A rhodamine–dansyl conjugate as a FRET based sensor for Fe3+ in the red spectral region

A new fluorescent resonance energy transfer (FRET) based fluorescent probe (compound 1) containing a dansyl unit as a donor and rhodamine 101 as an acceptor was developed to detect Fe3+ from other transition metal ions through ratiometric sensing in organic-aqueous solutions. Fe3+ induced a ring-opening reaction of the spirolactam rhodamine moiety of 1 resulting in the formation of a fluorescent derivative that can serve as the FRET acceptor. Ratiometric sensing of Fe3+ was accomplished by plotting the fluorescence intensity ratio at 605 nm and 515 nm versus ferric ion concentration. The probe displayed a linear response to Fe3+ in the range of 5.5–25 μM with a detection limit of 0.64 μM. A 1:1 stoichiometry for the 1–Fe3+ complex was formed with an association constant of 1.74×104 M−1. The probe also exhibited a large Stokes shift (225 nm) which can eliminate backscattering effects of excitation light.