Oxygen-saturated Solution Research Articles

Decatungstate photocatalyzed oxygenation of methanol in acetonitrile under photostationary state conditions

Abstract Complementary techniques including laser flash photolysis have been used to investigate the mechanism of decatungstate photocatalyzed oxygenation of methanol in acetonitrile. A particular attention has been directed to determine the most important chemical and photophysical kinetic parameters of this peculiar reaction. Under continuous photolysis, kinetics of both oxygen consumption and hydrogen peroxide formation has been followed in a closed system. The reaction was stopped when a conversion of less than 10% in oxygen-saturated solutions was reached. Our work enlightens the following findings: (i) as expected on the basis of a hydrogen-atom-abstraction mechanism and relative C H bond dissociation energy, the reaction constant rate of reactive species wO with methanol is the weakest of all alcohols already studied, (ii) the methanol decatungstate photocatalyzed oxygenation in acetonitrile satisfies the photostationary state conditions only up to [CH3OH] ∼ 2.5 M, (iii) the role of peroxyl radical is crucial since it not only leads to the resulting products, but it also oxidizes the reduced form of decatungstate H+W10O325− to regenerate the catalyst, thus closing the catalytic cycle, and (iv) progressive establishment of anaerobic conditions results in both ending of the substrate conversion and the decay of hydrogen peroxide. Hence, these findings offer new insights into the nature of the rate-determining step in the photocatalytic cycle.

Oxygen Reduction Reaction Activity of Aerosol Jet Printed Layers of Graphite and Graphene

Graphene and graphene-based materials have shown great promise in a broad range of applications including electrochemical sensors, supercapacitors, and lithium ion batteries. However, practical realization of the potential utility of graphene requires development of low cost, high volume methods of material synthesis and manufacturing-ready methods of device fabrication. This work aims to address the second challenge by demonstrating the use of aerosol jet printing for the fabrication of catalytically active electrodes.Graphene and graphite were formulated into high concentration inks (10mg/mL). These inks were aerosol jet printed to create micron scale layers (2 – 10 microns). The catalytic activity of the aerosol jet printed electrodes was evaluated in oxygen saturated basic solution. It was found that the oxygen reduction reaction activity of the aerosol jet printed graphene electrode was ~ 5 times higher times than the activity of the same material when drop cast. Electrochemical impedance spectroscopy showed that the activity difference correlated with a large decrease in the charge transfer resistance of aerosol jet printed graphene as opposed to drop cast graphene. Further, the activity of the aerosol jet printed graphene electrode was ~ 3 times greater than the activity of the aerosol jet printed graphite. In contrast, the drop cast materials did not show a measurable difference in performance. These results indicate that aerosol jet printing is a viable way to deposit micron scale layers of graphene and related materials. In addition, the activity of the graphene electrode is strongly dependent on the deposition method, with aerosol jet deposited electrodes showing a strong performance improvement over their drop cast counterparts.

Luminescence and photosensitivity of gadolinium labeled hematoporphyrin monomethyl ether

Photodynamic therapy for deep-lying lesions needs an appropriate imaging modality, precise evaluation of tissue oxygen and an effective photosensitizer. Gadolinium based metalloporphyrins Gd(III)-HMME is proposed in this study as a potential multifunctional theranostic agent, as photosensitizer, ratiometric oxygen sensor and MRI contrast agent. The time resolved spectroscopy revealed the luminescence peak of Gd(III)-HMME at 710 and 779 nm with a lifetime of 64 μs in oxygen-free methanol to be phosphorescent. This phosphorescence is strongly dependent on dissolved oxygen concentration. Its intensity in oxygen saturated methanol solution is 21% of that in deoxygenated solution. The singlet oxygen quantum yields ΦΔ of HMME and Gd(III)-HMME in air saturated methanol solution were determined to be 0.79 and 0.40 respectively using comparative spectra method. These phenomena indicate that the oxygen sensibility and production of singlet oxygen of Gd(III)-HMME can fulfill the requirement of PDT treatment.

Non-additive voltammetric limiting currents of acrylamide with oxygen and hydrogen peroxide

The non-additivity of the reduction currents of acrylamide (AA) and oxygen and of AA and hydrogen peroxide is reported. These results were obtained using hydrodynamic voltammetry at a rotating mercury covered gold electrode in solutions of 0.10M tetramethylammonium hydroxide and 0.10M LiCl. In air and oxygen saturated solutions AA limiting currents were 6–40% lower than in oxygen free solutions. The decreases in the apparent AA currents were greater when the AA/O2 concentration ratio was smaller.An explanation for the non-additivity is proposed.These results have implications regarding the determination of AA using electrochemical detection when the analyte concentrations are small and rigorous exclusion of oxygen is difficult, e.g., HPLC–EC.

