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Semiconductor colloidal quantum dots (QDs) are promising fluorescent probes for biology. Initially synthesized in organic solvents, they can be dispersed in aqueous solution by noncovalent coating with amphiphilic macromolecules, which renders the particles hydrophilic and modifies their interactions with other biological compounds. Here, after coating QDs with an alkyl-modified polyacrilic acid, we investigated their colloidal properties in aqueous buffers by electrophoresis, electron microscopy, light scattering, and rate zonal centrifugation. Despite polymer dispersity and variation of the size of the inorganic nanoparticles, the polymer-dot complexes appeared relatively well-defined in terms of hydrodynamic radius and surface charge. Our data show that these complexes contain isolated QD surrounded by a polymer layer with thickness 8-10 nm. We then analyzed their interaction with giant unilamellar vesicles, either neutral or cationic, by optical microscopy. At neutral pH, we found the absence of binding of the coated particles to lipid membrane, irrespective of their lipid composition. An abrupt surface aggregation of the nanoparticles on the lipid membrane was observed in a narrow pH range (6.0-6.2), indicative of critical binding triggered by polymer properties. The overall features of QDs coated with amphiphilic polymers open the route to using these nanoparticles in vivo as optically stable probes with switchable properties.

Water-soluble polymers containing a few hydrophobic anchors are known to bind onto lipid vesicles and are used as stabilizers of liposome-based formulas. In contrast, polymers with high hydrophobicity destabilize the lipid bilayers. With macromolecules of intermediate hydrophobic/hydrophilic balance, a gradual sweep of the stabilization-destabilization capacity can be achieved and is considered as promising triggered systems for drug release, although the mechanism of permeabilization and membrane breakage using polymers is essentially conjectural to date. As a model system, we used short octyl-modified poly(acrylic acid)s (MW 8000 g/mol) sensitive to pH, temperature, and ionic strength in conjunction with small unilamellar vesicles mainly comprised of DPPC or egg-PC. Kinetics of vesicle fragmentation was followed using static and dynamic light scattering. Polymer adsorption was studied by nonradiative energy transfer between pyrene-labeled lipids and a naphthalene-modified polymer. The permeability of the vesicles was characterized by calcein leakage experiments. The key findings were (i) the lack of coupling between the density of bound polymer and the rate of disruption and (ii) the qualitative difference depending on whether the polymer contains or not isopropyl side groups. Point i relates to the increase of the rate of polymer adsorption with increasing bulk polymer concentration, while the breakage is essentially unaffected. Point ii relates to the stabilization of large membrane fragments (Stokes radius ca. 40 nm) in the presence of a polymer with no isopropyl side groups, while micelle-like assemblies (Stokes radius 8 nm) containing the lipids are obtained with an isopropyl-containing polymer of similar hydrophobicity. Both polymers display similar efficiency at disrupting small vesicles. The mechanism of polymer-induced disruption appears to differ markedly from the disruption steps now recognized for conventional (molecular) surfactant and is discussed on the basis of data obtained with different membrane fluidity, polymer structure, concentration, and hydrophilicity.

We present a real-time model-based line tracking approach with adaptive learning of image edge features that can handle partial occlusion and illumination changes. A CAD (VRML) model of the object to track is needed. First, the visible edges of the model with respect to the camera pose estimate are sorted out by a visibility test performed on standard graphics hardware. For every sample point of every projected visible 3D model line, a search for gradient maxima in the image is then carried out in a direction perpendicular to that line. Multiple hypotheses of these maxima are considered as putative matches. The camera pose is updated by minimizing the distances between the projection of all sample points of the visible 3D model lines and the most likely matches found in the image. The state of every edge's visual properties is updated after each successful camera pose estimation. We evaluated the algorithm and showed the improvements compared to other tracking approache

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We examined by fluorescence microscopy the permeabilization of giant vesicles by hydrophobically modified polyacrylates (called amphipols). Amphipols trigger permeabilization to FITC-dextran of egg-PC/DPPA vesicles with no breakage of the lipid bilayers. The polyanionic amphipols were passing through bilayers as shown by permeabilization of multilamellar vesicles. Remarkably, the vesicles were not simultaneously permeable but became leaky one after the other. Altogether, our observations suggest a random formation of pores having diameters above a few nanometers. Decreasing pH and increasing ionic strength and polymer concentration were increasing the rate of permeabilization. The rate and efficiency of permeabilization was compared to the rate and density of adsorption of amphipols onto lipid membranes (as estimated by titration calorimetry onto large unilamellar vesicles and neutron reflectivity measurements on tethered bilayers). The polymer adsorption layer is built up in a few minutes. We conclude that the rate-limiting step for permeabilization is not the adsorption from the bulk solution but relates to slow intramembrane reorganizations.

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