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Abstract: Rhamnolipids are glycolipid surfactants composed by a hydrophilic head of either one (mono-RL) or two (di-RL) rhamnose moieties coupled to hydroxyaliphatic chains that can be of different lengths. In spite of their importance in different fields of applications, as bioremediation processes for instance, self-aggregation physico-chemical properties of RLs are not unique. This because a variety of aggregates morphologies (shape and size) can either exist or coexist in aqueous dispersion due to mono-RL:di-RL molar ratio, hydrophobic tails length, pH and the presence of co-surfactants and additives. Recently, a theorethical approach reported the self-assembling morphologies of either pure mono or di-RL in aqueous environment, predicting the formation of spherical to ellipsoidal micelles to worm-like and disk-like aggregates depending on RL concentration and fatty acid chain length. In order to add new information to those previously available, the present work investigated the self-assembling properties of mono-RL-C10-C10 and di-RL-C10-C10 separately in aqueous dispersion by small angle X-Ray scattering (SAXS). A novel approach was applied to the data analysis coupling the scattering length density profiles of the RLs chemical groups and Monte Carlo simulations. Such an approach allowed us to infer about the preferred mono-RL and di-RL conformations that fit better in the self-assembling morphologies. In this way, we show that mono-RL-C10-C10 self-assembles into lamella-like aggregates coexisting with 30% of multi-lamella aggregates (circa of 5 closed stacked lamella) from a concentration ranging from 10 to 50 mM, with hydrophobic thickness of about 12 Å, a hydrated polar head thickness of 10 Å, and an area per glycolipid of 76 Å2. On the other hand, di-RL prefers to self-associate into flexible cylinder-like aggregates, from 70 mM to 110 mM concentration, with hydrophobic radius on the order of 7.5 Å, a hydrated polar shell of 21.5 Å, with hydropobic/polar interface of 110 Å2 per glycolipid. Interestingly, the parameters obtained from the best fitting to the experimental data associated to the volume fraction distribution of the chemical groups within the aggregates revealed that the hydrophobic chains are more disordered in mono-RL planar aggregates than in di-RL worm-like aggregates, as well as the hydration properties. Further, the addition of 100 mM NaCl in di-RL aqueous dispersion leads to the formation of longer worm-like aggregates. Taking together, this work opens a new avenue regarding characterization of biosurfactants self-assembling properties by using SAXS, also contributing to prepare more efficient biosurfactant dispersions depending on the desired applications in industrial sectors and bioremediation.

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Abstract: Over 50 million people in South Asia are exposed to groundwater contaminated with carcinogenic arsenic(III). Photocatalyst-adsorbent composite materials are popularly developed for removing arsenic in a single-step water treatment. Here, As(III) is oxidised to As(V), which is subsequently removed via adsorption. We previously developed a component additive surface complexation model (CA-SCM) to predict the speciation of arsenic adsorbed onto TiO 2/Fe2O3 under different environmental conditions, using surface complexes taken from studies of single-phase minerals. In this work, we critically evaluate this approach, using experimental observations of the surface structures of arsenic adsorbed onto TiO 2/Fe2O3. Extended X-ray absorption fine structure spectroscopy (EXAFS) indicates significant As(III) surface precipitation, and the possible formation of tridentate 3C complexes. EXAFS was unable to identify As binding modes for TiO 2 and Fe2O3 surface complexes simultaneously, highlighting the challenge of analysing composite surfaces. FTIR and zeta potential analysis indicate that As(III)-Fe2O3 surface complexes are protonated at neutral pH, whilst As(III)-TiO 2, As(V)-Fe2O3 and As(V)-TiO 2 surface complexes are negatively charged. Our study confirms the speciation predicted by CA-SCM, particularly As(III) surface precipitation, but also introduces the possibility of tridentate As(III) at acidic pH. This study highlights how experiment and modelling can be combined to assess surface complexation on composite surfaces.

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Abstract: Graphene quantum dots (GOQDs)-MoSx nanohybrids with different MoSx stoichiometries (x = 2 and 3) were prepared in order to investigate their chemical stability under hydrogen evolution reaction (HER) conditions. Combined photoemission/electrochemical (XPS/EC) measurements and operando X-ray absorption spectroscopy (XAS) were employed to determine the chemical changes induced on the MoSx-based materials as a function of the applied potential. This in situ characterization indicates that both MoS2 and MoS3 materials are stable under operating conditions, although sulfur terminal sites in the MoS3 nanoparticles are converted from S-dimer (S22−) to S-monomer (S2−), which constitute the first sites where the hydrogen atoms are adsorbed for their subsequent evolution. In order to complete the characterization of the GOQDs-MoSx nanohybrids, the composition and particle size were determined by X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD) and Raman spectroscopy; whereas the HER activity was studied by conventional electrochemical techniques.

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Abstract: Understanding the distribution of nanoparticles in skin layers is fundamentally important and essential for developing nanoparticle-based dermal drug delivery systems. In the present study, we provide insights into the distribution of gold nanorods (GNRs) functionalized with hydrophobic or hydrophilic ligands in human skin layers using synchrotron X-ray fluorescence (SR-XRF) spectroscopy, confocal microscopy, and transmission electron microscopy. The results confirmed the important role that the surface chemistry of GNRs plays in their penetration into the skin; the GNRs coated with polyethylene glycol were distributed into the skin layers to a greater extent than the GNRs coated with hydrophobic polystyrene thiol. In addition, SR-XRF analysis revealed that the spatial distribution of endogenous elements (phosphorus and sulfur) in skin layers demonstrated a significant “anti-correlation” relationship with that of GNRs. These results suggest possible association (via adsorption) between the GNRs and these two elements localized in skin, which can be valuable for understanding the penetration mechanism of gold nanoparticles into the skin.

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Abstract: Derivatives of fluorophore FITC (fluorescein isothiocyanate) are widely used in bioassays to label proteins and cells. An N-terminal leucine dipeptide is attached to FITC, and we show that this simple conjugate molecule is cytocompatible and is uptaken by cells (human dermal and corneal fibroblasts) in contrast to FITC itself. Co-localisation shows that FITC-LL segregates in peri-nuclear and intracellular vesicle regions. Above a critical aggregation concentration, the conjugate is shown to self-assemble into beta-sheet nanostructures comprising molecular bilayers.