The strong catalytic effect of Pb(ii) on the oxygen reduction reaction on 5 nm gold nanoparticles

Citrate-capped gold nanoparticles (AuNPs) of 5 nm in diameter are synthesized via wet chemistry and deposited on a glassy carbon electrode through electrophoresis. The kinetics of the oxygen reduction reaction (ORR) on the modified electrode is determined quantitatively in oxygen-saturated 0.5 M sulphuric acid solution by modelling the cathode as an array of interactive nanoelectrodes. Quantitative analysis of the cyclic voltammetry shows that no apparent ORR electrocatalysis takes place, the kinetics on AuNPs being effectively the same as on bulk gold. Contrasting with the above, a strong ORR catalysis is found when Pb(2+) is added to the oxygen saturated solution or when the modified electrode is cycled in lead alkaline solution such that lead dioxide is repeatedly electrodeposited and stripped off on the nanoparticles. In both cases, the underpotential deposition of lead on the gold nanoparticles is found to be related to the catalysis.

Laccase/AuAg Hybrid Glucose Microfludic Fuel Cell

In this work a hybrid microfluidic fuel cell was fabricated and evaluated with a AuAg/C bimetallic material for the anode and an enzymatic cathode. The cathodic catalyst was prepared adsorbing laccase and ABTS on Vulcan carbon (Lac-ABTS/C). This material was characterized by FTIR-ATR, the results shows the presence of absorption bands corresponding to the amide bounds. The electrochemical evaluation for the materials consisted in cyclic voltammetry (CV). The glucose electrooxidation reaction in AuAg/C occurs around − 0.3 V vs. NHE. Both electrocatalytic materials were placed in a microfluidic fuel cell. The fuel cell was fed with PBS pH 5 oxygen saturated solution in the cathodic compartment and 5 mM glucose + 0.3 M KOH in the anodic side. Several polarization curves were performed and the maximum power density obtained was 0.3 mWcm−2 .

Photochemical oxidation of a manganese(III) complex with oxygen and toluene derivatives to form a manganese(V)-oxo complex.

Visible light photoirradiation of an oxygen-saturated benzonitrile solution of a manganese(III) corrolazine complex [(TBP8Cz)Mn(III)] (1): [TBP8Cz = octakis(p-tert-butylphenyl)corrolazinato(3-)] in the presence of toluene derivatives resulted in formation of the manganese(V)-oxo complex [(TBP8Cz)Mn(V)(O)]. The photochemical oxidation of (TBP8Cz)Mn(III) with O2 and hexamethylbenzene (HMB) led to the isosbestic conversion of 1 to (TBP8Cz)Mn(V)(O), accompanied by the selective oxidation of HMB to pentamethylbenzyl alcohol (87%). The formation rate of (TBP8Cz)Mn(V)(O) increased with methyl group substitution, from toluene, p-xylene, mesitylene, durene, pentamethylbenzene, up to hexamethylbenzene. Deuterium kinetic isotope effects (KIEs) were observed for toluene (KIE = 5.4) and mesitylene (KIE = 5.3). Femtosecond laser flash photolysis of (TBP8Cz)Mn(III) revealed the formation of a tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state was shown to be the key, activated state that reacts with O2 via a diffusion-limited rate constant. The data allow for a mechanism to be proposed in which the tripseptet excited state reacts with O2 to give the putative (TBP8Cz)Mn(IV)(O2(•-)), which then abstracts a hydrogen atom from the toluene derivatives in the rate-determining step. The mechanism of hydrogen abstraction is discussed by comparison of the reactivity with the hydrogen abstraction from the same toluene derivatives by cumylperoxyl radical. Taken together, the data suggest a new catalytic method is accessible for the selective oxidation of C-H bonds with O2 and light, and the first evidence for catalytic oxidation of C-H bonds was obtained with 10-methyl-9,10-dihydroacridine as a substrate.

