Two-step mechanism for low-temperature oxidation of vacancies in graphene

We studied the oxidation of vacancies in graphene by abinitio atomistic thermodynamics to identify the dominant reaction mechanisms. Our calculations show that the low temperature oxidation occurs via a two-step process: Vacancies are initially saturated by stable O groups, such as ether (C-O-C) and carbonyl (C=O). The etching is activated by a second step of additional O2 adsorption at the ether groups, forming larger O groups, such as lactone (C-O-C=O) and anhydride (O=C-O-C=O), that may desorb as CO2 just above room temperature. Our studies show that the partial pressure of oxygen is an important external parameter that affects the mechanisms of oxidation and that allows us to control the extent of etching

Insights into the function of silver as an oxidation catalyst by ab initio, atomistic thermodynamics

To help understand the high activity of silver as an oxidation catalyst, e.g., for the oxidation of ethylene to epoxide and the dehydrogenation of methanol to formaldehyde, the interaction and stability of oxygen species at the Ag(111) surface has been studied for a wide range of coverages. Through calculation of the free energy, as obtained from density-functional theory and taking into account the temperature and pressure via the oxygen chemical potential, we obtain the phase diagram of O/Ag(111). Our results reveal that a thin surface-oxide structure is most stable for the temperature and pressure range of ethylene epoxidation and we propose it (and possibly other similar structures) contains the species actuating the catalysis. For higher temperatures, low coverages of chemisorbed oxygen are most stable, which could also play a role in oxidation reactions. For temperatures greater than about 775 K there are no stable oxygen species, except for the possibility of O atoms adsorbed at under-coordinated surface sites Our calculations rule out thicker oxide-like structures, as well as bulk dissolved oxygen and molecular ozone-like species, as playing a role in the oxidation reactions.Comment: 15 pages including 9 figures, Related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

Room temperature plasmonic lasing in a continuous wave operation mode from an InGaN/GaN single nanorod with a low threshold

It is crucial to fabricate nano photonic devices such as nanolasers in order to meet the requirements for the integration of photonic and electronic circuits on the nanometre scale. The great difficulty is to break down a bottleneck as a result of the diffraction limit of light. Nanolasers on a subwavelength scale could potentially be fabricated based on the principle of surface plasmon amplification by stimulated emission of radiation (SPASER). However, a number of technological challenges will have to be overcome in order to achieve a SPASER with a low threshold, allowing for a continuous wave (cw) operation at room temperature. We report a nano-SPASER with a record low threshold at room temperature, optically pumped by using a cw diode laser. Our nano-SPASER consists of a single InGaN/GaN nanorod on a thin SiO2 spacer layer on a silver film. The nanorod containing InGaN/GaN multi-quantum-wells is fabricated by means of a cost-effective post-growth fabrication approach. The geometry of the nanorod/dielectric spacer/plasmonic metal composite allows us to have accurate control of the surface plasmon coupling, offering an opportunity to determine the optimal thickness of the dielectric spacer. This approach will open up a route for further fabrication of electrically injected plasmonic lasers

Shrinking-Hole Colloidal Lithography: Self-Aligned Nanofabrication of Complex Plasmonic Nanoantennas

Plasmonic nanoantennas create locally strongly enhanced electric fields in so-called hot spots. To place a relevant nanoobject with high accuracy in such a hot spot is crucial to fully capitalize on the potential of nanoantennas to control, detect, and enhance processes at the nanoscale. With state-of-the-art nanofabrication, in particular when several materials are to be used, small gaps between antenna elements are sought, and large surface areas are to be patterned, this is a grand challenge. Here we introduce self-aligned, bottom-up and self-assembly based Shrinking-Hole Colloidal Lithography, which provides (i) unique control of the size and position of subsequently deposited particles forming the nanoantenna itself, and (ii) allows delivery of nanoobjects consisting of a material of choice to the antenna hot spot, all in a single lithography step and, if desired, uniformly covering several square centimeters of surface. We illustrate the functionality of SHCL nanoantenna arrangements by (i) an optical hydrogen sensor exploiting the polarization dependent sensitivity of an Au-Pd nanoantenna ensemble; and (ii) single particle hydrogen sensing with an Au dimer nanoantenna with a small Pd nanoparticle in the hot spot

Absorption Enhancement in Lossy Transition Metal Elements of Plasmonic Nanosandwiches

Combination of catalytically active transition metals and surface plasmons offers a promising way to drive chemical reactions by converting incident visible light into energetic electron-hole pairs acting as a mediator. In such a reaction enhancement scheme, the conversion efficiency is dependent on light absorption in the metal. Hence, increasing absorption in the plasmonic structure is expected to increase generation of electron-hole pairs and, consequently, the reaction rate. Furthermore, the abundance of energetic electrons might facilitate new reaction pathways. In this work we discuss optical properties of homo- and heterometallic plasmonic nanosandwiches consisting of two parallel disks made of gold and palladium. We show how near-field coupling between the sandwich elements can be used to enhance absorption in one of them. The limits of this enhancement are investigated using finite-difference time-domain simulations. Physical insight is gained through a simple coupled dipole analysis of the nanostructure. For small palladium disks (compared to the gold disk), total absorption enhancement integrated over the near visible solar AM 1.5 spectrum is 8-fold, while for large palladium disks, similar in size to the gold one, it exceeds three

