Abstract
Continuous monitoring of oxygen concentration is of great importance in many different areas of research which range from medical applications to food packaging. In the last three decades, significant progress has been made in the field of optical sensing technology and this review will highlight the one inherent to the development of oxygen indicators. The first section outlines the bioanalytical fields in which optical oxygen sensors have been applied. The second section gives the reader a comprehensive summary of the existing oxygen indicators with a critical highlight on their photophysical and sensing properties. Altogether, this review is meant to give the potential user a guide to select the most suitable oxygen indicator for the particular application of interest.
Highlights
Oxygen is by far one of the most important chemical species on earth since it is essential for life
The majority of the optical sensors developed for oxygen detection rely on quenching of the luminescence of an indicator dye by molecular oxygen [153]
Voraberger et al [224] developed such sensor which was used to measure oxygen concentration in a fermenter; the results showed good comparison with the ones obtained with a Clark-type electrode, which has the disadvantage to suffer from inadequate signal stability, slow response time, and electrical interferences
Summary
Oxygen is by far one of the most important chemical species on earth since it is essential for life. The majority of the optical sensors developed for oxygen detection rely on quenching of the luminescence of an indicator dye by molecular oxygen [153]. Methylene blue suffers from a slow reduction to its leuco form and a fast subsequent oxidation by oxygen present at low concentration; Hay et al [187] have chosen indicators which showed a much faster reduction after UV exposure, namely 2,2′-dicyano-1-1′dimethy-4,4′-dipyridinium dimesylate and thionine (4b and 5b, respectively, Fig. 3) This class of indicators shows a moderate sensitivity to oxygen and can be used even when the level increases up to 4 kPa. In general, irreversible absorption-based indicators work by producing a visible color change which is caused by chromogenic chemistry that involves oxidation of the leuco dye by molecular oxygen. Indicators for oxygen sensing not belonging to any of these categories can be found in the section called “Miscellaneous”
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