Development of a Quinazoline-Based Chelating Ligand for Zinc Ion and Its Application to Validation of a Zinc-Ion-Coordinated Compound

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A novel fluorescent chelating ligand, 2,4-[bis-(2-hydroxy-3-methoxybenzylidene)]-dihydrazinoquinazoline (HBQZ), was synthesized, and the fluorescence characteristics of its complex with metal ions were investigated. Among the 36 different metal ions tested in this study, it was found that HBQZ emits intense fluorescence at 506 nm with an excitation wavelength of 414 nm in the presence of Zn2+. The fluorescence intensity was almost constant in the pH range 3.5—10.5. Complexes of other metal ions with HBQZ did not show fluorescence, and the detection limit of Zn2+ was approximately 250 nM (16 ppb). The proposed method was applied to the validation test of a bioactive compound containing Zn2+ in its structure—an antibacterial and antifungal reagent, zinc pyrithione (ZnPT). In order to effectively release Zn2+ from ZnPT, a pretreatment procedure involving heating with H3PO4 at 100 °C for 60 min was adopted. Under these conditions, a linear calibration curve was obtained in the ZnPT concentration range of 0.79—15.7 μM (0.25—5.0 ppm); the correlation coefficient and the relative standard deviation were 0.996 and within 3.1% (n=5), respectively.