Miniature CO₂ sensors could be mass-produced inexpensively.

Nanocrystalline tin oxide (SnO₂) doped with copper oxide (CuO) has been found to be useful as an electrical-resistance sensory material for measuring the concentration of carbon dioxide in air. SnO₂ is an n-type semiconductor that has been widely used as a sensing material for detecting such reducing gases as carbon monoxide, some of the nitrogen oxides, and hydrocarbons. Without doping, SnO₂ usually does not respond to carbon dioxide and other stable gases. The discovery that the electrical resistance of CuO-doped SnO₂ varies significantly with the concentration of CO₂ creates opportunities for the development of relatively inexpensive CO₂ sensors for detecting fires and monitoring atmospheric conditions. This discovery could also lead to research that could alter fundamental knowledge of SnO₂ as a sensing material, perhaps leading to the development of SnO₂-based sensing materials for measuring concentrations of oxidizing gases.