RESUMEN

The optical density of pristine graphene is high and broad in the near infrared region of the electromagnetic spectrum positioning this material as a highly efficient photothermal agent for in vivo applications. In this study, surfactant assisted exfoliated graphene was incorporated within bulk lipid samples of varying lipid types: glyceryl monoether, glyceryl monooleate and phytantriol. The pristine graphene sheets did not disrupt the packing of the liquid crystals while being in sufficiently intimate contact to provide localized heating and induce phase transitions. The phase progressions induced through heating using NIR irradiation of the entrained graphene particles within the bulk liquid crystal were studied using SAXS and confirmed using polarized optical microscopy. Increases in apparent temperature experienced by the matrix of up to 50 °C were observed by establishing a SAXS versus bulk temperature calibration curve allowing in situ measurements. The studies demonstrate the potential for use of graphene as a photothermal actuator across a range of lipid based systems of interest in controlled drug delivery.

RESUMEN

Pristine graphene particles prepared using an aqueous phase exfoliation technique have been used to promote the stabilization of emulsions through adsorption at the oil-water interface. Highly localized phase separation of these ultrastable emulsions could, however, be induced through photothermal heating of the graphene particles at the interface exposed to near-infrared light. The graphene wettability, which is a key determinant in preventing droplet coalescence was altered through the adsorption of nonionic block copolymer surfactants. Varying the aqueous solution conditions influenced the hydration of the hydrophilic component of the surfactant providing a further opportunity to alter the overall particle wettability and, hence, stability of the emulsion. In this way, highly stable-oil-in water emulsions were produced with decane; however, water-in-oil emulsions were formed with toluene as the oil phase.

RESUMEN

Liquid-air foams have been stabilised using a suspension of graphene particles at very low particle loadings. The suspension was prepared through the liquid phase exfoliation of graphite in the presence of the non-ionic tri-block surfactant, Pluronic® F108. The graphene particles possess an extremely high aspect ratio, with lateral dimensions of between 0.1 and 1.3 µm as evidenced by TEM imaging. The particles were shown to exhibit a number of other properties known to favour stabilisation of foam structures. Particle surface activity was confirmed through surface tension measurements, suggesting the particles favour adsorption at the air-water interface. The evolution of bubble size distributions over time indicated the presence of particles yielded improvements to foam stability due to a reduction in disproportionation. Foam stability measurements showed a non-linear relationship between foam half-life and graphene concentration, indicative of the rate at which particles adsorb at bubble surfaces. The wettability of the graphene particles was altered upon addition of alkali metal chlorides, with the stability of the foams being enhanced according to the series Na(+)>Li(+)>K(+)>Cs(+). This effect is indicative of the relative hydration capacity of each salt with respect to the surfactant, which is adsorbed along the graphene plane as a result of the exfoliation process. Thus, surfactant exfoliated graphene particles exhibit a number of different features that demonstrate efficient application of high-aspect ratio particles in the customisation and enhancement of foams.

RESUMEN

Pristine graphene, its derivatives, and composites have been widely reported to possess antibacterial properties. Most of the studies simulating the interaction between bacterial cell membranes and the surface of graphene have proposed that the graphene-induced bacterial cell death is caused either by (1) the insertion of blade-like graphene-based nanosheets or (2) the destructive extraction of lipid molecules by the presence of the lipophilic graphene. These simulation studies have, however, only take into account graphene-cell membrane interactions where the graphene is in a dispersed form. In this paper, we report the antimicrobial behavior of graphene sheet surfaces in an attempt to further advance the current knowledge pertaining to graphene cytotoxicity using both experimental and computer simulation approaches. Graphene nanofilms were fabricated to exhibit different edge lengths and different angles of orientation in the graphene sheets. These substrates were placed in contact with Pseudomonas aeruginosa and Staphylococcus aureus bacteria, where it was seen that these substrates exhibited variable bactericidal efficiency toward these two pathogenic bacteria. It was demonstrated that the density of the edges of the graphene was one of the principal parameters that contributed to the antibacterial behavior of the graphene nanosheet films. The study provides both experimental and theoretical evidence that the antibacterial behavior of graphene nanosheets arises from the formation of pores in the bacterial cell wall, causing a subsequent osmotic imbalance and cell death.

