Showing papers in "ChemInform in 2004"

What Are Batteries, Fuel Cells, and Supercapacitors?

Abstract: Electrochemical energy conversion devices are pervasive in our daily lives. Batteries, fuel cells and supercapacitors belong to the same family of energy conversion devices. They are all based on the fundamentals of electrochemical thermodynamics and kinetics. All three are needed to service the wide energy requirements of various devices and systems. Neither batteries, fuel cells nor electrochemical capacitors, by themselves, can serve all applications.

Cubic AgPbmSbTe2+m: Bulk Thermoelectric Materials with High Figure of Merit.

Abstract: The conversion of heat to electricity by thermoelectric devices may play a key role in the future for energy production and utilization. However, in order to meet that role, more efficient thermoelectric materials are needed that are suitable for high-temperature applications. We show that the material system AgPb m SbTe 2+ m may be suitable for this purpose. With m = 10 and 18 and doped appropriately, n -type semiconductors can be produced that exhibit a high thermoelectric figure of merit material ZT max of ∼2.2 at 800 kelvin. In the temperature range 600 to 900 kelvin, the AgPb m SbTe 2+ m material is expected to outperform all reported bulk thermoelectrics, thereby earmarking it as a material system for potential use in efficient thermoelectric power generation from heat sources.

Electrochemical DNA Sensors

Abstract: Electrochemistry-based sensors offer sensitivity, selectivity and low cost for the detection of selected DNA sequences or mutated genes associated with human disease. DNA-based electrochemical sensors exploit a range of different chemistries, but all take advantage of nanoscale interactions between the target in solution, the recognition layer and a solid electrode surface. Numerous approaches to electrochemical detection have been developed, including direct electrochemistry of DNA, electrochemistry at polymer-modified electrodes, electrochemistry of DNA-specific redox reporters, electrochemical amplifications with nanoparticles, and electrochemical devices based on DNA-mediated charge transport chemistry.

Pyrolysis of Biomass to Produce Fuels and Chemical Feedstocks

Abstract: This review presents the summary of new studies on pyrolysis of biomass to produce fuels and chemical feedstocks. A number of biomass species, varying from woody and herbaceous biomass to municipal solid waste, food processing residues and industrial wastes, were subjected to different pyrolysis conditions to obtain liquid, gas and solid products. The results of various biomass pyrolysis investigations connected with the chemical composition and some properties of the pyrolysis products as a result of the applied pyrolysis conditions were combined. The characteristics of the liquid products from pyrolysis were examined, and some methods, such as catalytic upgrading or steam reforming, were considered to improve the physical and chemical properties of the liquids to convert them to economic and environmentally acceptable liquid fuels or chemical feedstocks. Outcomes from the kinetic studies performed by applying thermogravimetric analysis were also presented.

Sorghum Phytochemicals and Their Potential Impact on Human Health.

Abstract: Sorghum is a rich source of various phytochemicals including tannins, phenolic acids, anthocyanins, phytosterols and policosanols. These phytochemicals have potential to significantly impact human health. Sorghum fractions possess high antioxidant activity in vitro relative to other cereals or fruits. These fractions may offer similar health benefits commonly associated with fruits. Available epidemiological evidence suggests that sorghum consumption reduces the risk of certain types of cancer in humans compared to other cereals. The high concentration of phytochemicals in sorghum may be partly responsible. Sorghums containing tannins are widely reported to reduce caloric availability and hence weight gain in animals. This property is potentially useful in helping reduce obesity in humans. Sorghum phytochemicals also promote cardiovascular health in animals. Such properties have not been reported in humans and require investigation, since cardiovascular disease is currently the leading killer in the developed world. This paper reviews available information on sorghum phytochemicals, how the information relates to current phytonutrient research and how it has potential to combat common nutrition-related diseases including cancer, cardiovascular disease and obesity.

Chemical Sensing and Catalysis by One-Dimensional Metal-Oxide Nanostructures

Abstract: ▪ Abstract Metal-oxide nanowires can function as sensitive and selective chemical or biological sensors, which could potentially be massively multiplexed in devices of small size. The active nanowire sensor element in such devices can be configured either as resistors whose conductance is altered by charge-transfer processes occurring at their surfaces or as field-effect transistors whose properties can be controlled by applying an appropriate potential onto its gate. Functionalizing the surface of these entities offers yet another avenue for expanding their sensing capability. In turn, because charge exchange between an adsorbate and the nanowire can change the electron density in the nanowire, modifying the nanowire's carrier density by external means, such as applying a potential to the gate, could modify its surface chemical properties and perhaps change the rate and selectivity of catalytic processes occurring at its surface. Although research on the use of metal-oxide nanowires as sensors is still i...

