A series of fluorescent coumarin derivatives 2a-e were systematically designed, synthesized and studied for their Cu2+ sensing performance in aqueous media. The sensitivities and selectivities of the on-to-off fluorescent Cu2+ sensing signal were in direct correlation with the relative arrangements of the heteroatoms within the coordinating moieties of these coumarins. Probes 2b and 2d exhibited Cu2+ concentration dependent and selective fluorescence quenching, with linear ranges of 0-80 μM and 0-10 μM, and limits of detection of 0.14 μM and 0.38 μM, respectively. Structural changes of 2b upon Cu2+ coordination were followed by fluorescence titration, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), mass spectrometry, and single crystal X-ray diffraction on the isolated Cu2+-coumarin complex. The results revealed a 1:1 stoichiometry between 2b and Cu2+, and that the essential structural features for Cu2+-selective coordination are the coumarin C=O and a three-bond distance between the amide NH and heterocyclic N. Probe 2b was also used to determine copper (II) levels in aqueous soil extracts, with recovery rates over 80% when compared to the standard soil analysis method: inductively coupled plasma-mass spectrometry (ICP-MS).
Keywords: Cu2+ sensing; coumarin; fluorescent quenching; soil sensing.