J. Am. Chem. Soc. 131, 6692–6694 (2009)

Like a pen writing with molecular ink, the copper-coated tip of an atomic force microscope can attach molecules from a solution to anchored molecules on a surface through chemical reactions, forming patterns with lines as narrow as 50 nanometres.

Fraser Stoddart and his colleagues at Northwestern University in Evanston, Illinois, achieved this precision by exploiting the bond-forming reaction between two chemical groups: azides and alkynes. Molecules decorated with these groups link in the presence of copper, so the tip acts as a moving, solid catalyst. It could be used to write arrays of modified biomolecules onto a surface.

A related 'dip-pen' technique coats the tip with a copper-containing solution, but has a lower writing resolution of 300 nanometres.