Showing papers in "Langmuir in 2017"

Benchmarking Cellulose Nanocrystals: From the Laboratory to Industrial Production

Abstract: The renewability, biocompatibility, and mechanical properties of cellulose nanocrystals (CNCs) have made them an attractive material for numerous composite, biomedical, and rheological applications. However, for CNCs to shift from a laboratory curiosity to commercial applications, researchers must transition from CNCs extracted on the bench scale to material produced on an industrial scale. There are a number of companies currently producing kilogram to ton per day quantities of sulfuric acid-hydrolyzed CNCs as well as other nanocelluloses, as described herein. With the recent intensification of industrially produced CNCs and the variety of cellulose sources, hydrolysis methods, and purification procedures, the characterization of these materials becomes critical. This has further been justified by the past two decades of research that demonstrate that the CNC stability and behavior are highly dependent on the surface chemistry, surface charge density, and particle size. This work outlines key test method...

Janus Particle Synthesis, Assembly, and Application

Abstract: Janus particles are colloidal particles with more than a single type of surface chemistry or composition, ranging in size from hundreds of nanometers to a few micrometers. Like traditional colloids, they are large enough to be observed under optical microscopy in real time and small enough to diffuse by Brownian motion, but their interesting and useful new properties of directional interaction bring new research opportunities to the fields of soft matter and fundamental materials research as well as to applications in other disciplines and in technologies such as electronic paper and other multiphase engineering. In this review, a variety of methods that have been used to synthesize Janus particles are introduced. Following this, we summarize the use of Janus particles as basic units that assemble into novel structures and tune important material properties. The concluding sections highlight some of the technological applications, including recent progress in using Janus particles as microprobes, micromot...

Competition between Hydrogen Evolution and Carbon Dioxide Reduction on Copper Electrodes in Mildly Acidic Media.

Abstract: Understanding the competition between hydrogen evolution and CO2 reduction is of fundamental importance to increase the faradaic efficiency for electrocatalytic CO2 reduction in aqueous electrolytes. Here, by using a copper rotating disc electrode, we find that the major hydrogen evolution pathway competing with CO2 reduction is water reduction, even in a relatively acidic electrolyte (pH 2.5). The mass-transport-limited reduction of protons takes place at potentials for which there is no significant competition with CO2 reduction. This selective inhibitory effect of CO2 on water reduction, as well as the difference in onset potential even after correction for local pH changes, highlights the importance of differentiating between water reduction and proton reduction pathways for hydrogen evolution. In-situ FTIR spectroscopy indicates that the adsorbed CO formed during CO2 reduction is the primary intermediate responsible for inhibiting the water reduction process, which may be one of the main mechanisms b...

Highly Efficient Red-Emitting Carbon Dots with Gram-Scale Yield for Bioimaging.

Abstract: Carbon dots (CDs) are a new class of photoluminescent (PL), biocompatible, environment-friendly, and low-cost carbon nanomaterials. Synthesis of highly efficient red-emitting carbon dots (R-CDs) on a gram scale is a great challenge at present, which heavily restricts the wide applications of CDs in the bioimaging field. Herein, R-CDs with a high quantum yield (QY) of 53% are produced on a gram scale by heating a formamide solution of citric acid and ethylenediamine. The as-prepared R-CDs have an average size of 4.1 nm and a nitrogen content of about 30%, with an excitation-independent emission at 627 nm. After detailed characterizations, such strong red fluorescence is ascribed to the contribution from the nitrogen- and oxygen-related surface states and the nitrogen-derived structures in the R-CD cores. Our R-CDs show good photostability and low cytotoxicity, and thus they are excellent red fluorescence probes for bioimaging both in vitro and in vivo.

