A MOLECULAR piston has been spotted shuttling back and forth for the first time. The researchers behind the feat say the piston could be harnessed to drive future nanoscale machines or form part of a molecular memory in which the piston’s position represents digital 0s and 1s.
The piston works in conjunction with a synthetic organic molecule called a rotaxane. As it slides along the rotaxane’s rod-like backbone it comes to rest over one of two points: a naphthalene group towards one end of the backbone, or a sulphur-containing thiafulvalene group towards the other. Bulky blocking groups sit at each end of the backbone to stop the piston sliding off.
The molecular-scale machine made by chemist Fraser Stoddart at the University of California in Los Angeles uses oxidation and reduction reactions to send the piston back and forth. In other rotaxane molecules, the motion can be triggered by light or electricity.
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The piston comprises six carbon rings joined up to form a hoop-shaped positively charged ion. The piston’s preferred location is normally around the thiafulvalene. To move it to its alternate position around the naphthalene, the rotaxane molecule is oxidised by an iron compound that strips two electrons from the thiafulvalene, leaving behind two positive charges (see Graphic). These repel the positively charged piston, which then settles above the naphthalene, where it stays until a reducing reaction removes the positive charges from the thiafulvalene, allowing the piston to return.
That, at least, is how it works in a liquid suspension. Whether the piston could ever work in this way outside a fluid was unclear. Now Stoddart’s group has published the first evidence that the piston-like action is indeed achieved on a surface (Nano Letters, vol 4, p 2065).
Using a technique called X-ray photoelectron spectroscopy (XPS), the researchers were able to visualise the rotaxane molecule when the thiafulvalene was both charged or uncharged. Sure enough, XPS revealed that when the thiafulvalene was uncharged, the piston was at that end, and when it was charged the piston was over the naphthalene, 3.7 nanometres away. As it moved, it exerted a force of around 100 piconewtons. That is an order of magnitude greater than that produced by biological motor molecules like kinesin, which is involved in muscle contraction. Stoddart has already made the piston do some real work. He built a nanomachine that uses a pair of pistons to lift a gold cantilever.
