Fullerenes: fill 'em up or move 'em out.
Biophysicists seeking to understand how plants convert solar energy into useful chemical energy have discovered that fullerenes can mimic nature by ferrying electrons across membranes. Other scientists have encapsulated simple metal crystals inside the giant hollow polyhedra that also form during the production of carbon nanotubes (SN: 7/18/92, p.36).
The giant fullerenes seem to develop around the metal, says Rodney S. Ruoff at SRI International in Menlo Park, Calif. To make these filled particles, Ruoff, Donald C. Lorents, and their SRI colleagues first drilled a hole into a graphite rod, then filled the hole with lanthanum oxide. They shot sparks from the rod to another electrode a millimeter away, causing the rod to vaporize. Then they sent the deposits that formed to Shekhar Subramoney at the Du Pont Co. Experimental Station in Wilmington, Del., for examination with a transmission electron microscope.
"Shekhar called me from the darkroom at Du Pont because he was so excited about the negatives he was developing;' Ruoff recalls. The micrographs revealed a variety of nested carbon particles, many filled with crystals that turned out to be lanthanum dicarbide, he and his colleagues report in the Jan. 15 SCIENCE. Air's moisture destroys this carbide, but the carbon cages prevented that, they note.
SRI and Japanese researchers are encapsulating other materials, Ruoff adds.
He hopes that giant fullerene "containers" will one day protect potentially useful air-sensitive materials. Also, chemists may be able to modify the filled carbon cages and incorporate them into polymers to create new materials, Ruoff suggests.
David Mauzerall of Rockefeller University in New York City takes a much different tack in putting fullerenes to work. He and Kuo Chu Hwang at Rockefeller study photosynthesis -- in particular, how cells move electrons across membranes. This "charge transfer" ensures that when a donor molecule gets excited by light and lets go of an electron, that electron gets whisked away to someplace from which it cannot return. This transport and charge separation is important to successful energy storage for solar power, as well as to the survival of living organisms, Mauzerall says.
To mimic this process, the Rockefeller group and other researchers typically use various organic molecules to carry electrons from one side of a synthetic membrane to another. But Mauzerall decided to try fullerenes after noticing that their optical properties closely resemble those of light-activated porphyrins.
For their experiments, Hwang and Mauzerall mix the 70-carbon fullerene, [C.sub.70], with lipid molecules. Because they have a polar "head" and a nonpolar "tail," the lipid molecules arrange themselves into a two-layer film, with tails facing inward, trapping the carbon molecules. The researchers place two liquids - a light-sensitive one that donates electrons and another that can accept free electrons - on either side of this film.
Light from a slide projector bulb makes the light-sensitive liquid release electrons and makes the carbon molecules "photoactive" - more accepting of electrons. Thus light generates a current across the membrane.
"The fullerenes carry charges across membranes much better than porphyrins," says Mauzerall. This fullerene-based system is 40 times more efficient than the best synthetic charge-transfer system, the Rockefeller team reports in the Jan. 14 NATURE. The fullerene works quite fast-shuttling each electron in less than 20 microseconds, sustaining current for longer periods, and transferring more electrons before losing its photoactivity
Mauzerall and Hwang are now trying to determine whether a [C.sub.70] ferries each electron or whether several [C.sub.70] molecules straddle the membrane and, like stepping-stones, enable electrons to hop across.
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|Title Annotation:||uses for the all-carbon molecules|
|Date:||Jan 23, 1993|
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