into the regular lattice of the synthesized crystal, degrading the material properties of the substance. The goal of this research was to optimize the synthesis of phosphonic acid precursor ligands as well as to explore and optimize phosphonate MOF synthesis in an attempt to open the field up to greater interest from the MOF community. The results were mixed as we report successful optimization and prototyping of more traditional and newly synthesized phosphonic acid ligands but were unsuccessful in optimizing the synthesis of the MOF material. Phosphonic acid ligand synthesis was demonstrated to be versatile and thorough in its substitution, yielding variety of phosphonic acid species. Investigations into the mechanistic progression of MOF synthesis revealed numerous obstacles impending the formation of large, regular lattices that resulted in inconsistent reaction products. While some issues were able to be overcome, certain fundamental aspects of the system such as the denticity of the ligand and the reactivity of the metal constituent were viewed as insurmountable based on current synthetic methodologies.
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    Interaction of Colloidal Gold Nanoparticles with Model Serum Proteins: The Nanoparticle-Protein ‘Corona’ from a Physico-Chemical Viewpoint
    (2015-09-15) Dominguez-Medina, Sergio; Link, Stephan; Marti, Angel; Drezek, Rebekah
    When nanoparticles come in contact with biological fluids they become coated with a mixture of proteins present in the media, forming what is known as the nanoparticle-protein ‘corona’. This corona changes the nanoparticles’ original surface properties and plays a central role in how these get screened by cellular receptors. In the context of biomedical research, this presents a bottleneck for the transition of nanoparticles from research laboratories to clinical settings. It is therefore fundamental to probe these nanoparticle-protein interactions in order to understand the different physico-chemical mechanisms involved. This thesis is aimed to investigate the exposure of colloidal gold nanoparticles to model serum proteins, particularly serum albumin, the main transporter of molecular compounds in the bloodstream of mammals. A set of experimental tools based on optical microscopy and spectroscopy were developed in order to probe these interactions in situ. First, the intrinsic photoluminescence and elastic scattering of individual gold nanoparticles were investigated in order to understand its physical origin. These optical signals were then used to measure the size of the nanoparticles while in Brownian diffusion using fluctuation correlation spectroscopy. This spectroscopic tool was then applied to detect the binding of serum albumin onto the nanoparticle surface, increasing its hydrodynamic size. By performing a binding isotherm as a function of protein concentration, it was determined that serum albumin follows an anti-cooperative binding mechanism on negatively charged gold nanoparticles. This protein monolayer substantially enhanced the stability of the colloid, preventing their aggregation in saline solutions with ionic strength higher than biological media. Cationic gold nanoparticles in contrast, aggregated when serum albumin was present at a low protein-to-nanoparticle ratio, but prevented aggregation if exposed in excess. Single-molecule fluorescence microscopy revealed that under low protein-to-nanoparticle binding ratios, serum albumin irreversibly unfolds upon adsorption and spreads across the available nanoparticle surface area. Unfolded proteins then interact with one another, triggering nanoparticle aggregation. Fibrinogen and globulin also triggered aggregation when exposed to cationic nanoparticles. In an effort to relate these physico-chemical observations to relevant biological parameters, the uptake of protein coated gold nanoparticles by a model cancer cell line was investigated under different incubation conditions. Those nanoparticles pre-incubated with bovine serum albumin before fetal bovine serum were found to be uptaken three times more than those only incubated in serum.
