Oxidised porous silicon impregnated with Congo Red for chemical sensoring applications

A method for the impregnation of oxidised porous silicon (OPS) with Congo Red (CR), an anionic dye used as pH indicator, has been studied in order to obtain a pH optical sensor for gases. The distribution of CR inside the mesopores has been investigated by means of cross-section μ-Raman spectroscopy. The optical properties of the hybrid material have been characterised by means of UV-Vis spectroscopy in the specular reflectance mode. The as-prepared sample shows an absorption band centred on about 480 nm characteristic of the basic form of the indicator. The exposure to acidic vapour (HCl) causes the red-shift of the band to about 590 nm, where the absorption of the acid form of the dye is expected. Introduction of ammonia restores the initial condition evidencing a full reversibility of the process. waveguides, consisting in double layer structures of the hybrid material have been prepared and how the light is guided depending on the presence of acidic/basic vapours has been preliminary characterised.