Theoretical Study of the Exchange Coupling in Large Polynuclear Transition Metal Complexes Using DFT Methods

Abstract: Polynuclear transition metal complexes containing paramagnetic cations have caught the attention of many chemists, since they are one of the main fields of study in molecular magnetism and they play important roles in the reactivity of active sites of systems of biological interest. Theoretical methods based on density functional theory due to the possibility of handling large systems are especially indicated for studying the electronic structure of this kind of molecules. At the same time, such methods provide good accuracy to allow the calculation of the small energy differences involved in the exchange interactions. It is worth noting that theoretical methods are especially important in the study of the exchange interactions in complexes with a large number of paramagnetic centers because they can provide a more detailed analysis of the interactions than experimental data can. This fact is due to the limitations in obtaining exchange coupling constants from experimental measurements for large size systems of this kind. The knowledge of the exchange interactions that controls the ground state of the system is crucial to the understanding of magnetic properties such as the single-molecule magnet character or the reactivity of an active site in a biological system.

G-Quartets 40 Years Later: From 5′-GMP to Molecular Biology and Supramolecular Chemistry

Abstract: Molecular self-assembly is central to many processes in both biology and supramolecular chemistry. The G-quartet, a hydrogen-bonded macrocycle formed by cation-templated assembly of guanosine, was first identified in 1962 as the basis for the aggregation of 5'-guanosine monophosphate. We now know that many nucleosides, oligonucleotides, and synthetic derivatives form a rich array of functional G-quartets. The G-quartet surfaces in areas ranging from structural biology and medicinal chemistry to supramolecular chemistry and nanotechnology. This Review integrates and summarizes knowledge gained from these different areas, with emphasis on G-quartet structure, function, and molecular recognition.

Dendrimers in Drug Research

Abstract: Dendrimers are versatile, derivatisable, well-defined, compartmentalised chemical polymers with sizes and physicochemical properties resembling those of biomolecules e.g. proteins. The present critical review (citing 158 references) briefly describes dendrimer design, nomenclature and divergent/convergent dendrimer synthesis. The characteristic physicochemical features of dendrimers are highlighted, showing the effect of solvent pH and polarity on their spatial structure. The use of dendrimers in biological systems are reviewed, with emphasis on the biocompatibility of dendrimers, such as in vitro and in vivo cytotoxicity, as well as biopermeability, biostability and immunogenicity. The review deals with numerous applications of dendrimers as tools for efficient multivalent presentation of biological ligands in biospecific recognition, inhibition and targeting. Dendrimers may be used as drugs for antibacterial and antiviral treatment and have found use as antitumor agents. The review highlights the use of dendrimers as drug or gene delivery devices in e.g. anticancer therapy, and the design of different host–guest binding motifs directed towards medical applications is described. Other specific examples are the use of dendrimers as ‘glycocarriers’ for the controlled multimeric presentation of biologically relevant carbohydrate moieties which are useful for targeting modified tissue in malignant diseases for diagnostic and therapeutic purposes. Finally, the use of specific types of dendrimers as scaffolds for presenting vaccine antigens, especially peptides, for use in vaccines is presented.

