THE tiniest car in the world has gone for a drive. Made of a single molecule, the 鈥渧ehicle鈥� has four wheel-like paddles that rotate in the same direction when zapped with a beam of electrons.
鈥淭he molecule is autonomous,鈥� says Syuzanna Harutyunyan, a chemist at the University of Groningen in the Netherlands who worked on the mini motor vehicle. 鈥淵ou don鈥檛 need to touch it. Just give it energy and it鈥檚 capable of converting that energy into movement.鈥�
鈥淭he nanocar is autonomous. Just give it energy and it converts that into movement鈥�
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The nanocar could be used to transport miniature loads of cargo and to help unravel why tiny motors in nature tend to be so much more efficient than large-scale ones.
To create the nanocar, Harutyunyan and her team designed a molecule with a long central body and one pivoted paddle at each of four corners. The paddles are free to swing around in circles, not unlike wheels.
Ordinarily they arrange themselves so as to minimise crowding with the central body, as this costs the molecule the least amount of energy. But when the team applies a pulse of electrons to the 鈥渨heels鈥�, some gain energy and move a quarter turn.
In this new position, the wheels experience overcrowding against the body of the nanocar and will move to a more spacious position as soon as possible. They get this opportunity when the bonds holding the wheels to the body stretch, prompting the wheels to move another quarter turn in the same direction, to a more 鈥渃omfortable鈥� position. A further pulse of electrons repeats the process (see diagram).
Frigid temperatures of 7 kelvin (-266 掳C) help this clunky forward motion by effectively freezing the wheels in place except when excited by the electron pulse and during their subsequent self-adjustment. This keeps them from rolling backwards.
A nanocar had been built before but its wheels only spun in place, equivalent to placing a car on blocks to test it. By contrast, the new vehicle moves in a straight line (Nature, ).
It鈥檚 a slow road trip: it takes 10 pulses of electric fuel to move the vehicle 6 nanometres. The head of a pin is about a million nanometres wide.
Nonetheless, nanocar team member Karl-Heinz Ernst at the University of Zurich, Switzerland, is anxious to put it to work. 鈥淲e have a locomotive,鈥� he says, 鈥渂ut it鈥檚 time to put some cars at the back and pull them along.鈥�
Paul Weiss at the University of California, Los Angeles, says the car can help us unravel why tiny motors in nature, such as the motor proteins that move material around in cells, are so highly efficient. 鈥淭he reason we work at these small scales is so that we can really understand the motion and efficient energy conversion.鈥� He hopes this will lead to more efficient large-scale motors.
Famous knot, tied by 160 atoms
Just 160 atoms have been made to combine by tying themselves into the smallest version of the pentafoil knot ever made. It is also the most complicated knot ever achieved by a single molecule.
The knot, also known as the cinquefoil, or Solomon鈥檚 knot, is a 鈥減rime knot鈥�. Its woven star-shape contains five crossing points and cannot be built from smaller knots, similar to the way a prime number cannot be built by multiplying smaller numbers. A version features on the flags of Ethiopia and Morocco, giving it cultural as well as mathematical significance.
Chemists have previously created a prime knot called a trefoil, which has three crossing points. David Leigh at the University of Edinburgh, UK, and colleagues wove the pentafoil using 鈥渘eedles鈥� made of positively charged iron ions attached to long, skinny organic-molecule 鈥渢hreads鈥�.
When the team added negatively charged chloride ions, these ions became hubs, each attracting exactly five needle-and-thread compounds. In the process of arranging themselves around the central hub, the metal ions folded the organic molecules over one another, braiding them into a woven star shape. Finally, chemical bonds formed that connected the strands at the points of the star, turning the whole arrangement into a single molecule.