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Abstract:
A research team, led by UC Riverside's Ludwig Bartels, was the first to design a molecule that can move in a straight line on a flat surface. Now this team has found a way to attach cargo: two CO2 molecules, making the nano-walker a molecule carrier.

Walking Molecule Now Carries Packages

Riverside, CA | Posted on January 18th, 2007

The work will be published Thursday, Jan. 18 in "Science Express" and later in the print-version of the journal "Science."

"This is an unprecedented step forward towards the realization of molecular-scale machinery," said Bartels, associate professor of chemistry and a member of UCR's Center for Nanoscale Science and Engineering. "Our experiments show a means to transport molecules reliably. This will become as important to the molecular machinery of the future as trucks and conveyor belts are for factories of today."

The last paper Bartels and his team published on this subject generated a great deal of interest. It was included in the American Institute of Physics "Top 25 Physics Stories for 2005."

The new molecule carrier runs on a copper surface. It can pick up and release up to two carbon dioxide (CO2) molecules and carry them along its straight path. "Carrying a load slows the molecule down" explained Bartels. "Attachment of one CO2 molecule makes the carrier need twice as much energy for a step, and a carrier with two CO2s requires roughly three times the energy. This is not unlike a human being carrying heavy loads in one or both hands."

Bartels explained that using machines at the scale of single molecules will ultimately be the most efficient way to build objects or to deliver material.
"It resembles the way nature does it: the molecule carrier transports carbon dioxide across a surface," he said. "In the human body, the molecule hemoglobin carries oxygen from and carbon dioxide to the lungs, thereby allowing us to breathe - and to live."

Bartels cautions, however, that this research is still in its infancy. "In 2005 we invented the molecular walker, which moves in a straight line rather than hopping around in all directions as a normal molecule would do. Now it can carry a load."

Bartels said the continuing evolutionary process will take some time. "Ten years ago, a cell phone could just place calls, nothing else. Now it plays mp3-files, organizes your day, lets you send emails and browse the web." He said his team will be pursuing the next step for this molecule carrier. "Next, we would like to be able to make one go around corners, rotate its cargo or send out photons to tell us where it is."

The molecule carrier is anthraquinone, which consists of three fused benzene rings with one oxygen atom on each side. An organic compound, anthraquinone is widely used in the pulp industry for turning cellulose from wood into paper. It is also the parent substance of a large class of dyes and pigments. Its chemical formula is C14H8O2.

The UCR study used a scanning tunneling microscope in Bartels's laboratory that gives a precise picture of individual molecules. Experiments took place on a highly polished copper surface, cleaned so that only the desired molecules were present on it. An individual anthraquinone molecule appears in Bartels' microscope as an almost rectangular feature with slightly rounded edges.

All experiments of this study were conducted in the Bartels Lab by graduate students Greg Pawin, Ki-Young Kwon, Xing Lin and Tong Jiao as well as post-docs Kin Wong and Richard Fawcett and undergraduate student Urvinee Solanki. They had help in understanding details of the attachment process by Theoretical Physicists Sergey Stolbov and Talat Rahman of the University of Central Florida.
The US Department of Energy funded the research. Additional support came from the Petroleum Research Fund and the Air Force Office of Scientific Research. The San Diego Supercomputer Center provided computational resources.

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About UC Riverside
The University of California, Riverside is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment of about 17,000 is projected to grow to 21,000 students by 2010. The campus is proposing a medical school and already has reached the heart of the Coachella Valley by way of the UCR Palm Desert Graduate Center. With an annual statewide economic impact of nearly $1 billion, UCR is actively shaping the region's future. To learn more, visit www.ucr.edu or call (951) UCR-NEWS.

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