Electrophysiological toxicity testing of VEGF Trap‐Eye in an isolated perfused vertebrate retina organ culture model

Age-related macular degeneration (AMD) is the leading cause of visual impairment in Western nations. Since the discovery of the importance of vascular endothelial growth factor (VEGF) in the pathogenesis of neovascular AMD, anti-VEGF agents including pegaptanib, ranibizumab and bevacizumab provide a treatment option to improve vision in affected persons. VEGF Trap-Eye (Aflibercept) is a new agent available for the treatment of exudative AMD. The molecule is a receptor decoy with a longer half-life and a higher affinity to VEGF compared with ranibizumab or bevacizumab. The presented study has been designed to evaluate the short-term toxic effects of VEGF Trap-Eye on retinal function during and after direct exposure to the drug. Isolated bovine retinas were perfused with an oxygen-saturated nutrient solution, and the electroretinogram (ERG) was recorded using silver/silver chloride electrodes. A total of 0.5mg or 2mg VEGF Trap-Eye was added to the nutrient solution and retinas were exposed for 45min, followed by a washout period of 100min. The percentage of a- and b-wave reduction at the end of the washout was compared with the baseline values. Additionally, retinal whole mount cultures were exposed for 24hr to VEGF Trap-Eye, and the amount of apoptotic cells were determined using the terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphate-biotin nick end labelling (TUNEL) assay. During simulation of intraocular application, no significant reduction in the a-wave amplitude for 0.5mg (2.70%, p=0.37) and 2mg (3.84%, p=0.37) VEGF Trap-Eye and b-wave amplitude for 0.5mg (19.68%, p=0.17) and 2mg (24.1%, p=0.06) VEGF Trap-Eye was observed at the end of the washout. However, there were significant changes in a-wave and b-wave amplitudes directly after exposure to 0.5mg VEGF Trap-Eye (18.4%, p=0.004 and 43.1%, p=0.006, respectively). The presented data suggest that intraocular application of up to 2mg VEGF Trap-Eye does not induce irreversible toxic retinal damage. However, short-term results showed a negative effect directly after the application for 0.5mg and 2mg VEGF Trap-Eye.

Synthesis and Characterization of Palladium and Palladium–Cobalt Nanoparticles on Vulcan XC-72R for the Oxygen Reduction Reaction

A single-source approach was used to synthesize bimetallic nanoparticles on a high-surface-area carbon-support surface. The synthesis of palladium and palladium-cobalt nanoparticles on carbon black (Vulcan XC-72R) by chemical and thermal reduction using organometallic complexes as precursors is described. The electrocatalysts studied were Pd/C, Pd2Co/C, and PdCo2/C. The nanoparticles composition and morphology were characterized using inductively coupled plasma mass spectrophotometer (ICP-MS), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray fluorescence spectroscopy (EDS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) techniques. Electrocatalytic activity towards the oxygen reduction reaction (ORR) and methanol tolerance in oxygen-saturated acid solution were determined. The bimetallic catalyst on carbon support synthetized by thermal reduction of the Pd2Co precursor has ORR electrocatalytic activity and a higher methanol tolerance than a Pt/C catalyst.

Kinetics and Mechanism of Palladium Pressure-Cyanide Dissolution

At room temperature and pressures, the reaction between sodium cyanide and platinum group metals (PGMs) does not occur because of poor kinetics. However, at elevated temperatures, PGMs can be leached by sodium cyanide like the reaction of gold. However, few rate expression which describe the leaching of Palladium in cyanide solution has been developed. In this paper, the rate of Palladium dissolution was measured in pressure clear cyanide solution using a Palladium rotating disk. The data at different rotation speed, cyanide concentrations, temperature and oxygen pressure are obtained. The dissolution rates were independent of rotation speed for oxygen-saturated solutions between 100-400rpm. With increasing temperature the dissolution rate is increased. With increasing cyanide concentration and oxygen pressure, the dissolution rate first increased to a maximum value and then decreased.

Silica-pillared graphene sheets with iron–nitrogen units as an oxygen reduction catalyst

Graphene layers with silica pillars which widen the interlayer distance and functionalize the space as micropores have been prepared as one of the new porous carbon materials. In this study, the silica-pillared graphene with Fe–N units as a catalytic active site for cathodic oxygen reduction was formed as a new type of carbonaceous noble-metal-free oxygen reduction catalysts with the ordered pore structure. The formation of the pillared carbon was performed by introducing silylating reagents as the pillar source and also as the nitrogen source of the units in the graphite oxide interlayer spaces, introducing the Fe ions, and heat treatment in vacuum at 500°C. The X-ray diffraction (XRD) patterns showed the ordered layered structure. The X-ray absorption fine structure (XAFS) of the Fe-K edge showed that the unit was a square planar Fe–N4 moiety. The catalytic activity for the O2 reduction was demonstrated by measurements using rotating disk electrodes with the pillared carbon fixed on the surface and immersed in an oxygen-saturated acidic solution. The activity was enhanced by the combination of the nitrogen containing compound to the silylating reagent and using the nitrogen-rich silylating reagent.

