Comparison of Anionic, Cationic and Nonionic Surfactants as Dispersing Agents for Graphene Based on the Fluorescence of Riboflavin.

Fluorescence quenching is a valuable tool to gain insight about dynamic changes of fluorophores in complex systems. Graphene (G), a single-layered 2D nanomaterial with unique properties, was dispersed in surfactant aqueous solutions of different nature: non-ionic polyoxyethylene-23-lauryl ether (Brij L23), anionic sodium dodecylsulphate (SDS), and cationic hexadecyltrimethylammonium bromide (CTAB) and dodecyltrimethylammonium bromide (DTAB). The influence of the surfactant type, chain length and concentration, G total concentration and G/surfactant weight ratio on the fluorescence intensity of vitamin B2 (riboflavin) was investigated. The quality of the different G dispersions was assessed by scanning and transmission electron microscopies (SEM and TEM). A quenching phenomenon of the fluorescence of riboflavin was found for G dispersions in all the surfactants, which generally becomes stronger with increasing G/surfactant weight ratio. For dispersions in the ionic surfactants, the quenching is more pronounced as the surfactant concentration raises, whilst the non-ionic one remains merely unchanged for the different G/Brij L23 weight ratios. More importantly, results indicate that DTAB solutions are the optimum media for dispersing G sheets, leading to an up to 16-fold drop in the fluorescence intensity. Understanding the mechanism in fluorescence quenching of G dispersions in surfactants could be useful for several optical applications.

La creencia epistemológica constructivista sobre el conocimiento científico varía en función del año de formación en los estudiantes de Medicina pero no en los estudiantes de otras carreras de la salud / The constructivist epistemological belief about scientific knowledge varies according to the year of training in medical students but not in students of other health careers

Background: To optimize the teaching-learning process it is fundamental to know the representations that students have regarding knowledge. Epistemological beliefs are implicit theories that guide the practical actions of people. Aim: To characterize and compare epistemological beliefs regarding the nature and acquisition of scientific knowledge of health career students. Material and Methods: Between 2012 and 2013, 726 students coursing first, third or fifth year from six health careers answered a validated questionnaire that includes closed and open questions aimed to characterize their epistemological beliefs about scientific knowledge. Results: Irrespective of the career, when students had to select predefined answers, most of them appeared as constructivists (61%). On the other hand, when they had to argue, the majority seemed objectivist (47%). First-year medical students have the highest frequency of constructivist epistemological beliefs (56%). Paradoxically, the lowest percentage is found (34%) in the fifth year. The students of the health careers, in particular those of Medicine, recognize that knowledge is not acquired immediately (83%) and that its distribution is shared (92%). Conclusions: Discordance between selections and arguments suggests that epistemological sophistication is achieved declaratively but not practically. The lower proportion of students who presented constructivist beliefs in the fifth year compared to first year of Medicine could be associated with the pedagogical approaches used in the different cycles of the career.

[The constructivist epistemological belief about scientific knowledge varies according to the year of training in medical students but not in students of other health careers]. / La creencia epistemológica constructivista sobre el conocimiento científico varía en función del año de formación en los estudiantes de Medicina pero no en los estudiantes de otras carreras de la salud.

BACKGROUND: To optimize the teaching-learning process it is fundamental to know the representations that students have regarding knowledge. Epistemological beliefs are implicit theories that guide the practical actions of people. AIM: To characterize and compare epistemological beliefs regarding the nature and acquisition of scientific knowledge of health career students. MATERIAL AND METHODS: Between 2012 and 2013, 726 students coursing first, third or fifth year from six health careers answered a validated questionnaire that includes closed and open questions aimed to characterize their epistemological beliefs about scientific knowledge. RESULTS: Irrespective of the career, when students had to select predefined answers, most of them appeared as constructivists (61%). On the other hand, when they had to argue, the majority seemed objectivist (47%). First-year medical students have the highest frequency of constructivist epistemological beliefs (56%). Paradoxically, the lowest percentage is found (34%) in the fifth year. The students of the health careers, in particular those of Medicine, recognize that knowledge is not acquired immediately (83%) and that its distribution is shared (92%). CONCLUSIONS: Discordance between selections and arguments suggests that epistemological sophistication is achieved declaratively but not practically. The lower proportion of students who presented constructivist beliefs in the fifth year compared to first year of Medicine could be associated with the pedagogical approaches used in the different cycles of the career.

Quenching of fluorene fluorescence by single-walled carbon nanotube dispersions with surfactants: application for fluorene quantification in wastewater.

