The behavior of graphene oxide trapped at the air water interface.

Graphene oxide is a derivate of graphene obtained by oxidation of graphite and other carbonaceous materials. The more accepted structure consists in carbonyl and carboxyl groups located at the edge of the graphene network and hydroxyl and epoxy groups attached to the basal plane. The percentage of O-groups depends on the synthesis route and the material used as carbon source. In addition, highly oxidized fragments, called oxidative debris, OD, are produced during the oxidation process. These fragments are adsorbed onto the graphene oxide network and can be removed by alkaline washing. The purified material has lower O/C ratio than graphene oxide and its properties are also quite different. Due to its structure, graphene oxide can be adsorbed at the air-water interface of the aqueous solution by diffusion, Gibbs monolayers, or by spreading on a clean water subphase resulting in a Langmuir film. This review is intended to provide information on the importance of controlling the chemical composition, structure, size, and oxidative debris, on the manufacture of graphene oxide films. To this end the review shows the influence of the synthesis route and the starting material on the structure of graphene oxide and analyzes several examples of the behavior and properties of films prepared with different types of graphene oxides. The great variability of behaviors of graphene oxide films caused by the different structure of this material provides a great opportunity to fine-tune the properties of films according to the needs of different applications.

Contribution of resonance energy transfer to the luminescence quenching of upconversion nanoparticles with graphene oxide.

Upconversion nanoparticles (UCNP) are increasingly used due to their advantages over conventional fluorophores, and their use as resonance energy transfer (RET) donors has permitted their application as biosensors when they are combined with appropriate RET acceptors such as graphene oxide (GO). However, there is a lack of knowledge about the design and influence that GO composition produces over the quenching of these nanoparticles that in turn will define their performance as sensors. In this work, we have analysed the total quenching efficiency, as well as the actual values corresponding to the RET process between UCNPs and GO sheets with three different chemical compositions. Our findings indicate that excitation and emission absorption by GO sheets are the major contributor to the observed luminescence quenching in these systems. This challenges the general assumption that UCNPs luminescence deactivation by GO is caused by RET. Furthermore, RET efficiency has been theoretically calculated by means of a semiclassical model considering the different nonradiative energy transfer rates from each Er3+ ion to the GO thin film. These theoretical results highlight the relevance of the relative positions of the Er3+ ions inside the UCNP with respect to the GO sheet in order to explain the RET-induced efficiency measurements.

Development and In Vitro Evaluation of a Novel Drug Delivery System (Albumin Microspheres Containing Liposomes) Applied to Vancomycin.

A pharmaceutical vehicle based on the encapsulation of liposomes with unmodified albumin has been designed, formulated, and in vitro characterized. Microscopy was used to investigate particle morphology and dynamic light scattering to determine the size and zeta potential. Vancomycin was selected as a model drug for water-soluble and moderately albumin-bound products. The results indicated that regardless of the zeta potential of the liposomes these can be trapped within albumin microspheres. The zeta potential, drug entrapment efficacy, and drug delivery profile of the resulting microspheres were found to depend on the liposome composition and the conditions of flocculation. The protein concentration was observed to influence drug entrapment efficiency (from 13.17 ± 5.0% to 61.27 ± 4.54%), as did the zeta potential of the microspheres, which was also seen to depend on the initial charge of the liposomes. The relationship between the microsphere zeta potential or entrapment efficacy and the protein concentration used for flocculation was established. Regarding drug delivery, differences between microspheres prepared from cationic or anionic liposomes were observed. The combination of liposome versatility together with the drug-binding ability of albumin provides to a vehicle with multiple choices for theranostic delivery.

The role of oxidative debris on graphene oxide films.

We study the effect of oxidative impurities on the properties of graphene oxide and on the graphene oxide Langmuir-Blodgett films (LB). The starting material was grupo Antolín nanofibers (GANF) and the oxidation process was a modified Hummers method to obtain highly oxidized graphene oxide. The purification procedure reported in this work eliminated oxidative impurities decreasing the thickness of the nanoplatelets. The purified material thus obtained presents an oxidation degree similar to that achieved by chemical reduction of the graphite oxide. The purified and non-purified graphene oxides were deposited onto silicon by means of a Langmuir-Blodgett (LB) methodology. The morphology of the LB films was analyzed by field emission scanning microscopy (FE-SEM) and micro-Raman spectroscopy. Our results show that the LB films built by transferring Langmuir monolayers at the liquid-expanded state of the purified material are constituted by close-packed and non-overlapped nanoplatelets. The isotherms of the Langmuir monolayer precursor of the LB films were interpreted according to the Volmer's model.

Microrheology and characteristic lengths in wormlike micelles made of a zwitterionic surfactant and SDS in brine.

