Polyoxometalates in Analytical Sciences

Owing to the importance of antioxidants in the protection of both natural and man-made materials, a large variety of testing methods have been proposed and applied. These include methods based on inhibited autoxidation studies, which are better followed by monitoring the kinetics of oxygen consumption or of the formation of hydroperoxides, the primary oxidation products. Analytical determination of secondary oxidation products (e.g. carbonyl compounds) has also been used. The majority of testing methods, however, do not involve substrate autoxidation. They are based on the competitive bleaching of a probe (e.g. ORAC assay, β-carotene, crocin bleaching assays, and luminol assay), on reaction with a different probe (e.g. spin-trapping and TOSC assay), or they are indirect methods based on the reduction of persistent radicals (e.g. galvinoxyl, DPPH and TEAC assays), or of inorganic oxidizing species (e.g. FRAP, CUPRAC and Folin-Ciocalteu assays). Yet other methods are specific for preventive antioxidants. The relevance, advantages, and limitations of these methods are critically discussed, with respect to their chemistry and the mechanisms of antioxidant activity. A variety of cell-based assays have also been proposed, to investigate the biological activity of antioxidants. Their importance and critical aspects are discussed, along with arguments for the selection of the appropriate testing methods according to the different needs.