Low O2 Concentrations Research Articles

Tuning the band gap and nitrogen content of ZnOxNy thin films deposited by reactive HiPIMS

We show that it is possible to manipulate the stoichiometry and composition of the thin films deposited by reactive high power impulse magnetron sputtering (HiPIMS) by the design of the discharge pulsing scheme. This is illustrated for the case of the short-pulse HiPIMS of a pure Zn target in Ar/N2/O2 mixture gas (low content of O2), where the variation of the discharge pulsing frequency has led to depositions of crystalline ZnOxNy thin films with the nitrogen content and optical band gap values ranging from 0 to 6.2at.% and from 3.34 to 1.67eV, respectively. The fine control of nitrogen content in the deposited ZnOxNy thin films was obtained by the use of smooth transitions of the reactive HiPIMS discharge between the compound and metallic target sputtering modes. Thus, the yield of the sputtered metal atoms was controlled by the discharge pulsing frequency from low values, in the compound target sputtering mode, to large values, in the metallic target sputtering mode. The controlled amount of metal atoms in the gas phase and the limited amount of oxygen in the deposition chamber determined depositions of films with variable content of nitrogen.

The enhancement of hydrogen photoproduction in marine Chlorella pyrenoidosa under nitrogen deprivation

Previously, we found that marine Chlorella pyrenoidosa strain IOAC707S could generate hydrogen (H2) under nitrogen deprivation in seawater medium, without dark incubation, N2 flushing or addition of protonophore. In this work, the H2 yield of C. pyrenoidosa was enhanced, and the mechanism of H2 photoproduction was investigated. It was found that C. pyrenoidosa could produce H2 under hypoxic conditions, which indicated the hydrogenase of C.pyrenoidosa could tolerate a low concentration of O2. Nitrogen deprivation could lower the efficiency of photosyntem (PS) II photochemical activity and PSII oxygenic activities, which were favorable for rapid establishment of anoxia and efficient H2 photoproduction, and thus significantly enhance H2 photoproduction of C. pyrenoidosa. Pre-culture in nitrogen-deprived medium could further increase the H2 yield of C. pyrenoidosa, for its more severe inhibition of the oxygen-evolving complex, electron transport and photochemical efficiency of PSII, and PSII oxygenic activities, which facilitated the establishment of anaerobiosis and activation of hydrogenase.

Transcriptomic analysis of fruit stored under cold conditions using controlled atmosphere in Prunus persica cv. "Red Pearl".

Cold storage (CS) can induce a physiological disorder known as chilling injury (CI) in nectarine fruits. The main symptom is mealiness that is perceived as non-juicy fruit by consumers. Postharvest treatments such as controlled atmosphere (CA; a high CO2 concentration and low O2) have been used under cold conditions to avoid this disorder. With the objective of exploring the mechanisms involved in the CA effect on mealiness prevention, we analyzed transcriptomic changes under six conditions of “Red Pearl” nectarines by RNA-Seq. Our analysis included just harvested nectarines, juicy non-stored fruits, fruits affected for CI after CS and fruits stored in a combination of CA plus CS without CI phenotype. Nectarines stored in cold conditions combined with CA treatment resulted in less mealiness; we obtained 21.6% of juice content compared with just CS fruits (7.7%; mealy flesh). RNA-Seq data analyses were carried out to study the gene expression for different conditions assayed. During ripening, we detected that nectarines exposed to CA treatment expressed a similar number of genes compared with fruits that were not exposed to cold conditions. Firm fruits have more differentially expressed genes than soft fruits, which suggest that most important changes occur during CS. On the other hand, gene ontology analysis revealed enrichment mainly in metabolic and cellular processes. Differentially expressed genes analysis showed that low O2 concentrations combined with cold conditions slows the metabolic processes more than just the cold storage, resulting mainly in the suppression of primary metabolism and cold stress response. This is a significant step toward unraveling the molecular mechanism that explains the effectiveness of CA as a tool to prevent CI development on fruits.

Rate Constant for the OH + CO Reaction at Low Temperatures.

