OH Quencher Research Articles

Enhancement of Sm3+ Luminescence in Sol-Gel Silica Matrix: Effect of Ligands Phenyl Phosphinic Acid and Trioctylphosphine Oxide.

Sol-gel silica matrices singly doped with Sm3+ and co-doped with ligands phenyl phosphinic acid (PPIA) and trioctylphosphine oxide (TOPO) were fabricated and studied for their structural and spectroscopic behaviour. Structural studies were done by x-ray diffraction (XRD) and Fourier transform infra-red (FTIR) absorption analysis whereas spectroscopic behaviour was studied by ultraviolet - visible (UV-Vis) absorption, photoluminescence (PL) excitation, emission and time-correlated decay analyses. XRD studies exhibit the amorphous nature of the samples and FTIR studies corroborate the presence of the ligands in the silica matrix. UV-Vis absorption and PL studies show significant enhancement in the radiative parameters in ligand co-doped samples compared to singly doped Sm3+ sample. Judd Ofelt parameters estimated from the absorption spectra possess higher values compared to popular hosts; revealing higher asymmetry and covalency around the active Sm3+ ions. An enhancement in PL intensity by 50.38 times and yield by 52.57 times with the co-doping of the ligands PPIA and TOPO is reported. These enhancement in the radiative output in ligand co-doped samples is attributed to the modification of the silica network with the incorporation of the ligands, energy transfer from ligands to nearby Sm3+ ions as well as due to the shielding of Sm3+ ions from the OH quenchers in the silica matrix. The improved PL behaviour especially corresponding to 4G5∕2→6H9∕2 transitions occurring at 643nm suggests the potential of PPIA and TOPO co-doped Sm3+ silica matrix for diverge optical applications.

Polyphenol content, Antioxidant and Antimicrobial activities of Hyoscyamus albus L. Aerial extracts

This study involved the investigation of polyphenol and flavonoid content, the antioxidant and antimicrobial potential of Hyoscyamus albus extracts. Four sub-fractions were obtained by successive extraction by using methanol, chloroform, and ethyl acetate. The extracts' yields were counted and the total phenolic (TPC) and flavonoid content (TFC) were assessed via spectrophotometric methods. The extracts' antioxidant activity was investigated by using DPPH test, Total Antioxidant Capacity (TAC), Hydroxyl radical scavenging potency and β-Carotene/linoleic acid bleaching assay. The antimicrobial potential was valued against 08 strains of pathogenic bacteria and yeast. The results revealed that the CrE yielded the highest extracted value (13,34%) and the lowest percentage yield was that of EAE (1.46%). Interestingly, the EAE gave higher amounts of polyphenols (186.55 mg GAEq/gE), whereas the ChE showed the lowest content (45.19mg GAEq/gE). Notably, both the EAE and ChE fractions contained the highest levels of flavonoids, correlating with their antioxidant activity. Specifically, the EAE displayed the highest DPPH scavenging activity (p<0.001) with IC50 of 21μg/ml and revealed the strongest total antioxidant capacity (EC50 = 50μg/ml). While the CrE is regarded as an excellent OH quencher with a weaker IC50 close to the synthetic reference standard (p<0.001). However, ChE showed greater inhibition of β-carotene bleaching and impeded linoleic acid oxidation. Furthermore, the tested extracts exhibited different degrees of antimicrobial activity. The EAE was the only extract that proved effective against the yeast (Candida albicans). The current study confirmed the important antioxidant action, as well as the significant antimicrobial effects of Hyoscyamus albus extracts. These findings firmly underpin the traditional applications of this herb for treating ailments and infection, and could in fact be a source of natural antioxidant, and antibacterial compounds.

