Metallo-porphyrin zinc as gas sensitive material for colorimetric gas sensors on planar optical waveguides

The presented work focuses on the investigations of a metallo-porphyrin and its gasochromic behavior to different gases. Gasochromic materials change their color while they are exposed to a certain gas. So they offer the possibility to develop highly selective chemical gas sensors and gas sensing systems. The focus of this work is the characterization of the metallo-porphyrin 5, 10, 15, 20-tetraphenylporphyrin-zinc (ZnTPP). Nonetheless, there is a wide range of other possible metallo-porphyrins. When embedded into a polymeric matrix (PVC) a color change to the toxic gas NO₂ can be detected. To develop a stand-alone gas sensor, the porphyrin/PVC matrix is deposited onto a planar optical waveguide. The color change of the porphyrin dye can be detected in the evanescent field of the optical waveguide. Therefore, the light of a high power LED is coupled into the waveguide. The color change of the porphyrin is detectable with photodiodes as a variation of the out-coupled light intensity. The sensor shows no unwished sensitivities to CO₂ and CO and only low to NH₃. NO₂ is detectable with a resolution of 1 ppm.