Osaka-based researchers developed a new method to create films of porous metal–organic frameworks fully aligned on inorganic substrates. The method is simple, requiring only that the substrate and an organic linker are mixed under mild conditions, and fast, producing perfectly aligned films within minutes. The films oriented fluorescent dye molecules within their pores, and the fluorescence response of these dyes was switched on or off simply by rotating the material in polarized light.
Metal–organic frameworks, or MOFs, are highly ordered crystalline structures made of metal ion nodes and organic molecule linkers. Many MOFs can take up and store gases, such as carbon dioxide or hydrogen, thanks to their porous, sponge-like structures.
MOFs are also potential chemical sensors. They can be designed to change color or display another optical signal if a particular molecule is taken up into the framework. However, most studies on MOFs are performed on tiny single crystals, which is not practical for the commercial development of these materials.
Chemists have now come a step closer to making commercially viable sensors that contain highly ordered MOFs, thanks to the collaboration of an international team of researchers at Osaka Prefecture University, Osaka University and Graz University of Technology. The method will allow researchers to fabricate large tailor-made MOF films on any substrate of any size, which will vastly improve their prospects for commercial development.
In a study recently published in Nature Materials and highlighted on the cover and in the ‘News and Views’ section of the journal, the Osaka-based researchers report a one-step method to prepare thin MOF films directly on inorganic copper hydroxide substrates. Using this method, the researchers produced large MOF films with areas of more than 1 cm2 that were, for the first time, fully aligned with the crystal lattice of the underlying substrate.
Noting that microcrystals of copper hydroxide can be converted into MOFs by adding organic linker molecules under mild conditions, the researchers used the same strategy to create a thin MOF layer on larger copper hydroxide substrates. They carefully chose the carboxylic acid-based linker molecule 1,4-dibenzenedicarboxylic acid because it fit exactly to the spacing between the copper atoms on the substrate surface.
A MOF film began to grow on the copper hydroxide substrates within minutes of mixing it with the linker molecule, making this technique much easier and faster than previous step-wise approaches to build up MOF films. Using microscopy and X-ray diffraction techniques, the researchers found that the film was precisely oriented along the copper hydroxide lattice.
To demonstrate the unique optical behavior of their films, the researchers filled the MOF’s ordered pores with fluorescent molecules, which fluoresce when light is shone on them in a particular direction. When they shone polarized light on the ordered material, the researchers found that they could easily switch the fluorescence response on or off simply by rotating the material.