Photophysical investigations of a FRET-based bifluorophoric conjugate and its Hg(II) specific ratiometric ‘turn-on’ signalling

• Signalling pattern of bifluorophoric rhodamine based probe responds to Hg(II) ion.
• The higher sensitivity owes to ratiometric fluorescence signals through FRET.
• Stereo-electronic contribution of “piperazinyl-ethyl-amide” unit corroborates preferences.
• The excited state life-time measurements support FRET process.

The molecular probe L2 exhibited simultaneous chromogenic and fluorogenic dual mode signalling responses selectively in presence of Hg(II) ion among all the metal ions investigated in both organic and organic aqueous medium. The spiro-cyclic rhodamine signalling subunit undergoes complexation induced structural transformation to result in ratiometric fluorescence modulation through an operative FRET between 7-nitrobenz-2-oxa-1,3-diazole (NBD) and rhodamine moieties separated through an ethyl-piperazinyl linker. Its complexation induced signalling exhibited reversibility with various contrasting reagents such as bio-thiols, anions and competing chelating agents. Its bio-imaging on Escherichia coli has exploited its utility as a potential ratio-metric chemosensor for in-situ detection of Hg(II) ion in living cells. The reference probe L3, which has also exhibited high selectivity towards Hg(II) ions in rendering dual mode optical signal perturbation, puts insight towards complexation pattern at receptor segment as well towards individual contribution of operative photo-physical processes in the bifluorophoric chemosensory probe (L2).

Figure optionsDownload as PowerPoint slideThe bi-fluorophoric conjugate (L2) selectively detects Hg(II) through FRET-mediated ratiometric signals, responsive towards other metal ions through donor emission as a function of aqueous component in solvent; stereo-electronic contributions (L3) towards selectivity and energy transfer.