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 more easily manufactured and characterised than previously described organic transistor gas sensors. Resistance R is high but within the range of modern voltage buffers. The chemiresistor weakly responds to several gases, with either a small increase (NH₃ and H₂S) or decrease (SO₂) in R. However, the response is low at environmental pollution levels. The 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 do not interfere with the sensor. Above a threshold of ~87 ppb, the response to NO₂ becomes very strong. This response is presumably due to the doping of CuPc by the strongly oxidising NO₂, and is far stronger than for previously reported CuPc chemiresistors. We relate this to differences in the film morphology. Under 1 ppm NO₂, R drops by a factor of 870 vs. non-polluted air. An amount of 1 ppm NO₂ is far above the 'background' environmental pollution, thereby avoiding false alarms, but far below immediately life-threatening levels, thus giving time to evacuate. Our sensor is destined for leak detection in the nitrogen fertiliser industry, where NO₂ is an important intermediate.