Rate constants for the reaction of OH + CO → products (1) have been measured using laser photolysis/laser-induced fluorescence (LP/LIF) over the temperature range 193–296 K and at pressures of 50–700 Torr of Ar and N2. The reaction was studied under pseudo-first-order conditions, monitoring the decay of OH in the presence of a large excess of CO. The rate constants can be expressed as a combination of bimolecular and termolecular components. The bimolecular component was found to be temperature-independent with an expression given by kbi(T) = (1.54 ± 0.14) × 10(–13)[e(–(13±17)/T)] cm(3) molecule(–1) s(–1), with an error of one standard deviation. The termolecular component was fitted to the expression, kter = k0(T)[M]/[1 + (k0(T)[M]/k∞(T)] × 0.6({1+[log10(k0(T)[M]/k∞(T))]2}−1) where k0(T) = k0(300)(T/300)(−n) and k∞(T) = k∞(300)(T/300)(−m). The parameters for k0(T) were determined to be k0(300) = (6.0±0.5) × 10(−33) cm(6) molecule(–2) s(–1) in N2 and k0(300) = (3.4 ± 0.3) × 10(–33) cm(6) molecule(–2) s(–1) in Ar, with n = 1.9±0.5 and 2.0±0.4 in N2 and Ar, respectively. These parameters were determined using k0(T) and m from the NASA kinetics data evaluation (JPL Publication No. 10-6) since the experimental pressure range was far from the high-pressure limit. Addition of low concentrations of O2 had no discernible effect on the mechanism of the OH + CO reaction but resulted in secondary reactions which regenerated OH.

Diffusion MILD Combustion of Firing Pulverized-coal at a Pilot Furnace

This paper presents the computational investigation of the diffusion MILD (Moderate or Intense Low-oxygen Dilution) combustion of firing pulverized-coal at a pilot furnace. Dependences of the combustion performance are examined on the injection parameters, e.g., the fuel injection angle and the separation between the primary and secondary nozzles. Calculations are performed to examine the combustion characteristics, e.g., velocity and temperature distributions, CO and NO emissions, and char burnout. The validation of the modeling is carried out using the experimental measurements of Weber et al. (Proc. Combust. Inst. 30, 30 (2), 2623–2629 2005). Results reveal that an increase in the fuel injection angle or the separation reduces the peak temperature in the confluence region of the primary and secondary streams and thus the exhaust NO emission. During the coal MILD combustion, the (destroying) NO reburning mechanism is strongly involved so that the NO emission is below the NO production from the fuel-NO route, given that other routes of NO formation are unimportant. It is also found that the gasification reactions of char with CO2 and H 2O in coal MILD combustion appear to compensate for depression of the low O2 concentration on the coal burnout. In addition, several suggestions are made that could be useful for design of coal MILD burners.

Storage of fresh-cut swede and turnip in modified atmosphere: effects on vitamin C, sugars, glucosinolates and sensory attributes

Fresh-cut swede (Brassica napus L. var. napobrassica (L.) Rchb.) and turnip (Brassica rapa L. ssp. rapifera Metzg.) in modified atmosphere packaging (MAP) were stored for 5 or 10 d, at 5°C or 10°C. Two passive and one active (flushed with 5% O2 at sealing) modified atmospheres were tested. Vitamin C content was not influenced by the experimental parameters. The content of total sugar decreased during storage for both vegetables, and significant effects of time, temperature and atmosphere were observed for glucose, fructose and sucrose. Total aliphatic and indolic glucosinolates increased in swede during storage, while for turnip total aliphatic glucosinolates decreased and total indolic glucosinolates increased. Atmosphere composition affected glucosinolates, and the response varied between individual glucosinolates. Taste and flavour were more influenced by time and temperature, than by atmosphere in both vegetables. However, modified atmosphere with low O2 concentration reduced discolouration related changes, especially for turnip. Positive correlation was observed between sugar content and sweet taste for both vegetables. Sweet and bitter tastes were negatively correlated for swede. Low temperature and short storage time were the most important criteria to prevent changes of appearance, odours, taste and flavours, and contents of sugar and glucosinolates of fresh-cut swede and turnip.

