Composite lithium metal anode by melt infusion of lithium into a 3D conducting scaffold with lithiophilic coating

Lithium metal-based battery is considered one of the best energy storage systems due to its high theoretical capacity and lowest anode potential of all. However, dendritic growth and virtually relative infinity volume change during long-term cycling often lead to severe safety hazards and catastrophic failure. Here, a stable lithium–scaffold composite electrode is developed by lithium melt infusion into a 3D porous carbon matrix with “lithiophilic” coating. Lithium is uniformly entrapped on the matrix surface and in the 3D structure. The resulting composite electrode possesses a high conductive surface area and excellent structural stability upon galvanostatic cycling. We showed stable cycling of this composite electrode with small Li plating/stripping overpotential (<90 mV) at a high current density of 3 mA/cm² over 80 cycles.Nowadays the increasing demand for portable electronic devices as well as electric vehicles raises an urgent need for high energy density batteries. Lithium (Li) metal anode has long been regarded as the “Holy Grail” of battery technologies, due to its light weight (0.53 g/cm³) (1), lowest anode potential (−3.04 V vs. the standard hydrogen electrode) (1), and high specific capacity (3,860 mAh/g vs. 372 mAh/g for conventional graphite anode) (1). It possesses an even higher theoretical capacity than the recently intensely researched anodes such as Ge, Sn, and Si (2–10). In addition, the demand for copper current collectors (9 g/cm³) in conventional batteries with graphite anodes can be eliminated by employment of Li metal anodes, hence reducing the total cell weight dramatically. Therefore, Li metal could be a favorable candidate to be used in highly promising, next-generation energy storage systems such as Li−sulfur battery and Li−air battery.The safety hazard associated with Li metal batteries, originating from the uncontrolled dendrite formation, has become a hurdle against the practical realization of Li metal-based batteries (11, 12). The sharp Li filaments can pierce through the separator with increasing cycle time, thus provoking internal short-circuiting (12). Most previous academic research to settle this bottleneck focuses on solid electrolyte interphase (SEI) stabilization/modification by introducing various additives (13–17). These electrolyte additives interact with Li quickly and create a protective layer on the Li metal surface, which helps reinforce the SEI (13–17). Furthermore, recent study in our group has also shown the employment of interconnected hollow carbon spheres (18) and hexagonal boron nitride (19) as mechanically and chemically stable artificial SEI which effectively block Li dendrite growth.In addition to the notorious Li dendrite formation, another significant factor that contributes considerably to the battery short-circuiting is the volume change of Li metal during electrochemical cycling, which is usually overlooked (20, 21). During battery cycling, Li metal is deposited/stripped without a host material. Thus, the whole electrode suffers from a virtually infinite volume change (ratio of Li metal volume at completely charged state versus at the completely discharged state is infinite) compared with the finite volume expansion of several common anodes for lithium ion batteries such as Si (∼400%) (6) and graphite (∼10%) (19). As a result, the mechanical instability induced by the virtually infinite volumetric change would cause a floating electrode/separator interface as well as an internal stress fluctuation (21). However, little attention has been paid to the volume fluctuation problem of the “hostless” Li. We propose that a host scaffold to trap Li metal inside can effectively reduce the volume change of the whole electrode and therefore maintain the electrode surface.Herein, we report a newly designed Li–scaffold composite anode and its effectiveness on addressing the safety issue of traditional hostless Li metal electrode. The preexisting scaffold serves as a rigid host with Li uniformly confined inside to accommodate the electrode-level virtually infinite volume change of Li metal during cycling. To create the composite electrode as we designed in Fig. 1A, we need to find a suitable porous material to host the Li metal. An ideal scaffold for Li encapsulation should have the following attributes: (i) mechanical and chemical stability toward electrochemical cycling; (ii) low gravimetric density to achieve high-energy density of the composite anode; (iii) good electrical and ionic conductivity to provide unblocked electron/ion pathway, enabling fast electron/ion transport; and (iv) relatively large surface area for Li deposition, lowering the effective electrode current density and the possibility of dendrite formation. By considering these aspects, we choose carbon-based porous materials to provide the required features. Specifically, an electrospun carbon fiber network (11) was used as an example to illustrate the capability of this composite anode to sustain the volume fluctuation and shape change during each electrochemical cycle.Open in a separate windowFig. 1.Schematic and optical images of Li encapsulation by melt infusion. (A) Schematic illustration of the design of a Li–scaffold composite. (B) Li wetting property of various porous materials with and without the Si coating. (C) Time-lapse images of Li melt-infusion process for lithiophilic and lithiophobic materials. See the Supporting Information for full movies.How to encapsulate Li metal inside the porous carbon scaffold presents a major challenge. Compared with many of the battery electrode materials which can be fabricated via various synthetic processes, manufacturing of Li metal-based μm- and nanostructures are very difficult due to the high reactivity of Li (1, 12). Previously, studies on Li encapsulation aimed to entrap Li through electrochemical deposition. However, the lack of spatial control of the deposition and unsmooth Li surface due to dendritic Li formation impeded such progress (13, 22). Therefore, development of versatile and simple approaches for encapsulating Li inside porous carbon or other scaffold to create Li-based composite electrodes is highly desired.Li metal possesses a low melting point of 180 °C; it would liquefy into molten Li under anaerobic atmosphere when heated to its melting point. Inspired by the fact that water could be absorbed into a hydrophilic porous structure, we develop a new strategy: melt infusion of molten Li into a “lithiophilic” matrix, which has low contact angle with liquid Li. A porous material with a thin layer of lithiophilic coating has excellent wettability with liquefied Li and thus could function as the host scaffold for Li entrapment. In this study, the aforementioned electrospun carbon fiber network modified with lithiophilic coating–silicon (Si), was used as the scaffold for Li encapsulation. Li easily and quickly flows into the fiber layer region and occupies the empty spaces between each single fiber. The resulting composite structure, denoted as Li/C, remains both mechanically and chemically stable under galvanostatic cycling; moreover, it provides a stable electrode/electrolyte interface. The effective anode current density could also be reduced due to an enlarged surface area for Li nucleation process, which in turn causes superior electrochemical performances under the same test conditions. To summarize, in contrary to the hostless Li metal, the as-proposed Li/C composite anode is able to accommodate the volume variation and therefore mitigate the potential safety hazard; moreover, the reduced current density, rooted to larger surface area, also triggers a greatly improved electrochemical performance, with stable cycling of over 2,000 mAh/g for more than 80 cycles at a high current density of 3 mA/cm².     

Structure-based cleavage mechanism of Thermus thermophilus Argonaute DNA guide strand-mediated DNA target cleavage

