It’s every chemist’s dream — arranging and rearranging molecular pieces into controlled structures. Mark Allendorf (8324) and other chemists at Sandia have made that dream a reality through a highly successful Laboratory Directed Research and Development project that has yielded exciting results and a few surprises.
Those molecular tinker toys are metal-organic frameworks or MOFs, a new class of nanoporous materials. “For many years people have tried to make porous materials out of a combination of organic and inorganic pieces but were never able to achieve permanent porosity,” says Mark.
Then came MOFs. These materials, formed from seemingly modest chemical components, have generated big results for Mark and his team. An LDRD project to study MOFs as a discovery platform for confined space chemistry has, in less than three years, resulted in outside funding, two patent applications, six publications with more in submission, and three major findings.
MOFs were discovered in 1999 by Omar Yaghi, who was a professor at the University of Michigan at the time (he has been at UCLA since 2006). MOFs are positively charged metal cations that are linked to anionic organic groups, which are called a coordination bond. Yaghi learned that by properly selecting the structure of the organic component and the coordination geometry of the metal, he could condense them into crystals with specific morphologies that preserve their porosity.
“The first MOF that Yaghi reported was pretty spectacular. It has a surface area more than three times that of industrial-grade molecular sieves, such as zeolites,” says Mark. “It really does look like tinker toys.”
In late 2005, with a little less than $500,000 in annual LDRD funding for three years, Mark, Blake Simmons (8755), Jeffery Greathouse (6316), and several postdocs began brainstorming research avenues to pursue. The problem was not coming up with ideas, but finding the resources to pursue all of their ideas.
“We felt like kids in a candy store, with all of this low-hanging fruit,” Mark adds.
A new class of scintillators
One of the first areas they pursued was a fluorescent framework, which could be used as a sensing mechanism. Postdoc Christina Bauer created two new MOFs, both of which were fluorescent and one of which was porous, a result that Mark says was satisfying in its own right.
Then, at a talk she gave on the new MOFs, Patrick Doty (8772) observed that they contain stilbene, a well-known scintillator. This got him wondering if the new MOFs could be used as a scintillator.
Patrick tested the MOFs and the results were better than anyone had anticipated. They not only scintillated, but they performed so well that they turned out to be the first truly new class of scintillators found in decades.
“What is really exciting is that these structures have scintillation built into the framework, but because they are porous you can add other materials,” says Mark. “For example, you could shift the wavelength of the scintillation light to a more convenient area, making it easier to detect, or add helium-3 (He-3) gas to make radiation detection more efficient.”
Once again, the researchers had that kid-in-a-candy-store feeling, as the new scintillators opened up a whole raft of ideas that would be difficult to pursue with other materials. Earlier this year, the Defense Threat Reduction Agency (DTRA) agreed to fund a Sandia proposal to build a better understanding of the fundamental processes controlling neutron-generated scintillation in MOFs. This is a particularly important research area because neutrons are emitted by fissionable material but are difficult to detect using current materials that can only detect the highest energy or slowest neutrons.
“In less than three years, the original LDRD had already pulled in outside funding, which is pretty unusual,” Mark says.
Microcantilevers springboard chemical detection
Another big result was in chemical detection. Some MOFs can shrink by up to 10 percent when water is added without actually changing their crystal structure.
“For a sensor, if you take a material that changes its dimension when it absorbs something and mate it with another material, the absorption will create a stress at the interface between the two materials,” explains Mark. “You can then detect that stress using a piezo resistance sensor.”
He formed a collaboration with Peter Hesketh, a mechanical engineering professor at Georgia Tech who works on microcantilevers (approximately 40 microns wide by 100-300 microns long) machined using MEMS technology. As Mark explains, the challenge of microcantilevers is making them responsive to specific stimuli. One method is to put organic polymers on top of the microcantilever, but those materials are not porous.
“MOFs are porous and rigid in comparison with an organic polymer. We hypothesized that if we could glue an MOF layer onto the microcantilever surface so that there was no slip, then we would be able to very sensitively detect absorbed molecules,” he says.
After nearly two years of work, the researchers were able to apply a uniform layer of MOFs onto the microcantilever. It then took several months to prove that they had indeed accomplished their task. The small size of microcantilevers means that most analytical techniques don’t work.
The results, again, were stunning. The MOF-coated microcantilever responds in less than a second to water vapor, ethanol, and methanol and does so differentially, meaning that the response is unique to the chemical detected. The researchers also found that if they baked out the water, they opened up binding sites in the MOF and turned on sensitivity to carbon dioxide.
“We see resistance changes in these sensors that are very fast and extremely reversible — almost square-wave type behavior,” says Mark. “As far as I know, this is the first time anyone has ever integrated an MOF with a real device. We not only built a sensor, but we also proved it responds reversibly and that it has actual selectivity by controlling the hydration state of the layer.”
The team submitted a patent application and a journal article describing the results is currently being reviewed.
A potential application is for breath analysis. A symptom of pulmonary distress, especially in children, is increased levels of nitric oxide and carbon monoxide in the breath. Mark envisions a device based on an MOF microcantilever that could detect dangerous levels in a doctor’s office or even at home. The team has submitted proposals to pursue this application to the National Institutes of Health.
Jeffery led the third major accomplishment from the original MOF LDRD — development of a computational tool that enables prediction of how MOFs would react when they adsorb gases. As Mark explains, this had been done before but always under the approximation that the framework atoms were fixed at their crystallographic coordinates.
Jeffery wanted to look at reactivity so he invented a flexible force field to allow MOF atoms to move during the simulation and to reproduce the structural changes that result from adsorption of various molecules.
“Our force-field approach has led to new insight into the physical and mechanical properties of MOFs, such as the unusual trend of negative thermal expansion,” he says.
The project team has published several papers, including two in the Journal of the American Chemical Society. The first paper, on water reactivity with MOFs, had 17 citations within the first 18 months of publication.
A bright future
The potential applications for MOFs seem to be limited only by the imagination of the researchers. Research is underway on chemical separations, other gas storage that could include CO2 sequestration, and catalysts. More distant applications include drug delivery devices, since some MOFs react with water and fall apart in aqueous solutions, and personal exposure monitors.
“This is the first time in my 22-year career that I’ve been involved in really hot science,” says Mark. “It’s pretty fun.”