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

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− (ϕ = 0.658) from that in the absence of F− (ϕ = 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.