Sensing of transition metals through fluorescence quenching or enhancement. A review

A series of fluorescent sensors for transition metal ions were synthesized by linking a light-emitting subunit, anthracene, to a polyaza chelating subunit, either a dioxotetraamine or a tetraamine. Sensing of the divalent cations CuII, NiII and ZnII was investigated through spectrofluorimetric titrations in acetonitrile–N water (4:1) solutions. The selective recognition of CuII and NiII among other transition and non-transition metal ions is signalled through full quenching of fluorescence; discrimination between the two ions can be achieved by performing titrations at controlled pH. The system containing the tetraamine fragment, whose interaction with CuII and NiII induces fluorescence quenching, is also sensitive to ZnII, but in this case the recognition is signalled through a fluorescence enhancement.