Stabilizing Li-O2 Batteries with Multifunctional Fluorinated Graphene.

As a full cell system with attractive theoretical energy density, challenges faced by Li-O2 batteries (LOBs) are not only the deficient actual capacity and superoxide-derived parasitic reactions on the cathode side but also the stability of Li-metal anode. To solve simultaneously intrinsic issues, multifunctional fluorinated graphene (CFx, x = 1, F-Gr) was introduced into the ether-based electrolyte of LOBs. F-Gr can accelerate O2- transformation and O2--participated oxygen reduction reaction (ORR) process, resulting in enhanced discharge capacity and restrained O2--derived side reactions of LOBs, respectively. Moreover, F-Gr induced the F-rich and O-depleted solid electrolyte interphase (SEI) film formation, which have improved Li-metal stability. Therefore, energy storage capacity, efficiency, and cyclability of LOBs have been markedly enhanced. More importantly, the method developed in this work to disperse F-Gr into an ether-based electrolyte for improving LOBs' performances is convenient and significant from both scientific and engineering aspects.

Single-Molecule Studies of the Linker Histone H1 Binding to DNA and the Nucleosome.

Linker histone H1 regulates chromatin structure and gene expression. Investigating the dynamics and stoichiometry of binding of H1 to DNA and the nucleosome is crucial to elucidating its functions. Because of the abundant positive charges and the strong self-affinity of H1, quantitative in vitro studies of its binding to DNA and the nucleosome have generated results that vary widely and, therefore, should be interpreted in a system specific manner. We sought to overcome this limitation by developing a specially passivated microscope slide surface to monitor binding of H1 to DNA and the nucleosome at a single-molecule level. According to our measurements, the stoichiometry of binding of H1 to DNA and the nucleosome is very heterogeneous with a wide distribution whose averages are in reasonable agreement with previously published values. Our study also revealed that H1 does not dissociate from DNA or the nucleosome on a time scale of tens of minutes. We found that histone chaperone Nap1 readily dissociates H1 from DNA and superstoichiometrically bound H1 from the nucleosome, supporting a hypothesis whereby histone chaperones contribute to the regulation of the H1 profile in chromatin.

Dynamics of nucleosome assembly and effects of DNA methylation.

The nucleosome is the fundamental packing unit of the eukaryotic genome, and CpG methylation is an epigenetic modification associated with gene repression and silencing. We investigated nucleosome assembly mediated by histone chaperone Nap1 and the effects of CpG methylation based on three-color single molecule FRET measurements, which enabled direct monitoring of histone binding in the context of DNA wrapping. According to our observation, (H3-H4)2 tetramer incorporation must precede H2A-H2B dimer binding, which is independent of DNA termini wrapping. Upon CpG methylation, (H3-H4)2 tetramer incorporation and DNA termini wrapping are facilitated, whereas proper incorporation of H2A-H2B dimers is inhibited. We suggest that these changes are due to rigidified DNA and increased random binding of histones to DNA. According to the results, CpG methylation expedites nucleosome assembly in the presence of abundant DNA and histones, which may help facilitate gene packaging in chromatin. The results also indicate that the slowest steps in nucleosome assembly are DNA termini wrapping and tetramer positioning, both of which are affected heavily by changes in the physical properties of DNA.

On-chip manipulation of single microparticles, cells, and organisms using surface acoustic waves.

Techniques that can dexterously manipulate single particles, cells, and organisms are invaluable for many applications in biology, chemistry, engineering, and physics. Here, we demonstrate standing surface acoustic wave based "acoustic tweezers" that can trap and manipulate single microparticles, cells, and entire organisms (i.e., Caenorhabditis elegans) in a single-layer microfluidic chip. Our acoustic tweezers utilize the wide resonance band of chirped interdigital transducers to achieve real-time control of a standing surface acoustic wave field, which enables flexible manipulation of most known microparticles. The power density required by our acoustic device is significantly lower than its optical counterparts (10,000,000 times less than optical tweezers and 100 times less than optoelectronic tweezers), which renders the technique more biocompatible and amenable to miniaturization. Cell-viability tests were conducted to verify the tweezers' compatibility with biological objects. With its advantages in biocompatibility, miniaturization, and versatility, the acoustic tweezers presented here will become a powerful tool for many disciplines of science and engineering.

Dark-field illumination on zero-mode waveguide/microfluidic hybrid chip reveals T4 replisomal protein interactions.