Asunto(s)

RESUMEN

A method for preparing hydrogen bonded multilayer thin films comprised of layer pairs of surfactant stabilized graphene and an anionic polyelectrolyte is described. The films were constructed at low pH using the Layer-By-Layer (LbL) technique, where the adsorption of the cationic polyelectrolyte, polyethyleneimine (PEI) is followed by the sequential alternating adsorption of the anionic polyelectrolyte, polyacrylic acid (PAA) and anionic graphene sheets modified with Pluronic® F108, a polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO) surfactant. Quartz Crystal Microbalance (QCM) measurements indicate that film formation was driven by hydrogen bonding between the carboxylic acid group of the PAA and ethylene oxide unit present in the surfactant. QCM measurements and Raman spectra showed evidence of non-linear and linear growth at low and high numbers of adsorbed layers respectively, suggesting overall superlinear film growth. Atomic Force Microscopy (AFM) Quantitative Nanomechanical Mapping (QNM) measurements of the films indicated that the reduced Young's Modulus of the films decreased with increasing numbers of adsorbed layers, reaching a bulk value of 6.07-32.3 MPa for samples with greater than 300 layers of surfactant stabilized graphene and PAA. The films were also shown to deteriorate partially with aqueous solutions at neutral and basic pH. The thin films exhibited features advantageous for use in coatings, such as pH responsiveness in addition to different mechanical properties, surface roughness, and internal structures based on the number of layers adsorbed.

RESUMEN

Lipid packing is intimately related to the geometry of the lipids and the forces that drive self-assembly. Here, the photothermal response of a cubic liquid-crystalline phase formed using phytantriol in the presence of low concentrations of pristine graphene was evaluated. Small-angle X-ray scattering showed the reversible phase changes from cubic to hexagonal to micellar due to localized heating through irradiation with near-infrared (NIR) light and back to cubic after cooling.

Asunto(s)

RESUMEN

Research into the structure, properties and applications of graphene has moved at a tremendous pace over the past few years. This review describes one aspect of this research, that of the incorporation of graphene particles with a range of polymers to create novel hybrid materials with increased functionality such as improved conductance, increased strength and introduced biocompatibility or cytotoxicity. This review focuses on dispersing graphene in polymer matrices, both insulating and conducting. Additionally, a brief discussion of carbon based platelet production methods is given in order to provide context on the subsequent use of this family of materials such as graphene, graphene oxide (GO) and reduced graphene oxide (rGO) incorporated into polymeric thin films.

RESUMEN

A method for the modification of surface properties through the deposition of stabilized graphene nanosheets is described. Here, the thickness of the film is controlled through the use of the layer-by-layer technique, where the sequential adsorption of the cationic polyethyleneimine (PEI) is followed by the adsorption of anionic graphene sheets modified with layers of polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO) surfactants. The graphene particles were prepared using the surfactant-assisted liquid-phase exfoliation technique, with the low residual negative charge arising from edge defects. The buildup of the multilayer assembly through electrostatic interactions was strongly influenced by the solution conditions, including pH, ionic strength, and ionic species. Thereby, not only could the thickness of the film be tailored through the choice of the number of bilayers deposited but the viscoelastic properties of the film could also be modified by changing solution conditions at which the different species were deposited. The quartz crystal microbalance was used to measure the mass of graphene and polyelectrolyte immobilized at the interface as well as to probe the energy dissipated in the adsorbed layer.

RESUMEN

The adsorption isotherms and aggregate structures of adsorbed surfactants on smooth thin-film surfaces of mineral oxides have been studied by optical reflectometry and atomic force microscopy (AFM). Films of the mineral oxides of titania, alumina, hafnia, and zirconia were produced by atomic layer deposition (ALD) with low roughness. We find that the surface strongly influences the admicelle organization on the surface. At high concentrations (2 × cmc) of cetyltrimethylammonium bromide (CTAB), the surfactant aggregates on a titania surface exhibit a flattened admicelle structure with an average repeat distance of 8.0 ± 1.0 nm whereas aggregates on alumina substrates exhibit a larger admicelle with an average separation distance of 10.5 ± 1.0 nm. A wormlike admicelle structure with an average separation distance of 7.0 ± 1.0 nm can be observed on zirconia substrates whereas a bilayered aggregate structure on hafnia substrates was observed. The change in the surface aggregate structure can be related to an increase in the critical packing parameter through a reduction in the effective headgroup area of the surfactant. The templating strength of the surfaces are found to be hafnia > alumina > zirconia > titania. Weakly templating surfaces are expected to have superior biocompatibility.

