Litmus paper is a fairly common tool in at least school chemistry labs as a means to measure the pH of a solution. With a drop of the solution, chemicals in the paper react and change color depending on the presence of positive or negative ions. Researchers at Rice University and MIT have created a polymer that also changes color, but does so in response to specific ions, instead of just the count of a particular ion.
The polymer is a combination of hydrophobic polystyrene and hydrophilic poly(2-vinyl pyridine) or P2VP. When in solution these compounds remain separated, but once applied to a surface and allowed to dry, they self-assemble into the alternating nanolayers of a photonic material. Normally the polymer is transparent, but as ions enter the material, the photonic properties of it change, causing some colors to be reflected.
Potentially this could be used to detect specific chemicals, such as those released as food spoils or as multiband optics in laser-driven systems. Whatever it is used for though, it will be cheap as the researchers can already produce enough of it to cover a football field for only one hundred dollars.