Development of porphyrin dispersed sol–gel films as HCl sensitive optochemical gas sensor

The porphyrin dispersed sol–gel composite films were prepared from the different molar ratios of methyltriethoxysilane (MTEOS) to tetraethoxysilane (TEOS), M/T (=4, 8, 16 and 32), and optochemical detection of hydrogen chloride (HCl) gas (0.5–5.0 ppm) was examined by using the photo-reflection spectral technique. The optical spectra of the films showed an intense Soret-band at λmax of 419.3 nm for M/T = 4–16 and 418.3 nm for M/T = 32. The sensitivity of the sensors measured at the Soret-band strongly depended on the M/T ratio of the sol–gel films and to 0.5 ppm it resulted in the order of M/T = 8 > 4 ≈ 16 > 32. From the thermogravimetric (TG) analysis, it was found that M/T = 4 contained a predominantly large amount of water adsorption. It is predicted that M/T = 8 has the highest adsorption capacity of HCl to exhibit the highest sensitivity. However, at high concentration of HCl (≥2.8 ppm) M/T = 4 exhibited a comparable sensitivity to M/T = 8 probably due to the large amount of water adsorption property of M/T = 4. In spite of the high sensitivity to HCl gas, the M/T = 8 sensor was almost silent to the 0.5 ppm SO2 exposure.