A novel fluorescent 'off-on-off' probe for relay recognition of Zn2+ and Cu2+ derived from N,N-bis(2-pyridylmethyl)amine

A metal-to-metal relay recognition was discovered with sequence specificity via a fluorescence 'off-on-off' phenomenon. We present here the synthesis of a new N,N-bis(2-pyridylmethyl)amine-based fluorescent sensor termed NBPA and its application as a selective relay probe for sensing of free trace zinc and copper ions. The addition of Zn2+ to NBPA causes a drastic enhancement of fluorescent intensity by the formation of a 1 : 1 NBPA-Zn2+ complex whereas other cations have no response. Moreover, Cu2+ can induce fluorescence quenching in the NBPA-Zn2+ system by the formation of a 1 : 1 NBPA-Cu2+ complex confirmed by Job's Plot. The signal change of the sensor is based on the chelation-enhanced fluorescence (CHEF) effect of NBPA-Zn2+ with the inhibition of the photoinduced electron transfer (PET) effect and the paramagnetic nature of Cu2+ to the NBPA-Zn2+ system. Under optimized conditions, the fluorescence intensity is linear to the concentration of Zn2+ and Cu2+ separately, exhibiting good linearities from 1.0 x 10(-6) M to 8.0 x 10(-6) M and 1.0 x 10(-6) M to 1.0 x 10(-5) M (R-2 = 0.9996 and R-2 = 0.9914) with detection limits of 7.89 x 10(-7) M and 1.27 x 10(-7) M, respectively. Furthermore, the rapid, selective feature enables the proposed sensor to be promising in monitoring Zn2+ and Cu2+ in biological and environmental research with good accuracy and precision.