The Design and Synthesis of Fluorescent Coumarin Derivatives and Their Study for Cu(2+) Sensing with an Application for Aqueous Soil Extracts

A series of fluorescent coumarin derivatives 2a–e were systematically designed, synthesized and studied for their Cu(2+) sensing performance in aqueous media. The sensitivities and selectivities of the on-to-off fluorescent Cu(2+) 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 Cu(2+) 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 Cu(2+) 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 Cu(2+)-coumarin complex. The results revealed a 1:1 stoichiometry between 2b and Cu(2+), and that the essential structural features for Cu(2+)-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).