Cathodic reaction kinetics and its implication on flow-assisted corrosion of aluminum alloy in aqueous ethylene glycol solution

The cathodic reaction kinetics and anodic behavior of Al alloy 3003 in aerated ethylene glycol–water solution, under well-controlled hydrodynamic conditions, were investigated by various measurements using a rotating disk electrode (RDE). The transport and electrochemical parameters for cathodic oxygen reduction were fitted and determined. The results demonstrate that the cathodic reaction is a purely diffusion-controlled process within a certain potential region. The experimentally fitted value of diffusion coefficient of oxygen is 3.0 × 10⁻⁸ cm² s⁻¹. The dependence of cathodic current on rotation speed was in quantitative agreement with Levich equation. At potentials more positive than the diffusion controlled region, the cathodic process was controlled by both diffusion and electrochemical kinetics. The electrochemical reaction rate constant, k ₀, was determined to be 1.1 × 10⁻⁹ cm s⁻¹. There is little effect of electrode rotation on anodic behavior of Al alloy during stable pitting. However, fluid hydrodynamics play a significant role in formation of the oxide film and the Al alloy passivity. An enhanced electrode rotation would increase the mass-transfer rate of solution, and thus the oxygen diffusion towards the electrode surface for reduction reaction. The generated hydroxide ions are favorable to the formation of Al oxide film on electrode surface.     

Surface hydrophobicity alteration of fractionated paraffin wax, crude by-product polyolefin wax and their blend

The surface compositions of crude by-product polyolefin wax (wax K) from a naphtha cracking unit, fractionated commercial paraffin wax (wax J) and their blend (wax M) were compared, under various conditions, by using FT-IR. Then a practical criterion was theoretically derived by back-of-the-envelope-calculation to estimate the diffusivity for the surface enrichment of hydrophilic functional groups. The hydrophobicity in terms of chemical structure and functional groups was altered by changing the environment as well as by blending wax K with wax J. The surface properties of wax J and wax M turned out more hydrophobic than that of wax K within the experimental period of water-submerged condition, even though the structure and functional groups on the surfaces of all waxes under ambient atmospheric condition are extremely similar. Thus the blend of wax M behaved unlike wax K but similarly to wax J in terms of alteration of hydrophobicity. Such values of diffusivity as 1.35×10⁻¹¹ cm²/s and 1.96×10⁻¹¹ cm²/s are established for wax J and wax M, respectively, under 1 day water-submerged condition. Furthermore, under 1 week water-submerged condition, the values of their diffusivity for wax J, wax K and wax M are estimated as 3.06×10⁻¹² cm²/s, 7.23×10⁻¹¹ cm²/s and 1.50×10⁻¹¹ cm²/s, respectively. The obtained values of diffusivity are consistent, in terms of the order of magnitude, with the value of diffusion coefficient of 5.75×10⁻¹¹ cm²/s obtained, using a very complicated empirical-formula, by Chen et al. for the investigation of surface-modifier enrichment. Thus, the suggested criterion may be very useful and practically applied to clarify the process of surface enrichment from general polymer-blends.     

Polymeric ionic liquid membranes as electrolytes for lithium battery applications

A new kind of polymeric ionic liquid (PIL) membrane based on guanidinium ionic liquid (IL) with ester and alkyl groups was synthesized. On addition of guanidinium IL, lithium salt, and nano silica in the PIL, a gel PIL electrolyte was prepared. The chemical structure of the PIL and the properties of gel electrolytes were characterized. The ionic conductivity of the gel electrolyte was 5.07 × 10⁻⁶ and 1.92 × 10⁻⁴ S cm⁻¹ at 30 and 80 °C, respectively. The gel electrolyte had a low glass transition temperature (T _g ) under −60 °C and a high decomposition temperature of 310 °C. When the gel polymer electrolyte was used in the Li/LiFePO₄ cell, the cell delivered 142 mAh g⁻¹ after 40 cycles at the current rates of 0.1 C and 80 °C.     

