In 2013, the Syrian military allegedly launched sarin gas rockets into a rebel-held town, killing hundreds. After diplomats brokered a deal to eradicate the weapons, international organizations began the dangerous job of destroying them. One roadblock to chemical weapons disposal is that heat and humidity quickly break down enzymes that can disable the deadly chemicals. Now, researchers have developed a highly stable compound that can inactivate nerve agents like sarin in a matter of minutes.
To create the compound, chemists Omar Farha and Joseph Hupp of Northwestern University in Evanston, Illinois, turned to nature for inspiration. Bacteria produce enzymes called phosphotriesterases, which deactivate certain pesticides and chemically related nerve gases at lightning speed, wiping out weapons in milliseconds. But such enzymes are fragile and easily degraded. The chemists set out to reproduce the mechanism by which phosphotriesterase breaks down these chemicals to create a humanmade catalyst that would survive the most inhospitable conditions.
They started with metal-organic frameworks (MOFs), a recently developed class of porous compounds composed of metals arranged in a crystalline network linked by carbon-based molecules. MOFs are highly adaptable materials: Scientists can switch out the metals or the linkers to optimize the material for a variety of applications, such as carbon dioxide capture and hydrogen or methane storage. And because MOFs are porous, they have large surface areas that can rapidly create chemical bonds, making them good candidates for catalysts.
In the natural enzyme, phosphotriesterase, two zinc atoms act as so-called Lewis acids, which accept electrons to bind with the nerve agent. Once the agent has bonded, hydrolysis occurs—a water molecule attacks the agent, slicing and dicing essential chemical bonds, thereby deactivating it. The scientists designed a MOF with a similar structure, but they replaced the zinc with zirconium, which likewise behaves as a Lewis acid and makes for an ultrastable MOF.
They tested their compound, known as NU-1000, on a pesticide chemically similar to nerve agents but safer to work with. NU-1000 deactivated half of the pesticide within 15 minutes—the fastest decomposition of this chemical ever achieved with a MOF and three times as fast as the group's previous MOFs. The team then sent the compound to a U.S. Army facility equipped to study toxic nerve agents, where military scientists measured a half-life of 3 minutes for destroying the nerve agent GD, which is even more toxic than sarin, they report online today in Nature Materials. The new compound obliterates GD 80 times faster than previous MOFs.
NU-1000 improves on previous MOFs in part because the pores between its metal nodes are larger, allowing the chemical agent to penetrate within the MOF structure and interact with zirconium throughout. And the reaction doesn’t use up or degrade NU-1000, so relatively small quantities are sufficient, and the compound is reusable. “The MOF approach is very unique and very promising,” says chemist Banglin Chen of the University of Texas, San Antonio, calling it a “really great result.”
But more work is necessary to make NU-1000 ready for action. The natural enzyme, phosphotriesterase, is between 1000 and 100,000 times faster, says biochemist Frank Raushel of Texas A&M University, College Station. “In order for [MOFs] to be practical in any sense, they are certainly going to have to get better,” he says. NU-1000, for instance, is not fast enough for use in gas masks, where air must be purified quickly enough for a soldier to breathe.
The findings will guide the design of more advanced catalysts, Farha says. By modeling the chemical reaction, the group was able to understand how the catalyst worked, which highlighted the importance of large pores and the benefits of zirconium-based MOFs. “After learning from all these design rules, now we know what to do next,” Farha says. He hopes that they will eventually be able to predict how well such catalysts will function before even making them.
“It sets the stage for the future,” Raushel says. “They are going in the right direction.”