Fluorescence sensing of metal ions in solution using a morpholine-containing phenolic Mannich base of...

A phenolic Mannich base derived from 1'-hydroxy-2'-acetonaphthone (HAN) as a substrate and morpholine as an amine reagent was synthesized and structurally characterized. The sensing ability toward various metal ions of the s-, p- and d-block of this molecule that has the binding site for metal ions in the starting ortho-hydroxyphenone preserved was examined. Interaction between this phenolic Mannich base and Al³⁺, Cr³⁺, Cu²⁺ and Co²⁺ leads to modifications of the sensing molecule's absorption spectrum. Fluorescence spectroscopy showed that Al³⁺ acts as a fluorescence enhancer, whereas Cu²⁺ functions as a fluorescence quencher for the aminomethylated derivative. The phenolic Mannich base may be employed either as a sensitive "turn-on" chemosensor for Al³⁺ or as a sensitive "turn-off" chemosensor for Cu²⁺. However, in the presence of these ions at identical concentrations, the Mannich base becomes a selective chemosensor for Al³⁺. The sensing ability of this phenolic Mannich base toward rare earth ions showed that Eu³⁺, Dy³⁺ and Gd³⁺ induce changes in the absorption spectrum of the Mannich base. Fluorescence spectroscopy showed that the response of the sensing molecule toward Eu³⁺ and Dy³⁺ is weak, and this phenolic Mannich base may be used as a "turn-off" chemosensor for these two lanthanide ions only in a narrow concentration range (1-16 × 10^-5 M).