Colloidal Particles at a Range of Fluid–Fluid Interfaces

Abstract: The study of solid particles residing at fluid–fluid interfaces has become an established area in surface and colloid science recently, experiencing a renaissance since around 2000. Particles at interfaces arise in many industrial products and processes such as antifoam formulations, crude oil emulsions, aerated foodstuffs, and flotation. Although they act in many ways like traditional surfactant molecules, they offer distinct advantages also, and the area is now multidisciplinary, involving research in the fundamental science and potential applications. In this Feature Article, the flavor of some of this interest is given on the basis of recent work from our own group and includes the behavior of particles at oil–water, air–water, oil–oil, air–oil, and water–water interfaces. The materials capable of being prepared by assembling various kinds of particles at fluid interfaces include particle-stabilized emulsions, particle-stabilized aqueous and oil foams, dry liquids, liquid marbles, and powdered emulsions.

Comparison of MoS2, WS2, and Graphene Oxide for DNA Adsorption and Sensing

Abstract: Interfacing DNA with two-dimensional (2D) materials has been intensely researched for various analytical and biomedical applications. Most of these studies have been performed on graphene oxide (GO) and two metal dichalcogenides, molybdenum disulfide (MoS2) and tungsten disulfide (WS2); all of them can all adsorb single-stranded DNA. However, they use different surface forces for adsorption based on their chemical structures. In this work, fluorescently labeled DNA oligonucleotides were used and their adsorption capacities and kinetics were studied as a function of ionic strength, DNA length, and sequence. Desorption of DNA from these surfaces was also measured. DNA is more easily desorbed from GO by various denaturing agents, whereas surfactants yield more desorption from MoS2 and WS2. Our results are consistent with the fact that DNA can be adsorbed by GO via π–π stacking and hydrogen bonding, and MoS2 and WS2 mainly use van der Waals force for adsorption. Finally, fluorescent DNA probes were adsorbed b...

Improving the Stability and Size Tunability of Cesium Lead Halide Perovskite Nanocrystals Using Trioctylphosphine Oxide as the Capping Ligand

Abstract: Recently, all-inorganic cesium lead halide (CsPbX3, X = Cl, Br, and I) nanocrystals (NCs) have drawn wide attention because of their excellent optoelectronic properties and potential applications. However, one of the most significant challenges of such NCs is their low stability against protonic solvents. In this work, we demonstrate that by incorporating a highly branched capping ligand, trioctylphosphine oxide (TOPO), into the traditional oleic acid/oleylamine system, monodisperse CsPbX3 NCs with excellent optoelectronic properties can be achieved at elevated temperatures (up to 260 °C). The size of such NCs can be varied in a relatively wide range. The capping of TOPO on NCs has been verified through Fourier transform infrared spectroscopy measurement. More importantly, the presence of TOPO can dramatically improve the stability of CsPbX3 NCs against ethanol treatment. After ethanol treatment for 100 min, the emission intensity of the TOPO-capped sample dropped only 5%, whereas that of non-TOPO-capped ...

One-Pot Hydrothermal Synthesis of Carbon Dots with Efficient Up- and Down-Converted Photoluminescence for the Sensitive Detection of Morin in a Dual-Readout Assay.

Abstract: Blue luminescent carbon dots (CDs) with a high photoluminescence (PL) quantum yield (48.3 ± 5.3%) were prepared by the one-pot hydrothermal reaction of citric acid with poly(ethylenimine) (PEI). The CDs display bright PL, narrow emission spectra, pH-dependent PL intensity, high photostability, and up-converted luminescence. The CDs exhibit a quenching of both down- and up-conversion PL in the presence of morin and thus serve as useful probes for morin detection. Both down- and up-conversion measurements allow the quantification of concentrations from 0 to 300 μmol/L with a detection limit of 0.6 μmol/L, and this dual-mode detection increases the reliability of the measurement. The proposed method of determination is simple, sensitive, and cost-effective, with potential applications in clinical and biochemical assays.

Nanofiltration Membrane with a Mussel-Inspired Interlayer for Improved Permeation Performance.

