Electrooxidation and inhibition of the antibacterial activity of oxytetracycline hydrochloride using a RuO<Subscript>2</Subscript> electrode

This work reports on the electrochemical oxidation of oxytetracycline hydrochloride (OTCH) [(4S,4aS,5aS,6S,12aS)-4-dimethylamino-1,4,4a,5, 5a,6,11,12a-octahydro-3,6,10,12,12a-hexahydroxy-6-methyl-1,11-dioxonaphthacene-2-carboxamide] on a RuO₂ electrode (DSA^®) by cyclic voltammetry and electrolysis. The electrocatalytic efficiency of the electrode material was investigated as a function of different aqueous buffer solutions with pH values of 2.10 and 5.45 as supporting electrolytes. Spectrophotometric studies have shown that OTCH is stable in such solutions. The electrochemical degradation of OTCH is pseudo-first order at both pH values investigated with rate constants, k, of 9.9 × 10^?5 s^?1 (pH 2.10) and 1.9 × 10^?4 s^?1 (pH 5.45) at 21 ± 1 °C. Microbiological studies with Staphylococcus aureus ATCC 29213 have shown that OTCH lost antibacterial activity after 120 min of electrolysis at 50 mA cm^?2.     

Electrooxidation of acetaldehyde on platinum-modified Ti/Ru0.3Ti0.7O2 electrodes

In this study, the electrochemical oxidation of acetaldehyde was investigated at activated massive DSA^® electrodes in acid medium, using differential electrochemical mass spectrometry (DEMS) and high-performance liquid chromatography (HPLC). The electrodes were prepared either by platinum electrodeposition or by depositing a highly nanodispersive-supported catalyst (Pt and Pt-Ni) over electrode surfaces with a Ti/Ru_0.3Ti_0.7O₂ nominal composition. Bulk electrolysis shows evidence of CO₂ and acetic acid formation. The electrocatalytic efficiency of the electrode material was also investigated as a function of the amount of catalyst added over the DSA^® electrode surface. The presence of RuO₂-active sites on the DSA^® substrate plays an important role in the reaction overall efficiency. The addition of platinum to DSA^® enhances the oxidation of acetaldehyde to CO₂. The role of the substrate on the direct activation of acetaldehyde oxidation is discussed on the basis of the direct application of the metal nanoparticle catalyst over conductive oxide surface based on Magneli phase (mixture of Ti_nO_2n−1 and other phases) from Ebonex^®.     

Studies on electrochemical oxidation of azithromycin and Hemomycin at gold electrode in neutral electrolyte

The aim of the present study was to examine the oxidative properties and an assay of azithromycin and Hemomycin^® at a gold electrode in neutral electrolyte using cyclic linear sweep voltammetry. The maximum value of the current of the oxidation peak of pure azithromycin and azithromycin from Hemomycin^® at 0.6 V versus SCE in 0.05 M NaHCO₃ and in a mixture methanol −0.05 M NaHCO₃ (1:1) at a scan rate of 50 mV s⁻¹ is a linear function of the concentration in the range of 0.235-0.588 mg/cm³. HPLC analysis of the bulk of electrolyte confirmed the data obtained by analysis of the values of the current peak concerning the concentration of antibiotic in the investigated concentration range. The role of methanol, when present, is discussed. In the case of azithromycin, the presence of methanol leads to higher current peak values. However, in the case of Hemomycin^®, methanol should be avoided because of its inhibiting influence on the qualitative and quantitative determination of azithromycin and on the azithromycin/lactose separation.     

Effect of concentration and temperature on electrochemical oscillations during sulfide oxidation on Ti/Ta2O5-IrO2 electrodes

The impact of current density, concentration, and temperature on the potential oscillations observed during the anodic oxidation of sulfide on Ti/Ta₂O₅-IrO₂ oxide electrodes is investigated. Electrochemical methods including: cyclic voltammetry, linear voltammetry, linear galvanic voltammetry, galvanostatic technique and electrochemical impedance spectroscopy are used in this study. The observed potential oscillations are caused by the periodic formation/removal of sulfur from the electrode surface. Increasing current density has the effect of increasing the frequency of oscillations as well as increasing the onset potential. Both increasing concentration and temperature gave rise to significant increases in current density prior to the onset of potential oscillations. The activation energy for electrochemical oxidation of sulfide was estimated from the temperature studies. Lifetime experiments were also conducted and demonstrated that potential oscillations have a detrimental impact on the electrode lifetime.     

