Thursday, December 15, 2005Volume 5, Issue 7
Stopping to Smell the Scientific Flowers: A Tour Through Tom Mallouk’s Lab
By Walt Mills
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In Tom Mallouk’s lab in the Chemistry Building on the University Park campus, Justin Youngblood, a postdoctoral scholar from North Carolina State, is heating chemicals behind a glass-enclosed cabinet, part of an experiment to improve the light absorption capacity of solar cells. "It’s like natural photosynthesis on a chip," says Mallouk, Dupont Professor of Materials Chemistry and Physics, about the research Youngblood is doing. "The process is like a Rube Goldberg machine. The first step is driven by light, then the next steps are driven by releasing small amounts of energy, like someone walking down a flight of stairs."
At another table, graduate student Neal Abrams works on dye-sensitized solar cells. Converting sunlight to usable energy has long been a research interest in the Mallouk group. In order to improve the efficiency of solar cells, it is necessary to utilize a wider spectrum of visible light. A process for slowing the light down would allow for the capture of light energy in the infrared part of the spectrum, Mallouk says. Using an approach inspired by the wings of butterflies, whose brilliant colors are produced by the diffraction of light rays through mesoscopic structures within their wings, Mallouk’s group is using photonic crystals spheres about ½ the wavelength of light to scatter and slow the photons of red light.
"In nanotechnology there are big problems we are trying to crack," Dr. Mallouk explains as we make our way through the lab. "In our lab, we stop and smell the flowers, in a scientific sense. We support students who are learning the scientific method and laboratory techniques. We rarely solve the biggest problems, but we make progress on significant problems."
One of the students learning the scientific method is Laura Hoch, a sophomore working with nanoparticles to clean up underground toxic wastes. She is running a solution of elemental iron colloid with various support materials through a column of sand to see how much of it makes its way through. Hoch and graduate student Bianca Hydutsky are looking for colloids with particles that won’t clump together.
"Laura has already found a very interesting carbon-based colloid with good possibilities," Mallouk tells me. "We are designing particles that can do environmental remediation. Kepone and Mirex are carcinogens that accumulate in fish. For decades, companies used these chemicals that went into the groundwater. It’s why Spring Creek (in Centre County) is a catch and release stream. The chemicals can spread out through acres of ground. The choices are to dig the polluted earth up or use chemicals to detoxify, which is what we do."
This is a technology that has already been used in the field, he says. In Trenton , New Jersey, a company performed a full-scale remediation on a contaminated area the size of a football field, using 4500 pounds of nano iron. The remediation resulted in a dramatic lessening of the pollution at a cost one-tenth that of excavating and trucking the contaminated material away.
We finished the tour back at the table where Justin Youngblood’s chemicals were bubbling away. Dr. Youngblood showed me a membrane about the size of a quarter, with countless holes that he said were just1/250th the size of a human hair. With this membrane as a mold, they can grow tiny totem pole-like nanowires, with stripes made from different materials. Nanowires as small as 10 nm in diameter can be used to make electrical, electrochemical, and other connections.
Mallouk and colleagues in the Center for Nanoscale Science have used striped nanowires of gold and platinum to create nanoscopic motors. In a 2005 article in Chemical Communications, University of Groningen chemistry professor Ben Feringa called the experiment "a beautiful demonstration of how a designed nanoscale object can undergo movement powered by a chemical fuel."
The chemical fuel in question is very unexotic and inexpensive: hydrogen peroxide, available on the shelves at any drugstore.
In his office, Mallouk pulls up a movie of the nanoscale catalytic motors and pumps on his computer. The motor is about the same size as the biological motor in an infectious bacteria called Listeria, he says. The tiny rods move through the liquid like bacteria in the cytoplasm of a cell. When two rods meet and attach, they spin like a pinwheel. The rods can be steered by a magnet, he says, and the next step, working with Ayusman Sen, professor of chemistry and head of the chemistry department, is to introduce chemotaxis into the nanomotors − what bacteria do to move towards their food.
On the computer screen, a tiny gear appears with the letters PSU inside. This is the work of Jeff Catchmark, whom Mallouk calls a wizard with patterning and small structures. The gear spins, its platinum-coated tips reacting chemically with the hydrogen peroxide. "We are now3008/15 at the point where we can really start thinking of making little motors that run by catalysis," Mallouk says. To me this looks like significant progress on a pretty big problem.
Contributing to the molecular motor project were professors Auysman Sen, Vincent Crespi and Mallouk; Penn State Nanofabrication Facility manager Jeffrey Catchmark; postdoc Paul Lammert; and Ph.D. students Yanyan Cao, Timothy Kline, Yang Wang, Shaymala Subramanian and Walter Paxton. Their work was funded by the National Science Foundation as part of the Materials Research Science and Engineering Center (MRSEC).
Tom Mallouk is DuPont Professor of Materials Chemistry and Physics, and director of the Center for Nanoscale Science. He can be contacted at tom@chem.psu.edu
