Copper Phthalocyanine Chemiresistors as Industrial NO2 Alarms

We present a chemiresistor sensor for NO2 leaks. The sensor uses the organometallic semiconductor copper(II)phthalocyanine (CuPc) and is manufactured and characterised easier than previously described organic transistor gas sensors. Resistance R is high but within range of modern voltage buffers. The chemiresistor weakly responds to several gases, with either a small increase (NH3, H2S), or decrease (SO2) of R. However, response is low at environmental pollution levels. Response to NO2 also is near- zero for permitted long-term exposure. Our sensor is therefore not suited for environmental monitoring, but acceptable environmental pollutant levels also do not interfere with the sensor. Above a threshold of ~ 87 ppb, response to NO2 becomes very strong. Response presumably is due to doping of CuPc by the strongly oxidising NO2, and is far stronger than for previously reported CuPc chemiresistors. We relate this to differences in film morphology. Under 1 ppm NO2, R drops by a factor of 870 vs. non- polluted air. 1 ppm NO2 is far above ‘background’ environmental pollution, avoiding false alarms, but far below immediately life-threatening levels, giving time to evacuate. Our sensor is destined for leak detection in the nitrogen fertiliser industry, where NO2 is an important intermediate.