The ability of zero-mode waveguides (ZMWs) to guide light energy into subwavelength-diameter cylindrical nanoapertures has been exploited for single-molecule fluorescence studies of biomolecules at micromolar concentrations, the typical dissociation constants for biomolecular interactions. Although epi-fluorescence microscopy is now adopted for ZMW-based imaging as an alternative to the commercialized ZMW imaging platform, its suitability and performance awaits rigorous examination. Here, we present conical lens-based dark-field fluorescence microscopy in combination with a ZMW/microfluidic chip for single-molecule fluorescence imaging. We demonstrate that compared to epi-illumination, the dark-field configuration displayed diminished background and noise and enhanced signal-to-noise ratios. This signal-to-noise ratio for imaging using the dark-field setup remains essentially unperturbed by the presence of background fluorescent molecules at micromolar concentration. Our design allowed single-molecule FRET studies that revealed weak DNA-protein and protein-protein interactions found with T4 replisomal proteins.

Insights into Okazaki fragment synthesis by the T4 replisome: the fate of lagging-strand holoenzyme components and their influence on Okazaki fragment size.

In this study, we employed a circular replication substrate with a low priming site frequency (1 site/1.1 kb) to quantitatively examine the size distribution and formation pattern of Okazaki fragments. Replication reactions by the T4 replisome on this substrate yielded a patterned series of Okazaki fragments whose size distribution shifted through collision and signaling mechanisms as the gp44/62 clamp loader levels changed but was insensitive to changes in the gp43 polymerase concentration, as expected for a processive, recycled lagging-strand polymerase. In addition, we showed that only one gp45 clamp is continuously associated with the replisome and that no additional clamps accumulate on the DNA, providing further evidence that the clamp departs, whereas the polymerase is recycled upon completion of an Okazaki fragment synthesis cycle. We found no support for the participation of a third polymerase in Okazaki fragment synthesis.

Fluorinated carbon as high-performance cathode for aqueous zinc primary batteries.

Fluorinated carbon (CFx) has been extensively served as promising positive electrode material for lithium primary batteries due to its high energy density. However, there are comparatively far less reports about the use of CFx on other battery systems, let alone on the research of aqueous batteries. Herein in this study, we employed CFx as the cathode active for aqueous zinc batteries for the first time and systematically investigated its electrochemical behavior under a series of aqueous zinc-ion electrolytes. As is discovered that the F/C ratio (the x value in CFx) of CFx have significant effects on the electrochemical performance of aqueous Zn/CFx batteries. Specifically, CF0.85 exhibits excellent electrochemical property with delivering a remarkable discharge capacity of 503 mA h g-1 and energy density of 388 W h kg-1 (at a current rate of 30 mA g-1 under temperature of 25 °C), much better than several other CFx electrode with F/C ratio of 0.70, 0.95, and 1.10, respectively. Besides, it also exhibits decent temperature performance with discharge capacities of 550 mA h g-1 at 50 °C and 460 mA h g-1 at 0 °C under current density of 30 mA g-1. Furthermore, the electrochemical discharge mechanism based on conversion reaction was further uncovered by applying XPS, XRD, SEM and EDS elemental analysis characterization techniques. In conclusion, these results demonstrate the potential application value of CFx in aqueous zinc primary batteries.

In-situ texturing hollow carbon host anchored with Fe single atoms accelerating solid-phase redox for Li-Se batteries.

Se-based cathodes have caught tremendous attention owing to their comparable volumetric capacity and better electronic conductivity to S cathodes. However, its low utilization ratio and sluggish redox kinetics due to the high reaction barrier of solid-phase transformation from Se to Li2Se limit its practical application. Herein, an in-situ texturing hollow carbon host by gas-solid interface reaction anchored with Fe single-atomic catalyst is designed and prepared for advanced Li-Se batteries. This Se host presents high pore volume of 1.49 cm3 g-1, Fe single atom content of 1.53 wt%, and its specific structure protects single-atomic catalyst from the destructive reaction environment, thus balancing catalytic activity and durability. After Se loading by reduction of H2SeO3, this homogenous Se-based cathode delivers a superior rate capacity of 431.3 mA h g-1 at 4C, and great discharge capacity of 301.8 mA h g-1 after 1000 cycles at 10C, with high Li-ion diffusion coefficient and capacitance-contributed ratio. The distribution of relaxation times analysis verifies solid-phase transformation mechanism of this cathode and density functional theory calculations confirm the adsorption and bidirectionally catalysis effect of Fe single-atomic catalyst. This work provides a new strategy to prepare high-efficient Se cathode associated with non-noble metal single atoms for high-performance Li-Se batteries.

