A PET based pH-responsive bridged organic scaffold exhibiting sensitivity for lysosome and Hg²⁺ ion through fluorescence Off-On-Off response

A 1,5-bis(1,4-piperazinyl-naphthalimide-N-ethylmorpholine) pyridine based flexible fluorescence probe 7 is designed, synthesized, and its photophysical behavior has been examined in different media. The photophysical behavior of probe 7 in phosphate (PBS) buffer (10 % DMSO) showed pH sensitivity and selectivity for Hg2+ ion in solution and live cell with a change in naked-eye sensitive visible color. In acidic medium (pH <= 6) probe 7 showed enhanced (similar to 21 fold) "turn-On" emission, while at pH >= 7.4 probe 7 remained in the "turn-Off" state. The observed fluorescence "turn-On" response is attributed to inhibition in photoinduced electron transfer (PET) process. Jobs plot analysis revealed a 1:2 stoichiometry for an interaction between probe 7 and Hg2+ ion with good limit of detect (LOD) 5.78 x 10(-8) M (57 nM). The reversibility in complexation and the mode of interaction were confirmed by EDTA and through NMR and HRMS spectroscopic data. MTT assay and cell imaging studies showed biocompatibility of the probe 7 to detect lysosomes and Hg2+ ion in A549 cells. For analytical application paper strip test was performed to detect Hg2+ ion with naked-eye sensitive visible color change.