According to Official Guinness Records,
The tightest knotted structure is 2.5 nanometres per crossing, achieved by David Leigh, Jonathan J. Danon, Alexander J. Stephens (all UK), Steffen L. Woltering (Germany), Jean-Francois Lemonnier (France), Anneke Krüger (South Africa) and Iñigo J. Vitorica-Yrezabal (Spain), in University of Manchester, Manchester, United Kingdom, on 12 January 2017.Knotting is a scientific technique that can induce changes in the molecule's properties. Only four prime knot formations have been created out of more than 6 billion: the trefoil, the figure-eight, the pentafoil and now the 819. 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. This approach enables two-step assembly of a molecular 819 knot featuring eight non-alternating crossings in a 192-atom closed loop ~20 nanometers in length. Therefore the knot has 24 atoms (~2.5 nanometers) per crossing.
For a complete list of 2017 records, please visit 2017 Guinness Records in United Kingdom.