Red-emitting CO2 sensors with tunable dynamic range based on pH-sensitive azaphthalocyanine indicators

Abstract A new group of optical CO 2 sensors based on red-emitting pH-sensitive zinc azaphthalocyanine (Zn-AzaPc) indicators embedded into polyurethane polymeric matrices is presented. The effects of particular Zn-AzaPc indicator, type of a polymeric matrix and a base on sensing properties are investigated. All the indicators are virtually non-fluorescent in the deprotonated form (in the absence of CO 2 ) and are “switched on” in presence of the analyte. Zn-AzaPc without any phenolate moiety is used as a positive control (always-ON state). Among the indicators with one, two and four phenolate receptors, the dye with one receptor is the most suitable due to brightest emission in ON state. Sufficient water content is found to be particularly important in case of the hydrophobic Zn-AzaPc indicators and leads to the necessity of employing hydrophilic HydroThane™ matrices instead of conventional ethylcellulose matrix. Among them HydroThane™ 25 showed the most promising results with the highest increase in fluorescence intensity between OFF/ON states (12×), reasonably high fluorescence quantum yields in ON state (0.071) and the most reliable and reproducible response to CO 2 in sensing range 0–95 kPa pCO 2 with the limited temperature-dependency in the range 15–35 °C. Importantly, sensitivity significantly increased with shortening of the alkyl chains of a base, i.e. in order TOAOH  2 sensors for different applications. Finally, fiber-optic sensors compatible with a commercially available compact phase fluorometer as well as materials for 2-wavelength sensing and imaging are prepared and possibility of a referenced read-out is demonstrated.