We describe the fabrication and characterization of an optical CO2 sensor based on the change in fluorescence lifetimes due to fluorescence resonance energy transfer from a pH-insensitive donor, sulforhodamine 101, to a pH-sensitive acceptor, either m-cresol purple or thymol blue, entrapped in an ethyl cellulose film. A phase transfer agent allows incorporation of the dyes and water into the film, while providing an initially basic environment for the acceptor. Diffusion of CO2 into the water entrapped in the film produced carbonic acid, causing a pH-dependent decrease in the spectral overlap of the acceptor absorbance with the donor emission, and decreased energy transfer, resulting in increased SR101 donor lifetimes. The lifetime changes were detected as a change in the phase of the emission, relative to the modulated excitation, and were insensitive to excitation intensities and emission signal levels. In addition to an externally modulated 442-nm light source, we excited the sensor with a directly modulated 635-nm laser diode and detected the anti-Stokes emission. The CO2 sensor is not fragile and can provide stable readings for weeks. The use of fluorescence resonance energy transfer, along with the simple entrainment procedure, allows facile change of the CO2 response range through change of the acceptor dye and the use of laser diode excitation sources.
