A rhodamine-based "off-on" colorimetric and fluorescent chemosensor for Cu(II) in aqueous and non-aqueous media

A new rhodamine derivative (RhB-NSal) bearing an electron-withdrawing group -NO2 at the 5-position of 2-hydroxyphenyl moiety was synthesized and its sensing behaviors for Cu(2+) in acetonitrile and aqueous acetate-buffer/acetonitrile (2/3, v/v, pH 4.8) media were investigated. In each medium, significant absorption and fluorescence enhancements accompanied by an instant color change from colorless to pink were observed for RhB-NSal upon addition of Cu(2+). RhB-NSal binds with Cu(2+) forming a 1:1 stoichiometric complex with an association constant of 6.72 (±0.03) × 10(4) M(-1) and 4.23 (±0.03) × 10(4) M(-1), respectively. RhB-NSal displayed high selectivity for Cu(2+) over possibly competitive metal ions except that Fe(3+) and Bi(3+) ion can respectively bring about a little interference in absorption and fluorescence with its sensing for Cu(2+). In dry acetonitrile, pronounced enhancements in the absorbance and emission of RhB-NSal were induced by Cu(2+), with a detection limit of 0.49 μM, exhibiting higher sensitivity than that of a known analogue bearing no substituent on its phenol ring, RhB-Sal. In aqueous solution, RhB-NSal displayed likewise a high selectivity but a lower sensitivity for Cu(2+) than that in acetonitrile, with a detection limit of 14.98 μM, still more sensitive than RhB-Sal in absorption. By virtue of these properties, RhB-NSal could be used as an "Off-On" fluorescent and colorimetric chemosensor for Cu(2+) in acetonitrile medium, and be developed to be a promising candidate of "Off-On" eye-naked chemosensor for Cu(2+) in a weakly acidic aqueous medium.