Switching selectivity between Pb2+ and Hg2+ ions through variation of substituents at xanthene end; 'turn-on' signalling responses by FRET modulation

Abstract

Few rhodamine based probes (L-1-L-4) that consist of a similar 2-(aminoethyl)-pyridine unit at their carboxamide end but vary with substituents attached to the N-atom at their xanthene end were synthesized. Rhodamine G based probes L-1-L-3 have shown preferential chromogenic and fluorogenic 'turn-on' spectral responses in the presence of Pb2+ ions, where one of the two ethyl substituted secondary amino groups attached to the xanthene core either remains un-substituted (as in L-1) or is functionalized with a bulky aromatic group (as in L-2) or a long alkyl chain (as in L-3). On the contrary, the L-4 probe that incorporates two ethyl-substituents at both N-atoms attached to the xanthene core has selectively exhibited a dual mode spectral amplification in the presence of Hg2+ ions. The reversible selective dual mode signalling pattern of bifluorophoric L-2 in the presence of Pb2+ ions is because of the perturbation of the combined PET (photo-induced electron transfer) inhibition and FRET (fluorescence resonance energy transfer) initiation processes. The observed ratiometric signalling pattern enabled it to detect Pb2+ ions at a low concentration level, even in living organisms such as E. coli. The altered selectivity in the signalling pattern infers a modulated stereo-electronic environment for metal ion coordination, which in turn is caused by induced amine rigidity at the xanthene end.