Investigating the reactivity of TiOx and BDD anodes for electro-oxidation of organic pollutants by experimental and modeling approaches

The reactivity of two non-active anodes (BDD and TiOx) for the removal of organic pollutants from water was investigated and compared. A mathematical model for electro-oxidation of organic compounds was developed by considering (i) an analytical expression of the spatial distribution of •OH concentration at non-active anodes, (ii) mass transport through the diffusion boundary layer and (iii) both direct electron transfer and •OH-mediated oxidation for degradation of organic pollutants. The model was calibrated from experimental data using, for example, terephthalic acid as •OH probe molecule or ethanol as •OH quencher during paracetamol degradation. Calculation of the fraction of current ascribed to the different possible reactions as well as a sensitivity analysis allowed for highlighting the key mechanisms and limiting phenomena. Degradation of organics occurs almost exclusively through •OH-mediated oxidation under mass transport limitation and for sufficiently high reaction rate constants. Improvement of degradation kinetics strongly depends on mass transport enhancement of organic compounds to the reaction zone at the electrode surface. Competition between the mother molecule and degradation by-products for reaction with •OH did not significantly affect degradation kinetics under mass transport limitation. Direct electron transfer can potentially become important in presence of large amounts of •OH scavengers or for organic compounds that react slowly with •OH (k < 105 m3 mol−1 s−1). Results also showed that the implementation of a suitable •OH quenching experiment required the use of a concentration of ethanol of at least 100 mM for experiments performed at 5 mA cm−2 and with a concentration of target pollutant of 0.1 mM.

On the photolysis branching ratio of methyl ethyl ketone

The methyl ethyl ketone (MEK) photolysis branching ratio (α) was re-evaluated by an end product analysis and box model simulations with the Master Chemical Mechanism (MCM). Using light emitting diodes centered at 285 nm or 315 nm, MEK was irradiated in the presence of nitric oxide and oxygen to produce peroxyacetic and peroxypropanoic nitric anhydride, CH3C(O)O2NO2 (PAN) and C2H5C(O)O2NO2 (PPN), which were quantified by gas chromatography. Box model simulations indicated that PPN is partially produced as a secondary product from chemistry initiated by reaction of the hydroxyl radical (OH) with MEK. Under NOx-limited experimental conditions or in the presence of ethane as an OH quencher, the product distribution observed required α = (7 ± 1)% + (1.1 ± 0.7) × 10−4 × (T-298) for 250K < T < 300K (2σ uncertainty), independent of pressure (at pressures > 266 hPa) and consistent with current IUPAC recommendations.

Graphene quantum dots as singlet oxygen producer or radical quencher - The matter of functionalization with urea/thiourea.

Due to their low cost and possible green synthesis, high stability and resistance to photobleaching, graphene quantum dots (GQDs) can be considered as one of the class of carbon nanomaterials which may have great potential as an agent for photosensitized oxygen activation. In such a way, GQDs can be used as a theranostic agent in photodynamic therapy. In this work pristine GQDs, GQDs irradiated with gamma rays and GQDs doped with N and N, S atoms are produced using a simple, green approach. By using different techniques (AFM, HR-TEM, SEM-EDS, FTIR, XRD, PL and UV-Vis) we investigated structural and optical properties of the new types of GQDs. We showed that GQDs functionalized with thiourea (GQDs-TU) completely lost the ability to produce singlet oxygen (1O2) upon photoexcitation while functionalization with urea (GQDs-U) improves the capability of GQDs to produce 1O2 upon the same conditions. Thus, presented GQDs modification with urea seems like a promising approach for the production of the efficient photosensitizer. On the opposite, GQDs-TU are efficient OH quencher. Due to high singlet oxygen production and low cytotoxicity below 100 μg/mL against HeLa cells, GQDs-U is a good candidate as an agent in photodynamic therapy at this concentration.

Enhanced Perfluorooctanoic Acid Degradation by Electrochemical Activation of Sulfate Solution on B/N Codoped Diamond.

