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 (CH ₂)_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 1e 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 1e has a sufficient selectivity for practical pH monitoring in the presence of Li ⁺, Na⁺, K⁺, Cs⁺, Ca²⁺, Mg²⁺, Ba²⁺, Cu²⁺, Pb²⁺, Ni ²⁺, Zn²⁺ and Cd²⁺. The coordination chemistry of these complexes was studied by UV-Vis, fluorescence and ¹H NMR. This chemosensor displayed high selectivity fluorescence enhancement toward Pb²⁺ ions in the presence of the metals ions mentioned in CH ₃CN 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 1×10^-5 M for Pb(II) ions (R=0.9968) was obtained and detection limit resulted of 5.0×10^-8 M.