RESUMEN

Single and few layer molybdenum disulfide (MoS2) was exfoliated from the bulk form through a liquid phase exfoliation procedure. Highly concentrated suspensions were prepared that were stabilized against reaggregation through adsorption of nonionic polymers to the sheet surface. These exfoliated particles showed strong photoluminescence at an energy of 1.97 eV which is in the visible-light region. These exfoliated MoS2 sheets were then used to catalyze the degradation of a model dye upon exposure to visible light.

RESUMEN

Single and few layered tungsten disulphide (WS2) nanoparticles were prepared using a surfactant assisted ultrasonication exfoliation technique with concentrations of up to 0.4 mg/mL. The lateral dimension of the particles was in the range of 100-250 nm. The exfoliated WS2 was stabilised against re-aggregation through adsorption of a tri-block non-ionic polymeric surfactant (PEO-PPO-PEO). These nanoparticles were characterised by absorption, Raman and photoluminescence spectroscopy (PL). Broadening of the E2g peak in the Raman spectrum was observed due to phonon confinement within a single layer of WS2. The exfoliated particles have significantly different properties than the bulk WS2 material, in particular, the emergence of strong photoluminescence at 1.97 eV in energy coincidental with the excitonic peak in the UV-Vis spectrum. The emergent PL emission suggests that the monolayer WS2 is a direct gap material analogous to other dichalcogenides such as MoS2.

RESUMEN

Highly concentrated suspensions of graphene stabilized with surfactant were prepared using ultrasonic exfoliation. Concentrations of up to 1.5% w/w (15 mg/mL) were achieved through the continuous addition of the surfactant during the exfoliation process. Previous methods typically add the surfactant only once, prior to the commencement of sonication. The vast increase in the available solid-liquid interfacial area through delamination results in the rapid depletion of the surfactant from solution through adsorption. This leads to a change in the liquid-vapor surface tension outside of the optimum range for the efficient production of graphene sheets. By continuously replacing the surfactant to lower the surface tension during sonication and the production of the graphene surface area, the concentration of particles was significantly increased. Cationic, anionic, and nonionic surfactants were studied and all showed significant increases in the concentration of graphene produced using this continuous addition method.

RESUMEN

The adsorption of nonionic surfactants such as poly(ethylene oxide) alkyl ether surfactant (C(n)E(m)) and polysorbate (commonly referred to as Tween) was studied at the titania-water interface using optical reflectometry and atomic force microscopy (AFM). Previous reports have indicated little to no adsorption of these surfactants to titania, however under certain conditions, the surface excess was high. Typically significant adsorption was only observed when the titania surface was not strongly hydrated, that is, at point of zero charge and under low ionic strength conditions. For these amorphous titania surfaces prepared using atomic layer deposition, the pzc was at pH 5.1. Furthermore, the adsorbed amount of nonionic surfactant decreased with increasing ionic strength. This was attributed to the increased hydration of the titania interface from the specific adsorption of ions inhibiting the adsorption of the strongly hydrated ethylene oxide headgroup of the surfactant. AFM force measurements at the pzc in the presence of the surfactant as a function of concentration confirmed adsorption of the surfactant. Furthermore, soft contact imaging suggests that there was a spherical aggregate adsorbed layer structure above the critical surface aggregation concentration at pH 5.1 and no additional background electrolyte. No structure was observed at lower concentrations or under other solution conditions.

