“Turn-On” Fluorescent Biosensors for High Selective and Sensitive Detection of Al³⁺ Ion

A series of new compounds (1-4) based on pyrrole hydrazone Schiff bases were designed and synthesized. The interactions of these new compounds with metal ions and their fluorescent recognition were investigated. All compounds showed “turn-on” fluorescence in the presence of Al³⁺ in aqueous solution. Their sensing behaviors with Al³⁺ were studied using photophysical experiments, ESI-MS spectrometry analysis, ¹H NMR titration, and DFT calculation. The detection limits of 1-4 for the analysis of Al³⁺ were found to reach a 10⁻⁸ M level in aqueous solution, which are far lower than the WHO guidelines for drinking water (7.41 mM for Al³⁺). A high selectivity test paper has been fabricated for Al³⁺ detection based on sensor 3. Theoretical calculations (DFT) have been carried out to elucidate the configuration of 1-4 and their Al complexes and rationalize experimental absorption data.