SENSITIVITY STUDIES ON OPTICAL CARBON-DIOXIDE SENSORS BASED ON ION-PAIRING

A recent approach to optical carbon dioxide sensing makes use of ion pairs consisting of a pH indicator dye anion (D-) and an organic quaternary cation (Q(+)). The resulting 'dry' (as opposed to the better-known buffer-based 'wet' sensor chemistry) or 'plastic' sensor membranes comprise a pH indicator dye immobilized along with a quaternary cation, and an additional amount of quaternary hydroxide (Q(+)OH(-)) within a polymer layer on a polyester support. Such sensors are applicable to optical sensing of carbon dioxide both in dry gases, where they have an extremely short response time, and in liquid samples. The sensitivity, or degree of response of a sensor to a given analyte concentration, is a vital parameter for describing its performance. In this work, it is shown that the sensitivity of carbon dioxide sensors, based on the ion-pairing method, is largely governed by the molar concentration ratio between the ion pair and the free quaternary hydroxide. We have found that the sensitivity of such optrodes, as well as their degree of linearity, can be adjusted by varying the (Q(+)D(-))/(Q(+)OH(-)) ratio. Certain environmental conditions, such as high temperature and the presence of acidic vapours, and also changes due to membrane aging, can shift the dynamic range to lower pCO(2), too. We discuss probable causes for the sensitivity shift.