1,720,957 research outputs found
Carbon nanodots and molecular machines as bottom-up approaches to nanotechnology
No full textThe field of nanotechnology, a broad discipline committed to the design, control and manipulation of matter at the nanoscale, has advanced tremendously in the last decades. The final goal of nanotechnology is the development of novel materials, possessing at least one dimension between 1-100 nm. The controlled manipulation of materials at the nanoscale allows conveying new features, which can be dramatically different from those of their corresponding bulk counterparts. Two strategies can be adopted for the fabrication of nanomaterials. Top-down approach consists in the miniaturization of larger materials into nanomaterials; while bottom-up method relies on controlled reactions of atoms and molecular precursors to assemble complex nanomaterials. Interest in the latter strategy is growing due to the possibility to fabricate tailored nanomaterials with fine-tuned properties from molecularly engineered “building blocks”.
The fil rouge of this thesis is the use of a rational bottom-up approach to build novel structures at the nanoscale, using two different strategies: a traditional bottom-up approach to obtain materials with nanometric dimensions, and a supramolecular-based approach to build molecular systems performing tasks at the nanoscale. Chapter 1 provides, initially, a general overview of nanotechnology and of respective top-down and bottom-up strategy. An emphasis on the bottom-up methods will be given. After, the synthetic bottom-up approaches for the synthesis of carbon nanomaterials, and in particular carbon nanodots, will be described, and some notable examples will be discussed. Lastly, a complementary approach to the fabrication of nanomaterials will be described, based on supramolecular chemistry. A considerable attention will be given to the synthesis and functioning of one of the most common family of systems used in this domain: artificial molecular machines. Through some specific examples, their operation will also be discussed. Chapter 2 presents the synthesis, purification, and characterization of a family of atropoisomeric carbon nanodots, via bottom-up microwave-assisted method. Contrary to other hydrophilic carbon nanodots, these nanoparticles display solubility in organic solvents. Depending on the chirality of the enantiomer employed, these chiral particles show specular profile in the UV-Visible region, as detected by electronic circular dichroism. Remarkably, one class of these carbon nanodots shows circularly polarized luminescence. Contrary to the literature precedents on carbon nanodots, this advanced optical property is observed in solution, without needing any chiral external matrix. As evidenced by morphological and chiroptical experiments, the axial chirality is transferred from molecular to the nanoscale. In this way, this property is directly encoded within nanomaterial structure, without the need for post-functionalization steps. Chapter 3 presents the synthesis and characterization of a symmetric and an asymmetric molecular axle. Both bear a recognition site for a macrocycle, allowing the formation of pseudorotaxane. Its terminal part can be dynamically stoppered, gaining control on the pseudorotaxane formation. The threading/dethreading operation in response to acid-base inputs was studied, confirming machine operation according to a ratchet mechanism, leading to the energetically-demanding trapping of the macrocycle in a high-energy state. The peculiar choice of molecular stopper allowed controlling the dethreading kinetics, ranging from obtaining kinetically-trapped out-of-equilibrium state to rapid equilibration. The solvent in which the operation occurs is fundamental in controlling the dethreading kinetics. Tuning this parameter, the machine can experience either a rapid equilibration or an observable dissipative relaxation, revealing the directional exploration of a square reaction network underlying machine operation, which can be repeated multiple times in situ.The field of nanotechnology, a broad discipline committed to the design, control and manipulation of matter at the nanoscale, has advanced tremendously in the last decades. The final goal of nanotechnology is the development of novel materials, possessing at least one dimension between 1-100 nm. The controlled manipulation of materials at the nanoscale allows conveying new features, which can be dramatically different from those of their corresponding bulk counterparts. Two strategies can be adopted for the fabrication of nanomaterials. Top-down approach consists in the miniaturization of larger materials into nanomaterials; while bottom-up method relies on controlled reactions of atoms and molecular precursors to assemble complex nanomaterials. Interest in the latter strategy is growing due to the possibility to fabricate tailored nanomaterials with fine-tuned properties from molecularly engineered “building blocks”.
