A smart rhodamine-pyridine conjugate for bioimaging of thiocyanate in living cells

A rhodamine-pyridine conjugate, REDA-2PC, can selectively detect NCS- in human embryonic kidney 293 (HEK293) cells. DFT calculations suggest that this is due to non-covalent interactions (hydrogen bonding N-H center dot center dot center dot NCS-, pi-pi stacking) and long range electrostatic forces acting between the sulfur atom of the NCS- anion and the NEt2 unit of REDA-2PC. A visible light excitable probe allows fluorescence and naked eye detection of nanomolar NCS-, much lower than normal NCS- levels in the human body. A lock and key sensing mechanism is established and compared with the corresponding fluorescein derivative as a model compound.