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 Cu2+ 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 Cu2+. RhB-NSal binds with Cu2+ forming a 1:1 stoichiometric complex with an association constant of 6.72 (±0.03) × 104 M−1 and 4.23 (±0.03) × 104 M−1, respectively. RhB-NSal displayed high selectivity for Cu2+ over possibly competitive metal ions except that Fe3+ and Bi3+ ion can respectively bring about a little interference in absorption and fluorescence with its sensing for Cu2+. In dry acetonitrile, pronounced enhancements in the absorbance and emission of RhB-NSal were induced by Cu2+, 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 Cu2+ 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 Cu2+ in acetonitrile medium, and be developed to be a promising candidate of “Off-On” eye-naked chemosensor for Cu2+ in a weakly acidic aqueous medium.