Furan/thiophene-based fluorescent hydrazones as fluoride and cyanide sensors

Four dual optical signalings of fluorescent hydrazone Schiff bases were synthesized from 1,8-naphthalimide hydrazide and substituted furan and thiophene rings, employing ethanol as green solvent. All synthesized molecules can detect F- and CN- with fast response with naked eye color change and quenching of fluorescence. Most common competitive anions have paltry interferences during the optical sensing of F-, while only nitro and methyl furan substituted provided good selectivity to CN- in THF. Substituents on heterocyclic directly affect fluoride capturing sensitivity, namely, electron donating group provides more sensitivity than with electron withdrawing groups. H-1 NMR confirms the H-bonding between sensor molecule and F-/CN-. The detection limits of the four-sensor molecules were found below 0.3 ppm for F- and CN- detection. The magnitude of fluorescence quenching was estimated through Stern-Volmer plots. Test strips experimentation revealed the on-site solid-state detection efficacy of the sensors. Addition of Cu(II) ions to nitro and methyl furan substituted, resulted in selective discrimination between F- and CN- in THF. Computational studies prove the agreement of reactivity for four optical molecules interaction with F- and show that substituent at furan/thiophene does not affect the sensitivity, this is contrary to traditional school of thoughts.