The influence of the lipophilic base in solid state optical pCO2 sensors

Solid-state optical pCO2 sensors comprise mostly a pH indicator dye and a quaternary ammonium hydroxide incorporated in a hydrophobic polymer membrane. In this study we investigated a solid-state optical pCO2 sensor in which the quaternary ammonium ion was replaced by the neutral phosphazene base P1-t-Oct (PBO) and compared it with a conventional system containing tetraoctylammonium hydroxide (TOA+OH−). The basic character of PBO is due to the unshared electron pair of the tertiary nitrogen atom linked to the phosphorus atom by a double bond. The phosphazene base forms together with water the buffer system of the sensor. Both sensor types have dynamic ranges between 0 and 50 hPa pCO2. They show a completely reversible sensor response. A strong cross-sensitivity towards relative humidity (RH) of the analyte gas was found for the PBO-containing sensor. The ratio protonated form/deprotonated form (HD/D−) of the indicator dye increases continuously with decreasing RH. At the same time the sensitivity towards pCO2 decreases. In the case of the TOA+OH−-containing sensor the ratio HD/D− stays nearly constant at RH values above 20%. At lower RH, however, it decreases significantly as well as the sensitivity towards pCO2. The deviant behaviour of the two sensor types at the removal of water is due to the nature of the respective organic base and the resulting influence on the polarity within the sensor matrix.