Kinetic modeling of CO(ad) monolayer oxidation on carbon-supported platinum nanoparticles

We present a theoretical study of CO(ad) electrooxidation on Pt nanoparticles. Effects of size and surface texture of nanoparticles on the interplay of relevant kinetic processes are investigated. Thereby, strong impacts of particle size on electrocatalytic activities, observed in experiments, are rationalized. Our theoretical approach employs the active site concept to account for the heterogeneous surface of nanoparticles. It, moreover, incorporates finite rates of surface mobility of adsorbed CO. As demonstrated, the model generalizes established mean field or nucleation and growth models. We find very good agreement of our model with chronoamperometric current transients at various particle sizes and electrode potentials (Maillard, F.; Savinova, E. R.; Stimming, U. J. Electroanal. Chem., in press, doi:10.1016/j.jelechem.2006.02.024). The full interplay of on-site reactivity at active sites and low surface mobility of CO(ad) unfolds on the smallest nanoparticles ( approximately 2 nm). In this case, the solution of the model requires kinetic Monte Carlo simulations specifically developed for this problem. For larger nanoparticles (>4 nm) the surface mobility of CO(ad) is high compared to the reaction rate constants, and the kinetic equations can be solved in the limiting case of infinite surface mobility. The analysis provides an insight into the prevailing reaction mechanisms and allows for the estimation of relevant kinetic parameters.     

Effects of catechins on superoxide and hydroxyl radical

Superoxide (O2-.) was reduced by the addition of superoxide dismutase (SOD: O2-. scavenger) and catechins. In competitive reactions utilizing different concentrations of spin-trap agent, the IC50 values of each sample were changed. With regard to the Cu2+/H2O2 and Fe2+/H2O2 reaction systems, metal chelater, hydroxyl radical (.OH) scavenger and catechins eliminated the levels of .OH. For the Cu2+/H2O2 reaction systems, the IC50 for .OH scavenger changed, but that for metal chelater and catechins did not. However, for the Fe2+/H2O2 reaction system, the IC50 for .OH scavenger and catechins changed, whereas that for metal chelater did not. The ESR signal for free Cu2+ was changed by addition of metal chelater and catechins. In the spectrophotometer experiments, it was confirmed that the CuCl2 spectrum was changed by addition of metal chelater and catechins but not by .OH scavenger. Conversely, the FeSO4 spectrum was not changed by addition of .OH scavenger or catechins, but was altered by metal chelater. Lipid peroxidation was inhibited by catechins in a concentration-dependent manner. Therefore, it was suggested that the catechins did not scavenge directly the generated .OH from the Cu2+/H2O2 reaction system, but inhibited the generation of .OH by acting on the Cu2+/H2O2 reaction system. On the other hand, with respect to the .OH generated from the Fe2+/H2O2 reaction system, it was suggested that the catechins had a direct scavenging capacity of the .OH, but had little chelating activity of iron. It was confirmed that catechins have the ability to scavenge for O2-. as well as .OH and to inhibit the generation of .OH by chelation with metal ions.     

Quasi-simultaneous determination of antioxidative activities against superoxide anion and nitric oxide by a combination of sequential injection analysis and flow injection analysis with chemiluminescence detection

A method that combines sequential injection analysis (SIA), flow injection analysis and chemiluminescence (CL) detection was developed for the quasi-simultaneous determination of antioxidative activities against superoxide anion and nitric oxide (NO). The antioxidative activity was expressed as the decrease in luminol CL intensity caused by the quenching of or NO by an antioxidant. The SIA system consisted of two syringe pumps, two selection valves, two holding coils, an HPLC pump to deliver luminol solution, and a CL detector. Operation of the syringe pumps and multiport valves was controlled automatically using a personal computer with appropriate software. A hypoxanthine (HX)-xanthine oxidase (XOD) system was used for the generation of , and (±)-(E)-4-methyl-2-[(E)-hydroxyimino]-5-nitro-6-methoxy-3-hexenamide (NOR1) was employed as NO donor agent. The repeatability of the method was evaluated with 35.2 μg ml⁻¹ L-ascorbic acid, and the relative standard deviations (RSD) of the antioxidative activities were less than 3.8%. The quasi-simultaneous determination of the antioxidative activities in one sample was completed within 2.0 min. The antioxidative activities of some antioxidants and commercially available supplements containing certain antioxidants were successfully determined using this system. The proposed system is rapid and reproducible, and thus may be useful for the screening of functional foods, supplements and pharmaceutical formulations that exhibit antioxidative activity. Figure The system that utilizes a combination of SIA and FIA with CL for the quasi-simultaneous determination of antioxidative activity against a NO and b . SP1, 2: syringe pump, HC1, 2: holding coil, MV1, 2: multi-port valve, P: pump, D: chemiluminescence detector, I: integrator, M1, 2: mixing tee, NOR1: (±)-(E)-4-methyl-2-[(E)- hydroxyimino]-5-nitro-6-methoxy-3-hexenamide, HX: hypoxanthine, XOD: xanthine oxidase.     