The fil rouge of this thesis is the use of a rational bottom-up approach to build novel structures at the nanoscale, using two different strategies: a traditional bottom-up approach to obtain materials with nanometric dimensions, and a supramolecular-based approach to build molecular systems performing tasks at the nanoscale. Chapter 1 provides, initially, a general overview of nanotechnology and of respective top-down and bottom-up strategy. An emphasis on the bottom-up methods will be given. After, the synthetic bottom-up approaches for the synthesis of carbon nanomaterials, and in particular carbon nanodots, will be described, and some notable examples will be discussed. Lastly, a complementary approach to the fabrication of nanomaterials will be described, based on supramolecular chemistry. A considerable attention will be given to the synthesis and functioning of one of the most common family of systems used in this domain: artificial molecular machines. Through some specific examples, their operation will also be discussed. Chapter 2 presents the synthesis, purification, and characterization of a family of atropoisomeric carbon nanodots, via bottom-up microwave-assisted method. Contrary to other hydrophilic carbon nanodots, these nanoparticles display solubility in organic solvents. Depending on the chirality of the enantiomer employed, these chiral particles show specular profile in the UV-Visible region, as detected by electronic circular dichroism. Remarkably, one class of these carbon nanodots shows circularly polarized luminescence. Contrary to the literature precedents on carbon nanodots, this advanced optical property is observed in solution, without needing any chiral external matrix. As evidenced by morphological and chiroptical experiments, the axial chirality is transferred from molecular to the nanoscale. In this way, this property is directly encoded within nanomaterial structure, without the need for post-functionalization steps. Chapter 3 presents the synthesis and characterization of a symmetric and an asymmetric molecular axle. Both bear a recognition site for a macrocycle, allowing the formation of pseudorotaxane. Its terminal part can be dynamically stoppered, gaining control on the pseudorotaxane formation. The threading/dethreading operation in response to acid-base inputs was studied, confirming machine operation according to a ratchet mechanism, leading to the energetically-demanding trapping of the macrocycle in a high-energy state. The peculiar choice of molecular stopper allowed controlling the dethreading kinetics, ranging from obtaining kinetically-trapped out-of-equilibrium state to rapid equilibration. The solvent in which the operation occurs is fundamental in controlling the dethreading kinetics. Tuning this parameter, the machine can experience either a rapid equilibration or an observable dissipative relaxation, revealing the directional exploration of a square reaction network underlying machine operation, which can be repeated multiple times in situ
Transfer of Axial Chirality to the Nanoscale Endows Carbon Nanodots with Circularly Polarized Luminescence
Full text linkWe report the synthesis, purification and characterization of chiral carbon nanodots starting from atropoisomeric precursors. The obtained atropoisomeric carbon nanodots are soluble in organic solvents and have good thermal stability, which are desirable features for technological applications. The synthetic protocol is robust, as it supports a number of variations in terms of molecular doping agents. Remarkably, the combination of axially chiral precursors and 1,4‐benzoquinone as doping agent results in green‐emissive carbon dots displaying circularly polarized luminescence. Dissymmetry factors of |3.5|×10(−4) are obtained in solution, without the need of any additional element of chirality. Introducing axial chirality expands the strategies available to tailor the properties of carbon nanodots, paving the way for carbon nanoparticles that combine good processability in organic solvents with engineered advanced chiroptical properties
Solvent-Free Pd-Catalyzed Heteroaryl–Aryl Coupling via C–H Bond Activation for the Synthesis of Extended Heteroaromatic Conjugated Molecules
Full text linkDirect arylation of thienopyrrolodione,
diketopyrrolopyrroles, benzodithiophene derivatives, and fluorinated
heteroarenes with functionalized aryl iodides is proven in solvent-free
and non-anhydrous conditions. The reaction is performed in the presence
of air and tolerates several functional groups on both the coupling
partners, enabling a convenient synthesis of extended heteroaromatic
conjugated molecules
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
Dispelling 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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