Electrochemical oxidation based on SO4•- and •OH generated from sulfate electrolyte is a cost-effective method for degradation of persistent organic pollutants (POPs). However, sulfate activation remains a great challenge due to lack of active and robust electrodes. Herein, a B/N codoped diamond (BND) electrode is designed for electrochemical degradation of POPs via sulfate activation. It is efficient and stable for perfluorooctanoic acid (PFOA) oxidation with first-order kinetic constants of 2.4 h-1 and total organic carbon removal efficiency of 77.4% (3 h) at relatively low current density of 4 mA cm-2. The good activity of BND mainly originates from a B and N codoping effect. The PFOA oxidation rate at sulfate electrolyte is significantly enhanced (2.3-3.4 times) compared with those at nitrate and perchlorate electrolytes. At sulfate, PFOA oxidation rate decreases slightly in the presence of •OH quencher while it declines significantly with SO4•- and •OH quenchers, indicate both SO4•- and•OH contribute to PFOA oxidation but SO4•- contribution is more significant. On the basis of intermediates analysis, a proposed mechanism for PFOA degradation is that PFOA is oxidized to shorter chain perfluorocarboxylic acids gradually by SO4•- and •OH until it is mineralized.

Using DOM fraction method to investigate the mechanism of catalytic ozonation for real wastewater

Resin fraction of dissolved organic matters (DOM) was used to investigate the mechanism of catalytic ozonation with a commercial catalyst treating petrochemical secondary effluent (PSE). The results showed hydrophilic substances (HI) and hydrophobic acids (HoA) were the dominated fractions in PSE, accounting for about 47% and 36% of the total dissolved organic carbon (DOC). HoA could be more easily oxidized by ozone than HI. It may be ascribed to the highly reactive structures of HoA with high SUVA254. Besides, HoA could be transformed into HI during ozonation. Compared to ozonation, catalytic ozonation could enhance more removal of HI than HoA. Radical scavenger experiment verified that HoA and HI were removed by both ozone molecules and hydroxyl radical (OH). The adsorption of DOM by catalyst kept a dynamic balance process between oxidation and re-adsorption during catalytic ozonation, triggering a cyclic reaction between adsorbed DOM and adsorbed ozone as well as decomposed OH from the adsorbed ozone. In addition, DOM and ozone adsorbed onto the surface of the catalyst simultaneously could protect the quencher of OH caused by high concentration of anions (mainly Cl− and SO42−) in the bulk solution and significantly improve the catalytic performance. The fractions reaction and transformation, dynamic process between oxidation and re-adsorption, role of adsorption and OH quencher protection were clearly interpreted. For catalytic ozonation, the key point is to achieve an equilibrium for the adsorption of organics and oxidation ability. The proposed mechanism can give the direction for synthesis or modification of catalyst for real wastewater treatment.

Hydroxyl radicals and reactive chlorine species generation via E+-ozonation process and their contribution for concentrated leachate disposal

Concentrated leachate is collected from two-stage anaerobic/aerobic-membrane bioreactor (MBR) and nanofiltration (NF) unit in landfill leachate treatment process and featured with non-biodegradation compounds and high salts content. E+-ozonation process was developed and introduced to deal with concentrated leachate, to make good use of chlorine ions (Cl−) and the associated O2 during ozonation process. 74.5% and 69.7% of COD and TOC were removed in E+-ozonation under the optimum conditions with initial pH of 9 and current density of 30 mA∙cm−2, and the corresponding enhancement factors were 1.36 and 1.13, compared to the sum of the electrolysis and ozonation individual. The co-generation of reactive oxygen species (ROS) and reactive chlorine species (RCS) made a complementary effect on the degradation of refractory substances presented in concentrated leachate. 10.1% of total COD removal was improved by non-radical RCS generated in situ and 6.5% of total COD removal was achieved by ●OH-enhanced and Cl/ClO-emerged. With the help of electrolysis, the OH quencher of Cl− was converted into non-radical RCS (such as Cl−, ClO− and Cl2) at anode, and the un-reacted O2/O3 was reduced into OH at the cathode simultaneously. E+-ozonation offers a synergetic degradation potential for special wastewater with refractory matters and high Cl− content.

