A rhodamine-B-based turn-on fluorescent sensor for biological iron(III)

Rhodenal, a turn-on fluorescent sensor for iron, derived from rhodamine-B, was synthesized and characterized. Its electronic absorption spectrum in ethanol has bands at 410 (ε = 500 cm− 1 mol− 1 L), 316 (ε = 14 × 103 cm− 1 mol− 1 L), and 272 nm (ε = 32 × 103 cm− 1 mol− 1 L), and it is pH sensitive with two pKa values (1.2 and 2.8). It reacts promptly with Fe3 +, with a pesudo-first order rate constant of 1 × 103 s− 1, with an equilibrium constant of 1.3 × 106 mol− 1 L, forming a violet complex (λmax = 540 nm) with a fluorescent emission at 582 nm, with a higher sensitivity and selectivity compared to those of Fe2 +, Ba2 +, Al3 +, K+, Ca2 +, Ni2 +, Co2 +, Cd2 +, Cr3 +, Hg2 +, Mg2 +, Mn2 +, Na+, or Cu2 +. Biological assays with B16-F10 showed high permeability of plasma membrane to rhodenal and, according to confocal microscopy results, the fluorescent source originated from within the cell revealed iron pools within cytoplasmic and nuclear sub-compartments, making rhodenal a very good prospective Fe3 + sensor, in aqueous solution and especially for biological iron pool detection.