A combined hydrothermal/hydrogen reduction method has been developed for the mass production of helical carbon nanotubes (HCNTs) by the pyrolysis of acetylene at 475 °C in the presence of Fe(3)O(4) nanoparticles. The synthesized HCNTs have been characterized by high-resolution transmission electron microscopy, scanning electron microscopy, X-ray diffraction analysis, vibrating sample magnetometry, and contact-angle measurements. The as-prepared helical-structured carbon nanotubes have a large specific surface area and high peroxidase-like activity. Catalysis was found to follow Michaelis-Menten kinetics and the HCNTs showed strong affinity for both H(2)O(2) and 3,3',5,5',-tetramethylbenzidine (TMB). Based on the high activity, the HCNTs were firstly used to develop a biocatalyst and amperometric sensor. At pH 7.0, the constructed amperometric sensor showed a linear range for the detection of H(2)O(2) from 0.5 to 115 μM with a correlation coefficient of 0.999 without the need for an electron-transfer mediator. Because of their low cost and high stability, these novel metallic HCNTs represent a promising candidate as mimetic enzymes and may find a wide range of new applications, such as in biocatalysis, immunoassay, and environmental monitoring.
Recently, many approaches were applied for assembling graphene sheets into a three-dimensional structure. However, it is still a great challenge to obtain a three-dimensional macroporous graphene network with high mechanical strength after drying. Herein, an ammonia strengthened three-dimensional graphene aerogel was prepared. Based on graphene chemistry and ice physics, the mechanical strength of graphene aerogel was improved greatly when the graphene hydrogel was treated by ammonia solution at an ambient temperature. The results demonstrated that the three-dimensional structure of graphene aerogels was destroyed thoroughly without ammonia solution treatment; conversely, the three-dimensional structure was maintained and the compressive strength was improved to 152 kPa at the static load after it was treated by ammonia solution at 90 °C for only 1 h. This phenomenon is due to two reasons: (1) the low freezing point of ammonia solution, which effectively retarded its freezing and then kept the porous structure undestroyed; (2) the reaction between ammonia and graphene hydrogel, which brought some covalent bonds among graphene sheets. We believe our efforts may pave the way for the development and application of three-dimensional graphene based materials.
principle, the corrosive issues of the sodium polysulfi des still exist at this modest temperature. Therefore, a desirable solution of safety problems is to develop a room temperature (RT) Na-S batteries.Due to low ion conductivity of the solid electrolyte and contact problems between electrodes and current collectors, the confi guration design of HT Na-S batteries may not be for RT Na-S batteries. Consequently, several attempts have been made to use polymer electrolytes in RT Na-S batteries. [6][7][8] Unfortunately, poor cycle life and low capacity were displayed. Liquid electrolytes, which have been demonstrated with good ion conductivity and widely used in lithium-ion batteries and even in Li-S batteries, [9][10][11][12] are probably suitable for RT Na-S batteries. However, as is well known, metal Na has similar chemical characteristics to Li, the S cathode in RT Na-S batteries will face the same issues observed in Li-S batteries as: i) low active material utilization, ii) poor cycle life, and iii) low Coulumbic effi ciency. [13][14][15] These drawbacks arise mainly from insulating nature of S, dissolution of polysulfi de intermediates in liquid electrolytes and large volume change during charge/ discharge. [16][17][18][19] Signifi cant advances have been achieved for Li-S batteries in decade years. The strategies include coating S with conductive polymers or carbon materials, infusing S into porous carbon, and employing different organic electrolytes. [20][21][22][23][24][25] Among these, one of the most attractive strategies is to encapsulate S with a self-supporting and void-containing carbon matrix. [20][21][22]26,27 ] Although the approach of carbon matrix supporting S can signifi cantly improve the S utilization and restrain the solubility of lithium polysulfi des, especially by infusing S into the micropores of carbon materials, the C/S composite cathodes showed outstanding cycling life even using carbonate electrolytes, but the S loading is limited to around 30% in the C−S composite due to the insulating nature of S 2 , Li 2 S 2 and Li 2 S and the side reaction between high-order polysolfi des and electrolyte, which leads to low overall capacity. [ 21 ] In order to increase the S loading of the S-based cathode, a high-cost electrolyte with linear and cyclic ethers, such as bis-(trifl uoromethane) sulfonimide lithium (LiTFSI) in dimethoxyethane and dioxolane and tetra(ethylene glycol) dimethyl ether, is generally adopted in the Li-S cells. [28][29][30] Although the use of high-cost ether-based electrolyte in Li-S batteries can increase S loading, it also generates shuttle reaction, reducing the cycle life and Coulumbic effi ciency. Recently, transition metal (Co, Ni and Cu etc.) sulfi des have been investigated as cathode materials for Li-S batteries and shown fairly stable cycle life, which are attributed to chemical stabilization of S. [31][32][33][34][35][36] The similar features between Na and Li bring us to consider whether it is possible
High stable C/S composites are fabricated by a novel high-temperature sulfur infusion into micro-mesoporous carbon method following with solvent cleaning treatment. The C/S composite cathodes show high Coulombic efficiency, long cycling stability and good rate capability in the electrolyte of 1.0 M LiPF6 + EC/DEC (1:1 v/v), for instance, the reversible capacity of the treated C/S-50 (50% S) cathode retains around 860 mAh/g even after 500 cycles and the Coulombic efficiency is close to 100%, which demonstrates the best electrochemical performance of carbon-sulfur composite cathodes using the carbonate-based electrolyte reported to date. It is believed that the chemical bond of C-S is responsible for the superior electrochemical properties in Li-S battery, that is, the strong interaction between S and carbon matrix significantly improves the conductivity of S, effectively buffers the structural strain/stress caused by the large volume change during lithiation/delithiation, completely eliminates the formation of high-order polysulfide intermediates, and substantially avoids the shuttle reaction and the side reaction between polysulfide anions and carbonate solvent, and thus enables the C/S cathode to use conventional carbonate-based electrolytes and achieve outstanding electrochemical properties in Li-S battery. The results may substantially contribute to the progress of the Li-S battery technology.