Degradation of organic compounds during the corrosion of ZVI by hydrogen peroxide at neutral pH: Kinetics, mechanisms and effect of corrosion promoting and inhibiting ions

The corrosion of zero valent iron (ZVI) by hydrogen peroxide (H2O2) generates hydroxyl (⋅OH) and other radical oxygen species (ROS) that degrade organic materials. To better understand the factors that govern the ROS formation during the H2O2-induced corrosion, we investigated the degradation of an organic probe compound (acesulfame (ACE)) in slurries of ZVI powder in unbuffered laboratory water at pH 6.5 ± 0.5. Chloride ions accelerated the corrosion of ZVI by H2O2 and the formation ROS and, therefore, the degradation of organic materials. Conversely, slowing corrosion by phosphate buffer inhibited ROS formation and the degradation of organic compounds. The rate of H2O2 decomposition was correlated with the liberation of Fe2+(aq) and the ACE degradation rate. The kinetics of H2O2 decomposition was pseudo-first-order and zero-order at low (<0.04 mM/mg) and high [H2O2]/[ZVI] initial ratios, respectively, and was consistent with Langmuir kinetics. The H2O2 decomposition rate was proportional to the ZVI reactive surface area (SA) and nearly independent of the extent of ZVI oxidation, the presence of a Fe2+(aq) chelating agent, and ⋅OH quenchers (methanol and tert-butanol). Kinetic data suggest a mechanism involving rapid cathodic reduction of H2O2 at the metallic ZVI surface which causes the liberation of Fe2+(aq) that generate ⋅OH via the homogeneous Fenton reaction. The stoichiometric efficiency (SE) of organics degradation ranged from 0.0008% to 0.014% and increased with decreasing H2O2 decomposition rate.

Visible Light Sensitized Production of Hydroxyl Radicals Using Fullerol as an Electron-Transfer Mediator.

Fullerenes and their derivatives are known to photosensitize the production of singlet oxygen (1O2), but their role in generating hydroxyl radical (•OH) under visible light has not been reported. Here, we demonstrate that fullerol can mediate the electron transfer from Rhodamine B dye to O2 under visible light irradiation, achieving simultaneous dye decolorization and •OH-induced degradation of 4-chlorophenol. The hydroxyl radical is proposed to be produced via a consecutive reduction of molecular oxygen by fullerol anion radical, which is formed through the electron transfer from the dye to the triplet state of fullerol. Mechanistic investigations using various probe reagents such as superoxide dismutase (superoxide quencher), t-butanol (•OH quencher), and coumarin (•OH probe) provided indirect evidence for the generation of •OH under visible light. Furthermore, spin trapping technique directly detected the oxidizing species such as •OH, HO2•, and 1O2 in the visible light irradiated solution of RhB/fullerol mixture. It was proposed that the photochemical oxidation mechanism depends on pH: •OH production is favored at acidic pH through fullerol-mediated sequential electron transfer while 1O2 is generated as a main oxidant at neutral and alkaline condition through the energy-transfer process. Therefore, the photochemical oxidation can be switchable between •OH-driven and 1O2-driven mechanism by a simple pH adjustment.

Effects of inorganic oxidants on kinetics and mechanisms of WO3-mediated photocatalytic degradation

