UCLA supramolecular chemists report in the journal Science an artificial molecular machine that functions like a nanoelevator.
"Such nanoscale robotic devices could find use in slow-release drug delivery systems and in the control of chemical reactions within nanofluidic systems conducted in laboratories on a chip," said Jovica Badjic, the lead author of the March 19 Science article and postdoctoral researcher in the laboratory of Fraser Stoddart, holder of the Fred Kavli Chair in nanosystems sciences and director of the California NanoSystems Institute at UCLA.
In Badjic's incrementally staged design of the nanoelevator — a rig-like construct with three legs embracing an interlocked deck-like component — can be made to move between two levels, he had to get the matching components to fit together just perfectly.
The challenge is similar to one that nature has already solved in creating the multivalent interactions that exist between cells for the purpose of communicating information throughout the body.
"The first step in the synthesis can be likened to learning how to put a glove on one's hand blindfolded. You will make countless mistakes but eventually you find out by trial and error how to get the match just right. That's how the multivalency gets expressed during my template-directed synthesis," Badjic said.
In order to demonstrate the operation of the elevator, the UCLA chemists entered into collaboration with an Italian team at the University of Bologna: professor Vincenzo Balzani, assistant professor Alberto Credi and graduate student Serena Silvi.
The elevator is about 3.5 nanometers in diameter and 2.5 nanometers in height. Each leg of the rig has two stations — one, a strong one, which relies on hydrogen bonds, and another much weaker one. The strong hydrogen bonds between the rig and the deck can be destroyed by taking a proton away from each leg one at a time with base. The result is a stepwise movement of the deck down to the now preferred stations lower down the rig. By taking steps one at a time, the elevator is more reminiscent of a legged animal than it is of a passenger elevator. The deck can be returned to the top level by the addition of acid. The elevator has been made to go up and down 10 times by the consecutive addition of acid and base, respectively.
Although it has been commented in the article that distance traveled by the deck is just a little less than one nanometer — 1,000 times smaller than the thickness of a human hair — and that the force generated could be as much as 200 picoNewtons, Stoddart urged caution with respect to this claim, based on calculations carried out in Bologna, until it is backed up by experiments, and also by an all-encompassing theory.
A common theme of Stoddart's research is the quest for a better fundamental understanding of self-assembly and molecular recognition processes in chemical systems. He has been working for more than a quarter of a century on using this growing understanding to develop template-directed protocols that rely upon such processes to create molecular switches and motor-molecules. Underlying his bottom-up approach to the construction of functioning nanosystems is Stoddart's philosophy of transferring concepts from biology into chemistry.
Despite the rarefied scientific atmosphere, Stoddart's highly specialized world might be more akin to that of an engineer or an artist than a scientist. In fact, in his quest to create mechanoelectrochemical systems, Stoddart likens himself to the painter who creates abstracts, rather than one who produces landscapes and portraits.
"Constructing artificial molecular machines is a pursuit that allows the chemist to enter the world of the engineer," Stoddart said. It is not an area of research for the faint-hearted. Many chemists and engineers have their misgivings about where all the effort is going to lead to in the fullness of time. Stoddart's answer to the skeptics is to draw a comparison between natural and unnatural systems using the action of flight.
Birds, bees and bats have been flying around for a long time. It is only in the past 100 years that humankind has learned how to fly. Prior to the first demonstration of manned flight, there were many great scientists and engineers who said it was impossible.
"Building artificial molecular machines and getting them to operate is where airplanes were a century ago," Stoddart said. "We have come a long way in the last decade, but we have a very, very long way to go yet to realize the full potential of artificial molecular machines."
"The main reason for doing this kind of nanoscience is that it is intellectually and technically challenging," said Badjic, who was one of five recipients March 17 of a UCLA Chancellor's Award for Postdoctoral Research.
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