Conformational analysis of the complex of 2-methyl-1,3,2-dioxaborinane with hydroxyl anion

Studies of the conformationsl transformations of the complex of 2-methyl-1,3,2-dioxaborinane with the hydroxyl anion performed by the quantum-chemical method HF/6-31+G(d) showed that they are more versatile than those in the initial boric ester, include the existence of six minima, five transition states, and are close to the conformational behavior of 1,3-dioxanes.     

Factor analysis of in vitro antitumor activities of platinum complexes

Factor analysis was applied to the data matrix of in vitro growth inhibitory activities of 52 platinum complexes against 9 tumor cell lines, L1210, P388, Lewis lung, AH66, AH66F, HeLa S3, KB, HT-1197 and HT-1376 cell lines. Three factors were obtained by the principal factor analysis method. After the varimax rotation of these three factors, tumor cell lines were classified into four groups according to their factor loadings. The platinum complexes were characterized by the factor scores. Cisplatin was situated in an extreme position as compared with the other platinum complexes. In vivo antitumor activities of the platinum complexes were tested against L1210 and LL murine tumor models. The in vivo activity against L1210 showed a negative correlation with that against LL. Factor 2 scores of the complexes obtained by factor analysis of in vitro antitumor activities showed a good correlation with these in vivo antitumor activities. Then, the structure-factor 2 score relationships among platinum complexes were analyzed by the Free-Wilson method. From this analysis, structure-activity relationships for carrier ligands and leaving groups are proposed. Factor analysis is suggested to be a useful method to establish an efficient screening system for platinum complexes.     

Hemibonding of hydroxyl radical and halide anion in aqueous solution

Molecular geometries and properties of the possible reaction products between the hydroxyl radical and the halide anions in aqueous solution were investigated. The formation of two-center three-electron bonding (hemibonding) between the hydroxyl radical and halide anions (Cl, Br, I) was examined by density functional theory (DFT) calculation with a range-separated hybrid (RSH) exchange-correlation functional. The long-range corrected hybrid functional (LC-ωPBE), which have given quantitatively satisfactory results for odd electron systems and excited states, was examined by test calculations for dihalogen radical anions (X(2)(-); X = Cl, Br, I) and hydroxyl radical-water clusters. Equilibrium geometries with hemibonding between the hydroxyl radical and halide anions were located by including four hydrogen-bonded water molecules. Excitation energies and oscillator strengths of σ-σ* transitions calculated by the time-dependent DFT method showed good agreement with observed values. Calculated values of the free energy of reaction on the formation of hydroxyl halide radical anion from the hydroxyl radical and halide anion were endothermic for chloride but exothermic for bromide and iodide, which is consistent with experimental values of equilibrium constants.     

Outer-sphere oxidation of the superoxide radical anion

Efforts to use the Marcus model to describe oxidations of the superoxide radical anion (O(2-)) by transition-metal complexes have failed dramatically, with discrepancies between theory and experiment spanning 13 orders of magnitude. As a result, the prevailing view is that these reactions involve some complex interactions that are not yet understood. We now show that once the familiar form of the Marcus cross relation (MCR) is modified to allow for the relatively small size of O(2-), excellent agreement is obtained between theory and experiment. This simple finding dispels the decades of uncertainty surrounding these reactions and provides a reliable method for determining whether oxidations of (O2)- occur via inner- or outer-sphere pathways. More generally, the modified MCR is applicable to any homogeneous electron-transfer process characterized by significant differences in size between electron donors and acceptors.     

