Ordinarily chemists can't mix together their reactants until the instant they're ready for the reaction to proceed. But now researchers have developed a way to encapsulate highly reactive chemicals with carbon nanotubes inside nylon microshells, place them in a mixture with other reactants, store the mixture as long as they wish, and then use laser light to burst the capsules, initiating the chemical reaction when and where it's needed. The system could be used for printing, for controlled drug delivery inside the body, or in industrial chemical synthesis.
"We've developed a way to have incompatible chemicals in a single container, and an external means to make them react," says Jean Fréchet, an organic chemist at the University of California, Berkeley.
Light activated: These microcapsules, made of nylon, can be used to separate highly reactive chemicals in the same liquid. The capsules burst when illuminated by a laser, mixing the chemicals.
Using the encapsulation system, says Alex Zettl, professor of physics at the university who's working on the chemical capsules with Fréchet, "you can place the chemicals exactly where you want them, leave them, and then use the laser as a knob to initiate a reaction on demand."
The Berkeley researchers use an established reaction to create the capsules. They mix the chemical to be encapsulated with a small amount of carbon nanotubes and the precursors for making nylon, while continuously stirring. The stirring causes the nylon to form spheres that capture the nanotubes and the reactant. By varying the stir rate, the Berkeley chemists can vary the size of the resulting capsules from about 100 to 1,000 micrometers. When they aim a laser at a capsule, the carbon nanotubes absorb the light, heating up the liquid inside and causing it to expand until it explodes, releasing the contents. "The novelty is not the particle itself, but the fact that it can be addressed by a cheap laser," says Fréchet. This is possible because carbon nanotubes -- the blackest known substance -- absorb a broad spectrum of light very efficiently.
Microcapsules are commonly found in detergents, where they separate soap from other ingredients until mixed with water, and in carbon-free copy paper, where they separate inks that react when the capsules are burst by pressure from a pen. But there hasn't been a good way to separate reactive liquids from each other, and to precisely control their release.
In a paper published in Journal of the American Chemical Society, the researchers describe using the light-triggered microcapsules to control two different types of reactions.
The laser control offers "many degrees of freedom in depositing something reactive," says Jeffrey Leon, a chemist in the electronics division of Henkel, a company headquartered in Düsseldorf, Germany. Protecting highly reactive chemicals from each other in a liquid, and then being able to carefully control their release, says Leon, "has been a goal for a long time." The research is in its early stages and has many potential applications. "The next step is to make the capsules as small as possible," which could enable use of the laser-triggered capsules in printing electronic materials, Leon says.
The Berkeley researchers are working on swapping out the carbon nanotubes for dyes that absorb much narrower bands of light. This would allow for another degree of control over chemical release: lasers of different wavelengths could be used to release different chemicals.
By Katherine Bourzac
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