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    Investigating the Amyloid-beta aggregation and oxidation using metal complexes
    (2018-03-22) Jiang, Bo; Marti, Angel
    Amyloid-β (Aβ), a short peptide which self-assembles into large aggregates, was observed in the gray matter of Alzheimer’s patients. The aggregates in oligomeric and fibrillar forms were proven to be toxic to our brain. Based on this, many groups have taken on the task of investigating the aggregation process of Aβ. Previous work in our lab has shown that metal complexes can be used as a new family of photoluminescence probes for the detection of Aβ aggregates. More specifically, Dr. Amir Aliyan in our research group showed that [Re(CO)3(dppz)(Py)]+ exhibits a secondary light-switching response in the presence of Aβ fibrils upon UV irradiation, and at the same time performs oxidation on Aβ fibrils. This thesis focuses on investigating the interactions between Aβ and rhenium complexes and also probing Aβ oligomerization using ruthenium complexes. Chapter 1 is an introduction of the photoluminescence probes for the detection of Aβ aggregates, and the previous work of our lab on the metal complexes. Chapter 2 details the interactions between Aβ fibrils and [Re(CO)3(dppz)(Py)]+. Job plot and binding assay were used to determine the dissociation constant Kd as 4.2 ± 0.6 μM. Molecular dynamics simulations were used to propose a binding site for [Re(CO)3(dppz)(Py)]+ on Aβ fibrils is at a hydrophobic cleft between Val18 and Phe20. Due to the fact that Aβ fibrils are oxidized by [Re(CO)3(dppz)(Py)]+ after UV irradiation, the binding site was studied using the oxidation site as a chemical footprint. In addition, the study of the photooxidation of Aβ monomers showed that after UV irradiation His13 and Tyr10 are also prone to be oxidized by [Re(CO)3(dppz)(Py)]+. In order to further study the secondary light-switching behavior of [Re(CO)3(dppz)(Py)]+, functional groups were used to simulate the amino acids of Aβ and we found the photoluminescence of [Re(CO)3(dppz)(Py)]+ was enhanced in the presence of imidazole and dimethyl sulfide in SDS solutions but not in buffer. In addition, the quantum yield of singlet oxygen produced by [Re(CO)3(dppz)(Py)]+ upon UV irradiation, power flux of the irradiation source, and the quantum yields of photooxidation were determined. In chapter 3, I reported using photoluminescence anisotropy of [Ru(bpy)2(dpqp)]2+ for the detection of Aβ oligomerization. Aβ oligomers are believed to form immediately following monomers, however they are invisible to fluorescence sensors, such as Thioflavin T. Given that photoluminescence anisotropy is sensitive to the rotational correlation time of molecules, it is useful for monitoring the formation of biomolecule aggregates. We found that Aβ oligomers start to form from time zero with a steady increase in anisotropy that plateaus after 48 hours. The real-time monitoring of Aβ oligomers is of great importance for understanding the kinetics of aggregation, the forces that bring peptides together and study their inhibition. The formation of Aβ oligomers was supported by Western Blot analysis.
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    Photoactive inorganic molecules for the next generation of photoluminescent probes and materials
    (2019-12-06) Ogle, Meredith McDowell; Marti, Angel
    Photoluminescent molecules come in many forms from organic molecules to metal complexes to nanomaterials and have a large range of applications as dyes, sensors, catalysts and more. In this thesis three distinct areas will be covered: temperature sensors, nanomaterial antennae, and photooxidation catalysts. Photoluminescent sensors can report on environmental changes at the molecular level. Chapter 1 summarizes the literature on photoluminescent temperature probes and discusses the different reporting techniques and molecular mechanisms. Due to different photophysical properties of the dyes, probes can have temperature dependent changes in emission intensity, a ratio of two emissions, peak emission wavelength, or lifetime of emission. Each of these can be measured by a spectrometer or microscope and some can be seen by eye. The probes with the least number of confounding variables are metal complexes that have variable lifetime with temperature. Therefore, Chapter 2 describes the development of iridium (III) complexes as phosphorescent temperature probes and the challenges of biocompatibility and lifetime determination in living cell using available equipment. Chapter 3 discusses the design of a fluorescent boron-dipyrromethene temperature probe and its implementation as a live cell thermometer. We modified a viscosity probe to solely report on temperature in high viscosity environments like the cell membrane. This probe is non-toxic, membrane permeable and retained by live cells. The fluorescent lifetime is predictively quenched by molecular vibrations as temperature increases and live cell temperature was measured with fluorescent lifetime imaging microscopy. Graphene quantum dots are nanoflakes of graphene isolated from oxidized coal. These nanoparticles are heterogenous by nature of the top down synthesis which leads to broad emission bands and low quantum yields compared to traditional quantum dots. Chapter 4 explores the photophysical properties of these nanoparticles and their use as antennae for lanthanide cations. The energy transfer from graphene quantum dots to lanthanides, and subsequent quantum yield for the emission for the rare earth, was most efficient in the ultraviolet range. This is in contrast with the well know Kasha-Vavilov rule, and shows promise toward development of anti-counterfeit dyes. Metal organic frameworks, a class of mesoporous scaffolds, have been developed as gas storage devices, sensors, and photocatalyst. Chapter 5 describes initial experiments that aims to create a rhenium (I) doped MOF as a photo-oxidation catalyst that is easily recovered from the reaction mixture. A rhenium carbonyl complex, a known singlet oxygen sensitizer, was doped into a highly stable MOF at different concentrations. Photoluminescent characterization determined the optimal concentration to use as a catalyst. Current work focuses on optimizing oxidation reaction conditions and increasing substrate scope.
