Optical pH sensor based on the absorption of antenna generated europium luminescence by bromothymolblue in a sol–gel membrane

A new up to the visible range absorbing and at 460 nm fluorescing 4-trifluoromethylcarbostyril derivative of diethylenetriaminepentaacetic acid (DTPA) was synthesized. After formation of an europium (III)-complex intense antenna mediated lanthanide luminescence was observed. Without loss of the antenna function, the complex was entrapped into various sol–gel and ormosil (organically modified siloxanes) matrices. Luminescence spectra were then found to be independent of pH changes in the range 1–10. No leaching was observed and photostability was excellent. This stability allowed us to study the influence and pH response of bromothymolblue (BTB) as co-immobilized non-fluorescent indicator by fluorescence. A single sensor layer based on tetramethoxysilane (TMOS) doped with the Eu+3-complex and about 3.5 equivalents BTB was found to be most appropriate for purposes of sensing pH over the broad pH range 5–9. The apparent pKa was 7.29 at ionic strength 0.1 and 6.05 at ionic strength 1.0 (KCl).