Chemists at Italy's University of Bologna, UCLA and the California NanoSystems Institute have designed and constructed a molecular motor of nanometer size that does not consume fuels; their nano motor is powered only by sunlight. The research, federally funded by the National Science Foundation, will be published Jan. 31 in Proceedings of the National Academy of Sciences (PNAS).
The nano motor can work continuously without any external interference, and operates without consuming or generating chemical fuels or waste, said Fraser Stoddart, UCLA's Fred Kavli Professor of NanoSystems Sciences and director of the institute.
"We design and make sunlight-powered nano motors and then 'test drive' them much as an engineer would a new motorcar," Stoddart said. "It is as if we had managed to get a solar-powered motor car onto the road and running."
Precisely how light-powered nano motors will be used in the future is not yet clear, Stoddart said, but he listed a number of possible areas for applications: nanoelectronics, molecular computers and nano valves that perhaps could be used for the delivery of anti-cancer drugs and other medications.
"The achievement reported in PNAS is the culmination of a research effort lasting a quarter of a century and involving hundreds of students and millions of dollars," Stoddart said.
What is the nano motor's composition, and how does it work?
The nano motor is a multi-component molecular-scale system called a rotaxane, a mechanically interlocked molecule consisting of one or more rings trapped on a rod by bulky stoppers at either end, in a manner reminiscent of an abacus. The system is built up from two separate molecular components: a dumbbell-shaped one, which is more than six nanometers long, and a ring component of a diameter of approximately 1.3 nanometers. The ring component is trapped on the rod portion of the dumbbell by two very bulky stoppers attached to the ends of the rod so that, although the ring can move along the rod, it cannot go over the stoppers at the ends. The rod portion of the dumbbell contains two "stations" that can be called "A" and "B."
"It is the attractiveness between the ring and stations A and B that assists us in making the molecules in the first place," Stoddart said. "The attractiveness for these two stations (A and B) lives on in the two-state or bistable rotaxane after it has been made. The final requirement in the design of the nano motor is that the ring prefers, in the starting state of the molecule, to surround one of the two stations, let us say A. In order to induce the ring to move from A to B we have to make A temporarily a less desirable station such that the ring will spontaneously migrate to station B.
"The linear nano motor works as follows. The absorption of sunlight by one of the two stoppers, a light-harvesting one, causes the transfer of one electron to station A, which is deactivated as far as wanting the ring to encircle it. As a consequence, the ring moves to its second port of call, station B. Station A is subsequently reactivated by the return of the transferred electron to the light-harvesting stopper, and the ring moves back to this station.
"The system operates according to a four-stroke cycle which is reminiscent of an internal combination engine in a motor car: 1) light excitation and subsequent transfers of an electron ("combustion"), 2) displacement of the ring along the rod from A to B ("piston displacement"), 3) removal of the electron received by station A ("exhaust removal"), and 4) relocation of the piston. The motions executed by the nano motor are quite rapid: a full cycle is carried out in less than one-thousandth of a second, which means that the motor can operate at a frequency of 1000 Hertz — a speed that is equivalent, using the car engine analogy, to 60,000 rpm."
Where does the research go from here?
"The research will have to go in the direction of taking the nano motors and putting them on surfaces and into membranes," Stoddart said. "The science and engineering to achieve these objectives is daunting and tough, but the rewards are correspondingly great and satisfying. The kind of nanotechnology that will emerge from these nano motors still requires a lot of fundamental work. The nano motors are extremely sophisticated in their design."
The nano motor was designed, assembled and run by the research groups at UCLA and the University of Bologna working closely together.
The international research team included Vincenzo Balzani, Alberto Credi and Margherita Venturi from the University of Bologna's Photochemistry and Supramolecular Chemistry Group.
The research also was funded by Italy's Ministry of Education, University and Research, and by the European Union.