Microscopy: Bonds imaged by AFM last year may be an artifact of the technique, leaving the discovery in question

Jyllian Kemsley

The sighting of hydrogen bond interactions in atomic force microscopy (AFM) images, highlighted in C&EN last year, may be an experimental artifact. The images published by a group of researchers in China appear to show electron density where hydrogen bonds would connect 8-hydroxyquinoline molecules (Science 2013, DOI: 10.1126/science.1242603). But what the group imaged may have been caused by the interaction of the AFM tip with the potential energy surface between the molecules, according to work published this year by a separate team led by Sampsa Hämäläinen and Peter Liljeroth of Aalto University School of Science, in Finland, and Ingmar Swart of Utrecht University, in the Netherlands (Phys. Rev. Lett. 2014, DOI: 10.1103/physrevlett.113.186102). “We’re not saying there can be no contribution from hydrogen bonds,” Swart said, “but we show that you can also have contrast when there is no bond at all.” Swart and his colleagues used AFM to study tetramers of bis(p-pyridyl)acetylene molecules. The tetramers are held together by intermolecular C–H∙∙∙N hydrogen bonds and bring two nitrogen atoms on separate molecules within 3 Å of each other. The nitrogens should have no bonding interaction, yet AFM images appear to show a bond between the atoms. The results emphasize that researchers must be cautious about interpreting highly processed microscopy images, said James K. Gimzewski of the University of California, Los Angeles, and California NanoSystems Institute. “The nano is invisible, and images of it should be treated with care.”