Response of carbon assimilation and chlorophyll fluorescence to soybean leaf phosphorus across CO2: Alternative electron sink, nutrient efficiency and critical concentration

To evaluate the response of CO2 assimilation rate (PN) and various chlorophyll fluorescence (CF) parameters to phosphorus (P) nutrition, soybean plants were grown in controlled environment with sufficient (0.50mM) and deficient (0.10 and 0.01mM) phosphate (P) supply under ambient and elevated CO2 (aCO2, 400 and eCO2, 800μmolmol−1, respectively). Measurements were made at ambient (21%) and low (2%) O2 concentrations. Results showed strong correlation of leaf P concentration with PN and CF parameters. The P deficiency showed parallel decreases in PN, and CF parameters including quantum efficiency (Fv′/Fm′), quantum yield of photosystem II (ΦPSII), electron transport rate (JF), and photochemical quenching (qP). The Fv′/Fm′ decreased as a result of greater decline in maximal (Fm′) than minimal (Fo′) fluorescence. The eCO2 stimulated PN especially under higher leaf P concentrations. Low O2 also stimulated PN but only at aCO2. The photosynthetic carbon reduction (PCR, signified by PN) and photorespiratory carbon oxidation cycles (PCO, signified photorespiration as indicated by ratio of JF to gross PN and % increase in PN at 2% O2) was the major electron sinks. However, the presence of alternative electron sink was also evident as determined by the difference between the electron transport calculated from chlorophyll fluorescence and gas exchange measurements. Alternative electron sink declined at lower leaf P concentration suggesting its minor role in photochemical energy consumption, thus dissipation of the excess excitation pressure of PSII reaction center under P deficiency. The JF/PG and % increase in PN at 2 versus 21% O2 remained consistent across leaf P concentration suggesting PCO cycle as an important mechanism to dissipate excess excitation energy in P deficient leaves. The severe decline of Fv′/Fm′, ΦPSII, JF and qP under P deficiency also suggested the occurrences of excess radiant energy dissipation by non-photochemical quenching mechanisms. Critical leaf P concentration (CLPC) needed to achieve 90% of the maximum value was greater for PN than CF parameters. Moreover, CLPC was always higher at eCO2 suggesting increased sensitivity of soybean to P deficiency under eCO2. An increased phosphorus utilization efficiency of PN and CF parameters was also achieved but with the expense of net CO2 assimilation in P-deficient leaves.

Quality of guava (Psidium guajava L. cv. Pedro Sato) fruit stored in low-O2 controlled atmospheres is negatively affected by increasing levels of CO2

Guava is a climacteric fruit, extremely perishable, that has a short shelf life. Controlled atmosphere (CA) and cold storage can be used to extend guava shelf life. However, few studies have been conducted to evaluate the effects of CA storage on guavas, especially with high levels of carbon dioxide (CO2). Therefore the objective of this study was to evaluate the quality changes of ‘Pedro Sato’ guava fruit during CA storage with atmospheres containing low concentrations of O2 and increasing levels of CO2. For that, ‘Pedro Santo’ guavas were stored at 12.2°C for up to 28 days in atmospheres with low oxygen (O2) concentration (5kPa) and increasing level of CO2 (1, 5, 10, 15 or 20kPa CO2) in order to evaluate the fruit quality changes. It was possible to identify the relationships among quality variables in response to atmosphere composition and storage duration which were related to modifications due to ripening (ratio SS/TA, TSS, RS, TA), ripening and early CO2 injury modifications (pH, SS, chromaticity), and CO2 injury modifications (hue angle, firmness, soluble pectin). A clear CO2 injury occurred in fruit stored in 5kPa O2+15kPa CO2 and 5kPa O2+20kPa CO2 after 28 days at 12.2°C, with increasing pH values and soluble pectin content, which were inversely related to fruit firmness. ‘Pedro Sato’ guavas should be stored in atmospheres with 5kPa O2 and no more than 5kPa CO2 in order to prevent CO2 damage. Further investigation is necessary to identify the metabolites responsible for the onset of responses to high CO2 in guava fruit.