We report on crystal structures of ternary Thermus thermophilus Argonaute (TtAgo) complexes with 5′-phosphorylated guide DNA and a series of DNA targets. These ternary complex structures of cleavage-incompatible, cleavage-compatible, and postcleavage states solved at improved resolution up to 2.2 Å have provided molecular insights into the orchestrated positioning of catalytic residues, a pair of Mg²⁺ cations, and the putative water nucleophile positioned for in-line attack on the cleavable phosphate for TtAgo-mediated target cleavage by a RNase H-type mechanism. In addition, these ternary complex structures have provided insights into protein and DNA conformational changes that facilitate transition between cleavage-incompatible and cleavage-compatible states, including the role of a Glu finger in generating a cleavage-competent catalytic Asp-Glu-Asp-Asp tetrad. Following cleavage, the seed segment forms a stable duplex with the complementary segment of the target strand.Argonaute (Ago) proteins, critical components of the RNA-induced silencing complex, play a key role in guide strand-mediated target RNA recognition, cleavage, and product release (reviewed in refs. 1–3). Ago proteins adopt a bilobal scaffold composed of an amino terminal PAZ-containing lobe (N and PAZ domains), a carboxyl-terminal PIWI-containing lobe (Mid and PIWI domains), and connecting linkers L1 and L2. Ago proteins bind guide strands whose 5′-phosphorylated and 3′-hydroxyl ends are anchored within Mid and PAZ pockets, respectively (4–7), with the anchored guide strand then serving as a template for pairing with the target strand (8, 9). The cleavage activity of Ago resides in the RNase H fold adopted by the PIWI domain (10, 11), whereby the enzyme’s Asp-Asp-Asp/His catalytic triad (12–15) initially processes loaded double-stranded siRNAs by cleaving the passenger strand and subsequently processes guide-target RNA duplexes by cleaving the target strand (reviewed in refs. 16–18). Such Mg²⁺ cation-mediated endonucleolytic cleavage of the target RNA strand (19, 20) resulting in 3′-OH and 5′-phosphate ends (21) requires Watson–Crick pairing of the guide and target strands spanning the seed segment (positions 2–2′ to 8–8′) and the cleavage site (10′–11′ step on the target strand) (9). Insights into target RNA recognition and cleavage have emerged from structural (9), chemical (22), and biophysical (23) experiments.Notably, bacterial and archaeal Ago proteins have recently been shown to preferentially bind 5′-phosphoryated guide DNA (14, 15) and use an activated water molecule as the nucleophile (reviewed in ref. 24) to cleave both RNA and DNA target strands (9). Structural studies have been undertaken on bacterial and archaeal Ago proteins in the free state (10, 15) and bound to a 5′-phosphorylated guide DNA strand (4) and added target RNA strand (8, 9). The structural studies of Thermus thermophilus Ago (TtAgo) ternary complexes have provided insights into the nucleation, propagation, and cleavage steps of target RNA silencing in a bacterial system (9). These studies have highlighted the conformational transitions on proceeding from Ago in the free state to the binary complex (4) to the ternary complexes (8, 9) and have emphasized the requirement for a precisely aligned Asp-Asp-Asp triad and a pair of Mg²⁺ cations for cleavage chemistry (9), typical of RNase H fold-mediated enzymes (24, 25). Structural studies have also been extended to binary complexes of both human (5, 6) and yeast (7) Agos bound to 5′-phosphorylated guide RNA strands.Despite these singular advances in the structural biology of RNA silencing, further progress was hampered by the modest resolution (2.8- to 3.0-Å resolution) of TtAgo ternary complexes with guide DNA (4) and added target RNAs (8, 9). This precluded identification of water molecules coordinated with the pair of Mg²⁺ cations, including the key water that acts as a nucleophile and targets the cleavable phosphate between positions 10′-11′ on the target strand. We have now extended our research to TtAgo ternary complexes with guide DNA and target DNA strands, which has permitted us to grow crystals of ternary complexes that diffract to higher (2.2–2.3 Å) resolution in the cleavage-incompatible, cleavage-compatible, and postcleavage steps. These high-resolution structures of TtAgo ternary complexes provide snapshots of distinct key steps in the catalytic cleavage pathway, opening opportunities for experimental probing into DNA target cleavage as a defense mechanism against plasmids and possibly other mobile elements (26, 27).     

Coulomb interaction,phonons, and superconductivity in twisted bilayer graphene

The polarizability of twisted bilayer graphene, due to the combined effect of electron–hole pairs, plasmons, and acoustic phonons, is analyzed. The screened Coulomb interaction allows for the formation of Cooper pairs and superconductivity in a significant range of twist angles and fillings. The tendency toward superconductivity is enhanced by the coupling between longitudinal phonons and electron–hole pairs. Scattering processes involving large momentum transfers, Umklapp processes, play a crucial role in the formation of Cooper pairs. The magnitude of the superconducting gap changes among the different pockets of the Fermi surface.

Twisted bilayer graphene (TBG) shows a complex phase diagram which combines superconducting and insulating phases (1, 2) and resembles strongly correlated materials previously encountered in condensed matter physics (3–6). On the other hand, superconductivity seems more prevalent in TBG (7–11), while in other strongly correlated materials magnetic phases are dominant.The pairing interaction responsible for superconductivity in TBG has been intensively studied. Among other possible pairing mechanisms, the effect of phonons (12–19) (see also ref. 20), the proximity of the chemical potential to a van Hove singularity in the density of states (DOS) (21–25) and excitations of insulating phases (26–28) (see also refs. 29–31), and the role of electronic screening (32–35) have been considered.In the following, we analyze how the screened Coulomb interaction induces pairing in TBG. The calculation is based on the Kohn–Luttinger formalism (36) for the study of anisotropic superconductivity via repulsive interactions. The screening includes electron–hole pairs (37), plasmons (38), and phonons (note that acoustic phonons overlap with the electron–hole continuum in TBG). Our results show that the repulsive Coulomb interaction, screened by plasmons and electron–hole pairs only, leads to anisotropic superconductivity, although with critical temperatures of order T_c ∼ 10⁻³ to 10⁻² K. The inclusion of phonons in the screening function substantially enhances the critical temperature, to T_c ∼ 1 to 10 K.     

Probing the metabolic water contribution to intracellular water using oxygen isotope ratios of PO4

Knowledge of the relative contributions of different water sources to intracellular fluids and body water is important for many fields of study, ranging from animal physiology to paleoclimate. The intracellular fluid environment of cells is challenging to study due to the difficulties of accessing and sampling the contents of intact cells. Previous studies of multicelled organisms, mostly mammals, have estimated body water composition—including metabolic water produced as a byproduct of metabolism—based on indirect measurements of fluids averaged over the whole organism (e.g., blood) combined with modeling calculations. In microbial cells and aquatic organisms, metabolic water is not generally considered to be a significant component of intracellular water, due to the assumed unimpeded diffusion of water across cell membranes. Here we show that the ¹⁸O/¹⁶O ratio of PO₄ in intracellular biomolecules (e.g., DNA) directly reflects the O isotopic composition of intracellular water and thus may serve as a probe allowing direct sampling of the intracellular environment. We present two independent lines of evidence showing a significant contribution of metabolic water to the intracellular water of three environmentally diverse strains of bacteria. Our results indicate that ∼30–40% of O in PO₄ comprising DNA/biomass in early stationary phase cells is derived from metabolic water, which bolsters previous results and also further suggests a constant metabolic water value for cells grown under similar conditions. These results suggest that previous studies assuming identical isotopic compositions for intracellular/extracellular water may need to be reconsidered.Metabolic water, more precisely defined as an isotopically distinct flux of O (and H) produced during metabolism (1), has been studied extensively as an alternative water source contributing to body water in animals, such as desert mammals, insects, and migrating birds (2–6), but does not easily lend itself to direct measurement. In recent years, interest in metabolic water has been extended to its oxygen isotopic composition (¹⁸O/¹⁶O ratio or δ¹⁸O value) and contribution to body water because this information is crucial to the interpretation of biomineral [e.g., carbonate—CaCO₃—and phosphate—Ca₃(PO₄)₂] oxygen isotopic compositions used heavily in paleoclimate/paleohydrological research (7–14). This includes biomineral shells of aquatic marine organisms that are preserved in the geologic record and used to infer Earth’s climate history (15–19).A core assumption of applications of biomineral oxygen isotopic compositions to infer environmental conditions is that the ¹⁸O:¹⁶O ratio is controlled by exchange of oxygen isotopes between oxyanions comprising the biomineral and ambient water in bodily fluids (i.e., body water) (9, 20–24). In multicellular eukaryotic organisms, body water includes all water found in various body compartments (e.g., intravascular/intercellular) and bodily fluids (e.g., blood plasma, urine, breath vapor), including water produced by metabolism, and is averaged over the entire organism (25, 26). Blood is the largest reservoir of body water in mammals. Accordingly, most previous studies have been based on measurements of total body water in blood, in urine, or in breath CO₂ that has exchanged and presumably equilibrated with body water, and, thus, not on direct measurement of intracellular water.A strong linear relationship has been observed between δ¹⁸O values of body water determined from mammal blood and δ¹⁸O values of extracellular (ingested) water, which is equivalent to local meteoric water (1, 27–30). A contribution of metabolic water to body water can be detected using this linear relation based on deviation of the slope from a value of 1 (7, 8, 28, 30–32). Based on slopes and modeling calculations, the percentage of metabolic water has been determined to vary between 7% and 56% among different mammal species (7, 8, 30–34).In contrast to macroorganisms and mammals, microorganisms are primarily unicellular (e.g., bacteria and archaea), and, thus, body water is equivalent to intracellular water, which has traditionally been assumed to be identical to water in the surrounding extracellular medium (35), without consideration of a metabolic water component.Here we present two independent lines of evidence for a significant contribution of metabolic water to the intracellular water pool of bacterial cells, based on a new approach for detecting the contribution of metabolic water by direct sampling of the intracellular environment through measurement of the ¹⁸O:¹⁶O ratio of PO₄ (i.e., δ¹⁸O_PO4) in intracellular molecules (e.g., DNA) and also PO₄ in total biomass. Our findings are consistent with recent results of Kreuzer-Martin et al. (27, 36) indicating a metabolic water contribution to the intracellular water of Escherichia coli cells of as much as 70% during the log phase of growth and up to 27% for cells in the stationary phase. Knowledge of the amount and also the isotopic composition of a metabolic water component of body/intracellular water is important to studies of cell physiology and metabolism in which the ¹⁸O:¹⁶O ratio of both water and oxyanions such as CO₂/CO₃ and PO₄ are used to track metabolic processes and reaction pathways.     