This study evaluates the capacity of various inorganic oxidants (IO4−, HSO5−, S2O82−, H2O2, and BrO3−) to act as alternative electron acceptors for WO3-mediated photocatalytic oxidation. Combination with IO4− drastically increased the rate of photocatalytic degradation of 4-chlorophenol by WO3, while the other oxyanions only negligibly improved the photocatalytic activity. The extent of the photocatalytic performance enhancement in the presence of inorganic oxidants correlated well with the efficiencies for: (1) hydroxylation of benzoic acid as an OH probe, (2) dechlorination of dichloroaceate as a hole scavenger, and (3) water oxidation with O2 evolution. The results suggest that the promoted charge separation primarily causes kinetic enhancement in photocatalytic degradation using the WO3/IO4− system. In marked contrast to the substrate-dependent activity of the photochemically activated IO4− (generating selective IO3), the efficiency of the WO3/IO4− system for photocatalytic degradation did not sensitively depend on the type of target organic compound, which implies the existence of a minor contribution of the photocatalytic reduction pathway associated with the production of IO3 as a secondary oxidant. On the other hand, the insignificant inhibitory effect of methanol as an OH quencher may reveal the possible involvement of SO4− in the improved photocatalytic activity of the WO3/HSO5− system. The alternative use of platinized WO3, where the interfacial electron transfer occurs in a concerted step, achieved a highly accelerated photocatalytic oxidation in the presence of HSO5− and polyoxometalates as electron scavengers. In particular, the surface loading of nanoscale platinum appeared to retard the reaction route for SO4− generation associated with a one-electron transfer.

Degradation of antibiotic norfloxacin in aqueous solution by visible-light-mediated C-TiO2 photocatalysis

A visible-light-mediated C-TiO2 photocatalytic process (Vis/C-TiO2) was employed to degrade antibiotic norfloxacin. The influences of catalyst dosage, initial probe compound concentration and solution pH levels on the decay performance and reaction kinetics were investigated and optimized. Based on the experimental results, an equation was established to predict the observed rate constant under neutral pH. In addition, the decay rate was accelerated under weak alkali in the presence of moderate OH− anions. Hydroxyl radical was confirmed to play a major role in the Vis/TiO2 process, where in the presence of OH quencher and electron acceptor, retardation and improvement were found respectively. Furthermore, an original schematic diagram describing the surface property of C-TiO2 was built and further verified, in which, NH4+ cations normally served as hole scavengers showed a negligible effect because the adsorbed OH− formed a barrier for NH4+ ions to approach the holes, and the F− anions presented a significant suppression on norfloxacin decay due to the formation of hydrogen bond (OH⋯F) around the C-TiO2 surface. Besides, the recycling and sedimentation tests justified that the Vis/C-TiO2 process is a cost-effective and feasible way for wastewater treatment.

The control of root growth by reactive oxygen species in Salix nigra Marsh. seedlings

The production of reactive oxygen species (ROS) in specific regions of Salix seedlings roots seems essential for the normal growth of this organ. We examined the role of different ROS in the control of root development in Salix nigra seedlings, and explored possible mechanisms involved in the regulation of ROS generation and action. Root growth was not significantly affected by OH quenchers, while it was either partially or completely inhibited in the presence of H 2O 2 or O 2 − scavengers, respectively. O 2 − production was elevated in the root apex, particularly in the subapical meristem and protodermal zones. Apical O 2 − generation activity was correlated to a high level of either Cu/Zn superoxide dismutase protein as well as carbonylated proteins. While NADPH-oxidase (NOX) was probably the main source of O 2 − generation, the existence of other sources should not be discarded. O 2 − production was also high in root hairs during budding, but it markedly decreased when the hair began to actively elongate. Root hair formation increased in the presence of H 2O 2 scavengers, and was suppressed when H 2O 2 or peroxidase inhibitors were supplied. The negative effect of H 2O 2 was partially counteracted by a MAPKK inhibitor. Possible mechanisms of action of the different ROS in comparison with other plant model systems are discussed.

