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Metal-substituted phthalocyanine (MPc) thin films have been well documented as sensitive materials for detection of nitrogen dioxide (NO2) gas. The mechanism for detection is such that a MPc thin film acting as the electron donor possesses a strong chemical adsorption affinity towards NO2 molecules acting as electron acceptors. The resulting charge-carrier complex formed significantly increases MPc thin-film conductivity. Sensors based on iron (II) phthalocyanine (FePc) thin films were fabricated by physical vapor deposition of FePc onto gold interdigitated electrodes patterned on an oxidized silicon substrate. Sensor exposure to NO2 concentrations ranging from 0.25 to 2 ppm under an ultrahigh-purity nitrogen environment was conducted. Results show that room temperature as-deposited 400-nm FePc films with no post-deposition heat treatment produced a sensor that can reliably differentiate NO2 concentrations in the range of 0.5 to 2 ppm NO2 under a nitrogen atmosphere, via passive monitoring of relative resistance change.