Abstract: A mussel-inspired interlayer of polydopamine (PDA)/polyethylenimine (PEI) is codeposited on the ultrafiltration substrate to tune the interfacial polymerization of piperazine and trimesoyl chloride for the preparation of thin-film composite (TFC) nanofiltration membranes (NFMs). This hydrophilic interlayer results in an efficient adsorption of piperazine solution in the substrate pores. The solution height increases with the PDA/PEI codeposition time from 45 to 135 min due to the capillary effect of the substrate pores. The prepared TFC NFMs are characterized with thin and smooth polyamide selective layers by ATR/IR, XPS, FESEM, AFM, zeta potential, and water contact angle measurements. Their water permeation flux measured in a cross-flow process increases to two times as compared with those TFC NFMs without the mussel-inspired interlayer. These TFC NFMs also show a high rejection of 97% to Na2SO4 and an salt rejection order of Na2SO4 ≈ MgSO4 > MgCl2 > NaCl.

Liquid Marbles, Elastic Nonstick Droplets: From Minireactors to Self-Propulsion.

Abstract: Liquid marbles are nonstick droplets wrapped by micro- or nanometrically scaled colloidal particles, representing a platform for a variety of chemical, biological, and microfluidics applications. Liquid marbles demonstrate elastic properties and do not coalesce when bounced or pressed. The effective surface tension and Young modulus of liquid marbles are discussed. Physical sources of the elasticity of liquid marbles are considered. Liquids and powders used for the fabrication of liquid marbles are surveyed. This feature article reviews properties and applications of liquid marbles. Liquid marbles demonstrate potential as microreactors, microcontainers for growing micro-organisms and cells, and microfluidics devices. The Marangoni-flow-driven self-propulsion of marbles supported by liquids is addressed.

Accurate Characterization of the Pore Volume in Microporous Crystalline Materials.

Abstract: Pore volume is one of the main properties for the characterization of microporous crystals. It is experimentally measurable, and it can also be obtained from the refined unit cell by a number of computational techniques. In this work, we assess the accuracy and the discrepancies between the different computational methods which are commonly used for this purpose, i.e, geometric, helium, and probe center pore volumes, by studying a database of more than 5000 frameworks. We developed a new technique to fully characterize the internal void of a microporous material and to compute the probe-accessible and -occupiable pore volume. We show that, unlike the other definitions of pore volume, the occupiable pore volume can be directly related to the experimentally measured pore volumes from nitrogen isotherms.

Solid–Liquid Work of Adhesion

Abstract: We establish a tool for direct measurements of the work needed to separate a liquid from a solid. This method mimics a pendant drop that is subjected to a gravitational force that is slowly increasing until the solid–liquid contact area starts to shrink spontaneously. The work of separation is then calculated in analogy to Tate’s law. The values obtained for the work of separation are independent of drop size and are in agreement with Dupre’s theory, showing that they are equal to the work of adhesion.

Revisiting Nitrogen Species in Covalent Triazine Frameworks.

Abstract: Covalent triazine frameworks (CTFs) are porous organic materials promising for applications in catalysis and separation due to their high stability, adjustable porosity, and intrinsic nitrogen functionalities. CTFs are prepared by ionothermal trimerization of aromatic nitriles; however, multiple side reactions also occur under synthesis conditions, and their influence on the material properties is still poorly described. Here we report the systematic characterization of nitrogen in CTFs using X-ray photoelectron spectroscopy. With the use of model compounds, we could distinguish several types of nitrogen species. By combining these data with textural properties, we unravel the influence that the reaction temperature, the catalyst, and the monomer structure and composition have on the properties of the resulting CTF materials.

Understanding the Seed-Mediated Growth of Gold Nanorods through a Fractional Factorial Design of Experiments

Abstract: Since the development of simple, aqueous protocols for the synthesis of anisotropic metal nanoparticles, research into many promising, valuable applications of gold nanorods has grown considerably, but a number of challenges remain, including gold-particle yield, robustness to minor impurities, and precise control of gold nanorod surface chemistry. Herein we present the results of a composite fractional factorial series of experiments designed to screen seven additional potential avenues of control and to understand the seed-mediated silver-assisted synthesis of gold nanorods. These synthesis variables are the amount of sodium borohydride used and the rate of stirring when producing seed nanoparticles, the age of and the amount of seeds added, the reaction temperature, the amounts of silver nitrate and ascorbic acid added, and the age of the reduced growth solution before seed nanoparticles are added to initiate rod formation. This statistical experimental design and analysis method, besides determining w...

pH-Responsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with a Conventional Zwitterionic Surfactant.