The effect of Nafion sulfonate surface concentration on cationic polyacrylamide adsorption

Perfluorosulfonate ionomer membranes (PFSIs) were cast from Nafion^® propanol-water solutions of varying equivalent weights (950, 1100, and 1200 Da) and all annealed at 100 °C under vacuum. Sulfonate group surface concentration on the various membranes and its effect on the adsorption of a cationic polyacrylamide were investigated utilizing X-ray photoelectron spectroscopy (XPS). The cationic polyacrylamide is employed in the microelectronics industry for electroless printed circuit fabrication and may be used to pattern electrodes directly onto the PFSI surfaces for fuel cell miniaturization. Sulfonate surface concentration was found to be well correlated to the sulfonate bulk concentration obtained from the known equivalent weights of the Nafion^® solutions. Adsorbed cationic polyacrylamide surface coverage was found to be independent of the immersion time in the adsorbate solution. A wide variation in adsorbate coverage (from 0 to 60% of the surface) occurred over a comparatively small variation in the sulfonate repeat unit surface concentration (from 0 to 20% of the total Nafion^® repeat units). Adsorbate coverage goes through a maximum as the sulfonate concentration increases and is consistent with a random, enthalpy-driven adsorption process. Greater sulfonate concentration and lower associated adsorbate coverage was detected on cast membranes than previously found with commercially prepared membranes.     

Nanocomposites from poly(ethylene-co-methacrylic acid) ionomers: effect of surfactant structure on morphology and properties

A detailed study of the structure-property relationships for nanocomposites prepared using melt processing techniques from a sodium ionomer of poly(ethylene-co-methacrylic acid) and a series of organoclays is reported. Transmission electron microscopy, X-ray scattering, stress-strain behavior, and Izod impact analysis were used to evaluate the nanocomposite morphology and physical properties. Four distinct surfactant structural effects lead to improved levels of exfoliation and higher stiffness for these nanocomposites: higher number of alkyl tails on the amine rather than one, longer alkyl tails instead of shorter ones, use of 2-hydroxy-ethyl groups as opposed to methyl groups on the ammonium ion, and an excess amount of the amine surfactant on the clay instead of an equivalent amount. These trends are opposite of what has been seen in nylon 6 based nanocomposites but are similar to those observed in nanocomposites formed from LDPE and LLDPE. Although some organoclays were exfoliated better than others, none of the ionomer-based nanocomposites exhibited exfoliation levels as great as those seen in nylon 6 nanocomposites; nevertheless, these nanocomposites offer promising improvements in performance and may be particularly interesting for barrier applications.     

Preparation of Ir0.3Sn(0.7-x)Ti x O2 Electrodes by the Polymeric Precursor Method: Characterization and Lifetime Study

Thin film electrodes of nominal composition Ir_0.3Sn_(0.7-x)Ti _x O₂ (0 ≤ x ≤ 0.7) were prepared by decomposition of polymeric precursors. The solutions used to prepare the electrodes were obtained by mixing of the precursor salts with a mixture of ethylene glicol and citric acid. The films were burned at 400 °C and characterized by X-ray diffraction, scanning electronic microscopy, energy dispersive X-ray spectroscopy and cyclic voltammetry. The electrodes were submitted to high anodic current density in order to evaluate their lifetime in perchloric acid solution. Results show that the electrodes present compositions similar to that of the precursor solutions, suggesting that there is no loss of tin during the calcination step. The electrodes had large surface area and higher lifetime in comparison with electrodes of similar composition prepared by other methods. The possible mechanisms involved in deactivation of the electrodes are discussed.     