Rhodium nanoparticles for ultraviolet plasmonics

The nonoxidizing catalytic noble metal rhodium is introduced for ultraviolet plasmonics. Planar tripods of 8 nm Rh nanoparticles, synthesized by a modified polyol reduction method, have a calculated local surface plasmon resonance near 330 nm. By attaching p-aminothiophenol, local field-enhanced Raman spectra and accelerated photodamage were observed under near-resonant ultraviolet illumination, while charge transfer simultaneously increased fluorescence for up to 13 min. The combined local field enhancement and charge transfer demonstrate essential steps toward plasmonically enhanced ultraviolet photocatalysis.This work has been supported by NSF-ECCS-12-32239. This work was partially supported by the Army’s In-house Laboratory Innovative Research program. Financial support from USAITCA (project no. W911NF-13-1-0245) and MICINN (Spanish Ministry of Science and Innovation, project no. FIS2013- 45854-P) is also acknowledged

New Insights into the Mechanism of Visible Light Photocatalysis

ABSTRACT: In recent years, the area of developing visible-lightactive photocatalysts based on titanium dioxide has been enormously investigated due to its wide range of applications in energy and environment related fields. Various strategies have been designed to efficiently utilize the solar radiation and to enhance the efficiency of photocatalytic processes. Building on the fundamental strategies to improve the visible light activity of TiO2-based photocatalysts, this Perspective aims to give an insight into many contemporary developments in the field of visible-light-active photocatalysis. Various examples of advanced TiO2 composites have been discussed in relation to their visible light induced photoconversion efficiency, dynamics of electron− hole separation, and decomposition of organic and inorganic pollutants, which suggest the critical need for further development of these types of materials for energy conversion and environmental remediation purposes

Redoxnetzwerke des Malariaerregers Plasmodium : Validierung von Schlüsselenzymen für neue chemotherapeutische Ansätze