Abstract: pH-responsive oil-in-water Pickering emulsions were prepared simply by using negatively charged silica nanoparticles in combination with a trace amount of a zwitterionic carboxyl betaine surfactant as stabilizer. Emulsions are stable to coalescence at pH ≤ 5 but phase separate completely at pH > 8.5. In acidic solution, the carboxyl betaine molecules become cationic, allowing them to adsorb on silica nanoparticles via electrostatic interactions, thus hydrophobizing and flocculating them and enhancing their surface activity. Upon increasing the pH, surfactant molecules are converted to zwitterionic form and significantly desorb from particles’ surfaces, triggering dehydrophobization and coalescence of oil droplets within the emulsion. The pH-responsive emulsion can be cycled between stable and unstable many times upon alternating the pH of the aqueous phase. The average droplet size in restabilized emulsions at low pH, however, increases gradually after four cycles due to the accumulation of NaCl. Experime...

Unraveling the Growth Mechanism of Silica Particles in the Stöber Method: In Situ Seeded Growth Model

Abstract: In this work, we investigated the kinetic balance between ammonia-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) and subsequent condensation over the growth of silica particles in the Stober method Our results reveal that, at the initial stage, the reaction is dictated by TEOS hydrolysis to form silanol monomers, which is denoted as pathway I and is responsible for nucleation and growth of small silica particles via condensation of neighboring silanol monomers and siloxane network clusters derived thereafter Afterward, the reaction is dictated by condensation of newly formed silanol monomers onto the earlier formed silica particles, which is denoted as pathway II and is responsible for the enlargement in size of silica particles When TEOS hydrolysis is significantly promoted, either at high ammonia concentration (≥095 M) or at low ammonia concentration in the presence of LiOH as secondary catalyst, temporal separation of pathways I and II makes the Stober method reminiscent of in situ seeded

Highly Efficiently Delaminated Single-Layered MXene Nanosheets with Large Lateral Size

Abstract: Single layered Ti3C2(OH)2 nanosheets have been successfully fabricated by etching its Ti3AlC2 precursor with KOH in the presence of a small amount of water. The OH group replaced the Al layer within the Ti3AlC2 structure during etching, and Ti3C2(OH)2 nanosheets could be easily and efficiently achieved through a simple washing process. The delaminated single-layered nanosheets are clearly revealed by atomic force microscopy to be several micrometers in lateral size. Interestingly, the exfoliated Ti3C2(OH)2 nanosheets could be restacked to form a new layer-structured material after drying. When redispersing this restacked Ti3C2(OH)2 materials in water again, it could be re-delaminated easily only after shaking for several hours. The easy delamination and restacking properties, coupled with intrinsic metallic conductivity and hydrophilicity, make it an ideal two-dimensional building block for fabricating a wide variety of functional materials.

NLDFT Pore Size Distribution in Amorphous Microporous Materials.

Abstract: The pore size distribution (PSD) is one of the most important properties when characterizing and designing materials for gas storage and separation applications. Experimentally, one of the current standards for determining microscopic PSD is using indirect molecular adsorption methods such as nonlocal density functional theory (NLDFT) and N2 isotherms at 77 K. Because determining the PSD from NLDFT is an indirect method, the validation can be a nontrivial task for amorphous microporous materials. This is especially crucial since this method is known to produce artifacts. In this work, the accuracy of NLDFT PSD was compared against the exact geometric PSD for 11 different simulated amorphous microporous materials. The geometric surface area and micropore volumes of these materials were between 5 and 1698 m2/g and 0.039 and 0.55 cm3/g, respectively. N2 isotherms at 77 K were constructed using Gibbs ensemble Monte Carlo (GEMC) simulations. Our results show that the discrepancies between NLDFT and geometric P...

