A multi-ion detection Schiff base chemosensor: Turn-off and ratiometric fluorescence sensing of Ni²⁺, Co²⁺, Cd²⁺, and Cu²⁺ ions

Abstract

A novel Schiff-based fluorescent sensor, (Z)-2-((1H-indol-3-yl)methylene)-N'-((Z)-(1H-indol-3-yl)methylene)hydrazine-1-carbothiohydrazide (NF16) for multi-ion sensing was designed and synthesized to detect Ni²⁺, Co²⁺, Cd²⁺, and Cu²⁺ ions in DMSO and water systems. The fluorophore detects the metal ions through fluorescence turn-off and ratiometric sensing facilitated by chelation-enhanced quenching and intramolecular charge transfer mechanisms. The sensor NF16 exhibited relatively low detection limits of 2.363 × 10^–6, 4.1 × 10^–5, 2.040 × 10^–6, and 1.649 × 10^–6 M for Cu²⁺, Cd²⁺, Ni^2+, and Co²⁺, respectively. The binding constants for the NF16-metal complexes were 4.112 × 10⁵, 8.383 × 10³, 5.123 × 10^3, and 8.398 × 10³ M^-1, respectively, for NF16-Cu²⁺, NF16-Cd²⁺, NF16-Ni^2+, and NF16-Co²⁺complexes. The mass spectra and Job's plot analysis confirmed a 2:1 (NF16: metal ion) stoichiometric binding ratio between NF16 and the metal ions. All the experimental outcomes strongly propose that NF16 is a highly effective sensor for Cu²⁺, Cd²⁺, Ni^2+, and Co²⁺. In the metal ion interference studies, it is observed that the detection of Cu²⁺ ions is not affected by the presence of other metal ions, including Cd²⁺, Ni^2+, and Co²⁺ ions, indicating that NF16 has more selectivity towards Cu²⁺ ions, and it could be a possible candidate for Cu²⁺ ion sensing applications.