Solvatochromic betaine dyes as optochemical sensor materials: detection ofpolar and non-polar vapors

Solvatochromic effects in the UV/VIS spectra of dyes can be applied to the monitoring of solvents in the gaseous phase. Within the class of solvatochr omic dyes Reichhardt's betaine reveals an outstanding hypsochromic effect o n protic solvents with a band shift of up to 350 nm. For vapor detection, t he sensor behavior was optimized according to the linearity of the response signal, the sensitivity and a minimum influence of humidity by hydrophobiz ing siloxanes as additives. Furthermore, the dyes were immobilized under mi ld conditions by a sol-gel process which generates a high porosity for easy analyte access. The detection principle could be further extended to aprot ic, polar solvents by embedding the dyes in protic matrices that form hydro gen bonds to the betaine oxygen. Then, the analyte disturbs the hydrogen bo nds resulting in a bathochromic band shift resembling a back titration. Fin ally, the betaine phenol blue even allows the detection of halogenated hydr ocarbons that lack a pronounced functionality but nonetheless a hyperchromi c effect occurs during analyte exposition. Mechanistic aspects of these sol vent absorption processes were traced by mass-sensitive measurements. (C) 2 000 Elsevier Science B.V. All rights reserved.