Development of optical sensors ("optodes") for carbon dioxide and their application to modified atmosphere packaging (MAP)

Summary This thesis reports on the design of new types of planar optical sensors (often referred to as optodes) for oxygen and carbon dioxide, respectively. Such sensors are of highest interest for monitoring oxygen and CO2 in modified atmospheric packaging (MAP). The oxygen sensors are based on the quenching effect that oxygen exerts on the fluorescence (or luminescence in general) of certain fluorophores. In the particular case, luminophores out of the group of ruthenium dipyridyl and ruthenium phenanthroline complexes have been used. These have absorption maxima at around 450 nm which make them compatible with blue light emitting diodes (LEDs) which can acts as low-cost light sources for fluorescence excitation. Their luminescence is far red-shifted and peaks at around 620 nm. Both the intensity and the decay time of the luminescence of the ruthenium complexes is strongly affected by molecular oxygen, and this finding forms the basis for the sensing scheme applied here. The sensor for carbon dioxide (CO2) is based on a quite different analytical approach. CO2 is a weakly acidic gas that can change the pH of a buffer contained inside a gas-permeable sensor membrane. The acidification of such an "immobilised" buffer resulting from exposure to CO2 is indicated by a fluorescent pH indicator. Several possibilities were considered, of which the measurement of fluorescence intensity, fluorescence decay time, fluorescence energy transfer, and of "dual luminophore referenced luminescence" (DLRF) came into closer consideration. A careful analysis of the merits of each method was performed which led to the following conclusions: (a) In case of oxygen sensors it is found that the measurement of luminescence intensity is slightly inferior to measurement of fluorescence decay time. However, in view of the substantial additional costs for decay time based sensors it is recommended that luminescence intensity be measured which still gives adequate results for many MAP applications. (b) The situation is different for CO2 sensors where the superiority of DLRF became quite obvious after a short time. Energy transfer is second to it yet much better than the other two methods. The method of DLRF is therefore described in some detail (chapter 3) both in theory and in terms of practical implementation and choice of materials (chapter 4). The last experimental chapter (no. 5) refers to the use of microparticles and nanoparticles which are shown to be useful reference materials in optical sensors. These are made from sol-gels and modified sol-gels and have unique properties including narrow particle distribution and high luminescent brightness. In order to test the sensor obtained in the studies summarised before, all sensor types were tested for the parameters and properties that are of relevance in context with MAP. In conclusion, it can be said that the two types of optical presented here represent a novel type of sensors for MAP applications, with several advantages over existing sensor types. In particular, the costs for sensor preparation, the ease of depositing the material on package materials, and the possibility of reading the signal through an optically transparent package material (usually colourless plastic), thereby avoiding the risk of bacterial contamination (as a result of opening the package) are substantial advantages that make the results of this work highly promising. I am confident that further studies (to be performed in the respective industry) will demonstrate the enormous potential of this new method