Selective determination of catechin among phenolic antioxidants with the use of a novel optical fiber reflectance sensor based on indophenol dye formation on nano-sized TiO₂.

The optical sensor for "tea catechins" was built by immobilizing 2,2'-(1,4-phenylenedivinylene)bis-8-hydroxyquinoline (PBHQ) on TiO₂ nanoparticles (NPs). The sensor worked by "indophenol blue" dye formation on PBHQ-immobilized TiO₂ NPs as a result of p-aminophenol (PAP) autoxidation with dissolved O₂ at pH 10. Among quercetin, rutin, naringenin, naringin, gallic acid, caffeic acid, ferulic acid, p-coumaric acid, catechin, epicatechin, epicatechin gallate, epigallocatechin, epigallocatechin gallate, and trolox, only catechin group antioxidants delayed the color formation on NPs, as measured by the reflectance signal at 710 nm. For quantitative analysis, reflectance signal versus time was recorded, and the difference between the areas under curve (ΔAUC) in the presence and absence of catechin was correlated (r = 0.98) to catechin concentration. The selectivity of the sensor for catechins was shown in tea infusions compared to other plant extracts and was ascribed to the nonplanar structure of catechin interfering with the formation of perfectly conjugated indophenol blue on TiO₂ surface.