A capnometer adaptor includes a nanostructure sensor configured to selectively respond to a gaseous constituent of exhaled breath, such as to carbon dioxide. In certain embodiments, the adaptor includes an airway adaptor having at least one channel configured for the passage of respiratory gas; at least one nanostructure sensor in fluid communication with the passage, the sensor configured to selectively respond to at least one gaseous constituent of exhaled breath comprising carbon dioxide; and electronic hardware connected to the nanostructure sensor and configured to provide a signal indicative of a response of the sensor to the at least one gaseous constituent of exhaled breath. The sensor may be provided as a compact and solid-state device, and may be adapted for a variety of respiratory monitoring applications.