University of Manchester researchers, from the birthplace of graphene, have managed to create the most tightly knotted physical structure ever recorded. This involves braiding together multiple molecular strands. The outcome is a tighter and more complex knot than has ever physically been measured. The structure is small, on the nanoscale. In a sense it is the molecular equivalent of the Gordian Knot (from the time of Alexander the Great and often used as a metaphor for an intractable problem).
The structure, which is in the form of a knot, has eight crossings in a 192-atom closed loop. This is some 20 nanometres long, meaning that it is very small – no larger than 20 millionths of a millimeter. The molecular knot was created through a process called ‘self-assembly’, whereby molecular strands are interwoven around metal ions which form at crossing points. The ends of the strands are fused together using a chemical catalyst. With the finishes structure, octahedral iron(II) ions control the relative positions of the three strands at each crossing point in a circular triple helicate, while structural constraints on the ligands determine the braiding connections.
The way that a material is woven is the key to its potential strength. This way spider silk can be twice as strong as steel, at least in relation to the same amounts of material when the two (spider silk and high-grade alloy steel) are compared in stress and strength tests. Through braiding polymers the researchers were aiming to tap into this strength.
The aim of the study was to inform about how knotting affects strength and elasticity of materials. The longer-term aim is to create stronger structures through the weaving together of polymer strands, from which new types of materials can be generated.
The lead researcher, Professor David Leigh from Manchester’s School of Chemistry notes in a research briefing: “Tying knots is a similar process to weaving so the techniques being developed to tie knots in molecules should also be applicable to the weaving of molecular strands.”
The academic goes on to list applications where such a super-strong weave will be useful. Today most bullet-proof vests and body armor are made of Kevlar. This is a type of plastic consisting of rigid molecular rods aligned in a parallel structure. It should be possible to make such materials even stronger through the new knotting technique, by interweaving polymer strands together. The configuration has the potential to create tougher, lighter and more flexible.
The research is published in the journal Science and it is titled “Braiding a molecular knot with eight crossings.”
