NIR squaraine dyes for dual colorimetric and fluorescent determination of Fe, Cu, and Hg ions.

Four benzoindolenine-based squaraine dyes (SQs), which have the advantages of intense visible and near-infrared (NIR) absorption and emission ( 663-695 nm, 686-730 nm) were synthesized and characterized by UV-vis absorption, fluorescent emission spectrophotometry, FTIR, NMR and HRMS analysis. Among them, BBSQ showed excellent performance, which exhibited high selectivity to Fe, Cu, and Hg in acetonitrile solution even in the presence of other competitive metal ions, accompanied by obvious color change easily detected by the naked eye. The detection limit was 14.17 μM for Fe and 6.06 μM for Cu. Most importantly, the response mechanism of BBSQ to Fe, Cu, and Hg involves the coordination of BBSQ and metal ions through the O atom on the central squarate ring, N atom, and olefin π bond of BBSQ and has been demonstrated by Job's plot, FTIR, and H NMR titration analyses. Furthermore, BBSQ was applied successfully to detect Fe, Cu, and Hg in thin-layer chromatography (TLC) plates with good precision and is quite promising for the quantitative detection of Fe and Cu ions in water samples.