A turn-on Rhodamine B-indole based fluorogenic probe for selective sensing of trivalent ions

An indole functionalized Rhodamine based fluorophoric probe (L) rendered selective colorimetric as well as turn-on fluorescence responses toward trivalent metal cations (Al3+, Fe3+ and Cr3+) over a variety of divalent and monovalent metal ions in 1:1 methanol-water mixed solvent. These selective colorimetric and fluorescence responses were explained as a consequence of spirolactam ring opening due to the protonation of L, where the lower Ksp values of M3+ (Al3+, Fe3+ and Cr3+) assisted the generation of H+ through hydrolysis in mixed aqueous medium. The turn-on fluorescence response of L towards M3+ is also visually detectable in test paper strips.