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Special Report - Advanced Nanoscale Materials: Putting Science at Your Fingertips

Supercritical fluids—making nanoparticles easy

It's not a liquid. It's not a gas. It's a supercritical fluid. Although it looks like a liquid, it has unique properties that allow scientists to work with it in ways they can't with liquids. Researchers at Pacific Northwest National Laboratory are using supercritical fluids as solvents in a process that creates nanoparticles. Nanoparticles, by virtue of their minute size, have different properties than the material from which they are generated. For example, an iron particle smaller than two to three nanometers will catalyze reactions differently than a larger iron particle. These distinct properties allow scientists to use nanoparticles in new applications in fields as diverse as waste cleanup and biomedical equipment.

In the supercritical process, PNNL scientists apply heat and pressure to carbon dioxide, a solvent chosen for its environmental friendliness. When the supercritical point is reached, the CO2 changes from a liquid to a supercritical phase. Next, scientists dissolve other substances into the supercritical CO2.

The dissolved material is then sprayed through a nozzle using a process called rapid expansion of carbon dioxide solutions (RESS). The ability to be sprayed is a unique property of supercritical CO2. "All the CO2 evaporates and, because the supercritical fluid becomes a gas so quickly in the spraying process, RESS inherently generates nanoparticles," said John Fulton, who leads PNNL's supercritical fluids research.

The RESS process was patented by the Laboratory in the late 1980s. Two recent innovations have resulted in new applications for the process. First, scientists have discovered that by charging the nanoparticles as they leave the nozzle and creating an electric field in the spray container, the particles can be easily collected on a nearby surface. Scientists also have discovered that by applying a beam of light to the plume of nanoparticles, they can make the particles react to create different substances.

Recently, researchers working on waste remediation discovered that ground iron mixed with contaminated earth would convert chlorinated hydrocarbons—a waste material—to non-harmful chemicals. But they didn't know how the chemistry worked. Because iron does not dissolve in CO2, Fulton and his team dissolved a related metal, iron carbonyl, in supercritical CO2 and sprayed it into a bright ultraviolet beam. The light reacted with the iron carbonyl nanoparticles, turning them into iron oxide and iron metal nanoparticles.

Waste remediation scientists are now using the iron metal nanoparticles to analyze the chemistry of the iron-chlorinated hydrocarbon reaction.

Scientists also are using supercritical fluids and RESS to apply Teflon to vascular stents, a tool used to open arteries in the heart. Typically, the human body rejects the stainless steel stents by growing tissue around the stent. "The body recognizes foreign surfaces and builds up tissue to get rid of them and that reclogs the artery," Fulton said. Collaborating with industry partners, PNNL scientists have successfully used the RESS process to coat the stents with a nanoparticle matrix of Teflon and a drug that prevents tissue buildup. "By coating the stent with this mixture, we not only have a surface that is compatible with the body, but also a drug that releases over a long time," Fulton said.

One of the properties of supercritical fluids is that they can be sprayed. Scientists at PNNL are using a process called rapid expansion of carbon dioxide solutions (RESS) to spray materials dissolved in supercritical CO2 through a nozzle to generate nanoparticles.

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