In science,science, a walking molecule is an atomic structure or molecule that is able to move along a surface, through the action of induced movement,movement, by alternating its appendices, driven by external energy source. [1]

History
In September of 2005 a research team led German-born, American physical chemist Ludwig Bartels at the University of California Riverside designed a molecule that can walk in a straight line on a flat surface, like a little human being. [2] The news release for the results of this project, first published in the Physics Review Letters, was entitled ‘Molecule Walks Like a Human.’ [3] This tiny walking molecule, shown above, achieves this effect by mimicking the motor dynamics of a human walking. [4] B

The molecule's motion, according to Bartels’ research team, was extremely stable: "in tests, DTA took more than 10,000 steps without losing its balance once. Our work proves that molecules can be designed deliberately to perform certain dynamic tasks on surfaces," said research leader Ludwig Bartels. "Similar to a human walking, where one foot is kept on the ground while the other moves forward and propels the body, our molecule always has one linker on a flat surface, which prevents the molecule from stumbling to the side or veering off course."

DTA
This tiny walking-molecule, called 9,10-dithioanthracene or ‘DTA’, has two linkers that act like little mobile feet. DTA, is an organic molecule composed of a coal tar derivative called anthracene linked to a pair of sulfur-bearing functional groups on either side (referred to as "linkers"), which serve as the molecule's "feet". When the compound is heated on a flat copper surface, the linkers raise up, alternating from side to side, and propel the molecule forward.

Thus, obtaining its energy by heat supplied to it (similar to human molecules,molecules, obtaining their heat from the sun), the molecule moves such that only one of the linkers is lifted from the surface; the remaining linker guides the motion of the molecule and keeps it on course. Without the assistance of non-rails or nano-groves for guidance, the little molecule is able to alternate the motion of its two ‘feet’ so to walk in a straight line. These published results generated a great deal of interest and made the American Institute of Physics ‘Top 25 Physics Stories for 2005.’

Molecular carrier
In January of 2007, interestingly, Bartels’ research team found a way to attach cargo: two CO2 molecules (load), making the nano-walker a molecule carrier or worker. 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.’ [5]

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.’ This depiction, of human molecules looking at and studying simpler walking molecules, from a humorous perspective, might be analogous to how English naturalist Charles Darwin felt when, in 1838, he first looked at the great chimpanzees and realized they were related to us. [1]

References
1. Thims, Libb. (2008). The Human Molecule, (section: "Free will", pgs. 59-61), (preview). Morrisville, NC: LuLu.
2. Melville, Kate. (2005). “Novel ‘Walking’ Molecule Mooted For Nano-Abacus”, ScienceAGoGo.com, Sept. 27.
3. Kwon, K.; Wong, K.; Pawin, G.; Bartels, L.; Stolbov, S.; Rahman, T., Unidirectional Adsorbate Motion on a High-Symmetry Surface: ‘Walking’ Molecules can Stay the Course. Phys. Rev. Lett. (2005), in press (Oct).
4. University of California Riverside Newsroom (2005). “Molecule Walks Like Human”.
5. Staff writer (2007). “Walking Molecule Now Carries Packages”, PhysOrg, Jan. 18.