STRATEGIES TO DESIGN PH OPTODES WITH LUMINESCENCE DECAY TIMES IN THE MICROSECOND TIME REGIME

We present the first optical pH sensor with luminescence decay times l onger than 1 mu s. It is based on radiationless energy transfer from a luminescent ruthenimn(II) complex as the donor to a colored pH indica tor as the acceptor. The metal-ligand complex ruthenium(II) tris-4,4'- diphenyl-2,2'-bipyridyl was selected as the donor, and bromothymol blu e and a reactive azo dye were selected as pH-sensitive accepters. Stra tegies for coimmobilization of transition metal complexes and pH indic ators into a hydrogel matrix were evaluated and discussed. pH transiti on intervals between 7 and 9 allow the measurement in the physiologica l range and also under marine conditions. Signal changes of up to 50% over 2.5 pH units are observed, depending on the respective acceptor c ontent in the sensing film. Luminescence lifetime measurements of the sensing films were performed in the frequency domain with a blue LED a s the light source. The effect of molecular oxygen (acting both as a q uenching and a bleaching agent, thereby limiting the accuracy and the long-term stability of such sensors) was found to be crucial for pract ical applications. Since leaching of the acceptor also limits the stab ility of the sensing films, several strategies to reduce this effect w ere evaluated.