Real-Time Ammonia and Humidity Monitoring with Ultra-Fast Conductometric Sensors Based on Porphyrin and Phthalocyanine Complexes

Organic semiconductors such as porphyrins and phthalocyanines are attracting a wide range of researchers due to their versatile electrical properties and sensing performances in conductometric sensors. In this study, we investigate two types of π-extended porphyrins, which share the same macrocyclic structure but differ in their central metal. These porphyrins are employed as sublayers in bilayer heterojunction devices, with the lutetium bisphthalocyanine complex, LuPc₂, serving as the common top layer. Remarkably, the central metal in the porphyrin macrocycle significantly influences the solubility of the materials and, consequently, the surface topography of the resulting bilayer heterojunction devices. This structural variation translates into distinct electrical and sensing performances. The device incorporating nickel as the metal center (AM2) demonstrates superior sensitivity toward NH₃, with a relative response (RR) of approximately -7% at 90 ppm, an ultrafast response time of about 9 s, and an impressive limit of detection (LOD) of 250 ppb. In contrast, the device that has zinc as the metal center in the sublayer (AM3) exhibits an RR value of approximately -0.9% at 90 ppm, with t₉₀ of approximately 120 s and an LOD of 2 ppm. Both devices are evaluated under randomly varying NH₃ concentrations and RH values. The results show that the AM2-based sensor allows real-time monitoring of NH₃, while the AM3-based sensor provides an average concentration over time. On the other hand, the AM2-based sensor exhibits slow kinetics under RH exposure, while the AM3-based sensor precisely mirrors the pattern of random RH changes generated by the software, demonstrating its exceptional responsiveness and accuracy in tracking humidity fluctuations. These findings underscore the critical role of the metal center in tuning the electrical and sensing properties of the heterojunction devices.