A new optical luminescent-lifetime pH sensor based on the sol-gel doped europium chelate has been developed. Near-UV to visible (350.400 nm) excitation of the europium(III) chelate of a 4-trifluoromethylcarbostyril derivative of diethylenetriaminepentaacetic acid (DTPA) exhibits a typical lanthanide line-like emission, the highest peak being at 615 nm. Due to antenna-mediated effects, the heterocyclic chromophore (parent emission at 450 nm) transfers its energy to the close europium having isolated very weak quantum efficiency. The complex was co-immobilized with the non-fluorescent pH indicator bromothymol blue (BTB) in a semi-permeable sol-gel layer. It can be shown, that not only the line emission spectra but also decay lifetimes depend strongly on pH changes. Since the sensitive dyestuff is highly stable and not quenched by oxygen, a sensor in the most interesting pH range of 4-9.5 could be produced, having its highest sensitivity at around physiological pH
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