Low density MOF makes perfect home for enzymes

Using uranium atoms and organic linkers, scientists at Northwestern University have built the lowest-density metal-organic framework (MOF) ever made.

Directed by design rules developed by the scientists, the uranium atoms and organic linkers self-assemble into a beautiful crystal – a large, airy three-dimensional (3D) net of very roomy and useful pores. Each pore is so roomy, in fact, that the scientists have been able to nestle a large enzyme inside – no small feat. The material can thus act as a protective scaffold for enzymes.

"We are building with one-atom precision," said Omar Farha, a research professor of chemistry in the Weinberg College of Arts and Sciences who led the research. "Our material begins at the level of individual atoms, measured by angstroms, and ends on the hundreds of microns level, where we can hold the small crystal with tweezers and see it with the naked eye." Farha and his colleagues report their findings in a paper in Science.

The material is very light, despite being made from uranium, one of the heaviest elements in the periodic table (the scientists used uranium-238, a nonradioactive isotope). This is because the uranium atoms are so far away from each other in the structure that the MOF has a very low density.

"It is counterintuitive," said Farha, who specializes in the rational design of MOFs for catalysis, sensing and storage applications. "This material has not been seen or predicted before. Despite its simple beginnings, our MOF's structure has an unparalleled structural complexity. And we've just scratched the surface of building sophisticated structures using simple building blocks."

"This material has not been seen or predicted before. Despite its simple beginnings, our MOF's structure has an unparalleled structural complexity. And we've just scratched the surface of building sophisticated structures using simple building blocks."Omar Farha, Northwestern University

The discovery demonstrates the benefits of creating general design rules for the self-assembly of open, complex structures from simple building blocks while also highlighting the potential of actinides such as uranium in materials synthesis, Farha said. Researchers can now apply this new set of design rules for self-assembly to synthesize various novel materials with complex structures.

MOFs are well-ordered, lattice-like crystals; the nodes of the lattices are metals, which are connected by organic molecules. The new MOF, called NU-1301, is made up only of uranium oxide nodes and tricarboxylate organic linker units. This simplicity makes the MOF industrially relevant.

In addition to its structural complexity, NU-1301 has a very high surface area, large pore volume and water stability, and can be used to separate small organic molecules and large biological molecules such as enzymes based on their sizes and charges.

Each unit cell – the basic unit that repeats in three dimensions to create the crystal structure – is composed of 816 uranium nodes and 816 organic linkers. One unit cell measures 173Å across, and it keeps repeating itself. The unit cells assemble into pentagons and hexagons, eventually growing into a crystal that can reach a quarter of a millimeter wide.

This story is adapted from material from Northwestern University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.