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 (0.5-5.0ppm) was examined by using the photo-reflection spectral technique. The optical spectra of the films showed an intense Soret-band at lambda(m) 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 0.5ppm it resulted in the order of M/T=8 > 4 approximate to 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. (c) 2005 Elsevier B.V. All rights reserved.