Fixation of laccase enzyme into polypyrrole, assisted by chemical interaction with modified magnetite nanoparticles: A facile route to synthesize stable electroactive bionanocomposite catalysts

Effective bio-electrocatalysts require stable immobilization of sufficient amounts of the bioactive component. In this study, a novel and efficient method for specific binding of laccase enzyme onto magnetite nanoparticles (NPs) is presented. The interaction between the chemically modified magnetite NPs and the enzyme was evidenced by both infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). Subsequently, the enzyme-coated magnetite NPs were successfully incorporated into polypyrrole (PPy) matrix during galvanostatic electropolymerization. The encapsulation of laccase covered NPs was proved by EQCN, TEM, and FT-IR spectroscopy; whereas the electrochemical behaviour of the formed bionanocomposite was characterized by cyclic voltammetry. In oxygen saturated solution a cathodic charge surplus was observed, related to the electrochemical reduction of oxygen. This surplus was two times higher in the case of the laccase containing layer compared to its only magnetite containing counterpart. Kinetic aspects of the oxygen reduction reaction (ORR) on the laccase containing films were investigated by hydrodynamic voltammetry, and the four-electron route was found to be exclusive, which is promising from the fuel cell perspective. Such synergistic combination of inorganic NPs and enzymes may open new avenues in the application of these bio-nanocomposite materials.

Visible-Light-Induced Oxygenation of Benzene by the Triplet Excited State of 2,3-Dichloro-5,6-dicyano-p-benzoquinone

Photocatalytic oxygenation of benzene to phenol occurs under visible-light irradiation of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) in an oxygen-saturated acetonitrile solution of benzene and tert-butyl nitrite. The photocatalytic reaction is initiated by photoinduced electron transfer from benzene to the triplet excited state of DDQ.

Reversible growth of UO2 nanoparticles in aqueous solutions through 7 MeV electron beam irradiation

Uranium dioxide (UO2) nanoparticles of rod-shaped structures with diameter 80nm and length 500nm, have been grown in aqueous solutions of uranyl nitrate with electron beam irradiation using a 7MeV linear electron accelerator. These nanoparticles were found to be stable under de-aerated condition; however they decomposed upon exposure to air or oxygen. The bimolecular rate constant for decomposition of these nanoparticles has been estimated to be 4.3×10−1M−1s−1, in the aerated and oxygen saturated solutions, assuming these to be of pseudo-first order processes. The nitrogen-purged decomposed solution again produced the black coloured UO2 sol upon such irradiation and these reversible phenomena could be repeated for several cycles. There was also decomposition of the UO2 nanoparticles along with evolution of hydrogen upon addition of acid in the de-aerated condition.

Leaching and electrochemistry of gold, silver and gold–silver alloys in cyanide solutions: Effect of oxidant and lead(II) ions

The leaching and electrochemistry of gold, silver and gold–silver alloys in cyanide solutions has been investigated using air, oxygen and/or hydrogen peroxide as oxidant in the presence or absence of lead(II) ions. It has been found that both the gold–silver oxidation half reaction and the oxygen reduction half reaction occur more readily with increasing silver content in the alloy, and both half reactions are significantly enhanced by the under-potential deposition of lead onto the metal surface. However, the under-potential deposition of lead can be inhibited by high leach potentials, resulting in no beneficial effect for the cyanidation under these conditions (e.g. cyanidation in oxygen saturated solutions). A selective dissolution of gold over silver was found for low silver content alloys, which diminished with increasing silver content. The leach rates were less than the oxygen diffusion limited rate other than for gold alloyed with silver in air saturated solutions with lead(II) ions present. A low concentration of hydrogen peroxide had no positive effect on the leach rate as oxygen reduction occurs preferential to hydrogen peroxide reduction; enhanced hydrogen peroxide reduction could occur at leached surfaces resulting in enhanced leach rates. It has also been found that the gold leach rate predicted by an Evans' diagram could be misleading and a combination of the measured leach point (rate and potential) and Evans' diagram revealed some important information occurring during the leaching.

