A real-time tripodal colorimetric/fluorescence sensor for multiple target metal ions

In this work we report on the synthesis of a pyridyl/phenolic/benzothiazole functionalized colorimetric receptor (BPAP) that can selectively recognise Fe(III) and Fe(II) ions in aqueous medium with visible, naked eye colour changes. Moreover, BPAP behaves as a ‘turn-on’ fluorescent chemosensor, showing an excellent sensitivity towards Zn2+ and Cd2+ cations in water by exhibiting fluorometric change. BPAP exhibited a visible detection limit of 5.5 μM and 2.5 μM for Fe3+ and Fe2+, respectively, and a fluorometric selectivity towards Zn2+ and Cd2+ ions with lower detection limits of 205 nM and 642 nM. To understand the mode of binding to the sensor, both the nude sensor and the Zn-complex were isolated and characterized by X-ray crystallography. In Zn-BPAP, the ligand acts as neutral tridentate through Npyridine, Namine and Namide donor atoms. The N atom of benzothiazole group and the OH group of the phenole unit are not coordinated. Computational DFT study was performed on the free ligand BPAP and complex with Zn2+ to investigate the interaction site, to calculate the energies of the frontier molecular orbitals and to corroborate some of the experimental results.