Chemical sensors are used in a large variety of applications where manual chemical analysis is at a disadvantage. Chemical analysis may be difficult to perform, for example, because the sample is too small, because the measurement results are needed immediately to close a feedback loop, or because the measurements have to be performed in remote locations. One way to construct a chemical sensor is by measurement of a material property that reversibly changes upon exposure to the chemical species of interest. The property must be easily measurable (in practice, only optical and electrical material properties seem to meet this criterion), and it must change considerably when the material is exposed to the target species, but it must not depend on exposure to any other species or on temperature (that is, it must not be cross-sensitive). Practical sensors meet only a relevant subset of these criteria; that is, they can be used over only a certain range of temperatures or in a certain concentration range.