S and edge-opened graphite oxide, chemical reduction of graphene oxide and deposition of S. [23][24][25] Because the S in these graphene-S composites is not completely encapsulated by graphene, the polysulfi de intermediates still slowly dissolve into electrolyte resulting in progressive cycling decay. The ideal structure for the carbon-S composites is to intercalate S atoms or molecules into a graphite interlayer to form S intercalated graphite compounds, thus maximizing S loading and minimizing the dissolution of polysulfi des. However, only a small amount of S can be intercalated into graphite even under the conditions of high pressure and/or high temperature due to small layer spacing (planar distance ≈0.34 nm). [ 26 ] Because of the large interlayer distance, expanded graphite has been investigated as host to embed S. The expanded graphiteembedded sulfur nanocomposites were normally synthesized by two-step reactions: thermal reduction of graphite oxides in H 2 /Ar at a high temperature (450 °C) and fl owed by S meltdiffusion at a low temperature of ≈155 °C. [ 27,28 ] Since S vapor can reduce graphite oxide (GO) at a high temperature, in this work we report a novel one-step method to synthesize S intercalated graphite by S in situ reducing GO and intercalating into the reduced graphite oxide (RGO) under vacuum at 600 °C. Figure 1 schematically depicts the preparation process of the RGO/S composite. At room temperature, sulfur exists mainly in the form of cyclooctasulfur (S 8 ), as heating the mixture of graphite oxide and S 8 to 600 °C, the large molecule S 8 will be broken into smaller chain species S 2 . [ 29,30 ] Due to the large interplanar distance of GO, these S 2 molecules can intercalate into GO to deoxygenate GO and form SO 2 gas. [ 31 ] Further S 2 intercalation into RGO will form S intercalated graphite compounds. By manipulating the interlayer distance of graphite oxide through controlling the degree of oxidation of graphite, [ 32 ] the S intercalation level can be maximized. However, the S 2 molecules deposited on the external surface and the edges of RGO interlayer may recombine to form cyclo-S 8 when the temperature is cooling down from 600 °C to room temperature. Due to the high solubility of CS 2 to S 8 , the surface S 8 can be removed using CS 2 solvent at ease. Here, we demonstrate that the RGO/S composites with 52% S loading show high capacities and long cycling stabilities. The CS 2 -wash treatment can further enhance the cycling stability of RGO/S composites. Almost no capacity decline can be observed for CS 2 -washed RGO/S composites in 225 cycles. Figure 2 a shows the X-ray diffraction (XRD) patterns of pure S, pristine graphite, GO and RGO/S composite. The XRD pattern of S exhibits very sharp and strong peaks throughout the entire diffraction range, indicating a well-defi ned crystal S 8 structure. Graphite exhibits a sharp peak at 2 θ = 26.6° corresponding to the diffraction of (002) plane with interlayer distance of ≈0.34 nm. [ 33 ]
Plasma membranes can sense the stimulations and transmit the signals from extracellular environment and then make further responses through changes in locations, shapes or morphologies. Common fluorescent membrane markers are not well suited for long time tracking due to their shorter retention time inside plasma membranes and/or their lower photostability. To this end, we develop a new bipolar marker, Mem-SQAC, which can stably insert into plasma membranes of different cells and exhibits a long retention time over 30 min. Mem-SQAC also inherits excellent photostability from the BODIPY dye family. Large two-photon absorption cross sections and long wavelength fluorescence emissions further enhance the competitiveness of Mem-SQAC as a membrane marker. By using Mem-SQAC, significant morphological changes of plasma membranes have been monitored during heavy metal poisoning and drug induced apoptosis of MCF-7 cells; the change tendencies are so distinctly different from each other that they can be used as indicators to distinguish different cell injuries. Further on, the complete processes of endocytosis toward Staphylococcus aureus and Escherichia coli by RAW 264.7 cells have been dynamically tracked. It is discovered that plasma membranes take quite different actions in response to the two bacteria, information unavailable in previous research reports.
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