RESUMEN

Exfoliated graphene particles stabilised by the cationic polyelectrolyte polyethyleneimine (PEI) were used in conjunction with an anionic polyelectrolyte, poly(acrylic acid), to construct multilayers using the layer-by-layer technique on a silica substrate. In the first adsorption step, the surface excess of the cationic graphene was dependent on the overall charge on the nanoparticle which in turn can be tuned through modifying solution pH as PEI has weakly ionisable charged amine groups. The adsorbed amount onto the silica surface increased as the solution pH increased. Subsequently, a layer of PAA was adsorbed on top of the cationic graphene through electrostatic interaction. The multilayer could be assembled through this alternate deposition, with the influence of solution conditions investigated. The pH of the adsorbing solutions was the chief determinant of the overall adsorbed amounts, with more mass added at the elevated pH of 9 in comparison with pH 4. Atomic force microscopy confirmed that the graphene particles were adsorbed to the silica interface and that the surface coverage of the disc-like nanoparticles was complete after the deposition of five graphene-polyelectrolyte bi-layers. Furthermore, the graphene nanoparticles themselves could be modified through the consecutive addition of the oppositely charged polymers. A multilayered assembly of negatively charged graphene sheets modified with a bi-layer of PEI and PAA was also deposited on a silica surface with adsorbed PEI.

RESUMEN

Stable graphene suspensions were prepared through ultrasonic exfoliation followed by surface modification with the cationic polyelectrolyte poly(ethyleneimine) (PEI). The stability of the suspensions was found to be dependent upon the pH of the solution and the molecular weight of the PEI adsorbed. For the graphene sheets with adsorbed PEI with a molecular weigh of 600 Da, the particles were stabilised through an increased electrostatic repulsion at low pH inferred from in an increase in the measured zeta potential of the particles. However, the graphene with higher molecular weight PEI (70 kDa) was stable over a comparatively larger pH range through a combination of electrostatic repulsion at low pH and steric repulsion at elevated pH. Thus, solution conditions allowing the control of the colloidal sized graphene particles can be easily tuned through judicious management of solution conditions as well as polymer layer properties.

RESUMEN

The adsorption of nonionic surfactants to the silica-water and cellulose-water interfaces was studied using optical reflectometry (OR) and soft-contact atomic force microscopy imaging. The polyethylene oxide alkyl ethers C(14)E(6) and C(16)E(8) were shown to readily adsorb to both interfaces. The kinetics of the adsorption process as well as the equilibrium surface excess was determined using OR. In agreement with previous studies, the short headgroup surfactant C(14)E(6) adsorbed to a greater extent than the longer headgroup surfactant C(16)E(8) on silica. This trend was also observed for the cellulose-water interface. The structure of the adsorbed surfactant layer above the critical surface aggregation concentration (csac) was visualized using the soft contact imaging technique for both interfaces. On the silica surface, the layer structure for both surfactants mostly showed spherical aggregates, however, with some elongation into rods being more prevalent for C(14)E(6). Similar structures were observed on the cellulose surface, but imaging was more difficult due to the soft gel-like nature of the cellulose thin film in water. This suggests that these surfactants adsorb in a cooperative fashion above the critical micelle concentration (cmc) with a similar interaction between surfactant headgroup and surface for both silica and cellulose. No evidence was seen for the penetration of surfactant molecules into the cellulose surface or any solubilization of the interface.

RESUMEN

The interaction forces between silica surfaces with adsorbed layers of polyethyleneimine (PEI) were measured using colloidal probe microscopy as a function of solution conditions and PEI molecular weight. The surface coverage of polymer, as determined from optical reflectometry, was a significant factor influencing the extension of the chains away from the interface and also the development of adhesion between the surfaces. For the high molecular weight PEI (70 kDa), the adhesion passed through a maximum as a function of pH. The magnitude and position of this maximum was dependent on the surface excess of adsorbed PEI. The greatest adhesion was observed for the highest surface coverage of 0.88 mg m(-2) at pH 11 where the force-distance curves on approach indicated the presence of a significant steric layer. Furthermore, the forces on separation under these conditions indicated strong bridging of PEI chains across the interface contributing to the enhanced adhesion. However, at lower surface coverage, no bridging was observed but the adhesion was still significantly greater than in the absence of an adsorbed layer of PEI. The adhesion at lower surface coverage was indicative of a charge-patch mechanism. The measured values of adhesion correlated very well with the observed yield stress of concentrated dispersions of silica in the presence of adsorbed layers of PEI. Thus, the molecular mechanisms probed during surface forces measurements can be used to predict the ensemble behaviour of the many particles dispersed in an aqueous medium which is of particular importance in minerals processing.