Linking fecal bacteria in rivers to landscape,geochemical, and hydrologic factors and sources at the basin scale

Linking fecal indicator bacteria concentrations in large mixed-use watersheds back to diffuse human sources, such as septic systems, has met limited success. In this study, 64 rivers that drain 84% of Michigan’s Lower Peninsula were sampled under baseflow conditions for Escherichia coli, Bacteroides thetaiotaomicron (a human source-tracking marker), landscape characteristics, and geochemical and hydrologic variables. E. coli and B. thetaiotaomicron were routinely detected in sampled rivers and an E. coli reference level was defined (1.4 log₁₀ most probable number⋅100 mL⁻¹). Using classification and regression tree analysis and demographic estimates of wastewater treatments per watershed, septic systems seem to be the primary driver of fecal bacteria levels. In particular, watersheds with more than 1,621 septic systems exhibited significantly higher concentrations of B. thetaiotaomicron. This information is vital for evaluating water quality and health implications, determining the impacts of septic systems on watersheds, and improving management decisions for locating, constructing, and maintaining on-site wastewater treatment systems.Water quality degradation influenced by diffuse sources at large watershed scales has been difficult to describe. Human modifications of natural landscapes can permanently alter hydrologic cycles and affect water quality (1, 2). Deforestation (3) and increased impervious surface area (4) have been linked with decreased infiltration and thus increased surface runoff. Overland flows concentrate pollutants and rapidly transport them down gradient where they eventually enter surface water systems and affect water quality (5, 6). A number of models have been developed to calculate overland and surface water flows (7, 8) and nutrient/chemical transport (9), but few studies have focused on microbial movement from land to water, particularly nontraditional fecal indicator bacteria that can be used to track human sources of pollution.Microbial contamination poses one of the greatest health risks to swimming areas, drinking water intakes, and fishing/shellfish harvesting zones where human exposures are highest (10–12). These highly visible areas often receive more attention than sources of contamination because identifying the origin of pollution in complex watersheds requires costly comprehensive investigation of environmental and hydrologic conditions across temporal and spatial scales (13). Grayson et al. (14) suggest using a “snapshot” approach that captures water quality characteristics at a single point in time across broad areas to provide information frequently missed during routine monitoring. Compared with long-term comprehensive investigations, the snapshot approach reduces the number of samples, cost, and personnel required to examine pollution sources.Escherichia coli concentrations are commonly used to describe the relative human health risk during water quality monitoring in lieu of pathogen detection. Studies attempting to trace pollution in water back to a specific land use with E. coli have rarely produced definitive conclusions (15, 16). Using molecular approaches, specific source targets can be isolated in complex systems and have recently been used to investigate land use and water quality impairments (17). Furtula et al. (18) demonstrated ruminant, pig, and dog fecal contamination in an agriculturally dominated watershed (Canada) using Bacteroides markers. The Bacteroides thetaiotaomicron α-1–6 mannanase (B. theta) gene has a high human specificity (19–22), but no studies to date have linked its presence to land use patterns.Reference conditions have been established for minimally disturbed environments based on measurements of macroinvertebrates, fish, and diatoms (23–25), but microbial reference conditions have not been adequately explored or defined. Based on 15 unimpaired California streams, microbial reference conditions for E. coli [1.0 log₁₀ most probably number (MPN)⋅100 mL⁻¹] and enterococci (1.2 log₁₀ MPN⋅100 mL⁻¹) were defined as being below state water quality thresholds (26). In the Great Lakes, a human health threshold of 2.37 log₁₀ E. coli MPN⋅100 mL⁻¹ (27), or a level equally protective of human health, has been adopted by all state governments. However, this health-associated reference level was derived from epidemiological studies undertaken at beaches throughout the United States (28, 29) with limited knowledge of local implications.In response to water quality degradation from human stressors and the poorly understood microbial conditions in large-scale fresh water systems such as the Great Lakes basin, this paper aims to (i) examine the spatial distribution of E. coli and a human specific source marker (B. theta) in 64 river systems that drain most of the state’s Lower Peninsula under baseflow conditions, (ii) identify baseflow reference levels of fecal contamination in rivers, and (iii) determine how key chemical, physical, environmental, hydrologic, and land use variables are linked to river water quality at large scales.     

Physical and virtual water transfers for regional water stress alleviation in China

Water can be redistributed through, in physical terms, water transfer projects and virtually, embodied water for the production of traded products. Here, we explore whether such water redistributions can help mitigate water stress in China. This study, for the first time to our knowledge, both compiles a full inventory for physical water transfers at a provincial level and maps virtual water flows between Chinese provinces in 2007 and 2030. Our results show that, at the national level, physical water flows because of the major water transfer projects amounted to 4.5% of national water supply, whereas virtual water flows accounted for 35% (varies between 11% and 65% at the provincial level) in 2007. Furthermore, our analysis shows that both physical and virtual water flows do not play a major role in mitigating water stress in the water-receiving regions but exacerbate water stress for the water-exporting regions of China. Future water stress in the main water-exporting provinces is likely to increase further based on our analysis of the historical trajectory of the major governing socioeconomic and technical factors and the full implementation of policy initiatives relating to water use and economic development. Improving water use efficiency is key to mitigating water stress, but the efficiency gains will be largely offset by the water demand increase caused by continued economic development. We conclude that much greater attention needs to be paid to water demand management rather than the current focus on supply-oriented management.The geographical mismatch between freshwater demand and available freshwater resources is one of the largest threats to sustainable water supply in China (1) and throughout the world. It is well-known that China has a temperate south and an arid north (2). The North China Plain shows the greatest water scarcity, with per capita water availability under 150 m³/y (3–5). At the same time, this area is home to 200 million people and provides more than one-half of China’s wheat and one-third of its maize (6). Recognizing such a mismatch, China has been developing over 20 major physical water transfer projects with a total length of over 7,200 km (6), including the world’s largest—the South–North Water Transfer Project (SNWTP) (7). Three routes are projected in the SNWTP, which will ultimately transfer 44.8 Gm³ water from the Yangtze River Basin to the Huang-Huai-Hai River Basin annually, of which 14.8 Gm³ is for the East Route, 13 Gm³ is for the Middle Route, and 17 Gm³ is for the West Route (7). After completion of the three routes, the transferred water is projected to amount to 30.5% of total water withdrawal in the Huang-Huai-Hai River Basin in 2012 (the latest available statistic) (8).Apart from these major physical water transfer projects, there is another solution to remedy regional water scarcity—so-called virtual water (9–11). The virtual water concept, first introduced by Allan (12), is the water required for the production of goods and services along their supply chains (13). Based on this concept, water-scarce regions import water-intensive products instead of producing them locally, thus conserving local water resources (12, 14). Because the SNWTP has proved highly controversial in its potential impacts on both exporting and importing river ecosystems and its huge capital cost (∼$60 US billion), scholars have suggested that the North China Plain should, instead, reduce the export of water-intensive products or even import virtual water from southern China (11, 13, 15–17). An important question is if such redistributions can be effective in mitigating regional water stress in China.To answer this question, we report here on our quantification of China’s physical and virtual water flows at the provincial level for the year 2007. We have used the most recent interregional trade data and evaluated the associated impacts on water stress. To calculate virtual water flows, we have calculated water use throughout the entire supply chain in China. The study focused on 30 provincial-level administrative regions (provinces, autonomous regions, and municipalities—for simplicity, they are referred to as provinces) (names are shown in SI Appendix, Fig. S1) in mainland China where data were available. The volume of physical water transfer for each province was acquired through the Water Resources Bulletin of the studied provinces (4). To study virtual water flows, we incorporated the direct water use of 30 economic sectors of each province into an environmental extended multiregion input–output (MRIO) model (18, 19) (Methods). An MRIO model distinguishes production structure, technology, and consumption for each study area and shows flows of goods and services between and within regions; thus, it is ideally suited for measuring interregional virtual water flows (20, 21). The virtual water trade generated by final consumption was evaluated using the emissions embodied in trade method (22). Water stress was evaluated using the water stress index (WSI) (10, 23, 24). Moderate, severe, and extreme water stresses occur when the ratio of the annual freshwater withdrawal to the renewable freshwater resource is 20–40%, 40–100%, and over 100%, respectively.     