We study the Brownian motion of probe particles embedded in a wormlike micellar fluid made of a zwitterionic surfactant N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (TDPS), sodium dodecyl sulfate (SDS), and salty water to get structural and dynamical information of the micellar network. The motion of the probe particles was tracked with diffusing wave spectroscopy, and the mean square displacement as a function of time for the particles was obtained. This allowed us to obtain the long-time diffusion coefficient for microspheres moving in the micellar network and the cage size where each particle is harmonically bound at short times in that network. The bulk mechanical susceptibility of the fluid determines the response of the probe particles excited by the thermal stochastic forces. As a consequence, the mean square displacement curves allowed us to calculate the elastic (storage) and the viscous (loss) moduli as a function of the frequency. From these curves, spanning a wide frequency range, we estimated the characteristic lengths as the mesh size, the entanglement length, the persistence length, and the contour length for micellar solutions of different zwitterionic surfactant concentration, surfactant ratio ([SDS]/[TDPS]), salt concentration, and temperature. Mesh size, entanglement length, and persistence length are almost insensitive to the change of these variables. In contrast, the contour length changes in an important way. The contour length becomes shorter as the temperature increases, and it presents a peak at a surfactant ratio of ∼0.50-0.55. When salt is added to the solution, the contour length presents a peak at a salt concentration of ∼0.225 M, and in some solutions, this length can reach values of ∼12 µm. Scission energies help us to understand why the contour length first increases and then decreases when salt is added.

The wormlike micellar solution made of a zwitterionic surfactant (TDPS), an anionic surfactant (SDS), and brine in the semidilute regime.

Structural and dynamical properties of a micellar solution are studied mainly through examining its rheological behavior in the semidilute regime. The micellar solution is made of a zwitterionic surfactant N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, sodium dodecyl sulfate, and salty water. In particular, we are interested in how the system is affected when the ionic strength of the media is modified by adding salt. Until recently, it was known that this solution forms wormlike micelles. In a range of chemical composition, the solution behaves as a viscoelastic Maxwellian fluid at low frequencies. We present measurements of the elastic (storage) modulus and the viscous (loss) modulus varying the surfactant ratio (R = [SDS]/[TDPS]), and how the Maxwellian relaxation time abruptly increases when the NaCl concentration is also varied. Reptation and breaking/recombination times were estimated. The effect of temperature in the viscoelastic solution is also studied. Shear stress versus shear rate flow curves were measured under shear and stress control, for different micellar solutions with different composition, brine concentration, and temperature, showing a nonlinear behavior. Flow curves present two branches, one corresponding to high viscous fluid and another to low viscous fluid, separated by a stress plateau. We were able to develop a master dynamic phase diagram, which summarizes the nonlinear behavior by appropriately reducing the rheological variables. In the stress plateau, the micellar solution presents gradient shear banding, which was observed with the scattered light of a sheet of light perpendicular to the fluid flow velocity in the gap of a transparent Couette rheometer.

A rheological study in the dilute regime of the worm-micelle fluid made of zwitterionic surfactant (TDPS), anionic surfactant (SDS), and brine.

We present a rheological study for a system made of zwitterionic surfactant N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (TDPS), sodium dodecyl sulfate (SDS), and water (0.5M NaCl). We found that this system forms wormlike micelles. This study is focused in the dilute regime below the overlap concentration, where micelles are not entangled. The overlap concentration was determined using dynamic light scattering. The behavior of the apparent viscosity and the shear stress, both as a function of the shear rate, was determined for different zwitterionic surfactant concentrations, temperatures, and two surfactant ratios (R=[SDS]/[TDPS]). The shear-thickening transition and its temperature dependence was also studied. Finally, we were able to observe the shear-induced structures by using the scattered light from a sheet of light perpendicular to the flow that is installed in the gap of a transparent Couette cell filled with the micellar fluid.

Surface properties of mixed monolayers of sulfobetaines and ionic surfactants.

To study the influence of the head group in the properties of the mixed monolayers adsorbed at the air-water interface, the surface tension and surface potential of binary mixtures of surfactant have been determined as a function of the surfactant composition. Experiments were carried out with anionic-zwitterionic sodium dodecyl sulfate and dodecyl dimethyl ammoniopropane sulfonate (SDS/DDPS), and cationic-zwitterionic dodecyl trimethylammonium bromide and dodecyl dimethyl ammoniopropane sulfonate (DTAB/DDPS), and dodecyl trimethylammonium bromide and tetradecyl dimethyl ammoniopropane sulfonate (DTAB/TDPS). It was shown that mixed monolayers of cationic-zwitterionic surfactant exhibit small negative deviations of ideal behavior, whereas for SDS/DDPS monolayers show strong negative deviation from the ideality. Deviations of ideal behavior are interpreted by regular solution theory. The surface potential values agree very well with the concentration of the ionic component at the interface. The dynamic surface tension values show that the adsorption kinetics on the interface is a diffusion-controlled process. In monolayers with significant deviation of the ideal behavior, anionic-zwitterionic, there is some evidence of intermolecular attractions after diffusion of both surfactants at the interface.