A retrievable and highly selective fluorescent sensor for detecting copper and sulfide

A new selective fluorescent sensor for Cu2+ and S2−, 2-hydroxy-N′-((quinolin-2-yl)methylene)benzohydrazide (HL), based on 2-methylquinoline derivative has been designed, synthesized and evaluated. The fluorescence of the sensor HL was quenched by Cu2+ with a 1:1 binding ratio, behaving as an “on–off” type sensor even in the presence of a wide range of biological cations. Once binding with Cu2+, it can display high selectivity for S2−. Among the various anions, only sulfide anion induces the revival of fluorescence of HL, resulting in “off–on” type sensing of sulfide anion. The signal transduction occurs via reversible formation–separation of complex L–Cu and CuS. With the addition of Cu2+, sensor HL give rise to a colorless to yellow color change. The resulting yellow solution switches to colorless immediately upon the addition of S2−; however, no changes were observed in the presence of other anions, including CN−, NO3−, P2O74−, various forms of sulfate, and some other reactive sulfur species (RSS) including SCN−, l-methionine (l-Me) and l-cysteine (l-Cys). Notably, the color change is so distinct that the recycling process can be seen clearly by the naked eye.