ANALYST FULL PAPER THE www.rsc.org/analyst Sol–gel based optical carbon dioxide sensor employing dual luminophore referencing for application in food packaging technology Christoph von Bültzingslöwen, a Aisling K. McEvoy, a Colette McDonagh, a Brian D. MacCraith,* a Ingo Klimant, b Christian Krause c and Otto S. Wolfbeis c a Department of Physics/National Centre for Sensor Research, Dublin City University, Glasnevin, Dublin 9, Ireland. E-mail: Brian.MacCraith@dcu.ie b Institute of Analytical Chemistry, Graz University of Technology, 8010 Graz, Austria c University of Regensburg, Institute of Analytical Chemistry, Chemo- and Biosensors, 93040 Regensburg, Germany Received 29th July 2002, Accepted 1st October 2002 First published as an Advance Article on the web 18th October 2002 An optical sensor for the measurement of carbon dioxide in Modified Atmosphere Packaging (MAP) applications has been developed. It is based on the fluorescent pH indicator 1-hydroxypyrene-3,6,8-trisulfonate (HPTS) immobilised in a hydrophobic organically modified silica (ormosil) matrix. Cetyltrimethylammonium hydroxide was used as an internal buffer system. Fluorescence is measured in the phase domain by means of the Dual Luminophore Referencing (DLR) sensing scheme which provides many of the advantages of lifetime-based fluorometric sensors and makes it compatible with established optical oxygen sensor technology. The long-term stability of the sensor membranes has been investigated. The sensor displays 13.5 degrees phase shift between 0 and 100% CO 2 with a resolution of better than 1% and a limit of detection of 0.08%. Oxygen cross-sensitivity is minimised (0.6% quenching in air) by immobilising the reference luminophore in polymer nano-beads. Cross-sensitivity towards chloride and pH was found to be negligible. Temperature effects were studied, and a linear Arrhenius correlation between ln k and 1/T was found. The sensor is stable over a period of at least seven months and its output is in excellent agreement with a standard reference method for carbon dioxide analysis. Introduction Food products are very often packed under a protective atmosphere of nitrogen, oxygen and carbon dioxide. Often, but not always, the exclusion of oxygen is preferred in order to inhibit growth of aerobic spoilage organisms, whereas carbon dioxide is typically used in food packs to decrease bacterial growth rates. The composition of the protective atmosphere, however, depends on the type of food and the delivery stage of the food item. 1 The major advantages of MAP technology include increased food safety, an extended shelf life and, in some instances, enhanced visual appearance of the products. Because package integrity is an essential requirement for the quality of MAP food, leakage detection is a very important part of MAP technology. Currently, food packs are mostly sampled destruc- tively by extracting the atmosphere with a needle probe and delivering it to an electrochemical fuel cell for oxygen analysis, followed by infrared absorption spectrometry for carbon dioxide measurement. 2 If a package fails the quality control test, an expensive process of back checking and re-packing is required. 1 This means of testing is not only destructive, leading to large losses every year, but it also only allows for random sampling of the food packs, so that 100% quality control is not possible. In order to achieve 100% non-destructive quality control, a desirable solution is a sensor strip located inside the pack in such a way that it can be sampled by a hand-held scanning device from the outside. For such a solution it is necessary to find optical sensor membranes for oxygen and carbon dioxide that can be used in these sensor strips. They should be capable of detecting changes over the whole range of encountered concentrations (0–100% for CO 2 ) with sufficient resolution (±2%). In the past, some approaches for both oxygen and carbon dioxide sensors have been used in food packaging technology, but these have generally been in the form of tablets or sachets, mostly relying on colorimetric leak indicators. 3–6 An approach which is more compatible with industrial demands would consist of sensor membranes that are printed on the packaging material and provide an exact measure of both analyte gases at any given stage in the packaging and delivery process. Thin films doped with luminescent indicators are very suitable for this approach, and have often been used for the design of oxygen optodes. In particular, ruthenium polypyridyl complexes have been widely employed for oxygen sensors based on luminescence quenching. 7–11 These fluorophores, with lifetimes in the range of microseconds, offer the possibility for phase-fluorometric decay time analysis as a robust and accurate measurement technology, which is already well established and compatible with low-cost LED’s and photo- diodes. 7,8,11 In order to minimise the expenditure of the industrial scanning process, the optoelectronic technology required for both analytes should ideally be the same. Therefore, the aim of this work was to develop a carbon dioxide sensor capable of detecting CO 2 between 0 and 100% with a resolution of at least ±2%, and which is compatible with the above-mentioned oxygen decay-time analysis instrumenta- tion. Most reported fluorescence-based optical carbon dioxide sensors rely on the intensity change of a luminescent pH indicator such as 1-hydroxypyrene-3,6,8-trisulfonate (HPTS), but the very short decay times of such species cannot be measured by the low-cost phase modulation techniques used for oxygen sensors. 12–14 Ruthenium complexes with pH-sensitive ligands, which were used for the design of lifetime-based CO 2 sensors, have been reported. 15 However, their decay times are This journal is © The Royal Society of Chemistry 2002 1478 Analyst, 2002, 127, 1478–1483 DOI: 10.1039/b207438a