TODAY MARCH 20, 2023

Nano-rods self-assemble into nano-rings

Picture: Gold nanorods form a ring. The insert in the center shows a single nanorod coated with polymer molecules.
A team of Rice University chemists lead by Eugene Zubarev have observed an unusual and potentially useful phenomenon: tiny particles, known as “gold nanorods” spontaneously assemble themselves into perfect rings (see picture with an image of a ring and a schematic drawing of a single nanorod in the center). The researches covered gold nanorods with a layer of a polymer, thus making them solvable in chloroform. As the chloroform solution, placed on a substrate, evaporates, its temperature reduces thus causing condensation of water from the air. These newly formed droplets of water act as templates for nanorods and organize them into circles.
Hybrid AuNR(PS)n nanorods spontaneously organize into well-defined ring-like arrays when a drop of their solution in a nonpolar solvent is dried on a hydrophobic substrate. The process is highly reproducible and generates thousands of well-defined rings of nanorods. This work will be published as a cover article in the March 19, 2007 edition of the journal Angewandte Chemie. Potential applications of the method include novel single-electron nanodevices, sensitive optical sensors, and structures with unusual optical characteristics. Multiple control experiments reveal that water droplets, which condense on the surface of evaporating nonpolar solvent, template the formation of rings. However, the rings form only if there is a dense shell of polymer arms attached to the surface of nanorods. The polymer shell ensures their high solubility and keeps the rods in solution until they concentrate around the water droplets. The approach is very simple and offers a new way to organize inorganic nanostructures of various sizes and shapes into ring-like assemblies.
The cylindrical superstructures are composed of silver nanoparticles with V-shaped amphiphilic arms. The short rod-like and spherical assemblies are made of gold nanoparticles with the same V-shaped amphiphilic arms.  The self-assembly occurs upon slow addition of water to solution of nanoparticles in organic solvent called tetrahydrofuran. The resulting mixture is then dialyzed against pure water in order to remove organic solvent and obtain a pure aqueous solution of the superstructures (they remain in water without any precipitation, just like micelles).
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