Development of a fluorescent chelating ligand for gallium ion having a quinazoline structure with two Schiff base moieties

A novel chelating ligand, 2,4-bis-(2,4-dihydroxybenzylidene)]-dihydrazinoquinazoline (DBHQ), was synthesized, and the fluorescence characteristics of its complex with metal ions were investigated.Thirty-five different metal ions were tested for the emission of fluorescence in the presence of DBHQ in aqueous solutions in a pH range of 3.0-10.5 (at a difference of 0.5 for each metal).It was observed that DBHQ fluoresces intensely at 470 nm with an excitation wavelength of 405 nm in the presence of Ga³⁺ or Al³⁺ in the pH range 3.0-4.0. The other metal ions did not show fluorescence with DBHQ. Although the presence of Cu²⁺, Co²⁺ and Fe³⁺ decreased the fluorescence intensity of DBHQ-Ga³⁺, the addition of a fluoride ion (NaF) recovered the fluorescence by masking the interfering ions. In addition, the fluoride ions were found to enhance the sensitive determination of Ga³⁺ because the fluorescence intensity of DBHQ-Ga³⁺ was further increased approximately 2.5-fold in the presence of F⁻ (? = 0.658) from that in the absence of F⁻ (? = 0.401). The fluoride ions also masked the Al³⁺ ions, which emit fluorescence on chelation with DBHQ. Therefore, a selective and sensitive detection of Ga³⁺ was achieved by using DBHQ in the presence of F⁻. The detection limit of Ga³⁺ was approximately 50 nmol L⁻¹ (3.5 ppb). The proposed method was applicable to determine Ga³⁺ in river water.