Highly permeable artificial water channels that can self-assemble into two-dimensional arrays

Bioinspired artificial water channels aim to combine the high permeability and selectivity of biological aquaporin (AQP) water channels with chemical stability. Here, we carefully characterized a class of artificial water channels, peptide-appended pillar[5]arenes (PAPs). The average single-channel osmotic water permeability for PAPs is 1.0(±0.3) × 10⁻¹⁴ cm³/s or 3.5(±1.0) × 10⁸ water molecules per s, which is in the range of AQPs (3.4∼40.3 × 10⁸ water molecules per s) and their current synthetic analogs, carbon nanotubes (CNTs, 9.0 × 10⁸ water molecules per s). This permeability is an order of magnitude higher than first-generation artificial water channels (20 to ∼10⁷ water molecules per s). Furthermore, within lipid bilayers, PAP channels can self-assemble into 2D arrays. Relevant to permeable membrane design, the pore density of PAP channel arrays (∼2.6 × 10⁵ pores per μm²) is two orders of magnitude higher than that of CNT membranes (0.1∼2.5 × 10³ pores per μm²). PAP channels thus combine the advantages of biological channels and CNTs and improve upon them through their relatively simple synthesis, chemical stability, and propensity to form arrays.The discovery of the high water and gas permeability of aquaporins (AQPs) and the development of artificial analogs, carbon nanotubes (CNTs), have led to an explosion in studies aimed at incorporating such channels into materials and devices for applications that use their unique transport properties (1–9). Areas of application include liquid and gas separations (10–13), drug delivery and screening (14), DNA recognition (15), and sensors (16). CNTs are promising channels because they conduct water and gas three to four orders of magnitude faster than predicted by conventional Hagen–Poiseuille flow theory (11). However, their use in large-scale applications has been hampered by difficulties in producing CNTs with subnanometer pore diameters and fabricating membranes in which the CNTs are vertically aligned (4). AQPs also efficiently conduct water across membranes (∼3 billion molecules per second) (17) and are therefore being studied intensively for their use in biomimetic membranes for water purification and other applications (1, 2, 18). The large-scale applications of AQPs is complicated by the high cost of membrane protein production, their low stability, and challenges in membrane fabrication (1).Artificial water channels, bioinspired analogs of AQPs created using synthetic chemistry (19), ideally have a structure that forms a water-permeable channel in the center and an outer surface that is compatible with a lipid membrane environment (1). Interest in artificial water channels has grown in recent years, following decades of research and focus on synthetic ion channels (19). However, two fundamental questions remain: (i) Can artificial channels approach the permeability and selectivity of AQP water channels? (ii) How can such artificial channels be packaged into materials with morphologies suitable for engineering applications?Because of the challenges in accurately replicating the functional elements of channel proteins, the water permeability and selectivity of first-generation artificial water channels were far below those of AQPs (SI Appendix, Table S1) (20–25). In some cases, the conduction rate for water was much lower than that of AQPs as a result of excess hydrogen bonds being formed between the water molecules and oxygen atoms lining the channel (20). The low water permeability that was measured for first-generation water channels also highlights the experimental challenge of accurately characterizing water flow through low-permeability water channels. Traditionally, a liposome-based technique has been used to measure water conduction, in which the response to an osmotic gradient is followed by measuring changes in light scattering (26, 27) or fluorescence (28). The measured rates are then converted to permeability values. These measurements suffer from a high background signal due to water diffusion through the lipid bilayer, which, in some cases, can be higher than water conduction through the inserted channels, making it challenging to resolve the permeability contributed by the channels (29). Thus, there is a critical need for a method to accurately measure single-channel permeability of artificial water channels to allow for accurate comparison with those of biological water channels. Furthermore, first-generation artificial water channels were designed with a focus on demonstrating water conduction and one-dimensional assembly into tubular structures (20–24), but how the channels could be assembled into materials suitable for use in engineering applications was not explored. To derive the most advantage from their fast and selective transport properties, artificial water channels are ideally vertically aligned and densely packed in a flat membrane. These features have been long desired but remain a challenge for CNT-based systems (4).Here we introduce peptide-appended pillar[5]arene (PAP; Fig. 1) (30) as an excellent architecture for artificial water channels, and we present data for their single-channel permeability and self-assembly properties. Nonpeptide pillar[5]arene derivatives were among first-generation artificial water channels (1, 23). Pillar[5]arene derivatives, including the one used in this study, have a pore of ∼5 Å in diameter and are excellent templates for functionalization into tubular structures (31–34). However, the permeability of hydrazide-appended pillar[5]arene channels was low (∼6 orders of magnitude lower than that of AQPs; SI Appendix, Table S1). We addressed the challenges of accurately measuring single-channel water permeability and improving the water conduction rate over first-generation artificial water channels by using both experimental and simulation approaches. The presented PAP channel contains more hydrophobic regions (30) compared with its predecessor channel (23), which improves both its water permeability and its ability to insert into membranes. To determine single-channel permeability of PAPs, we combined stopped-flow light-scattering measurements of lipid vesicles containing PAPs with fluorescence correlation spectroscopy (FCS) (35, 36). Stopped-flow experiments allow the kinetics of vesicle swelling or shrinking to be followed with millisecond resolution and water permeability to be calculated, whereas FCS makes it possible to count the number of channels per vesicle (36, 37). The combination of the two techniques allows molecular characterization of channel properties with high resolution and demonstrates that PAP channels have a water permeability close to those of AQPs and CNTs. The experimental results were corroborated by molecular dynamics (MD) simulations, which also provided additional insights into orientation and aggregation of the channels in lipid membranes. Finally, as a first step toward engineering applications such as liquid and gas separations, we were able to assemble PAP channels into highly packed planar membranes, and we experimentally confirmed that the channels form 2D arrays in these membranes.Open in a separate windowFig. 1.Structure of the peptide-appended pillar[5]arene (PAP) channel. (A) The PAP channel (C₃₂₅H₃₂₀N₃₀O₆₀) forms a pentameric tubular structure through intramolecular hydrogen bonding between adjacent alternating d-l-d phenylalanine chains (d-Phe-l-Phe-d-Phe-COOH). (B) Molecular modeling (Gaussian09, semiempirical, PM6) of the PAP channel shows that the benzyl rings of the phenylalanine side chains extend outward from the channel walls (C, purple; H, white; O, red; N, blue). (C and D) MD simulation of the PAP channel in a POPC bilayer revealed its interactions with the surrounding lipids. The five chain-like units of the channel are colored purple, blue, ochre, green, and violet, with hydrogen atoms omitted. In C, the POPC lipids are represented by thin tan lines; in D, water is shown as red (oxygen) and white (hydrogen) van der Waals spheres.     

