NIR squaraine dyes for dual colorimetric and fluorescent determination of Fe(3+), Cu(2+), and Hg(2+) ions

Four benzoindolenine-based squaraine dyes (SQs), which have the advantages of intense visible and near-infrared (NIR) absorption and emission (λ(abs/max) 663–695 nm, λ(em/max) 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(3+), Cu(2+), and Hg(2+) 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(3+) and 6.06 μM for Cu(2+). Most importantly, the response mechanism of BBSQ to Fe(3+), Cu(2+), and Hg(2+) 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 (1)H NMR titration analyses. Furthermore, BBSQ was applied successfully to detect Fe(3+), Cu(2+), and Hg(2+) in thin-layer chromatography (TLC) plates with good precision and is quite promising for the quantitative detection of Fe(3+) and Cu(2+) ions in water samples.