Nitric Oxide And Oxygen: Actions And Interactions In Health And Disease

Nitric oxide (NO) inhibits cell respiration reversibly and in competition with O2 through the inhibition of the mitochondrial cytochrome c oxidase (Complex IV). At concentrations lower than those required to inhibit respiration, endogenous NO enhances the reduction of the electron transport chain, thus enabling cells to maintain their O2 consumption. This however facilitates the release of superoxide anion, which initiates the transcriptional activation of NF-kB as an early signal of a stress response. Through free radical formation, long-term inhibition of mitochondrial respiration by NO leads to persistent inhibition of Complex I. This is dependent on the S-nitrosylation of a specific thiol in the active form of this protein. S-nitrosylation of Complex I might indicate the early stages of a pathological process. Inhibition of mitochondrial respiration by low concentrations of NO at critical O2 concentrations also leads to prevention of the stabilization of hypoxia-inducible factor-1α (HIF-1α) due to the redistribution of O2 towards non-respiratory O2-dependent targets. This prevents the cell from registering a state of hypoxia at low O2 concentrations. On the other hand, at higher concentrations such as those generated in certain forms of cancer, NO increases the expression of HIF-1α by an action most probably involving a free radical mechanism. It is likely that the interactions between oxygen and NO, either at the mitochondria or in the cell in general, play a role in the initiation and development of neoplastic transformation and spreading. The ways in which these interactions operate remain unclear and are likely to vary from cancer to cancer.

Kinetic Investigations of the Role of Factor Inhibiting Hypoxia-inducible Factor (FIH) as an Oxygen Sensor

The hypoxia-inducible factor (HIF) hydroxylases regulate hypoxia sensing in animals. In humans, they comprise three prolyl hydroxylases (PHD1-3 or EGLN1-3) and factor inhibiting HIF (FIH). FIH is an asparaginyl hydroxylase catalyzing post-translational modification of HIF-α, resulting in reduction of HIF-mediated transcription. Like the PHDs, FIH is proposed to have a hypoxia-sensing role in cells, enabling responses to changes in cellular O2 availability. PHD2, the most important human PHD isoform, is proposed to be biochemically/kinetically suited as a hypoxia sensor due to its relatively high sensitivity to changes in O2 concentration and slow reaction with O2. To ascertain whether these parameters are conserved among the HIF hydroxylases, we compared the reactions of FIH and PHD2 with O2. Consistent with previous reports, we found lower Km(app)(O2) values for FIH than for PHD2 with all HIF-derived substrates. Under pre-steady-state conditions, the O2-initiated FIH reaction is significantly faster than that of PHD2. We then investigated the kinetics with respect to O2 of the FIH reaction with ankyrin repeat domain (ARD) substrates. FIH has lower Km(app)(O2) values for the tested ARDs than HIF-α substrates, and pre-steady-state O2-initiated reactions were faster with ARDs than with HIF-α substrates. The results correlate with cellular studies showing that FIH is active at lower O2 concentrations than the PHDs and suggest that competition between HIF-α and ARDs for FIH is likely to be biologically relevant, particularly in hypoxic conditions. The overall results are consistent with the proposal that the kinetic properties of individual oxygenases reflect their biological capacity to act as hypoxia sensors.

Modified atmosphere packaging of precooked vegetables: Effect on physicochemical properties and sensory quality

This study aims at verifying the effect of three modified atmosphere packaging (MAP) conditions, all with high CO2 and residual or low O2 contents (%O2/%CO2: 0/40; 2.5/40 and 2.5/60), on the quality preservation of several species of precooked vegetables (cabbage, carrots, green beans and bell peppers). The study was carried out for different storage periods (up to 28days and 6 sampling periods). Physicochemical parameters (pH, acidity, moisture and ash contents, antioxidant activity, colour, and texture), microbial growth, organoleptic properties and consumer acceptability were assessed. Concerning physicochemical parameters and microbial growth only slight changes without any consistent tendency were observed. This was also confirmed by the trained panel that could not discriminate samples with different storage times. Best preservation conditions were obtained with 0%O2/40%CO2, promoting a shelf life extension of almost 12days more comparing to commercial conditions presently used.

