We present the Stanford Question Answering Dataset (SQuAD), a new reading comprehension dataset consisting of 100,000+ questions posed by crowdworkers on a set of Wikipedia articles, where the answer to each question is a segment of text from the corresponding reading passage. We analyze the dataset to understand the types of reasoning required to answer the questions, leaning heavily on dependency and constituency trees. We build a strong logistic regression model, which achieves an F1 score of 51.0%, a significant improvement over a simple baseline (20%). However, human performance (86.8%) is much higher, indicating that the dataset presents a good challenge problem for future research. The dataset is freely available at https://stanford-qa.com.

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The aim of the UniProt Knowledgebase is to provide users with a comprehensive, high-quality and freely accessible set of protein sequences annotated with functional information. In this article, we describe significant updates that we have made over the last two years to the resource. The number of sequences in UniProtKB has risen to approximately 190 million, despite continued work to reduce sequence redundancy at the proteome level. We have adopted new methods of assessing proteome completeness and quality. We continue to extract detailed annotations from the literature to add to reviewed entries and supplement these in unreviewed entries with annotations provided by automated systems such as the newly implemented Association-Rule-Based Annotator (ARBA). We have developed a credit-based publication submission interface to allow the community to contribute publications and annotations to UniProt entries. We describe how UniProtKB responded to the COVID-19 pandemic through expert curation of relevant entries that were rapidly made available to the research community through a dedicated portal. UniProt resources are available under a CC-BY (4.0) license via the web at https://www.uniprot.org/.

Rechargeable lithium metal batteries are considered the "Holy Grail" of energy storage systems. Unfortunately, uncontrollable dendritic lithium growth inherent in these batteries (upon repeated charge/discharge cycling) has prevented their practical application over the past 40 years. We show a novel mechanism that can fundamentally alter dendrite formation. At low concentrations, selected cations (such as cesium or rubidium ions) exhibit an effective reduction potential below the standard reduction potential of lithium ions. During lithium deposition, these additive cations form a positively charged electrostatic shield around the initial growth tip of the protuberances without reduction and deposition of the additives. This forces further deposition of lithium to adjacent regions of the anode and eliminates dendrite formation in lithium metal batteries. This strategy may also prevent dendrite growth in lithium-ion batteries as well as other metal batteries and transform the surface uniformity of coatings deposited in many general electrodeposition processes.

Exosomes are 40–100 nm nano-sized vesicles that are released from many cell types into the extracellular space. Such vesicles are widely distributed in various body fluids. Recently, mRNAs and microRNAs (miRNAs) have been identified in exosomes, which can be taken up by neighboring or distant cells and subsequently modulate recipient cells. This suggests an active sorting mechanism of exosomal miRNAs, since the miRNA profiles of exosomes may differ from those of the parent cells. Exosomal miRNAs play an important role in disease progression, and can stimulate angiogenesis and facilitate metastasis in cancers. In this review, we will introduce the origin and the trafficking of exosomes between cells, display current research on the sorting mechanism of exosomal miRNAs, and briefly describe how exosomes and their miRNAs function in recipient cells. Finally, we will discuss the potential applications of these miRNA-containing vesicles in clinical settings.

No abstract

Butenes and butadiene, which are useful intermediates for the synthesis of polymers and other compounds, are synthesized traditionally by oxidative dehydrogenation (ODH) of n-butane over complex metal oxides. Such catalysts require high O 2 /butane ratios to maintain the activity, which leads to unwanted product oxidation. We show that carbon nanotubes with modified surface functionality efficiently catalyze the oxidative dehydrogenation of n-butane to butenes, especially butadiene. For low O 2 /butane ratios, a high selectivity to alkenes was achieved for periods as long as 100 hours. This process is mildly catalyzed by ketonic C=O groups and occurs via a combination of parallel and sequential oxidation steps. A small amount of phosphorus greatly improved the selectivity by suppressing the combustion of hydrocarbons.Transition metal oxides have been widely used as catalysts for the conversion of butane to C 4 alkenes, important industrial precursors for producing synthetic rubbers, plastics, and a number of industrially important chemicals. Despite a great deal of research, alkene selectivity in the current butane-to-butadiene process is severely limited (1). One important reason is that the unsaturated products are much more readily oxidized to CO 2 than is the starting alkane. The chemical complexity of polyvalent metal oxides, although found to be necessary for catalytic activity, impedes satisfactory selectivity through isolation of active sites (2-6). For this reason, the origin of the catalytic activity is debated, and there is as yet no generally accepted picture of the reaction mechanism (7, 8).Carbon materials have been reported to catalyze the oxidative dehydrogenation (ODH) of an aromatic molecule, ethylbenzene. However, conventional carbons, in particular activated carbon, underwent unavoidable deactivations due to coking or combustion (9-12). Recently, it was shown that only wellnanostructured carbons are stable and coke-free catalysts for styrene synthesis (12, 13). Activation of C-H bonds in the ethyl group is considered to be coordinated by the ketonic carbonyl (C=O) group. Ethylbenzene has an aromatic moiety that enables relatively facile activation. Here, we report on surfacemodified carbon nanotubes (CNTs) as a high-performance catalyst for the ODH of the much less active butane. Relative to metal-based catalysts, CNTs displayed an enhanced selectivity to C 4 alkenes, especially butadiene.We conducted the reaction at 400° or 450°C with an O 2 /butane ratio of 2.0. The product mixture contained only 1-butene, 2-butene, butadiene, CO 2 , CO, and residual reactants; the resulting carbon balance was 100 ± 3% ( fig. S1A) (14). In a blank experiment without catalyst, the alkene yield was as low as 0.9%. Over pristine CNTs, 88.9% of the converted butane was burnt, yielding 1.6% alkenes (Fig. 1A). Considering the intensive stability of CNTs in O 2 ( fig. S1B) (14), we conclude that the CO 2 during the reaction mainly originated from the oxidation of the hydrocarbon feedstock and not from burning of t...

Fluorescence spectroscopy is widely used in biological research. Until recently, essentially all fluorescence experiments were performed using optical energy which has radiated to the far-field. By far-field we mean at least several wavelengths from the fluorophore, but propagating far-field radiation is usually detected at larger macroscopic distances from the sample. In recent years there has been a growing interest in the interactions of fluorophores with metallic surfaces or particles. Near-field interactions are those occurring within a wavelength distance of an excited fluorophore. The spectral properties of fluorophores can be dramatically altered by near-field interactions with the electron clouds present in metals. These interactions modify the emission in ways not seen in classical fluorescence experiments. In this review we provide an intuitive description of the complex physics of plasmons and near-field interactions. Additionally, we summarize the recent work on metal-fluorophore interactions and suggest how these effects will result in new classes of experimental procedures, novel probes, bioassays and devices.