A new off-on-off sensor for sequential detection of Al3+ and Cu2+ with excellent sensitivity and selectivity based on different sensing mechanisms

A new simple Schiff base, (E)-N'-(2-hydroxybenzylidene)imidazo[2,1-b]thiazole-6-carbohydrazide (X), was designed and synthesized based on salicylaldehyde and imidazo[2, 1-b]thiazole. X could be used as a sensor to identify Al3+ through a significant fluorescence enhancement because of CHEF and inhibition of PET process and then to detect Cu2+ through a highly efficient quenching behavior due to the paramagnetic quenching. The sensor showed a high selectivity for Al3+ and Cu2+ in methanol solution at pH = 5 and was not disturbed by other competing metal ions. Furthermore, based on the equation 3 sigma/slope, the detection limits of sensor for Al3+ and Cu2+ were calculated to be 3.1 x 10(-10) M and 9.8 x 10(-11) M, respectively. Additionally, the association constants of X[Al3+] and X[Cu2+] were also determined to be 2.3 x 10(4) M-1 and 6.1 x 10(4) M-1 on basis of Benesi-Hildebrand equation. In addition, the titration experiment of fluorescence and mass spectrometry showed that sensor was combined with Al3+ or Cu2+ both in 1:1 ratio. Moreover, the optimized structure and energy calculations of X, X[Al3+] and X[Cu2+] were obtained by Gaussian software based on the basis set of B3LYP/6-31 G(d) and B3LYP/LANL2DZ. And, the sensor successfully detected Al3+ and Cu2+ in the real water sample with a satisfactory recovery (91.4 % - 107.1 %) and RSD values (0.66 % - 1.91 %).