Recycling and regeneration of used perfluorosulfonic membranes for polymer electrolyte fuel cells

This paper describes a method for the recycling and regeneration of used perfluorosulfonic Nafion^® (Dupont) membranes by dissolution and recasting. The dissolution of the used Nafion^® membranes from polymer electrolyte fuel cells is realized using dimethyl sulfoxide as a solvent under atmospheric pressure and 190 °C. A mechanically robust membrane can be reproduced by a recast process of the dissolved Nafion^® solution at 170 °C. The recycled membrane has shown a good crystalline structure and high mechanical strength. Membrane properties, including water uptake, exchange capacity and resistance are similar to that of the as-received Nafion^® 115 membrane. Fuel cells prepared by the recycled membrane demonstrate a comparable performance to that of the fresh fuel cell.     

Kinetics and mechanism of the formation of doped skeletal copper catalysts: the effect of zincate compared to undoped and chromate-doped systems

Addition of zincate to the leach liquor for the preparation of skeletal copper increases the copper surface area; however it does not stabilize the structure against rearrangement. The leaching kinetics have been studied using a rotating disc electrode (RDE) at 269–293 K in 2–8 M NaOH and 0.0005–0.1 M Na₂ZnO₂. Zincate ions precipitate as zinc oxide, due to the local consumption of hydroxide ions near the leach front as the aluminium dissolves. This oxide hinders the aluminium dissolution, slowing the leaching rate. It also hinders copper dissolution/redeposition and prevents copper diffusion, thus reducing the structural rearrangement significantly, and causing the formation of a much finer copper structure with increased surface area. The zinc oxide redissolves as the leach front passes, releasing the copper to rearrange once more, thereby allowing the surface area to decrease with time. The activation energy for leaching was found to be 84 ± 6 kJ mol^–1.     

Adsorption of N,N′-dimethylthiourea at a mercury/aqueous solution of NaClO4 interface; dependence on the supporting electrolyte concentration

Adsorption of N,N′-dimethylthiourea (DMTU) on mercury electrode from 0.1, 1 and 5 M NaClO₄ was studied as the function of electrode charge density and adsorbate bulk concentration. In the study, the experimental data obtained from the measurements of differential capacity of double layer were used, the measurements of zero charge potential and surface tension at the zero charge potential. In each system studied the values of the relative surface excess increase with an increase of the concentration of N,N′-dimethylthiourea and NaClO₄. The adsorption parameters were obtained from the Frumkin, virial and modified Flory-Huggins isotherms. It was found that the values of free adsorption energy, interactions constants and integral capacity depends on the supporting electrolyte concentration. The strength of the surface bond formed between N,N′-dimethylthiourea and the electrode surface and the influence of water present on the electrode surface in the obtained results of calculations were discussed.     

Morphological, structural and rheological properties of beta-cyclodextrin based polypseudorotaxane gels

Supramolecular self-assembled gels of polypseudorotaxane (PPR) were made through sequential inclusion complexations between beta-cyclodextrin (β-CD) and reverse Pluronic^® (RPL, or PPG-PEG-PPG). This study aimed to disclose the differences in morphological, structural and rheological properties of the PPR gels made in citric acid and other conditions. β-CD/RPL mixture in citric acid solution incubated at 25 °C (β-CD/RPL-C-25) had the most homogeneous, fine and continuous structure, the lowest crystallite size, and the highest gel strength and thixotropy. It was hypothesized that the citric acid partially interfered with the crystal growth of PPR, and thus led to high degree of crosslinking which explained its largest gel strength and thixotropy. β-CD/RPL mixtures in water incubated at 65 °C (β-CD/RPL-W-65) underwent PPR rearrangement, resulting in larger crystallites and a more integrate net-like structure, therefore yielding higher gel strength as compared with β-CD/RPL mixture in water incubated at 25 °C.     