Fluorescence Enhancement Effect for the Determination of Nucleic Acids With Morin–NanoTiO<Subscript>2</Subscript>

It is found that nucleic acids can greatly enhance the fluorescence intensity of morin–nanoTiO₂. Under optimum conditions, the enhanced fluorescence intensity of the system is in proportion to the concentration of nucleic acids in the range of 2.0 × 10⁻⁸ to 2.2 × 10⁻⁷ g mL⁻¹ for calf thymus DNA (ctDNA) and 1.0 × 10⁻⁸ to 2.5 × 10⁻⁷ g mL⁻¹ for yeast RNA (yRNA). The detection limits are 4.8 × 10⁻⁹ g mL⁻¹ for ctDNA and 1.2 × 10⁻⁹ g mL⁻¹ for yRNA, respectively. This method has satisfactorily been used for the determination of nucleic acids in actual sample.     

Electrocatalytic oxidation of glutathione at carbon paste electrode modified with 2,7-<Emphasis Type="Italic">bis</Emphasis> (ferrocenyl ethyl) fluoren-9-one: application as a voltammetric sensor

Electrooxidation of glutathione (GSH) was studied at the surface of 2,7-bis (ferrocenyl ethyl) fluoren-9-one modified carbon paste electrode (2,7-BFEFMCPE). Cyclic voltammetry (CV), double potential-step chronoamperometry, and differential pulse voltammetry (DPV) were used to investigate the suitability of this ferrocene derivative as a mediator for the electrocatalytic oxidation of GSH in aqueous solutions with various pH. Results showed that pH 7.00 is the most suitable pH for this purpose. At the optimum pH, the oxidation of GSH at the surface of this modified electrode occurs at a potential of about 0.410 V versus Ag|AgCl|KCl_sat. The kinetic parameters such as electron transfer coefficient, α = 0.61, and rate constant for the chemical reaction between GSH and redox site in 2,7-BFEFMCPE, k _h = 1.73 × 10³ cm³ mol⁻¹ s⁻¹, were also determined using electrochemical approaches. Also, the apparent diffusion coefficient, D _app, for GSH was found to be 5.0 × 10⁻⁵ cm² s⁻¹ in aqueous buffered solution. The electrocatalytic oxidation peak current of GSH showed a linear dependence on the glutathione concentration, and linear calibration curves were obtained in the ranges of 5.2 × 10⁻⁵ M to 4.1 × 10⁻³ M and 9.2 × 10⁻⁷ M to 1.1 × 10⁻⁵ M with cyclic voltammetry and differential pulse voltammetry methods, respectively. The detection limits (3σ) were determined as 1.4 × 10⁻⁵ M and 5.1 × 10⁻⁷ M for the CV and DPV methods, respectively. This method was also examined as a selective, simple, and precise new method for voltammetric determination of GSH in real sample such as hemolysed erythrocyte.     

Simple Determination of Deoxycholic and Ursodeoxycholic Acids by Phenolphthalein-β-Cyclodextrin Inclusion Complex

An expeditious colorimetric methodology for the determination of the deoxycholic acid (DCA) and of the ursodeoxycholic acid (UDCA) in pharmaceutical formulations is reported. The method is based on their competitive complexation reaction with a color indicator to form β-cyclodextrin-inclusion complexes. Several pH color indicators were tested, but phenolphthalein (PHP) showed the best interaction with the β-cyclodextrin (β-CD) with an inclusion yield higher than 95%. The best concentrations of β-cyclodextrin to form inclusion complexes were 1.24 × 10⁻³ and 6.2 × 10⁻⁴ M at pH 9.5 and 10.5. Statistical analysis of the results showed that the pH had a significant effect on the DCA determination and that high β-CD-PHP inclusion complex concentrations had a significant negative effect on the UDCA determination (p < 0.05). The limit of detection and limit of quantification were 3.94 × 10⁻⁵ and 1.31 × 10⁻⁴ M for DCA (range: 6.1 × 10⁻⁶–3.13 × 10⁻³ M), 4.08 × 10⁻⁵  and 1.36 × 10⁻⁴ M for UDCA (range: 6.05 × 10⁻⁶–3.88 × 10⁻⁴ M). This simple and cheap method showed high stability and feasible instrumentation.     

