Optical sensor for gaseous ammonia with tuneable sensitivity

The detection of gaseous ammonia using an optical evanescent wave sensor coated with a PVC film containing a chromogenic calixarene (nitrophenylazophenol calix[4]arene) is described. On addition of various concentrations of ammonia gas, the absorbance maximum shifts to 500 nm, which is characteristic of deprotonation of the chromogenic group, The sensitivity of the sensor to ammonia varied depending on whether the free ligand or lithium complex was used in the membrane, Intermediate sensitivities can be generated by varying the mole ratio of lithium to calixarene. The response time of the lithium complex measured at 520 nm to a 5-50 ppm change in ammonia concentration is fast (several minutes to steady state) but the signal is slow to return to the baseline when the gas is replaced by nitrogen.