Charge Properties of TiO2 Nanotubes in NaNO3 Aqueous Solution.

Charging of material surfaces in aqueous electrolyte solutions is one of the most important processes in the interactions between biomaterials and surrounding tissue. Other than a biomaterial, titania nanotubes (TiO2 NTs) represent a versatile material for numerous applications such as heavy metal adsorption or photocatalysis. In this article, the surface charge properties of titania NTs in NaNO3 solution were investigated through electrophoretic mobility and polyelectrolyte colloid titration measuring techniques. In addition, we used high-resolution transmission electron microscopy imaging to determine the morphology of TiO2 NTs. A theoretical model based on the classical density functional theory coupled with the charge regulation method in terms of mass action law was developed to understand the experimental data and to provide insights into charge properties at different physical conditions, namely, pH and NaNO3 concentration. Two intrinsic protonation constants and surface site density have been obtained. The electrostatic properties of the system in terms of electrostatic potentials and ion distributions were calculated and discussed for various pH values. The model can quantitatively describe the titration curve as a function of pH for higher bulk salt concentrations and the difference in the equilibrium amount of charges between the inner and outer surfaces of TiO2 NTs. Calculated counterion (NO3-) distributions show a pronounced decrease of NO3- ions for high bulk pH (both inside and outside TiO2 NT) because of the strong electric field. With the decrease of bulk pH or the increase of the salt concentration, NO3- is able to accumulate near the TiO2 NTs surfaces.

How to unveil self-quenched fluorophores and subsequently map the subcellular distribution of exogenous peptides.

Confocal laser scanning microscopy (CLSM) is the most popular technique for mapping the subcellular distribution of a fluorescent molecule and is widely used to investigate the penetration properties of exogenous macromolecules, such as cell-penetrating peptides (CPPs), within cells. Despite the membrane-association propensity of all these CPPs, the signal of the fluorescently labeled CPPs did not colocalize with the plasma membrane. We studied the origin of this fluorescence extinction and the overall consequence on the interpretation of intracellular localizations from CLSM pictures. We demonstrated that this discrepancy originated from fluorescence self-quenching. The fluorescence was unveiled by a "dilution" protocol, i.e. by varying the ratio fluorescent/non-fluorescent CPP. This strategy allowed us to rank with confidence the subcellular distribution of several CPPs, contributing to the elucidation of the penetration mechanism. More generally, this study proposes a broadly applicable and reliable method to study the subcellular distribution of any fluorescently labeled molecules.

Self-assembly of ovalbumin into amyloid and non-amyloid fibrils.

We study the fibrillation pathway of ovalbumin protein and report the simultaneous formation of several types of fibrils, with clear structural and physical differences. We compare the fibrillation mechanisms at low pH with and without salt, and follow the kinetics of fibrils growth by atomic force microscopy (AFM), static and dynamic light scattering (SLS, DLS), and small-angle X-ray scattering (SAXS). We show that, among the morphologies identified, long semiflexible amyloid fibrils (type I), with persistence length Lp∼3 µm, Young's modulus E∼2.8 GPa, and cross-ß structure are formed. We also observe much more flexible fibrils (type III, Lp∼63 nm), that can assemble into multistranded ribbons with time. They show significantly lower intrinsic stiffness (1.1 GPa) and a secondary structure, which is not characteristic of the well-ordered amyloids, as determined by circular dichroism (CD), wide-angle X-ray scattering (WAXS), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). In between these two main classes of fibrils, a third family, with intermediate flexibility (type II, Lp∼300 nm), is also resolved.