Passive chemiresistor sensor based on iron (II) phthalocyanine thin films for monitoring of nitrogen dioxide

Nitrogen dioxide (NO2) is a highly toxic oxidizing gas generally produced as a byproduct of combustion processes and also commonly stored in high concentrations as dinitrogen tetroxide (N2O4) for military and industrial use. A passive, unpowered sensor for the detection of NO2 gas was achieved using iron phthalocyanine (FePc), an organometallic molecular crystal. FePc acts as an electron donor in the presence of nitrogen dioxide gas, forming a charge carrier complex that effectively dopes the FePc. The resulting decrease in resistivity was monitored to determine ambient NO2 concentrations. FePc thin film sensors were manufactured via physical vapor deposition of FePc powder onto gold inter-digitated electrodes patterned on oxidized silicon substrates. Gas exposure tests were conducted in a Teflon PTFE test cell, with sensor resistance monitored in real time. Up to four orders of magnitude change in resistance was observed upon saturation to 100 parts per million (ppm) NO2 equilibrated in Nitrogen (N2) gas; sensors were tested over an extreme diurnal temperature range of −46−46 to 71 °°C, demonstrating two orders of magnitude change at 71 °° C and up to four orders of magnitude change at −46−46 °°C. Thermal cycling results across this temperature range indicated that sensor base resistance is mostly a function of temperature. Using normalized sensor resistance output plotted vs. time, NO2 concentrations in the range of 0.5–2 ppm was consistently detectable. A passive FePc based NO2 sensor can provide passive long-term monitoring of toxic NO2 levels with high cost-effectiveness and minimal maintenance under a large operating temperature range.