Protein binding onto surfactant-based synthetic vesicles

Synthetic vesicles were prepared by mixing anionic and cationic surfactants, aqueous sodium dodecylsulfate with didodecyltrimethylammonium or cetyltrimethylammonium bromide. The overall surfactant content and the (anionic/cationic) mole ratios allow one to obtain negatively charged vesicles. In the phase diagram, the vesicular region is located between a solution phase, a lamellar liquid crystalline dispersion, and a precipitate area. Characterization of the vesicles was performed by electrophoretic mobility, NMR, TEM, and DLS and we determined their uni-lamellar character, size, stability, and charge density. Negatively charged vesicular dispersions, made of sodium dodecylsulfate/didodecyltrimethylammonium bromide or sodium dodecylsulfate/cetyltrimethylammonium bromide, were mixed with lysozyme, to form lipoplexes. Depending on the protein/vesicle charge ratio, binding, surface saturation, and lipoplexes flocculation, or precipitation, occurs. The free protein in excess remains in solution, after binding saturation. The systems were investigated by thermodynamic (surface tension and solution calorimetry), DLS, CD, TEM, 1H NMR, transport properties, electrophoretic mobility, and dielectric relaxation. The latter two methods give information on the vesicle charge neutralization by adsorbed protein. Binding is concomitant to modifications in the double layer thickness of vesicles and in the surface charge density of the resulting lipoplexes. This is also confirmed by developing the electrophoretic mobility results in terms of a Langmuir-like adsorption isotherm. Charges in excess with respect to the amount required to neutralize the vesicle surface promote lipoplexes clustering and/or flocculation. Protein-vesicle interactions were observed by DLS, indicating changes in particle size (and in their distribution functions) upon addition of LYSO. According to CD, the bound protein retains its native conformation, at least in the SDS/CTAB vesicular system. In fact, changes in the alpha-helix and beta-sheet conformations are moderate, if any. Calorimetric methods indicate that the maximum heat effect for LYSO binding occurs at charge neutralization. They also indicate that enthalpic are by far the dominant contributions to the system stability. Accordingly, energy effects associated with charge neutralization and double-layer contributions are much higher than counterion exchange and dehydration terms.     

Colour centre production in yttria-stabilized zirconia by X-ray and electron irradiations: effect of yttria content

Yttria-stabilized zirconia (YSZ) single crystals (for 9.5 and 18 mol% yttria) were irradiated at room temperature (RT) by X-rays (W white spectrum) and 2.5-MeV electrons. The growth curves of the so-called T-centre (for trigonal centre, i.e. Zr³⁺ sitting in a trigonal symmetry site) were studied as a function of absorbed dose, or irradiation time, by UV–visible optical absorption (OA) spectroscopy and X-band electron paramagnetic resonance spectroscopy. The defect concentration at saturation and the production rate are increased by a factor around two for 18 mol% yttria with respect to 9.5 mol%. Defect decay was then followed after irradiation by OA spectroscopy as a function of ageing time at RT. Growth and decay curves of the T-centre are modelled on the basis of rate equations of charge-exchange reactions with the zirconium lattice ions. Increase in yttrium content is thought to decrease hole trapping on Zr³⁺ ions, thereby enhancing T-centre formation.     

Recrystallization of hexagonal silicon carbide after gold ion irradiation and thermal annealing

Silicon carbide (SiC) single crystals with the 6H polytype structure were irradiated with 4.0-MeV Au ions at room temperature (RT) for increasing fluences ranging from 1?×?10¹² to 2?×?10¹⁵ cm^?2, corresponding to irradiation doses from ~0.03 to 5.3 displacements per atom (dpa). The damage build-up was studied by micro-Raman spectroscopy that shows a progressive amorphization by the decrease and broadening of 6H-SiC lattice phonon peaks and the related growth of bands assigned to Si–Si and C–C homonuclear bonds. A saturation of the lattice damage fraction deduced from Raman spectra is found for ~0.8?dpa (i.e. ion fluence of 3?×?10¹⁴ cm^?2). This process is accompanied by an increase and saturation of the out-of-plane expansion (also for ~0.8?dpa), deduced from the step height at the sample surface, as measured by phase-shift interferometry. Isochronal thermal annealing experiments were then performed on partially amorphous (from 30 to 90%) and fully amorphous samples for temperatures from 200 °C up to 1500 °C under vacuum. Damage recovery and densification take place at the same annealing stage with an onset temperature of ~200 °C. Almost complete 6H polytype regrowth is found for partially amorphous samples (for doses lower than 0.8 dpa) at 1000 °C, whereas a residual damage and swelling remain for larger doses. In the latter case, these unrelaxed internal stresses give rise to an exfoliation process for higher annealing temperatures.     