The Neuroprotective Potential of Rho-Kinase Inhibition in Promoting Cell Survival and Reducing Reactive Gliosis in Response to Hypoxia in Isolated Bovine Retina

Aims: To investigate the outcomes of Rho-kinase inhibition in the electrophysiological ex vivo model of the isolated perfused vertebrate retina under hypoxia. Methods: Bovine retinas were perfused with an oxygen saturated nutrient solution with or without the Rho-kinase inhibitor H-1152P. The retinas were stimulated repeatedly until stable amplitudes were reached and the electroretinogram was recorded at five minute intervals. Hypoxia was induced for 15, 30, and 45 minutes, after which the oxygen saturation was restored. The extent of the cell damage and glial reactivity was determined by Ethidium homodimer-1 staining, immunohistochemistry, and Western blot. Results: Hypoxia caused a time-dependent reduction of the b-wave amplitudes, which could not be prevented by the H-1152P. Although the Rho-kinase inhibitor maintained higher b-wave amplitudes, these effects did not reach statistical significance. Hypoxia also resulted in an increase in cell damage and the activation of the glial cells in the untreated retinas whereas the administration of H-1152P significantly reduced the extent of these events. Conclusion: H-1152P exerted a neuroprotective effect against necrosis on the isolated bovine retina under hypoxia together with a reduction in glial cell reactivity. However, the inhibitor could not prevent the hypoxia induced retinal dysfunction possibly due to the interference with synaptic modulation.

Intense Long-Lived Fluorescence of 1,6-Diphenyl-1,3,5-Hexatriene: Emission from the S<sub>1</sub>-State Competes with Formation of O<sub>2</sub> Contact Charge Transfer Complex

The fluorescence kinetics of 1,6-diphenyl-1,3,5-hexatriene (DPH) dissolved in cyclohexane was investigated as a function of temperature, concentration and 355 nm excitation pulse energy. At concentrations above 2.5 μM and excitation energies above 1 mJ a long-lived, very intense emission, which appears within less than 5 ns and lasts up to 70 ns, is observed. During the first 50 ns the decay does not follow an exponential but rather a linear behaviour. In oxygen saturated solutions the long-lived emission is suppressed and solely short-lived fluorescence with τ 1-state and competes with the formation of DPH-O2 contact charge-transfer complexes and intersystem crossing which both quench the fluorescence. Our investigations show that even the small amount of oxygen dissolved in nitrogen saturated solutions has a distinct influence on the fluorescence kinetics of DPH.

Self sensitized photooxidation of tetrathienyltetrathiafulvalenes: synthesis of thienyl substituted 1,2,5,8-tetrathiecine-6,7-dione, a new heterocyclic system

Irradiation of tetrathienyltetrathiafulvalenes in an oxygen saturated solution gives tetrathienyl substituted 1,2,5,8-tetrathiecine-6,7-diones, via singlet oxygen generation followed by cycloaddition onto the TTF double bond and a final rearrangement process. The thermal and photochemical behaviour and reductive methylation of this new class of macro-heterocyclic system have also been investigated.

Investigation of the pulsed electrochemical deposition of ZnO

The influence of pulse parameters on the morphology of ZnO prepared by pulsed cathodic electrodeposition from oxygen-saturated aqueous ZnCl2 solution on ITO (indium tin oxide)/glass substrates was investigated. It was found that the ratio between the pulse and the pause duration has a crucial influence on the crystal growth, reaching the highest density of the films with pause/pulse-ratios between 0.25 and 1. Longer pauses cause an Ostwald-like ripening of the ZnO crystals and therewith a strong change in the crystal morphology from roundly shaped to hexagonal. Also the hydrophilicity of the substrate resulting from pre-treatment has a crucial influence on the deposited films, leading to films only consisting of few large and separately grown ZnO crystals for highly hydrophilic substrates and an increasing fraction of small densely grown ZnO crystals with increasing hydrophobicity.

Study on Pressure-Cyanide Dissolution of Metal Platinum Powder

A research study has been undertaken to develop the fundamentals of a method for the direct dissolution of Metal Platinum. At room temperature and pressures, the reaction between sodium cyanide and platinum group metals (PGMs) does not occur because of poor kinetics. However, at elevated temperatures between 20°C and 180°C, PGMs can be dissolved by sodium cyanide like the reaction of gold. In this work, the dissolution of Platinum was measured in pressure clear cyanide solution. The data at different cyanide concentrations, different temperature and different oxygen pressure are obtained. With increasing cyanide concentration and oxygen pressure, the dissolution first increased to a maximum value and then decreased. With increasing temperature the dissolution is increased. The dissolution was found to have a relation of the cyanide and oxygen level. The dissolution was independent of rotation speed for oxygen-saturated solutions and cyanide concentrations above 5 mol.m-3 and was well below chemical reaction-limited for cyanide and oxygen.