Selective recognition of luteolin and quercetin based on the specific interaction of ortho-dihydroxy substituents with a zinc(ii) complex

Selective recognition of ortho-dihydroxy-substituted flavonoids has been developed based on their specific interaction with the fluorescent complex, 4-(methylphenyl)-2,2′:6′,2′′-terpyridine–Zn(II) (mptpy–Zn(II)). Six related flavonoids, including quercetin, naringenin, genistein, luteolin, daidzein, and morin, which have hydroxyl substituents in different positions, have been investigated. It has been observed that the ortho-3′,4′-dihydroxy substituents in luteolin and quercetin play an important role in the fluorescence quenching of mptpy–Zn(II) as ternary complexes are formed between luteolin/quercetin and mptpy–Zn(II). The quenching constants and fluorescence lifetime measurements indicated that the quenching was static. As the easy oxidation of 3-hydroxy and 4′-hydroxy in quercetin reduced the number of ortho-dihydroxy groups, luteolin had a greater quenching efficiency than quercetin under the same conditions. Furthermore, this fluorimetric method was successfully applied in the determination of the luteolin content in Lamiophlomis capsules from two factories, with satisfactory results compared with an HPLC method.