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

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 & mu;M and 0.62 & mu;M (in water) for Zn2+ and Cd2+ ions, respectively. In both DMSO/HEPES buffer and DMSO/water (1:1), LOD values were identically 1.67 & mu;M (for Zn2+) and 1.93 & mu;M (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.

Automated summary

The probe LH2 was investigated for its sensing application towards Zn2+ and Cd2+ ions in different aqueous systems. It showed a turn-on response for both ions in water and a ratiometric change in fluorescence maximum in DMSO/HEPES buffer and DMSO/water. The probe has a low detection limit and a good pH tolerance range.