A selective ‘turn-on’ fluorescent chemosensor for detection of Al3+ in aqueous medium: Experimental and theoretical studies

A Schiff-base compound, 2-((E)-1-(2-(dimethylamino)ethylimino)ethyl)phenol (HL), has been synthesized and characterized by different spectroscopic methods The chemosensor shows selective “turn–on” cyan colour fluorescence response to Al3+ ion over a wide variety of other competing metal ions such as Zn2+, Cu2+, Co2+, Ni2+, Cd2+, Fe2+, Mg2+, Hg2+, Mn2+, Ca2+, Pb2+, Fe3+, Cr3+, Na+ and K+. The fluorescence emission intensity at 462 nm displayed by the chemosensor, increases progressively during the addition of Al3+ ion until saturation point, with over 20 fold intensity enhancement. Additionally, the linear range for the chemosensor has been found to be 0.5 × 10−6 to 10 × 10−6 mol/L, the detection limit of the chemosensor is 4.32 × 10−6 M and the stability constant is 2.1 × 104 M−1. Similar fluorescence behaviour is also observed in a variety of solvents and solvent mixtures in the presence of Al3+. The fluorescent emission has also been detected when a sample solution of HL containing Al3+ is irradiated by UV light. UV–vis titration experiment has established that the chemosensor HL, binds to Al3+ in 2:1 ratio. The Density Functional Theory (DFT) and Time Dependent-Density Functional Theory (TD–DFT) calculations were performed on HL, L− and its aluminium complex which give insight into the geometric, electronic, thermodynamic and optical properties of chemosensor-Al3+ interactions.