CuSO4/H2O2-Triggered Polydopamine/Poly(sulfobetaine methacrylate) Coatings for Antifouling Membrane Surfaces

Abstract: Mussel-inspired polydopamine (PDA) coatings have been broadly exploited for constructing functional membrane surfaces. One-step codeposition of PDA with antifouling polymers, especially zwitterionic polymers, has been regarded as a promising strategy for fabricating antifouling membrane surfaces. However, one challenge is that the codeposition is usually a slow process over 10 h or even several days. Herein, we report that CuSO4/H2O2 is able to notably accelerate the codeposition process of PDA with poly(sulfobetaine methacrylate) (PSBMA). In our case, PSBMA is facilely anchored to the polypropylene microporous membrane (PPMM) surfaces within 1 h with the assistance of PDA because of its strong interfacial adhesion. The PDA/PSBMA-coated PPMMs show excellent surface hydrophilicity, high water permeation flux (7506 ± 528 L/m2·h at 0.1 MPa), and an outstanding antifouling property. Moreover, the antifouling property is maintained after the membranes are treated with acid and alkali solutions as well as organ...

SERS Quantification and Characterization of Proteins and Other Biomolecules.

Abstract: Changes in protein expression levels and protein structure may indicate genomic mutations and may be related to some diseases. Therefore, the precise quantification and characterization of proteins can be used for disease diagnosis. Compared with several other alternative methods, surface-enhanced Raman scattering (SERS) spectroscopy is regarded as an excellent choice for the quantification and structural characterization of proteins. Herein, we review the main advance of using plasmonic nanostructures as SERS sensing platform for this purpose. Three design approaches, including direct SERS, indirect SERS, and SERS-encoded nanoparticles, are discussed in the direction of developing new precise approaches of quantification and characterization of proteins. While this Review is focused on proteins, in order to highlight concepts of SERS-based sensors also detection of other biomolecules will be discussed.

Generation and Stability of Bulk Nanobubbles.

Abstract: Recently, extremely small bubbles, referred to as nanobubbles, have drawn increased attention due to their novel properties and great potential for various applications. In this study, a novel method for the generation of bulk nanobubbles (BNBs) was introduced, and stability of fabricated BNBs was investigated. BNBs were created from CO2 gas with a mixing method; the chemical identity and phase state of these bubbles can be determined via infrared spectroscopy. The presence of BNBs was observed with a nanoparticle tracking analysis (NTA). The ATR-FTIR spectra of BNBs indicate that the BNBs were filled with CO2 gas. Furthermore, the BNB concentration and its ζ-potential were about 2.94 × 108 particles/mL and −20 mV, respectively (24 h after BNB generation with a mixing time of 120 min). This indicates the continued existence and stability of BNBs in water for an extended period of time.

Controllable Synthesis of Mesoporous Sulfur-Doped Carbon Nitride Materials for Enhanced Visible Light Photocatalytic Degradation.

Abstract: Mesoporous sulfur-doped graphitic carbon nitride (MCNS) materials were successfully synthesized using thiourea as a low-cost precursor and SiO2 gel solution as a template through a simple thermal condensation method. The effects of three synthesis key factors, namely, the reaction temperature, the reaction time, and the weight ratio of SiO2/thiourea, and also their interactions on the removal rate of methyl orange (MO) were investigated using response surface methodology, and the samples were subjected to several characterization techniques. Results showed that the optimized physicochemical properties could be achieved for the MCNS samples by controlling the synthesis key factors, and it was found that the reaction temperature and the reaction time had significant influences on the MO photocatalytic removal. Among bulk graphitic carbon nitride (g-C3N4), CN (undoped g-C3N4), CNS (sulfur-doped g-C3N4 without template), and TiO2 (Degussa P25) samples, the optimized MCNS-4 illustrated the highest photocatalyt...

