Influence of ammonia on the conductivity of Nafion membranes

The effect of NH₃ and NH₄⁺ poisoning on the conductivity of Nafion membranes was investigated via electrochemical impedance spectroscopy. The conductivities of membranes prepared with different NH₄⁺ compositions were measured in deionized water at room temperature and compared to those at 80 °C in a gas phase for various relative humidities. The liquid-phase conductivity decreased linearly with an increase in the NH₄⁺ composition in the membrane (y_NH4⁺), with that of the NH₄⁺-form having a conductivity 25% that of the H⁺-form. The gas-phase conductivity of the NH₄⁺-form, on the other hand, declined by 66–98% relative to the H⁺-form depending on humidity. The conductivities of fresh membranes in the presence of gas-phase NH₃ at different humidities were also studied. The conductivity decreased with time-on-stream and reached the same conductivity at a given humidity regardless of the NH₃ concentration, but the time to reach steady-state varied with NH₃ concentration. The y_NH4⁺ at steady-state conductivity was equivalent for all the NH₃ concentrations studied. The kinetics of conductivity decrease was slower at higher humidities. The humidity and y_NH4⁺ appear to have a concerted effect on the conductivity. The quantitative conductivity data under practical fuel cell conditions should be useful for future fuel cell modeling.     

Coconut husk extract: antibacterial properties and its application for shelf-life extension of Asian sea bass slices

Antibacterial properties of ethanolic coconut husk extracts (ECHE) prepared using varying ethanol concentrations as the extracting media against Pseudomonas aeruginosa, Escherichia coli, Vibrio parahaemolyticus, Listeria monocytogenes and Staphylococcus aureus was studied. ECHE60, prepared using 60% ethanol, had similar minimum inhibitory concentration and minimum bactericidal concentration to those extracted using 80% and 100% ethanol (P ˂ 0.05). When Asian sea bass (Lates calcarifer) slices were treated with ECHE60 at 200 and 400 ppm, the quality changes were monitored in comparison with the control (without treatment) during 12 days of refrigerated storage (4 °C). The shelf life of sea bass treated with 400 ppm ECHE60 was extended to 9 days, whereas the control had a shelf life of 3 days. Lipid oxidation was lowered in ECHE60 treated samples than the control during storage. Therefore, ECHE60 could inhibit both spoilage and pathogenic bacteria and also extended the shelf life of sea bass slices.     

Preparation and characterization of nanosized silver phosphate loaded hydroxyapatite by single step co-conversion process

The preparation and characterization of silver phosphate nanoparticles loaded hydroxyapatite aiming to enhance the bactericidal performance by a single step co-conversion technique using low temperature phosphorization in the presence of various silver nitrate concentration (AgNO₃, ranging 0.001–0.1 M) was performed. Characterization by using X-ray diffraction, infrared spectroscopy and transmission electron microscopy showed that hydroxyapatite and silver phosphate were the main phases in all converted samples and the microstructure comprised the distribution of spherical-shaped silver phosphate nanoparticles within the cluster of hydroxyapatite nanocrystals. Total silver content (ranging 0.09–5.6%) in the converted samples was found to increase with increasing silver nitrate content. Flexural modulus and strength of converted samples remained unchanged for samples using silver nitrate between 0.001 and 0.01 M, but decreased at greater silver nitrate concentration. Antibacterial activity of two selected samples (0.001 and 0.005 M AgNO₃) against two bacterial strains (Pseudomonas aeruginosa and Staphylococcus aureus) was observed since 100% reduction of viable cells after 24 h contact was detected. Cytotoxic potential by MTT assay of sample using 0.001 M AgNO₃ was only observed at 24 h extraction, but was seen at all extraction periods (24–72 h) for sample using 0.005 M AgNO₃.     

Influence of stabilising agents on the properties of liposomal encapsulated ethanolic coconut husk extract

Characteristics of liposomal encapsulated ethanolic coconut husk extract (LE-ECHE) as influenced by different stabilising agents (SA) were investigated. LE-ECHE was prepared using four liposome formulations including soy phosphatidylcholine (SPC) alone and SPC mixed with cholesterol (CHL), tween 80 (TWE) or glycerol (GLY) at a ratio of 4 : 1 as lipid phase (LP) at a concentration of 60 µmol mL⁻¹. Particle size, zeta potential, poly-dispersity index, encapsulation efficiency (EE) and suspension stability (SS) of LE-ECHE samples were markedly influenced by SA (P < 0.05). Antioxidant activity of ECHE was retained after entrapment in liposome, regardless of SA used. However, SA affected the antioxidant activities of LE-ECHE differently. Antibacterial properties of ECHE were enhanced after encapsulation in liposome, irrespective of SA. Therefore, liposome with glycerol or cholesterol could serve as a delivery based system for improving the antibacterial properties with high stability, while lowering the undesirable colour of ECHE.     