Three Pseudomonas putida FNR Family Proteins with Different Sensitivities to O2

The Escherichia coli fumarate-nitrate reduction regulator (FNR) protein is the paradigm for bacterial O2-sensing transcription factors. However, unlike E. coli, some bacterial species possess multiple FNR proteins that presumably have evolved to fulfill distinct roles. Here, three FNR proteins (ANR, PP_3233, and PP_3287) from a single bacterial species, Pseudomonas putida KT2440, have been analyzed. Under anaerobic conditions, all three proteins had spectral properties resembling those of [4Fe-4S] proteins. The reactivity of the ANR [4Fe-4S] cluster with O2 was similar to that of E. coli FNR, and during conversion to the apo-protein, via a [2Fe-2S] intermediate, cluster sulfur was retained. Like ANR, reconstituted PP_3233 and PP_3287 were converted to [2Fe-2S] forms when exposed to O2, but their [4Fe-4S] clusters reacted more slowly. Transcription from an FNR-dependent promoter with a consensus FNR-binding site in P. putida and E. coli strains expressing only one FNR protein was consistent with the in vitro responses to O2. Taken together, the experimental results suggest that the local environments of the iron-sulfur clusters in the different P. putida FNR proteins influence their reactivity with O2, such that ANR resembles E. coli FNR and is highly responsive to low concentrations of O2, whereas PP_3233 and PP_3287 have evolved to be less sensitive to O2.

Contribution of Opioid Receptors to the Cytoprotective Effect of the Adaptation to Chronic Hypoxia at Anoxia/Reoxygenation of Isolated Cardiomyocytes.

We examined the role of opioid receptors in the cardioprotective effect of chronic continuous normobaric hypoxia in the model of anoxia/reoxygenation of isolated cardiomyocytes. Adaptation to hypoxia was provided by placing the rats for 21 days in an atmosphere with low O2 content. In intact rats, anoxia/reoxygenation of cardiomyocytes caused death of 23% cells and increased lactate dehydrogenase release from cardiomyocytes. In adapted rats, anoxia/ reoxygenation of cardiomyocytes caused death of only 2.5% cells and the release of lactate dehydrogenase decreased by 25%. Preincubation of the cells with opioid receptor blocker naloxone (300 nM) for 25 min eliminated the adaptive decrease in cell survival and reduction in lactate dehydrogenase release. Hence, opioid receptors of the cardiomyocytes contribute to the cytoprotective effect of chronic normobaric hypoxia.

Experimental study on preheated combustion of pulverized semi-coke

In a test rig, pulverized semi-coke was preheated to 850oC in a circulating fluidized bed (CFB) and then combusted at 1100oC in a down-fired combustor (DFC). Experiments were conducted to reveal the effects of three secondary air nozzle cases (co-axial jet, top circular jet and wall circular jet) on the NO emission. The results show that the optimized secondary air nozzle can reduce NO emission. O2 concentration profile is the major factor affecting NO generation and emission, which is led by the secondary air nozzle. The lower O2 concentration led to the generation of lower initial NO. The NO emission at the exit of the DFC was reduced from 189 to 92 mg/m3 (@ 6% O2) with the decrease of initial generation. The peak of NO at 100 mm below the nozzle should be attributed to the oxidization of NH3 in the syngas, rather than the oxidization of fuel-N in the char. The low and well-distributed O2 concentration contributes to the reduction of initial NO, which helps to reduce the NO emission. The combustion efficiencies of the cases of the co-axial jet, the top circular jet, and the wall circular jet are 97.88%, 98.94% and 98.74%, respectively.

