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.