Physical evidence of predatory behavior in Tyrannosaurus rex

Feeding strategies of the large theropod, Tyrannosaurus rex, either as a predator or a scavenger, have been a topic of debate previously compromised by lack of definitive physical evidence. Tooth drag and bone puncture marks have been documented on suggested prey items, but are often difficult to attribute to a specific theropod. Further, postmortem damage cannot be distinguished from intravital occurrences, unless evidence of healing is present. Here we report definitive evidence of predation by T. rex: a tooth crown embedded in a hadrosaurid caudal centrum, surrounded by healed bone growth. This indicates that the prey escaped and lived for some time after the injury, providing direct evidence of predatory behavior by T. rex. The two traumatically fused hadrosaur vertebrae partially enclosing a T. rex tooth were discovered in the Hell Creek Formation of South Dakota.One of the most daunting tasks of paleontology is inferring the behavior and feeding habits of extinct organisms. Accurate reconstruction of the lifestyle of extinct animals is dependent on the fossil evidence and its interpretation is most confidently predicated on analogy with modern counterparts (1–6). This challenge to understanding the lifestyle of extinct animals is exemplified by the controversy over the feeding behavior of the Late Cretaceous theropod Tyrannosaurus rex (3, 7–17). Although predation and scavenging have often been suggested as distinct feeding behavior alternatives (3, 7–9, 11–17), these terms merit semantic clarification. In this study, predation is considered a subset of feeding behavior, by which any species kills what it eats. Although the term “predator” is used to distinguish such animals from obligate scavengers, it does not imply that the animal did not also scavenge.Ancient diets can be readily reconstructed on the basis of the available evidence, although their derivation (e.g., predation or scavenging behavior) often remains elusive. Speculation as to dinosaur predation has ranged from inferences based on skeletal morphology, ichnofossils such as bite marks, coprolites, stomach contents, and trackways and, by more rarely, direct predator–prey skeletal associations (3, 4, 18–23).Direct evidence of predation in nonavian dinosaurs other than tyrannosaurids has been observed in rare instances, such as the Deinonychus–Tenontosaurus kill site of the Cloverly Formation where the remains of both were found in close association along with shed teeth (9, 24), and the “fighting dinosaurs” from the Gobi Desert, in which a Velociraptor and Protoceratops were found locked in mortal combat (9, 17). The evidence on tyrannosaurids is more limited. Putative stomach contents, such as partially digested juvenile hadrosaur bones, have been reported in association with tyrannosaurid remains (3, 12, 18). This latter instance only represents physical evidence of the last items consumed before the animal’s death, an indicator of diet but not behavior.Mass death assemblages of ornithischians frequently preserve shed theropod teeth (6, 22, 24). Lockley et al. (23) suggest such shed teeth are evidence of scavenging behavior. It is widely argued that T. rex procured food through obligate scavenging rather than hunting (11, 14, 25–27) despite the fact that there is currently no modern analog for such a large bodied obligate scavenger (26). Horner (25) argued that T. rex was too slow to pursue and capture prey items (14) and that large theropods procured food solely through scavenging, rather than hunting (11, 25). Horner also suggested that the enlarged olfactory lobes in T.rex were characteristic of scavengers (25). More recent studies (28, 29) determined the olfactory lobes of modern birds are “poorly developed,” inferring that enlarged olfactory lobes in T. rex are actually a secondary adaptation for predation navigation “to track mobile, dispersed prey” (30). T. rex has a calculated bite force stronger than that of any other terrestrial predator (7), between 35,000 and 57,000 Newtons (30, 31), and possible ambulatory speeds between 20 and 40 kph (7, 15, 16), documenting that it had the capability to pursue and kill prey items.Healed injuries on potential prey animals provide the most unequivocal evidence of survival of a traumatic event (e.g., predation attempt) (3, 32, 33), and several reports attribute such damage to T. rex (4, 17, 19, 20). These include broken and healed proximal caudal vertebral dorsal spines in Edmontosaurus (17) and healed cranial lesions in Triceratops (4, 19). Although the presence of healed injuries demonstrates that an animal lived long enough after the attack to create new bone at the site of the damage (a rare occurrence in the fossil record) (19), the healing usually obliterates any clear signature linking the injury to a specific predator. Bite traces (e.g., raking tooth marks on bone and puncture wounds in the bones of possible prey animals) attributed to T. rex (2, 4, 19) are ambiguous, because the damage inflicted upon an animal during and after a successful hunt mirrors feeding during scavenging. This makes distinction between the two modes of food acquisition virtually impossible with such evidence (3, 34–38).Tooth marks, reported from dinosaur bone-bearing strata worldwide (e.g., 2–4, 8, 19, 20, 39, 40), are further direct evidence of theropod feeding behavior, attributed by some to specific theropod groups (2, 4, 19, 20). Happ (19) and Carpenter (17) identified theropods to family and genus by matching spaces to parallel marks (traces) with intertooth distance. Happ (19) described opposing conical depressions on a left supraorbital Triceratops horn that was missing its distal third (tip), attributing them to a bite by either a T. rex or a crocodilian. Happ (19) stated that the spacing of the parallel marks present on the left squamosal of the same individual matched the intertooth distance of tyrannosaurids. The presence of periosteal reaction documents healing. This contrasts with the report by Farlow and Holtz (3) and again by Hone and Rauhut (20) of the same Hypacrosaurus fibula containing a superficially embedded theropod tooth. Absence of bone reaction precludes confident attribution to predation.Two coalesced hadrosaur (compare with Edmontosaurus annectens) caudal vertebrae were discovered in the Hell Creek Formation of Harding County, South Dakota (40). Archosaur fauna identified in this site include crocodiles, dinosaurs, and birds (41). Physical evidence of dental penetration and extensive infection (osteomylitis) of the fused vertebral centra and healing (bone overgrowth) document an unsuccessful attack by a large predator. A tooth crown was discovered within the wound, permitting identification of the predator as T. rex. This is unambiguous evidence that T. rex was an active predator, fulfilling the criteria that Farlow and Holtz (3) advanced. As T. rex comprises between 1% and 16% of the Upper Cretaceous dinosaurian fauna in Western North America (41–45), its status as a predator or obligate scavenger is nontrivial and could have significant implications for paleoecological reconstructions of that time period. The present contribution provides unique information demonstrating the ecological role for T. rex as that of an active predator. Despite this documentation of predatory behavior by T. rex, we do not make the argument that T. rex was an obligate predator. Like most modern large predators (27, 45) it almost certainly did also scavenge carcasses (9, 16).     

Tryptophan-to-heme electron transfer in ferrous myoglobins

It was recently demonstrated that in ferric myoglobins (Mb) the fluorescence quenching of the photoexcited tryptophan 14 (*Trp¹⁴) residue is in part due to an electron transfer to the heme porphyrin (porph), turning it to the ferrous state. However, the invariance of *Trp decay times in ferric and ferrous Mbs raises the question as to whether electron transfer may also be operative in the latter. Using UV pump/visible probe transient absorption, we show that this is indeed the case for deoxy-Mb. We observe that the reduction generates (with a yield of about 30%) a low-valence Fe–porphyrin π [Fe^II(porph^●−)] -anion radical, which we observe for the first time to our knowledge under physiological conditions. We suggest that the pathway for the electron transfer proceeds via the leucine 69 (Leu⁶⁹) and valine 68 (Val⁶⁸) residues. The results on ferric Mbs and the present ones highlight the generality of Trp–porphyrin electron transfer in heme proteins.Electron transfer plays a fundamental role in many biological systems (1–3) ranging from photosynthetic proteins (4) to iron–sulfur (5), copper (6), and heme (7, 8) proteins. It was demonstrated that electron transfer can be used to produce from heme proteins in situ drugs with antimalarial activity (9) and it might have a role in protein folding (2). In general, electron transfer in proteins can occur over long distances (>10 Å) by hopping through different residues, thus reducing the time that would be needed for a single step tunneling from the donor to the acceptor (10–12). Aromatic amino acids and Tryptophan (Trp) in particular can act as a relay in such processes (13–19). Trp also acts as a phototriggered electron donor, e.g., in DNA repair by photolyase (16–18) and in cryptochromes (20, 21). When no obvious electron acceptors are present, excited Trp or (*Trp) still displays shorter lifetimes than its nanosecond decay times in solution (22, 23). This is due to its strong tendency to act as an electron donor, undergoing electron transfer toward the protein’s backbone as in the case of apo-myoglobin mutants (24), small cyclic peptides (25), and human γ–d-crystallin (26). It is interesting to note that in wild-type horse heart (WT-HH) apo-myoglobin the fluorescence lifetime of the two *Trp residues was reported to be comparable to that in water (27), demonstrating the absence of deactivation mechanisms, either by energy or by electron transfer.The protein visible absorption spectrum is dominated by their cofactors, e.g., heme or flavins, whereas the UV absorption in the region between 250 nm and 300 nm is mainly due to the three aromatic amino acids, Trp, tyrosine (Tyr), and phenylalanine (Phe) (28), with Trp having the highest molar extinction coefficient. The high sensitivity of Trp to the local environment and the possibility to correlate it with its fluorescence response (28) have led to its widespread use as a local natural probe of protein structure and dynamics in time-resolved fluorescence resonance energy transfer (FRET) studies, and it has emerged as the “spectroscopic ruler” in such studies (28–30). FRET is mediated by dipole–dipole coupling between a donor *Trp and an acceptor molecule, and its rate is inversely proportional to the sixth power of the distance between them and to the relative orientation of their dipoles.Myoglobin (Mb) is a small heme protein composed of ∼150 residues (31) arranged in eight α-helices (from A to H) (SI Appendix, Fig. S7), whose biological function is to store molecular oxygen in muscles of vertebrates (32). This is accomplished by its prosthetic group: a Fe–Protoporphyrin IX complex bound to the protein structure via the proximal histidine (His⁹³) (SI Appendix, Fig. S7). Both ferric and ferrous hemes tend to bind small diatomic molecules (e.g., O₂, CO, NO, and CN) at the Fe site. Mb has two Trp residues that are situated in the α-helix A: Trp⁷ toward the solvent and Trp¹⁴ within the protein and closer to the heme (SI Appendix, Fig. S7) (33). Previous time-resolved fluorescence studies on various Mb complexes have reported decay times (SI Appendix, Table S1) of ∼120 ps and ∼20 ps, for *Trp⁷ and *Trp¹⁴, respectively (34–38). These decay times appear invariant with respect to the ligand and the oxidation state of the iron ion in the heme. They were attributed to *Trp-to-porphyrin energy transfer via FRET over different donor–acceptor distances (37, 38) [the Trp⁷-Heme and Trp¹⁴-Heme center-to-center distances are 21.2 Å and 15.1 Å, respectively (33, 39) (SI Appendix, Fig. S7)]. We recently showed, using ultrafast 2D-UV and visible transient absorption (TA) spectroscopy, that in the ferric myoglobins (MbCN and MbH₂O) the relaxation pathway of *Trp¹⁴ involves not only a *Trp-to-heme FRET but also an electron transfer from the *Trp to the heme (40) in a ratio of approximately 60–40%. One can expect that due to its ferric character, the heme is a strong electron acceptor in these cases, and indeed our study showed the formation of an Fe^II heme.However, the invariance of *Trp decay times in ferric and ferrous Mbs (SI Appendix, Table S1) suggests that similar electron transfer processes may also occur in ferrous Mbs. In this event, questions arise as to (i) whether a formally Fe^I heme is formed, which has to date been observed only in cryo-radiolysis experiments (41, 42), or (ii) whether the electron localizes on the porphyrin ring or even on the ligand that binds to the Fe ion. Theoretical investigations have suggested that an iron porphyrin anion radical can be formed (43–45).To address these questions, here we present a UV-pump/visible-probe TA study of ferrous Mbs. In the latter case with apical diatomic ligands, e.g., MbNO and MbCO, heme photoexcitation leads to dissociation of the ligand, followed by its recombination to the heme, which can be both geminate (the ligand stays inside the protein scaffold) and nongeminate (the ligand migrates out of the protein scaffold) (46–48). For the NO ligand, recombination timescales are typically ∼10 ps, ∼30 ps, and ∼200 ps (46, 47), whereas for CO they span up to the millisecond range (46, 49–51). The presence of recombination timescales in the order of *Trp decay times leads to additional signal contributions, which complicate the analysis of the data. These problems are avoided using deoxy-Mb, which has a penta-coordinated heme bound only to the His⁹³. Upon heme photoexcitation, the system recovers to the ground state within a few picoseconds (46, 52). This allows investigating the *Trp–heme interaction without any overlapping contributions.We show here that just as in the ferric Mbs (40), also in deoxy-Mb does *Trp¹⁴ partly decay to the heme by electron transfer, competing with the FRET pathway. We find that the transferred electron is localized on the porphyrin ring, contrary to the ferric case where it resides on the metal center. This is due to the highly negative reduction potential of the Fe^II/Fe^I couple (53, 54), which is close to the porphyrin reduction potential (55). To our knowledge, this is the first report of a low-valent myoglobin, under physiological conditions.The experimental setup, the sample preparation, and the data analysis are described in SI Appendix.     

