Probing the Dissociation State of Acid−Base Indicators by Time-Resolved Lanthanide Luminescence: A Convenient Transduction Scheme for Optical Chemical Sensors

The paper describes a new method for the optical measurement of pH based on time-resolved luminescence. The intense line emission of a europium terpyridine chelate is strongly quenched by bromothymol blue (BTB). Since only the blue base form acts as a quencher, the decay time of the luminescence directly reflects the protonation state of BTB and, hence, the pH of the solution. The experimental results are in good agreement with a theoretical model that accommodates both the Henderson−Hasselbalch equation and the Stern−Volmer law. Decay time-based response curves have the same sigmoidal shape as conventional titration curves, but their apparent p K a is shifted to lower pH, depending on the total concentration of BTB used. All luminescence decay curves featured single-exponential decay. With 8 μmol/L BTB, decay times decreased from 1172 μs at pH 2.60 to 120 μs at pH 8.94, and the apparent p K a shifted from 7.0 to 6.1. A Stern−Volmer quenching constant of 9.55 × 10 5 was obtained for the base form of BTB. The method appears particularly useful as a transduction scheme for optical chemical sensors, since it makes conventional indicator dyes available to decay time-based sensing.