A team from MIT has published a paper in which they describe a bomb detection experiment conducted with spinach plants. The plants, which are sensitive to the presence of “nitro-aromatics” (key components of several bomb-making substances), have carbon-nanotube-based nanoparticles embedded in their leaves that give off infra-red light. A Raspberry Pi with an IR camera is used to detect this light. You can read the full Nature Materials article here, a slightly less complicated explanation at livescience.com here and an even simpler explanation over at the BBC.