New dual emission fluorescent sensor for pH and Pb(II) based on bis(napfthalimide) derivative

This paper describes a novel dual emission bis-1,8-naphthalimide sensor for selective determination of pH and Pb2+ ions. The influence of the variability in the backbone that links the two fluorophores (naphthalimides) as a function of pH and metal ions was studied by UV-visible and fluorescence spectroscopy. Compounds 1(a-d) with different length alkyl linkers (CH2)(n) (n = 1, 2,4 and 6) showed no excimer formation in aqueous solution. Fluorescence emission of these derivatives varied in a narrow range of pH (5-8) and was only slightly influenced by the addition of metal ions in CH3CN solutions. However, derivative le with amino-containing spacer (CH2-NH-CH2) showed excimer emission in aqueous solution, a wide response to pH (2.5-9.5) and fluorescence enhancement with selective behavior towards metal ions. The pH sensor based in derivative le has a sufficient selectivity for practical pH monitoring in the presence of Li+, Na+, K+, Cs+, Ca2+, Mg2+, Ba2+, Cu2+, Pb2+, Ni2+, Zn2+ and Cd2+. The coordination chemistry of these complexes was studied by UV-Vis, fluorescence and H-1 NMR. This chemosensor displayed high selectivity fluorescence enhancement toward Pb2+ ions in the presence of the metals ions mentioned in CH3CN solutions. Competitive assays show that a 1-fold of metal cations in each case, compared with Pb2+ ions, results in less than +/- 5% fluorescence intensity changes. Linear calibration up to 1 x 10(-5) M for Pb(II) ions (R = 0.9968) was obtained and detection limit resulted of 5.0 x 10(-8) M. (c) 2012 Elsevier B.V. All rights reserved.