Enzymatic Biofuel Cells for Implantable and Microscale Devices

Abstract: Introduction Technology for electrical power generation using enzyme catalysts, established four decades ago, has recently received increased attention associated with demand for micro-scale and implantable power supplies. The main challenges, namely the fragility of enzyme molecules, characteristic low current density, and poor fundamental understanding of redox biocatalysis, are currently being addressed from a variety of research perspectives, to take advantage of enzyme selectivity, low temperature and moderate pH activity, and manufacturability in small-scale devices. Such an effort benefits from four decades of multidisciplinary research in biosensors and related bioelectrochemical fields. This review paper summarizes the current state of enzymatic biofuel cell research in the context of foreseeable applications and assesses the future prospects of the technology. Emphasis is placed on device performance and engineering aspects, with a view toward practical portable power devices based on enzymatic biofuel cells. Research in biocatalytically modified electrodes, particularly for sensor applications, has provided a significant technological underpinning for current biofuel cell development. There exists significant overlap in technical requirements between sensors and biofuel cells, including chemical and mechanical stability, selectivity, and cost of materials. However, these two technologies diverge in the areas of current density, cell potential and stability. There exists extensive review literature in the area of biological fuel cells. Notably, Palmore and Whitesides summarized biological fuel cell concepts and performance up until about 1992. More recently, Katz and Willner discussed recent progress in novel electrode chemistries for both microbial and enzymatic fuel cells. We do not duplicate these valuable contributions, but instead focus on the strengths and weaknesses of state-of-art materials in the context of specific classes of applications, and point to areas where additional knowledge is currently needed to exploit biological fuel cells. With some exceptions, we focus on contributions made after 1992. Biofuel cells have traditionally been classified according to whether the catalytic enzymes were located inside or outside of living cells. If living cells are involved the system is considered to be microbial, and if not it is considered enzymatic. Although microbial fuel cells posses unique features unmatched by enzymatic cells, such as long-term stability and fuel efficiency, the power densities associated with such devices are typically much lower owing to resistance to mass transfer across cell membranes. Thus, microbial fuel cells are expected to find limited application in smallscale electronic devices. This review will focus on enzymatic biofuel cells. While such cells typically demonstrate reduced stability due to the limited lifetime of extracellular enzymes, and are typically unable to fully oxidize fuels, they allow for substantial concentration of catalysts and removal of mass transfer barriers and provide higher current and power densities, approaching the range of applicability to microand mini-scale electronics applications. Applications and Requirements The range of possible applications for biofuel cells may be broken down into three main subclasses: 1. Implantable power, such as micro-scale cells implanted in human or animal tissue, or larger cells implanted in blood vessels. 2. Power derived from ambient fuels or oxidants, mainly plant saps and juices, but extending to sewage and other waste streams. 3. Power derived from conventional fuels including hydrogen, methanol or higher alcohols. Classes 1 and 2 are closely related. The fuels available for implantable power, such as blood borne glucose or lactate, are ambient in the sense that they are present in a physiological environment in the absence of a fuel cell device. One major distinction between these two classes is that the ambient-fueled cell need not be implanted, and focuses on plantor waste-derived fuels, whereas the implantable cell focuses on animal-derived fuels and is present within the physiological system. Class 3 is unique in that this class competes with well-established conventional fuel cell technology. To a greater or lesser extent, all three classes share the fundamental technical requirements of high power density and high activity.

Agostic Interactions from a Computational Perspective: One Name, Many Interpretations

Abstract: The agostic bond defines an intramolecular interaction where a σ bond is geometrically close to an electron deficient centre (often a transition metal). The computational studies on this energetically weak interaction are reviewed and discussed. Various types of σ bonds have been considered (C-H, C-C, Si-H, Si-C, B-H). It is suggested that a C-X bond in which X carries a lone pair should preferably not be viewed as agostic. The factors that contribute to his occurrence are discussed. In particular, the agostic interaction is very sensitive to steric effects. Explanations based on molecular orbital analysis, electron delocalization and topological analysis of the electron density are presented.

Layered Double Hydroxides (LDHs)

Abstract: In this chapter we present an overview of what we regard as the most significant areas of recent activity in layered double hydroxide chemistry, using documents known to us through mid-2002, with occasional later examples. While we have attempted to give credit to the initiators in each area, we make no claims to ascribing priority (itself an uncertain matter in a fast-moving field) and will no doubt have overlooked many seminal papers; for this we can only ask our colleagues for their understanding. The division into sections and subsections is, we hope, self-explanatory. Significant results can of course have implications in more than one area, and we have therefore attempted to reduce duplication by crossreferencing.

A High-Performance Cathode for the Next Generation of Solid-Oxide Fuel Cells.