Effect of varying lanthanide local coordination sphere on luminescence properties illustrated by selected inorganic and organic rare earth complexes synthesized in sol–gel host glasses

Inorganic and organic ligands were carefully selected to illustrate the effect of modifications in the local field environment around the rare earth lanthanide (III) on its emission properties. In this article two strategies were employed to enhance emission of lanthanides encapsulated in sol–gel glass. (i) Changing the symmetry around the lanthanide, which was diagnosed by changing the local environment around the lanthanide using different inorganic counter ions (acetate, nitrate and chloride) these ligands differ in their affinity toward the lanthanide first coordination sphere. The ligand that penetrates the lanthanide more results in more asymmetric environment and thus results in higher emission. The aim of this part was to demonstrate the change of symmetry on emission in the absence of energy transfer. Our results indicate that the acetate ion has the highest affinity toward the first coordination sphere followed by the nitrate while the chloride showed the lowest affinity. Penetration by the ligands ofthe lanthanide also results in removing OH quenchers surrounding the lanthanide and this further explains the boost in emission. (ii) A bulky organic ligand that forms a complex with the lanthanide is used. The organic ligand separates the lanthanide ion from inner O–H oscillators. In this case the chelating organic chromophore with suitable photophysical properties was employed to sensitize the lanthanide and thus energy transfer occurs via the antenna effect. The organic ligand absorbs UV light, then energy is transferred to the lanthanide and finally the lanthanide emits in the visible region. The first coordination environment surrounding europium was controlled by the ligand selection and the outer sphere was modified by doping the synthesized complexes in an optically transparent sol–gel glass host. The glass network carefully prepared by sol–gel process is effective in preventing free oxygen and water from attacking lanthanide -complexes without loss of luminescence. Emission spectroscopy measurements of the doped silica specimens confirmed the variation of Eu (III) emission depending on the first coordination sphere surrounding the europium ion. The encapsulation of the europium complexes was performed for two reasons: (i) to improve the stability of red phosphor with efficient and high color-purity characteristics under ultraviolet excitation and (ii) this work provides a framework for preparing transparent composite glasses that are robust hosts to study the fundamental interactions between nano-materials and light.

Regioselectivity of oxidation by positive holes (h +) in photocatalytic aqueous transformations

The photocatalytic transformation of 4-hydroxybenzylalcohol (HBA) on ZnO or TiO 2 leads to a mixture of three photoproducts: 4-hydroxybenzaldehyde (HBZ), 3,4-dihydroxybenzylalcohol (DHBA) and hydroquinone (HQ). In the presence of isopropanol (iPrOH), used as OH quencher, the formation of DHBA is completely inhibited and the formation of HBZ is partly reduced. The product least affected by iPrOH is HQ. However, the oxidation of HBA by OH, produced in the Fenton reaction, leads to HBZ and DHBA, the formation of HQ being almost negligible. These results suggest that positive holes and hydroxyl radicals have different regioselectivities in the photocatalytic transformation of HBA. HQ results from the oxidation by h +, DHBA from the reaction with OH and HBZ is produced by both species.

Absolute rate constant and branching fractions for the atomic hydrogen + hydroperoxyl radical reaction from 245 to 300 K

The total rate constant and the branching fractions for the reactions H + HO/sub 2/ ..-->.. OH + OH (k/sub 1a/), ..-->.. O + H/sub 2/O (k/sub 1b/), ..-->.. H/sub 2/ + O/sub 2/ (k/sub 1c/) have been determined by using resonance fluorescence detection of radical and atomic species. For the total rate constant measurement, pseudo-first-order conditions were used with HO/sub 2/ concentrations in large excess over initial atomic hydrogen. The result is (8.7 +/- 1.5) x 10/sup -11/ cm/sup 3/ molecule/sup -1/ s/sup -1/, independent of temperature from 245 to 300 K. For the branching fraction measurements, product yields of OH and atomic oxygen were measured by adding HO/sub 2/ to an excess of atomic hydrogen. Atomic oxygen yields decreased when OH quenchers, such as water or CO, were added. A similar reduction in atomic oxygen yields was observed in the H + NO/sub 2/ reaction. Evidence is presented that the product yields observed with added water are free of interference from secondary reactions. The resulting branching fractions are k/sub 1a//k/sub 1/ = (0.90 +/- 0.04), k/sub 1b//k/sub 1/ = (0.02 +/- 0.02), and k/sub 1c//k/sub 1/ = (0.08 +/- 0.04), independent of temperature from 245 to 300 K.