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    Theranostic gold nanoshells and nanomatryoshkas for cancer therapy
    (2015-01-20) Ayala-Orozco, Ciceron; Halas, Naomi; Marti, Angel; Nordlander, Peter; Joshi, Amit
    This dissertation describes the synthesis of multifunctional gold nanoparticles designed for therapy and diagnosis of cancer (theranostics), and the evaluation of their therapeutic efficacy and bioimaging of tumors in mice. The design of these metallic nanoparticles is aimed to incorporate imaging agents (MRI contrasts and fluorophores) in compact structures with dimensions below 100 nm while keeping their NIR-light-absorbing properties and optimum surface chemistry to enhance accumulation in tumor. The therapeutic response of these metallic nanoparticles is derived from the photoexcitation of their plasmon resonance, the collective oscillation of the conduction band electrons, which was advantageously utilized to enhance the quantum yield of fluorophores resonant in the NIR where the penetration of light is maximal in biological tissue and minimally destructive. Gold nanoshells as absorbers of NIR light can convert the absorbed light into heat consequently causing hyperthermia in the surrounding medium which leads to tumor cell death. To extent the application of previously developed theranostic nanoshells to the highly lethal pancreatic cancer, chapter 2 describes a magneto-fluorescent theranostic nanocomplex targeted to neutrophil gelatinase associated lipocalin (NGAL) receptor in pancreatic cancer. Gold nanoshells (SiO2-Au core-shell nanoshell) resonant at 810 nm were encapsulated in silica epilayers doped with iron oxide and the NIR dye ICG, resulting in a theranostic gold nanoshells, which provided contrast for both T2 weighted MRI and NIR fluorescence optical imaging. The large size of this complex (200 nm) potentially can hinder the accumulation in tumor. Seeking to reduce the size of the theranostic nanoparticles, chapter 3 presents the sub-100 nm Au nanomatryoshkas (Au/SiO2/Au). Au nanomatryoshkas are strong light absorbers with 77% absorption efficiency while the nanoshells are weaker absorbers with only 15% absorption efficiency. After an intravenous injection of Au nanomatryoshkas followed by a single NIR laser dose of 2 W/cm2 for 5 min, 83% of the tumor-bearing mice appeared healthy and tumor free >60 days later, while only 40% of mice treated with nanoshells survived the same period. The smaller size and larger absorption cross section of Au nanomatryoshkas combine to make this nanoparticle more effective than Au nanoshells for photothermal cancer therapy. Chapter 4 presents the therapeutic efficacy in mice bearing large (>1000 mm3) and highly aggressive triple negative breast tumors. To equip the Au nanomatryoshkas with imaging contrast agents, fluorophores were encapsulated in the internal SiO2 layer of the Au/SiO2/Au matryoshkas as described in chapter 5. We observed strong fluorescence enhancements of the NIR dyes Cy7 and IR800. This behavior can be understood by taking into account the near field enhancement induced by the Fano resonance of the nanomatryoshka, which is responsible for enhanced absorption of the fluorophores incorporated into the nanocomplex. The combination of compact size and enhanced light emission with internal encapsulation of the fluorophores for increased biocompatibility suggests outstanding potential for this type of nanoparticle complex in biomedical applications as it is investigated and presented in chapter 6.