Abstract: Fuel cells directly and efficiently convert chemical energy to electrical energy. Of the various fuel cell types, solid-oxide fuel cells (SOFCs) combine the benefits of environmentally benign power generation with fuel flexibility. However, the necessity for high operating temperatures (800–1,000 °C) has resulted in high costs and materials compatibility challenges. As a consequence, significant effort has been devoted to the development of intermediate-temperature (500–700 °C) SOFCs. A key obstacle to reduced-temperature operation of SOFCs is the poor activity of traditional cathode materials for electrochemical reduction of oxygen in this temperature regime2. Here we present Ba_(0.5_Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-delta) (BSCF) as a new cathode material for reduced-temperature SOFC operation. BSCF, incorporated into a thin-film doped ceria fuel cell, exhibits high power densities (1,010 mW cm^(-2) and 402 mW cm^(-2) at 600 °C and 500 °C, respectively) when operated with humidified hydrogen as the fuel and air as the cathode gas. We further demonstrate that BSCF is ideally suited to 'single-chamber' fuel-cell operation, where anode and cathode reactions take place within the same physical chamber. The high power output of BSCF cathodes results from the high rate of oxygen diffusion through the material. By enabling operation at reduced temperatures, BSCF cathodes may result in widespread practical implementation of SOFCs.

Photochemical Activity of Nitrogen-Doped Rutile TiO2(110) in Visible Light.

Abstract: TiO2(110) single crystals, doped with nitrogen via an NH3 treatment at 870 K, have been found to exhibit photoactivity at photon energies down to 2.4 eV, which is 0.6 eV below the band-gap energy for rutile TiO2. The active dopant state of the interstitial nitrogen that is responsible for this effect exhibits an N (1s) binding energy of 399.6 eV and is due to a form of nitrogen that is probably bound to hydrogen, which differs from the substitutional nitride state with an N (1s) binding energy of 396.7 eV. Optical absorption measurements also show enhanced absorption down to 2.4 eV for the NH3-treated TiO2(110). A co-doping effect between nitrogen and hydrogen is postulated to be responsible for the enhanced photoactivity of nitrogen-doped TiO2 materials in the range of visible light.

Polytriazoles as Copper(I)‐Stabilizing Ligands in Catalysis.

Abstract: Polytriazolylamines were synthesized by the copper(I)-catalyzed ligation of azides and alkynes. The C3-symmetric derivative, TBTA, was shown to be a powerful stabilizing ligand for copper(I), protecting it from oxidation and disproportionation, while enhancing its catalytic activity.

Virtual Screening Using Protein—Ligand Docking: Avoiding Artificial Enrichment.

Abstract: This study addresses a number of topical issues around the use of protein−ligand docking in virtual screening. We show that, for the validation of such methods, it is key to use focused libraries (containing compounds with one-dimensional properties, similar to the actives), rather than “random” or “drug-like” libraries to test the actives against. We also show that, to obtain good enrichments, the docking program needs to produce reliable binding modes. We demonstrate how pharmacophores can be used to guide the dockings and improve enrichments, and we compare the performance of three consensus-ranking protocols against ranking based on individual scoring functions. Finally, we show that protein−ligand docking can be an effective aid in the screening for weak, fragment-like binders, which has rapidly become a popular strategy for hit identification. All results presented are based on carefully constructed virtual screening experiments against four targets, using the protein−ligand docking program GOLD.

Polymer Brushes via Surface‐Initiated Polymerizations

Abstract: Polymer brushes produced by controlled surface-initiated polymerization provide a route to surfaces coated with well-defined thin polymer films that are covalently bound to the substrate. All of the major controlled polymerization techniques have been applied to the synthesis of polymer brushes and examples of each are presented here. Many examples of brush synthesis in the literature have used the living atom transfer radical polymerization (ATRP) system, and in this tutorial review a particular focus is given to examples of this technique.

The Bioavailability of Chemicals in Soil for Earthworms

Abstract: The bioavailability of chemicals to earthworms can be modified dramatically by soil physical/chemical characteristics, yet expressing exposure as total chemical concentrations does not address this problem. In order to understand the effects of modifying factors on bioavailability, one must measure and express chemical bioavailability to earthworms in a consistent, logical manner. This can be accomplished by direct biological measures of bioavailability (e.g., bioaccumulation, critical body residues), indirect biological measures of bioavailability (e.g., biomarkers, reproduction), or indirect chemical measures of bioavailability (e.g., chemical or solid-phase extracts of soil). If indirect chemical measures of bioavailability are to be used, they must be correlated with some biological response. Bioavailability can be incorporated into ecological risk assessment during risk analysis, primarily in the estimation of exposure. However, in order to be used in the site-specific ecological risk assessment of chemicals, effects concentrations must be developed from laboratory toxicity tests based on exposure estimates utilizing techniques that measure the bioavailable fraction of chemicals in soil, not total chemical concentrations.