Salmonella bacteriophage diversity reflects host diversity on dairy farms

Salmonella is an animal and human pathogen of worldwide concern. Surveillance programs indicate that the incidence of Salmonella serovars fluctuates over time. While bacteriophages are likely to play a role in driving microbial diversity, our understanding of the ecology and diversity of Salmonella phages is limited. Here we report the isolation of Salmonella phages from manure samples from 13 dairy farms with a history of Salmonella presence. Salmonella phages were isolated from 10 of the 13 farms; overall 108 phage isolates were obtained on serovar Newport, Typhimurium, Dublin, Kentucky, Anatum, Mbandaka, and Cerro hosts. Host range characterization found that 51% of phage isolates had a narrow host range, while 49% showed a broad host range. The phage isolates represented 65 lysis profiles; genome size profiling of 94 phage isolates allowed for classification of phage isolates into 11 groups with subsequent restriction fragment length polymorphism analysis showing considerable variation within a given group. Our data not only show an abundance of diverse Salmonella phage isolates in dairy farms, but also show that phage isolates that lyse the most common serovars causing salmonellosis in cattle are frequently obtained, suggesting that phages may play an important role in the ecology of Salmonella on dairy farms.     

Effect of ammonium ion distribution on Nafion conductivity

Nafion^® conductivity in a proton exchange membrane fuel cell (PEMFC) with the fuel stream containing ammonia is mainly affected by the ammonium ion composition and operating conditions. In this study, the effect of ammonium ion distribution on Nafion conductivity was investigated for the first time. The conductivities of two kinds of contaminated membranes having uniform and non-uniform ammonium ion distributions were studied. To simulate a membrane with a well-defined ammonium ion concentration profile, three individual Nafion membranes containing known amounts of ammonium ions were physically stacked together. The uniform and non-uniform cases represented membranes having three layers with the same yN⁺H₄ or step changes in concentration, respectively. Under fuel cell operations, the conductivities of non-uniformly poisoned membranes were ca. 1.07-1.86 times larger than those of uniformly poisoned membranes, depending on humidity, contamination level, and ammonium ion distribution. Consequently, the performance prediction of a cationic-poisoned PEMFC needs to consider any concentration gradients that may exist in MEA. The liquid-phase conductivities of composite membranes were also studied and the results show that conductivity measurements performed in deionized water are not representative of what exists under fuel cell conditions due to rapid redistribution of ions in the Nafion via the liquid phase.     

Persistent Listeria monocytogenes subtypes isolated from a smoked fish processing facility included both phage susceptible and resistant isolates

Contamination of Ready-To-Eat foods with Listeria monocytogenes can typically be traced back to post-processing contamination from environmental sources; contamination is often linked to subtypes that persist in food associated environments. Although phage-based biocontrol strategies have been proposed for controlling this pathogen, information on the efficacy of phage treatment against diverse L. monocytogenes subtypes from food associated environments is still limited. We identified subtypes that were repeatedly found (“persistent”) in a smoked fish processing facility by using EcoRI ribotyping data for isolates obtained in 1998–2009. PFGE analysis of 141 isolates (9 ribotypes) supported persistence for up to 11 years. Characterization of selected isolates, representing persistent subtypes, against a panel of 28 listeriaphages showed a wide range of likelihood of phage susceptibility, ranging from 4.6% (for 7 ribotype DUP-1043A isolates) to 95.4% (for 7 ribotype DUP-1044A isolates). In challenge studies with 10⁵ and 10⁶ CFU/ml L. monocytogenes, using phage cocktails and a commercial phage product at different phage-host ratios, one isolate (ribotype DUP-1043A) was not affected by any treatment. A reduction in L. monocytogenes counts of up to 4 log units was observed, after 8 h of treatment, in isolates of two ribotypes, but subsequent re-growth occurred. Survivor isolates obtained after 24 h of treatment showed decreased susceptibility to individual phages included in the phage cocktail, suggesting rapid emergence of resistant subtypes.     

Prediction of the effective conductivity of Nafion in the catalyst layer of a proton exchange membrane fuel cell

In a previous study, a simple acid catalyzed reaction (esterification) was found to predict excellently conductivity of a membrane contaminated with NH₄⁺ or Na⁺. Since measurement of the conductivity of Nafion in a catalyst layer is problematic, being able to predict this conductivity for various formulations and fuel cell conditions would be advantageous. In this study, the same methodology as before was used to examine the proton availabilities of supported Nafion (Nafion on carbon and on Pt/C), as exists in the catalyst layer used in a PEMFC, during impurity exposure (e.g., NH₃) as a means for prediction of its conductivity. It was found that the effect of NH₃ exposure on the proton composition (yH+) of supported Nafion was similar to that of N-211 under the same conditions. Determined values of yH+ were then used to estimate the effective conductivity of an ammonium-poisoned cathode layer using the correlation developed and the agglomerate model. The predicted conductivities were matched with the results available in the literature. This technique would be useful for the optimization of catalyst design and for fuel cell simulation, since it provides many benefits over conventional performance test procedures.     

Pt Alloy Electrocatalysts for Proton Exchange Membrane Fuel Cells: A Review

Both the CO poisoning problem on the anode and the slow oxygen reduction reaction kinetics on the cathode lead to significantly decreased output power and energy utilization efficiency and remain main obstacles hindering commercialization of proton-exchange membrane fuel cells (PEMFCs). A promising means to mitigate CO poisoning and to improve the oxidation reduction reaction (ORR) activity is through the use of platinum alloy catalysts. This article reviews recent developments in Pt alloy catalyst utilization and addresses activity comparisons and the relationship between activity and structure characteristics. The mechanisms for improved CO-tolerance and ORR activity are also discussed. Finally, theoretical studies on Pt alloy catalysts are discussed briefly.