A CO₂ sensor based on Pt-porphyrin dye and FRET scheme for food packaging applications

An optochemical CO₂ sensor is described which uses a phosphorescent reporter dye PtTFPP and a colourimetric pH indicator α-naphtholphthalein incorporated in plastic matrix together with a phase transfer agent tetraoctyl- or cetyltrimethylammonium hydroxide. Good spectral overlap of PtTFPP emission and α-naphtholphthalein absorption at around 650 nm provides efficient energy transfer from PtTFPP via FRET mechanism, thus reducing its phosphorescence lifetime. Experiments were carried out to optimize the composition and working characteristics of such a sensor for the measurement of headspace CO₂ in foods packaged under modified atmosphere. The resulting thin film sensor coatings showed robust optical responses to CO ₂, response (1 min, 99.9%) and recovery times (4 min, 99.9%) from 0 to 100% CO₂. As expected, the CO₂ sensor showed significant cross-sensitivity to O2 quenching, and this was accounted for by a tandem O2 sensor having similar phosphorescent characteristics but no CO ₂ sensitivity. Upon storage in an open container, the sensors were stable for at least 14 days at 4 °C and 50 days at -20 °C, however at room temperature they were seen to deteriorate within 7 days loosing colour and sensitivity to CO₂. At the same time, in food and modified atmosphere environment the sensor retained its sensitivity to CO₂ for 21 days at 4 °C which is sufficient for many packaged products. Migration of sensor components examined with a standard set of standard food stimulants was only detectable in 95% ethanol and in olive oil for PtTFPP (0.06-0.13 μg/ml) and in 50% and 95% ethanol for α-naphtholphthalein (9.86-21.65 μg/ml).