Electrochemical stability of thin film LiMn2O4 cathode in organic electrolyte solutions with different compositions at 55 °C

A survey of the electrochemical stability of electrostatic spray deposited thin film of LiMn₂O₄ was performed in LiClO₄-EC-PC, LiBF₄-EC-PC, and LiPF₆-EC-PC solutions at 55 °C. The solution resistance, the surface film resistance, and the charge-transfer resistance were all found to depend on the electrolyte composition. Among the LiX-salts studied, the lowest charge transfer-resistance, and surface layer resistance were obtained in LiBF₄-EC-PC solution. There is no major influence of the electrolyte solution compositions upon lithium ion transport in the LiMn₂O₄ bulk at 55 °C. The diffusion coefficient of lithium in the solid phase varied within 10⁻¹⁰-10⁻⁸ cm² s⁻¹ in the three solutions. In general, it seems that in LiBF₄ solutions, the surface chemistry is the most stable in the three solutions examined, and hence the electrode impedance in LiBF₄ solutions was the lowest. In LiPF₆ solutions, HF seems to play an important role, and thus, the electrode impedance is relatively high due to the precipitation of surface LiF.     

Encapsulation efficiency of solid lipid hybrid particles prepared using the PGSS technique and loaded with different polarity active agents

The manufacture of particulate hybrid carriers containing a glyceryl monostearate (Lumulse^® GMS-K), a waxy triglyceride (Cutina^® HR), silanized TiO₂ and different active agents (caffeine, glutathione or ketoprofen) was investigated with the aim of producing controlled drug delivery systems based on solid lipid particles. Particles were obtained using the supercritical PGSS^® (particles from gas saturated solutions) technique. Experiments were performed at 13 MPa and 345 K, according to previous measurements of lipid melting points. Solid lipid particles were loaded with silanized TiO₂ and caffeine, glutathione or ketoprofen in percentages of 6-7 wt% for the mineral filler and 4.2, 5.6 and 16.1 wt% for the respective drugs. The particles obtained were analyzed in the solid state by thermogravimetric and X-ray diffraction analysis and scanning electron microscopy. Drug contents in the precipitated lipid samples and their elution profiles were studied by HPLC. Hydrophobic drugs, such as ketoprofen, were more efficiently encapsulated in the lipophilic lipidic matrix than hydrophilic drugs, such as caffeine and glutathione.     

Influence of nature, concentration and pH of buffer acid-base system on rate determining step of the electrochemiluminescence of Ru(bpy)3 with tertiary aliphatic amines

The electrogenerated chemiluminescence (ECL) of Ru(bpy)₃²⁺ (bpy = 2,2′-bipyridyl) with tertiary aliphatic amines as co-reactants, was theoretically and experimentally studied as a function of the pre-equilibria involved in the ammonium proton lost and in relation to the nature of the rate determining step. Transient potential steps were used with a 3-mm glassy carbon disk electrode or carbon fiber ultramicroelectrodes array to investigate emission behavior in a variety of aqueous solution types, containing phosphate, tartrate and phthalate acid-base systems at differing pH values. The emission of Ru(bpy)₃²⁺ resulting from the reaction with n-tripropylamine (TPrA), tri-isobutylamine (TisoBuA), n-tributylamine (TBuA), methyl-di-n-propylamine (MeDPrA) and triethylamine (TEtA) in varying acid-base media was interpreted on the basis of the quoted pre-equilibria, ammonium pK_a being known. The nature of the rate determining steps changes depending on pH. Above pH ≈ 5 the amine neutral radical formation is the rate determining step and, is independent of pH with rate constant close to 10³ s⁻¹; below pH ≈ 5 the rate determining step becomes the deprotonation of the ammonium ion, operated by different bases present in solution. Different amines in the same acid-base system showed analogous ECL behavior, conditioned by the chosen acid base system. A single amine in different acid-base systems showed different kinetic behaviors, due to the dissociation constants of the chosen buffers. The concentration of the acid-base system also played an important role and influenced emission intensity and shape. ECL emission were simulated by finite difference methods, implementing a previously proposed mechanism by including the relevant pre-equilibria. Simulation may also give estimates of the pK_a values of the ammonium ions. An ion pair formation between R₃N⁺ and the mostly charged species present in solution is hypothesized to explain the contradictory experimental results concerning the reaction mechanism of the proton lost of the radical cation.     

