SciCombinator

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Concept: Oxidizing agent

254

The permanent ice cover of Lake Vida (Antarctica) encapsulates an extreme cryogenic brine ecosystem (-13 °C; salinity, 200). This aphotic ecosystem is anoxic and consists of a slightly acidic (pH 6.2) sodium chloride-dominated brine. Expeditions in 2005 and 2010 were conducted to investigate the biogeochemistry of Lake Vida’s brine system. A phylogenetically diverse and metabolically active Bacteria dominated microbial assemblage was observed in the brine. These bacteria live under very high levels of reduced metals, ammonia, molecular hydrogen (H(2)), and dissolved organic carbon, as well as high concentrations of oxidized species of nitrogen (i.e., supersaturated nitrous oxide and ∼1 mmol⋅L(-1) nitrate) and sulfur (as sulfate). The existence of this system, with active biota, and a suite of reduced as well as oxidized compounds, is unusual given the millennial scale of its isolation from external sources of energy. The geochemistry of the brine suggests that abiotic brine-rock reactions may occur in this system and that the rich sources of dissolved electron acceptors prevent sulfate reduction and methanogenesis from being energetically favorable. The discovery of this ecosystem and the in situ biotic and abiotic processes occurring at low temperature provides a tractable system to study habitability of isolated terrestrial cryoenvironments (e.g., permafrost cryopegs and subglacial ecosystems), and is a potential analog for habitats on other icy worlds where water-rock reactions may cooccur with saline deposits and subsurface oceans.

Concepts: Oxygen, Carbon dioxide, Water, Hydrogen, Redox, Oxidizing agent, Nitrogen, Carbon

182

The emergence of oxygen-producing (oxygenic) photosynthesis fundamentally transformed our planet; however, the processes that led to the evolution of biological water splitting have remained largely unknown. To illuminate this history, we examined the behavior of the ancient Mn cycle using newly obtained scientific drill cores through an early Paleoproterozoic succession (2.415 Ga) preserved in South Africa. These strata contain substantial Mn enrichments (up to ∼17 wt %) well before those associated with the rise of oxygen such as the ∼2.2 Ga Kalahari Mn deposit. Using microscale X-ray spectroscopic techniques coupled to optical and electron microscopy and carbon isotope ratios, we demonstrate that the Mn is hosted exclusively in carbonate mineral phases derived from reduction of Mn oxides during diagenesis of primary sediments. Additional observations of independent proxies for O2-multiple S isotopes (measured by isotope-ratio mass spectrometry and secondary ion mass spectrometry) and redox-sensitive detrital grains-reveal that the original Mn-oxide phases were not produced by reactions with O2, which points to a different high-potential oxidant. These results show that the oxidative branch of the Mn cycle predates the rise of oxygen, and provide strong support for the hypothesis that the water-oxidizing complex of photosystem II evolved from a former transitional photosystem capable of single-electron oxidation reactions of Mn.

Concepts: Photosynthesis, Oxygen, Carbon dioxide, Evolution, Hydrogen, Redox, Oxidizing agent, Carbon

168

Tween 80 (polysorbate 80) has been used as a reducing agent and protecting agent to prepare stable water-soluble silver nanoparticles on a large scale through a one-pot process, which is simple and environmentally friendly. Silver ions can accelerate the oxidation of Tween 80 and then get reduced in the reaction process. The well-ordered arrays such as ribbon-like silver nanostructures could be obtained by adjusting the reaction conditions. High-resolution transmission electron microscopy confirms that ribbon-like silver nanostructures (approximately 50 nm in length and approximately 2 mum in width) are composed of a large number of silver nanocrystals with a size range of 2 to 3 nm. In addition, negative absorbance around 320 nm in the UV-visible spectra of silver nanoparticles has been observed, probably owing to the instability of nanosized silver colloids.

