ABSTRACT
Superparamagnetic nanoparticles were encapsulated in liposomes prior to the stepwise adsorption of polyelectrolytes of opposite charges commonly known as the layer-by-layer (LbL) technique. Magnetic fields allow a fast separation of coated liposomes from unbound polyelectrolytes. The coated particles were characterized by dynamic light scattering (DLS), cryo-TEM, AFM, and zeta-potential techniques. The presence of magnetic nanoparticles and the polyelectrolyte shell opens the possibility of their magnetic manipulation and targeting by applying an external magnetic or electric field.
Subject(s)
Electrolytes/chemistry , Liposomes , Magnetics , Microscopy, Atomic Force , Microscopy, Electron, Transmission , Spectrometry, FluorescenceABSTRACT
Nanoparticle brushes: Complex nanostructures can be formed by self assembly of amphiphilic CdSe/CdS core-shell nanoparticles that bear a brushlike layer of poly(ethylene oxide) chains. This route is based on controlling the volume fractions of hydrophilic and hydrophobic moieties within the particles and allows the formation of micellar, cylindrical, or vesicular nanoobjects (see picture).
ABSTRACT
In aqueous solution, poly(2-vinylpyridine-b-ethylene oxide) spontaneously forms bilayer vesicles, the size of which can be tailored by extrusion through polycarbonate membranes. However, their size can be even more precisely influenced by subjecting them to a specific cooling/warming process proceeding through a cylinder-vesicle shape transition. The thermotropic alterations of the polymer aggregates and the topological pathways of the cylinder-vesicle transition were followed by dynamic light scattering (DLS) and cryo-electron microscopy (cryo-TEM). Upon cooling the vesicles to 4degreesC, there is a transition of the vesicles to basketlike aggregates and their further disintegration to wormlike micelles. Rewarming of the dispersion results in the reformation of vesicles via intermediate discoid and octopus-like structures. The variation of incubation times at 4 and 25degreesC, heating rate, polymer concentration, and ionic strength allows tailored preparation of unilamellar and almost monodisperse vesicles with diameters between 60 and 500 nm. Furthermore, fluorescently labeled dextrans, which were used as model drugs of differing molar mass, could be easily and stably encapsulated during the thermotropic formation of vesicles from wormlike micelles.
Subject(s)
Phase Transition , Polyethylene Glycols/chemistry , Polyvinyls/chemistry , Temperature , Cryoelectron Microscopy , Desiccation , Microscopy, Electron, Transmission , Molecular Structure , Particle Size , Water/chemistryABSTRACT
The molecular exchange of tracer molecules through the membranes of dispersed vesicles of the block copolymer poly(2-vinylpyridine-block-ethylene oxide) was studied by using NMR spectroscopy combined with pulsed field gradients. The hydrodynamic radius of the tracer molecules was varied systematically to obtain a permeability profile of the vesicle membrane. In addition, the effect of system parameters, such as temperature, pH value, vesicle size, and thickness of the vesicle membrane, was studied. In the case of rapid exchange with average residence times significantly smaller than 10 s, the permeation is observed under equilibrium conditions and the data are analyzed by using a simple analytical approach. For slow exchange processes with average residence times above 10 s, the permeation is monitored in a time-resolved measurement under nonequilibrium conditions. Generally, the transmembrane exchange rate of the tracer clearly depends on its hydrodynamic radius. The characteristics of this dependence indicate the presence of two different mechanisms of membrane penetration, one dominating for smaller and one for larger tracer molecules, respectively. The exchange rate also shows a significant dependence on the bilayer thickness and on the vesicle diameter. By contrast, no variation of the membrane permeability with the temperature or the pH value could be detected as long as the vesicles remain stable.
Subject(s)
Ethylene Oxide/chemistry , Membranes, Artificial , Polyethylene Glycols/chemistry , Polyvinyls/chemistry , Transport Vesicles/metabolism , Diffusion , Hydrogen-Ion Concentration , Kinetics , Microscopy, Electron, Transmission , Polyvinyl Alcohol/chemistry , Temperature , Time Factors , WaterABSTRACT
The pH-induced release of hydrophilic dyes from poly(2-vinylpyridine-b-ethylene oxide) (P2VP-PEO) block copolymer vesicles is investigated. The structure of the vesicles is characterized using small-angle neutron scattering (SANS) and cryo-electron microscopy (cryo-TEM). A decrease of the pH below 5 leads to protonation and dissolution of the poly-2-vinylpyridine blocks which induces rupture and dissolution of the vesicle membrane. Details of the rupture, dissolution, and release process are studied by fluorescence video microscopy, gel electrophoresis, and high-performance ultrafiltration.