Electrochemical determination of thiamazole at a multi-wall carbon nanotube modified glassy carbon electrode

A simple and convenient method is described for voltammetric determination of thiamazole, a commonly used anti-hyperthyroid drug, based on its electrochemical oxidation at a multi-wall carbon nanotube modified glassy carbon electrode. Under optimized conditions, the proposed method exhibited acceptable analytical performances in terms of linearity (over the concentration range from 1.0 × 10⁻⁷ to 5.0 × 10⁻⁴ mol L⁻¹, r = 0.9983), detection limit (3.0 × 10⁻⁸ mol L⁻¹) and reproducibility (RSD = 2.64%, n = 10, for 5.0 × 10⁻⁵ mol L⁻¹ thiamazole). To further validate its possible application, the method was used for the quantification of thiamazole in pharmaceutical formulations and biological fluids.     

Drying kinetics and effective moisture diffusivity of purslane undergoing microwave heat treatment

The effects of microwave drying on moisture content, moisture ratio, drying time and effective moisture diffusivity of purslane leaves (Portulaca oleracea L.) were investigated. By increasing the microwave output power (180–900W) and the sample amounts (25–100 g), the drying time decreased from 43 to 12.5 minutes and increased from 27 to 54 minutes, respectively. To determine the kinetic parameters, the drying data were fitted to various models based on the ratios of the differences between the initial and final moisture contents and equilibrium moisture content versus drying time. Among the models proposed, the semi-empirical Midilli et al. model gave a better fit for all drying conditions applied. By increasing the microwave output power and decreasing the sample amount, the effective moisture diffusivity values ranged from 5.913×10⁻¹¹ to 1.872×10⁻¹⁰ m²/s and from 9.889×10⁻¹¹ to 3.292×10⁻¹¹ m²/s, respectively. The activation energy was calculated using an exponential expression based on the Arrhenius equation.     

Sensory Thresholds of Selected Phenolic Constituents from Thyme and their Antioxidant Potential in Sunflower Oil

The odor detection thresholds of carvacrol (5-isopropyl-2-methyl-phenol), thymol (2-isopropyl-5-methyl-phenol) and p-cymene 2,3-diol (2,3-dihydroxy-4-isopropyl-1-methyl-benzene) in sunflower oil, determined by the three-alternative, forced-choice procedure, were 30.97, 124 and 794.33 mg kg⁻¹, respectively. Sunflower oil containing 13, 70, or 335 mg kg⁻¹ of carvacrol, thymol or p-cymene 2,3-diol, respectively, was judged to be similar (P < 0.01) in taste and odor to its antioxidant-free counterpart. The rate constant of sunflower oil oxidation, measured from the increase in peroxide value during storage at 25 °C, was 9.2 × 10⁻⁹ mol kg⁻¹ s⁻¹ while the rate constants were 9.3 × 10⁻⁹, 9.8 × 10⁻⁹, and 4.3 × 10⁻⁹ mol kg⁻¹ s⁻¹ in the presence of 13 mg kg⁻¹ carvacrol, 70 mg kg⁻¹ thymol, and 335 mg kg⁻¹ p-cymene 2,3-diol, respectively. At a level of 335 mg kg⁻¹, p-cymene 2,3-diol did not impart flavor taints and effected a 46.7% reduction in the rate of oxidation of sunflower oil. These findings indicate that the diphenolic p-cymene 2,3-diol could potentially replace synthetic antioxidants and is a valuable addition to the antioxidants used by the food industry in its quest to meet consumer demands for synthetic-additives-free and ‘natural’ foods.     

Homogeneous electrocatalytic oxidation of <Emphasis Type="SmallCaps">d</Emphasis>-penicillamine with ferrocyanide at a carbon paste electrode: application to voltammetric determination

The electrooxidation of d-penicillamine (d-PA) was studied in the presence of ferrocyanide as a homogeneous mediator at the surface of a carbon paste electrode in aqueous media using cyclic voltammetry (CV) and chronoamperometry. Under optimum pH in CV the oxidation of d-PA occurs at a potential about 380 mV less positive than that in the absence of ferrocyanide. The catalytic oxidation peak current was dependent on the d-PA concentration and a linear calibration curve was obtained in the ranges 4.0 × 10⁻⁵–2.0 × 10⁻³ M and 8.0 × 10⁻⁶–1.8 × 10⁻⁴ M of d-PA with CV and differential pulse voltammetry (DPV) methods, respectively. The detection limits (3σ) were determined as 1.9 × 10⁻⁵ and 3.2 × 10⁻⁶ M by CV and DPV methods. This method was also used for the determination of d-PA in pharmaceutical preparations by the standard addition method.     

