Ratiometric detection of Zn(II) and Cd(II) ions by a benzobisimidazole based probe in aqueous systems and theoretical calculations

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The probe 2,6-di(2-pyridyl)-1,5-dihydroimidazo[4,5-f]benzimidazole (LH2) was investigated for its sensing application towards metal ions. This probe can detect Zn2+ and Cd2+ ions in three different aqueous systems viz., water, DMSO/HEPES buffer (1:1, pH = 7.34, rt), and DMSO/water (1:1, rt). In water a “turn-on” response was observed for both metal ions, whereas in the later two solvent systems, a ratiometric change in fluorescence maximum was observed. The detection limit of this probe is as low as 0.3 μM and 0.62 μM (in water) for Zn2+ and Cd2+ ions, respectively. In both DMSO/HEPES buffer and DMSO/water (1:1), LOD values were identically 1.67 μM (for Zn2+) and 1.93 μM (for Cd2+). This probe has a good pH tolerance range which is 4–8 (for Zn2+) and 4–11 (for Cd2+). Lehrer and Chipman plot and Job’s plot suggested a 2:1 binding ratio between metal ion and LH2. The high binding constants obtained from the Lehrer and Chipman method are suggestive of the formation of a very stable complex in the solution. The binding of Zn2+ and Cd2+ ions to the probe was further confirmed by the NMR titration method which indicated the intensity of N–H protons is halved upon addition of the respective metal ion solution in DMSO‑d6. Reversibility of the detection was also established using Na2EDTA solution. DFT/TD-DFT calculation supported the experimentally observed red-shift of the absorption band.