Study on a highly selective fluorescent chemosensor for Cu2+ and its direct sensing for proton based on 1,3,4-oxadiazole

A photoinduced electron transfer cation sensor, oxadiazole-bridge-bis(N,N-bis(2-pyridylmethyl)amine), was designed and prepared. This sensor displayed an “off-on-off” fluorescent switch for proton and an “on-off” fluorescent switch for Cu2+. The fluorescence emission of the sensor was quenched upon addition of Cu2+ and proton in CH3CN/H2O solution (v/v, 4:1) due to electron transfer between the N,N-bis(2-pyridylmethyl)amine and the oxadiazole. The sensor presented high selectivity for Cu2+ over other metal ions and determined Cu2+ concentration in a linear fashion from 5×10−7 M to 8×10−5 M. Stern-Volmer analysis showed the binding stoichiometry to be 1:2 (host-guest) with a binding constant of 1.8×1010 M−2, calculated using the Benesi-Hilderbrand equation. The response of the sensor to Cu2+ was instantaneous and reversible, and the usual anions did not influence the selectivity for Cu2+.