Materials imaging: Dyeing to see a difference

Metal–organic frameworks (MOF) may look like ordinary salt crystals; but they are anything but.  Because of their ultrahigh porosity, surface area and tuneable properties, MOFs are have been touted as the future materials for a wide range of applications, including catalysis, separation and gas storage. Recent research can help us further understand the compound. 

MOFs are a class of structural materials made from metal ions and organic linkers. The constituents interlock to form a crystalline lattice full of empty spaces.

The properties of MOFs are determined by the presence of metal ions, organic linkers, as well as functional groups attached to these organic linkers. Recent studies have shown that a single MOF can accommodate up to 18 functional groups without altering its structure, but elucidating the distribution of the groups remains a challenge. 

Stefan Wuttke from the University of Munich in Germany, and KACST affiliated Omar Yaghi, from the Lawrence Berkeley National Laboratory in the United States along with co-workers, have developed a fluorescence imaging-based method for visualizing functional groups and defects inside a MOF. Yaghi is also the co-director of the Center for Nanomaterials for Clean Energy Applications (CENCEA), a collaboration between KACST and the University of California, Berkley.  

The method involves adding a fluorescent dye to organic linkers so that they too become fluorescent, and then substituting conventional linkers within a MOF with fluorescent ones. Because the fluorescence of each linker is strongly affected by its environment, by observing the exponential decay rate, or lifetime, of the fluorescence, it becomes possible to map the functional groups and defects inside the MOF. 

The technique allows subtle differences, or heterogeneities, inside a single crystal to be mapped in a precise manner. By using this technique, the researchers were able to determine the chemical diversity, number, relative position and ratio of various functional groups inside the MOF UiO-67. 

The chemical industry needs to separate gases such as carbon dioxide and carbon monoxide for synthesis. Recent studies have shown that MOFs with certain combination of functional groups show up to 400 percent better selectivity for carbon dioxide over carbon monoxide. Such materials would be perfect for use in gas separation, given that they do not require electricity to operate. However, the mechanisms underlying the selectivity are not well understood. The researchers hope that through the use of the fluorescence imaging-based method, they will gain better insights into how functional groups affect selectivity and other properties in MOFs.

1. Schrimpf, W., Jiang, J., Ji, Z., Hirschle, P., Lamb, D. C., et al. Chemical diversity in a metal–organic framework revealed by fluorescence lifetime imaging. Nature Communications 9:1647 (2018).| article