Concepts: Electron, Hydrogen, Redox, Oxidizing agent, Nanotechnology, Reducing agent, Polysorbate 80, Polysorbate

157

Rapid development of agriculture and fossil fuel combustion greatly increased US reactive nitrogen emissions to the atmosphere in the second half of the 20th century, resulting in excess nitrogen deposition to natural ecosystems. Recent efforts to lower nitrogen oxides emissions have substantially decreased nitrate wet deposition. Levels of wet ammonium deposition, by contrast, have increased in many regions. Together these changes have altered the balance between oxidized and reduced nitrogen deposition. Across most of the United States, wet deposition has transitioned from being nitrate-dominated in the 1980s to ammonium-dominated in recent years. Ammonia has historically not been routinely measured because there are no specific regulatory requirements for its measurement. Recent expansion in ammonia observations, however, along with ongoing measurements of nitric acid and fine particle ammonium and nitrate, permit new insight into the balance of oxidized and reduced nitrogen in the total (wet + dry) US nitrogen deposition budget. Observations from 37 sites reveal that reduced nitrogen contributes, on average, ∼65% of the total inorganic nitrogen deposition budget. Dry deposition of ammonia plays an especially key role in nitrogen deposition, contributing from 19% to 65% in different regions. Future progress toward reducing US nitrogen deposition will be increasingly difficult without a reduction in ammonia emissions.

Concepts: Carbon dioxide, Ammonia, Hydrogen, Redox, Oxidizing agent, Nitrogen, Nitric oxide, Ammonium nitrate

61

Thin-film elastomers (elastic polymers) have a number of technologically significant applications ranging from sportswear to medical devices. In this work, we demonstrate that graphene can be used to reinforce 20 micron thin elastomer films, resulting in over 50% increase in elastic modulus at a very low loading of 0.1 wt%, while also increasing the elongation to failure. This loading is below the percolation threshold for electrical conductivity. We demonstrate composites with both graphene oxide and reduced graphene oxide, the reduction being undertaken in-situ or ex-situ using a biocompatible reducing agent in ascorbic acid. The ultrathin films were cast by dip moulding. The transparency of the elastomer films allows us to use optical microscopy image and confirm the uniform distribution as well as the conformation of the graphene flakes within the composite.

Concepts: Carbon dioxide, Hydrogen, Redox, Oxidizing agent, Electrochemistry, Nitrogen, Vitamin C, Reducing agent

28

The oceans at the start of the Neoproterozoic Era (1,000-541 million years ago, Ma) were dominantly anoxic, but may have become progressively oxygenated, coincident with the rise of animal life. However, the control that oxygen exerted on the development of early animal ecosystems remains unclear, as previous research has focussed on the identification of fully anoxic or oxic conditions, rather than intermediate redox levels. Here we report anomalous cerium enrichments preserved in carbonate rocks across bathymetric basin transects from nine localities of the Nama Group, Namibia (∼550-541 Ma). In combination with Fe-based redox proxies, these data suggest that low-oxygen conditions occurred in a narrow zone between well-oxygenated surface waters and fully anoxic deep waters. Although abundant in well-oxygenated environments, early skeletal animals did not occupy oxygen impoverished regions of the shelf, demonstrating that oxygen availability (probably >10 μM) was a key requirement for the development of early animal-based ecosystems.

Concepts: Photosynthesis, Carbon dioxide, Water, Oxidizing agent, Animal, Sedimentary rock, Carbon, Animals

28

Understanding and improving in vivo materials related to signal stability and preservation for active chemical sensor and biosensor transduction systems is critical in achieving implantable medical sensors for long-term in vivo applications. During human in vivo clinical testing of an implantable glucose sensor based on a glucose sensitive hydrogel, post-explant analysis showed that the boronate recognition element had been oxidized from the fluorescent indicator, causing a rapid loss of signal within hours after implant. Additional wet-bench analytical evidence and reproduction in vitro suggests reactive oxygen species, particularly hydrogen peroxide (H2 O2 ), stemming from natural inflammatory response to the material, to be the cause of the observed oxidative de-boronation. A 3-nm thick deposition of metallic platinum (Pt) placed by plasma sputtering onto the porous surface of the hydrogel, showed immediate protection from sensor signal loss due to oxidation both in vitro and in vivo, greatly extending the useful lifetime of the implantable glucose sensor from 1 day to an expected ≥6 months. This finding may represent a new strategy to protect an implanted material and/or device from in vivo oxidative damage, leading to much improved overall stability and reliability for long-term applications. © 2012 Wiley Periodicals, Inc. J Biomed Mater Res Part A, 2013.