Nanostructured Systems Containing Rutin: In Vitro Antioxidant Activity and Photostability Studies

The improvement of the rutin photostability and its prolonged in vitro antioxidant activity were studied by means of its association with nanostructured aqueous dispersions. Rutin-loaded nanocapsules and rutin-loaded nanoemulsion showed mean particle size of 124.30 ± 2.06 and 124.17 ± 1.79, respectively, polydispersity index below 0.20, negative zeta potential, and encapsulation efficiency close to 100%. The in vitro antioxidant activity was evaluated by the formation of free radical ·OH after the exposure of hydrogen peroxide to a UV irradiation system. Rutin-loaded nanostructures showed lower rutin decay rates [(6.1 ± 0.6) 10⁻³ and (5.1 ± 0.4) 10⁻³ for nanocapsules and nanoemulsion, respectively] compared to the ethanolic solution [(35.0 ± 3.7) 10⁻³ min⁻¹] and exposed solution [(40.1 ± 1.7) 10⁻³ min⁻¹] as well as compared to exposed nanostructured dispersions [(19.5 ± 0.5) 10⁻³ and (26.6 ± 2.6) 10⁻³, for nanocapsules and nanoemulsion, respectively]. The presence of the polymeric layer in nanocapsules was fundamental to obtain a prolonged antioxidant activity, even if the mathematical modeling of the in vitro release profiles showed high adsorption of rutin to the particle/droplet surface for both formulations. Rutin-loaded nanostructures represent alternatives to the development of innovative nanomedicines.     

Horizontal nutrient fluxes and food safety in urban and peri-urban vegetable and millet cultivation of Niamey,Niger

Urban and peri-urban agriculture (UPA) has often been accused of being nutrient inefficient and producing negative externalities. To investigate these problems for the West African capital Niamey (Niger), nutrient inputs through fertilizer and manure to 10 vegetable gardens and 9 millet fields and nutrient offtakes through harvests were quantified during 24 months, and contamination of irrigation water and selected vegetables with faecal pathogens and heavy metals was determined. Annual partial horizontal balances for carbon (C), nitrogen (N), phosphorus (P) and potassium (K) amounted to 9,936 kg C ha⁻¹, 1,133 kg N ha⁻¹, 223 kg P ha⁻¹ and 312 kg K ha⁻¹ in high input vegetable gardens as opposed to 9,580 kg C ha⁻¹, 290 kg N ha⁻¹, 125 kg P ha⁻¹ and 351 kg K ha⁻¹ in low input gardens. In high input millet fields, annual surpluses of 259 kg C ha⁻¹, 126 kg N ha⁻¹, 20 kg P ha⁻¹ and 0.4 kg K ha⁻¹ were recorded, whereas surpluses of 12 kg C ha⁻¹, 17 kg N ha⁻¹, and deficits of −3 kg P ha⁻¹ and −3 kg K ha⁻¹ were determined for low input fields. Counts of Salmonella spp. and Escherichia coli yielded above threshold contamination levels of 7.2 × 10⁴ CFU 25 g⁻¹ and 3.9 × 10⁴ CFU g⁻¹ in lettuce irrigated with river water and fertilized with animal manure. Salmonella counts averaged 9.8 × 10⁴ CFU 25 g⁻¹ and E. coli 0.6 × 10⁴ CFU g⁻¹ for lettuce irrigated with wastewater, while these pathogens were not detected on vegetables irrigated with pond water. These results underline the need for urban gardeners to better adjust the nutrients applied to crop requirements which might also reduce nutrient accumulations in the soil and further in the edibles parts of the vegetables. Appropriate pre-treatment of irrigation water would help improve the quality of the latter and enhance the food safety of vegetables determined for the urban markets.     