Viscosity of deeply supercooled water and its coupling to molecular diffusion

The viscosity of a liquid measures its resistance to flow, with consequences for hydraulic machinery, locomotion of microorganisms, and flow of blood in vessels and sap in trees. Viscosity increases dramatically upon cooling, until dynamical arrest when a glassy state is reached. Water is a notoriously poor glassformer, and the supercooled liquid crystallizes easily, making the measurement of its viscosity a challenging task. Here we report viscosity of water supercooled close to the limit of homogeneous crystallization. Our values contradict earlier data. A single power law reproduces the 50-fold variation of viscosity up to the boiling point. Our results allow us to test the Stokes–Einstein and Stokes–Einstein–Debye relations that link viscosity, a macroscopic property, to the molecular translational and rotational diffusion, respectively. In molecular glassformers or liquid metals, the violation of the Stokes–Einstein relation signals the onset of spatially heterogeneous dynamics and collective motions. Although the viscosity of water strongly decouples from translational motion, a scaling with rotational motion remains, similar to canonical glassformers.Water, considered as a potential glassformer, has been a long-lasting topic of intense activity. Its possible liquid–glass transition was reported 50 years ago to be in the vicinity of 140?K (1, 2). However, ice nucleation hinders the access to this transition from the liquid side. Bypassing crystallization requires hyperquenching the liquid at tremendous cooling rates, ca. 10⁷?K ? s^?1 (3). As a consequence, many questions about supercooled and glassy water and its glass–liquid transition remain open (4–7).As an example, crystallization of water is accompanied by one of the largest known relative changes in sound velocity, which has been attributed to the relaxation effects of the hydrogen bond network (8, 9). Indeed, whereas the sound velocity is around 1,400 m⋅s−1 in liquid water at 273?K, it reaches around 3,300 m⋅s−1 in ice at 273?K and a similar value in the known amorphous phases of ice at 80?K (10). Such a large jump is usually the signature of a strong glass, i.e., one in which relaxation times or viscosity follow an Arrhenius law upon cooling. However, pioneering measurements on bulk supercooled water by NMR (11) and quasi-elastic neutron scattering (12), as well as recent ones by optical Kerr effect (8, 9), reveal a large super-Arrhenius behavior between 340 and 240?K, similar to what is observed in fragile glassformers (13, 14). The temperature dependence of the relaxation time is well described by a power law (8, 9), as expected from mode-coupling theory (15, 16), which usually applies well to liquids with a small change of sound velocity upon vitrification. Based on these and other observations, it has been hypothesized that supercooled water experiences a fragile-to-strong transition (17). This idea has motivated experimental efforts to measure dynamic properties of supercooled water and has received some indirect support from experiments on nanoconfined water (18–20) and from simulations (21, 22).In usual glassformers, many studies have focused on the coupling or decoupling between the following dynamic quantities: viscosity (η) and self or tracer diffusion coefficients for translation (D_t) and rotation (D_r). If objects as small as molecules were to follow macroscopic hydrodynamics, one would expect that the preceding quantities would be related through the Stokes–Einstein (SE), D_t ∝ T/η, and Stokes–Einstein–Debye (SED), D_r ∝ T/η, relations, where T is the temperature. These relations are indeed obeyed by many liquids at sufficiently high temperature. However, they might break down at low temperature. Pioneering experiments were performed by the groups of Sillescu (23–25) and Ediger (26–28) where a series of molecular glassformers were investigated. SE relation is obeyed at sufficiently high temperature but violated around 1.3T_g, where T_g is the glass transition temperature, thus indicating decoupling between translational diffusion and viscosity. In contrast, it was observed for ortho-terphenyl (23, 24, 26) that rotational diffusion and viscosity remain strongly coupled (i.e., obey the SED relation) even very close to T_g. A corollary is that translational and rotational diffusion decouple from each other at low temperature. These observations imply that deeply supercooled liquids exhibit spatially heterogeneous dynamics (29–31). Dynamic heterogeneities have been confirmed by direct observations of several single fluorescent molecules immersed in ortho-terphenyl (32) or nanorods immersed in glycerol (33). Physically different systems also show analogous behavior. Colloids near the colloidal glass transition violate SE but obey SED (34). In the metallic alloy Zr₆₄Ni₃₆, SE relation is even violated without supercooling, more than 35% above the liquidus temperature (35). This has also been related to the emergence of dynamic heterogeneities (36).For water, SE already breaks down at ambient temperature, which corresponds to around 2.1?T_g (T_g ? 136?K). Molecular dynamics simulations (37–39) have proposed that this occurs concurrently to dynamic heterogeneities caused by a putative liquid–liquid critical point. However, SE and SED also fail by application of high pressure at 400?K (40) where no liquid–liquid transition is expected. To gain more insight, the test of SE and SED in supercooled water deserves further investigation. Translational self-diffusion coefficient D_t (41) and rotational correlation time τ_r (assumed to scale as 1/D_r) (42) have thus been measured down to the homogeneous crystallization temperature (238?K) at ambient pressure. Their comparison reveals a decoupling between rotation and translation that increases with supercooling (42), similar to glassformers. However, viscosity data are needed for a direct test of SE and SED relations. Quite surprisingly, there are only two sets of data for the viscosity η at significant supercooling. Using Poiseuille flow in capillaries, Hallett (43) reached 249.35?K, and Osipov et al. (44) reached 238.15?K. However, the two sets disagree below 251?K, with an 8% difference at 249?K, beyond the reported uncertainties. The measurements in ref. 44 are suspected of errors (45) because of the small capillary diameter used. Here we report η at ambient pressure down to 239.27?K. Our study completes the knowledge of the main dynamic parameters of water down to the homogeneous crystallization limit and allows us to check the coupling of viscosity to molecular translation or rotation, as has been done for usual glassformers.     