Biopolymer-Targeted Adsorption onto Halloysite Nanotubes in Aqueous Media

Abstract: Studies on the adsorption of biopolymers onto halloysite nanotubes (HNTs) in water were conducted. Three polymers with different charges—anionic (pectin), neutral (hydroxypropyl cellulose), and cationic (chitosan)—were chosen. The thermodynamic parameters for the adsorption of polymers onto the HNT surface were determined by isothermal titration calorimetry (ITC). The experimental data were interpreted based on a Langmuir adsorption model. The standard variations in free energy, enthalpy, and entropy of the process were obtained and discussed. Turbidimetry was used to evaluate the stability of functionalized nanoparticles in water. The ζ-potential clarified the surface charge properties of functionalized nanotubes upon polymer adsorption. The interaction of modified nanotubes with polymers led to the formation of a colloidal system with tunable stability and surface properties, which offers different perspectives on new applications of these dispersions, such as carriers for substances to be released in r...

Surface Interaction of Water-in-Oil Emulsion Droplets with Interfacially Active Asphaltenes

Abstract: Adsorption of interfacially active components at the water/oil interface plays critical roles in determining the properties and behaviors of emulsion droplets. In this study, the droplet probe atomic force microscopy (AFM) technique was applied, for the first time, to quantitatively study the interaction mechanism between water-in-oil (W/O) emulsion droplets with interfacially adsorbed asphaltenes. The behaviors and stability of W/O emulsion droplets were demonstrated to be significantly influenced by the asphaltene concentration of organic solution where the emulsions were aged, aging time, force load, contact time, and solvent type. Bare water droplets could readily coalesce with each other in oil (i.e., toluene), while interfacially adsorbed asphaltenes could sterically inhibit droplet coalescence and induce interfacial adhesion during separation of the water droplets. For low asphaltene concentration cases, the adhesion increased with increasing asphaltene concentration (≤100 mg/L), but it significant...

Mesoporous Silica Nanoparticles Capped with Graphene Quantum Dots for Potential Chemo–Photothermal Synergistic Cancer Therapy

Abstract: In this study, mesoporous silica nanoparticles (MSNs) have been successfully capped with graphene quantum dots (GQDs) to form multifunctional GQD–MSNs with the potential for synergistic chemo–photothermal therapy. The structure, drug-release behavior, photothermal effect, and synergistic therapeutic efficiency of GQD–MSNs to 4T1 breast cancer cells were investigated. The results showed that GQD–MSNs were monodisperse and had a particle size of 50–60 nm. Using doxorubicin hydrochloride (DOX) as a model drug, the DOX-loaded GQD–MSNs (DOX–GQD–MSNs) not only exhibited pH- and temperature-responsive drug-release behavior, but using near-infrared irradiation, they efficiently generated heat to kill cancer cells. Furthermore, GQD–MSNs were biocompatible and were internalized by 4T1 cells. Compared with chemotherapy and photothermal therapy alone, DOX–GQD–MSNs were much more effective in killing the 4T1 cells owing to a synergistic chemo–photothermal effect. Therefore, GQD–MSNs may have promising applications in ...

Fusogenic Liposomes as Nanocarriers for the Delivery of Intracellular Proteins.

Abstract: Direct delivery of proteins and peptides into living mammalian cells has been accomplished using phospholipid liposomes as carrier particles. Such liposomes are usually taken up via endocytosis where the main part of their cargo is degraded in lysosomes before reaching its destination. Here, fusogenic liposomes, a newly developed molecular carrier system, were used for protein delivery. When such liposomes were loaded with water-soluble proteins and brought into contact with mammalian cells, the liposomal membrane efficiently fused with the cellular plasma membrane delivering the liposomal content to the cytoplasm without degradation. To explore the key factors of proteofection processes, the complex formation of fusogenic liposomes and proteins of interest and the size and zeta potential of the formed fusogenic proteoliposoms were monitored. Intracellular protein delivery was analyzed using fluorescence microscopy and flow cytometry. Proteins such as EGFP, Dendra2, and R-phycoerythrin or peptides such as...

