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 -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. Planar 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. © 2004 Elsevier B.V. All rights reserved.