Sensing parts per million levels of gaseous NO2 by a optical fiber transducer based on calix[4]arenes

Calixarenes are interesting building blocks in supramolecular receptor design. They can be easily functionalized to give the desired guest binding and sequestration properties. We demonstrate here the use of simple alkylated calixarenes as novel NO2 sensors. Upon reacting with gaseous NO2, alkylated calixarenes form stable calixarene-NO+ (nitrosonium) complexes that have a deep purple color. This specific and selective formation of the colored complex was used to develop a fiber optic based colorimetric NO2 sensor. Several alkylated calixarenes are used and tested as sensing materials. The calixarene compound was immobilized on a fine mesh silica–gel coated thin layer chromatography plate. The sensing plate was coupled with a fiber optic based photodetector. Gas samples were sampled in a manner where they impinged on the surface of sensing plate. The light transmission through the plate was continuously monitored. For a 5 min sample, the limit of detection was 0.54 ppmv with 1,3-alternate O-hexyl calix[4]arene (1a). There were no significant response differences between different conformations of calixarenes such as 1,3-alternate or cone. This chemistry can form the basis of a colorimetric sensor that relies on extant filter tape technology.With calixarenes however, such a reaction is potentially reversible – color formed upon reaction with NO2 can be reversed by flushing the sensing plate by purified air. While we found that the removal of the developed color can be accelerated by simultaneous heating and suction, permitting the reuse of the same sensing area multiple times, we also observed that the sensitivity gradually decreased. The nitrosonium calixarene derivative tends to transform to the nitrated form; this process is catalyzed by light. Several methylated calixarenes were synthesized and tested but a fully satisfactory solution has proven elusive.