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Anew fluorescent probe for turn-on detection of pHusing 2,6-bis-[(1-H-benzoimidazol-2-ylmethylimino)-methyl]-4-methylphenol: imaging application in living cells

Uday Chand Saha, Sushil KumarMandal, Siddhartha Pal, AnisurRahmanKhuda-Bukhsh, Pabitra Chattopadhyay, Koushik Dhara

A newly designed, easy to make and simple molecule, 2,6-bis-[(1-Hbenzoimidazol- 2-ylmethylimino)-methyl]-4-methylphenol (HL1) behaves as a highly pH-responsive selective fluorescent probe in a Britton- Robinson buffer at 25°C. The pH titration indicates that the fluorescence intensity increases more than 36-fold in the pH range of 5.0-8.5 with pKa value of 6.99 (±0.09). This enhancement of fluorescence intensity with increase in pH is accounted by the formation of the phenoxide form, L1, generated by the deprotonation of HL1 in higher pH. The DFT calculations reveal that the energy difference between the HOMO-LUMO of L1 is less than that of HL1 and it is evidenced by the red shift observed in the UV-vis absorption studies experimentally. The presence of the biologically relevant ions i.e. Na+, K+, Ca2+ etc. and the other important metal ions e.g. Cr3+, Mn2+, Pb2+etc. do not show any significant contribution in the fluorescence behavior. The fluorescent imaging of HeLa cells also demonstrates that the designed probe has great value in monitoring intracellular H+ within living cells as it has no significant cytotoxic effect.