Fluorinated aggregated nanocarbon with high discharge voltage as cathode materials for alkali-metal primary batteries.

Due to its exceptionally high theoretical energy density, fluorinated carbon has been recognized as a strong contender for the cathode material in lithium primary batteries particularly valued in aerospace and related industries. However, CF x cathode with high F/C ratio, which enables higher energy density, often suffer from inadequate rate capability and are unable to satisfy escalating demand. Furthermore, their intrinsic low discharge voltage imposes constraints on their applicability. In this study, a novel and high F/C ratio fluorinated carbon nanomaterials (FNC) enriched with semi-ionic C-F bonds is synthesized at a lower fluorination temperature, using aggregated nanocarbon as the precursor. The increased presence semi-ionic C-F bonds of the FNC enhances conductivity, thereby ameliorating ohmic polarization effects during initial discharge. In addition, the spherical shape and aggregated configuration of FNC facilitate the diffusion of Li+ to abundant active sites through continuous paths. Consequently, the FNC exhibits high discharge voltage of 3.15 V at 0.01C and superior rate capability in lithium primary batteries. At a high rate of 20C, power density of 33,694 W kg-1 and energy density of 1,250 Wh kg-1 are achieved. Moreover, FNC also demonstrates notable electrochemical performance in sodium/potassium-CF x primary batteries. This new-type alkali-metal/CF x primary batteries exhibit outstanding rate capability, rendering them with vast potential in high-power applications.

Protecting Li-metal anode with LiF-enriched solid electrolyte interphase derived from a fluorinated graphene additive.

As the holy-grail material, the Li-metal anode has been considered the potential anode of the next generation of Li-metal batteries (LMBs). However, issues of undesirable dendrite growth and unsatisfactory reversibility of the Li-plating/stripping process during the electrochemical cycling impede further application of LMBs. Herein, we innovatively introduce fluorinated graphene (F-Gr) species as a sacrificial effective electrolyte additive into EC/EMC-based electrolyte, which effectively triggers LiF-enriched (composition) and organic/inorganic species uniform-distributed (structure) SEI film architecture that features robustness and denseness, as well as good stability. With the F-Gr additive, efficient Li-metal anode protection (dendrite-free morphology on Li-metal surface and improved Li plating/stripping reversibility during electrochemical cycling) and significantly enhanced long-term lifespan of LMBs is achieved. Remarkably, classical electrochemical techniques, combined with the surface-sensitive characterizations (XPS and TOF-SIMS), comprehensively and systematically highlight critical structure-activity relationships between the SEI architecture (both composition and structure) and electrochemical performance. These techniques provide deep insights into the optimal electrolyte designation of Li-metal anode in LMBs.

Boosting the Electrochemical Performance of Primary and Secondary Lithium Batteries with Mn-Doped All-Fluoride Cathodes.

Transition metal fluorides are potentially high specific energy cathode materials of next-generation lithium batteries, and strategies to address their low conductivity typically involve a large amount of carbon coating, which reduces the specific energy of the electrode. In this study, MnyFe1-yF3@CFx was generated by the all-fluoride strategy, converting most of the carbon in MnyFe1-yF3@C into electrochemical active CFx through a controllable NF3 gas phase fluorination method, while still retaining a tightly bound graphite layer to provide initial conductivity, which greatly improved the energy density of the composite. This synergistic effect of nonfluorinated residual carbon (∼11%) and Mn doping ensures the electrochemical kinetics of the composite. The loading mass of the active substance had been increased to 86%. The theoretical and actual discharge capacity of MnyFe1-yF3@CFx composite was up to 765 mAh g-1 (pure FeF3 is 712 mAh g-1) and 728 mAh g-1, respectively. The discharge capacity at the high-voltage (3.0 V) platform was more than three times higher than that of the non-Mn-doped composite (FeF3@CFx).

The human lagging strand DNA polymerase δ holoenzyme is distributive.

