Scientists tie the nano-knot

What do a clove hitch, a sheet bend and a sheepshank all have in common? They are of course, as any former scout will tell you, all knots. But I bet they couldn’t tie an 819 knot: at less than a millionth of a millimetre across, it’s the world’s smallest knot and it’s just been tied by a team at the University of Manchester. They made the molecular tangle in a test tube using a sequence of carefully-controlled chemical reactions that used iron catalysts to bend and entwine short strings of carbon-rich molecules. Tom Crawford heard how from lead author David Leigh...

David - What my group’s done is tied the smallest, tightest knot that’s been tied to date. So the knot has eight crossings in 192 atom strand and that makes it the tightest knotted physical structure ever made.

Tom - How small is this thing?

David - The width of the molecular strand is just half a nanometer, so that’s less than a millionth of a millimeter. So that’s ten thousand times thinner than a human hair. And the length of the molecular strand if it was opened out is just 20 nanometers, so that’s five hundred times smaller than a red blood cell, one of the smallest cells in the body.

Tom - Wow! Tiny then?

David - Very, very small indeed - yeah.

Tom - And when you say a knot, do you mean like me tying my shoelace, or like a fisherman’s knot?

David - Yeah, it’s exactly the same principle, but in mathematics a knot actually describes a closed loop. So this is exactly like the sorts of knots that you would tie in your shoelace or a fisherman would tie except it’s got no end, so the ends have been fused together.

Tom - So, if I tied my shoelace as I normally do and then the two straight bits that are left, if they were fused together?

David - Yeah, if you just glued those together then you’d get what a mathematician would call a closed knot.

Tom - What does the knot actually look like?

David - Obviously it’s too small to see; it’s very tiny. But if you use a technique to look at the positions of the atoms, which we can do very precisely with a technique called X-ray crystallography, you can see that it looks a little bit like a four leaf clover with the strands wrapping round the outside of the leaves of the four leaf clover, and then they cross over and under each other eight times.

Tom - That leads me quite nicely in the next question actually - how did you make these knots?

David - You can’t simply tie molecular strands in the same sort of way that you would tie a strand in the big world into a knot, they’re just too small to grab hold of the ends. So what we use is a technique called self-assembly in which the molecular strands are woven around metal ions. The metal ions are sticky in certain places and positions of the ions and the strands, the building blocks, wrap around those in a precise way forming the crossing points in the right places, just like happens in knitting. Then once all the pieces are assembled in the the right way, then we use a chemical catalyst to fuse the ends of those strands together to close the loop and form the completed knot.

Tom - So now that you’ve made this knot, what uses do you see for it going forwards?

David - Knotting, of course, is also a very similar process to weaving. And so it should be possible for us to use the same techniques that we’ve used for knotting molecules to actually weave molecular strands, and in that way we hope to make strong, flexible, light materials out of molecular strands.

Tom - How does weaving lead to stronger materials, for example?

David - The benefits of weaving fabrics we can see in our big world, of course, mankind has been doing it ever since we moved out of caves and used knotting and weaving for making fabric, but also for making tools and materials. It allows fabrics to stretch in different directions, to hold their shape and to be light and strong and flexible. And examples of where this might be useful - weaving on a molecular level might be say kevlar, which is a type of super-strong plastic which is used in bulletproof vests and knife-proof body armour. Kevlars chemical structure is, basically, a whole lot of tiny straight rods that pack very closely together - a bit like pencils stuffed tightly in a pencil box. What we may be able to do is actually weave strands of materials instead of having things that are packed closely together and maybe that will lead to lighter, stronger, and more flexible materials than having them all packed tightly together like they currently are.