Redistribution of soil water by a saprotrophic fungus enhances carbon mineralization

The desiccation of upper soil horizons is a common phenomenon, leading to a decrease in soil microbial activity and mineralization. Recent studies have shown that fungal communities and fungal-based food webs are less sensitive and better adapted to soil desiccation than bacterial-based food webs. One reason for a better fungal adaptation to soil desiccation may be hydraulic redistribution of water by mycelia networks. Here we show that a saprotrophic fungus (Agaricus bisporus) redistributes water from moist (–0.03 MPa) into dry (–9.5 MPa) soil at about 0.3 cm⋅min⁻¹ in single hyphae, resulting in an increase in soil water potential after 72 h. The increase in soil moisture by hydraulic redistribution significantly enhanced carbon mineralization by 2,800% and enzymatic activity by 250–350% in the previously dry soil compartment within 168 h. Our results demonstrate that hydraulic redistribution can partly compensate water deficiency if water is available in other zones of the mycelia network. Hydraulic redistribution is likely one of the mechanisms behind higher drought resistance of soil fungi compared with bacteria. Moreover, hydraulic redistribution by saprotrophic fungi is an underrated pathway of water transport in soils and may lead to a transfer of water to zones of high fungal activity.Drought is one of the most important and frequent abiotic stresses in terrestrial ecosystems (1). With respect to soil processes, soil desiccation limits microbial activity and decreases soil enzyme activity (2), carbon mineralization (3, 4) and nitrogen mineralization (5). In addition, drought can also alter soil microbial community composition (2, 6).During desiccation and dry periods, soil fungal communities and fungal-based food webs are better adapted to drought than bacterial communities and bacteria-based food webs (7, 8). Bacteria are more strongly restricted than fungi (9), as bacterial activity needs a constant supply of water (10). One reason for the better adaptation of fungi compared with bacteria to low soil water potentials is seen in their strong cell walls, preventing water losses (1). The strength of fungal cell walls can even be enhanced by cross-linking of polymers and thickening under stress. Another reason for the better adaptation of fungi to soil desiccation might be hydraulic redistribution of water by mycelia networks. Hydraulic redistribution is defined as the passive transport of water in soils through organisms along a gradient in soil water potential and was first observed for plant roots (11). Hydraulic redistribution through plant roots improves plant survival and nutrient uptake by extending the life span and activity of roots (12) and by favoring decomposition of soil organic matter (13). Mycorrhiza fungal hyphae can also relocate water along gradients in soil water potential (12, 14–16). In addition, some studies reported the transport of nutrients and water over larger distances (>1 m) by saprotrophic fungal hyphae in nonsoil systems. Further, water leakage from hyphae into dry growth medium was observed (17). The water transport in hyphae was attributed to gradients in osmotic potentials (18–22).Saprotrophic fungi are main regulators of soil nutrient cycling, litter decomposition, and soil respiration due to their specific enzymatic activities (23, 24) and due to the high density of hyphae in soil (up to 800 m⋅g⁻¹ soil) (25), and especially in litter layers. The ability of saprotrophic fungi to distribute water would provide a direct and fast connection between water and nutrient sources in soils that would be hardly accessible to bacteria. This could have an enormous impact on decomposition processes under drought conditions.Here, we show the potential of the saprotrophic fungus Agaricus bisporus for hydraulic redistribution and impact of water redistribution on carbon mineralization in a desiccated soil.     

In situ investigation of water on MXene interfaces

A continuum of water populations can exist in nanoscale layered materials, which impacts transport phenomena relevant for separation, adsorption, and charge storage processes. Quantification and direct interrogation of water structure and organization are important in order to design materials with molecular-level control for emerging energy and water applications. Through combining molecular simulations with ambient-pressure X-ray photoelectron spectroscopy, X-ray diffraction, and diffuse reflectance infrared Fourier transform spectroscopy, we directly probe hydration mechanisms at confined and nonconfined regions in nanolayered transition-metal carbide materials. Hydrophobic (K⁺) cations decrease water mobility within the confined interlayer and accelerate water removal at nonconfined surfaces. Hydrophilic cations (Li⁺) increase water mobility within the confined interlayer and decrease water-removal rates at nonconfined surfaces. Solutes, rather than the surface terminating groups, are shown to be more impactful on the kinetics of water adsorption and desorption. Calculations from grand canonical molecular dynamics demonstrate that hydrophilic cations (Li⁺) actively aid in water adsorption at MXene interfaces. In contrast, hydrophobic cations (K⁺) weakly interact with water, leading to higher degrees of water ordering (orientation) and faster removal at elevated temperatures.

Geologic clays are minerals with variable amounts of water trapped within the bulk structure (1) and are routinely used as hydraulic barriers where water and contaminant transport must be controlled (2, 3). These layered materials can exhibit large degrees of swelling when intercalated with a hydrated cation (4). Fundamentally, water adsorption at exposed interfaces and transport in confined channels is dictated by geometry, morphology, and chemistry (e.g., surface chemistry, local solutes, etc.) (5). Understanding water adsorption and swelling in natural clay materials has significant implications for understanding water interactions in nanoscale layered materials. At the nanoscale, the ability to control the interlayer swelling and water adsorption can lead to more precise control over mass and reactant transport, resulting in enhancement in properties necessary for next-generation energy storage (power and capacity) (6–8), membranes (selectivity, salt rejection, and water permeability), catalysis (9–13), and adsorption (14).Two-dimensional (2D) and multilayered transition-metal carbides and nitrides (MXenes) are a recent addition to the few-atom-thick materials and have been widely studied in their applications to energy storage (6, 9, 15, 16), membranes (13), and adsorption (17). MXenes (Mn+1XnTx) are produced via selective etching of A elements from ceramic MAX (Mn+1 AX_n) phase materials (11, 18). The removal of A element results in thin Mn+1 X_n nanosheets with negative termination groups (Tx −). MXene’s hydrophilic and negatively charged surface properties promote spontaneous intercalation of a wide array of ions and compounds. Cation intercalation properties in MXenes have been vigorously explored due to their demonstrated high volumetric capacitance, which may enable high-rate energy storage (6, 19). In addition, their unique and rich surface chemistry may enable selective ion adsorption, making them promising candidates for water purification and catalytic applications (20–22).Water and ion transport within multilayered MXenes is governed by the presence of a continuum of water populations. The configuration of water in confined (interlayer) and nonconfined state (surface) influences the material system’s physical properties (13, 23–27). However, our current understanding of water–surface interactions and water structure at the molecular scale is incomplete due to limited characterization approaches (28). Most modern observations are limited to macroscopic measurements (e.g., transport measurement, contact angle, etc.), which do not capture the impact of local heterogeneity due to surface roughness, surface chemistry, solutes, etc. (29). Herein, we address this gap via combining theory with an ensemble of direct and indirect interrogation techniques. Water structure and sorption properties at MXene interfaces are directly probed by using ambient-pressure X-ray photoelectron spectroscopy (APXPS), X-ray diffraction (XRD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). APXPS enables detection of local chemically specific signatures and quantitative analysis at near-ambient pressures (30). This technique provides the ability to spatially resolve the impact of surface chemistry and solutes on water sorption/desorption at water–solid interfaces. Model hydrophobic (e.g., K⁺) and hydrophilic (e.g., Li⁺) cations were intercalated into the layers via ion exchange to systematically probe the impacts of charged solutes on water orientation and sorption. Prior reports suggest that water within the confined interlayer transforms from bulk-like to crystalline when intercalated with bulky cations (31, 32). Furthermore, it has been demonstrated that water ordering is correlated with ion size (33, 34). Here, we expand upon this early work and examine the role that solute hydrophobicity and hydrophilicity impacts water adsorption on solid interfaces. Water mobility within the interlayer is impacted by the hydration energy of that cation. Results shed light on the intertwined role that surface counterions and terminating groups play on the dynamics of hydration and dehydration.     

