A simple metal ion displacement-type turn-on fluorescent probe for the detection of halide ions in 100% water - Spectroscopic and TD-DFT investigations

The enzyme cofactor pyrroloquinoline quinone (PQQ) has been employed for sequential ON-OFF-ON type fluorescent detection of Hg(II) and halide ions (I-, Br- and Cl-) in 100% water. The fluorophore PQQ detects Hg(II) by fluorescence quenching. Fluorescence titration experiments reveal that the binding constant for the formation of Hg(II)-PQQ complex is found to be 5.4x10(4) M-1. Job's plot analysis suggests a 1:1 binding between PQQ and Hg(II). The limit of detection (LOD) of PQQ for Hg(II) is as low as 7.9 mu M. The mode of coordination of Hg(II) with PQQ has been established using FT-IR, 1H NMR and mass spectral techniques, which indicates that Hg(II) binds to the binding site 1 of PQQ. This in-situ generated Hg(II)-PQQ complex exhibits excellent turn-on fluorescence to halide ions and thus acts as a sequential fluorescent probe. The mechanism of sensing of halide ions involves displacement of Hg(II) ion from the Hg(II)-PQQ complex by these halide ions and the order of displacement efficiency is found to be I- > Br- > Cl-. Interestingly this is the same order as of their binding ability with Hg(II) ion. Absorption and emission behaviour of PQQ are investigated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations.

Automated summary

The enzyme cofactor PQQ can be used for sequential detection of Hg(II) and halide ions in water. It detects Hg(II) through fluorescence quenching and exhibits excellent turn-on fluorescence to halide ions.