Improvement of hydrogen production by reduction of the photosynthetic oxygen in microalgae cultures of Chlamydomonas gloeopara and Scenedesmus obliquus

One of the most significant challenges in hydrogen (H2) production by microalgae is the incompatibility between oxygenic photosynthesis and anaerobic hydrogen production due to the high sensitivity of hydrogenase to oxygen (O2). One strategy for mitigating this problem is to decrease the concentration of photosynthetic O2 as soon as it is produced. This work proposes that if the photosynthetic O2 in the culture medium is depleted will be possible to produce hydrogen. To this end, we studied the effects of 4 chemical reducing agents (cysteine, sodium sulfite, ascorbic acid, and hydroquinone) on the growth and consumption of photosynthetic O2 in cultures of Chlamydomonas gloeopara and Scenedesmus obliquus. All reducing agents were tested at 3 concentrations in triplicate. Then, anaerobic hydrogen production was improved through photosynthetic oxygen consumption using a chemical reducing agent. Of the compounds, only cysteine did not have inhibitory effects against algal growth (even stimulating it) and maintained low O2 concentrations for 192 h. In descending order of activity, hydroquinone, ascorbic acid, and sodium sulfite inhibited the growth of both algal species at the concentrations that were tested. Thus, cysteine was used to induce the anaerobic H2 production in algal cultures with a sulfur-containing TAP medium. Hydrogen production was between 3 and 5 times greater when photosynthetic O2 was eliminated with cysteine than when the sulfur-deprived protocol was applied. Also, algal cultures with cysteine experienced similar growth and carbohydrate metabolism as those in TAP medium, indicating that direct and indirect photolysis supplies electrons to hydrogenase for H2 production. Based on our results, oxygenic photosynthesis and hydrogen production were reconciled for over 40 h by capturing the produced O2 by a reducing agent.

THE EFFECT OF HEAT TREATMENT ON POST-HARVEST QUALITY OF FRESH-CUT NECTARINE

The work investigate the effect of short-term heat treatment on quality of fresh-cut nectarines ‘Orion’. This treatment may have effects on the fruit beyond their stated purpose because a high temperature stress can trigger changes in plant tissue that affect many physiological processes. These processes include inhibition of ethylene production and other ripening and senescence related processes, induction of defense compounds against pathogens, attack and induction of resistance to other stresses, including low temperature stress. The result of the treatment is to maintain fruit quality following the heat treatment. In this work, a pre-cutting heat treatment at 40°C for 50 min and modified atmosphere packaging (5 kPa O2 and 5 kPa CO2) storage was evaluated. The quality-affecting parameters were evaluated by physical and chemical methods (firmness, colour, pH, soluble solids content and titratable acidity) and by the evaluation of physiological aspects (PPO activity) as well as the changes in headspace gas composition during storage for 11 days at 4°C. Significantly lower concentrations of O2 in the package atmosphere were recorded for heat-treated slices. In contrast, hot water treatment did not reduced firmness loss. Moreover, an insignificant effect of heat treatment and modified atmosphere on chemical composition (total soluble solids and titratable acidity) and colour parameters was observed. Also for the polyphenoloxidase activity no significant changes was recorded.

Increased Turnover at Limiting O2 Concentrations by the Thr(387) → Ala Variant of HIF-Prolyl Hydroxylase PHD2.

PHD2 is a 2-oxoglutarate, non-heme Fe(2+)-dependent oxygenase that senses O2 levels in human cells by hydroxylating two prolyl residues in the oxygen-dependent degradation domain (ODD) of HIF1α. Identifying the active site contacts that determine the rate of reaction at limiting O2 concentrations is crucial for understanding how this enzyme senses pO2 and may suggest methods for chemically altering hypoxia responses. A hydrogen bonding network extends from the Fe(II) cofactor through ordered waters to the Thr(387) residue in the second coordination sphere. Here we tested the impact of the side chain of Thr(387) on the reactivity of PHD2 toward O2 through a combination of point mutagenesis, steady state kinetic experiments and {FeNO}(7) EPR spectroscopy. The steady state kinetic parameters for Thr(387) → Asn were very similar to those of wild-type (WT) PHD2, but kcat and kcat/KM(O2) for Thr(387) → Ala were increased by roughly 15-fold. X-Band electron paramagnetic resonance spectroscopy of the {FeNO}(7) centers of the (Fe+NO+2OG) enzyme forms showed the presence of a more rhombic line shape in Thr(387) → Ala than in WT PHD2, indicating an altered conformation for bound gas in this variant. Here we show that the side chain of residue Thr(387) plays a significant role in determining the rate of turnover by PHD2 at low O2 concentrations.

