Efficient and Selective Colorimetric Sensing of Bisulfate, Ferrous, and Cupric Ions by a Pyrazole-Derived Schiff Base

Pyrazole-derived Schiff bases are mostly designed for the optical sensing of ionic species. In this research, a pyrazole-based Schiff base, 4-(4-methoxybenzylideneamino)-1,5-dimethyl-2-phenyl-1H-pyrazol-3(2H)-one (MBz-AAP), has been studied for its optical sensing properties towards various ionic species, including heavy metal ions and anions. Among these species, this probe exhibits a color change from colorless to maroonish-red for Fe²⁺ and Cu²⁺ ions, and colorless to yellow for \({\text{HSO}}_{4}^{ - }\) ions. These optical changes are found to be consistent with red shifts in the ultraviolet-visible absorption spectrum of MBz-AAP from 334 to 504, 510, and 387 nm for Fe²⁺, Cu²⁺, and HSO₄^– ions, respectively. Limits of detection (LOD) of MBz-AAP for Fe²⁺, Cu²⁺, and \({\text{HSO}}_{4}^{ - }\) ions are calculated to be 2.45, 3.34, and 2.24 μM, respectively. These LODs for Fe²⁺ and Cu²⁺ are much lower than their allowable limits in drinking water, i.e., 5.36 and 31.5 μM, respectively. Moreover, the stoichiometric binding ratios of Fe²⁺, Cu²⁺, and \({\text{HSO}}_{4}^{ - }\) with MBz-AAP are determined to be 1 : 1, with binding constants of 5.0 × 10³, 1.6 × 10³, and 7.05 × 10³ M^–1, respectively.