Exploring diaminomaleonitrile-based fluorescence turn-off and turn-on probes for zinc (II) detection

Zn(II) is an essential dietary and industrial metal ion whose imbalances disrupt physiological functions. Therefore, developing an accurate and sensitive method of Zn(II) detection in real samples is necessary. In this study, two fluorescence probes H₂L1 and H₂L2 were synthesized by condensation reaction of diaminomaleonitrile and 5-R-salicylaldehide (R is –OH for H₂L1 and −F for H₂L2 ) in ethanol with 1:1 ratio ; characterized by ¹H NMR, FTIR, PXRD, UV–Vis, and fluorescence (FL) spectroscopy, supported by DFT computations. Under identical conditions, robes’ substituent effects were evaluated on photophysical responses, before and after addition of Zn(II) ion in buffered ethanol. Both probes recognize Zn(II) ion through coordination bonding involving the imine- N , the ethanol- O , the mono-deprotonated amine-N^- and the phenolate-O^-. Under optimal conditions (i.e., at RT, pH 7.4, 0.17 μM of probe, and 0.0–85 μM of Zn²⁺), H₂L1 showed linear fit (R² > 0.9963), FL turn-off (Φ = 0.407 to 0.156), weaker binding (Ka = 2.1 μM⁻¹), and lower sensitivity (LOD = 60 μM), attributed to its electron transfer from H₂L1 to Zn(II) via σ-donation. In contrast, H₂L2 demonstrated linear fit (R² > 0.9926), FL tur-on (Φ = 0.954 to 0.982), stronger binding (Ka = 10 μM⁻¹), and superior sensitivity (LOD = 0.0045 μM), which is far below the reported literatures. These are due to the electron transfer from Zn(II) to H₂L2 via strong π-back bonding. The binding of each probe with Zn(II) ions were found to be reversible with sequential addition of EDTA. Particularly, the properties of probe H₂L2 encouraged us to study its practical applicability to determine Zn(II) ions which-spiked (at 0.152 μM, 0.367 μM and 0.550 μM) with tap-water samples, achieved excellent recovery rates (92.46–98.77 %) and low relative standard deviations (RSD = 2.3–4.1 %, n = 5), confirming its practical applicability in environmental monitoring.