Reactions on Demand
Microcapsules isolate reactants until a laser bursts the bubble
Source: “Chemicals on Demand with Phototriggerable Microcapsules”
Jean M.J. Fréchet et al.
Journal of the American Chemical Society 131: 13586-13587
Results: Researchers at the University of California, Berkeley, enclosed highly reactive chemicals in polymer microcapsules. They showed that the capsules can be burst using light from a laser, allowing the chemicals to escape and react with each other to form a desired product.
Why it matters: The microcapsules will enable chemists to place reactants in precise locations before triggering them to react. Scientists could also initiate reactions at precisely timed intervals. These techniques could be useful in applications such as timed drug delivery inside the body, printing, and self-healing materials.
Methods: The chemical to be encapsulated is mixed with a small quantity of carbon nanotubes and with the chemical precursors of nylon, which form nylon microspheres as the mixture is stirred. As they form, the spheres capture the nanotubes and the chemical reactant. When the researchers shine a red laser on the capsules, the nanotubes absorb the energy and heat up until the capsules burst. In a proof-of-concept experiment, the researchers made microcapsules that contained a special catalyst and suspended them in a reactive liquid. The microcapsule protected the catalyst and the liquid from reacting; however, when a laser was used to burst the capsules, the catalyst was released and quickly transformed the liquid into a solid.
Next steps: The group is testing reaction capsules that contain dyes instead of nanotubes to absorb the laser energy; the dyes respond to specific bandwidths of light, such as red, green, or blue. This could allow scientists to control reactions more precisely by shining different colors of light at different times.
Superacids are the key to assembling nanotubes into large structures
Source: “True solutions of single-walled carbon nanotubes for assembly into macroscopic materials”
Matteo Pasquali et al.
Nature Nanotechnology, published online November 1, 2009
Results: Rice University researchers have developed a way to arrange carbon nanotubes into large structures, including fibers hundreds of meters long, by dissolving them in a “superacid.”
Why it matters: Assembling carbon nanotubes into well-ordered materials such as long fibers has proved challenging; though lining them up in a flowing solution seemed like a promising approach, nanotubes don’t dissolve in conventional solvents. The new processing methods could be used to manufacture materials such as electrical transmission lines that are stronger and more conductive than the metal ones used today.
Methods: The researchers tried dissolving nanotubes in acids of varying strengths and found that in stronger acids, the tubes arrange themselves into a liquid crystalline phase in which they are well aligned. After developing a theoretical model to explain what conditions, including acid strength, are necessary to control the phase transitions, they were able to produce liquid crystal solutions that can easily be used to form long, high-quality fibers. Making them involves shooting the nanotube-acid mixture through a nozzle similar to a shower head and removing the acid with a coagulant, causing the nanotubes to bind together.
Next steps: To realize the promise of the assembly methods, researchers will need to develop ways to manufacture solutions of carbon nanotubes that have uniform properties. Transmission lines, for example, would need to be made from a batch containing mostly conducting nanotubes, with as few semiconducting nanotubes as possible.