Preparation, characterization and electrocatalytic properties of poly(luminol) and polyoxometalate hybrid film modified electrodes

Hybrid films composed of poly(luminol) and nanometer-sized clusters of polyoxometalate, SiMo₁₂O₄₀⁴⁻ and PMo₁₂O₄₀³⁻ have been prepared in acidic aqueous solutions. These films are stable and electrochemically active, and produced on glassy carbon, platinum, gold and transparent semiconductor tin oxide electrodes. The electrochemical quartz crystal microbalance and cyclic voltammetry were used to study in situ growth of the hybrid poly(luminol)/SiMo₁₂O₄₀⁴⁻ and poly(luminol)/PMo₁₂O₄₀³⁻. Both the poly(luminol)/SiMo₁₂O₄₀⁴⁻ and poly(luminol)/PMo₁₂O₄₀³⁻ hybrid films showed four redox couples and the electrochemical properties were compared to SiMo₁₂O₄₀⁴⁻ and PMo₁₂O₄₀³⁻. When transferred to various acidity aqueous solutions, the four redox couples and the formal potentials of two hybride film were observed to be pH-dependent. The electrocatalytic reduction of ClO₃⁻, BrO₃⁻, IO₃⁻, S₂O₈²⁻ and NO₂⁻ by a poly(luminol)/PMo₁₂O₄₀³⁻ hybrid film in an acidic aqueous solution showed an electrocatalytic reduction activity of IO₃⁻ > BrO₃⁻ and ClO₃⁻. The electrocatalytic oxidation of dopamine and epinephrine by a poly(luminol)/PMo₁₂O₄₀³⁻ hybrid film was also investigated.     

Adsorption and desorption of Ni on Na-montmorillonite: Effect of pH, ionic strength, fulvic acid, humic acid and addition sequences

Humic substances and clay minerals have been studied intensively because of their strong complexation and adsorption capacities. In this work, adsorption of Ni²⁺ on Na-montmorillonite was studied using batch technique under ambient conditions. Effect of pH, ionic strength, solid content, humic acid (HA), fulvic acid (FA) and the addition sequences of fulvic acid/Ni²⁺/montmorillonite on Ni²⁺ adsorption was also investigated. The results indicate that adsorption of Ni²⁺ on montmorillonite are strongly dependent on pH and ionic strength. The adsorption of Ni²⁺ is mainly dominated by surface complexation and ion exchange. The adsorption–desorption hysteresis suggests that the adsorption of Ni²⁺ is irreversible. The thermodynamic parameters (ΔH, ΔS, and ΔG) are calculated from the temperature dependence, and the results suggest that the adsorption reaction is endothermic and spontaneous. The presence of FA and the addition sequences of FA/Ni²⁺/montmorillonite do not influence the adsorption of Ni²⁺ on FA bound montmorillonite hybrids. Montmorillonite is a suitable candidate for pre-concentration and solidification of Ni²⁺ from large volume of solutions.     

Electrooxidation of acetaldehyde on carbon-supported Pt, PtRu and Pt3Sn and unsupported PtRu0.2 catalysts: A quantitative DEMS study

The oxidation of acetaldehyde on carbon supported Pt/Vulcan, PtRu/Vulcan and Pt₃Sn/Vulcan nanoparticle catalysts and, for comparison, on polycrystalline Pt and on an unsupported PtRu_0.2 catalyst, was investigated under continuous reaction and continuous electrolyte flow conditions, employing electrochemical and quantitative differential electrochemical mass spectroscopy (DEMS) measurements. Product distribution and the effects of reaction potential and reactant concentration were investigated by potentiodynamic and potentiostatic measurements. Reaction transients, following both the Faradaic current as well as the CO₂ related mass spectrometric intensity, revealed a very small current efficiency for CO₂ formation of a few percent for 0.1 m acetaldehyde bulk oxidation under steady-state conditions on all three catalysts, the dominant oxidation product being acetic acid. Pt alloy catalysts showed a higher activity than Pt/Vulcan at lower potential (0.51 V), but do not lead to a better selectivity for complete oxidation to CO₂. C–C bond breaking is rate limiting for complete oxidation at potentials with significant oxidation rates for all three catalysts. The data agree with a parallel pathway reaction mechanism, with formation and subsequent oxidation of CO_ad and CH _{x, ad} species in the one pathway and partial oxidation to acetic acid in the other pathway, with the latter pathway being, by far, dominant under present reaction conditions.     