Microcalorimetric studies on the dismutation of superoxide anion catalyzed by superoxide dismutase

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AFM characterization of dendrimer-stabilized platinum nanoparticles

This work describes the use of atomic force microscopy (AFM) to measure the size of dendrimer-stabilized Pt nanoparticles (Pt DNs) deposited from aqueous solutions onto mica surfaces. Despite considerable previous work in this area, we do not fully understand the mechanisms by which PAMAM dendrimers template the formation of Pt DNs. In particular, Pt DN sizes measured by high-resolution transmission electron microscopy (HRTEM) are reported to be larger than expected if one assumes that each PAMAM molecule templates one spherical Pt nanoparticle. AFM provides a vertical height measurement that complements the lateral dimension measurement from HRTEM. We show that AFM height measurements can distinguish between "empty" PAMAM and Pt DNs. If the complexation of Pt precursor with PAMAM is prematurely terminated, AFM images and feature height distributions show evidence of arrested precipitation of Pt colloids. In contrast, sufficient Pt-PAMAM complexation time leads to AFM images and height distributions that have relatively narrow, normal distributions with mean values that increase with the nominal Pt:PAMAM ratio. The surface density of features in AFM images suggest that these Pt DNs reside on the mica surface as two-dimensional surface aggregates. These observations are consistent with an intradendrimer templating mechanism for Pt DNs. However, we cannot determine if the mechanism obeys a fixed loading law because we do not have definitive information about Pt DN shape. A second peak in the Pt DN height distribution appears when the Pt loading exceeds about 66% of PAMAM's theoretical capacity for Pt. Excluding these secondary particles, the dependence of mean feature height on the Pt:PAMAM ratio follows a power-law relationship. Also considering the magnitudes of the measured mean height values, the data suggest that Pt DNs exist as ramified, noncompact aggregates of Pt atoms interspersed within the PAMAM framework.     

Surfactant-free preparation of supported cubic platinum nanoparticles

A novel method has been developed for preparing supported cubic platinum nanoparticles. Carbon monoxide and hydrogen are used to reduce platinum precursors present at a solid-gas interface and to control the shape of the growing Pt nanoparticles. By avoiding the use of any organic agents in the synthesis, cubic Pt particles free of hydrocarbons are formed, thereby avoiding possible contamination of the catalyst surface. The approach used is simple and readily scalable.     

Quasi-elastic light scattering of platinum dendrimer-encapsulated nanoparticles

Platinum dendrimer-encapsulated nanoparticles (DENs) containing an average 147 atoms were prepared within sixth-generation, hydroxyl-terminated poly(amidoamine) dendrimers (G6-OH). The hydrodynamic radii (R(h)) of the dendrimer/nanoparticle composites (DNCs) were determined by quasi-elastic light scattering (QLS) at high (pH ～10) and neutral pH for various salt concentrations and identities. At high pH, the size of the DNC (R(h) ～4 nm) is close to that of the empty dendrimer. At neutral pH, the size of the DNC approximately doubles (R(h) ～8 nm) whereas that of the empty dendrimer remains unchanged. Changes in ionic strength also alter the size of the DNCs. The increase in size of the DNC is likely due to electrostatic interactions involving the metal nanoparticle.     

CO-assisted synthesis of finely size-controlled platinum nanoparticles

We present a surfactant-free approach for synthesizing size-dependent carbon supported Pt nanoparticles with mean sizes ranging from 4.8 to 1.7 nm by increasing ratios of CO/Ar. In this work, gas stabilizer exhibits the originality on the design of the supported size-controllable clusters.     

Sensitive electrochemical detection of superoxide anion using gold nanoparticles distributed poly(methyl methacrylate)-polyaniline core-shell electrospun composite electrode

In the present communication, a novel composite nanofibrous electrode is developed for the detection of superoxide anion (O(2)˙(-)) in phosphate buffered saline (PBS). The composite fiber electrode is fabricated by dispersing gold nanoparticles onto poly(methyl methacrylate) (PMMA)-polyaniline (PANI) core-shell electrospun nanofibers. The constructed architecture is proven to be a favorable environment for the immobilization of the enzyme, superoxide dismutase (SOD). Direct electron transfer is achieved between SOD and the electrode with an electron transfer rate constant of 8.93 s(-1). At an applied potential of +300 mV, PMMA/PANI-Au(nano)/SOD-ESCFM shows highly sensitive detection of O(2)˙(-). In addition to this, quantification of different activities of SOD is realized at PMMA/PANI-Au(nano)/SOD-ESCFM. These analytical features offer great potential for construction of the third-generation O(2)˙(-) biosensor.     

