Rhodamine-based fluorescent off–on sensor for Fe³⁺– in aqueous solution and in living cells: 8-aminoquinoline receptor and 2: 1 binding

8-Aminoquinoline (8-AQ) and 2-aminopyridine (AmPyr) both as ionophoric agents were robustly used in development of a cation sensor. To evaluate the effect of complexation with cations, flexible 2-aminopyridine and rigid 8-AQ moiety were directly introduced into rhodamine chromophore and two candidate fluorescent sensors were constructed successfully. Rh-AQ with a rigid 8-AQ fragment exhibited turn-on fluorescence and color responses to Fe³⁺ ion over other metal ions related to biology in aqueous solution. The sensor Rh-AmPyr with a flexible 2-aminopyridine did not give positive responses to cations although with a similar binding cavity as on Rh-AQ. Furthermore, the 2: 1 recognition mode of Rh-AQ with Fe³⁺ was proved according to the 1D and 2D COSY H–H NMR experiments. The live cell imaging experiments demonstrated that Rh-AQ could be successfully applied as a bioimaging agent for monitoring Fe³⁺ in living cells.