A Highly Selective Chemosensor for Al(III) and Zn(II) and Its Coordination with Metal Ions

This paper reports a fluorescence chemosensor, N-(benzimidazol-2-yl)­salicylaldimine (H₂L), for Zn­(II) and Al­(III) ions. H₂L has high selectivity for Al­(III) in dimethyl sulfoxide (DMSO) and for Zn­(II) in N,N-dimethylformamide (DMF). In methanol, Zn­(II) and Al­(III) could also be distinguished by H₂L with different excitation wavelengths. The fluorescent species [Zn­(HL)­(H₂O)­(CH₃OH)]⁺, [Zn­(HL)­(H₂O)­(DMF)]⁺, [Al­(HL)₂(OH)­(H₂O)], and [Al­(HL)­(OH)₂(H₂O)­(DMSO)] formed in solution were established by a combination of experimental and theoretical methods, including Job’s plot, ¹H NMR titration, electrospray inonization mass spectrometry (ESI-MS), and B3LYP-SCRF/6-31­(d) and TD-B3LYP-SCRF/6-31G* density functional theory methods. The results show that Zn­(II) and Al­(III) are all coordinated to the imine nitrogen atom and the hydroxyl oxygen atom from H₂L, which is the same as the M²⁺ ions in the obtained mononuclear complexes [M­(HL)₂(CH₃OH)₂] (where M = Cd, Ni, Co, and Mg). The detection limits of H₂L for Zn­(II) were 5.98 μM in methanol and 5.76 μM in DMF, while the detection limits of H₂L for Al­(III) were 3.3 μM in methanol and 5.25 μM in DMSO. Furthermore, it is also confirmed that H₂L has low toxicity for HeLa cells and could be used to detect Zn­(II) and Al­(III) ions in living cells by bioimaging.