Characterization of the Intrinsic Water Wettability of Graphite Using Contact Angle Measurements: Effect of Defects on Static and Dynamic Contact Angles

Abstract: Elucidating the intrinsic water wettability of the graphitic surface has increasingly attracted research interests, triggered by the recent finding that the well-established hydrophobicity of graphitic surfaces actually results from airborne hydrocarbon contamination. Currently, static water contact angle (WCA) is often used to characterize the intrinsic water wettability of graphitic surfaces. In the current paper, we show that because of the existence of defects, static WCA does not necessarily characterize the intrinsic water wettability. Freshly exfoliated graphite of varying qualities, characterized using atomic force microscopy and Raman spectroscopy, was studied using static, advancing, and receding WCA measurements. The results showed that graphite of different qualities (i.e., defect density) always has a similar advancing WCA, but it could have very different static and receding WCAs. This finding indicates that defects play an important role in contact angle measurements, and the static contact...

Droplet Motion on a Shape Gradient Surface.

Abstract: We demonstrate a facile method to induce water droplet motion on an wedge-shaped superhydrophobic copper surface combining with a poly(dimethylsiloxane) (PDMS) oil layer on it. The unbalanced interfacial tension from the shape gradient offers the actuating force. The superhydrophobicity critically eliminates the droplet contact line pinning and the slippery PDMS oil layer lubricates the droplet motion, which makes the droplet move easily. The maximum velocity and furthest position of droplet motion were recorded and found to be influenced by the gradient angle. The mechanism of droplet motion on the shape gradient surface is systematically discussed, and the theoretical model analysis is well matched with the experimental results.

Synthesis of a Multifunctional Graphene Oxide-Based Magnetic Nanocomposite for Efficient Removal of Cr(VI)

Abstract: A novel magnetic nanocomposite was synthesized using graphene oxide (GO), polyethylenimine (PEI), and Fe3O4 to removal hexavalent chromium (Cr(VI)) from water and soil. Therein, GO was functionalized with plenty of −NH2 by the modification of PEI through an amidation reaction, and the resulting GO/PEI reacted with FeSO4·7H2O and NaBH4 to obtain RGO/PEI/Fe3O4 (the optimal one is designated as ORPF) through an oxidation–reduction reaction. ORPF could effectively adsorb Cr(VI) through electrostatic attraction, and the adsorbed Cr(VI) ions were partially reduced to trivalent chromium (Cr(III)) with low toxicity by RGO (π electron). Afterward, the resulting ORPF-Cr could be conveniently removed from water with a magnet, achieving the maximum Cr(VI) removal capacity of 266.6 mg/g. Importantly, ORPF, once carried by sponge particles, could efficiently remove Cr(VI) from soil, and the resulting mixture could be facilely collected with a magnet on a filter net. Besides, the leaching experiment suggested that, when...

1H NMR Shows Slow Phospholipid Flip-Flop in Gel and Fluid Bilayers

Abstract: We measured the transbilayer diffusion of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in large unilamellar vesicles, in both the gel (Lβ′) and fluid (Lα) phases. The choline resonance of headgroup-protiated DPPC exchanged into the outer leaflet of headgroup-deuterated DPPC-d13 vesicles was monitored using 1H NMR spectroscopy, coupled with the addition of a paramagnetic shift reagent. This allowed us to distinguish between the inner and outer bilayer leaflet of DPPC, to determine the flip-flop rate as a function of temperature. Flip-flop of fluid-phase DPPC exhibited Arrhenius kinetics, from which we determined an activation energy of 122 kJ mol–1. In gel-phase DPPC vesicles, flip-flop was not observed over the course of 250 h. Our findings are in contrast to previous studies of solid-supported bilayers, where the reported DPPC translocation rates are at least several orders of magnitude faster than those in vesicles at corresponding temperatures. We reconcile these differences by proposing a defect...