Imaging breakthrough for superstructures

Editorial

Rebecca Pool

Thursday, July 13, 2017 - 11:45
Image: A new metallic-organic framework is made up of uranium oxide nodes and tricarboxylate organic linkers. [Northwestern University]
 
Claiming two world firsts, researchers from Pacific Northwest National Laboratory and colleagues have used low-dose electron microscopy to image a metallic-organic framework down to the atomic level.
 
The bottom-up construction of intricate superstructures from simple building blocks remains one of the most difficult challenges in chemistry, while imaging these structures to high resolution has also eluded researchers.
 
For example, past MOF analysis has taken place using x-ray diffraction, providing only a general overview of the structure.
 
This lack of structural detail has thwarted development of materials that can be used, for example, as molecular sieves and filters to remove pollutants from various sources.
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But now, in a breakthrough for chemistry, researchers have built a porous uranium-based MOF - NU-1301 - comprising 10 uranium nodes and 7 tricarboxylate ligand linkers.
 
As Pacific Northwest National Laboratory researcher, Dr Nigel Browning, points out, the organic linkers are sensitive to the electron beam, so while electron microscopy could provide a detailed image, the MOF would be damaged in the process.
 
With this in mind, Browning and colleagues turned to low-dose scanning transmission electron microscopy imaging to take images near the atomic scale.
 
They directed a dose of one electron per angstrom squared into the MOF, studying the metal nodes in Z-contrast imaging mode.
 
Using this method, they were able to image the arrangement of thousands of uranium rings, discovering that the structure has a 173.3 angstrom cubic unit cell enclosing 816 uranium nodes and 816 organic linkers; the largest unit cell found for any non-biological material.
 
According to Browning, the next step in the research is to grow the MOF inside the electron microscope to document how the framework self-assembles.
 
The researchers will also continue to examine the complex structures of NU-1301.
 
Research is published in Science.
 
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