Ageing and thermal recovery of paramagnetic centers induced by electron irradiation in yttria-stabilized zirconia

We have used electron spin resonance spectroscopy to study the defects induced in yttria-stabilized zirconia (YSZ) single crystals by 2.5-MeV electron irradiations. Two paramagnetic centers are produced: the first one with an axial d 111 symmetry is similar to the trigonal Zr 3+ electron center (T center) found after X-ray irradiation or thermo-chemical reduction, whereas the second one is a new oxygen hole center with an axial d 100 symmetry different from the orthorhombic O m center induced by X-ray irradiation. At a fluence around 10 18 e/cm 2 , both centers are bleached out near 600 v K, like the corresponding X-ray induced defects. At a fluence around 10 19 e/cm 2 , defects are much more stable, since complete thermal bleaching occurs near 1000 v K. Accordingly, ageing of as-irradiated samples shows that high-dose defects at more stable than the low-dose ones.     

Thermo-stimulated luminescence of x-ray- and alpha-irradiated yttria-stabilized zirconia

Yttria-stabilized zirconia (ZrO2 : Y3+) single crystals (with 9.5 mol% Y2O3) were irradiated with x-rays and α particles. Thermally stimulated luminescence (TSL) data show a main broad peak centred at ～500-550 K in the glow curves of all irradiated samples. The TSL peak maximum temperature is consistent with the characteristic recovery temperature (～450 K) of colour centres (T centres) deduced from isochronal annealing curves measured by electron paramagnetic resonance (EPR) spectroscopy. However, the trap-depth energies (ranging between 0.8 and 1.2 eV) deduced from the initial rise of partially cleaned TSL peaks (and from a rough approximation using Urbach's formula) are much larger than the activation energies for defect recovery of 0.3 eV deduced from the EPR data. A second TSL peak centred at ～350-450 K found in freshly irradiated samples is seen to decay substantially in aged samples. The processes involved in TSL are discussed in relation to the defect annealing processes, and available defect-level energy and TSL data.     

Interplay between thermodynamics and kinetics in the capping of InAs/GaAs(001) quantum dots

A microscopic picture for the GaAs overgrowth of self-organized quantum dots is developed. Scanning tunneling microscopy measurements reveal two capping regimes: the first being characterized by a dot shrinking and a backward pyramid-to-dome shape transition. This regime is governed by fast dynamics resulting in island morphologies close to thermodynamic equilibrium. The second regime is marked by a true overgrowth and is controlled by kinetically limited surface diffusion processes. A simple model is developed to describe the observed structural changes which are rationalized in terms of energetic minimization driven by lattice mismatch and alloying.     

Optical spectroscopy study of modifications induced in cerium dioxide by electron and ion irradiations

UV-visible absorption spectroscopy and Raman spectroscopy were used to study damage production in cerium dioxide epitaxial films and polycrystalline sintered samples after irradiation with electrons for three energies to span the threshold displacement energies of cerium and oxygen atoms, and 2.4-MeV Cu ions. Neither amorphization nor specific colour-centre bands were detected. Evolutions of the refractive index were derived from the interference fringes in the optical transmission spectra of epilayers after irradiation. No significant change of the refractive index occurred for the 1.0-MeV electron irradiation, whereas a maximum decrease by 28?±?8% was deduced for the 1.4-MeV and 2.5-MeV energies. These modifications are consistent with ballistic damage on the cerium sublattice for high electron energies producing Ce³⁺ ions. However, no significant change of refractive index was found for the Cu ion irradiation. This likely stems from the high rate of Frenkel pair recombination in the collision cascades induced by more energetic recoils than for the electron irradiations, combined with electronic excitations and hole capture on Ce³⁺ ions. This study reveals modifications of the electronic structure upon irradiation that could take place in other non-amorphizable oxide systems.     

Magnetic properties and phase stability in Co-Ni base alloy thin films

Co-Ni base thin films were deposited by reactive radio frequency magnetron sputtering in N₂+Ar atmosphere at different partial pressure ratios. Only the nitrogen content in the films changes when changing . Amorphous magnetic films can be obtained for compositions around N/Co=1–1.5, whereas higher N₂ contents lead to non-magnetic nitride phases. Significant changes in structural and magnetic properties are observed upon heating thin films in a TG(M) apparatus. Only the sample having a N/Co=1.5, when heat treated, appears to be made of a stable ferromagnetic phase. The N content is appropriate to keep Cr, Al and N, which exert an adverse effect on the magnetization of Co-Ni base alloys, out of the ferromagnetic phase in the form of separated nitride phases. PACS 68.55.-a; 68.60.Dv; 68.18.Jk; 75.70.Ak; 75.75.+a; 85.70.-w     

Energy loss of deuterons in 3He gas: a threshold effect

The energy loss of deuterons in ³He gas was measured at E _d = 15 to 100 keV using the ³He pressure dependence of the ³He(d,p)⁴He cross-section at a given incident energy. At the highest energies, the observed energy loss is in good agreement with a standard compilation. However, with decreasing energy the experimental values drop steadily below the theoretical values and near E _d = 18 keV they drop sharply (within 1 keV) reaching the domain of nuclear stopping power. This threshold behavior is due to the minimum 1s → 2s electron excitation of the He target atoms, i.e. it is a quantum effect. Some consequences are discussed.