A fluorescence “Turn-on” Schiff base probe based on naphthylcarbazone for selective detection of Mg2+/Cd2+ ions and DNA binding: Synthesis, characterization and DFT/TD-DFT studies

A compound with a Schiff base structure (H) of the “Turn-On” type was designed and synthesized using 3‑hydroxy-2-naphthylcarbazone and 2-hydroxyacetophenone as raw materials. The structure of H was systematically characterized using IR spectroscopy, HRMS, ¹H NMR, and ¹³C NMR. This study illustrated that the probe H can rapidly and highly selectively recognize both Mg²⁺ and Cd²⁺, with detection limit (LOD) of 4.9 × 10⁻⁸ mol⋅L⁻¹ and 1 × 10⁻⁸ mol⋅L⁻¹, respectively. Moreover, fluorescence spectroscopy and ultraviolet-visible spectroscopy indicated that the combination of probe H and Mg²⁺/Cd²⁺ produced an obvious fluorescence enhancement effect which was negligibly affected by other metals cations. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations revealed that Mg²⁺/Cd²⁺ions coordinated with the imine nitrogen and carbonyl oxygen of the Schiff base, triggering an intramolecular charge transfer, which led to fluorescence enhancing of Schiff base H. Furthermore, the interaction between H and zebrafish sperm DNA (zf-DNA) was investigated using spectroscopic experiments, sodium ion effects assays, ssDNA/dsDNA binding assays, viscosity measurements, and molecular docking calculations. The findings revealed that the primary mode of interaction between H and zf-DNA involved both groove-face binding and intercalation, which was spontaneous and mediated by hydrogen bonds, CH interactions, π-π stacking, and van der Waals forces. In summary, the Schiff base probe H developed in this study demonstrates high sensitivity and selectivity for the detection of Mg²⁺ and Cd²⁺ ions, even in complex environmental matrices. Furthermore, this work advances our understanding of Schiff base–DNA interactions and provides an innovative strategy for designing multifunctional sensors with potential applications in biomacromolecule detection and anticancer drug development.