Responses of methanotrophic activity, community and EPS production to CH4 and O2 concentrations in waste biocover soils

Biocover soils are known to be a good alternative material to mitigate CH4 emissions from landfills to the atmosphere. In this study, 16 treatments with four O2 concentrations (∼0%, 5%, 10% and 21%) and four CH4 concentrations (i.e. 1%, 10%, 20% and 50%) were conducted to estimate extracellular polymeric substances (EPS) production, methanotrophic activity and community in response to CH4 and O2 concentrations in waste biocover soil (WBS). When the CH4 concentration was saturated for CH4 oxidation in the WBS, the continuous exposure of CH4 above the saturated concentrations could not obviously enhance CH4 oxidation activity. In the WBS, extracellular protein (ECP) production was negatively related with the tested CH4 concentrations, while both ECP and extracellular polysaccharides (ECPS) productions were positively related with the tested O2 concentrations. Cloning and terminal restriction fragment length polymorphism analyses showed that type I methanotrophs (Methylocaldum, Methylococcaceae, Methylomicrobium and Methylobacter) and type II methanotrophs (Methylosinus) dominated in the WBS. Among them, Methylocaldum and/or Methylococcaceae were sensitive to low O2 concentrations of ∼0%. Methylobacter had propensity to grow at low O2 concentrations of ∼0% and 5%, while Methylosinus preferred environments with high concentrations of CH4 (⩾10%) and O2 (21%). In the tested five environmental variables of ECPS, O2, EPS, CH4 and ECP, only ECPS and O2 concentrations had significant effect on the methanotrophic communities. These results suggested that O2 concentration in landfill covers should be paid more attention to optimize and sustain CH4 oxidation for mitigating CH4 emission from landfills.

Experimental evidence of the generation of gaseous SiO as precursor for the growth of SiOx nanowires

The vapor–liquid–solid process is a fundamental and widely used mechanism for the growth of nanowires. In this article, experimental observations have been carried out to explain the growth mechanism of silicon-based nanowires using a solid-bulk silicon source as unique precursor. The synthesis consisted of a thermal treatment at 900°C under an Ar-H2 atmosphere with a low residual O2 concentration. The presence of SiO in gaseous phase, originated via the active oxidation of the Si substrate, is shown as the principal factor responsible for the nanowires growth, via a process commonly termed as solid–vapor–liquid–solid.

Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the oxygen reduction reaction

Functionalized carbon nanotubes (CNTs) using three aminobenzene acids with different functional groups (carboxylic, sulphonic, phosphonic) in para position have been synthesized through potentiodynamic treatment in acid media under oxidative conditions. A noticeable increase in the capacitance for the functionalized carbon nanotubes mainly due to redox processes points out the formation of an electroactive polymer thin film on the CNTs surface along with covalently bonded functionalities. The CNTs functionalized using aminobenzoic acid rendered the highest capacitance values and surface nitrogen content, while the presence of sulfur and/or phosphorus groups in the aminobenzene structure yielded a lower functionalization degree. The oxygen reduction reaction (ORR) activity of the functionalized samples was similar to that of the parent CNTs, independently of the functional group present in the aminobenzene acid. Interestingly, a heat treatment in N2 atmosphere with a very low O2 concentration (3125 ppm) at 800 °C of the CNTs functionalized with aminobenzoic acid produced a material with high amounts of surface oxygen and nitrogen groups (12 and 4% at., respectively), that seem to modulate the electron-donor properties of the resulting material. The onset potential and limiting current for ORR was enhanced for this material. These are promising results that validates the use of electrochemistry for the synthesis of novel N-doped electrocatalysts for ORR in combination with adequate heat treatments.