An 'OFF-ON' fluorescent chemosensor based on rhodamine 6G-2-chloronicotinaldehyde for the detection of Al<SUP>3+</SUP> ions: Part II

Abstract

A novel, optical rhodamine-2-chloronicotinaldehyde-type chemosensor (R6CN) was designed, synthesized and characterized as a reversible switch. R6CN displayed high selectivity toward Al3+ from various metal ions, including Al3+, Li+, K+, Cs+, Mg2+, Ca2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Hg2+, Pb2+ and the resultant complex [R6CN-Al3+]. The ring-opening mechanism of the rhodamine spirolactam was induced by Al3+ binding, and the 1:1 stoichiometric structure between R6CN and Al3+ was adequately supported by the Job-plot evaluation, optical titration, FT-IR and H-1 NMR results. Theoretical calculations and modeling simulations were performed using Material Studio 4.3 suite (VAMP), and the results supported the formation of a 1:1 complex between R6CN and Al3+. The fluorescence quantum yield of R6CN-Al3+ (Phi(f) = 92.33%) was very high compared to that of the bare ligand. The detection limit for Al3+ was 4.28 x 10(-9) M, and a significant color change from almost colorless to pale-pink occurred in the presence of Al3+. In turn, the R6CN-Al3+ complex acted as a selective chemosensor toward N-3(-) among various anions, including F-, Cl-, Br-, I-, NO3-, CH3COO-, ClO4-, CN-, SCN-, HSO4-, HPO4- and PF6-, in acetonitrile media. Moreover, the R6CN-Al3+ complex also exhibited a high selectivity and sensitivity toward the azide anion upon the addition of Al3+, and the color reversed back to colorless when the two ions were present together in solution. At last, R6CN was productively applied to the PEGDMA polymer to sense Al3+ ions, which was analyzed using FT-IR, fluorescence confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) images. (C) 2015 Elsevier B.V. All rights reserved.