An efficient excited-state proton transfer fluorescence quenching based probe (7-hydroxyquinoline) for sensing trivalent cations in aqueous environment

7-hydroxyquinoline (7-HQ) exists as enol and keto forms and is interconverted through isomerization resulting in the 402 nm along with 327 nm absorption band in water. Upon excitation, an enormous Stoke shifted green fluorescence (FL) is observed without the contribution of normal FL of 7-HQ. This is due to a fast proton transfer reaction in the first excited-state, which relaxed in 2.60 ns +/- 0.01 ns. An efficient FL quenching is observed in the presence of selected metal ions at 400 nm excitation. The Stern-Volmer (S-V) plots of FL quenching are nearly linear, and the slopes of the S-V plots are different in two different regions of Al3+ ions concentrations. In contrast, the FL lifetime is roughly constant and is independent in the Al3+ or Fe3+ ions concentration. Based on the steady-state and time-resolved results, the mechanism of quenching is predominantly static. The presence of metal ions inhibited keto formation and tautomerization, supporting the hydrogen-bonding between 7-HQ and water instead of forming zwitterion species in the ground and excited states. With substantially higher quenching constant, single exponential FL decay, linear change in absorbance, and commercial availability demonstrate this probe as a potential candidate for real-world applications. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

7-hydroxyquinoline can exist in different forms and exhibit unique fluorescence properties in water, with metal ions causing fluorescence quenching. The presence of metal ions inhibits keto formation and tautomerization of 7-HQ.