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

Nitrogen dioxide (NO(2)) is a highly toxic oxidizing gas generally produced as a byproduct of combustion processes and also commonly stored in high concentrations as dinitrogen tetroxide (N(2)O(4)) for military and industrial use. A passive, unpowered sensor for the detection of NO(2) 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 NO(2) 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) NO(2) equilibrated in Nitrogen (N(2)) gas; sensors were tested over an extreme diurnal temperature range of 46 to 71 degrees C, demonstrating two orders of magnitude change at 71 degrees C and up to four orders of magnitude change at 46 degrees 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, NO(2) concentrations in the range of 0.5-2 ppm was consistently detectable. A passive FePc based NO(2) sensor can provide passive long-term monitoring of toxic NO(2) levels with high cost-effectiveness and minimal maintenance under a large operating temperature range. (C) 2010 Elsevier B.V. All rights reserved.