Electrochemical modification of steel by platinum nanoparticles

The aim of this study was to synthesize low-concentration catalysts with a highly developed surface based on nanodispersed platinum deposited onto the steel surface electrochemically modified with the use of an ionic liquid. Sintered fibers of austenitic steel without pretreatment (steel 1) and etched with hydrochloric acid to remove surface oxides (steel 2) were used as substrates. 1-Butyl-4-methylimidazolium acetate [BMIM][Ac] ionic liquid was used. Variation of the current intensity and anodic treatment time leads to the formation of different structures at the steel surface. For steel 2, optimal conditions leading to self-organization, namely, formation of hexagonal structures, have been selected. It has been demonstrated that formation of nanostructures at the steel surface can occur without the participation of fluoride ions. Low-concentration (Pt/steel 2) catalysts with a uniform distribution of platinum nanoparticles over the surface were prepared via galvanostatic deposition from an aqueous solution of H₂PtCl₆.     

Preparation of low-molecular-weight carboxymethyl chitosan and their superoxide anion scavenging activity

Low-molecular-weight carboxymethyl chitosans (CMCTSs) were prepared by oxidative degradation method involving hydrogen peroxide (H₂O₂) without or with microwave radiation. Viscosity determination and end group analysis were applied to measure molecular weights of CMCTSs. Effects of concentration of H₂O₂ and degradation time on molecular weights of CMCTSs were studied. The degradation process of CMCTSs will be accelerated with the aid of microwave radiation and degradation time may be reduced greatly. The superoxide anion scavenging activity of CMCTSs was evaluated by application of flow injection chemiluminescence technology. The 50% inhibition concentrations (IC₅₀s) of CMCTSs A, B, and C (1130, 2430 and 4350 Da) were 10.36, 17.57, and 23.38 mg/mL, respectively. The above results showed that CMCTSs with lower molecular weight had better superoxide anion scavenging activity.     

Determination of hydroxyl radical by capillary electrophoresis and studies on hydroxyl radical scavenging activities of Chinese herbs

High-performance capillary electrophoresis (CE) with electrochemical detection (ED) was employed to determine hydroxyl radicals in the Fenton reaction. Hydroxyl radicals can react with salicylic acid to produce 2,3-dihydroxy benzoic acid and 2,5-dihydroxy benzoic acid, which can be analyzed by CE-ED. Based on this principle, hydroxyl radicals were determined indirectly. In a 20 mmol/L phosphate running buffer (pH 7.4), 2,3-dihydroxy benzoic acid and 2,5-dihydroxy benzoic acid would elute simultaneously from the capillary within 6 min. As the working electrode, a 300 m diameter carbon-disk electrode exhibits good responses at +0.60 V (vs. SCE) for the two analytes. Peak currents of the two analytes are additive. Excellent linearity was obtained in the concentration range from 1.0×10^-3 mol/L to 5.0×10^-6 mol/L for 2,3-dihydroxy benzoic acid. The detection limit (S/N=3) was 2.0×10^-6 mol/L. This method was successfully applied for studying hydroxyl radical scavenging activities of Chinese herbs. It is testified that Apocynum Venetum L., Jinkgo bibola L., Morus alba L. and Rhododendron dauricum L. have strong hydroxyl radical scavenging activities.     

Diffusion of platinum ions and platinum nanoparticles during photoreduction processes using the transient grating method

The photoreduction process of PtCl(6)2- to Pt nanoparticles in poly(N-vinyl-2-pyrrolidone) solutions upon UV light irradiation was investigated by monitoring the change in the diffusion coefficient (D). The D values of chemical species during UV irradiation was measured by the laser-induced transient grating (TG) method. The TG signal of the PtCl(6)2- solution before UV irradiation was composed of three kinds of contributions, the thermal grating, the species grating due to the creation of PtCl4(2-), and the species grating due to the depletions of PtCl6(2-). Upon UV irradiation of the solution, the species grating signal due to PtCl6(2-) diminished and then the TG signal of Pt nanoparticles gradually appeared. This result indicates that the gradual clustering of Pt0 atoms into Pt nanoparticles occurs after all PtCl(6)2- ions are photochemically reduced to PtCl(4)2- and subsequently transformed to Pt0 atoms with a short delay. With increasing time of the UV irradiation, the TG signal intensity increased, while D of the Pt nanoparticles did not change. This suggests that the number of Pt nanoparticles increases, but the size of the Pt nanoparticles with the polymer layer is unchanged, in the course of the UV irradiation.