Farha’s team tested the fabric’s efficacy in conditions that would be fairly realistic for an active duty soldier, dirtying it with diesel and artificial sweat, for example. These contaminants did not significantly lower its performance. In fact, sweaty fabric performed better than clean fabric—probably because of the extra water.
MOF-808 belongs to a larger class of molecules known as metal-organic frameworks, which chemists have begun to use to more precisely control chemical reactions. Broadly speaking, these frameworks consist of metal atoms linked to chains of organic molecules to form cage-like crystalline structures, which can be put in a powder form. Chemists can tune the properties of these structures to attract specific molecules like water. You can think of these molecules as being like folded-up accordions: extensive surfaces fitted into compact spaces. This expansive surface area allows MOF-808, for example, to collect a lot of water relative to its size. Just a dime-sized dollop of metal-organic frameworks comprises about two football fields’ worth of surface area, says chemist Yuzhang Li of Stanford University.
Once these molecules get stuck inside the cage, chemists can then direct them to interact in a desired way. Researchers have designed more than 50,000 types of metal-organic frameworks, each a potential stage for a particular set of chemical reactions. In particular, chemists want to use these customized cages for storing gases—perhaps for trapping carbon dioxide produced at a coal plant, or storing hydrogen gas for fuel cells.
Farha’s fabric coating also uses a polymer called polyethylenimine, which glues the metal-organic framework to the cloth evenly. But achieving this uniform layer was a bit of a fluke. Chemists don’t have a detailed picture of how a metal-organic framework attaches to a surface, so they’re still not clear on the best way to make the molecules stick.
Li has developed a technique for photographing metal-organic frameworks that could help answer this question. In Li’s method, he triggers the metal-organic framework to undergo a chemical reaction, and then plunges it into liquid nitrogen. Then, he photographs the framework under a microscope. The method, known as cryogenic electron microscopy, is adapted from a similar technique in biology. It freezes the chemical reaction in time, allowing a chemist to study the reaction frame by frame. Li’s team used the technique to image a carbon dioxide molecule trapped inside a metal-organic framework. These more detailed images could lead researchers to design frameworks that perform specific chemical reactions better, says Li.