An improved azo chromophore for optical NO2 sensing

In previous studies, the response to NO2 of a number of substituted azobenz enes was characterised for sensitivity and kinetics over a range of tempera tures. These studies indicated that the optimal sensor of this sort must be based on a chromophore with a binding energy for the target gas of approxi mately 70 kJ mol(-1). Here, we present data for the best chromophore which has resulted, dispersed in an addition-cure polysiloxane matrix. We charact erise the activation energy for the binding of NO2 to the azobenzene sites and report measurements of the response time of thin films from which the d iffusion constant of NO2 in the matrix is deduced. We report on the behavio ur of the films when operated in the cycled mode. We present quartz-crystal microbalance data allowing absolute calibration of the fractional site occ upancy in a related system.