Synthesis of a triphenylamine-based “ON-OFF” type fluorescent probe: Cu2+ detection, DNA binding studies and theoretical calculations

A Cu²⁺ fluorescent probe molecule (S) of the “ON-OFF” type was designed and synthesized using 4-(diphenylamino)benzaldehyde and carbohydrazide as starting materials. The structure of S was systematically characterized using IR spectroscopy, HRMS, ¹H NMR, and ¹³C NMR. Spectroscopic experiments demonstrated that the binding stoichiometry of S to Cu²⁺ was 1:1, with remarkable selectivity and sensitivity for detecting Cu²⁺. Moreover, the detection limit of 3.5 × 10⁻⁸ mol⋅L⁻¹ was significantly lower than those reported for previously developed fluorescent sensors targeting Cu²⁺. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations revealed that Cu²⁺ ions coordinated with the imine nitrogen and carbonyl oxygen of the Schiff base, triggering a photoinduced electron transfer (PET) effect that quenched the fluorescence of Schiff base S. Furthermore, the interaction between S and zebrafish sperm DNA (zf-DNA) was investigated using a combination of spectroscopic methods, DNA binding assays (with both single- and double-stranded DNA), viscosity measurements, and molecular docking simulations. The findings revealed that the primary mode of interaction between S and zf-DNA was groove-face binding, which occurred spontaneously and was governed by hydrogen bonds, CH interactions, π-π stacking, and van der Waals forces. In summary, the developed Schiff-base probe S demonstrates high sensitivity and selectivity for Cu²⁺ detection in complex environmental matrices. Moreover, this study advances the understanding of Schiff-base-DNA interactions and introduces a novel strategy for designing multifunctional sensors applicable to biomacromolecule detection and anticancer drug development.