Optical gas sensing by semiconductor nanoparticles or organic dye molecules hosted in the pores of mesoporous siliceous MCM-41

The presence of different gas atmospheres is optically detected by use of molecular sieves modified with metal oxide clusters or dye molecules. The reversible redox behaviour of tin dioxide clusters, embedded in the regular pores of mesoporous siliceous MCM-41 in reducing and oxidizing atmospheres (CO, H2, NH3, O2) is studied by in-situ diffuse reflectance (DR) UV/Vis spectroscopy. By impregnation with Sn precursors a carpet of SnO2 clusters is formed on the inner pore walls of the MCM-41 due to strong interactions of the tin oxide species with the silanol groups of the MCM-41 matrix. The response time of hosted two-dimensional SnO2 layers for the registration of CO by optical detection is short and concentrations down to 20 ppm CO in air and 50 ppm of H2 or NH3 in air or Ar, respectively, can be monitored. Hydrocarbons adsorbed in the pores of siliceous MCM-41 alter, depending on their chain length, the optical scattering of the mesoporous matrix. Since the gas adsorption depends on the partial pressure of the hydrocarbons this effect can be used to detect hydrocarbons and to monitor their concentration with the help of embedded light absorbing guests. This sensing, however, is only possible with powdered samples and by measuring in diffuse reflectance according to the Kubelka–Munk formalism. The concentration of SO2 in a gas atmosphere can be deduced from the quenching of the fluorescence of rhodamine dye molecules anchored in the pores of siliceous MCM-41.