The fluorescence of fluorene in aqueous solutions of surfactants of different natures, anionic sodium dodecylsulphate (SDS), cationic cetyltrimethyl ammonium chloride (CTAC) and non-ionic polyoxyethylene-23-lauryl ether (Brij 35), as well as in single-walled carbon nanotube (SWCNT) dispersions in these surfactants, has been studied and compared. A fluorescence quenching phenomenon has been observed in the presence of SWCNT, the effect being stronger for dispersions in CTAC, related to the improved dispersion capability of this surfactant as revealed by microscopic observations and its stronger adsorption onto the SWCNT surfaces as inferred from the Raman spectra. SWCNT interact with fluorene causing a fluorescence quenching. The fluorescence intensity ratio, calculated in the absence and in the presence of SWCNT, follows the Stern-Volmer equation. For the CTAC concentration that provides the highest quenching effect, the analytical characteristics of the fluorimetric method like sensitivity, detection and quantification limits, repeatability, reproducibility and robustness have been calculated. Results demonstrate that it is possible to determine fluorene in a fortified wastewater sample in aqueous solutions of CTAC and SWCNT/CTAC dispersions, showing recoveries close to 100 %. The quenching effect found in this work could be useful for the development of an optical device that uses SWCNT-based receptors for fluorene detection and quantification in aqueous surfactant solutions. Graphical abstract Distribution of fluorene between single-walled carbon nanotubes and micelles.

Antioxidant, antibacterial and ACE-inhibitory activity of four monofloral honeys in relation to their chemical composition.

Different monofloral honeys from Castilla-La Mancha (Spain) have been studied in order to determine their main functional and biological properties. Thyme honey and chestnut honey possess the highest antioxidant capacity, which is due to their high vitamin C (in thyme honey) and total polyphenolic content (in chestnut honey). On the other hand, chestnut honey showed high antimicrobial activity against Staphylococcus aureus and Escherichia coli, whilst others had no activity against S. aureus and showed very small activity against E. coli. Moreover it was found that the antimicrobial activity measured in chestnut honey was partly due to its lysozyme content. In addition the angiotensin I-converting enzyme (ACE) inhibitory activity was measured, and the ACE inhibition is one mechanism by which antihypertensive activity is exerted in vivo. All the types of honey showed some activity but chestnut honey had the highest ACE inhibitory activity.

Vitamin C and sugar levels as simple markers for discriminating Spanish honey sources.

UNLABELLED: In this work, 7 Spanish honeys with different botanical origins were studied. The honey origins were rosemary, chestnut, lavender, echium, thyme, multifloral, and honeydew. The chemical compounds determined were ascorbic acid (vitamin C), hydroxymethylfurfural, and major sugar contents (glucose and fructose). The physicochemical parameters, pH, conductivity, moisture, free acidity, and color, were also measured. Vitamin C is an important antioxidant in food, and the possibility to use it as discriminate parameter among different honeys was studied. The determination of vitamin C in honey samples was carried out by 2 different methods, volumetric and chromatographic comparing the results by both statistically. Vitamin C content was higher in thyme honeys than in the other types; however a wide dispersion in the values was found. Through a linear discriminant analysis (LDA), conductivity, glucose, fructose, and vitamin C content were the most important discriminant parameters. PRACTICAL APPLICATION: Vitamin C content in different honey sources has been determined by a simple and rapid chromatographic method (less than 3 min) in honeys from 6 botanical origins. The results together with glucose and fructose content and some physicochemical parameters have been studied in order to discriminate the botanical origin of honeys and in the future certified their quality. A statistical LDA was applied to the data, and differentiation of honey sources was possible with very good agreement. The vitamin C content found in thymus honeys was significantly higher than in other types. This fact makes vitamin C a special marker for thymus honeys that have a higher antioxidant effect than the others giving it special properties. The identification of honey sources is essential for beekeepers in order to certify honeys for consumers.

mtDNA microevolution in Southern Chile's archipelagos.

The genetic variability of four predominantly Indian populations of southern Chile's archipelagos was examined by determining the frequencies of four mitochondrial DNA haplogroups that characterize the American Indian populations. Over 90% of the individuals analyzed presented Native American mtDNA haplogroups. By means of an unweighted group pair method with arithmetic mean (UPGMA) dendrogram, a principal component analysis (PCA) derived from a distance matrix of mtDNA, and the exact test of population differentiation, we are able to prove the existence of a North-South cline. The populations in the northern part of the archipelagos are genetically similar to the Huilliche tribe, while the groups from the South are most closely related to the Fueguino tribe from the extreme South of Chile, and secondarily to the Pehuenche and Mapuche, who are found to the North and East of Chiloé archipelago. These results are consistent with a colonization of the southern archipelagos from Tierra del Fuego. We evaluate the evolutionary relationships of the population of the Chiloé area to groups from other geographic areas of Chile, using analysis of molecular variance (AMOVA). Three Amerindian clusters are identified: one formed by the Aymará and Atacameño, a second by the Huilliche, and a third including the Mapuche, Pehuenche, and Fueguino tribes, and the population inhabiting the South of the Chiloé arcipelago. These groups exhibit a North-South gradient in the frequency of haplogroup B, confirmed by F(ST) tests.