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A mathematical analysis of inorganic nanoparticles explains why they form complex structures with a layer of hydrophilic polymer chains. Stephan Förster, Marija Nikolic, Charlotta Olsson, Andrea Salcher, Andreas Kornowski, Andreas Frömsdorf, and Horst Weller, of the University Hamburg, and Anja Rank and Rolf Schubert of the University of Freiburg, Germany, and their colleagues describe how amphiphilic molecules, ones with a hydrophilic and a hydrophobic end, can spontaneously form capsules or bilayers in aqueous solution. The phenomenon is exploited in the action of soaps and detergents which enclose particles or oil droplets within tiny capsules of surfactant, rendering them water soluble. Cell membranes also follow the same organising principles in which amphiphilic lipids molecules form a bilayer by aggregating so that their hydrophobic tails are sandwiched with their hydrophilic heads protruding into the aqueous environment surrounding the cell. The researchers have now made hybrid particles composed of a water-insoluble inorganic nanoparticle of cadmium selenide and cadmium sulfide at the core surrounded by a bristle-like layer of hydrophilic polyethylene oxide polymer chains. They attach the nanoparticles to the polymer through amino groups. Depending on how densely the polymer bristles are packed on the surfaces of the spheres determines the nature of the superstructures formed in aqueous solution. If they are tightly packed, the hybrid particles remain in solution as individual entities, because the bristles repel each other. If the bristles are not too close together, then aggregates containing two (dimers) or three (trimers) particles can form. If the bristles are very sparse, then long thread-like aggregates containing many particles strung together can form and become branched with occasional Y-junctions. The researchers explain that for the particles to aggregate in this way, the bristles on the surfaces of the inorganic cores have to undergo a significant reorganization. They must move out of each other's way to free up additional points of contact so that the particles' cores can become attached to each other. Aggregation then continues until the resulting ensemble is wholly surrounded by a sufficiently dense layer of bristles. This resulting dense layer of bristles prevents the addition of any further particles to the aggregate. It is also possible to make extended curved layers, these require the cores to have an even smaller number of polymer bristles. Ultimately, these curved layers become closed resulting in the formation of capsule-like vesicles in which the walls of the vesicle consist of a monolayer of the bristly particles. The team used confocal microscopy and tranmission electron microscopy (TEM) to characterise their products. "These experiments demonstrate that amphiphilic nanoparticles undergo surfactant-like self-assembly. When the polymer/nanoparticle ratio is decreased, that is, the surface area A is increased, a structural sequence of spheres - cylinders - networks - vesicles is observed," the researchers explain, "which exactly parallels the behaviour known for surfactants and lipids, except that the vesicle wall consists of a nanoparticle monolayer instead of the bilayer structure known for liposomes." These polymer-coated spheres offer a new, simple method for the controlled production of quite complex superstructures with possible technological applications, the researchers explain. The vesicles could be used to encapsulate small molecules for targeted drug delivery, protection of medical imaging contrast agents or as ordered liquid crystalline phases for the production of nanostructured hybrid materials. Related links:
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