A benzoxazole-based fluorescent ‘off-on-off’ probe for cascade recognition of cyanide and Fe3+ ions

This article introduced a benzoxazole-based (NT) photoluminescence 'off–on-off' probe for sequential detection of cyanide (CN–) and iron (III) (Fe3+) ions, respectively in H2O/DMSO (1:1 v/v) medium. Probe NT quickly produced a strong blue fluorescence due to the introduction of CN– ions, found to be 1:1 binding stoichiometry, mainly due to the disruption of excited state intramolecular proton transfer (ESIPT) because of deprotonation of hydroxy (–OH) functional group present in the salicylimine moiety. The resulting deprotonated NT is further employed for detecting Fe3+ ions by turn-off fluorescence response and binding affinity found in the 1:3 stoichiometry ratio. The detection thresholds for CN– and Fe3+ ions are estimated to be 23 nM and 5.70 μM, respectively, much lower than the permissible maximum contamination level predicted by Environmental Protection Agency (EPA). For the practical utilization of our fluorescence sensor NT as an on-site detector, we have demonstrated a paper strip-based test kit experiment to quantify traces of CN– ions in stocks of related analytes. The ability of NT and deprotonated NT to detect CN– and Fe3+ ions, respectively, demonstrated that it might be a sensing tool for real sample analysis.