Polymerase δ is widely accepted as the lagging strand replicative DNA polymerase in eukaryotic cells. It forms a replication complex in the presence of replication factor C and proliferating cell nuclear antigen to perform efficient DNA synthesis in vivo. In this study, the human lagging strand holoenzyme was reconstituted in vitro. The rate of DNA synthesis of this holoenzyme, measured with a singly primed ssM13 DNA substrate, is 4.0 ± 0.4 nucleotides. Results from adenosine 5'-(3-thiotriphosphate) tetralithium salt (ATPγS) inhibition experiments revealed the nonprocessive characteristic of the human DNA polymerase (Pol δ) holoenzyme (150 bp for one binding event), consistent with data from chase experiments with catalytically inactive mutant Pol δ(AA). The ATPase activity of replication factor C was characterized and found to be stimulated ∼10-fold in the presence of both proliferating cell nuclear antigen and DNA, but the activity was not shut down by Pol δ in accord with rapid association/dissociation of the holoenzyme to/from DNA. It is noted that high concentrations of ATP inhibit the holoenzyme DNA synthesis activity, most likely due to its inhibition of the clamp loading process.

Synthesis and characterization of fluorinated carbon nanotubes for lithium primary batteries with high power density.

The synthesis and characterization of fluorinated carbon nanotubes have been carried out under an inert gas containing fluorine. All of the samples have been characterized by x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), solid-state nuclear magnetic resonance ((13)C and (19)F ss-NMR) and transmission electron microscopy (TEM) techniques. The comparison of the effects of various experimental parameters on the structure of fluorinated materials allows the disclosure of the fluorination mechanism. It is shown that fluorine was intercalated into the outer part of the carbon nanotubes initially where graphene layers were coaxial within a distance of 0.60 nm. In contrast, the inner part of the carbon nanotubes was not intercalated. The electrochemical performance such as discharge capacity as a cathode for a primary lithium battery has also been investigated. The samples with a F/C ratio of 0.75 exhibited the best performance, namely high energy and power densities. The highest specific energy density and specific power density were 1147 Wh kg(-1) and 8998 W kg(-1), respectively, at a current density of 4 A g(-1).

Single-molecule studies of DNA replisome function.

Fast and accurate replication of DNA is accomplished by the interactions of multiple proteins in the dynamic DNA replisome. The DNA replisome effectively coordinates the leading and lagging strand synthesis of DNA. These complex, yet elegantly organized, molecular machines have been studied extensively by kinetic and structural methods to provide an in-depth understanding of the mechanism of DNA replication. Owing to averaging of observables, unique dynamic information of the biochemical pathways and reactions is concealed in conventional ensemble methods. However, recent advances in the rapidly expanding field of single-molecule analyses to study single biomolecules offer opportunities to probe and understand the dynamic processes involved in large biomolecular complexes such as replisomes. This review will focus on the recent developments in the biochemistry and biophysics of DNA replication employing single-molecule techniques and the insights provided by these methods towards a better understanding of the intricate mechanisms of DNA replication.

Evolution in the supramolecular complexes between poly(phenylene ethynylene)-based polyelectrolytes and octadecyltrimethylammonium bromide as revealed by fluorescence correlation spectroscopy.

Poly(phenyleneethynylene)-based conjugated polyelectrolytes (PPE-SO(3)(-)) are a class of polyions with rigid backbones. This work uses fluorescence correlation spectroscopy to study how the diffusion of complexes, formed between a PPE-SO(3)(-) polyelectrolyte and octadecyltrimethylammonium bromide (OTAB) surfactant molecules, changes with OTAB concentration below its critical micelle concentration. The dependence of the hydrodynamic radius of the complexes on the OTAB concentration has three regimes. In the low concentration regime ( C(OTAB)/ C(monomer) < 6), the complex has a size comparable to that of the polymer in deionized water. In the intermediate concentration regime (6 < C(OTAB)/ C(monomer) < 400), the complexes have the largest size and substantial heterogeneity. In the high concentration regime (400 < C(OTAB)/ C(monomer) < 1800), the complexes have a size that is about three times larger than that in the low concentration regime. These results elucidate features of the self-assembly of a polyelectrolyte and an ionic surfactant and show that the C(OTAB)/ C(monomer) concentration ratio controls the composition of polyelectrolyte/surfactant complexes.

Charge density effects on the aggregation properties of poly(p-phenylene-ethynylene)-based anionic polyelectrolytes.