Microscopic origins of the crystallographically preferred growth in evaporation-induced colloidal crystals

Unlike crystalline atomic and ionic solids, texture development due to crystallographically preferred growth in colloidal crystals is less studied. Here we investigate the underlying mechanisms of the texture evolution in an evaporation-induced colloidal assembly process through experiments, modeling, and theoretical analysis. In this widely used approach to obtain large-area colloidal crystals, the colloidal particles are driven to the meniscus via the evaporation of a solvent or matrix precursor solution where they close-pack to form a face-centered cubic colloidal assembly. Via two-dimensional large-area crystallographic mapping, we show that the initial crystal orientation is dominated by the interaction of particles with the meniscus, resulting in the expected coalignment of the close-packed direction with the local meniscus geometry. By combining with crystal structure analysis at a single-particle level, we further reveal that, at the later stage of self-assembly, however, the colloidal crystal undergoes a gradual rotation facilitated by geometrically necessary dislocations (GNDs) and achieves a large-area uniform crystallographic orientation with the close-packed direction perpendicular to the meniscus and parallel to the growth direction. Classical slip analysis, finite element-based mechanical simulation, computational colloidal assembly modeling, and continuum theory unequivocally show that these GNDs result from the tensile stress field along the meniscus direction due to the constrained shrinkage of the colloidal crystal during drying. The generation of GNDs with specific slip systems within individual grains leads to crystallographic rotation to accommodate the mechanical stress. The mechanistic understanding reported here can be utilized to control crystallographic features of colloidal assemblies, and may provide further insights into crystallographically preferred growth in synthetic, biological, and geological crystals.

As an analogy to atomic crystals, colloidal crystals are highly ordered structures formed by colloidal particles with sizes ranging from 100 nm to several micrometers (1–6). In addition to engineering applications such as photonics, sensing, and catalysis (4, 5, 7, 8), colloidal crystals have also been used as model systems to study some fundamental processes in statistical mechanics and mechanical behavior of crystalline solids (9–14). Depending on the nature of interparticle interactions, many equilibrium and nonequilibrium colloidal self-assembly processes have been explored and developed (1, 4). Among them, the evaporation-induced colloidal self-assembly presents a number of advantages, such as large-size fabrication, versatility, and cost and time efficiency (3–5, 15–18). In a typical synthesis where a substrate is immersed vertically or at an angle into a colloidal suspension, the colloidal particles are driven to the meniscus by the evaporation-induced fluid flow and subsequently self-assemble to form a colloidal crystal with the face-centered cubic (fcc) lattice structure and the close-packed {111} plane parallel to the substrate (2, 3, 19–23) (see Fig. 1A for a schematic diagram of the synthetic setup).Open in a separate windowFig. 1.Evaporation-induced coassembly of colloidal crystals. (A) Schematic diagram of the evaporation-induced colloidal coassembly process. “G”, “M”, and “N” refer to “growth,” “meniscus,” and “normal” directions, respectively. The reaction solution contains silica matrix precursor (tetraethyl orthosilicate, TEOS) in addition to colloids. (B) Schematic diagram of the crystallographic system and orientations used in this work. (C and D) Optical image (Top Left) and scanning electron micrograph (SEM) (Bottom Left) of a typical large-area colloidal crystal film before (C) and after (D) calcination. (Right) SEM images of select areas (yellow rectangles) at different magnifications. Corresponding fast-Fourier transform (see Inset in Middle in C) shows the single-crystalline nature of the assembled structure. (E) The 3D reconstruction of the colloidal crystal (left) based on FIB tomography data and (right) after particle detection. (F) Top-view SEM image of the colloidal crystal with crystallographic orientations indicated.While previous research has focused on utilizing the assembled colloidal structures for different applications (4, 5, 7, 8), considerably less effort is directed to understand the self-assembly mechanism itself in this process (17, 24). In particular, despite using the term “colloidal crystals” to highlight the microstructures’ long-range order, an analogy to atomic crystals, little is known regarding the crystallographic evolution of colloidal crystals in relation to the self-assembly process (3, 22, 25). The underlying mechanisms for the puzzling—yet commonly observed—phenomenon of the preferred growth along the close-packed <110> direction in evaporation-induced colloidal crystals are currently not understood (3, 25–29). The <110> growth direction has been observed in a number of processes with a variety of particle chemistries, evaporation rates, and matrix materials (3, 25–28, 30), hinting at a universal underlying mechanism. This behavior is particularly intriguing as the colloidal particles are expected to close-pack parallel to the meniscus, which should lead to the growth along the <112> direction and perpendicular to the <110> direction (16, 26, 31)^*.Preferred growth along specific crystallographic orientations, also known as texture development, is commonly observed in crystalline atomic solids in synthetic systems, biominerals, and geological crystals. While current knowledge recognizes mechanisms such as the oriented nucleation that defines the future crystallographic orientation of the growing crystals and competitive growth in atomic crystals (32–34), the underlying principles for texture development in colloidal crystals remain elusive. Previous hypotheses based on orientation-dependent growth speed and solvent flow resistance are inadequate to provide a universal explanation for different evaporation-induced colloidal self-assembly processes (3, 25–29). A better understanding of the crystallographically preferred growth in colloidal self-assembly processes may shed new light on the crystal growth in atomic, ionic, and molecular systems (35–37). Moreover, mechanistic understanding of the self-assembly processes will allow more precise control of the lattice types, crystallography, and defects to improve the performance and functionality of colloidal assembly structures (38–40).     

High-pressure superconducting phase diagram of 6Li: Isotope effects in dense lithium

We measured the superconducting transition temperature of ⁶Li between 16 and 26 GPa, and report the lightest system to exhibit superconductivity to date. The superconducting phase diagram of ⁶Li is compared with that of ⁷Li through simultaneous measurement in a diamond anvil cell (DAC). Below 21 GPa, Li exhibits a direct (the superconducting coefficient, α, T_c ∝ M^?α,  is positive), but unusually large isotope effect, whereas between 21 and 26 GPa, lithium shows an inverse superconducting isotope effect. The unusual dependence of the superconducting phase diagram of lithium on its atomic mass opens up the question of whether the lattice quantum dynamic effects dominate the low-temperature properties of dense lithium.Light elements (low Z) and their compounds have been the subject of many recent studies for their potential as high-temperature superconductors (e.g., refs. 1–5). Due to their low mass, the physical properties of the low-Z compounds can be strongly influenced by zero-point effects (lattice quantum dynamics) (6), and mass-related isotope effects may be present in their thermodynamics of vibrational degrees of freedom. Such effects will influence the superconducting properties of these materials. Dependence of superconductivity on isotopic variations of low-Z compounds can be used to probe and determine the magnitude of mass-related effects. This in turn allows better development of models to determine their superconducting properties.Under ambient pressure, lithium is the lightest elemental metallic and superconducting system, and it exhibits one of the highest superconducting transition temperatures of any elemental superconductor under compression (7–11). Despite the large mass difference between the stable isotopes of lithium (∼15%), isotope effects in superconductivity of lithium have not been studied before.In systems with long-range attractive potential, the ratio of lattice zero-point displacements to interatomic distances may increase under compression (increase to the Lindemann ratio at high densities), provided they retain their long-range interactions (12, 13). (This is opposed to systems with short-range interactions, e.g., helium, in which the lattice becomes more classical under compression.) In these systems, more deviations from the static lattice behavior are expected at higher densities. At sufficiently low temperatures, where thermal energy is small, lattice quantum dynamics can play a more dominant role in the bulk properties. Sound velocity measurements on stable isotopes of lithium at 77 K and up to 1.6 GPa show that quantum solid effects in lithium, at least in the pressure range studied, do not decrease as a function of pressure (14). Raman spectroscopy studies between 40 and 123 GPa and at 177 K report a reduced isotope effect in high-frequency vibrational modes of Li, which may be related to quantum solid behavior (15). Up to this point, no experiments have reported a comparison of any physical properties of lithium isotopes at low temperatures and high pressures concurrently. Because the superconducting transition of lithium occurs in a relatively low temperature range (16–18), studying its superconducting isotope effect provides excellent conditions to search for zero-point lattice effects and their evolution as a function of pressure.In the present study, we have measured the superconducting isotope effects in the stable isotopes of lithium under pressure. Lithium is a simple metallic system that is expected to exhibit conventional phonon-mediated superconductivity and a well-defined superconducting isotope effect with nominal pressure dependence of the relative T_c values (19) [according to the model by Treyeva and Trapezina (19), using theoretical values of Coulomb coupling constant, μ*(P) (20) by Christensen and Novikov and equation of state (EOS) of lithium (21) by Hanfland et al., assuming similar structures for the two isotopes, α should not vary by more than 10% for 15?GPa < P < 25?GPa]. Because phonon-mediated superconductivity depends on lattice and electronic properties of a material, any unusual isotopic mass dependence of the superconducting phase diagram can be indicative of the effects of large lattice dynamics on electronic and/or structural properties.