Simultaneous determination of dopamine and uric acid on nafion/sodium dodecylbenzenesulfonate composite film modified glassy carbon electrode

A novel electrochemical sensor has been constructed by using a glassy carbon electrode (GCE) coated with nafion/sodium dodecylbenzenesulfonate (SDBS). Differential pulse voltammetry (DPV) was used to study the electrochemical behaviors of dopamine (DA) and uric acid (UA). An optimum of 5 mM SDBS together with 0.05 wt% of nafion was used to improve the resolution and the determination sensitivity successfully. In 0.1 M phosphate buffer solution (pH 6.5), the modified electrode exhibited high electrocatalytical activity toward the oxidation of DA and UA with obvious reduction of overpotential. Compared with bare GCE, the modified electrode resolved the voltammetric response of DA and UA into two well-defined voltammetric peaks by DPV, which can be used for simultaneous determination of these species in mixture. The peak currents obtained from DPV were linearly related to the concentrations of DA and UA in the ranges of 4.0 × 10⁻⁷–8.0 × 10⁻⁵ M and 4.0 × 10⁻⁶–8.0 × 10⁻⁴ M, respectively. The detection limit of DA and UA (signal-to-noise ration was 3) were 5.0 × 10⁻⁸ and 4.0 × 10⁻⁷ M, respectively.     

Electrochemical determination of phloroglucinol using a carbon nanotube modified electrode enhanced by surfactant

A novel technique is utilized to detect trace amounts of phloroglucinol. In pH 5.0, 0.1 mol L⁻¹ HAc–NaAc buffer solution, phloroglucinol exhibited a stable and sensitive oxidation signal at a glassy carbon electrode modified with multi-wall carbon nanotube. By using the surfactant cetyl pyridinium chloride, the electrochemical response was greatly enhanced. The mechanism was systematically explored. In the range 9.0 × 10⁻⁷–3.0 × 10⁻⁴ mol L⁻¹, the oxidation peak currents of phloroglucinol have a linear relationship with concentration: the limit of detection was estimated to be 2.5 × 10⁻⁷ mol L⁻¹ (S/N = 3). The method was adopted to detect the content of phloroglucinol injection, and the recovery was from 97.5% to 103.0%.     

Investigation of N<Subscript>2</Subscript>-fixation on polyaniline electrodes in methanol by electrochemical impedance spectroscopy

The ac response of polyaniline thin films on platinum electrodes was measured at different dc potentials during the N₂-fixation in methanol + LiClO₄ electrolyte with 0.03 mol L⁻¹ H₂SO₄ for the first time. The optimum film thickness was found to be 1.5 μm, N₂-pressure 50 bar and an optimum electrolysis potential of −0.12 V (NHE). The diffusion coefficients for N₂ into the polymer film was found to be (5 ± 2)×10⁻⁹ cm² s⁻¹.     

Development of a new biosensor for mediatorless voltammetric determination of hydrogen peroxide and its application in milk samples

A new biosensor for the voltammetric detection of hydrogen peroxide was developed based on immobilization of catalase on a clinoptilolite modified carbon paste electrode using bovine serum albumin and glutaraldehyde. The biosensor response was evaluated according to electrode composition, reaction time, solution pH and temperature. The voltammetric signals were linearly in proportion to H₂O₂ concentration in the range 5.0 × 10⁻⁶–1.0 × 10⁻³ M with a correlation coefficient of 0.9975. The detection limit is 8.0 × 10⁻⁷ M and the relative standard deviation for 4.0 × 10⁻⁴ M hydrogen peroxide was 1.83% (n = 6). The biosensor exhibited high sensitivity, and it was determined that it could be used for more than 2 months. In addition, the biosensor was successfully applied for the determination of hydrogen peroxide in milk samples.     

Electroanalysis of urinary l-dopa using tyrosinase immobilized on gold nanoelectrode ensembles

