“Turn on/off” proton transfer based fluorescent sensor for selective detection of environmentally hazardous metal ions (Zn2+, Pb2+) in aqueous media

Sensing ability of fluorescence based chemosensor: Indole-7-carboxaldehyde (I7C) for environmentally hazardous metal ions (Xn+=Zn2+, Pb2+, Co2+, Ni2+, Cu2+, Cd2+, Cr3+, and Fe3+) has been investigated. Based on excited state proton transfer (ESPT) phenomenon, the sensing capability of I7C is verified experimentally (UV-vis, FTIR, NMR) and theoretically (DFT, TD-DFT) in aqueous media. Chemical hardness (η), electrophilicity (ω) and electronic chemical potentials (μ) confirm the selective reactivity of I7C (Nc and Nt conformers) in the presence of metal ions by the formation of (1:1) metal ion: I7C complex (MX, MX⁎). MX, MX⁎ are found to display excellent sensing capability for Zn2+ and Pb2+ ions. “Turn-On” response for Zn2+ is observed through the appearance of a new enhanced fluorescence at ~430 nm. By strong blue emission I7C establishes its strong candidature as “blue emitter”. “Turn-Off” response is observed for Pb2+ through the quenching of the existing fluorescence peak.