The dynamic response of optical oxygen sensors and voltammetric electrodes to temporal changes in dissolved oxygen concentrations

Accurately measuring dissolved oxygen concentrations in fresh and salt water environments has long been an interdisciplinary priority. Many methodologies exist, including two very promising new ones, optical fluorescence quenching optrodes (or optodes) and solid-state voltammetric electrodes. In this study we compare the responsiveness of these two techniques to dynamic changes in dissolved oxygen concentrations, using traditional methods of polarographic oxygen sensors and Winklerʹs chemical titrations to corroborate the measurements. Advantages of the optrode system include simplicity of operation and high sensitivity when changes in oxygen concentrations are small (<(delta)10 (mu)M O2 min^-1). Advantages of voltammetry include equally high sensitivity, independent of rate of change of oxygen concentration, and the capability of simultaneously measuring other chemical species. Both systems have the capability of producing high-resolution, continuous oxygen profiles by collecting and reporting real-time data. Kinetic estimates of the binding and dissociation constants for the ruthenium–oxygen complex in the optrode revealed a relatively long half-life of the Ru2+–O2 complex for dissociation (t1/2 of 43.2 s to equilibrate from a 100 to 0% O2 saturation change). In contrast, the Ru2+–O2 association constant was 5.0 times faster (t1/2 of 8.6 s to equilibrate from a 0 to 100% O2 saturation change). Therefore, because of these differential kinetics, researchers should take care when using a ruthenium-based optrode to measure real-time dissolved oxygen concentrations undergoing temporal variability.