A novel chromogenic molecular sensing platform for highly sensitive and selective detection of Cu^sup 2+^ ions in aqueous environment

A new chelate-type of FBO probe based on a spirocycle fluorescein hydrazone derivative was developed for the rapid, ultrasensitive, and highly selective detection of Cu2+ ions in aqueous media. Sensor FBO and its copper complex were characterized by elemental, magnetic moment and TGA-DTA analyses, FT-IR, 1H NMR and 13C NMR spectroscopies, and Tandem MS spectrometry. Probe FBO showed excellent selectivity and sensitivity only for Cu2+ among other biologically and environmentally important metal ions in EtOH-H2O (2:1, v/v, 20 mM Tris-HCl, pH 7.4). The binding interaction of FBO to the Cu2+ ion resulted in the spirolactam ring-opening mechanism associated with the colorimetric ‘turn-on’ response in the UV–vis absorption spectra above 350 nm. This molecular sensing platform established a chromogenic method for quantification and visualization of Cu2+ ions with the naked eye by forming a 1:1 receptor-copper complex under physiological pH conditions. Furthermore, the detection limit of FBO for Cu2+was determined to be as low as 7.4 nM, which is much lower than the limits set by the U.S. EPA and WHO. More importantly, chemosensor FBO was successfully applied for the direct detection of Cu2+ ions in aqueous media.