The performance of an amperometric biosensor constructed by associating tyrosinase (Tyr) enzyme with the advantages of a 3D gold nanoelectrode ensemble (GNEE) is evaluated in a flow-injection analysis (FIA) system for the analysis of l-dopa. GNEEs were fabricated by electroless deposition of the metal within the pores of polycarbonate track-etched membranes. A simple solvent etching procedure based on the solubility of polycarbonate membranes is adopted for the fabrication of the 3D GNEE. Afterward, enzyme was immobilized onto preformed self-assembled monolayers of cysteamine on the 3D GNEEs (GNEE-Tyr) via cross-linking with glutaraldehyde. The experimental conditions of the FIA system, such as the detection potential (−0.200 V vs. Ag/AgCl) and flow rates (1.0 mL min⁻¹) were optimized. Analytical responses for l-dopa were obtained in a wide concentration range between 1 × 10⁻⁸ mol L⁻¹ and 1 × 10⁻² mol L⁻¹. The limit of quantification was found to be 1 × 10⁻⁸ mol L⁻¹ with a resultant % RSD of 7.23% (n = 5). The limit of detection was found to be 1 × 10⁻⁹ mol L⁻¹ (S/N = 3). The common interfering compounds, namely glucose (10 mmol L⁻¹), ascorbic acid (10 mmol L⁻¹), and urea (10 mmol L⁻¹), were studied. The recovery of l-dopa (1 × 10⁻⁷ mol L⁻¹) from spiked urine samples was found to be 96%. Therefore, the developed method is adequate to be applied in the clinical analysis.     

Selective determination of dopamine in the presence of ascorbic acid using ferrocenyl-tethered PAMAM dendrimers modified glassy carbon electrode

Determination of dopamine (DA) in the absence and presence of ascorbic acid (AA) by ferrocenyl-tethered PAMAM G3 dendrimers (Fc-D) modified glassy carbon electrode (GCE) was reported. The modified electrode was characterized with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Factors influencing the detection processes were optimized and kinetic parameters were calculated. The sensor exhibited excellent catalytic activities for the oxidation–reduction reactions of DA and eliminated the interference of AA. Under optimal condition, the linear range of 1 × 10⁻⁵–1.5 × 10⁻³ mol L⁻¹ and the detection limit of 4.7 × 10⁻⁶ mol L⁻¹ was obtained. This study provides a new idea for the determination of DA in the presence of AA.     

An Effective Amperometric Biosensor Based on Gold Nanoelectrode Arrays

A sensitive amperometric biosensor based on gold nanoelectrode array (NEA) was investigated. The gold nanoelectrode array was fabricated by template-assisted electrodeposition on general electrodes, which shows an ordered well-defined 3D structure of nanowires. The sensitivity of the gold NEA to hydrogen peroxide is 37 times higher than that of the conventional electrode. The linear range of the platinum NEA toward H₂O₂ is from 1 × 10⁻⁶ to 1 × 10⁻² M, covering four orders of magnitudes with detection limit of 1 × 10⁻⁷ M and a single noise ratio (S/N) of four. The enzyme electrode exhibits an excellent response performance to glucose with linear range from 1 × 10⁻⁵ to 1 × 10⁻² M and a fast response time within 8 s. The Michaelis–Menten constant km and the maximum current density i _max of the enzyme electrode were 4.97 mM and 84.60 μA cm⁻², respectively. This special nanoelectrode may find potential application in other biosensors based on amperometric signals.     

Diffusivity of bacteria

The effects of motility and aggregation on the diffusion coefficient for bacteria were studied in an aqueous system. The effects of cell concentrations, capillary tube sizes, and dilution rates on the diffusion coefficient were examined. In general, motile cells can diffuse about 1000 times faster than non-motile cells.Pseudomonas aeruginosa, a motile cell, andKlebsiella pneumoniae, a non-motile cell, were used for this research. Diffusion coefficients were measured by the capillary tube assay developed by Adler [1969]. From this procedure the diffusion coefficient ofPseudomonas aeruginosa was 2.1×10⁻⁵ (standard deviation: 1.0× 10⁻⁵) cm²/s and that ofKlebsiella pneumoniae was 0.9×10⁻⁵ (standard deviation : 0.5 × 10⁻⁵) cm²/s. The diffusion coefficient ofPseudomonas aeruginosa was about 2.3 times higher than that ofKlebsiella pneumoniae. The Stokes-Einstein equation could not be used for estimating the diffusion coefficients forKlebsiella pneumoniae andPseudomonas aeruginosa. The experimental value for the diffusion coefficient ofKlebsiella pneumoniae was about 2000 times higher than that (4.5×10⁻⁹ cm₂/s) obtained from the Stokes-Einstein equation. This discrepancy was due to the aggregation ofKlebsiella pneumoniae.