Conductimetric microsensors integrating phthalocyanine nanolayers : high-performance sensing devices for the measurement of NO2 at ppb level in atmosphere

Metallophthalocyanines constitute an extended class of molecular organic semiconductors that the extrinsic electronic conductivity can be strongly modulated by the adsorption of oxidiz-ing species. More especially, taking into con-sideration the great number of pi-electron delo-calized on each macrocyle, adsorbed oxidizing pollutants like NO2 and O3 induce high p-type conductivity variations whereas adsorbed re-ducing pollutants like CO, NH3 or H2S not lead to significant response in the same range of concentrations. If previous results report the high sensitivity of phthalocyanine-based mi-crosensors to NO2 associated to a limit of detec-tion and resolution closed to few ppb, O3 re-mains the major interfering pollutant leading to substantial cross-sensitivity and long-term deg-radation of sensing layer by ozonolysis process. Moreover, the effect of sensing layer thickness on metrological performances as well as effects of humidity on response have never been inves-tigated in this range of concentrations.This paper deals with the last sensing perfor-mances of conductimetric microsensors inte-grating nanolayer of phthalocyanines as sens-ing membrane operating at low working tem-perature. Investigations on layer thickness in-fluence on sensor sensitivity established that 100nm is the optimized thickness and that thicker layers are not appropriate. With the benefit of a high signal/noise ratio, such mi-crosensors are highly efficient to measure NO2 in the 4-20 ppb range of concentration with a high level of repeatability and reproducibility, whatever the concentration variations in air, increasing or decreasing (figure 1). The limit of detection was assessed to 0,4 ppb while the se-lectivity is high, the response to the most inter-fering gas, i.e. ozone, remaining two times lower than this measured for the same concen-tration of NO2 (figure 2). Study of the effect of humidity highlights that highest responses were obtained for moderate relative humidity. Below 40% of relative humidity and above to 70%, microsensor response remains of weak magnitude. Hysteresis, mid-term stability and reproducibility of microsensors will be at last discussed