A new class of highly dispersed VOx catalysts on mesoporous silica: Synthesis, characterization, and catalytic activity in the partial oxidation of ethanol

The morphology of vanadium oxide supported on a titania-modified mesoporous silica (MCM-41), obtained by means of a careful grafting process through atomic layer deposition, was studied using a variety of characterization techniques. The X-ray diffraction (XRD) together with transmission electron microscopy (TEM), ⁵¹V nuclear magnetic resonance (⁵¹V-NMR), Raman, FTIR and DRS-UV/Vis results showed that the vanadia species are extremely well dispersed onto the surface of the mesoporous support; the dispersion being stable upon thermal treatments up to 400 °C. Studies of the catalytic activity of these materials were performed using the partial oxidation of ethanol as a probe reaction. The results indicate an intrinsic relationship between dispersion, the presence of a TiO₂–VO_x phase, and catalytic activity for oxidation and dehydration.     

Low-temperature preferential oxidation of CO in a hydrogen rich stream (PROX) over Au/TiO2: Thermodynamic study and effect of gold-colloid pH adjustment time on catalytic activity

By simulating CO and H₂ oxidations at thermodynamic equilibrium and studying the catalytic oxidations over Au/TiO₂, preferential oxidation of CO in a H₂ rich stream (PROX) was investigated. During the simulation, at least two cases under different gaseous feeds, H₂/CO/O₂/N₂ = 50/1/0.5/48.5 or 50/1/1/48 (vol.%) were examined under the assumption of an ideal gas and one atmosphere pressure in the reactor. It was found that the addition of 1% O₂ (the latter case) effectively reduced CO concentration to less than 100 ppm in the temperature range between 0 and 90 °C. This range narrowed to between 0 and 50 °C with the addition of 3% H₂O and 15% CO₂ in the feed. The thermodynamic study suggests that 1% CO in a H₂ rich system can be decreased to below 100 ppm within those low temperature ranges, if there is no substantial adsorptions onto the catalyst surface and the reactions rapidly reach equilibrium. During the catalysis reaction study, a well-pH adjusted Au/TiO₂ catalyst was found very active for PROX. CO conversions at the reactor outlet were close to those at equilibrium. Au/TiO₂ used in this work was prepared via deposition-precipitation (DP) method. The influence of gold colloid pH (at 6) adjustment time on gold loading, gold particle size and chloride residue on TiO₂ surface was detected by atomic absorption (AA), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). A pH adjustment time of at least 6 h for the preparation of gold colloids at room temperature was demonstrated to be essential for the high catalytic activity of Au/TiO₂. This was attributed to the smaller gold particle and the less chloride residue on the catalyst surface.     

On the capacity fading of LiCoO2 intercalation electrodes:: the effect of cycling, storage, temperature, and surface film forming additives

The electrochemical behavior and surface chemistry of LiCoO₂ intercalation cathodes as a function of cycling and storage at 25, 45, and 60 °C was studied. The standard solutions for this work comprised ethylene carbonate (EC), ethyl-methyl carbonate (EMC), (1:2) and 1 M LiPF₆. The effect of two surface film-forming additives, vinylene carbonate (VC) and an organo-borate complex (denoted as Merck's AD25) in solutions was also explored. We analyzed temperature-dependent processes of surface film formation on the cathodes, which increase their impedance upon cycling and storage, thus making their electrochemical kinetics sluggish. We also analyzed cobalt dissolution from the cathodes at 25, 45 and 60 °C. The apparent capacity fading of the LiCoO₂ electrodes is attributed mostly to changes on their surface, rather than to bulk degradation. There are signs that the presence of HF in solutions may play a major negative role. Hence, as the electrode's surface/solution volume ratio is higher, the capacity fading of the LiCoO₂ electrodes should be lower. The main tools for this study were cyclic voltammetry, chronopotentiometry, impedance spectroscopy, electrochemical quartz crystal microbalance (EQCM), IR-spectroscopy, XRD, XPS, and SEM.