This work shows that low charge density poly(p-phenylene-ethynylene)s (PPE-SO3Na-L and PPE-CO2Na-L), which feature sulfonate and carboxylate groups on every other phenyl ring, form aggregates in water, whereas high charge density poly(p-phenylene-ethynylene)s (PPE-SO3Na-H and PPE-CO2Na-H), which possess sulfonate or carboxylate groups on every phenyl ring, do not aggregate in water. The formation of aggregates of PPE-SO3Na-L and PPE-CO2Na-L is demonstrated by comparing the concentration and temperature dependence of their steady-state spectra in water to that in DMSO, in which the two polymers do not aggregate. For the weak polyelectrolytes PPE-CO2Na-H and PPE-CO2Na-L, the solution pH was changed to vary the charge density. In addition, the cationic surfactant, octadecyltrimethyl ammonium, is shown to dissociate the low charge density polymer aggregates and to form supramolecular complexes with each of the different polyelectrolytes. Fluorescence correlation spectroscopy was applied to provide insight into the sizes of aggregates under different solution conditions.

High-Power-Density, High-Energy-Density Fluorinated Graphene for Primary Lithium Batteries.

Li/CFx is one of the highest-energy-density primary batteries; however, poor rate capability hinders its practical applications in high-power devices. Here we report a preparation of fluorinated graphene (GFx) with superior performance through a direct gas fluorination method. We find that the so-called "semi-ionic" C-F bond content in all C-F bonds presents a more critical impact on rate performance of the GFx in comparison with sp2 C content in the GFx, morphology, structure, and specific surface area of the materials. The rate capability remains excellent before the semi-ionic C-F bond proportion in the GFx decreases. Thus, by optimizing semi-ionic C-F content in our GFx, we obtain the optimal x of 0.8, with which the GF0.8 exhibits a very high energy density of 1,073 Wh kg-1 and an excellent power density of 21,460 W kg-1 at a high current density of 10 A g-1. More importantly, our approach opens a new avenue to obtain fluorinated carbon with high energy densities without compromising high power densities.

Solvation and aggregation of polyphenylethynylene based anionic polyelectrolytes in dilute solutions.

The absorption and fluorescence properties of a polyphenylethynylene based conjugated polyelectrolyte with sulfonate solubilizing groups (PP2) are shown to change dramatically with solution conditions because of the equilibrium between unaggregated and aggregated forms of the polymer. The fluorescence of PP2 is strongly quenched on addition of counterions such as Na+, K+, Li+, and TBA+, an effect which arises from the creation of salt stabilized aggregates. The formation of aggregates has been further corroborated by concentration and temperature studies in water and comparisons to dimethylsulfoxide solvent, in which the polymer does not aggregate. In aqueous solutions, the addition of the cationic surfactant, octadecyltrimethyl ammonium, causes the polymer aggregates to dissociate and creates polymer/surfactant aggregates that have spectral properties like that of the unaggregated polymer.

The effect of ionic strength on the electron-transfer rate of surface immobilized cytochrome C.

Horse heart cytochrome c was immobilized on four different self-assembled monolayer (SAM) films. The electron tunneling kinetics were studied in the different assemblies as a function of the ionic strength of the buffer solution using cyclic voltammetry. When cytochrome c is electrostatically immobilized, the standard electron exchange rate constant k0 decreases with the increase of the solution's ionic strength. In contrast, the protein covalently attached or ligated has a rate constant independent of the ionic strength. The inhomogeneity of electrostatically immobilized cytochrome c increases with the increase of the solution's ionic strength whereas that of the covalently attached protein is independent of the ionic strength. A comparison of these different electron-transfer behaviors suggests that the thermodynamically stable geometry of cytochrome c in the electrostatic assemblies is also an electron transfer favorable one. It suggests that the surface charges of cytochrome c are capable of guiding it into geometries in which its front surface faces the electron-transfer partner. The inhomogeneity observed in this study indicates that a distribution of cytochrome c orientations and thus a distribution of electron transfer rate constants exists.

On the electron transfer mechanism between cytochrome C and metal electrodes. Evidence for dynamic control at short distances.

Cytochrome c was coordinatively bound to self-assembled monolayers of pyridine-terminated alkanethiols on Au and Ag electrodes. The mechanism of heterogeneous electron transfer of the immobilized protein was investigated by cyclic voltammetry and time-resolved surface-enhanced resonance Raman spectroelectrochemistry. The temperature, distance, and overpotential dependencies of the electron transfer rates indicate a change of mechanism from a tunneling controlled reaction at long distances (thicker films) to a solvent/protein friction controlled reaction at smaller distances (thinner films).