RESUMEN

The surface energy of lignin films spin-coated onto oxidized silicon wafer has been determined from contact angle measurements of different test liquids with varying polar and dispersive components. Three different lignin raw materials were used, a kraft lignin from softwood, along with milled wood lignin from softwood and hardwood. Infrared and (31)P NMR spectroscopy was used to identify any major functional group differences between the lignin samples. No significant difference in the total solid-vapor surface energy for the different lignin films was observed; however, the polar component for the kraft lignin was much greater than for either of the milled wood lignin samples consistent with the presence of carboxyl groups and higher proportion of phenolic hydroxyl groups as shown by quantitative (31)P NMR on the phosphitylated samples. Furthermore, the total surface energy of lignin of 53-56 mJ m(-2) is of a similar magnitude to cellulose, also found in the wood cell wall; however, cellulose has a higher polar component leading to a lower contact angle with water and greater wettability than the milled wood lignin. Although lignin is not hydrophobic according to the strictest definition of a water contact angle greater than 90 degrees, water may only be considered a partially wetting liquid on a lignin surface. This supports the long-held belief that one of the functions of lignin in the wood cell wall is to provide water-proofing to aid in water transport. Furthermore, these results on the solid-vapor surface energy of lignin will provide invaluable insight for many natural and industrial applications including in the design and manufacture of many sustainable products such as paper, fiberboard, and polymer composite blends.

RESUMEN

The structure of adsorbed aggregates of the cationic surfactant cetyltrimethylammonium bromide (C(16)TAB) at the cellulose-water interface was determined using soft-contact atomic force microscopy imaging. C(16)TAB was adsorbed to the surface from a solution with a concentration above the critical micelle concentration. Imaging of the surfactant aggregate layer showed predominantly spherical micellar structures at the cellulose-water interface with some areas of short rodlike aggregates. These structures are similar to those previously observed for C(16)TAB adsorbed to other hydrophilic surfaces such as silica; hence, a similar mechanism for the arrangement of C(16)TAB on cellulose is suggested, in agreement with previous neutron reflectivity data. The buildup of the surfactant layer proceeds via direct micelle adsorption to the uncharged cellulose surface. This occurs through polar interactions between the C(16)TAB headgroup and the hydrophilic substrate to form the observed admicelle structures.

RESUMEN

Typically, the adhesion between cellulose surfaces under aqueous conditions is very poor. Often, adsorbed polymers such as polyvinylamine (PVAm) are used to increase the wet strength; however, this provides only a minimal increase in the adhesion energy. Here, the adhesion between cellulose surfaces with adsorbed layers of phenylboronic acid derivatized polyvinylamine has been studied using colloidal probe microscopy as a function of pH. The adhesion due to the phenylboronic acid (PBA) groups grafted on the polyvinylamine backbone is almost 30 times greater, providing a new, exciting class of polymers using covalent linkages to improve the strength of the joint between cellulose surfaces. The measured surface forces on approach provided key information on the molecular conformation of the polymers at the cellulose-solution interface. At low pH, the three polymers tested, PVAm, PVAm-Ph (with pendant phenol groups), and PVAm-PBA (with phenylboronic acid groups) all had a relatively flat conformation at the interface, which is in agreement with the predictions based upon theory for highly charged polyelectrolytes adsorbing to an oppositely charged interface. With increasing pH, the charge on the polymers is reduced, eventually resulting in a more expansive conformation at the interface at pH 10 and above with the development of a steric interaction force. The onset of this steric force correlates well with the observed significant increase in the pull-off force upon separation of the cellulose surfaces. Furthermore, a greater increase in the adhesion was observed for PVAm-PBA in agreement with previous studies using macroscopic cellulose surfaces. This is attributed to the formation of boronic acid esters between the polymer and the cis diol groups on the cellulose surface.