Concepts: Oxygen, Hydrogen, Redox, Oxidative stress, Oxidizing agent, Oxidative phosphorylation, Reactive oxygen species, Hydrogen peroxide

28

Reaction of L(0)NiBr(2) with 2 equiv of NaH yielded the Ni(II) hydride complex [(L(•-))Ni(μ-H)(2)Ni(L(•-))] (1) (L = [(2,6-iPr(2)C(6)H(3))NC(Me)](2); L(0) represents the neutral ligand, L(•-) is its radical-anionic form, and L(2-) denotes the dianion) in good yield. Stepwise reduction of complex 1 led to a series of nickel hydrides. Reduction of 1 with 1 equiv of sodium metal afforded a singly reduced species [Na(DME)(3)][(L(•-))Ni(μ-H)(2)Ni(L(•-))] (2a) (DME = 1,2-dimethoxyethane), which contains a mixed-valent core [Ni(μ-H)(2)Ni](+). With 2 equiv of Na a doubly reduced species [Na(DME)](2)[L(2-)Ni(μ-H)(2)NiL(2-)] (3a) was obtained, in which each monoanion (L(•-)) in the precursor 1 has been reduced to L(2-). By using potassium as the reducing agent, two analogous species [K(DME)(4)][(L(•-))Ni(μ-H)(2)Ni(L(•-))] (2b) and [K(DME)](2)[L(2-)Ni(μ-H)(2)NiL(2-)] (3b) were obtained. Further treatment of 3b with 2 equiv of K led to a trinuclear complex [K(DME)(THF)](2)K(2)[L(2-)Ni(μ-H)(2)Ni(μ-H)(2)NiL(2-)] (4), which contains one Ni(II) and two Ni(I) centers with a triplet ground state. When 1 and 3a were warmed in toluene or benzene, respectively, three reverse-sandwich dinickel complexes, [(L(•-))Ni(μ-η(3):η(3)-C(7)H(8))Ni(L(•-))] (5) and [Na(DME)](2)[L(2-)Ni(μ-η(3):η(3)-C(6)H(5)R)NiL(2-)] (6: R = CH(3); 7: R = H), were isolated. Reaction of 1 with Me(3)SiN(3) gave the N(3)-bridged complex [(L(•-))Ni(μ-η(1)-N(3))(2)Ni(L(•-))] (8). The crystal structures of complexes 1-8 have been determined by X-ray diffraction, and their electronic structures have been fully studied by EPR/NMR spectroscopy.

Concepts: Hydrogen, Redox, Oxidizing agent, Ion, Hydride, Sodium, Reducing agent, Sodium hydride

28

A mixture of ferrous ions and hydrogen peroxide, known as Fenton’s reagent, is an effective oxidant and has been widely used in various industrial applications; however, there is still controversy about what the oxidizing agents are and how they are produced. In this study, we have determined minimum free-energy paths (MFEPs) from Fenton’s reagent to possible oxidizing agents such as hydroxyl radicals and ferryl-oxo species by combining ab initio molecular dynamics simulations and an MFEP search method. Along the MFEPs, representative free-energy profiles of the Fenton reaction were elucidated. Based on the free-energy profiles, we revealed that the reaction producing ferryl-oxo species from Fenton’s reagent is more energetically favorable than that yielding a free hydroxyl radical, by 24.4 kcal mol(-1), which indicates that the ferryl-oxo species is the primary oxidizing agent in reactions of Fenton’s reagent. Moreover, we clarified that the ferryl-oxo species is favorably formed via a two-step reaction pathway, which reaches the product through a dihydroxyiron(IV) intermediate. The energetics charting the free-energy profiles provided valuable information for a comprehensive understanding of Fenton reactions. We concluded that a ferryl-oxo species produced from Fenton’s reagent serves as the primary oxidizing agent in the Fenton reaction.

Concepts: Oxygen, Redox, Oxidizing agent, Radical, Hydrogen peroxide, Fenton's reagent, Hydroxyl radical, Oxidizing agents

27

Best of both worlds: Ptx Pdy , Pt, and Pd aerogels with high surface area and porosity can be synthesized in a controlled fashion by a straightforward and environmentally benign strategy. These materials, which are highly active and stable catalysts for the oxygen reduction reaction in PEFC cathodes, combine the high stability of extended surfaces with the high surface area of nanoparticles.

Concepts: Photosynthesis, Carbon dioxide, Iron, Hydrogen, Oxidizing agent, Electrochemistry, Cellular respiration, Nitrogen