Die Malaria-assozierte Pathologie wird durch die Entwicklung von Plasmodien in den Erythrozyten ihres Wirtes verursacht. Um ein reduzierendes intrazelluläres Milieu in diesem Sauerstoff-reichen Umfeld zu erhalten, haben Malaria-Parasiten ein komplexes antioxidatives Netzwerk entwickelt, welches auf zwei zentralen Elektronen-Donatoren beruht, dem Glutathion und dem Thioredoxin. Die intrazellulären Spiegel dieser beiden redox-aktiven Peptide in reduzierter – und damit reduzierender Form – werden durch die entsprechenden NADPH-abhängigen Flavoenzyme Thioredoxinreduktase (TrxR) und Glutathionreduktase (GR) aufrechterhalten. Da Katalase und eine klassische Selen-abhängige Glutathionperoxidase in Plasmodium fehlen, spielen die auf den Enzymen Thioredoxin- und Glutathionreduktase basierenden Systeme eine besondere Rolle. Weitere Bestandteile der antioxidativen Abwehr sind verschiedene Mitglieder der Thioredoxin-Familie, vor allem das Plasmodien-spezifische Plasmoredoxin (Plrx), welches als zusätzliche Verteidigungslinie der Parasiten gegen oxidativen Stress diskutiert wird. Im Rahmen der vorliegenden Arbeit wurden antioxidative Proteine der Parasiten untersucht, die als Zielmoleküle für die rationale Medikamentenentwicklung von Bedeutung sind. Um zu klären, welche der beteiligten Proteine sich aufgrund einer essentiellen Funktion in den Blutstadien-Parasiten insbesondere dafür eignen, wurden die zugrunde liegenden Gene dreier Redox-Proteine mit Methoden der reversen Genetik untersucht. Dies geschah durch stabile Gen-Inaktivierung unter Nutzung von Integrations- und replacement-Strategien, im Falle der Glutathionreduktase ergänzt durch Gen-Komplementation mit dem entsprechenden Ortholog. Die Targetvalidierung individueller redox-aktiver Proteine ist eine unerlässliche Voraussetzung für die rationale Entwicklung neuer effektiver Antimalaria-Medikamente. Die erfolgreiche Herstellung von Plasmoredoxin knock out-Mutanten in Nagetiermalaria-Modellparasiten und die phänotypischen Analysen im Verlauf des Lebenszyklus offenbarten in vitro und in vivo keine essentiellen Funktionen dieses Proteins. Dieses Ergebnis kann mit funktioneller Redundanz innerhalb der Mitglieder der Thioredoxin-Familie erklärt werden, somit bietet sich eine weitere Arzneimittelentwicklung allein auf der Basis Plasmoredoxin-spezifischer Inhibitoren nicht an. Weiterhin konnten Thioredoxinreduktase-defiziente P. berghei-Mutanten erzeugt werden. Eine systematische Analyse dieser Parasiten im Verlauf des Plasmodium-Lebenszyklus zeigte an keiner Stelle eine lebensnotwendige Funktion der Thioredoxinreduktase. Dies steht im Gegensatz zu früheren in vitro-Studien bei P. falciparum, welche diesem Enzym eine essentielle Funktion zuschrieben. Da ein praktikables in vivo-Modell für humanpathogene Malaria-Parasiten nicht verfügbar ist, verdeutlicht das hier vorgestellte Ergebnis der Targetvalidierung für Thioredoxinreduktase die Bedeutung funktioneller Studien in Nagetiermalaria-Modellen für die präklinische Entwicklung neuer Medikamente gegen Malaria. Für eines der viel versprechendsten drug targets von Malaria-Parasiten, der Glutathionreduktase, konnte auf genetischer Ebene der Beweis erbracht werden, dass durch den Verlust der Funktion dieses Enzyms die Parasiten nicht lebensfähig sind. Somit nimmt die Glutathionreduktase in dem komplexen antioxidativen Netzwerk von Plasmodium eine zentrale und essentielle Rolle ein. Zusätzlich wurde in dieser Arbeit die molekulare Wirkungsweise des seit mehr als 100 Jahren klinisch genutzten Medikaments Methylenblau auf Malaria-Parasiten untersucht. Hierbei zeigte sich, dass diese Verbindung neben ihren inhibierenden Eigenschaften interessante Charakteristika als subversives Substrat verschiedener Disulfidreduktasen besitzt. Bei Anwesenheit von Methylenblau werden diese Enzyme zu pro-oxidativen, H2O2-produzierenden Verbindungen, welche die reduzierenden zellulären Bedingungen herausfordern, statt sie zu bewahren.Malaria-associated pathology is caused by the continuous expansion of Plasmodium parasites inside host erythrocytes. To maintain a reducing intracellular milieu in this oxygen-rich environment, malaria parasites have evolved a complex antioxidative network based on two central electron donors, glutathione and thioredoxin. The intracellular levels of these redox-active peptides in reduced and thus reducing forms are maintained by the respective NADPH-dependent flavoenzymes thioredoxin reductase (TrxR) and glutathione reductase (GR). As catalase and classical selenium-dependant glutathione peroxidase are absent in Plasmodium, the systems based on thioredoxin reductase and glutathione reductase play a prominent role. Further components of the antioxidative defense comprise different members of the thioredoxin family, especially Plasmodium-specific plasmoredoxin which has been discussed as an additional defense line against oxidative stress. In the framework of this thesis, antioxidative parasite proteins that are of interest as targets for rational drug development were investigated. To clarify which proteins are particularly suitable as drug target due to an essential function for blood stage parasites, the respective genes of three redox proteins were studied employing reverse genetics. This was achieved by stable gene inactivation with integration and replacement strategies, in the case of glutathione reductase supplemented by gene complementation of the respective ortholog. Target validation of individual redox-active proteins is an indispensable prerequisite for the rational development of new and effective antimalarial drugs. The successful generation of plasmoredoxin knockout mutants in the rodent model malaria parasite and phenotypic analysis during life cycle progression revealed both in vitro and in vivo a non-vital function of this protein. This finding can be explained by functional redundancy among the members of the thioredoxin family and discourages future drug discovery efforts that aim at specifically targeting plasmoredoxin. Furthermore P. berghei thioredoxin reductase-deficient parasites could be generated in this thesis. A systematic phenotypic analysis of this mutant throughout the Plasmodium life cycle demonstrated no essential function for thioredoxin reductase at any phase of the parasite life cycle. This is in contrast to previous in vitro studies which attributed an essential role to thioredoxin reductase in P. falciparum. As a feasible in vivo model for human pathogen malaria parasites is not available, the presented target validation result of thioredoxin reductase highlights the importance of functional studies in rodent malaria models to guide preclinical development of novel antimalaria intervention strategies. For one of the most promising drug targets of malaria parasites, glutathione reductase, this work provides the genetic proof that in the case of this enzyme loss of function results in non-viable malaria parasites in vivo. Hence, glutathione reductase occupies an essential position in the complex antioxidant network of Plasmodium parasites. Furthermore, the molecular mode of action of methylene blue, which has been clinically used as an antimalarial drug for over 100 years, against malaria parasites has been investigated. It could be shown that this compound – besides its inhibitory potential – has interesting characteristics as a subversive substrate of different disulfide reductases. In the presence of methylene blue, they turn into pro-oxidant, H2O2-producing enzymes which challenge the reducing cellular milieu that they are meant to protect in the absence of this perturbing drug