A sensitive colorimetric and “OFF-ON” fluorescent sensor for cyanide ions based on ESIPT-AIE mechanism from a fluorene-hydroxybenzaldehyde Schiff base

Abstract

(E)-2-(((9H-Fluoren-2-yl)imino)methyl)phenol (FIP) is an ortho-hydroxy aromatic imine Schiff base that exhibits intramolecular hydrogen bonding. Its spectral characteristics, including absorption and fluorescence, were analyzed in solvents of varying polarities. FIP exhibited selective recognition of cyanide ions (CN⁻) in a DMF/H₂O buffer (7:3) at pH 7, showing significant changes in both absorption and “OFF-ON” fluorescent emission in the presence of competing anions. ¹H NMR and FT-IR studies indicate that the interaction with CN⁻ and OH⁻ involves a deprotonation process, leading to the formation of the keto tautomer of FIP, which plays a crucial role in the OFF-ON emission response: the ESIPT-ICT process is activated by OH⁻, while the ESIPT-AIE process is triggered by CN⁻. Notably, FIP exhibits high sensitivity for CN⁻ in fluorescent detection, with a limit of detection (LOD) of 6.5 nM, significantly lower than the absorbance detection (LOD = 97 nM). The sensor’s key attributes include selectivity, rapid response time, a large Stokes shift, and the ability to monitor CN⁻ levels in tap water, making it a user-friendly option for on-site applications.