A New Fluorescence Probe Based on Self-absorption for Detection of Co²⁺ and Viscosity

A novel Schiff base compound, 3-hydroxy-(2-hydroxy-3,5-dichlorobenzylidene)-2-naphthohydrazide (HDN), was successfully synthesized through the condensation of 3-hydroxy-2-naphthoylhydrazide with 3,5-dichlorosalicylaldehyde. The molecular structure was comprehensively characterized using NMR, HR-MS, and FT-IR analysis. Spectroscopic investigations revealed that HDN demonstrates a selective "turn-off" fluorescent response towards Co²⁺ ions in a PBS: DMF (4:6, v/v) medium with minimal interference from competing metal ions. Mechanistic investigations, including Job's plot analysis and ¹H NMR titration, confirmed a 1:1 binding stoichiometry between HDN and Co²⁺. Density functional theory (DFT) calculations suggested that fluorescence quenching is due to enhanced self-absorption effects upon complex formation. A linear calibration curve was established between fluorescence intensity and Co²⁺ concentration within the range of 0.1-10 µM, with a detection limit of 5.57 × 10^-8 mol/L. Additionally, HDN exhibited significant viscosity-sensitive emission behavior during chitosan cross-linking processes. These findings establish HDN as a dual-functional probe for both environmental Co²⁺ monitoring and real-time viscosity detection in polymer matrix systems.