Investigating the Time Response of an Optical pH Sensor Based on a Polysiloxane-Polyethylene Glycol Composite Material Impregnated with a pH-Responsive Triangulenium Dye

Determining the time it takes a sensor to report a change in the concentration of its target analyte may appear to be an easy task, but it is not. The dynamic characteristic of a sensor is determined by all components in the sensor system and the hydrodynamics of the sample. Here, the dynamic properties of an optical pH sensor were determined using the IUPAC-recommended activity step method in experimental setups that can determine sensor-limited response times longer than 5 s. In order to do so, experimental setups for the injection and for the dipping method of determining the sensor time response were developed, tested, and shown to be able to determine time-response curves with 1 s time resolution. This time resolution is shown to be sufficient for determining dynamic characterization of this optical pH sensor. The sensor chemistry-limited time-response curves were analyzed using curve fitting. It was found that the optode response time is limited by diffusion of protons within the sensor material when the proton concentration is reduced and limited by diffusion from the bulk to the boundary layer at the optode surface when proton concentration is increased. The latter is dependent on the magnitude of the change in analyte concentration and cannot be reported as a single response time. The investigation of the time response of the optical pH sensor reveals detailed information of the sensor chemistry, but does not yield a single response time of the sensor capable of describing the dynamic sensor characteristics of the optical pH sensor system.