1,720,959 research outputs found
Functionalization of mesoporous MCM-41 with aminopropyl groups by co-condensation and grafting: a physico-chemical characterization
No full textThis work reports on the synthesis and characterization of NH2-MCM-41, a well-known hybrid material commonly used in biomedical and biotechnological applications, based on mesoporous silica and aminopropyl functionalities. Samples were prepared by post-synthesis grafting and by one-pot co- condensation methods, to achieve a relatively high organic loading (around 12% wt), and were characterized in terms of porosity, thermal stability and distribution of the aminopropyl moieties in the silica framework. The results suggest that grafting brings about an almost complete consumption of surface silanols, with structurally defined functional groups mainly located inside the material pores. In contrast, co-condensation results in lower surface area and thermal stability, with ink-bottle-like pores. This suggests that the aminopropyl groups are not only linked to the pores inner surface but could be located in the pore walls or at their entrance
Pd Nanoparticle Catalyzed Heck Arylation of 1,1-Disubstituted Alkenes in Ionic Liquids. Study on Factors Affecting the Regioselectivity of the Coupling Process
No full textThe Heck reaction of neutral or electron-rich aryl bromides with the 1,1-disubstituted olefins butyl methacrylate and R-methylstyrene catalyzed by Pd nanoparticles in tetrabutylammonium bromide as solvent and tetrabutylammonium acetate as base leads to a prevalent formation of the terminal olefin. In contrast, reaction of p-bromoacetophenone
leads to the internal olefin. Whereas the solvent stabilizes the metal nanoclusters, the base is responsible for a fast neutralization of the PdH impeding the hydride readdition to reaction products and avoiding the olefin interconversion. The terminal olefins are efficiently converted into the more stable internal E isomers by using tetrabutylammonium pivalate as catalyst
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Pd nanoparticles catalyzed Heck arylation of 1,1-disubstituted alkenes in ionic liquids. a study on factors affecting the regioselectivity of the coupling process
No full textDispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Stabilization of quercetin flavonoid in MCM-41 mesoporous silica: positive effect of surface functionalization
Full text linkAntioxidants can prevent UV-induced skin damage mainly by neutralizing free radicals. For this purpose, quercetin (Q) is one of the most employed flavonoids even if the potential usefulness is limited by its unfavorable physicochemical properties. In this context, mesoporous silica (MCM-41) is herein proposed as a novel vehicle able to improve the stability and performance of this phenolic substrate in topical products. Complexes of Q with plain or octyl-functionalized MCM-41 were successfully prepared with different
weight ratios by a kneading method, and then, they were characterized by XRD, gas-volumetric (BET),
TGA, DSC, and FTIR analyses. The performances of the different complexes were evaluated in vitro in
terms of membrane diffusion profiles, storage and photostability, antiradical and chelating activities.
The physicochemical characterization confirmed an important host/guest interaction due to the formation of Si-OH/quercetin hydrogen-bonded adducts further strengthened by octyl functionalization through van der Waals forces. The immobilization of Q, particularly on octyl-functionalized silica, increased the stability without undermining the antioxidant efficacy opening the way for an innovative employment of mesoporous composite materials in the skincare field
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