In the air of factory floors and laboratories, invisible solvent molecules drift. One of them, acetonitrile, has a faint smell, but at high concentrations it can harm health. Typical sensors for it are bulky and expensive, but here we have a thin crystalline film that responds on its own to the presence of vapor.

This crystal is no ordinary one; it's made of chains of iron atoms and organic molecules. Between the chains are voids where acetonitrile molecules slip in, like guests squeezing into a cramped entryway. When a "guest" arrives, the chains nudge apart slightly, and the crystal changes color and its ability to conduct electricity. Remove the vapor, the molecules leave, and the crystal reverts to its former self.

This material is a coordination polymer, meaning long chains in which metal atoms are linked by organic "bridges." Its main advantage is reversibility: the cycle of "capture vapor, signal, release vapor — return" repeats many times without degradation. Potentially, the same approach could detect other volatile organic compounds by simply tweaking the composition of the "entryway."